摘要:

~~~~~~ Diffusion of transition metals (Co,Ni) and its effects on sol-gel derived ZrO, polymorphic stabilities Hua C. Zeng* and Min Qian Department of Chemical Engineering, Faculty of Engineering, National University of Singapore, 10 Kent Ridge Crescent, Singapore 0511 Transition-metal-incorporated ZrO, gel matrices have been prepared by an impregnation method. The polymorphic forms of the studied gels at room temperature were determined by Fourier-transform IR (FTIR) spectroscopy and X-ray diffraction (XRD) techniques, and the phase transformation behaviour of the gels was investigated with using differential thermal analysis (DTA) methods. For both undoped and doped ZrO, gels, DTA reveals metastable tetragonal-monoclinic-tetragonal-cubic phase transformations in the heating process up to 1400 "C.A high-temperature tetragonal-monoclinic transition is also observed in the cooling process. For the metal-loaded ZrO, gels, it is found that the diffusing metal cations stabilize the low-temperature tetragonal phase. However, for high-temperature (900 "C)calcined gels, both as-prepared and metal-stabilized tetragonal phases are reduced substantially. Correlations between metal diffusion and gel polymorphic stabilities are also demonstrated. Zr0,-based materials have received increasing attention for a wide range of applications.'-34 Among the many materials processing techniques, the sol-gel method provides the pos- sibilities of fabricating ZrO, matrices in a tailored manner, and of large-scale industrial production.'-'The polymorphic forms and other chemical/physical properties of a ZrO, support can be designed and then engineered. More specifically, the crystallographic structure, materials density, porosity, texture, specific surface area, chemical and thermal stabilities can all be controlled to meet the desired applications.'-' In the area of polymorphic transformation, the Zr0,-based ceramic materials have been extensively studied to improve their mechanical strength and other physico-chemical proper- While monoclinic ZrO, (m) is extremely brittle at room temperature, other polymorphic types of the ZrO,, such as the low-temperature metastable tetragonal phase (t'), offer excellent mechanical There has been a wide range of ZrO, phase-transformation studies, ranging from material synthesis of a desired polymorphic effects of the stabilizing dopant^,'^,^^ tests for chemical and physical pr~perties,'~-l' and even chemical bonding and structural determination at the atomic 1e~el.I~ Using the various standard ceramic techniques, doping elements are homogeneously mixed with the zirconium and the phase-transformed products obtained are largely bulk phases.'-'' For chemical applications, Zr0,-derived materials are also the subject of considerable interest, since Zr0,-related catalyst supports have shown superior properties over conventional catalyst carriers in a variety of catalyst sy~tems.''-~~ More detailed investigations on these technologically important material systems are clearly needed.From the materials pro- cessing viewpoint, maximizing the surface area of a ZrO, catalyst support to provide more reaction sites is frequently de~irable.".~' However, from the chemical viewpoint, the sup- porting material (ZrO,) itself is also expected to participate in a chemical reaction to a certain e~tent.'''~~.~' In other words, different polymorphic forms of ZrO, matrices and their inter- actions with the catalyst would determine the ultimate catalytic performance. An ideal candidate for a ZrO, support thus needs to strike the balance between the above two aspects, i.e. surface area and catalyst/carrier overall reactivity for a specific reac- tion. As most catalytically active components, such as trans- ition metals and noble metals, are introduced to ZrO, supports by an impregnation method and then sintered at elevated temperatures, physical and chemical interactions between the ZrO, supports and active catalytic components during the calcination or reaction become a subject of importance not only for the heterogeneous catalysis but also for general, fundamental understanding of the ZrO, polymorphic trans- formation phenomena.For multi-component catalyst systems, the role of each individual metal is different. Whereas some act as active catalytic ingredients, others may function as promoters to provide the correct environment for the catalysts, which includes structural modification of the substrate through surface diffusion at the reaction temperature^.^' As part of our recent efforts in this area, two materials systems concerning the diffusion of transition metals on sol- gel-derived ZrO, matrices have been studied using X-ray photoelectron spectroscopy (XPS).30,31The surface elements (Co and Ni) selected have been demonstrated very recently to have a strong catalytic effect on the decomposition of the greenhouse gas nitrous oxide (N,0).28 In this paper, we report a systematic study on the metal (Co,Ni)-induced stabilizing effects on starting ZrO, matrices of tetragonal and monoclinic modifications.Unlike most reported cases, the stabilizing metals in the current study were introduced to the bulk phase of the matrix particles through an impregnation method. The investigation indeed showed the metal-induced tetragonal stab- ility (t") and effects on other phase transformations of the studied material systems.A combined metal system (Co+Ni) was also studied to examine the role of each metal at elevated temperatures, and their ability to stabilize the metastable tetragonal phase. Experimental The metastable tetragonal and monoclinic ZrO, matrices used in this work were prepared from the zirconium(1v) propoxide- acetylacetone-water-isopropyl alcohol system, as detailed previo~sly.~*~-~~*~'The key synthesis parameters for the as-prepared ZrO, gel (Z) are summarized in Table 1. It is Table 1 Synthesis parameters for the as-prepared ZrO, gel (Z) used in this work Zr(OPr), :Pr'OH 1:45 H,O :Zr(OPr), 3:l MeCOCH,OCMe :Zr(OPr), 1:l J.Muter. Chem., 1996, 6(3), 435-442 435 Table 2 Summary of the nomenclature of the starting gel samples and their metal-incorporated sample senes thermal treatment derived sample(s) parent gel metal@) loaded n sintering time/h AZ Z - 4 10 BZ Z - 9 10 ACZ AZ co 1 2 BCZ ACNZ BCNZ BZ AZ BZ co Co +Ni Co +Ni 1 1 1 2 2 2 ACZn ACZ co 3-9 5 BCZn BCZ co 3-9 5 ACNZn ACNZ BCNZn BCNZ important to mention that the metastable tetragonal ZrO, matrix (AZ) and monoclinic ZrO, (BZ) were produced by sintering the Z gel at 400 and 900 "C respectively for 10 h 31 The AZ and BZ gels were manually ground into fine powders, using a grinding time of 15 min The loading of the respective transition-metal ions, such as Co or Co +Ni, was then carried out by using the impregnation method This involved impregnating the well ground AZ and BZ gel powders in a 10 mol dm-3 aqueous solution of Co2+ (cobalt nitrate hexahydrate, >99 O%O, Fluka) or mixed ions (0 5 mol dm-3 Co2++O 5 mol dm-3 Ni2+, nickel nitrate hexa- hydrate, >99 09'0, Fluka) respectively for 4h under vigorous magnetic stirring Filtering and drying (100 "C, 2 h) then gave the metal-ion-loaded gel powders The resulting powders are hereafter denoted as ACZ, BCZ, ACNZ and BCNZ (A =tetragonal, B =monoclinic, Z =zirconia, C =cobalt and N =nickel), and were ready for further thermal treatment To investigate effects of the metal incorporation on the polymorphic transformation of the ZrO, matrices, the above prepared metal-loaded gel series were calcined at 300, 400, 500, 600, 700, 800 and 900 "C for 5 h in static air in a temperature-programmable furnace (Carbolite) The resultant gel samples are identified as ACZn, BCZn, ACNZn and BCNZn, where n=calcination temperature (in "C) for a designated sample Table 2 summarizes the nomenclature of the metal-loaded gel samples studied in this work, the respect- ive parent gels from which the sample series were derived are also listed for easy reference The crystallographic structure and polymorphic transform- ation of the gels upon calcination treatments were examined using FTIR spectroscopy, and the population of each thermo- stable or metastable phase at room temperature was quantified using XRD (Philips PW1710 diffractometer) Using the KBr pellet technique, the FTIR spectra of the samples were recorded with 4cm-' resolution on a computerized FTIR spectrometer (Shimadzu FTIR-8101 instrument) The sample KBr ratio adopted in this study was optimized according to the general approach that the strongest bands should have intensities in the transmission range 5-15% Typically, 1 2 mg of the finely ground ZrO, gel sample was well mixed with 260mg of dry, spectroscopic-grade KBr powder in a dry environment The spectrum background was corrected using a freshly prepared pure KBr pellet (260mg) for each recorded spectrum Forty scans were performed for each spectrum to obtain a good signal noise ratio as previously described in the literature 32 To investigate the effect of thermally driven metal diffusion on polymorphic stabilities of ZrO, gel powders, DTA studies were carried out on the two parent gels, AZ and BZ, as well as for all the metal-loaded ZrO, sample series listed in Table 2 The DTA measurements were performed on a Shimazu DTA- 50 instrument in a nitrogen atmosphere with a gas flow rate of 20ml min-' The temperature was scanned from room temperature to 1400 "C at a rate of 20 "C min-' and then back to the room temperature with the same scanning rate The sample mass was fixed at 20 0 mg in each DTA measurement Co +Ni 39 5 Co +Ni 39 5 Results FTIR, DTA and XRD studies for undoped ZrO, matrices As mentioned in the Experimental section, the starting meta- stable tetragonal and monoclinic ZrO, (AZ and BZ) gels were prepared by sintering the gel Z at different temperatures The particle size and polydispersity of ZrO, fabricated from similar material precursors have been investigated and described in detail by scanning electron microscopy, transmission electron rnicro~copy~~and by XRD and small angle X-ray scattering (SAXS) For the current material precursor system, an XRD-SAXS study showed that for xerogel powders, smooth elemental particles are in a more or less ordered internal arrangement ' According to the reported XRD results, fine- grained metastable tetragonal ZrO, of size ca 6 nm can be obtained after firing the xerogel at 500 "C 'The polymorphic information on the different ZrO, matrices was established by investigating their characteristic IR absorptions, based on the known literature 33 34 In Fig 1, two FTIR spectra of the starting ZrO, gel matrices (AZ and BZ) are displayed The Zr-0 vibration mode at 450 cm-' in the spectrum of sample AZ is a typical tetragonal phase absorption for the ZrO, matrix 33 34 In sample BZ, monoclinic ZrO, IR absorptions at 735, 657, 575, 500 and 420 cm-' can be clearly ident~fied,~~ indicating that the gel is predominantly in a monoclinic phase I 1250 1000 750 500 400 wavenum berkm-' Fig.1 FTIR spectra recorded for tetragonal and monoclinic blank ZrOz matrices (AZ and BZ) after sintering Z gel at 400 and 90O0C, respectively, for 2 h, longer calcination times (5-20 h) give similar FTIR spectra 436 J Muter Chern, 1996, 6(3),435-442 The above observations were also confirmed with the XRD results by examining characteristic diffraction peaks."," The thermal behaviour of the starting gels AZ and BZ were investigated by in situ DTA measurements. Fig.2 reports the results of the DTA investigation. There are a number of endothermic effects observed in the heating process for the AZ gel matrix. The large endothermic band at 550-1050°C can be deconvoluted into two components [marked (I) and (II)]. In the low-temperature sintered gel AZ, an endothermic band located at 495.7"C is assigned to the decomposition of the retained carbon-containing species.32 For the BZ gel, the heating and cooling curves share a similar trend.Sharp exother- mic peaks located at 890.3 (AZ) and 920.5 "C(BZ) are observed during the cooling process. The gel polymorphic forms are much more sensitive to the calcination temperature used than to the sintering time, as indicated by the DTA, FTIR and XRD findings. FTIR, XRD and DTA investigations for metal-loaded ZrO, matrices Crystallographic information on the calcined metal-loaded gels at room temperature is revealed by the FTIR and XRD studies. In particular, the ZrO, polymorphic transformation of gels upon calcination can be viewed from the evolution of the FTIR spectra. In Fig. 3 and 4, the FTIR spectra of the two different metal-loaded cases are presented. The Co-loaded tetragonal gel matrices (ACZn), shown in Fig.3(u), indicate the effect of the sintering temperature on the determination of the final polymorphic structure. When the calcination tempera- ture reaches 900 "C, the characteristic monoclinic IR absorp- tions are fully developed (ACZ9).33*34 Similar observations were obtained for the Co-Ni-loaded sample series (ACNZn) displayed in Fig. 4(4. For the metal-loaded monoclinic gels, on the other hand, the overall features of the monoclinic polymorphic form seem to be well maintained at room tem- perature, except for certain fine-absorption modifications in the FTIR spectra. Phase volume populations of the as-prepared and metal-stabilized tetragonal phases in the metal-loaded gel series BCZn and BCNZn vs.calcination temperature are presented in Fig. 5, which were calculated from the ratio of the XRD peak intensities: (t'+ t")% = III4IIII 200 600 1000 1400 TIT Fig. 2 DTA analysis for AZ and BZ gel powders from room tempera- ture to 1400°C; AZ and BZ gels are obtained after sintering Z gel at 400 and 900"C, respectively, for 5 h; arrows indicate the heating- cooling loops I I I I , J I I , I 1250 1000 750 500 400 1250 1000 750 500 400 waven um berlcm-' Fig. 3 FTIR investigations on the polymorphic structures of the Co-included ZrO, for: (a) ACZ and ACZn, and (b) BCZ and BCZn; the ACZn and BCZn were derived from ACZ and BCZ, respectively, after calcining at different temperatures (300-900 "C) for 5 h (1 1 l)t,,t,,/[1.6(1li), +(11l)t,,t,,].loThe two series follow a simi- lar trend except for the point of the BCNZ7 sample. DTA investigations on polymorphic transformation-related phenomena of the above sintered gel series are reported in Fig.6 and 7. Similar to those of Fig. 2, the metal-loaded tetragonal gels (ACZn, and ACNZn) give a range of endo- thermic bands during the heating process. In the ACZn series, the broad bands observed at 25-300 "Ccan be associated with the catalytic effects of Co cations on the decomposition of nitrogen-containing species which occurs continuously over this temperature range.,' In the cases of all the metal-loaded gels, the sharp exothermic peaks located at temperatures ranging from 932.2 (ACZ8) to 946.9 "C (BCNZ) are observed.For the original, predominantly monoclinic ZrO, matrices, new endothermic effects can be observed in the Ni incorporated gels [BCNZ-BCNZ7, Fig. 7(b)]. These newly emerged endo- thermic bands are also located at approximately 700 "C, reflecting the effects of metal diffusion on polymorphic stabilit- ies, which is the main topic of the current work and will be discussed in detail below. Discussion Metastable tetragonal-monoclinic transformation The polymorphic form of the undoped ZrO, matrices derived from the current materials system depends on the calcination temperature ~sed.',~,~~.~' It has been found that the as-prepared metastable low-temperature tetragonal form (t') is stabilized with increasing [acac]:[Zr(OPr),] ratio.' In the current work, the metastable tetragonal ZrO, is produced at 400°C for gel AZ.30-32 The absorption mode at 450 cm-' of the spectrum of AZ in Fig.1 confirms the predominant presence of the meta- stable tetragonal ZrO, phase in AZ, as it is markedly different J. Muter. Chem., 1996, 6(3), 435-442 437 1250 1000 750 500 400 1250 1000 750 500 400 wavenurnberJcm-' Fig. 4 FTIR studies on the polymorphic structures of the Co-N1- incorporated ZrO, for: (a) ACNZ and ACNZn, and (b) BCNZ and BCNZn; the ACNZn and BCNZn were derived from ACNZ and BCNZ, respectively, after sintenng at different temperatures (300-900 "C) for 5 h 54: : : : : : : : : I 0 1W2alJX1~~8007Maa>nx,lm, calcination temperaturePC Fig. 5 Volume percentages of the low-temperature tetragonal phases (t'"'') us.calcination temperature for the gel senes BCZn (+) and BCNZn (B) from the IR absorptions of the monoclinic gel BZ. The metastable tetragonal AZ transforms to the monoclinic form (BZ) over a range of temperatures with a broad endothermic band, maximum at 881.1"C (Fig. 2). The broadness of the peak may be attributable to a second process such as grain growth during heating of the AZ gel. For the metal-containing ZrO, gels, such as ACZn and ACNZn, the monoclinic transformation of the as-prepared metastable tetragonal phase will be modified owing to trans- ition-metal surface diffusion, reflected in changes in the thermal behaviour. Within each series of metal-loaded samples, the amount of doped metal(s) is a constant.Under normal circum- stances, at low sintering temperatures the metal-doped ZrO, 438 J. Mater. Chem., 1996,6(3), 435-442 \I m ACZ6 AV,1008 9 200 600 1000 1400 200 600 1000 1400 Fig. 6 Phase transforming investigations of Co-loaded ZrO, by DTA (25+1400+25"C) for the starting samples: (a) ACZ and ACZn, and (b)BCZ and BCZn phase is expected to be small and solely in the surface or near- surface regions, while at high temperatures the doped regions will be expanded but the dopant concentration should be lowered. Therefore, the doped phase and the metal concen- tration in the bulk matrices, which are two important factors affecting the final chemical constitution and structure of the gel matrix, depend heavily on both the initial surface concen- tration and the calcination temperat~re.,~ There is a metal concentration gradient for a Zr02 matrix which starts from the surface region and points perpendicularly to the bulk phase, owing to the thermally driven nature of the current metal cation incorporation.It had been found in our recent studies that the surface metal (Ni,Co) contents of the originally tetragonal ZrO, decrease sequentially with increasing calci- nation temperature^.^'.^^ Using X-ray photoelectron spec-troscopy (XPS), the found reduction in the M :Zr mole ratio (M =Ni or Co) can be as large as fourfold on the ZrO, matrix surfaces, and the final, thermodynamically stable M :Zr ratio is around 5mol% for both the Ni/Zr02 and Co/ZrO, sys- tem~.~',~~However, as XPS is a surface-sensitive techniq~e,~~,~~ a much lower M :Zr concentration ratio for the bulk phase is generally expected.The evolution of the FTIR spectra presented in Fig. 3 and 4 reveals the effect of the calcination temperature on the transformation of the as-prepared metastable tetragonal phase to the monoclinic phase. Metal diffusion, on the other hand, ACNZ \I-939.9 951 9 J l r . 1 1 . 1 200 600 1000 1400 200 600 1000 1400 TIT Fig. 7 Phase transformation studies of Co-Ni-loaded ZrO, by DTA (25+1400-+25 "C),starting with the samples: (a) ACNZ and ACNZn, and (b)BCNZ and BCNZn causes some subtle changes of the IR absorption bands, revealing a modification of the ZrO, phase structures.For ACZ8 [Fig. 4(u)], bands characteristic of monoclinic ZrO, are well recognizable at 8OO"C, indicated by the whole range of absorption modes at 735, 657, 575, 500 and 420cm-l. However, with the incorporation of both Co and Ni metals, all monoclinic features, such as the absorption band at 420 cm-' of ACNZ8 [Fig. 4(u)], are suppressed till a high calcination temperature of 900 "C is used [ACNZB, Fig. 4(a)]. The monoclinic absorption mode at 657 cm-' does not appear until 600°C in the ACNZn series CACNZ6 in Fig. 4(u)], whereas it can be detected as low as 500°C in the ACZn gel series [ACZ5 in Fig. 3(a)]. The above FTIR analysis shows the extra stability of the metastable tetragonal ZrO, phase gained by incorporating the transition-metal ions into the ZrO, matrices.Based on the experimental data of Fig. 6(a) and 7(4, note that when the calcination temperature is increased, the popu- lation or likeness of the metastable tetragonal phase for the metal-loaded gels is lessened. This is indicated by a reduction in the areas of the DTA endothermic bands, owing to meta- stable tetragonal to monoclinic transformation [Fig. 6(u) and 7(a)]. Fig. 8(u) summarizes the temperature profiles obtained from the DTA data reported in Fig. 6(u) and 7(a) for the two sample series ACZn and ACNZn. These temperatures corre- spond to the endothermic maxima which are associated with 1100 , 1000 900 0 8001% 0 200 400 600 800 1000 .-E i'loo (b) m--1 1 v - 8 ' i - l - l ' 0 200 400 600 800 1000 900 -800 -700 1'1'1.1. 0 200 400 600 BOO 1000 calcination temperaturePC Fig.8 Temperature profiles: (a) metastable tetragonal to monoclinic (T,,,,) then monoclinic to normal tetragonal (TmJ transformations observed for ACZn (+ ) and ACNZn (U)gel series; (b)monoclinic to normal tetragonal (TmJ transformation observed for BCZn (+) and BCNZn (U)sample series, and (c) variations of the component I (+ ;T,,+t,,m)and component I1 (U;TmJ in gel series BCNZn the metastable tetragonal to monoclinic transformation (?;,-,) and the monoclinic to high-temperature tetragonal transform- ation (T,,J. However, the above temperatures should be viewed mainly as endothermic behaviours of the respective gels during the phase transformation, rather than as well defined transformation temperatures, since the band is broad and the multiple processes are complex. Within the calcination temperature range 100-600 "C, all gels give similar T,,, values.The significant increase in ?;,,, starts at a calcination temperature of 700°C. In fact, above this temperature, the transformation is a monoclinic-tetragonal type (T,,J, which will be addressed in the next section. Monoclinic-high-temperature tetragonal and cubic transformations One distinct difference between the as-prepared metastable tetragonal ZrO, and metal-loaded ZrO, gels is that the second component (11) of the endothermic band in the DTA spectra is further from (I) in the cases of the metal-loaded gels [Fig.2 vs. Fig. 6(a) and 7(u)]. The component, after the metastable tetragonal-monoclinic transition, should be naturally related to the monoclinic to high-temperature tetragonal (t) transform-ati~n,~-l~although further verification is still needed by using J. Muter. Chem., 1996, 6(3), 435-442 439 high-temperature XRD and other relevant structural determi- nation techniques As discussed earlier, at high calcination temperatures, the surface metal ions will be well dispersed among the ZrO, matrices and the M Zr ratio will be reduced In view of the presence of metals in the bulk matrices, endothermic bands observed in the high-temperature range of most DTA spectra can be assigned accordingly to a metal- modified high-temperature tetragonal-cubic (c) transformation [T+cranging from 1183 6 (ACNZ8) to 1236 2°C (ACZ4) in Fig 2, 6 and 71, which occurs at a temperature much lower '' than that for the normal pure ZrO, Fig 8(b)presents temperature profiles related to the mono- clinic to high-temperature tetragonal transformation (T,, t) for the two sample series BCZn and BCNZn, which are deduced from Fig 6(b) and 7(b) Once again, note that the temperatures associated with the maxima of endothermic effects should not be viewed purely as transitional temperatures, since other thermal processes are present Similarly to the high calcination temperature region of Fig 8(a), it is found that the Tm+tfor BCZn is in general higher than that of BCNZn The variations within each set of BCZn and BCNZn are at a moderate level Note that at a calcination temperature of 900"C, the Tm+ values reported in Fig 8(a)and (b)for every sample pair (ACZ9 us BCZ9, and ACNZ9 us BCNZ9) are mutually merged Metal-incorporated tetragonal-monoclinic transformation The metal-induced extra stability for an originally metastable tetragonal phase has been demonstrated in the FTIR spectra of ACNZn sample series [Fig 4(a)] In this section, the effect of metal cations will be further explored on originally mono- clinic dominant ZrO, matrices (BCZn and BCNZn) Three basic processes are considered to be involved in the current gel sample-sintering or DTA measurements The first process, the conversion of the as-prepared metastable tetragonal phase to a monoclinic phase, is determined mainly by the thermal treatment,' i e the calcination temperature The second process is the diffusion of the metal cations into the ZrO, matrices, which retards the conversion process of the as-prepared meta- stable tetragonal phase This process is associated closely with the solid-state chemistry of the surface cations and the substrate ZrO, The third type, the conversion of the metal cation- modified (or stabilized) tetragonal phase to the monoclinic, depends on several parameters, such as cation content and calcination temperature These processes are indeed reflected in the (t' + t")% data shown in Fig 5 As BCZ and BCNZ were only heat-dried at 100°C for 2 h, insignificant metal cation diffusion should be expected The volume percentages of the initial metastable tetragonal phase (t'%) are thus estimated to be 18 5 and 22 1% respectively for the BCZn and BCNZn series (Fig 5) using the BCZ and BCNZ data (t"% ~0)In our previous studies, it was observed that meaningful cation diffusion starts at around 300 "C 30 31 At this temperature, both BCZ3 and BCNZ3 increase their (t'+ t")% values (Fig 5), owing to the stabilizing effect of the diffusing cations on the monoclinic phase (ie m+t") It has been reported for a yttria-doped tetragonal Zr02 polycrystal- line ceramic system that the rate of tetragonal to monoclinic degradation reaches a maximum at some intermediate tempera- ture between 100 and 500"C, which depends upon the yttria content and the grain size l4 By analogy, the reduction of (t' + t")% at 400 "C can be understood by recognizing it as an overall result of the above-mentioned three processes Except for BCNZ7, all samples above 400 "C show an increase-then- decrease pattern, indicating that the stabilizing effect of the metal cation has a maximum effect at around 600°C for both sample series This observation is further verified by the analysis of (t' + t")% decline at 700 "C for BCNZ7, which gives a lower value than BCZ7 at the same sintering temperature According to a phase diagram for the binary system 440 J Muter Chem, 1996, 6(3), 435-442 N~O-COO,~~Ni0 and COO phases coexist below 750°C over the mole fraction range 0 2-0 6 However, above this tempera- ture, the two simple oxides form the Ni0-Coo solid solution 37 It has been reported previously that the Ni cations diffuse significantly at 700 "C in the metastable tetragonal phase 30 Furthermore, it was also found that for the Co,Ni/ZrO, system sintered at 700 "C, XPS investigations revealed the presence of Co cations on the ZrO, surface via measuring the Co 2p3,, photoelectron The Ni 2p3,, photoelectron, nevertheless, was not at a detectable indicating a lack of Ni cations in the surface region 28 Using energy-dispersive analysis of X-rays (EDAX), however, it was found that both Co and Ni are still in the material system with an atomic ratio of 1 1 28 Combining all these observations, the fall in (t'+ t")% for the BCNZ7 sample can be ascribed to the strong diffusion of the Ni cations in BCNZ7, resulting in the insufficient metal content in the gel matrix for the tetragonal phase (t") The abnormal point at 700 "C is accompanied by a shift of the IR mode at 735 cm-l to 750 cm-' in the spectrum of BCNZ7 [Fig 4(b)], indicating that a structural change started at this calcining temperature 30 The high (t'+ t")% value observed for BCNZ8 is in agreement with the phase diagram37 that the two types of cation (Ni,Co) combine to form the solid solution (Ni0-Coo) above 750"C, which reduces the diffusion of the Ni cations throughout the phase Incidently, at 800"C, our XPS observations also revealed a surface enrichment of Ni cations on the ZrO, matrix, owing to a repulsion or segregation of the Ni cations from the ZrO, matrix when the metastable tetragonal-to- monoclinic transformation occurs 30 For the undoped monoclinic ZrO, gel BZ and the Co- containing gels BCZn, DTA investigations have shown a similar thermal behaviour for both heating and cooling pro- cesses [Fig 2 and 6(b)] However, for the low calcination temperature cases such as BCNZ and BCNZn (n= 3-7) shown in Fig 7(b), the newly emerged component (I) can be ascribed to the endothermic behaviour associated with the metal- stabilized tetragonal-metastable tetragonal-monoclinic trans-formation The observation here is consistent with the significant diffusion of Ni cations found at 700 "C which leaves insufficient metal cations to form the metal-stabilized tetragonal phase (t") 28 30 Displayed in Fig 8(c) are temperature profiles deduced from Fig 7(b) for the metal-incorporated tetragonal (t")-metastable tetragonal (t')-monoclinic transformation ( &,,t,,,) and the monoclinic-high-temperature tetragonal transformation ( Tm+J observed in the BCNZn sample series While component (11) (Tm+t)remains constant over the calcination temperature range 100-900 "C, component (I) (T,,+t,4m)also remains unaltered till 700°C As mentioned earlier, these temperatures of maxi- mum endothermic effect cannot be solely treated as well defined transforming temperatures owing to the complicity of the thermal processes In line with the above XRD analysis for the Ni diffusion mechanism at 700"C, the q,,,t,,mvalues for samples sintered below 700°C all are very similar (ca 710-720 "C), indicating again that this temperature is associ-ated with a compositional change and/or degradation of the metal-stabilized tetragonal phase (t") at this temperature Above the calcination temperature of 700 "C, the temperature of component (I) rises sharply and reaches the same common point as that of component (11)at 900 "C In fact, beyond this point, the monoclinic-high-temperature tetragonal transition becomes the dominant transition High-temperature tetragonal-monoclinic transformation In the current work, all samples in the DTA investigations were heated to 1400°C and then cooled to room temperature The major thermal processes involved in the heating routine can be ascribed to metal diffusion, grain growth, and low- temperature-high-temperature polymorphic transformations, which may occur concurrently.For samples heat-treated to above 900 "C, no 'out-diffusion, (or surface enrichment) of the metals is observed, indicating the formation of thermo-dynamically stable solid solutions between the 'in-diffusing, metals and the ZrO, mat rice^.^'.^' The final chemical composi- tion and the polymorphic properties that gels possess at high temperatures will thus determine the transformation tempera- tures during the cooling process. The sharp exothermic peaks located at temperatures ranging from 890.3 (AZ) to 946.9 "C (BCNZ) in Fig. 2, 6 and 7 are assigned to the high-temperature tetragonal-monoclinic trans-The assignment here is based on the FTIR observation that the monoclinic modification is a thermo-dynamically stable form at room temperature for samples which are calcined at above 900°C (Fig.3 and 4). This is further confirmed by the reversibility of the two polymorphic forms, since both the observed endothermic bands and the exothermic peaks are located in the same temperature region. In Fig.9(u) and (b), the temperature profiles of the high- temperature tetragonal-monoclinic transformation (T-,,) are plotted against calcination temperature, according to the DTA results (Fig. 2, 6 and 7). The effects of the metal diffusion on the tetragonal-monoclinic transformation depend on the chemical nature of the diffuser.For blank ZrO, matrices, such as AZ and BZ, q-, of the monoclinic gel is higher than that of the metastable tetragonal gel, i.e. T,,,(AZ) <T+m(BZ). This trend is observed in both the Co/ZrO, [T,,,(ACZn) <T,,,(BCZn)] and Co,Ni/ZrO, [T,,(ACNZn) <T+,( BCNZn)] systems, although the temperature gaps are noticeably reduced. For the gels with calcination temperatures higher than 800°C [Fig. 9(u) and (b)],further narrowing of the temperature gaps can be observed. In fact, both ACZ9 and BCZ9 possess a similar T,m, suggesting that the final forms of the two gels may have virtually the same structure. As for ACNZn and BCNZn, the temperature gap between each corresponding pair is wider than that between ACZn and BCZn.However, for the pair of ACNZ9 and BCNZ9, the gap is reduced again. This can be attributed to the high-temperature ncn 1 \ 930~ $0 200 400 600 800 1000 t 950 I= 940 930 0 200 400 600 800 1000 calcination temperaturePC Fig. 9 High-temperature tetragonal-monoclinic transforming tem-perature profiles (T,,,) observed for metal-incorporated samples: (a)ACZn (El) and BCZn (+ ), and (h)ACNZn (El) and BCNZn (+) treatment of the gels at 900 "C,which cancels out the difference created by metal cations at lower temperatures. Conclusions The polymorphic forms and phase transformation behaviours of the undoped and doped ZrO, gel matrices have been investigated by FTIR and DTA methods. The metastable te tr agonal-monoclinic-te tragonal-cu bic phase transform-ations have been observed sequentially in the heating process up to 1400°C for both the blank and the doped ZrO, gels.The metal diffusion and volume percentage of the low-tempera- ture tetragonal phases (t' +t") have been investigated and correlated. In the metal-incorporated Zr02 gel systems, it has been found that the diffusing metal cations stabilize the low- temperature tetragonal phase. However, a significant reduction in (t'+ t")% is observed at 700 "C for the Ni-containing ZrO, gels. The observed reduction at 700°C can be attributed to the conversion of t" to the t' phase due to in-diffusion of Ni cations, and therefore a low concentration of dopants in the matrices. For all gels calcined at 900 "C, the low-temperature as-prepared and metal-stabilized tetragonal phases are reduced significantly.The transformation temperatures of the high- temperature tetragonal-monoclinic transition for the blank ZrO, and the metal-doped gels have also been addressed in the current work. The authors gratefully acknowledge research funding (RP3920644) for the experimental study of sol-gel technology supported by the National University of Singapore and the technical assistance provided by Ms. W. Zhang. References 1 S. D. Ramamurthi, Z. Xu and D. A. Payne, J. Am. Ceram. SOC., 1990,73,2760. 2 R. D. Maschio, M. Filipponi, G. D. Soraru, G. Carturan and G. M. D. Felice, Ceram. Bull., 1992,71,204. 3 H. Vesteghem, A. Lecomte and A. Dauger, J.Non-Cryst. Solids, 1992,147/148,503. 4 G. Monros, J. Carda, M. A. Tena, P. Escribano, M. Sales and J. Alarcon, J. Non-Cryst. Solids, 1992, 147/ 148, 588. 5 R. Guinebretiere, A. Dauger, A. Lecomte and H. Vesteghem, J. Non-Cryst. Solids, 1992, 147/148, 542. 6 K. Yamada, T. Y. Chow, T. Horihata and M. Nagata, J. Non-Cryst. Solids,1988, 100, 316. 7 D. Chaumont, A. Craievich and J. Zarzycki, J. Non-Cryst. Solids, 1992,147/148,41. 8 Q.Xu and M. A. Anderson, J. Am. Ceram. SOC.,1994,77,1939. 9 S. Gutzov, J. Ponahlo, C. L. Lengauer and A. Beran, J. Am. Ceram. SOC.,1994,77, 1649. 10 R. Srinivasan, M. B. Harris, R. J. De Angelis and B. H. Davis, J.Mater. Res., 1988,3, 787. 11 R. Srinivasan, C. R. Hubbard, 0.B. Cavin and B. H. Davis, Chem.Mater., 1993,5, 27. 1~12 G. Skandan, H. Hahn, M. Roddy and W. R. Cannon, J. Am. Ceram.SOC.,1994,77, 1706. 13 P. Li, 1. W. Chen and J. E. Penner-Hahn, J. Am. Ceram. SOC.,1994, 77,118; 1281; 1289. 14 J. F. Jue, J. Chen and A. V. Virkar, J. Am. Ceram. SOC., 1991, 74, 1811. 15 Ph. Colomban and E. Bruneton, J. Non-Cryst. Solids, 1992, 147/148,201. 16 J. M. Mariot, S. Hare1 and C. F. Hague, Appl. Surf. Sci., 1993, 65166,337. 17 K. Nakashima, K. Takihira, T. Miyazaki and K. Mori, J. Am. Ceram. SOC.,1993,76,3000. 18 K. Tanabe, Mater. Chem. Phys., 1985,13,347. 19 P. D. L. Mercera, J. G. van Ommen, E. B. M. Doesburg, A. J. Burggraaf and J. R. H. Ross, Appl. Catal., 1991,71,363. 20 P. D. L. Mercera, J. G. van Ommen, E. B. M. Doesburg, A.J. Burggraaf and J. R. H. Ross, Appl. Catal., 1990,57, 127. 21 P. Turlier, H. Praliaud, P. Moral, G. A. Martin and J. A. Dalmon, Appl. Catal., 1985,19,287. 22 G. R. Gavalas, C. Phichitkul and G. E. Voecks, J. Catal., 1984, 88, 54. J. Mater. Chem., 1996, 6(3), 435-442 441 23 24 25 26 27 28 29 30 K E Smith, R Kershaw, K Dwight and A Wold, Mater Res Bull, 1987,22,1125 S Narayanan and G Sreekanth, J Chem SOC ,Faraday Trans 1, 1989,85,3785 P Marginean and A Olanu, J Catal, 1985,95, 1 L A Bruce and J F Mathews, Appl Catal, 1982,4,353 L A Bruce, G J Hope and J F Mathews, Appl Catal, 1983, 8,349 H C Zeng, J Lin, W K Teo, J C Wu and K L Tan, J Mater Res ,1995,10,545 J W Mayer and S S Lau, Electronic Materials Science, Maxwell-Macmillan, New York, 1990, p 183 H C Zeng, J Lin, W K Teo, F C Loh and K L Tan, J Non-Cryst Solids, 1995,181,49 31 H C Zeng, J Lin and K L Tan, J Mater Res, 1995,10, in press 32 H C Zeng and S Shi, J Non-Cryst Solids, 1995,185,31 33 J C Debsikdar, J Non-Cryst Solids, 1986,86,231 34 C M Phillippi and K S Mazdiyasni, J Am Cerarn SOC, 1971, 54,254 35 A B Christie, in Methods of Surface Analysis, ed J M Walls, Cambndge University Press, New York, 1988, p 127 36 T L Barr, J Vac Sci Techno1 A, 1991,9,1793 37 W D Kingery, H K Bowen and D R Uhlmann, Introduction to Ceramics, John Wiley and Sons, Singapore, 1991, ch 7, p 290 Paper 5/05436B, Received 14th August, 1995 442 J Muter Chem, 1996, 6(3), 435-442

ISSN:0959-9428    

DOI:10.1039/JM9960600435

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Preparation of uniform zinc oxide colloids by controlled double-jet precipitationt Qiping Zhongl and Egon Matijevik" Centerfor Advanced Materials Processing, Clarkson University, Box 5814, Potsdam NY 13699-5814, USA A new and simple method for the preparation of uniform zinc oxide colloidal particles of different morphologies by controlled double-jet precipitation is described. Scanning electron microscopy and X-ray diffraction data indicate that the solids generated under different conditions have identical crystallite structures, although they are internally composite. The growths of the subunits and of the particles themselves follow different patterns. Particles formed in the presence of a surfactant (sodium dodecyl sulfate) exhibit essentially the same overall characteristics, but the particles are microporous.Colloidal zinc oxide is of interest both for its many uses and for the fundamental understanding of the formation of colloidal metal oxides. The latter is aided by the properties of the zinc ion, which appears in only one oxidation state and it does not hydrolyse as readily as some other multivalent metal ions. Traditionally, zinc oxide has been used in rubber and adhesive industries. During the past three decades, the chemical industry has opened new markets for this material, such as in the production of catalysts, semiconductors, advanced cer-amics, to name a In these highly technical applications, particle size and shape become very important parameters that determine the physical properties of the product^.^,^ A number of techniques have been developed that yield particles of different, but uniform shapes, and of narrow size distribution in pm and sub-pm range^.^,^ It has been shown that the precipitation of zinc oxide from homogeneous solu- tions can yield particles of narrow size distribution and of different morphologies, yet their X-ray pattern is always characteristic of zincite.Thus, monodispersed ZnO colloids were obtained by ageing at elevated temperatures solutions of zinc nitrate to which various bases were added.' Solutions of Zn(NO,), heated in the presence of urea resulted in uniform spherical zinc basic carbonate, which at elevated temperatures decomposed to zinc oxide.g Hydrolysis of zinc salt solutions in polyols also produced zinc oxide," but the particles so obtained contained significant amounts of carbon from organic compounds, which were difficult to remove by washing.In a much more involved two-step process, Haile and Johnson also prepared monodispersed ZnO particle^.^ As a rule, the conditions that yield uniform dispersions tend to be rather restrictive, requiring low concentrations of reac- tants. Thus, it is desirable to develop methods that could generate large quantities of these materials. One such technique is controlled double-jet precipitation (CDJP), which has been primarily employed in the photographic industry for the production of silver halide crystals of narrow size distribution, as well as of well developed habit, precise internal composition, and epita~y.ll-'~ The rate of production of these materials is about 1 mol AgX dm-3 h-l in containers as large as 2000 dm3.The CDJP technique was also used to generate relatively large crystals of various other sparingly soluble salts, including metal oxalates and sulfate~.l~*~~ Recently, uniform nanosize pure and doped barium titanates were produced by this method in a very short time and in large quantities.16,17 This study describes the use of the CDJP process for the synthesis of uniform zinc oxide particles of different morpho- logies, and offers an explanation for the mechanism of their formation. t Supported by the Procter & Gamble Company, Cincinnati, OH. 1Part of a Ph.D. thesis.Experimental Materials All chemicals of reagent grade quality were used without further purification. In order to remove any possible particulate contaminants, stock solutions were filtered through 0.2 pm pore size Nuclepore membranes. Procedures The experimental setup, schematically represented in Fig. 1, consists of a 0.50 dm3 reactor equipped with a heating jacket. Into the reactor was added 0.10dm3 of water preheated to 90 "C, with a stirring rate of 500 rpm, followed by 0.10 dm3 each of a 0.02 mol dm-3 zinc nitrate (which was later increased to 0.1 mol dm-3) and of 1.6 mol dmP3 triethanolamine (TEA) solution, introduced simultaneously through glass tubes at a constant flow rate, controlled by peristaltic pumps. The system r Th Eea t I Fig.1 Schematic presentation of the controlled double-jet precipi- tation (CDJP) reactor: the 0.50dm3 container is equipped with a heating jacket, into which Zn(N03), and triethanolamine (TEA) solutions were introduced simultaneously, under a stirring rate of 500 rpm J. Muter. Chem., 1996, 6(3),443-447 443 Fig 2 SEM images (5 mm =1 pm) of zinc oxide particles prepared at 90 "C using CDJP by adding 0 10 dm3 each of 0 02 mol dm '?(NO,), and 1 6 mol dm TEA solutions into 0 10 dm3 H,O at flow rates 0 003 (a) 0 007 (b) 0 009 (c) 0 014 (d) 0 02 (e) and 0 6 (f)dm3 min was then kept for 30 min at 90 "C, while under continuous of 0 001 mol dm sodium dodecyl sulfate (SDS) solution was agitation The addition rates varied from 0 003 to 0 60 dm3 placed into the reactor instead of water min ' in different runs, in order to evaluate the influence of After ageing the particles were separated from the super- this parameter on the particle morphology and size (Table 1) natant solution by centrifugation and washed with water It was established that accelerating the stirring had no effect several times The powders were then dried in a vacuum oven In expenments dealing with the effect of a surfactant, 0 10 dm3 at 50 "C overnight 444 J Mater Chem 1996, 6(3),443-447 Table 1 The flow rate and the addition time of Zn(NO,), and TEA solutions into the 0.50 dm3 reactor, containing 0.10 dm3 water at 90 "C, in CDJP process' flow rate/dm3 min-' 0.003 0.007 0.009 0.014 0.02 0.6 0.009 addition time/min 33 15 11 7 5 0.17 11 'In all cases, the total added solution volume was 0.10 dm3 each of 0.02 mol dmP3 Zn(NO,), and 2.6 mol dmp3 TEA.In the presence of the surfactant. Characterization Particle morphology and size were determined from scanning electron micrographs (SEM). The crystal structure was charac- terized by X-ray diffraction, and the crystallite size was calcu- lated from the analysis of the line broadening features using the Scherrer formula." The specific surface area of the powder was determined by the nitrogen adsorption method (BET) using three-point measurements. Thermogravimetry (TG) and differential thermal analyses (DTA) were carried out at scan- ning rates of 20 and 10°C min-l, respectively. Results and Discussion Particle shapes The preliminary investigations showed that the shape and size of the ZnO particles depended strongly on the flow rate of zinc nitrate and TEA solutions (Fig.2). The slow addition of reactants resulted in longer, but smaller particles, as illustrated by samples (a) and (b)in Fig. 2. With increasing flow rate, the long axis decreased [(c),(d)and (e)]and, finally, nearly spherical larger particles with a diameter of 1500+300 nm were formed (f). In the presence of ca. mol dm-3 of SDS, only much smaller spherical particles precipitated, as shown in Fig. 3 (g), which were prepared as in Fig. 2(c), but in the presence of 0.001 mol dmP3 SDS. In order to increase the solid content of the resulting particles, the concentrations of the same volume (0.10 dm3) of reagents were increased to 0.1 mol dm-3 Zn(NO,),, 3mol dm-3 TEA, and 0.005mol dm-3 SDS.Identical particles as shown as sample (g) were produced in five times higher amounts. Once SDS was added, the process seemed to be insensitive to the addition rate and reagent concentrations over a broad range; spheres of 200 & 30 nm in diameter were obtained in all cases. However, neither the addition rate nor the surfactant had any effect on the crystal structure of the resulting particles. Fig. 3 SEM image (20 mm = 1 pm) of zinc oxide particle (g)prepared under the same conditions as sample (c), except the water was replaced by 0.10 dm3 of 0.001 mol dmP3 sodium dodecyl sulfate (SDS) solution Crystallite and particle size The X-ray diffraction patterns of samples (c), (f) and (g) (Fig.4) showed that in all cases the same crystalline material was produced, regardless of the geometric shape of the particles (in the absence or in the presence of the surfactant). The corresponding d-spacings matched well with reported values (Table2).19 The crystallite sizes, based on the broadening of the [1001 and [1011 reflections, depended on the conditions of the powder preparation (Fig. 5). Thus, the crystallites of sample (a)were 107 nm long, while those of sample (f) were much smaller (20 nm), although the latter constituted the largest particles. The precipitates synthesized in the presence of SDS [sample (g)]had the smallest crystallite size (15 nm).A model, based on crystal growth by bulk diffusion and the Gibbs-Thomson effect, suggested by several pre-dicted that the number of stable nuclei should increase with I'l.1'I'I' 1200 I 1000 800 400 200 0 20 30 40 50 60 70 80 26Ndegrees Fig. 4 XRD spectra of ZnO powder samples (c),(f)and (g) Table 2 X-Ray diffraction patterns for ZnO powders prepared by CDJP process compared to literature values'' sample d-spacings, at A= 1.5418 A 2.81 2.6 2.48 1.91 1.62 1.48 1.41 1.38 1.36 2.82 2.60 2.48 1.91 1.63 1.48 1.41 1.38 1.36 2.81 2.61 2.47 1.91 1.62 1.48 1.41 1.38 1.36 2.81 2.60 2.47 1.91 1.62 1.48 1.40 1.37 1.36 2.81 2.60 2.47 1.92 1.63 1.47 1.42 1.38 1.36 2.81 2.60 2.47 1.92 1.63 1.47 1.42 1.38 1.36 2.81 2.61 2.48 1.91 1.63 1.48 1.42 1.38 -2.82 2.60 2.48 1.91 1.63 1.48 1.42 1.38 1.36 2o 1T/-l-l-i0 5 10 15 20 25 30 550 600 650 reactant flow rate/103 dm3 min-' Fig.5 Crystallite dimensions as a function of the reagents addition flow rate of Zn(NO,), and TEA solutions: 0,dimension along [1011; 0,dimension along [1001 J. Mater. Chem., 1996, 6(3), 443-447 445 Table 3 Number of stable ZnO crystallites (2)as a function of reactant addition rate (R)of Zn(NO,), and TEA solutions sample R/mol s cm Z/no cm 100x10 2 4 x lolo 2 17 x lop8 7 9 x 1o1O 300x10 ’ 12 x loll 4 67 x lo-* 2 8 x lo1’ 6 67 x 10 ’ 7 5 x lo1’ 200x10 3 7 x lo’, increasing addition rate of reactants, resulting in a smaller particle size The validity of this model was experimentally tested by the double-jet precipitation of AgBr 2o 23 The number of stable ZnO crystallites per unit suspension volume was determined from the following mass balance [eqn (l)], 2=3 VmRt/4nr3 where 2 is the number of stable nuclei, V, the molar volume and r the crystallite radius of precipitated solids, R the rate of reactant addition per unit suspension volume, and t time Here, spherical morphology is assumed for simplicity Table 3 shows the effect of reactant addition rate on the number of stable ZnO crystallites (Z), while log-log plots of 2 us R for the experimental data given in Table2 are presented in Fig 6 Clearly, the crystallites of ZnO particles produced by CDJP followed the same rule, ie, the number of stable nuclei increased, while the crystallite size decreased with increasing addition rate of reactants It is essential, however, to recognize that the crystallite size differs from the final particle size, the latter actually increases with faster addition rates (Fig 2) Obviously, the proposed mechanism applies only to the case of single-crystal growth The precipitation of zinc oxide particles must follow a process that differs from the generation of internal subunits Recently, it has been recognized that in the majority of cases the formation of ‘monodispersed’ colloids proceeds through a two-stage mechanism the primary particles (as in the case of zinc oxide) are produced first, which then aggregate to larger final products 24 However, the relationship of the resulting morphology to the precursor particles has been explained only in a few cases, such as in the formation of haematite 25 26 There are two possible mechanisms involved in the aggregation, based either on diffusion-limited or reaction-limited pro-cesses2’ The subunits have to overcome an energy barrier to effectively collide with growing particles If the driving force is 12 0-I 11 5-E2“,11 0-0, 105-loo!, ., -1 . I -I -I 80 78 76 74 72 70 log (R/mol s-’ cm-3) Fig. 6 Number of stable ZnO crystallites as a function of the reactant addition flow rate of Zn(NO,)z and TEA solutions 446 J Muter Chem, 1996, 6(3), 443-447 0-final crystallite size along final crystallite size along dimension [100]/nm dimension [10l]/nm 107 81 76 65 63 63 47 46 37 33 26 25 high enough, aggregation is rapid and the rate is limited by the diffusional motion of the colliding subunits Thus, the highly concentrated small crystallites are aggregated to large spherical particles due to their faster Brownian motion and higher collision efficiency, such as in sample (f) (Fig 2, 5, and Table 3) In the case of less concentrated larger subunits, the driving force is low, and aggregation may only happen on the certain ‘active’ sites of crystallites, especially when the latter are asymmetric or are built with different geometric faces 28 30 This kind of oriented aggregation may lead to the formation and growth of columnar or needle type particles, as in the case of sample (a) (Fig 2) The ZnO particles indeed changed systematically from elongated [sample (a)] to spherical [sample (f)]with decreasing asymmetry and smaller size of crystallite subunits (Fig 5), which is expected from the aggregation theories Surface area A closer inspection of the scanning electron micrographs (Fig 2,3) shows that the surfaces of samples (f) and (g)appear rough, indicating formation by the aggregation of small crystal- lites, as supported by the X-ray analysis This nature of the particles is also reflected in the high specific surface area as determined by BET The particle size calculated from the BET data, based on a density of 5 6 g cm for zinc oxide, yielded a diameter of ca 710 nm (specific surface area, SSA, 1 5 m2 g-l) for sample (f) and of ca 28 nm (SSA, 38 m2 8-l) for sample (g) While BET measures the specific surface area of the particles, including pores, XRD depends only on the crystalline regions of the sample It is, therefore, not surprising that the sizes calculated from BET data are larger than those determined by XRD Obviously, sample (g)(prepared with SDS) was porous, which must have been formed by aggregation of a larger number of ’*05 I 975 1 t I.I 100 200 300 400 600 TI C Fig.7 Typical TG curve for the zinc oxide powder sample (a) at a heating rate of 20°C min in air Similar curves were recorded in all cases order to develop a better understanding of the reasons for 40 A35 TI"C Fig.8 DTA curve of samples (a),(f) and (8)(heating rate 10°C min-' in air) small particles, possibly incorporating some surfactant Sample (f) consisting of larger crystallites, may owe its BET area to the surface roughness of the particles Thermal behaviour TG and DTA curves are presented in Fig 7 and 8, respectively The endothermic peak occurred around 100"C and the related mass loss arose from the loss of water, which in all cases amounted to ca 3% The exothermic peak at ca 370 "C, which was rather weak for sample (a) but much more pronounced for samples (f)and (g), corresponded to the crystallization of zinc oxide The calcined samples were re-examined by SEM and XRD, which showed that the crystallite size of (f)increased from 25 nm to 31 nm, and that of sample (g)from 15 nm to 22 nm, but no change was observed with sample (a), which agreed well with the DTA data The size and the shape of the final particles, on the other hand, remained the same in all samples Conclusions This work demonstrated the feasibility of the preparation of monodispersed zinc oxide powders by controlled double-jet precipitation The morphology and the size of the particles were dominated by a single parameter, z e the flow rate of the reactants All particles were built from crystalline subunits, but the precursors and the final products followed different growth mechanisms Obviously, the nucleation, crystallite growth, and the aggregation processes should be distinguished in the forma- tion of these solids More systematic studies are necessary in varying particle sizes and morphologies Surfactant SDS greatly affected the properties of the prod- ucts obtained by CDJP The particles so obtained were micro- porous with a high specific surface area and were made up of aggregates of very small particles whose size is ca 28 nm References 1 A S Perl, Am Cerum Soc Bull, 1993,72, 122 2 M Haase, H Weller and A Henglein, J Phys Chem ,1988,92,482 3 L Spanhel and M Anderson, J Am Chem Soc , 1991,113,2826 4 S M Haile and D W Johnson, Jr, J Am Cerum SOC,1989, 72,2004 5 L Koudelka and J Horak, J Muter Scz , 1994,29,1487 6 E Matijevic, Chem Muter, 1993,5,412 7 E Matijevic, Lungmuzr, 1994, 10, 8 8 A Chittofrati and E Matijevic, Collozds Surf, 1990,48,65 9 M Castellano and E Matijevic, Chem Muter, 1989, 1, 78 10 D Jezequel, J Guenot, N Jouini and F Fievet, J Muter Res, 1995,10,77 11 I H Leubner, in Reprographzc Technology, American Chemical Society, Washington, DC, 1982 12 C R Berry, in The Theory of the Photographic Process, 4th edn, ed T H James, Macmillan, New York, 1977 13 J Stavek, M Sipek, I Hirasawa and K Toyokura, Chem Muter, 1992,4545 14 J Stavek, T Hamslik and V Zapletal, Muter Lett, 1990,9,90 15 H Muhr, R David, J Villermaux and P H Jezequel, Chem Eng Scz , 1995,50,345 16 Y S Her, E Matijevic and M C Chon, J Muter Res, in press 17 Y S Her, E Matijevic and M C Chon, J Muter Res, 1995, 10,3106 18 E Warren, X-Ray Dzfructzon, Addison-Wesley, Reading, MA, 1969 19 Fink Index to the Powder DzfSructzon File, ASTM, 1966, NO 5-0664 20 T Sugimoto, J Colloid Interface Scz ,1992,150,208 21 V G Loginov and N B Denisova, Zh Nauchn Przkl Fotogr Kznemutogr , 1975,20,231 22 I H Leubner, R Jagannathan and R S Way, Photogr Scz Eng, 1980,24,268 23 E Klein and E Moisar, Ber Bunsenges Phys Chem ,1963,67,348 24 J Stavek and J Ulrich, Cryst Res Technol, 1994,29,465 25 M Ocaiia, R Rodriguez-Clemente and C J Serna, Adv Muter, 1995,7,212 26 J K Bailey, C J Brinker and M L Mecartney, J Colloid Interface Scz , 1993,157,l 27 J Stavek and J Ulrich, Cryst Res Technol, 1994,29,763 28 Y A Brasley, V V Peisakhov and L Y Kaplun, Usp Nuuchn Fot , 1986,24,5 29 P H Karpnski and J S Wey, J Imug Scz ,1988,32,34 30 T Sugimoto, J Imug Scz , 1989,33,203 Paper 5/05437K, Received 14th August 1995 J Muter Chem , 1996, 6(3), 443-447 447

ISSN:0959-9428    

DOI:10.1039/JM9960600443

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Structure of a novel form of carbon: dehydropolycondensed adamantane? ~~~~~~~ ~ Jane S. Rigden," K. Jarmo Koivusaari,ta Robert J. Newport,*" David A. Green,b Graham Bushnell-Wye" and John Tomkinsond "Physics Laboratory, The University of Kent at Canterbury, Canterbury, UK CT2 7NR bChemistry Department, University of Reading, Whiteknights, Reading, UK RG6 6AD 'Daresbury Laboratory, Daresbury, Warrington, UK WA4 4AD dRutherford Appleton Laboratory, Chilton, Didcot, UK OX11 OQX The paper presents synchrotron X-ray diffraction and complementary transmission IR spectroscopy data on a series of products of adamantane in an attempt to elucidate the detailed nature of their structure. Although qualitative similarities between this work and earlier studies have been identified, the X-ray diffraction measurements, when coupled with IR absorption analysis, argue for a very different interpretation of the structural changes associated with the chemical processes involved in generating this suite of materials.It is clear that 1,3,5,7-tetrabromoadamantanehas indeed been formed from the adamantane precursor, but that there are some Br sites that are occupied by carbon atoms. 'Polymerisation' of these units into an amorphous network with no residual long-range ordering may then be induced through chemical processes; the Clo carbon units are broken up to form an amorphous network (or 'patchwork') of unit fragments. The corroborating presence of short hydrocarbon chains is also revealed. Once the material is heat treated it begins to revert to a graphitic structure, with the hydrocarbon chains being eliminated.Carbon is probably the most widely studied of the known elements. Various forms of amorphous carbon, and in particu- lar amorphous hydrogenated carbon (sometimes referred to as 'diamond-like' carbon, a-C:H), have become of increasing significance in recent years following the development of CVD- based deposition This family of materials con- tains a mixture of sp3, sp2 and (sometimes) spl carbon bonding, with the nature of the mixture of bonding depending on the conditions under which the a-C:H was prepared; the incorpor- ated hydrogen plays an important role and has itself been the subject of much st~dy.~-~ A paper by Kasatochkin et aL7 claiming a 'wet chemistry' route to an amorphous carbon, which they termed dehydropolycondensed adamantane, using adamantane as the precursor and based on its polymerisation through the removal of hydrogen, was therefore of considerable intrinsic interest.Their work presents an outline description only of the preparative chemistry involved, and the structural study is limited to IR transmission spectroscopy and the use of a low intensity laboratory X-ray source which, with some additional electron diffraction, provides qualitative data only. Given the suggested novelty of the material itself it is important to establish the nature of its structural parameters with pre- cision: the work reported here was undertaken with this aim. The precursor for the materials studied here is adamantane.The essence of the adamantane molecule's structure' is that the 10 carbon atoms form a cage as depicted in Fig. l(a); it consists of two distinct carbon atom sites: four C, atoms which lie at the vertices of a tetrahedron and six C, atoms which lie at the vertices of an octahedron. The overall cage has tetra- hedral symmetry. Four-fold coordination of the carbon atoms is maintained with the addition of the 16 hydrogen atoms as depicted in Fig. l(b). Although (highly crystalline) polyadamantane was first pre- pared more than 30 years agog by heating 3,3-dibromo-l,l- diadamantane with metallic sodium, the route adopted here and by Kasatochkin et aL7 avoids the problems of steric hindrance generated by the size of the bromine atoms, which are used to replace hydrogen atoms, and allows a more complete removal of hydrogen by employing a repeated process t Permanent address: Department of Physics, University of Oulu, Linnanmaa, PL 333, 90571 Oulu, Finland.of bromination (initially to 1,3,5,7-tetrabromoadamantane)fol-lowed by the removal of the halogen using metallic sodium. It is therefore expected that a large degree of bonding between the Clo cages will be possible, resulting in a three-dimensional (3D) polymer network. Sample Preparation Adamantane (99 +YO),aluminium bromide (98 +YO)and bro- mine were purchased from Aldrich, and the bromine was dried by distillation from phosphorus(v) oxide before use. A 30% m/m dispersion of sodium metal in toluene was purchased from Fluka.Glacial acetic acid, ethanol and toluene were 'AnalaR' grade and purchased from BDH. The first stage of the sample preparation process is the formation of tetrabromoadamantane from the adamantane precursor:" CloH16 +4 Br, +CloH12Br4 +4 HBr Adamantane (10.0 g) was added to a stirred mixture of alu- minium bromide (17.6 g) and dry bromine (40 cm3) in an ice bath. After the addition was complete and the initial reaction had subsided, the mixture was heated to reflux at 60°C for 24 h with stirring. The excess bromine was then distilled from the reaction flask, and the remaining solids were treated with aqueous sodium metabisulfite to remove the last traces of bromine and hydrolyse the aluminium bromide.The product was collected, washed with water and recrystallised from glacial acetic acid (a yield of 61%) to give a light tan product. This was shown to be 1,3,5,7-tetrabromoadamantaneby its IR spectrum (see below) and with supportive evidence from mass spectroscopy measurements and C,H,N combustion analysis. This is referred to as sample 1. The second stage is centred on the Wurtz reaction:" nCloHl,Br, +4nNa+(C,,H,,), +4n NaBr Tetrabromoadamantane (16.0 g) was added to a stirred disper- sion of sodium (6.5 g) in toluene (60 cm3) and heated to reflux for 48 h with stirring. After allowing the reaction mixture to cool, ethanol (50cm3) was added to remove the sodium, and the solids were collected and washed with toluene.The sodium bromide was removed by stirring the solids in warm (50°C) J. Mater. Chem., 1996,6(3), 449-454 449 n Fig. 1 (a) Diagrammatic representation of the adamantane C,, cage structure (after ref. 8); (b) conventional ball-and-stick view of adamantane, showing the decoration of the C,, cage structure with hydrogens; (c) the analogous view of the principal intermediate stage, tetrabromoadaman tane water for 20min, after which the remaining product was collected and dried. This product (an off-white solid; yield 75.5%) is insoluble in toluene and ethanol, and according to Kasatochkin et aL7 it is the product of the 3D polymerisation of adamantane at its four apices (we dispute their interpret- ation, see below). This is referred to as sample 2.A third stage involved heating sample 2 in argon by placing it in a Pyrex boat and heating it in a horizontal tube furnace at 400°C for 2 h, with a steady stream of argon passing through. The powder darkened and lost about one-third of its mass (i.e. a yield at this stage of 67.2%. Note that if heated further to 450 "C, the powder turns black, but its mass remains constant). This product is referred to as sample 3. In the final stage, sample 3 (1.05 g) was heated at reflux in bromine (60 cm3) for 24 h, and the residual bromine removed as in stage 1. This dark powder (brominated polyadamantane, 2.37 g) was stirred under an argon atmosphere in a bath of liquid sodium (17 g) at 140°C for 1 h. This is intended to remove the bromine atoms and to cross-link the polymer further. The sodium was removed with ethanol, and then water was added, and the resultant solid was collected.The product 450 J. Muter. Chem., 1996, 6(3), 449-454 was finally heated to 400°C in a tube furnace under vacuum for 2 h to yield a dark powder; this observation is in immediate contrast to that of Kasatochkin et d7who cite the product as being white. Experimental Methods The X-ray diffraction experiments were performed at the SRS, Daresbury Laboratory, UK, using a high intensity line with the synchrotron radiation being produced through a 5T superconducting wiggler. The diffraction experiment was per- formed on flat-plate samples in 8-200 transmission geometry, and at an X-ray wavelength of 0.62A (which was calibrated using the K-shell absorption edge of molybdenum).This transmission geometry was chosen as it simplifies many of the necessary corrections. The 28 angular range measured was chosen to be 2-130" in 0.2" steps, giving a nominaloQ range [Q =(471. sin O)/A, the wavevector transfer] of 0.4-23 A-'. The sample itself was held in a flat-plate container of 0.5mm thickness with Kapton foil windows. The diffraction data were normalised to allow for variations in incident flux, corrected for beam polarisation, background scattering effects and sample illuminated volume variation on rotation. The correction and normalisation procedures adopted were broadly those described by Huxley,12 though much simplified here given the dominance of Bragg scattering in all but one of the samples studied.General data analysis is based upon the text by Warren.13 One important feature of the experimental method is that the Warren-Mavel meth~d'~.'~ was adopted in order to suppress Compton sc!ttering: in this case a molybdenum foil (K-edge at A=0.620 A) was used at the position normally occupied by the X-ray detector (see Fig. 2) and the fluorescence intensity then measured. In this way, to a good approximation, only elastic scattering events are recorded since those X-rays scattered incoherently with an associated energy loss will be unable to excite the Mo K-edge fluorescence. IR spectra were measured on a Perkin-Elmer 1720-X FT IR spectrometer. The IR samples were prepared by grinding a small amount of the powder with dried potassium bromide, which was then pressed into 13 mm discs.Results and Discussion The X-ray diffraction and IR absorption spectra for sample 1 are shown in Fig. 3(u), (b)respectively. The diffraction pattern detector -c\l*-Mo f0ll / scattered beam Fig. 2 Warren-Mavel experimental geometry 0 12 0 10 - .?0 08 3 50 06@ h c.- v) 004 .- 0 02 0 00 I 0 2 4 6 8 10 07 06 s 0s (d cY 5'-04 2 03 02 O1 1' 0 1 I I 1 4000 3500 3000 2500 2000 1500 1000 500 wavenumberlcrn-' Fig. 3 (a) Diffraction spectrum for sample 1; (b) IR absorption spectrum for sample 1 is easily interpreted in terms of the expected dominance of 1,3,5,7-tetrabromoadamantane.The associated bond lengths and pairwise separations, derived by applying Bragg's equation to the observed diffraction peaks, are listed in Table 1 together with their chemical assignments; these may be compared to published crystallographic values for tetrabromoadamant-ane.l63'' The one peak that does not easily fit intq this interpretation is that associated with a distance of 3.86 A; this feature is present in the data of Kasatochkin et aL3 also, but was not discussed by them.However, the difficulty may be resolved in a relatively straightforward way if one assumes that some of the (nominally) Br sites are in fact occupied by carbon atoms, since the calculated distance fr9m this carbon atom to those in the nearest Clo 'cage' is 3.89 A.The IR data for sample 1 was interpreted largely on the basis of existing work on adamantane,'* though bearing in mind the obvious fact that the presence of the relatively massive Br atoms will cause some shift to peak positions. The band at 716 cm-l derives from the CH2 rocking frequency and appears in all aromatic hydrocarbons containing more than four methylene-like groups.lg Note the absence of a strong band at 2900 cm-' which confirms that the majority of the methine groups have been removed by bromination. The doublet at 1450 and 1441 cm-I is the associated CH2 defor- mation which normally appears at 1465 cm-', but when derived from a many-ring system2' may be seen with compo- nents in the regions 1450-1485 and 1436-1450cm-l.The non-adamantane feature at 486 cm-' is the Br-C fundamental stretching mode. Note the weakness of the CH2 stretching mode band shown in the range 2850-2930cm-'; this is probably due to the nature of the carbon ring structure in bromoadamantane given the inverse correlation noted by Boobyer and Weckherlin" between the size of other such structures and the observed absorption band intensity. Other than the usual water/CO bands, the remaining features in the spectrum are associated with C-C skeleton vibrations. Note the similarity between the spectrum shown here and that measured for 1,3,5,7-bromoadamantane by Sollot and Gilbert,22 we therefore confirm the general nature of the compound as outlined in the original Russian paper with the small differences noted above.The diffraction data and IR absorption spectrum for sample 2 are shown in Fig. 4(a) and (b)respectively. All traces of the ordered structure have disappeared with the removal of bromine and we are left with diffuse scattering from what is now an amorphous network. After removing the underlying background and self-scattering terms from the spectrum it may be Fourier transformed to reveal the pair correlations in real- space shown in Fig. 4(~).~Only two broad features are evident, centred at 1.4 and 2.48 A, which are strongly suggestive of a highly disordered network based on graphitic bonding. This sample corresponds reasonably well to the disordered polyada- mantane model suggested by Kasatochkin et ~l.,~though it is Table 1 Synopsis of diffraction data from all samples d(=1/2sinB)/A (Bragg) correlation length/A sample Q/k' measured [tables/calculated] (amorphous) assignment 1 1.38 4.55 C4.25, 4.731 C-Br 1.60 1.75 3.93 C3.8831 3.59 C3.5671 C-C (extra-C,, unit) c-c 2.22 2.52 2.83 [2.85] 2.49 C2.5221 C-Br c-c 2.93 2.14 C2.1671 C-H 2 3, 4 3.20 ca. 3.5 ca.4.2 ca. 2.9, ca. 4.9 1.1 1.82 2.11 2.98 3.49 3.64 4.22 4.59 1.96 [1.931 1.8 [1.781 1.5 [1.5451 5.7 (absent from 4) 3.41 C3.361 2.98 C2.841 2.11 C2.13, 2.031 1.80 C1.801 1.73 [1.681 1.49 [1.541 1.37 2.5, 1.4 C-Br (H-H?) c-c C-C-C, C-C (disordered graphitic) approx. graphitic, see text graphite (0.02) (across graphitic ring) graphite {loo}, { 101) graphite { 102) graphite { 004) graphite { 103) 4.7 1 1.33 5.15 5.48 5.96 1.22 C1.231 1.15 [1.16,1.14, 1.121 1.05 5.2, 4.2, ca.2.6, 1.42 graphite ( 110) graphite (112}, { l05}, (006) graphite (201) disordered graphitic (sample 3 only) J. Muter. Chem., 1996, 6(3), 449-454 451 -005 -I I I0 00 0 2 8 10wA-’ 08 I I 1 I I 1 I 075 1 07 0 65 0 45 04 0 35 I I I 403 ‘ 4000 3500 3000 2500 2000 1500 1000 500 1 t I f10101 wavenumber/cm-’ , , , z5 1 008 i \ -0 !z p 1002 v) F Y 1000- L- 6 LL 0 998 I I I I I I I I f 0 1 2 3 4 5 6 7 8 9 10 rIA Fig. 4 (a)Diffraction spectrum for sample 2, (b) IR absorption spectrum for sample 2, (c) Fourier transform of the spectrum shown in (a) after the removal of the underlying ‘background’ intensity using a low-order polynomial evident that the data presented here is of substantially higher quality since the correlation lengths these determined cannot easily be related directly to any likely physical model Furthermore, there is clear evidence in their diffraction data of residual crystallinity whilst the data we present here hhs no such crystalline contaminant phase Given the intrinsic form factor weighting of the X-ray diffraction data towards corre- lations involving the (high atomic number) bromin? atoms, the absence of any indication of the primary 193 A Br-C separation is strong evidence for the successful removal of all the bromine atoms and this in its turn might at first sight be said to provide the evidence that the mechanism associated with the polymerisation process centres on the linking of adamantane cages at one or more of the 1,3,5,7-apices However, the amorphous nature of the spectrum (which was also noted by Kasatochkin et al ’) must actually rule out this simple model since the continued existence of well defined Clo units would, however disordered the inter-unit angular corre- 452 J Mater Chem , 1996, 6(3), 449-454 lation, yield diffraction features associated with the intra-unit correlations (much as we observe for sample 1) As might be anticipated the IR spectrum now has no Br-C band at 486cm-’, and this is consistent with the increased intensity of the CH, stretching band at 2850-2930 cm-’ The feature which appeared at 716 cm-’ in Fig 3(b), and was due to the presence of several CH, groups associated with a broadly aromatic ring structure, has disappeared and is replaced by three peaks (at 754, 729 and 702 ern-') These are also assigned to rocking modes associated with CH, groups, but in this case the presence of the feature at 729 cm-I makes it highly likely that these groups, or at least many of them, are part of chain-like structures with varying numbers of carbon atoms involved It is well known23 24 that the frequency of this mode depends strongly on the chain length, with a frequency of 770cm-I being associated with a single unit, and the frequency falling rapidly towards 720 cm-I when two or more units form the chain, the presence of the bands observed here is therefore further evidence for the nature of the poljmerisation process, which would appear to include the formation of short chain-like hydrocarbon segments The band at 1345 cm-’ might conceivably be due to the presence of methyl groups (symmetric deformation), but it is more likely to be due to a C-H rocking mode Samples 3 and 4 appear to have strong similarities, the experimental diffraction and IR absorption spectra are shown in Fig 5(u),(b)and 6(a), (b)respectively The X-ray diffraction pattern shows a strong increase in ordering, which, although qualitatively similar to the work of Kasatochkin et a1,’ pro-vides evidence for a more complete removal of any residual amqrphous phase The broad diffraction peak centred at Q= 1 1 A-’ in Fig 5(a) shows that some well defined amorphous phase material remains at this penultimate stage, and it is instructive to generate a Fourier transform of this spectrum [Fig 5(c)] and to compare it with that shown for sample 2 in Fig 4(c) The presence of graphitic short-range ordering is present in both, but it is clear that the pair correlations (see Table 1)exist over much longer length scales in sample 3, with distances reminiscent of those expected for an extended graph- ite-like or hexagonal structure Indeed, a comparison of the published graphite interplanar distances’’ with the present data for both samples 3 and 4 (Table 2) is revealing, particularly when combined yith the fact that the bulk graphite intErlayer distance of 3 36A and the cross-ring distance of 284A may also be associated with features in the data [at 3 41(7) and 2 96 A respectively for sample 3 (and 4)] Kasatochkin et a17 attempted to fit their own data using a quadratic equation to describe the system in terms of a generic hexagonal system, which they then assign to a ‘hexagonal crystalline phase’ of polyadamantane Our diffraction data does not agree with theirs, and given the close correspondence of the major part of our spectra to that expected from bulk graphite we are drawn to the conclusion that the material is actually a distorted graphitic structure rather than a polymeric network of well defined adamantane Clo units, with the initial adamantane-like units being broken up beginning at the stage associated with sample 2 The distortion of the graphitic layers Table 2 Comparison betyeen measured and literature (ref 17)graphite interplanar distances (d/A) measured ref 17 3 41 3 36 2 11 2 13 1 80 180 173 1 68 1 49 1 54 122 123 114 112 105 105 0.35 -h v) -.-rO30 -@ 0.25 -v .-20.20 -v) -a, 0.15-C.-0.10 -00s -I I I0.00 ‘ 0 2 4 6 8 10 QIA-’ 0.75 I I 0.7 0 65 8 0.6 C c .c 0.55 v) s 0.5 CI 0.45 0.4 I 0.35 4000 3500 3000 2500 2000 1500 1000 500 wavenumberlcm-’1.003 III,,II, 4 ~1.001> c.-v) a, .-2 1.000 -0 0.999 .c v) E c 0.998 .-2 0 LL 0.9Y7 0 12 3 4 5 6 7 8 9 10 IfA Fig.5 (a)Diffraction spectrum for sample 3; (b)IR absorption spectrum for sample 3; (c) Fourier transform of the spectrum shown in (a) after the removal of the underlying ‘background’ intensity using a low-order polynomial would appear to be greater for sample 3 than for sample 4, as might be expected if we interpret the structural change as being due to a progressive graphitisation of sample 2 due to the heat treatment. The IR absorption data for samples 3 and 4 supports these conclusions. For sample 3, the disappearance of the ca. 730cm-1 features (and the relative strength of the aromatic CH, features at 701 and 750 cm-l) serves as evidence for the reduction in CH, hydrocarbon chains and the growth of more graphitic/aromatic structures.The CH, stretching mode at ca. 2900cm-’ is now very intense (relative to other bands), and the associated deformation mode at 1448cm-’ is also clearly seen, as is the C-H rocking mode at 1346cm-’; all of which supports the model of a return to (graphitic/aromatic)crystal-linity. The absorption spectrum from sample 4 shows the continuing trend, but with weaker features due to the decrease in the overall hydrogen content (see Table 3). Note that 0.40 t i -0.35 - v)c.-5 0.30 - d2 0.25 - Y Eo20 - (r 0.15 - .- 0.10 - 0.05 -‘ 0.00 I I I I I 0 2 4 6 8 10 0.55 QIA-’ 1 0.5 0.45 0.4 0.35a, - a .r 0.3 3 0.25E, c 0.2 0.15 ! I I I I I0.05 I 4000 3500 3000 2500 2000 1500 1000 500 wavenumberlcm-’ Fig.6 (a)Diffraction spectrum for sample 4; (b)IR absorption spectrum for sample 4 Table 3 C,H, N combustion analysis of the samples; sample 4 provided data with a high degree of variability (+0.01), but samples 1, 2 and 3 yield errors in the H :C ratio of +0.003 or lower sample H :C (mass%) 0.10: 1 0.12: 1 0.10: 1 (0.06:1) although the combustion analysis is able to reveal quantitative information only on the volatile/combustible carbon and hydrogen, it is possible to summize additional information related to the ‘residual’ mass percentage. A clear example of this is in relation to sample 1 where this mass corresponds well, within errors, to the expected bromine fraction; for other samples where reliable data were obtained the residual mass is assumed to be associated with impurities of bromine and/or unreacted reagents) and with some C-0 stretching contami-nation at 1719 cm-l.The feature at 870 cm-’ is a deformation mode associated with C-H, and those at 1615cm-’ and 1040cm-’ are associated with C-C modes.” The presence of these well defined absorption features is strongly indicative of a graphite-like material, though clearly with some residual hydrogen (the presence of which may be related to our observation of some methyl groups in sample 1: they would affect the polymerisation process since the methyl groups would not cross-link). Neutron diffraction and incoherent inelastic neutron spec-troscopy (IINS)would provide additional information on these structural transformations since neutron scattering methods are highly sensitive to correlations and vibrations involving hydrogen; in addition, neutron diffraction offers a wider dynamic range and therefore improved real-space resolution, J.Mater. Chem., 1996, 6(3), 449-454 453 which would be of value in the detailed study of the amorphous components, and IINS provides direct access to the true vibrational density of states The structural changes suggested by the X-ray diffraction and IR data may be associated with macroscopic effects such as the formation and collapse of voids within the matenal, small angle X-ray scattering (SAXS) studies are underway in an attempt to understand such effects Conclusions Although qualitative similarities between this work and earlier studies have been identified, the more precise X-ray diffraction measurements, when coupled to careful IR absorption analysis, have served to argue for a different interpretation of the structural changes associated with the chemical processes involved in generating this suite of materials It is clear that 1,3,5,7-tetrabromoadamantanehas indeed been formed from the adamantane precursor, but that there are some bromine sites that are occupied by carbon atoms ‘Polymerisation’ of these units into an amorphous network with no residual long-range ordering may then be induced through chemical processes, with the data suggesting that it is in fact not the case that C,, carbon units are linked at their apices via the 1,3,5,7-carbon sites (the model currently in the literature), but rather that the units are broken up to form an amorphous network (or perhaps ‘patchwork’) of unit fragments The corroborating presence of short hydrocarbon chains is also revealed Once the material is heat treated it begins to revert to a graphitic structure, though initially with a small amorphous component remaining The hydrocarbon chains are eliminated We wish to thank the SERC (now the EPSRC, UK) for its financial support and for access to the facilities of the Daresbury Laboratory, Mr A Fassam (UKC) for the provision of mass spectroscopic, C,H,N combustion, and initial DRIFT characterisation, and Dr P C H Mitchell (University of Reading, UK) for his supervision of the sample preparation processes K J K acknowledges support from the EU’s ERASMUS programme which allowed him to study in the UK References 1 J K Walters and R J Newport, J Phys Condens Matter, 1995,7, 1755, and references therein 2 J Robertson, Adu Phys , 1986,35,317 3 E g A H Lettington, in Diamond and diamond-lrkejlms and coat- ings, ed J C Angus, R E Clausing, L L Horton and P Koidl, Plenum, New York, 1991 4 F Janson, M Machonkin, S Kaplan and S Hark, J Vac Sci Techno1 A, 1985,3,605 5 P J R Honeybone, R J Newport, J K Walters, W S Howells and J Tomkinson, Phys Rev B, 1994,50,839 6 J K Walters, D M Fox, T M Burke, 0 D Weedon, R J Newport and W S Howells, J Chem Phys, 1994,101,4288 7 V I Kasatochkin, Yu P Kudryavtsev, V M Elizen, 0 I Egorova, A M Sladkov and V V Korshak, Dokl Akad Nauk USSR, 1976,231,1358 8 Advanced Inorganic Chemistry (3rd edition) F A Cotton and G Wilkinson, Wiley, London, 1972 9 H F Reinhardt, J Polym Sci B, 1964,2,567 10 G P Sollot and E E Gilbert, J Org Chem, 1980,45, 5405 11 A I Vogel, Textbook on Practical Organic Chemistry, 5th edn revised by B S Furniss, Longman, Harlow, 1989 12 D W Huxley, PhD Thesis, University of Kent at Canterbury, UK, 1991 13 B E Warren, X-Ray Diffvaction, Dover Publications, New York, 1990 14 B E Warren and G Mavel, Rev Sci Instrum, 1962,36,196 15 G Bushnell-Wye, J L Finney, J Turner, D W Huxley and J C Dore, Rev Sci Instrum Meth , 1992,63, 1153 16 International Tables for Crystallography, vol 3, ed C H MacGillary, Kynoch Press, Birmingham, 1968 17 Interatomic Distances, ed A D Mitchell, L C Cross and G D Rieck, Kynoch Press, Birmingham, 1958 18 The Aldrich Library of Infrared Spectra, 2nd edn, ed C J Pouchert, Aldrich Chemical Company, Milwaukee, 1975 19 L J Bellamy, The Infrared Spectra of Complex Molecules, 2nd edn ,Chapman and Hall, London, 1980 20 G Chiurdoglu, Bull SOC Chim Belg , 1958,67, 198 21 G J Boobyer and S Weckherlin, Spectrochim Acta Part A, 1967, 23,321 22 G P Sollot and E E Gilbert, J Org Chem, 1980,45,5405 23 J C Hawkes and A J Neale, Spectrochim Acta, 1960,16,633 24 R G Snyder and G H Schachtschnaider, Spectrochim Acta, 1963, 19,85 Paper 5/04432D, Received 6th July, 1995 454 J Mater Chem, 1996, 6(3),449-454

ISSN:0959-9428    

DOI:10.1039/JM9960600449

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Single-crystal X-ray structure analysis of Mn-substituted barium hexaaluminates as-grown and after reduction? Hiroshi Inoue,aMasato Machida,b Koichi Eguchi' and Hiromichi Arai"" 'Department of Materials Science and Technology, Graduate School of Engineering Sciences, K yushu University, 6-1 Kasugakoen, Kasuga, Fukuoka 81 6, Japan bDepartment of Materials Science, Faculty of Engineering, Miyazaki University, 1-1 Gakuen-Kibanadai-Nishi, Miyazaki 889-21, Japan An X-ray single-crystal structure analysis was performed on manganese-substituted barium hexaaluminate (Bao~780Mno~254Al,o~706017~153)with the p-alumina structure, which was grown by the floating zone method and reduced by subsequent evacuation at 1150 "C. Both as-grown and reduced crystals have a hexagonal structure (P63/mm~, 2=2) and their unit-cell dimensions are a =0.559 1 (l), c =2.2659( 2) nm and a =0.5587( 2), c =2.2656( 3) nm respectively. The structure refinement from the X-ray reflection data collected by using the w28 scan technique was carried out by the least-squares method to give a final R value of 0.03.The resultant structure was identical with that of p-alumina (Ba0~75A111~0017~25) reported by Iyi et a1.l Di- and tri-valent manganese partially and preferentially replaced Al( 2) sites in a tetrahedral environment in the spinel block. UV-VIS spectra showed that the evacuation treatment brought about the reduction of some of the Mn3+ to Mn2+. The charge compensation for the reduction was found to be effected by 02-defect formation in the mirror plane whereas the occupancies of oxygen sites inside the spinel block remained unchanged.Hexaaluminate compounds are important functional inorganic materials which have been studied extensively in a number of fields of application, such as superionic conductors, host crystals for fluorescence, lasers and nuclear waste disposal. We previously reported the application of powdered hexaaluminate compounds in high-temperature catalytic processes. The most prominent feature of this material is its thermal stability against sintering, which is quite useful in retaining the large surface area necessary for catalytic The Mn-substituted hexaaluminate also exhibits catalytic activity for oxidation reactions, because of the reduction-oxidation cycle of Mn partially substituting the A1 site.The charge compensation in the redox process seems to be accomplished by the non-stoichiometry of lattice oxygen. Thus, the reactivity of lattice oxygen plays a key role in the catalytic activity. However, none of the structural changes during the redox process have been elucidated to demonstrate the structure-dependent cataly- sis of this material. With regard to the oxidation state of Mn in the hexaaluminate lattice, Laville et al. reported the result of spectroscopic investigations of LaMg, -xMn,All,Ol, single crystal^.^ The absorption spectra of this material, as grown under a reducing atmosphere, were assigned to Mn2+, while later oxidation treatment produced Mn3+ species. Although this result implies that redox cycles between Mn2+ and Mn3+ exist in the hexaaluminate lattice, the accompanying oxygen non-stoichiometry was not described.We have performed single-crystal structural analysis of Mn- substituted hexaaluminates to evaluate the structural changes that occur during the redox process. Comparing the structural parameters of the sample as grown in air and after subsequent reduction treatment is useful as it provided a new insight into the structure-dependent catalytic properties of this material. This enabled us to study the reactivity of lattice oxygen of different crystallographic sites. Experimenta1 Manganese-substituted hexaaluminate single crystals were used for the X-ray structural analysis. For single-crystal growth, a t Single-crystal data are available from the Fachinformationszentrum Karlsruhe, D-76344 Eggenstein-Leopoldshafen, Germany.sintered rod was prepared from a cool, isostatically pressed, powder sample of Bao~80Mno~40Allo~,0017~3-cI by heating at 1450 "C. A single crystal of Mn-substituted barium hexaalumin- ate was grown by using a floating zone (FZ) apparatus (Nichiden Kikai Co.) equipped with a xenon arc lamp. Crystal growth was maintained at the constant rate of 3mm h-' in air. The as-grown transparent red crystals were subsequently evacuated at 1150°C in order to reduce the Mn species. The crystal turned transparent green upon this reduction treatment. Clear sections with no cracks were cut out from both as-grown and reduced crystals, and shaped into spheres (ca.0.3-0.2 mm in diameter), for use in X-ray diffraction measurements. X-Ray intensity data were collected on a Rigaku AFC5R diffractometer with graphite-monochromated Mo-Ka (A= 0.71069 A) radiation and a 12 kW rotating anode generator. Cell constants and an orientation matrix for data collection, obtained from a least-squares refinement using the setting angles of carefully centred reflections in the range 52.09 <28 <55.03" corresponded to a primitive hexagonal cell (Laue class 6/mmm) with cell parameters: a =0.5591(1)nm, c=2.2659(2) nm, V=0.6134(1) nm3, 2=2 for an as-grown red crystal and a =0.5587( 2) nm, c =2.2656( 3) nm, V= 0.6125(1)nm3, 2=2 for a reduced crystal. Based on the systematic absence of hhl, I # 2n, packing considerations, stat- istical analysis of intensity distribution, and the successful solution and refinement of the structure, the space group was determined to be P63/mm~ (no.194). The data were collected at temperature of 24+_ 1 "C using the w28 scan technique to a maximum 28 value of 120.4". Omega scans of several intense reflections, made prior to data collection, had an average width at half-height of 0.28" with a take-off angle of 6.0". Scans of (1.68 +0.30 tan 8)O were made at a speed of 16.Oomin-' (in omega). The weak reflections [I <lO.Oo(I)] were rescanned (maximum of 3 scans) and the counts were accumulated to ensure good counting statistics. Stationary background counts were recorded on each side of the reflection.The ratio of peak counting time to background counting time was 2: 1. The diameter of the incident beam collimator was l.Omm, the crystal to detector distance was 258mm, and the detector aperture was 9.0~13.0mm (horizontal x vertical). J. Muter. Chem., 1996, 6(3), 455-458 455 Intensities were collected by applying Lorentz polarization and absorption corrections (transmission factors 0 5345-1 2028 for an as-grown crystal and 09425-1 0444 for a reduced crystal) Of the 3700 reflections which were collected, 1997 were unique The intensities of three representative reflections were measured after every 150 reflections and showed no significant deviations from the mean The structure was solved by and expanded using Fourier techniques with secondary extinction correction Refinements were based on the values for barium hexaaluminate (Ba, 75Alll ,O,, 25) reported by Iyi et a1 ,6 including general temperature factors, positional par- ameters, and the occupation factors of all sites Full-matrix least-squares refinement minimized Cw(lF,1 -(Fc1)2, w = l/a2(F,) Neutral atom scattering factors were taken from Cromer and Waber8 The values for Af' and A7 used were J LtL,mEa +b --c +ak Fig.1 Refined crystal structure of Mn-substituted barium hexaalumin- ate with 95% probability ellipsoids those of Creagh and Mcauley' The final least-squares cycle converged with unweighted and weighted agreement factors of R=O029 and R,=O039 for an as-grown crystal, and R= 0026 and R,=O033 for a reduced crystal All calculations were performed using the TEXSAN crystallographic software package of the Molecular Structure Corporation Results and Discussion The crystal structure of Mn-substituted hexaaluminates refined in this study is shown in Fig 1 The final structural parameters of the as-grown Mn-substituted hexaaluminate crystal are listed in Table 1 The occupancy was expressed as the number per unit cell divided by 24 The refined crystal structure agrees essentially with that of Ba, 75Alll ,O,, 25, which was refined by Iyi et a1 to be a defective p-alumina type' The ideal p-alumina structure is constructed by alternate stacking along the c axis of a spinel block and a monoatomic interlayer plane (mirror plane) containing Ba2+ and oxide ions (Fig 1) According to the report by Iyi et al, however, barium p-alumina contains interstitial oxygen with barium ion vacancies in the mirror plane due to Frenkel defects of A1 ions Their structural model is constructed from a random distribution of two types of half unit cells, one containing a barium and an oxygen ion in the mirror plane, and the other containing interstitial aluminium and oxygen ions with no barium ions Such a complicated defect structure arises from a charge compensation mechanism for non-stoichiometry to accommo- date divalent cations into the p-alumina structure As is evident from the refined structural parameters in Table 1, barium ion occupies the 2(d) (Beevers-Ross) site in the mirror plane (z=O 25) Aluminum ions in the spinel block are distributed on Al(1) and Al(4) sites in an octahedral environment and on Al(2) and Al(3) sites in a tetrahedral environment An interstitial Al( 5) site is created by the Frenkel defect of Al(1) First refinements indicated the partial occu- pancy of Al(2) of about lo%, whereas no deviation for Al(3) and Al(4) was found To determine the site occupancy of Mn, therefore, the refinement was conducted by assuming that Al(1) and/or Al(5) contain Mn with the same coordinates, but the occupancies of Mn on these sites converged to zero In the subsequent refinement, therefore, Mn was placed on Al(2) with the same coordinates and the total occupancy of Al(2) and Mn was constrained to 1/6 Under these conditions, the occupancy of Mn on the Al(2) site converged to 00212 Such preferential replacement is consistent with the result on man- ganese-substituted lanthanum hexaaluminate reported by Gasperin et al loand also supported by the UV-VIS absorption results described below Among the seven types of oxygen Table 1 Final structural parameters of an as-grown crystal ~~~~~ atom position ~ ~ ~ occupancy' X Y Z Be: 0 0650( 1) 213 113 114 1135(2) 0 0212( 7) 113 213 0 02374( 4) 0 43(4) 0 4702( 12) 0 1455( 7) 0 1667 0 0833 0 02654 12) 0 83309(6) 113 113 0 0 840( 1) 213 213 0 66618( 12) 0 0 680(2) 0 10528(2) 0 02374(4) 0 17512(3) 0 0 1774(3) 0 465(5) 0 357(6) 0 537(4) 0 426(6) 07(1) 0 5000 0 5000 0 1667 0 1667 0 15683( 10) 0 5039( 1) 213 0 031366(20) 0 0078(2) 113 0 0 05013(4) 0 14734(4) 0 05676( 7) 0 14207(7) 0 674( 10) 0 568(9) 0 584( 9) 0 513(9) 0 0196( 15) 0 0667( 17) 0 0097( 16) 113 0 2949(9) 0 880(5) 213 0 7051(9) 0 760( 10) 114 114 114 1 04(4) 0 55( 7) 0 9(5) 'The weighted sum of the occupancies corresponds to the formula Bao,,,Mno,,,All,,o,Ol, 153 The thermal parameters are of the form 4 n2[U,l(ua*)2+ U22(bb*)2+ U3,(cc*),+2Ul,aa*bb*cos y +2~,,aa*cc*cos/3+ 2U,,bb*cc*cos a] 'Interstitials 456 J Muter Chem , 1996,6(3), 455-458 sites, O(l), 0(2), O(3) and O(4) are close packed in the spinel block, whereas 0(5),O(6) and O(7) are in the mirror plane.The interstitial O(7) ion is placed near the mid-oxygen site, coordinating with the interstitial Al(5) ion.At the first stage of refinement, the occupancies of O(1)-0(4) remained unchanged. In the subsequent refinements, therefore, occupan- cies of these sites were fixed and only those of O(5)-0(7) were refined. We also examined refinement of all parameters simul- taneously. However, the result showed a good agreement with the stepwise refinement described above. Note that O(5)-0(6) bondlengths and 0(5)-A1(3)-0(6) angles are very small, but these two types of 0 sites are not occupied at the same time. The average oxidation number of Mn was estimated to be +2.472 from the refined structural formula, Bao~780Mno~25,All,~706~17~153,by assuming that the valences of the other elements is Ba2+, A13+ and 0,-.The refined structural formula agreed with that determined by inductively coupled plasma optical emission spectrometry (ICP-AES).Gasperin et al. reported that the strong diffuse scattering observed on manganese-substituted lanthanum hexaaluminates leads to a systematic error in the compositional calculation." The diffuse scattering was also observed on Nd3 +-exchanged sodium P-aluminogallate as reported by Kahn et ul." Since these diffuse scatterings arise from coherent shifts of La3+ or Nd3+ in combination with a local ordering of the cation vacancy, hexaaluminates with trivalent large cations tend to suffer from the same problem in determination of the site occupancies. In case of barium hexaaluminates, however, no result has yet been reported on the diffuse scattering.Taking electrical neutrality into consideration, the decrease in positive charge brought about by manganese substitution for A13+ should be compensated by an increase in cation concentration and/or by a decrease in anion concentration. As about no significant change in the final positional parameters as shown in Table 2. However, thermal parameters of O(5) and O(7) for the as-grown crystal are higher than those for the reduced crystal. The reason for the increased thermal parameters is not clear at the present stage, but one possible explanation could be as follows. The occupancy of the Al(5) site in the as-grown crystal was lower than that in the reduced crystal. The electrostatic bond between Al(5) and 0 in the mirror plane [0(5)and/or 0(7)] in the as-grown crystal is, therefore, supposed to be weakened by the reduction to produce the increased thermal parameters. A slight change can be also seen in the oxygen stoichiometry as a result of the elimination of lattice oxygen during evacuation.This is reflected by decreased occupancies of the 0(5),O(6) and O(7) sites in the mirror plane, whereas those of O(1)-0(4) sites in the spinel block remained unchanged. The cumulative change of their occupancies corresponds to ca. 4% of the interlayer oxygen being eliminated during the evacuation. The resultant structural formula of the reduced crystal was calculated as Bao~786Mn,~238Allo~705017~120,which implies an average oxi- dation state for the manganese ion of +2.324.The results of the present structural analysis suggest that the apparent oxi- dation state of manganese ion in the hexaaluminate was reduced from +2.472 to +2.324 during evacuation at 1150"C. The change in the oxidation state of Mn can be confirmed from the colour of the single crystal, which was initially red and turned transparent green upon evacuation at 11 50 "C. The UV-VIS absorption spectra of both crystals (Fig. 2) showed four independent bands at 362, 378, 425 and 447nm, which are well explained by the Tanabe-Sugano interaction matrix of a d5 ion (Mn2+) in tetrahedral symmetry.' However, the red as-grown crystal showed a significant red-shift of the absorption edge (-400 nm), compared with the reduced crystal, due to the intense oxygen-to-Mn3+ charge transfer bands.Therefore, decoloration of the crystal accompanying the evacu- compared with the structural parameters of Bao~75Alll~oO17~25 refined by Iyi et ~l.,~increases in the occupancies of Ba and A1 and decreases in those of O(5)-0(7), were observed. This suggests that the charge compensation required by the manga- nese substitution was achieved by both mechanisms in the present system. In addition, it should be noted that the occupancy of the interstitial Al( 5)ion decreased, whereas that of the Al(1) ion in octahedral environment increased with a simultaneous decrease in the occupancy of the interstitial O(7) ion, which coordinates the interstitial Al( 5).Consequently, the charge compensation is mainly achieved by O(7) defect forma- tion, which leads to the displacement of the interstitial Al(5) to the original position, the Al( 1) site.Structural analysis was also performed on a single crystal after the reduction treatment. The reduction treatment brought ation treatment must be associated with the progress of the reduction of Mn3+ to Mn2+. Besides the oxygen non-stoichiometry, a small variation of occupancies can be seen on Ba and Mn(2) sites. One possible reason for these variation must be due to an error in the compositional determination of these minority species from crystallographic data. In addition, the diffraction data may have been influenced by the compositional fluctuation occur- ring during single-crystal growth.For the lowered Mn content of the reduced crystal, another possible reduction mechanism may be also pointed out. This mechanism consists of two stages: disproportionation of Mn3+ to Mn2+ and Mn4+, and subsequent combination of Mn4+ and two lattice oxygens to yield MnO, which is given off. Such a disproportionation Table 2 Final structural parameters of a reduced crystal atom position occupancy' X Y z Beqb 0.0655( 1) 213 113 114 1.153( 2) 0.0198( 6) 113 213 0.02373( 2) 0.34(3) 0.4680( 10) 0.83 308(5) 0.666 16( 10) 0.10520( 2) 0.460( 4) 0.1469 (6) 113 213 0.02373( 2) 0.370( 6) 0.1667 113 213 0.175 17( 3) 0.544( 4) 0.0833 0 0 0 0.42 1 (6) 0.0272( 9) 0.8408(8) 0.6816( 16) 0.1770( 3) 0.40(8) 0.5000 0.15672(9) 0.3 1344( 18) 0.05012(4) 0.656( 9) 0.5000 0.50401 (9) 0.00802( 18) 0.14733(4) 0.556(8) 0.1667 213 113 0.05660( 6) 0.578(9) 0.1667 0 0 0.14218(6) 0.484(8) 0.0191( 11) 113 213 114 0.18 (4) 0.0653(15) 0.2943 (7) 0.5886( 14) 114 0.81(6) O.O089( 13) 0.881(3) 0.762( 6) 114 0.5(3) 'The weighted sum of the occupancies corresponds to the formula Bao.,86Mno.238A1,0,705017.120.The thermal parameters are of the form: $ + U,,(CC*)~n2[U,, + U,,(~ZI*)~ +2U,,aa*bb*cos y+ 2U,,aa*cc*cos +2U,,bb*cc*cos a].Interstitials. J. Mater. Chem., 1996, 6(3), 455-458 457 I 1 I I 300 400 500 600 700 wavelengthlnm Fig. 2 Absorption spectra of Mn-substituted Ba-hexaalummate single crystal (a)As-grown, (b)after reduction is a well known feature of Sn2+ chemistry, eg 2Sn0+ Sno+ SnO, l2 However, the exact route of this compositional variation is not clear at the present stage The results of this study suggest that the Mn ions in the hexaaluminate can be reduced without structural deterioration The calculated oxidation number of Mn in the crystal as prepared in air, 2472, agrees with that in the powder sample of BaMnAll1019-d,, which was determined by thermogravime- try in an H2 flow to be 243 Thus, manganese tends to be in a mixed oxidation state between + 2 and + 3 in the hexaalum- inate matrix This is in contrast to the other 3d elements, z e , Co and Ni prefer divalent, whereas Fe prefers trivalent states The intermediate property of Mn must be the reason for easy reduction/oxidation, which cannot be attained by hexaalumin- ates partially substituted with other 3d elements The reduction/oxidation of Mn plays a key role in the catalytic activity of manganese-substituted hexaaluminate With regard to the site of Mn in the hexaaluminate structure, Laville et a2 implied from the spectroscopic characterization, that the oxidation of Mn2+ to Mn3+ in a lanthanum hexaalum- inate single crystal is accompanied by the migration of Mn ions from a tetrahedral to an octahedral environment However, the present study clearly indicates that Mn in barium hexaaluminate remains in the tetrahedral Mn( 2) site during the reduction/oxidation process From a kinetic point of view, this oxidation/reduction process without cation diffusion should facilitate reversibility Note also the charge compensation mechanism for the reduction of manganese-substituted hexaaluminate As shown in Table 2, reducing the as-grown crystal produced the oxygen defects, in particular, only on mirror plane sites, 0(5), O(6) and O(7) This may suggest that the oxygen atoms, which are loosely packed on the mirror plane, tend to be easily removed as compared to the close-packed oxygen atoms inside spinel blocks However, the differences of occupancies of these oxygen sites are too small to be regarded as significant and no other evidence was found to support the preferential O2-defect formation on the mirror plane Mirror plane oxygen in hexa- aluminate shows unique behaviour because of its interlayer character We previously reported the anisotropic oxygen self- diffusion in barium he~aaluminate,'~ l4 which is brought about by a preferential diffusion route in the mirror plane Large interatomic distances between Ba and interlayer oxygen [Ba-0(5) 0 32 nm, Ba-0(6) 0 31 nm] suggest that these oxygen ions in the mirror phane in a weak electrostatic potential are readily mobile with a small activation energy References 1 N Iyi, S Takekawa and S Kimura, J Solid State Chem, 1985, 59,250 2 M Machida, K Eguchi and H Arai, J Catal, 1987,103,385 3 M Machida, K Eguchi and H Arai, J Catal, 1989,120,377 4 M Machida, K Eguchi and H Arai, J Catal, 1990,123,477 5 F Laville, D Gouner, A M Lejus and D Vivien, J Solid State Chem, 1983,49,180 6 N Iyi, S Takekawa, Y Bando and S Kimura, J Solid State Chem , 1983,47,34 7 N Iyi, Z Inoue, S Takekawa and S Kimura, J Solid State Chem , 1984,52,66 8 D T Cromer and J T Waber, in International Tables for X-Ray Crystallography, Kynoch Press, Birmingham, 1974, vol IV, Table 2 2 A 9 D C Creagh and W J Mcauley, in Internatmnal Tables for X-Ray Crystallography, Kluwer, Boston, 1992, vol C, Table 4 2 6 8 10 M Gasperin, M C Same, A Kahn, F Laville and M Lejus, J Solid State Chem ,1984,54,61 11 A Kahn, G Aka and J Thery, J Solid State Chem, 1991,91,71 12 C Decroly and M Ghodsi, Comp Rend, 1965,261,2659 13 M Machida, T Shiomitsu, Y Shimizu, K Eguchi and H Arai, J Solid State Chem, 1991,95,220 14 M Machida, T Shiomitsu, K Eguchi, H Haneda and H Arai, J Mater Chem, 1992,2,455 Paper 51072455, Received 2nd November, 1995 458 J Mater Chem, 1996, 6(3), 455-458

ISSN:0959-9428    

DOI:10.1039/JM9960600455

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Surface and bulk properties, catalytic activities and selectivities in methane oxidation on near-stoichiometric calcium hydroxyapatites Shigeru Sugiyama,"" Toshimitsu Minami," Toshihiro Moriga," Hiromu Hayashi," Kichiro Koto,b Michie Tanaka" and John B. Moffatd "Department of Chemical Science and Technology, Faculty of Engineering, The University of Tokushima, Minamqosanjima, Tokushima, 770 Japan bFaculty of Integrated Arts and Sciences, The University of Tokushima, Minamijosanjima, Tokushima, 770 Japan "Shikoku Research Institute Inc., 2109 Yashima-nishi, Takamatsu, 761 -01 Japan dDepartmentof Chemistry and the Guelph- Waterloo Centre for Graduate Work in Chemistry, University of Waterloo, Waterloo, Ontario, Canada N2L 3Gl Various calcium hydroxyapatites [Calo~z(HP04)z( PO,)6 -,(OH), -,; z = 0, stoichiometric apatite (Ca/P = 1.67; atomic ratio); 0 < z 6 1, non-stoichiometric apatites ( 1.67> Ca/P > 1.50)], together with the apatites of Ca/P = 1.72, were analysed by powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and extended X-ray absorption fine structure (EXAFS) spectroscopy and their catalytic properties in the oxidation of methane at 973 K were examined in the presence and absence of tetrachloromethane as a gas-phase additive.The XRD patterns of each hydroxyapatite were the same. However, the nearest- neighbour distances of the Ca- 0 bond of stoichiometric and non-stoichiometric hydroxyapatites as estimated by EXAFS were 2.41,2.37,2.38 and 2.39 A for each catalyst of Ca/P = 1.72, 1.68, 1.64 and 1.58, respectively.In the oxidation of methane on each hydroxyapatite in the absence of CCl,, the conversion of methane was little influenced by the value of Ca/P but the selectivities to C2H6 and CO showed a maximum and minimum, respectively, at Ca/P = 1.68. On addition of CCl, into the feedstream, the selectivity to C02 on Ca/P = 1.68 and 1.72 decreased as the time on-stream increased while sharp decreases in conversion were observed with time on-stream on non-stoichiometric hydroxyapatites of Ca/P = 1.64 and 1.53, respectively, without suppression of the selectivity to C02. The catalytic activities of a variety of hydroxyapatites for the oxidative dehydrogenation of methane have been investi- gated in our laboratories.'-* Most re~ently,~.~ the oxidation of methane on stoichiometric calcium hydroxyapatite [Calo( PO,),(OH),] in the presence and absence of tetrachloro- methane has been reported.Although it is generally observed that the selectivity to C,, compounds and, in particular, to ethylene in the oxidative dehydrogenation process is increased on the addition of CCl, into the methane oxidation feedstream while that to ethane is decreased,'-13 the introduction of CCl, into the feedstream on stoichiometric hydroxyapatite did not increase the selectivities to C2 compounds but rather resulted in an enhancement to that of carbon monoxide with decrease of that to carbon di~xide.~.' It is well known that calcium hydroxyapatites [Calo-,( HPO,),( P04)6-z(OH)2-z (0dz 6 l)] often display compositionally dependent behaviour in catalytic reactions.For example, the dehydrogenation of alcohols is catalysed by stoichiometric hydroxyapatite (z = 0, Ca/P = 1 .67)14*15 while dehydrogenation and dehydration are observed on non-stoichiometric hydroxyapatites (0< z < 1, 1.67> Ca/P 31.50).14,16However the source of such differences remains unclear since calcium vacancies may be distributed in the hydroxyapatite latticel7-'' a low Ca/P value may result from the formation of a mixture of stoichiometric hydroxyapa- tite and a phosphate having a low calcium content.20~21 The crystallographic structures of calcium phosphate and of the stoichiometric hydroxyapatite are similar, thus making it difficult to detect the presence of these two phases by X-ray diffraction ( XRD).22,23 This is especially so for deficient apa- tite~,'~,~~which are generally poorly crystallized." In part as a consequence of the biological importance of hydroxyapatite, a number of XRD and transmission electron microscopic (TEM) studies have been The surface structure of hydroxyapatites has also been investigated by the appli- cation of atomic force microscopy30 and X-ray photoelectron spectroscopy ( XPS).3-8,31 In the present study, the oxidation of methane in the presence and absence of CCl, has been studied both on the stoichiometric and non-stoichiometric hydroxyapatites as well as on the apatite containing excess calcium (Ca/P = 1.72).XRD and XPS analyses together with extended X-ray absorption fine structure (EXAFS) spectroscopy have also been applied, the latter to clarify the Ca-0 bond distance and the local structure around the calcium atoms in each apatite.Although EXAFS and XRD analyses provide information only on bulk phases, the properties of the surface of each apatite on which the catalysis occurs are undoubtedly strongly influenced by those of the bulk phase. Experimenta1 Catalyst preparation Calcium hydroxyapatites (Ap1.72, AP1.68, Ap1.64 and AP1.53, where the subscripts represent the Ca/P atomic ratio of each apatite) were prepared from Ca(NO,), .4H@ (Wako Pure Chemicals, Osaka) and (NH4),HP04 (Wako) according to the procedure reported by Hayek and Ne~esely.~' In the present study, Ap1.68 is considered to be the stoichiometric hydroxyapa- tite.The resulting solids were calcined at 773 K for 3 h after drying at 353 K overnight. Particles of 0.35-1.75 mm were employed as a catalyst in the present study. Apparatus and procedure The catalytic experiments were performed in a fixed-bed con- tinuous-flow quartz reactor operated at atmospheric pressure. Details of the reactor design and catalyst packing procedure have been described elsewhere." Prior to reaction the catalyst J. Mater. Chem., 1996, 6(3),459-464 459 was calcined zn sztu in an oxygen flow (25 ml min-l) at 1048 K for 1 h The reaction conditions were as follows W=O 1 g, 0 25 g or 0 5 g, F =30 ml min-l, T= 973 K, p(CH,) =28 7 kPa, p(02)=4 1 kPa, and p(CCl,)=O or 0 17 kPa, the balance to atmosphere pressure was provided by helium Analysis and characterization The reactants and products were analysed with an on-stream gas chromatograph (Shimadzu GC-8APT) equipped with a TC detector and integrator (Shimadzu C-R6A) The columns used in the present study and the methods employed in the calculation of conversions and selectivities have been described previously l2 The surface areas of the catalysts (72 9, 65 3, 73 8 and 82 6 m2 8-l for Ap, 72, Ap, 68, Ap, 64, and Ap, 53, respectively) were measured with a conventional BET nitrogen adsorption apparatus (Shibata P-700) X-Ray photoelectron spectroscopy (XPS) (Shimadzu ESCA- 1000AX) used monochromated Mg-Ka radiation The binding energies were corrected using 285 eV for C 1s as an internal standard Argon-ion etching of the catalyst was carried out at 2 kV for 1 min with a sputtering rate estimated as cu 2 nm min-' for SiO, Powder X-ray diffraction (XRD) patterns were recorded with an MXP-18 (MAC Science Co) diffractometer, using monochromated Cu-Ka radiation Patterns were recorded over the range 26 5-60' The concentrations of Ca and P or Cl in each catalyst were determined in aqueous HNO, solutions by inductively coupled plasma (ICP) spectrometry (SPS-1700, Seiko) or ion chroma- tography (Dionex 20101) X-Ray absorption spectra near the Ca K-edge were measured by a laboratory EXAFS spectrometer (Technos, EXAC800) with a molybdenum rotating anode The first-order line diffracted by a Ge monochromater was used X-Ray intensities were monitored with an Si-Li solid-state detector The absorp- tion edge Apt was estimated to be about 10 The fixed time durations for the measurement at each point were 150s for the incident beam and 450s for the transmitted one, so that total photon count was >los Analyses of the EXAFS data were performed using the program (ver 22) provided by Technos Co Ltd In obtaining the EXAFS function ~(k),the background level was subtracted from the observed absorption spectra by using a Victreen fit, and the absorption spectra for the isolated atom was approximated by the cubic spline technique 33 Fourier transformation of ~(k)into real space yielded a cadial structure function $(r),where the k-range from 3 to 8 5 A-' was used For curve-fitting analysis, the first- neighbour distance range for $(r) was filtered with a smooth filtering window and transformed back to k-space ~'(k) Carrying out the non-linear least-squares calculations of Marquardt's method,34 ~'(k)was fitted wit9 an analytical EXAFS function in the k-range from 3 to 8 5 A-' Theoretical amplitudes and phase functions calculated with the spherical wave approach of Teo were used to correct the absorber phase shift and the back-scattering amplitude 35 Results and Discussion Methane oxidation on calcium hydroxyapatites In all catalytic experiments, the products were CO, CO,, C2H4 and C2H6 Water and hydrogen were also produced but are not reported here Carbon balances of 100& 5% were obtained in all experiments The effect of changes in the Ca/P value on the methane oxidation process at 973 K are illustrated in Fig 1 in the presence and absence of CCl, In the absence of CCl,, the conversion of methane changes relatively little with the value of Ca/P While the selectivity to C2 compounds and, in particular, ethane reaches a maximum on Ap168, that to Ca P Fig. 1 The effects of the Ca P ratio of calcium hydroxyapatites on the oxidation of methane at 973 K in the absence (A) and presence (B) of CCl, a, 0 5 h on-stream, b, 6 h on-stream Reaction conditions W=05 g, P=30ml min p(CH,)=28 7 hPa, p(o,)=4 1 kPa and p(CCl,)=O kPa or 0 17 kPa diluted with He (1) C, selectivity (YO),(21) C, selectivity (Y), (iu) CH, conversion and C,(in)0, conversion (YO), yield (YO) carbon monoxide is at a minimum regardless of time on- stream In contrast, in the presence of CC14 the conversions and selectivities are dependent on both the Ca/P value and the time on-stream, but particularly the latter With the non- stoichiometric catalysts, where Ca/P is <1 68, the conversion decreases markedly with increasing time on-stream, but with Ap, 53 the selectivities remain relatively constant on introduc- tion of Ccl4 With Ap164, no C2H4 or C2H6 is formed, after 6 h on-stream, whereas the selectivity to C02 increases signifi- cantly The selectivities to CO, C02, C2H4 and C2H6, obtained with the nominally stoichiometric catalyst, are similar after 0 5 h on-stream regardless of the presence of CCl, in the feedstream However with CCl, present, with the latter catalyst composition, the selectivity to CO has approximately doubled with a corresponding decrease to that of C02,after 6 h on- stream, and additionally, the conversion has decreased some- what With the catalyst containing an excess of calcium, Ap, 72, the conversions and selectivities at both times on-stream are similar with and without CC14 In the presence of CCl, and after 6 h on-stream the selectivity to CO on Ap, 72 increases with decrease in space time (W/F) (Fig 2) A decrease in conversion is particularly evident at the lowest value of W/F In the absence of CCl, no systematic changes are observed with W/F on either Ap, 72 or Ap, 68 In contrast, with Ap, 68 and added eel,, virtually all of the c, formed after 6 h on-stream is CO, regardless of the value of W/F,in contrast with the lower selectivities to CO where CC1, is not present With Ap, 53 and Ap, 64 no remarkable changes in the C1 and C2 selectivities are observed with decreasing W/F (Fig 3) in either the presence or absence of CCl, However, the substantial decreases in conversion after 6 h on-stream where CC14 is present are particularly noticeable with these two catalysts Properties of bulk phase and surface of fresh calcium hydroxyapa ti tes Since the XRD patterns of the four samples of calcium hydroxyapatite with Ca/P values of 153, 164, 168 and 172 460 J Muter Chem , 1996, 6(3), 459-464 W/F/10-2 g rnin rnl-' Fig.2 Effects of W/F in the absence (A) and presence (B) of CCl, on the oxidation of methane at 973 K on Ap,.,, and Ap,,,,. Symbols and reaction conditions are the same as those in Fig. 1 except W=O.1 g at W/F=0.33 x lo-' g min ml-', W=0.25 g at W/F=0.83 x lo-' g min ml-', and W=0.5 g at W/F=1.67 x lo-' g min ml-'. WIFII 0-2 g min ml-' Fig. 3 Effects of W/F in the absence (A) and presence (B) of CCl, on the oxidation of methane at 973 K on Ap,,, and Ap,.,,. Symbols and reaction conditions are the same as those in Fig. 2. are, not surprisingly, essentially indistinguishable (Fig. 4), no correlations with catalytic activity can be deduced. As has been shown, the catalytic activities can frequently be related to the bulk properties of the solid catalysts such as electronega- tivity of cation^^^,^^ or anion^.'^.^' With the catalysts under investigation in the present work, the structures of the catalysts, as demonstrated from the XRD patterns, are similar.Consequently, it may be possible to view each sample as the stoichiometric calcium hydroxyapatite containing an excess of either calcium or phosphorus, the latter present as the phos- phate, or alternatively, containing calcium or phosphorus vacancies. Stoichiometric calcium hydroxyapatite has a hexag- onal structure constructed from columns of Ca and 0 atoms which are parallel to the hexagonal axis as shown in Fig. 5.39 Three oxygen atoms of each PO4 tetrahedron are shared by one column, with the fourth oxygen atom attached to a neighbouring column.The hexagonal unit cell of calcium hydroxyapatite contains ten cations located on two sets of non-equivalent sites, four on site I (Ca,) and six on site I1 (Ca,,). The calcium ions on site I are aligned in columns, while those on site I1 are in equilateral triangles centred on the screw 2tVdegrees Fig. 4 XRD patterns of the fresh calcium hydroxyapatites: (a) Ap1.72; (b)Apl.68; (c)Ap1.64;(d) Ap1.53 Fig. 5 Stoichiometric calcium hydroxyapatite structure projected on the a,b plane39 axes. The site I cations are coordinated to six oxygen atoms belonging to different PO4 tetrahedra and also to three, relatively distant, oxygen atoms. The site I1 cations are coordi- nated to six oxygen atoms belonging to PO4 and one oxygen atom belonging to an hydroxy gro~p.~~,~' The Ca-0 bond distance of the stoichiometric calcium hydroxyapatite as obtained from neutron diffraction are summarized in Table 1,28 together with the coordination numbers based on Fig.5. In the oxidation of methane on the solid catalysts, it is generally accepted that the active sites are oxygen species, although their natures are uncertain and undoubtedly depend on the catalyst, and the activity is also strongly influenced by the electronega- tivity of the cations. Although at this time the location and environment of the excess of calcium or phosphate ions are unclear, it is expected that these ions would exert a perturbing influence on the species contained within the structure and in particular the Ca-0 bond distance which may, in turn, alter the electronegativities of the oxygen species.Therefore EXAFS analyses were applied to clarify the bond distance and the J. Muter. Chem., 1996, 6(3), 459-464 461 Table 1 Ca-0 distances' and coordination numbers in stoichiometnc hydroxyapatite distance/A 2 408 2 454 2 808 CNb 3 3 3 'Ref 28 Coordination number local structure around the calcium atoms Fig 6 shows the X-ray absorption near-edge structure (XANES) spectra of the fresh catalysts with Ca/P 172, 168, 164 and 1 53 The shape of the XANES spectra and the edge position for each catalyst are almost the same, indicating that the electronic configuration and site symmetry of the calcium in the present calcium hydroxyapatites are not significantly different Fig 7 shows the Fourier transforms of the EXAFS oscillation around the Ca K-edge of each fresh catalyst Phase shifts are not corrected in these spectra The strongest peaks in each spectrum correspond to a nea5eest-neighbour distance with separations between 2 3 and 2 8 A (Table 1) It is of interest to note that other-order distances appear to be different from each other It would be expected that the nearest-neighbour Ca-0 dis-tance has the strongest relationship to the catalytic activities Further, the data obtained at the nearest-neighbour distances are more precise owing to the relative independence of the harmonics Thus, the reliability of the phase shift and amplitude functions are tested at approximately the nearest distance by I' A 1 6020 Goso 4100photon energylev ...I ....1 1.1.11--v,= c3 -.......,, 11..a,,., 012345012345 riA Fig. 7 Founer transformation of k3 weighted EXAFS oscillation measured at 300 K near the Ca K-edge of the fresh calcium hydroxyapatites (a) Apl 72, (b)Apl 68, (c) Apl 64, (dl Apl 53 462 J Muter Chem ,1996,6(3), 459-464 2 707 2 358 2 345 2 514 2 384 1 1 2 2 1 fitting the observed EXAFS of each fresh catalyst Fig 8 shows optimum curve fitting for each hydroxyapatite, in which the solid lines represent the expenmental data and the closed circles represent the calculated results The results of the curve fitting analysis are shown in Table 2 The coordination number of calcium, set at 4 8 in the stoichiometric calcium hydroxyapa- tite (Ap, 68), shows the estimated average coordination number of oxygen to CaJ and Call at a distance of Ca-0 between 2 345 and 2 454 A (Table 1) Although cognisance of the esti- mated deviations should be taken, the nearest-neighbour dis- tances of the Ca-0 bond of each catalyst are in the order of Ap, 72 =-Ap, 68 <Ap, 64 <Ap, 53 It may be of interest to com- pare the order of the Ca-0 bond with that of the selectivity to carbon monoxide in the oxidation of methane in the absence of Ccl4 (Ap, 72 >Ap, 68 <Ap, 64 <Ap, 53 as shown in Fig 1) The surface properties of each fresh catalyst were examined by XPS (Table 3) The binding energies of Ca 2p and 0 1s before and after argon-ion etching were virtually identical regardless of the quantity of Ca Furthermore, no correlation is evident between the values of Ca/P and of O/P on the surface and those of the bulk Ca/P This indicates the difficulty in companng and contrasting surface properties of the calcium hydroxyapatites by application of XPS analyses only Properties of bulk phase and surface of the used calcium hydroxyapatites XRD, EXAFS and XPS analyses of the used catalysts were performed to investigate the results of the interaction of the FF 0 2 4 6 8102 4 6 810 klA-' Fig.8 Curve fitting of the fresh calcium hydroxyapatites Experimental Table 2 Results of curve-fitting analysis sample r/k Nb o/A2c Eo/eVd R/(%)" AP112 2 41 55 0 1042 6 741 77 68 2 37 48 0 0999 3 961 81 Apl 64 Apt 53 2 38 2 39 46 42 0 0975 0 0705 4 480 5 018 67 61 a Distance, estimated maximum deviation (+O 01) Coordination number, estimated maximum deviation (+10), except for 4 8 which is fixed Debye-Waller factor Threshold increment Reliability factor Table 3 Binding energies and relative concentrations in the fresh The formation of chlorinated species on the surface of Ap1.72 calcium hydroxyapatites is confirmed by XPS (Table 5).Since the binding energies of relative Cl 1s in Ap1.72, used in the presence of CCl,, together with binding energy/eV concentration those of the other elements, are similar to those found with AP1.68, Ap,.,, and Ap1,53, chlorapatite may be formed in minor Ca amounts on Ap1.72. Note that the quantities of chlorinated species found on the surface of the catalysts previously sample time"/min 2p3/, 2p1,, 0 1s P 2p Ca/P O/Ca employed in the methane reaction, where CCl, was present, AP1.72 0 347.4 351.0 531.3 133.3 1.30 2.40 appear to correlate with results obtained from these reactions. 347.7 351.0 531.7 133.6 1.42 2.36 On Ap,.,, and Ap1.53, with which substantial decreases in the 1 Ap1.68 0 346.8 350.2 530.8 132.6 1.16 2.37 conversion with increasing time on-stream were observed in 1 347.0 350.6 531.2 133.3 1.44 2.26 the presence of CCl,, the amounts of the chlorinated species Ap1.64 0 347.4 350.9 531.5 133.1 1.20 2.49 formed on the surface during the reaction are quite similar 1 348.1 351.5 532.2 134.1 1.38 2.21 Ap1.53 0 347.2 350.7 531.3 133.1 1.20 2.64 (Cl/Ca =0.22 and 0.20, respectively).The quantity of the chlori- 347.8 351.4 531.7 133.7 1.34 2.42 nated species on the surface is found to be a maximum for 1 Ap1.68 with which the largest selectivity to CO was obtained. " Etching time. In contrast, relatively small quantities of chlorinated species were detected on Ap1.72, with which relatively little changes in feedstream components and, in particular CCl,, on the bulk either the conversions or selectivities were observed. Since the and surface properties of these catalysts. order of the amount of the chlorinated species estimated by XRD patterns of the catalysts used in the methane conver- XPS (Ap1.68>Ap1.64 zApl.53 >Ap1.72) is identical to that sion process in the absence of CCl, show that the composition found for the bulk phase and the formation of chlorapatite corresponds to that of calcium hydroxyapatite and no trans- apparently results from ion exchange of the hydroxy group by formations of the catalyst are detected (Table 4).In contrast, C1- it appears that the detected chlorinated species on Ap,.,, complete conversion of the stoichiometric and calcium-is also chlorapatite. Since the quantities of chlorinated species deficient calcium hydroxyapatites (AP1.68, and Ap,.,, and found in the catalysts do not correlate with that of the nearest Ap1.53 to chlorapatite [Gal,(P04)6C12]) is observed after the distance of Ca-0, the ion exchange of the hydroxy group in methane oxidation reaction in the presence of CCl,.the catalyst by C1- ion does not appear to be significantly Surprisingly, chlorapatite is not detected in Ap1.72 although dependent on this distance, and consequently the effects pro- ion chromatographic analyses of C1 show the presence of duced by the addition of CCl, are evidently not dependent on chlorinated species. the Ca-0 separation. Table 4 Properties of bulk phase of the used catalysts" sample SAC phased Cl/Ca" Ca-Of AP1.72 20.1 & 0.8 -2.37( 1) 19.1k0.9 0.026k0.001 2.36( 1) Ap1.68 20.1 & 0.4 -2.37( 1) 15.3k0.3 0.143 0.007 2.36( 1) Apl 64 18.1 kO.1 -2.34( 1) 7.8t0.1 0.122 0.006 2.38(1) Ap1.53 9.4 & 0.2 -2.39( 1) 22.7 k0.3 0.116& 0.006 2.36( 1) 'Previously employed in obtaining the results reported in Fig.1 but after 6 h on-stream. A; absence of CC14. P; presence of CCI,. 'Surface area (m2 g-'). By XRD. Ca2+ by ICP and CI- by ion chromatography. By EXAFS. Values in parentheses are the estimated maximum deviations. Not analysed. Table 5 Binding energies and relative concentrations in the used catalysts' binding energy/eV relative Ca concentrations sample CCI4 t'/min 2P3/2 2Ptp 0 1s p 2P c1 1s Ca/P O/Ca Cl/Ca AP1.72 0 347.2 350.7 531.3 133.2 - 1.30 2.79 - 1 347.6 351.1 531.7 133.7 - 1.40 2.65 - APl.72 0 347.0 350.8 531.1 132.7 199.0 1.23 2.89 0.05 1 347.4 351.2 531.5 133.3 200.2 1.40 2.70 0.07 .68 0 347.3 350.9 531.5 133.0 - 1.37 2.8 1 - 1 347.6 351.1 531.7 133.5 - 1.32 2.57 - 0 347.3 351.0 531.2 133.2 198.8 1.19 2.04 0.35 1 347.8 351.4 531.9 133.7 199.6 1.26 1.96 0.35 Ap1.64 0 1 347.5 347.7 350.9 351.2 531.5 531.9 133.6 133.7 -- 1.27 1.43 2.73 2.54 -- Ap1.64 0 1 347.4 347.8 350.9 351.3 531.3 531.5 133.0 133.7 198.8 198.8 1.18 1.28 2.77 2.56 0.20 0.2 1 Ap1.53 0 1 347.2 347.7 350.8 351.3 531.4 531.7 133.1 133.5 -- 1.16 1.30 2.67 2.48 -- Apl.53 0 1 347.5 347.8 351.0 351.3 531.4 531.7 133.2 133.7 199.1 199.2 1.18 1.30 2.55 2.3 1 0.22 0.19 " Previously employed in obtaining results reported in Fig.1 but after 6 h on-stream. A; absence of CCI,. P; presence of CC14.Etching time. J. Muter. Chem., 1996,6(3), 459-464 463 Analyses of the Ca-0 distance of the used catalysts were also carned out using EXAFS (Table 4) Although the Ca-0 distances of the used catalysts are somewhat smaller than those in the corresponding fresh catalysts, probably owing to sintenng, no systematic variations were found either with or without CCl, Therefore the properties of the catalysts appear to be more strongly influenced by the bond lengths of the fresh catalysts 5 6 7 8 9 10 Y Matsumura, S Sugiyama, H Hayashi and J B Moffat, J Solid State Chem, 1995,114, 138 Y Matsumura, S Sugiyama, H Hayashi and J B Moffat, Catal Lett, 1995,30,235 S Sugyama, T Minami, H Hayashi, M Tanaka, N Shigemoto and J B Moffat, Energy Fuels, in the press S Sugiyama, T Minami, H Hayashi, M Tanaka, N Shigemoto and J B Moffat, J Chem SOC,Faraday Trans, 1996,92,293 S Ahmed and J B Moffat, Stud Surf Sct Catal, 1991,61,57 T Ohno and J B Moffat, Appl Catal, 1993,93,414 Conclusions 11 12 S Sugyama and J B Moffat, Energy Fuels, 1994,8,463 S Sugiyama, K Satomi, N Kondo, N Shigemoto, H Hayashi and J B Moffat, J Mol Catal, 1994,93,53 The conversion of methane in the absence of CCl, was little influenced by the value of Ca/P in each calcium hydroxyapatite, while the selectivity to CO reached a maximum at Ap,,, Over calcium-deficient hydroxyapatites (Ap, 64 and Ap, 53), a sharp decrease in the conversion was observed on addition of 13 14 15 16 17 R Voyatzis and J B Moffat, Energy Fuels, 1995,9,240 H Monma, J Catal, 1982,75,200 J A S Bett, L G Christner and W K Hall, J Am Chem SOC, 1967,89,5535 M Misono and W K Hall, J Phys Chem, 1973,77,791 L Winand and G Duyckaerts, Bull SOC Chim Belg ,1962,71,142 CCI, while a substantial increase of the selectivity to CO occurred on the stoichiometric calcium hydroxyapatite (Ap, 68) On Ap, 72, minor increases in the selectivity to CO on addition of CCl, were observed, particularly at shorter space times The Ca-0 bond distances of the fresh cataly$s as estimated ,by EXAFS folloy the order Aplaz within the estimated maximum deviation of 001 A, while the selectivity to carbon monoxide in the oxidation of methane in the absence of CCl, follows the order Ap, 72>Ap1 68 < Ap, 64<Ap1 53 The order of the atomic ratio of Cl/Ca both in the bulk phase and on the surface of the catalysts previously employed in the methane conversion in the presence of CCl, was found to be Ap, 68 >Ap, 64 M Ap, 53 >Ap, 72 From XRD analyses the chlorinated species formed during the reaction appear to be predominantly chlorapatite Some relationships between the effect of the introduction of CCl, in the feedstream (2 41 A)>Ap, 68 (2 37 A)<Ap, 64 (2 38 A)6Ap, 53 .(2 39 A) 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 E E Berry, J Inorg Nucl Chem , 1967,29,317 E E Berry, J Inorg Nucl Chem, 1967,29, 1585 D McConnel, Arch Oral Biol, 1965,10,421 W E Brown, J P Smith, J R Lehr and S W Franer, Nature, 1962,196,1950 S J Joris and C H Amberg, J Phys Chem ,1971,75,3167 H Monma, Shokubai, (in Japanese, Catalyst),1985,27,237 W E Brown, Nature, 1962,196, 1048 H Ji and P M Marquie, J Muter Sci Lett, 1991,10, 132 W J Landis, J Moradian-Oldak and S Weiner, Connect Tissue Res, 1991,25,181 W J Landis and M J Glimacher, J Ultrastruct Res, 1978, 63, 188 M I Kay, R A Young and A S Posner, Nature, 1964,204,1050 H C W Skinner, H T Hunt and J Griswold, J Phys E, Sci Instrum, 1980, 13,74 L M Siperko and W J Landis, Appl Phys Lett, 1992,61,2610 H Nishikawa, S Ikeda and H Monma, Bull Chem SOC Jpn, 1993,66,2570 E Hayek and H Newesely, Inorg Synth ? 1963,7,63 and the amount of chlorapatite formed during the oxidation of methane are apparent 33 34 F W Lytle, D E Sayer and E A Stern, Phys Rev B, 1975, 11,4825 D W Marquardt, J SOC Ind Appl Math, 1963,11,431 This work was partially funded by the Natural Sciences and Engineering Research Council of Canada to J B M ,to which 35 36 B K Teo, EXAF Basic Principles and Data Analysis, Spnnger-Verlag, Berlin, 1986, p 71 S Sugiyama and J B Moffat, Catal Lett, 1992,13,143 our thanks are due 37 S Sugiyama, K Satomi, H Hayashi, N Shigemoto and J B Moffat, Appl Catal A Gen, 1993,103,55 References 38 39 S Sugiyama and J B Moffat, Energy Fuels, 1993,7,279 T Suzuki, in Ion-Koukan (in Japanese, Ion Exchange), ed M Seno, M Abe and T Suzuki, Kodansha, Tokyo, 1991, p 141 1 Y Matsumura and J B Moffat, Catal Lett, 1993,17, 197 40 T Kanazawa, in Muki-Rin-Kagaku (in Japanese, Chemistry of 2 Y Matsumura and J B Moffat, J Catal, 1994,148,323 Inorganic Phosphates), Kodansha, Tokyo, 1985, pp 61-63 3 Y Matsumura, J B Moffat, S Sugryama, H Hayashi, N Shigemoto and K Saitoh, J Chem SOC,Furaday Trans, 1994, 90,2133 41 42 Y Amenomiya, V I Birss, M Goledzinowski, J Galuszka and A R Sanger, Catal Rev Sci Eng ,1990,32,163 J H Lunsford, Catal Today, 1990,6,235 4 Y Matsumura, S Sugiyama, H Hayashi, N Shigemoto, K Saitoh and J B Moffat, J Mol Catal, 1994,92,81 Paper 5/07380D, Recezued 9th Nouember, 1995 464 J Muter Chem , 1996,6(3), 459-464

ISSN:0959-9428    

DOI:10.1039/JM9960600459

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

On the crystallisation and nature of the microporous boron-aluminium 0x0 chloride BAC(10) Jihong Yu," Ruren XU,"" Jiesheng Chen," and Yong Yue,b "Key Laboratory of Inorganic Hydrothermal Synthesis, Department of Chemistry, Jilin University, Changchun, PR China Wuhan Institution of Physics, Academia Sinica, Wuhan, 430071, PR China A novel microporous boron-aluminium 0x0 chloride [BAC( 1011has been synthesized hydrothermally. The crystallisation of the material was studied by X-ray diffraction (XRD), inductively coupled plasma analysis (ICP), IR spectroscopy and magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy. Its basic building units are triangular B03, tetrahedral B04 and octahedral AlO,. As BAC( 10) crystallises, some of the B03 groups are transformed into B04 species in the solid.Adsorption measurements reveal that BAC( 10)has a microporous structure, whereas the fact that the C1- ions in BAC( 10) can be partially exchanged by Br -ions suggests that this compound possesses a cationic framework. Thermal decomposition studies indicate that BAC( 10) is stable up to 300 "C, and at temperatures above 325 "C the crystal structure of BAC( 10) collapses with the evolution of HCl and H20 from the sample. The term 'zeolite' designates a crystalline microporous alumi- nosilicate or silica with an open oxide framework.' Heteroatoms other than Si and A1 have been successfully incorporated into various zeolites, and there is growing number of new compounds with a microporous open framework. Notable examples are the M"'XVO4 families (M =Al, Ga, In; X =P, germanium dioxide^,^ metal sulfides* and trans- ition-metal oxide^.^,'^ Nevertheless, the framework of all these materials is negatively charged or neutral, and it will be intriguing to synthesize microporous materials with a positively charged framework.Recently, we reported the preparation of a new crystalline boron-aluminium chloride [B-C( l)]," the structure of which was constructed from B03, B04 and AlO, primary building units. Further investigation revealed that a novel microporous boron-aluminium 0x0 chloride [designated BAC( lo)], with an empirical molar composition of 0.5B20,-1 .0A1203. 0.4HC1.3.0H20,12 could be synthesized from a reaction mixture similar to that for B-C(1).Interestingly, the framework of BAC( 10) is found to be positively charged and the C1- anions in the compound can be exchanged, to a certain extent. In this paper, we describe the crystallisation behaviour and the nature of the BAC( 10) material. Experimental To follow the crystallisation of BAC( lo), a typical gel with an empirical composition of 7.0H3B03 :2.0AlC13 :2. 5Ca0:200H20 was heated in a PTFE-lined stainless steel autoclave at 160 "C under autogenous pressure. The solid products were recovered by filtration and drying at ambient temperature. The X-ray powder diffraction (XRD) patterns were recorded on a Rigaku 3DX diffractometer with Cu-Ka radiation. Inductively coupled plasma analysis (ICP) was performed on a Jarrzall-Ash 800 Mark-I1 ICP instrument, and the IR spectra were obtained on a Nicolet 5DX FTIR spectrometer."B and 27Al magic-angle spinning nuclear magnetic reson- ance (MAS NMR) spectra were recorded on a Bruker 400 spectrometer with a magnetic field strength of 4.7 T. The spinning rate was ca. 4 kHz, "B and 27Al spectra were recorded at 64.2 and 52.1 MHz, and the chemical shifts were relative to external standards of KBF, and CAI( H20),]3+, respectively. Water adsorption measurements were conducted on a CAHN 2000 vacuum electrobalance at 20 "C. The ion-exchange capacity of BAC(10) was obtained by stirring the powder sample in a 0.1 mol dm-3 NH,Br-water solution at 60°C for 2 h followed by analysing the C1 and Br contents in the exchanged and unexchanged samples on a Soda LC-601 ion chromatograph analyser.Prior to analysis, the solid sample was melted with NaOH and then dissolved in distilled water. Temperature-programmed decomposition and mass spectro- metric (TPDE-MS) analysis was carried out on a conventional flow-type TPD apparatus equipped with a mass spectrometer as the dete~tor.'~ Results and Discussion BAC(10) crystallises well from the typical gel formed from boric acid, aluminium trichloride, calcium oxide and water. The crystallisation curve, defined as the crystallinity based on the XRD results versus the crystallisation time for BAC( lo), is shown in Fig. 1. Similar to that for zeolites, the curve shape is of type 'S'. The crystallisation process involves three stages, i.e. nucleation, crystal growth and stabilisation.It is seen that the nucleation stage takes a very short time (< 2 h), which is suggestive of a rapid formation of BAC( 10) crystal nuclei. ICP analysis shows that the B:A1 ratio in the starting gel, as well as in the solid samples with different crystallinities, is close to that (ca. 0.5) for the final product. On the other hand, 100 [ 0 '* 0 10 20 30 40 50 60 70 80 crystallisation timeh Fig. 1 Crystallisation curve of BAC(10) J. Muter. Chem., 1996, 6(3), 465-468 465 the B A1 ratios in the liquid undergo no distinct change as the crystallisation time increases (see Table 1) Fig 2 shows the IR spectra of the solid samples with crystallinities of 0, 20, 50 and 95%, respectively The main feature of the IR spectrum for the starting gel is similar to that of the well crystallised product Note that during the crystallis- ation from crystallinity 0 to 95%, the intensities of the bands at 1356 and 1293 cm-', characteristic of trigonal BO3,l4 l5 decrease markedly, whereas that of the band at 1089cm-' (typical of tetrahedral B041617), increases gradually In addition, there is no band between 1150 and 1160 cm-' in the sample with 0 crystallinity, but the bands at 1150, 1152 and 1159 cm-l, attributable to tetrahedral B04,16 l7 appear in the samples with 20, 50 and 95% crystallinities, respectively This indicates that some of the triangular BO, groups are trans- formed into tetrahedral BO, species during the formation of the microporous boron-aluminium 0x0 chloride "B MAS NMR spectra of these samples with crystallinities of 0, 10, 20, 50 and 95% each exhibit two resonance signals appearing at approximately 6 18 and 14 5 relative to KBF, (Fig 3) The signals appearing at high field are mainly attn- buted to tetrahedrally coordinated boron (B4), and are charac- terised by their highly symmetric Gaussian shapes The low- field signals assigned to triangularly coordinated boron (B,) exhibit evident quadrupolar interaction l8 A considerable part of the signal intensity for B3 is embedded in the observed peak at the higher field, so that the signal at high field appears stronger than the one at low field (Fig 3) In order to calculate the B, B, ratios, a powder program is used to simulate the experimental "B NMR spectra Table 2 lists the simulation parameters and the B3 B, ratios in samples with different crystallinities Note that the experimentally observed chemical shifts in Fig 3 for B, are lower than the calculated ones in Table 2, due to quadrupolar interactions The simulated results indicate that the B, B, ratio decreases gradually from 40 to 2 3 with increasing the crystallinity from 0 to 95% Obviously, a part of the triangularly coordinated B, are transformed into Table 1 B A1 mole ratios in the solid phases with varying crystal- linities, and those in the accompanying liquid phases relative crystallinity (%) 0 10 20 50 95 solid 0 44 0 47 0 45 0 48 0 50 liquid 39 40 44 43 44 I I 2000 1600 1200 900 700 400 wavenumberkm-1 Fig.2 IR spectra of samples with crystallinities of (a) 0, (b) 20, (c) 50 and (d) 95% Fig. 3 "B MAS NMR spectra of samples with crystallinities of (a) 0, (b) 10, (c) 20, (d) 50 and (e) 95% tetrahedrally coordinated B, dunng the formation of BAC( 10) This result is in good agreement with the IR studies B03 and BO, groups with various BO, BO, ratios have also been observed in known borates such as boracite (C1Mg3B7013),19 hilgardite (Ca2B,0,C1 H20)20 and some other polyanions, among which are [B303(OH),] -,21 [B304(OH)3]2-,22 [B,05(OH)4]2-,23 [B@6(OH)4]-,24 [B607(OH)6]2-25 and [B,012(OH)]4-26 It is known that hilgardite is a three-dimensional open-framework borate, the building block of which is the pentaborate anion, [B501,]9- The channels are filled by Ca2+ and C1- ions and water molecules It is believed that the C1- anions and water molecules in the as-synthesized microporous BAC( 10) also reside in the open channels, as is the case for hilgardite Therefore, in microporous BAC( 10) the Cl- anions can be partly exchanged by Br- anions as described later in this paper The observed 27Al NMR spectra of the samples with various crystallinities exhibit a broad peak appearing at 6 ca 22 relative to [A1(H2O)6l3' (see Fig 4), typical of octahedrally coordinated A127 However, note that the peaks are not sym- metrical due to the quadrupolar interaction On the other hand, it is observed that the chemical shifts of the 27Al signals move upfield and the peaks become more or less symmetric with increasing the crystallinity This is probably due to the change to higher symmetry of the framework during the crystallisation of BAC( 10) Simulation of the 27Al NMR spectrum for the sample with the highest crystallinity [Fig 4(e)] indicates that two compo- nents, I e ,two Gaussian lines, can be assumed one appears at 6 3 25, accounting for 63 4% of the total intensity and being typical of octahedral A106, and the other at 6-7 96 with a relative intensity of 36 6% This implies that the microporous BAC( 10) probably contains octahedrally coordinated A1 in two different environments, and the C1- anions in the structure may play a role in determining these two environments The A1 coordination states in the as-synthesized BAC( 10) is remi- niscent of jeremejerite A16B5F3015 ,28 which contains edge- sharing A106 octahedra and corner-sharing Al(0,F)6 octahedra Nevertheless, the boron coordination states in BAC( 10) and jeremejerite are different the former contains triangular B03 and tetrahedral BO,, whereas in the latter there exist two types of crystallographycally independent BO, triangles H20 adsorption measurements indicate that hydration- 466 J Mater Chem, 1996,6(3), 465-468 Table 2 Simulation parameters and results for the "B NMR signals relative crystallinity (YO) parameters and results 0 component 1 quadrupole coupling constant/MHz 2.9 dipolar broadening/Hz 600 asymmetry parameter 0 relative intensity (%) 80 chemical shift, G/ppm 20 component 2 Gaussian :Lorentzian ratio 0.5 linewidth/Hz 900 relative intensity (%) 20 chemical shift, G/ppm 2.0 B, :B4 ratio 4.0 n I 1 I I I I 150 loo 50 0 -50 -100 s Fig.4 27Al MAS NMR spectra of samples with crystallinities of (a) 0, (b) 10, (c) 20, (d) 50 and (e) 95% dehydration of the highly crystalline BAC( 10) is reversible and the shape of the adsorption isotherm, similar to that of NaX and AlPO,-17, is of the Langmuir type, typical of micropore filling29*30(Fig. 5). However, the previously reported boron- aluminium 0x0 chloride B-C( 1 ), which was synthesised from a reaction mixture essentially the same as that for BAC(10) but at 200"C,11 shows no adsorption capacity at all for H20, suggesting that high crystallisation temperatures favour the formation of dense phases for this reaction system.3s -NaX ________ccc_---_----a_d_----30E__-APO-I7 c________---~c* m BAC( 10) w silicalite ~ _-_- --_______ccH 0.2 0.4 0.6 0.8 1.0 Plpo Fig. 5 H,O adsorption isotherms for dehydrated NaX, APO-17, BAC( 10) and silicalite 10 20 50 95 3.0 3.0 3.0 2.8 500 500 500 500 0 0 0 0.1 80 78 76 70 22 21 21 21 0.5 0.5 0.5 0.8 1000 900 900 600 20 22 24 30 2.0 2.0 2.0 2.0 4.0 3.5 3.2 2.3 Chemical analysis reveals the existence of a certain amount of C1- anions but no Ca2+ cations in the as-synthesized BAC( 10).Furthermore, ion-exchange studies indicate that about 30% C1- ions in BAC(10) can be exchanged by Br- ions, and the exchange isotherm shape (Fig. 6) is similar to that for zeolite^.^' This result strongly suggests that the micro- porous BAC( 10) has a cationic framework structure. Fig. 7 shows the X-ray powder diffraction patterns of the as-synthesized BAC( 10) and the samples treated at various temperatures. BAC(10) is stable up to at least 300"C, and when the temperature is raised to 325°C it collapses to an amorphous form. Further increasing of temperature converts BAC( 10) to the crystalline A14B20932 phase. The microporous BAC(10) has lower thermal stability than the previously reported B-C( l)," probably due to the easy destruction of the open-framework structure of the former.The TPDE-MS for the BAC(10) heated at 280°C is shown in Fig. 8. It is seen that the thermal decomposition of the sample results in the evolution of HC1 and H20. A small amount of H20 is produced within the temperature range 280-604°C and the evolution of HCl is mainly from 325 to 604°C when the collapse of the framework of BAC(10) takes place, as suggested by Fig. 7. The evolution of HC1 and H20 from the BAC( 10) leads to the collapse of its cationic frame- work structure. Conclusions During the crystallisation of the microporous boron-alu-minium 0x0 chloride BAC( lo), some of the triangularly coordi- nated boron atoms are transformed into tetrahedrally coordinated boron, whereas the A1 atoms in the BAC(10) framework remain octahedrally coordinated.BAC( 10) pos- 50+ Br-ICI-ratio in solution Fig. 6 Ion-exchange isotherm for BAC( 10) J. Mater. Chem., 1996, 6(3), 465-468 467 We gratefully acknowledge the National Natural Science 1 I I 10 20 30 40 28ldegrees Fig. 7 X-Ray powder diffraction patterns for BAC( 10) (a) as-synthe- sized and treated at (b) 200, (c) 300, (d) 325, (e) 800 and (f) 1000°C * \ HCI I I I 280 380 480 580 temperature/"C Fig. 8 TPDE-MS curve for BAC( 10) sesses a cationic framework structure and the C1- anions in the material can be partially exchanged by Br-anions BAC( 10) is stable up to 300 "C,and when the sample is heated at 325 "C, its crystalline structure collapses with the evolution of HC1 and H20 Foundation of China for financial support References 1 M E Davis and R L Lobo, Chem Mater ,1992,4,756 2 J B Parise, J Chem SOC Chem Commun, 1985,606 3 R Xu, J Chen and S Feng, Stud Surf Sci Catal, 1991,60,63 4 J Chen and R Xu, J Solid State Chem ,1989,80,149 5 G Yang, L Li, J Chen and R Xu, J Chem SOC Chem Commun, 1989,810 6 Y Xu, L Koh, L An, S Qiu and Y Yue, Stud Surf Sci Catal, 1994,84,2253 7 R H Jones, J Chen, J M Tomas, A George, M B Hurtsthouse, R Xu, S Li, Y Lu and G Yang, Chem Mater ,1992,4,808 8 R L Bedard, S T Wilson, L D Vail, J M Bennet and E M Flanigen, Stud Surf Sci Catal, 1989,49,375 9 R C Haushalter, K G Strohmaier and F W Lai, Science, 1989, 246,1289 10 L A Meyer and R C Haushalter, Inorg Chem ,1993,32,1579 11 J Yu, R Xu, Q Kan, Y Xu and B Xu, J Mater Chem, 1993,3,77 12 J Yu, K Tu and R Xu, Stud Surf Scz Catal , 1994,84,315 13 B Xu, in Application of In-Situ Methods in Catalysis Research, ed Q Xin, Beijing University Press, 1993, p 392 14 H Moenke, Mineral Spektren I ,Akademie-Verlag, Berlin, 1962 15 C E Weier, J Res Natn Bur Stand A, 1966,70,153 16 J Haber and V Szybalska, Discuss Faraday SOC ,1981,72,263 17 C E Weier and R A Schroeder, J Res Natn Bur Stand USA Sect A, 1964,68 18 C A Fyfe, L Berni, H G Clark, J A Davies, G C Gobbi, J S Hartman, P J Hayes and R E Wasilyshen, Adv Chem Ser, 1983,211,405 19 T Ito, N Morimoto and R Sadanaga, Acta Crystallogr, 1951, 4,310 20 S Ghose and C Wan, Am Mineral, 1979,64,187 21 V G Skvorstov, R S Tsekhanskii, A K Molodin, V P Doiganev and N S Rodionov, Zh Neorg Khim ,1982,27,2426 22 H Gode and A Veveris, Latvyas PSR Zinatnu Akad Vestzs Kim Ser ,1984,ll 23 R Janada, G Heller and J Pickardt, Z Kristallogr ,1981,154, 1 24 K H Woller and G Heller, Z Kristallogr , 1983, 164,237 25 G Heller and J Schellhas, Z Kristallogr ,1983,164,237 26 M Touboul, C Bois and D Amoussou, J Solid State Chem ,1983, 48,412 27 C A Fyfe, G C Gobbi, J Khnowski, J M Thomas and S Ramdas, Nature, 1982,96,530 28 C Rodellas and S Garcia-Blanco, Z Kristallogr ,1983,165,255 29 S Brunauer, L S Demng, W S Deming and K Teller, J Am Chem SOC ,1940,62,1723 30 S Brunauer, P H Emmet and K Teller, J Am Chem SOC, 1938, 60,309 31 D W Breck, Zeolite Molecular Sieves, Wiley, New York, 1974, p 529 32 H Scholze, Z Anorg Allg Chem, 1956,284 Paper 5/03350K, Received 25th May, 1995 468 J Mater Chem, 1996,6(3), 465-468

ISSN:0959-9428    

DOI:10.1039/JM9960600465

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Consumption of Sic whiskers by the AI-Sic reaction in aluminium-matrix Sic whisker composites Shy-Wen Lai and D. D. L. Chung* Composite Materials Research Laboratory, State University of New York at Buffalo, Buffalo, NY 142600-4400, USA The A1-Sic reaction in aluminium-matrix Sic composites made by liquid metal infiltration resulted in the formation of silicon, the amount of which increased with increasing Sic volume fraction, but the fraction of Sic consumed by the reaction increased with decreasing Sic volume fraction. For Sic whisker composites made at an infiltration temperature of 800 "C, the fraction of Sic consumed was 18,26 and 55% at Sic volume fractions of 0.31,0.23 and 0.10, respectively. The fraction of Sic whiskers consumed was inversely proportional to the volume fraction of Sic whiskers in the composite.The product of these two fractions provides a scale (called the reactivity index) that describes the Al-Sic reactivity. The index decreased with decreasing infiltration temperature and was higher for Sic whiskers than Sic particles. Even at an infiltration temperature of 670 "C, the fraction of Sic whiskers consumed was 26% at an Sic volume fraction of 0.10. In contrast, the fraction of Sic consumed was only 8.4% for a 55 vol.% Sic particle composite made at an infiltration temperature of 800 "C. The fractional consumption values were obtained by determining the silicon concentration in the aluminium matrix uia calorimetric measurement of the liquidus-eutectic temperature difference. Silicon carbide (Sic) is the most commonly used filler in aluminium for obtaining composites of high strength, high modulus and low density.Both particle and whisker forms of Sic are used for this purpose. Aluminium is the most commonly used metal matrix material because of its low density and low melting temperature, which makes composite fabrication by liquid metal infiltration and stir casting economical. The reactivity between Sic and A1 is well known:'-'' 4A1+ 3SiC +A14C, +3Si This reaction is detrimental to the strength of the filler-matrix interface, as A14C, is brittle.6,8 Moreover, it is detrimental to the filler, as Sic is consumed by the rea~tion.~ The reaction is only significant at elevated temperatures, which are encoun- tered during composite fabrication by liquid metal infiltration or stir casting, or during remelting after composite fabrication.The reaction may be greatly diminished by using A14 with more than 7 mass% Si as the matri~,~ but the low ductility of Si-containing A1 matrices causes the composite to be low in strength compared with the corresponding composite with pure A1 as the matrix." Although there have been numerous studies on the Al-Sic reaction, relatively little work has been reported on the fraction of Sic consumed by the reaction; the only such report was made by Lloyd and Jin,4 whose study was limited to a composite (with 6061 as the A1 matrix) containing 20vol.% Sic particles. They reported that the fraction of Sic consumed was 0% after composite fabrication by stir casting and sub- sequent extrusion, and >20% after subsequent remelting at >675 "C for 1 h.Open questions that remain pertaining to the fraction of Sic consumed include: (1) how does the Sic volume fraction affect the fraction of Sic consumed?; (2) how does the fraction of Sic consumed differ between Sic whiskers and Sic particles?; and (3) what is the fraction of Sic consumed in the case of composites fabricated by liquid metal infiltration instead of stir casting? The objective of this paper was to answer these questions. Experimental Composite fabrication Both Sic particles and whiskers were used as the reinforcement. The Sic particles used were kindly provided by Electro Abrasives, Corp.(Buffalo, NY, USA) (no. 1200-W). The Sic particle size ranged from 1 to 10 pm, with a mean size of 3 pm. The Sic whiskers used were obtained from Advanced Refractory Technologies, Inc. (Buffalo, NY, USA). The proper- ties of the Sic powder and whiskers are listed in Table 1. The composition of the Sic powder was found to be: 98.5% SIC, 0.5% SiO,, 0.3% Si, 0.08% Fe, 0.1% Al, 0.3% C; that of the Sic whiskers was: 99% Sic, 0.4% SiOz and 0.6% trace metal impurities. The metal used was aluminium (170.1), the tensile strength of which was 65 MPa. Its composition was A1 (99.77%), Fe (0.16%) and Si (0.07%) and its melting temperature was 660 "C. The metal-matrix composites were fabricated by vacuum infiltration of a liquid metal into a porous preform under an argon pressure. The preform was a green body comprising Sic powder or whiskers.During composite fabrication, the preform was placed at the bottom of a graphite mould. Above the preform was placed a ceramic cloth layer and then an alu- minium ingot. The ceramic cloth was used to prevent contact between the Sic preform and the aluminium melt before the infiltration pressure was applied. The fabrication parameters were explained in detail in previous papers.lO*ll In general, the infiltration temperature was 800 "C, while the infiltration press- ure was 2000 psi (13.8 MPa) for Al/SiC whisker composites and 6000 psi (41.4 MPa) for Al/SiC particle composites. With all other parameters unchanged, the minimum infiltration pressure increased with increasing volume fraction of the reinforcement.In the case of Al/SiC composites containing Table 1 Properties of Sic particles and whiskers particles whiskers mean diameter/pm 3.0 1.4 mean length/pm - 18.6 density/g 3.18 3.21 Young's modulus/GPa thermal conductivity/ W m-l K-' 90 400-440 400-440 - coefficient of thermal 3.4 - crystal structure Sic (mass%) SiOz (mass%) expansion/106 OC-' hexagonal (a) 98.5 0.5 cubic (p)99 0.4 J. Muter. Chem., 1996, 6(3), 469-477 469 10 vol.% whiskers, the effect of the infiltration pressure on the degree of infiltration was studied. As shown in Table 2, at an infiltration pressure of 45 psi (0.3 MPa), no liquid metal infil- tration occurred. With the infiltration pressure increased to 200 psi (1.4 MPa), the infiltration occurred completely (with less than 5% porosity in the resulting composites).A porosity of less than 0.5% was achieved by further increasing the pressure to 800 psi (5.5 MPa). The Sic preforms were prepared by wet casting, which involved compressing in a die a slurry containing Sic powder or whiskers, a liquid carrier (acetone) and a binder (an acid phosphate formed from aluminium hydroxide and phosphoric acid.I2-l4) For particulate preforms, the carrier :binder ratio was from 40 :1 to 45 :1 by volume, as this amount of binder was sufficient to maintain rigidity in the preform. Excessive amounts of binder caused the particulate preform to be not porous enough for subsequent liquid metal infiltration, even at 6000psi (41.4MPa). For whisker preforms (which have lower filler-volume fractions), the amount of binder in the preform was greater, corresponding to a carrier :binder ratio of 15 :1.The die allowed any excessive liquid to be squeezed out. After removal from the die, the compact was dried in a fume hood at room temperature for 3 h. After drying, which removed most of the acetone, the preform was fired by (i) placing the preform in a furnace at room temperature, (ii) heating to 510°C at a controlled rate of 1.4"C min-', (iii) holding at 510 "C for 3 h, and (iv) cooling in the closed furnace. Ref. 10, 12-14 give the phases formed in the preform due to the binder after heating the Sic particle and whisker preforms, respectively, at different temperatures.The binder content was ca. 0.1 mass% of the particle preform" and ca. 5 mass% of the whisker preform.13 Excessive heating and a non-uniform temperature distribution in the furnace had to be avoided, as they would cause quick evolution of acetone and thermal stresses, thus resulting in cracking during the firing. The preforms were cylinders, 4.00 cm in diameter, with a height- to-diameter ratio of 0.5, for all the composites used in this study. In this paper, 'composite cylinder' refers to the metal Table 2 Effect of the infiltration pressure on the degree of infiltration for the AI/SiC composite containing 10 vol.% whiskers infiltration pressure /Psi /MPa degree of infiltration porosity (YO) 45 0.3 none - 200 800 2000 1.4 5.5 13.8 complete complete complete <5 <2 <0.5 infiltrated cylindrical preform and 'excess aluminium' refers to the metal cast around the preform during infiltration.The pressure used during compression of the slurry was adjusted to vary the Sic volume fraction in the resulting preform. Composite characterization Metallography. Fig. l(a) and (b) show SEM images of the Al/SiC composite cylinders containing 10 vol.YO whiskers, which were fabricated with an infiltration pressure of 200 psi (1.4 MPa) and 2000 psi (13.8 MPa), respectively. The porosity (ca. 5%) was clearly observed for the Al/SiC fabricated at 200 psi (1.4 MPa). The porosity was diminished by increasing the infiltration pressure to 2000 psi (13.8 MPa).Fig. l(c) shows that, at an infiltration pressure of 2000 psi (13.8 MPa), the Al/SiC composite containing 31 vol.% Sic whiskers exhibited ca. 3Yo porosity. This indicates that the infiltration pressure should be increased with increasing volume fraction of Sic whiskers in order for complete infiltration to occur. Fig. 2 shows SEM photographs of both the excess aluminium and the edge regions of Al/SiC composite cylinders containing 10 and 23 vol. YOwhisker reinforcement. The presence of needle- shaped Si precipitates in the excess A1 regions [Fig. 2(b) and (d)] indicated that the A1-SIC reaction occurred in both composites. The higher concentration of the Si precipitates for the 23 vol.% Sic whisker composite than the 10 vol.% Sic whisker composite indicates that the extent of Al-SIC reaction was larger at the higher Sic whisker volume fraction.Both composites contained uniformly distributed Sic whiskers and are expected to have isotropic properties. Note that the Al/SiC composite containing 23 vol.% Sic whiskers shows a slightly higher porosity compared to the Al/SiC composite containing lOvol.% Sic. A similar morphology of Si precipitates in the form of eutectic A1-Si was observed (not shown) between the primary A1 dendrites in the excess aluminium around Al/SiC containing 55 vol.% Sic particles. This indicates that Si, a product of the Al-SIC reaction, diffused outward to the excess aluminium and formed a hypoeutectic Al-Si alloy.The amount of Si in the excess aluminium increased with increasing volume fraction of reinforcement in the Al/SiC. This also indicates that the Sic whiskers, though in a single crystal form, reacted with aluminium at high temperatures (during infiltration), just as the Sic particles (not single crystals) did. No Al-Si eutectic or dendritic morphology was microscopi- cally observed within the Al/SiC composite cylinders contain- ing either Sic particles or Sic whiskers. This negative observation may be due to the very small Sic filler spacing in the Al/SiC composite. In order to study the location of Si (reaction product) in Al/SiC, the Al/SiC particulate composite Fig. 1 SEM photographs of polished sections of various Al/SiC composites.(a) Al/SiC containing 10 vol.% Sic whiskers and fabricated with an infiltration pressure of 200 psi (1.4 MPa). Porosity ca. 5%. (b) AI/SiC containing 10 vol.% Sic whiskers and fabricated with an infiltration pressure of 2000 psi (13.8 MPa). Porosity <0.5 YO.(c) Al/SiC containing 31 vol.% Sic whiskers and fabricated with an infiltration pressure of 2000 psi (13.8 MPa). Porosity ca. 3%. 470 J. Muter. Chem., 1996,6( 3), 469-477 Fig. 2 SEM photographs of polished sections of AI/SiC whisker composites. (a) 10 voL% Sic whisker composite. (b) The excess aluminium surrounding the 10 vol.% Sic whisker composite cylinder. (c) 23 vol.% Sic whisker composite. (d) The excess aluminium surrounding the 23 vol.% SIC whisker composite cylinder. (55 vol.% particles) was etched with acids to completely remove the aluminium.Unlike the preform which showed clean isolated Sic particles (not shown), Fig. 3(a) shows that the Sic particles were connected to form a network. Fig. 3(b) shows an SEM image at a higher magnification, which shows that Si precipi- tated between adjacent Sic particles, thereby forming a net- work. Similarly, Fig.4 shows the SEM morphology of plain Sic whiskers and a fully etched Al/SiC whisker composite containing 23 vol.'30whiskers. Unlike the clear morphology in plain Sic whiskers [Fig. 4(a) and (c)], the Sic whiskers in the etched composite were connected by the Si precipitates [Fig. 4( b) and (d)]. Although not confirmed statistically, the Sic whiskers in the Al/SiC composites tended to have a smoother and rounder surface morphology compared with plain Sic whiskers.This may be due to the Al-Sic reaction at the ends and surface of the Sic whiskers during composite fabrication. X-Ray diffraction. X-Ray diffraction (XRD) was performed with Cu-Ka radiation in different regions of the Al/SiC particle composite cylinder in order to investigate the phase distri- bution. We had previously studied the non-uniform phase (Si, Fig. 3 SEM photographs of a fully etched Al/SiC composite containing 55 vol.% Sic particles. Si precipitated and bridged adjacent Sic particles. J. Mater. Chem., 1996, 6(3), 469-477 471 Fig. 4 SEM photographs of plain Sic whiskers [(a) and (b)] and a fully etched Al/SiC containing 23 vol YOSic whiskers [(c) and (d)].(a) and (b) show clear whisker surface morphology, (c) and (d) show that Si precipitated and bridged adjacent SIC whiskers Sic and A1&) distributions throughout the Al/SiC particle composite cylinder." For details of the experimental pro- cedures and notations of the locations within the composite cylinder, see ref. 10. Fig. 5 shows the XRD patterns of the excess aluminium and the edge region of the Al/SiC particle composite cylinder. Only A1 and Si peaks were present in the excess aluminium surrounding the composite cylinder. No peak was found in the excess aluminium, even though this region was next to the graphite mould. However, in the edge region of the composite cylinder, Al,C3 peaks were weakly present, due to the reaction between Sic and Al.In order to .-30 40 50 60 28/degrees Fig. 5 Powder XRD patterns of (a) the excess aluminium and (b) the edge region of Al/SiC containing 55 vol %Sic particles 0,Sic, 0, Si; A, Al, A,Al,C, 472 J. Muter. Chem., 1996, 6(3), 469-477 investigate the silicon distribution, all Al/SiC particulate and whisker composites were fully etched using an acid solution of HC1, H,SO, and distilled water (1:1:5). The acid solution leached away and Al, but not Sic and Si. Fig. 6 shows the XRD patterns of the Al/SiC whisker composites containing 10 and 31 vol.% Sic whiskers and 55 vol.% Sic particles. The Al/SiC composite containing 10 vol. YOSic whiskers exhibited 0rr I 200 0 0 30 40 so 60 2Bldegrees Fig.6 Powder XRD patterns of various Al/SiC composites containing (a) 10 vol YOSic whiskers, (b) 31 vol.% Sic whiskers, (c) 55 vol YOSic particles, and (d) lOvol.% Sic whiskers. The composite of (d) was made at an infiltration temperature of 670°C The composites of (a), (b) and (c) were made at an infiltration temperature of 800°C The patterns were taken from the centre of the bottom face of the composite cylinder 0,SIC, 0,Si larger Si peaks relative to the Sic peaks [Fig. 6(a)] than the Al/SiC whisker composite containing 31 vol.% whiskers [Fig. 6(b)]. This indicates that more Sic whiskers were con- sumed due to the A1-SIC reaction in the Al/SiC whisker composite containing less whiskers. Fig. 7 shows the Si :Sic intensity ratio in the bottom slice of each of the Al/SiC composite cylinders containing 10 and 31 vol.% Sic whiskers and 55 vol.% particles. All three composites exhibited a trend of increasing Si: Sic intensity ratio as the distance from the edge of the composite cylinder increased.In agreement with ref. 10, this means that the silicon, an A1-Sic reaction product, diffused outward to the excess aluminium during composite fabrication for all three composites. The Si out-diffusion occurred for both whisker and particulate Al/SiC composites. In order to investigate the actual Si content in the matrix of Al/SiC composites, two kinds of Si powder were mixed with either the Sic whiskers or Sic particles at various atomic Si:SiC ratios for XRD. One kind of Si was amorphous Si powder (Sicomill UN1346), which was obtained from KemaNord Industrikemi (Ljungaverk, Sweden).Although it was rated amorphous, clear Si XRD peaks were found. The other kind of Si powder was obtained by grinding a 99.9999% pure polycrystalline Si lump, which was obtained from Fisher Scientific Co. (Pittsburgh, PA, USA). Fig. 8 shows that the mixtures involving either Sic particles or Sic whiskers separ- ately gave linear relationships between the Si : Sic X-ray inten- sity ratio and the atomic Si: Sic ratio. The two kinds of Si powder gave consistent results. For the Sic particles, the slope of the plot of the Si:SiC X-ray intensity ratio against the Si: Sic atomic ratio was 2.2795, which was obtained by the least-squares method.For the Sic whiskers, the slope of the plot of the Si: Sic X-ray intensity ratio against the Si :Sic atomic ratio was 1.0604. Table 3 shows the Si content (in mass%) in the matrix of Al/SiC (bottom slice of the composite cylinder), as obtained from XRD results and the conversion provided by Fig. 8. The Si content in the matrix of Al/SiC increased with increasing distance from the edge of the com- posite cylinder, whether Sic whiskers or particles were used. The Si content in the matrix also increased with increasing volume fraction of Sic at each corresponding location of the composite cylinder. The highest Si content of 8.60 mass% was observed in the centre (20mm from the edge) of the Al/SiC particulate composite containing 55 vol.% Sic particles.Even this highest Si content was below the Si content of the A1-Si eutectic (12 mass%). Since the Si: Sic intensity ratio varied 5 0.7 }L .2 0.6 16i2 0.5 1 .-0.4 1 0 5 10 15 20 distance from edge of compositehnm Fig.7 Si:SiC integrated intensity ratio (from XRD) as a function of the distance from the cylindrical edge of: (a) Al/SiC containing 10 vol.% Sic whiskers; (b) Al/SiC containing 31 vol.% Sic whiskers; and (c) Al/SiC containing 55 vol.% Sic particles. The results were obtained from the bottom slice of each composite. 3.5 1 0 0.2 0,4 0.6 0.8 1 1.2 1.4 1.6 Si:SiC atomic ratio Fig. 8 Linear relationship of the Si:SiC X-ray intensity ratio and the Si:SiC atomic ratio for (a) Sic whiskers (y = 1.0604x), and (b) Sic particles (y=2.279x).O/O,Si powder A; +, Si powder B. Table 3 Comparison of the Si content (in mass%) obtained by XRD and DSC in the A1 matrix of Al/SiC (bottom slice) at each correspond- ing location distance from the edge of the composite cylinder/mm Sic volume experimental fraction method 0 5 10 15 20 0.10" XRD 4.42 4.46 4.43 4.39 4.66 DSC 4.62 4.90 4.83 5.04 5.06 0.31" XRD 4.45 5.07 5.27 5.66 5.63 DSC 5.78 6.10 6.42 6.67 6.85 0.55b XRD 8.52 9.21 9.51 10.3 - DSC 6.32 6.72 7.23 8.18 8.60 a Sic whiskers. Sic particles. throughout the composite cylinder, the value at the centre of the bottom slice was used to investigate the effect of the reinforcement amount on the extent of the A1-Sic reaction (the Si content in the matrix indicates the extent of the A1-Sic reaction.The Al-Sic reaction rate is expected to decrease with increasing extent of infiltration, as the aluminium matrix contains more Si as infiltration proceeds, thereby causing further A1-SIC reaction to diminish5). The extent of the A1-SIC reaction is expected to decrease with decreasing infiltration temperature. Fig. 6(d) shows the XRD pattern of the Al/SiC composite containing 10 vol.% Sic whiskers, which was fabricated at the infiltration temperature of 670°C. A low Si peak intensity indicates that the Al-Sic reaction occurred only slightly at this infiltration temperature. However, it had been shown in a previous study'' that incomplete infiltration occurred at the infiltration temperature of 670°C for Al/SiC containing a high volume fraction of reinforcement.The required infiltration temperature and press- ure are expected to increase with increasing volume fraction of the reinforcement. Although the XRD technique can be used for the investi- gation of phase distributions, this work is focused on the use of differential scanning calorimetry (DSC) to study the Si concentration variation throughout the Al/SiC particle or whisker composites, since DSC requires no sample etching and thus can provide more accurate composition analysis. Differential scanning calorimetry. A Perkin-Elmer differential scanning calorimeter (DSC 7) was used to measure the melting J.Muter. Chem., 1996, 6(3), 469-477 473 temperatures and the associated enthalpy changes of the phases or microconstituents in the matrix of the Al/SiC composite and in the excess aluminium around the composite cylinder Fig 9(a) shows DSC thermograms of the Al/SiC composite which contained 10 vol YOSic whiskers and was fabricated at the infiltration temperature of 670 "C The thermograms were obtained on cooling from 700 to 500°C at a cooling rate of 10°C min-' for three local regions in the bottom slice (1 mm thick) of the composite cylinder The three local regions were A (the excess aluminium adjacent to the edge of the composite, within 1 mm from the edge), B (the edge of the slice, within 1mm from the cylindrical edge, such that the region was within the composite) and C (the centre of the slice, within 05mm from the centre) Region A gave an exothermic peak (the liquidus) with its onset at 655 "C and a small solidus peak at about 630°C Regions B and C exhibited exothermic peaks (liquidus) with similar onsets at about 637 "C and exothermic peaks (eutectic) with onsets at about 570°C This indicates that, even at the infiltration temperature of 670"C, the silicon content (due to the Al-Sic reaction) in the aluminium matrix of the composite is greater than the silicon solubility (16 mass%) in the aluminium Both the edge and centre regions exhibited similar temperature differences (AT) between the liquidus and the eutectic, indicating that both regions had similar silicon contents in the aluminium matrix This also indicates the absence of a non-uniform Si phase distribution in Al/SiC containing 10 vol YOwhiskers and fabricated at the infiltration temperature of 670 "C Fig 9(b) shows DSC thermograms of the Al/SiC composite which contained 23 vol YOSic whiskers and was fabricated at the infiltration temperature of 800 "C The thermograms were obtained on cooling from 700 to 500°C at a cooling rate of 1 1 ii 30 -') , , , , , , , , , I Fig. 9 (a) DSC scans showing exothermic peaks upon cooling for the Al/SiC composite containing 10 vol YOSic whiskers and made at an infiltration temperature of 670 "C A, excess aluminium adjacent to the edge of the composite cylinder, B, edge of bottom slice within composite, C, centre of bottom slice of composite cylinder, L= liquidus, E = eutectic (b) DSC scans showing exothermic peaks upon cooling for the Al/SiC composite containing 23 vol YOSic whiskers and made at an infiltration temperature of 800 "C A, excess aluminium adjacent to edge of composite cylinder, B, edge of bottom slice within composite, C, mid-radius region of bottom slice of composite cylinder, D, centre of bottom slice of composite cylinder, L=liquidus, E = eutectic 10"C min-' for four local regions in the bottom slice (1mm thick) of the composite cylinder The four local regions were A (the excess aluminium adjacent to the edge of the composite, within 1mm from the edge), B (the edge of the slice, within 1 mm from the cylindncal edge, such that the region was within the composite), C (the half radius of the slice, within 0 5 mm from the centre) and D (the centre of the slice, within 0 5 mm from the centre) Region A exhibited exothermic peaks with onsets at 570 5 and 625 8 "C, region B exhibited exother- mic peaks with onsets at 570 1 and 615 0 "C, region C exhibited exothermic peaks with onsets at 570 6 and 609 2 "C, and region D exhibited exothermic peaks with onsets at 570 1and 607 6 "C The first peak for each region z e , the one at 570-571 "C, is attributed to the A1-Si eutectic invariant reaction at 577 "C The second peak for each region is attributed to the liquidus of the Al-Si alloy matrix No other peak was observed in all regions, indicating no contamination associated with the matrix-mould reaction The temperature difference (AT)between the first DSC peak (corresponding to the sohdus or the eutectic temperature) and the second DSC peak (corresponding to the liquidus) was used to evaluate the Si content in each region of the composite The AT values were 55 3, 44 8, 38 3 and 36 4 "C for regions A, B, C and D, respectively, of the Al/SiC whisker (23 vol % whiskers) composite [Fig 9( b)] The Al/SiC composite which contained 10 vol % whiskers and was fabricated at 800°C exhibited a similar trend in that AT decreased with increasing distance from the edge to the centre of the composite As shown in Table4, AT decreased with increasing distance from the edge to the centre of the composite cylinder for all composites except for the Al/SiC whisker composite fabncated at the infiltration temperature of 670°C This means that the Si content in the aluminium matrix (obtained from AT based on the Al-Si phase diagram) increased from the edge to the centre, as shown in Table 3, which also shows good agreement between the Si content based on DSC and that based on XRD The Si amount (based on AT) at each corresponding position in the aluminium matrix increased with increasing volume fraction of the reinforcement (Fig 10) The severity of the non-uniform Si phase distribution (z e, the average slope of Si vanation along the line from the edge to the centre) also increased with increasing volume fraction of the reinforcement Based on the approach outlined in ref 4,the relative amount of Sic which was consumed by the A1-Sic reaction was obtained Table 5 shows the percentage of Sic consumed (based on DSC results) for Al/SiC particulate and whisker composites fabncated at the infiltration temperature of 800 "C (with various Sic volume fractions) at the centre region of the bottom slice of each composite The lower the volume fraction of the Sic reinforcement, the higher the fraction of Sic consumed for a given infiltration temperature The A1-Sic reaction is detrimen- tal to the tensile properties of Al/SiC because of the decrease of the strength of the Sic reinforcement due to the A1-Sic reaction and the brittle interfacial reaction products The most severe Sic consumption occurred for the Al/SiC containing 10 vol % Sic whiskers, as 55 vol YOof the Sic reacted with aluminium Upon decreasing the infiltration temperature to 670"C, the Sic consumption was reduced to 26% Scratch/shear testing.For interfacial shear strength testing, the shear/scratch test was performed with a Teledyne Taber Model 139 shear/scratch tester, which was equipped with a Taber S-20 contour tungsten carbide shear tool The applied load on the tool was 1000 g One or two scratches were made on the specimen surface for each increment of 5 mm from the cylindrical edge of the composite, the more uniform scratch (groove) was used for the determination of the scratch width with the help of a 10 x magnifier The larger the scratch width, the lower was the shear strength Table6 shows the scratch width of Al/SiC composites at 474 J Muter Chem, 1996, 6(3), 469-477 Table 4 Liquidus-solidus temperature difference (AT,from DSC) and Si content (mass%, from XRD) in the A1 matrix of Al/SiC (bottom section) fabricated with a graphite mould at the infiltration temperature of 780 "C 9' distance from the edge of the composite cylinder/mm Sic volume infiltration fraction temperaturePC 0 5 10 15 20 0.10" 670 Si (mass%) AT 2.23 66.9 -- -- -- 2.39 65.7 0.10" 800 Si (mass%) AT 4.62 49.8 4.90 47.8 4.83 48.3 5.04 46.8 5.06 46.7 0.23" 800 Si (mass%) 5.32 6.15 6.23 6.25 6.50 AT 44.8 38.9 38.3 38.2 36.4 0.3 1" 800 Si (mass%) AT 5.78 41.5 6.10 39.3 6.42 37.0 6.67 35.2 6.85 33.9 OSb 670 Si (mass%) AT 3.26 59.5 3.86 55.2 3.92 54.8 C C C C 0.55b 800 Si (mass%) AT 6.32 37.7 6.72 34.8 7.23 31.2 8.18 24.4 8.60 21.4 " Sic whiskers.Sic particles. Incomplete infiltration region. various reinforcement volume fractions. The scratch width decreased with increasing volume fraction of the reinforcement, indicating that the shear strength of Al/SiC increased due to 8.5 the addition of Sic particles or whiskers. Negligible shear strength variation was observed within each composite, in u1g8 spite of the observed variation in Si content (Table 3). This g 7.5 indicates that a non-uniform Si distribution did not cause a Y non-uniform shear strength distribution, as reported in ref. 10. .xI! 7 .r 6.5 Tensile testing.Tensile testing was performed using a c. c hydraulic Mechanical Testing System (MTS) with a loading rate of 1201b min-l (534N min-') at room temperature.8 Samples in the shape of a dogbone were machined from the 6 5.5 f b composite cylinder, with the dogbone axis perpendicular to the axis of the composite cylinder, so the mechanical properties 5 measured were those near the centre of the composite cylinder. I I I4.5 ' J Young's modulus was measured using a strain gauge at low loads. The ductility was determined by drawing two parallel 0 5 10 15 20 distance from edge of composite/mm lines marking the gauge length on the sample and measuring the distance between the lines before and after tensile testing Fig.10 Silicon content (mass%, obtained from DSC) in the aluminium using calipers. matrix of various Al/SiC composites as a function of distance from Table 7 lists the tensile properties of Al/SiC whisker com- the composite cylindrical edge. (a) Al/SiC,, 55 vol.%; (b) Al/SiC,, 31 vol.%; (c) Al/SiC,, 23 vol.%; (d) Al/SiC,, 10 vol.% p=particles; posites. The yield strength, ultimate strength and modulus w =whiskers. increased while the ductility decreased with increasing Sic whisker volume fraction at the same infiltration temperature Table5 Silicon content (mass%) in the aluminium matrix (in the bottom centre region) and the percentage of the Sic consumed due to the Al-Sic reaction Sic volume Sic mass Si content in Si content in fraction of Sic reactivity fraction fraction composite (mass%)d A1 matrix (mass%) consumed (%) indexe O.lO"pb 0.117 8.17 2.39 25.8 0.0258 0.10" 0.117 8.17 5.06 54.7 0.0547 0.23" 0.262 18.35 6.50 25.9 0.0596 0.31" 0.348 24.37 6.85 18.3 0.0567 0.55' 0.592 41.44 8.60 8.4 0.0462 " Sic whiskers.Infiltration temperature =670 "C for this composite and 800 "C for the other four composites in this table. 'Sic particles. This refers to Si in Sic. Product of the fraction Sic consumed and the original Sic volume fraction. Table 6 Scratch width (mm) of Al/SiC particulate and whisker composites (standard deviation in parentheses) Distance from the edge of the composite cylinder/mm Sic volume fraction 0 5 10 15 20 0.10" 0.99( 0.02) 1.01 0.97 0.97 0.97 0.23" 0.80 0.75 0.80 0.80 0.80 OSb 0.59( 0.02) 0.59( 0.01) 0.58(0.02) 0.56(0.04) 0.56 'Sic whiskers.Sic particles. J. Mater. Chem., 1996, 6(3), 469-477 475 Table 7 Tensile properties of AI/SiC whisker composites fabncated at an infiltration pressure of 2000 psi (standard deviations based on three samples of each type in parentheses) Sic volume fraction infiltration temperaturePC yield strength/MPa 0 10 0 10 023 031 670 800 800 800 108 7(8 6) 105 7( 3 6) 156 6(7 6) 204 4( 39 2) of 800°C A decrease in the infiltration temperature from 800 to 670°C increased the ultimate strength and ductility (as expected from the decreased fraction of Sic consumed), whilst having little effect on yield strength or modulus The composite yield strength (gcy)was calculated by using eqn (1) l5 where omyis the matrix yield strength (65 MPa), S is the whisker aspect ratio (13 3 before composite fabrication) and V, is the matrix volume fraction If the Sic consumption caused the whisker diameter to decrease without affecting the whisker length, the aspect ratio would increase from 13 3 to the modified values listed in Table 8 Furthermore, the Sic consumption caused the whisker volume fraction to decrease to the modified values listed in Table 8 The yield strength calculated using the original whisker volume fraction and the original whisker aspect ratio, and the yield strength calculated using the modified whisker volume fraction and the modified whisker aspect ratio were both quite close to the corresponding measured yield strength (Table 8) The increase of the Sic whisker aspect ratio and the decrease of the whisker volume fraction, both caused by the Al-Sic reaction, affected the yield strength in opposite directions, so that the yield strength appeared unaffected by the A1-Sic reaction As a result, eqn (1) gave good theoretical fits to the measured yield strength, whether or not the effects of the Al-Sic reaction on the whisker aspect ratio and volume fraction were considered On the other hand, the ultimate strength is expected to be more affected by the interfacial reaction than the yield strength, but theoretical models for the ultimate strength are not available for the case of discontinuous fibres or whiskers that are not oriented Discussion For the same filler (z e ,Sic whiskers) and the same infiltration temperature, the fraction of Sic consumed increases with decreasing value of the onginal Sic volume fraction in the composite, as shown in Table 5 These two fractions are in fact inversely proportional to one another, as shown in Table 5 by the constancy of the product of the two fractions This product, called the reactivity index, provides a scale that describes the A1-Sic reactivity This means that the variation of the fraction of Sic consumed with the original Sic volume fraction was governed by geometry (probably the surface area of the ultimate strengt h/M Pa modulus/GPa ductility (%) 224 l(5 4) 181 3(8 6) 282 l(5 9) 309 7( 16 3) 93 O( 102) 97 6( 19 1) 106 7( 10 5)150 4 7 8(0 5) 26(15) 24(12) 10 whiskers per unit volume of the composite, since the reaction is more severe at the onset and decreases in severity as silicon is formed and enters the aluminium matnx) A decrease of the infiltration temperature decreased the reactivity index, as shown in Table 5 for the case of composites containing 0 10 vol % of Sic whiskers The reactivity was lower for Sic particles than Sic whiskers for the same infiltration tempera- ture, in spite of the single-crystal nature of the whiskers This reactivity difference is attributed to the much larger (by a factor of 6) surface area per unit volume of the composite for the Sic whiskers compared to the Sic particles at the same volume fraction Although the Si content was highest in the bottom centre region of the composite cylinder (Tables 3 and 4), the Sic consumption was most severe at the very edge of the composite cylinder The non-uniform Si distribution was merely a consequence of the out-diffusion of the silicon in the liquid A1-Si state to the aluminium melt surrounding the preform during composite fabrication (Fig 2 and 3) The fraction of Sic consumed (Table 5) obtained by using the Si content in the bottom centre region of the composite cylinder thus reflects the true Al/SiC reactivity Even though the Sic whiskers or particles were partially coated by the binder used in preform preparation and the binder concentration was higher in the Sic whisker preforms than the Sic particle preforms, the reactivity between the Sic whiskers and the A1 matnx was higher than that between the Sic particles and the A1 matnx This means that the presence of the binder did not attenuate the reactivity much, if any This paper provides the fraction of Sic consumed in Al/SiC composites made by liquid metal infiltration, in contrast to earlier work4 on Al/SiC composites made by stir casting The non-uniform Si distribution reported here applies to com-posites made by liquid metal infiltration, but not to those made by stir casting The method used in this work for determining the fraction of Sic consumed is essentially the same as that used in ref 4 However, there are two differences One difference is that we measured the liquidus-eutectic temperature difference, AT, whereas the liquidus temperature alone was measured in ref 4 The advantage of measuring AT is that it avoids the effect of the temperature scanning rate Another difference is related to the calculation of the fraction of Sic consumed based on the silicon content in the aluminium matnx The calculation yields the fraction of Sic consumed as the expression used in ref 4 multiplied by the factor [1 -(Sic volume fraction)] In other Table 8 Comparison of the measured and calculated values of the yield strength of Al/SiC whisker composites [calculated values based on either the original Sic volume fraction together with the onginal whisker aspect ratio (13 3), or the modified Sic volume fraction together with the modified whisker aspect ratio] yield strength/MPa original Sic volume infiltration fraction of Sic modified Sic modified calculated fraction temperaturePC consumed (YO) volume fraction ratio original modified measured 0 10 0 10 023 0 31 670 25 8 0074 15 4 108 2 102 1 108 7( 8 6) 800 54 7 0045 19 8 108 2 942 105 7(3 6)800 25 9 0 17 15 4 1643 1501 156 6(7 6) 800 18 3 025 14 7 198 9 185 9 204 4( 39 2) J Mater Chem , 1996, 6(3),469-477 words, a factor is missing in ref.4, so that the values given in ref. 4 must be corrected in order to obtain the correct value. Ref. 4 reported that the fraction of Sic consumed is 37% (ie., 29% after the correction) for an Sic particle 6061 Al-matrix composite containing 20 vol.% Sic after remelting at 800°C for 1 h.In this work, the fraction of Sic consumed was 26% for a Sic whisker composite containing 23 vol.% Sic. Thus, the results of ref. 4 and this work are comparable. Conclusion The A1-SIC reaction in Al/SiC composites made by liquid metal infiltration caused the reaction product Si to have a non-uniform distribution in the resulting composite, such that the Si concentration decreased from the centre to the edge of the composite (the edge is the interface between the composite and the excess aluminium cast around the composite). The non-uniformity increased in severity as the Sic whisker volume fraction increased and as the infiltration temperature increased; it was present at a Sic volume fraction of 10% when the infiltration temperature was 8OO"C, but was absent when the infiltration temperature was 670 "C.However, the non-uniform Si distribution, if any, did not result in a non-uniform mechan- ical property distribution.The amount of reaction product Si increased with increasing Sic volume fraction, but the fraction of Sic consumed by the reaction increased with decreasing original Sic volume frac- tion, such that the two fractions were inversely proportional to one another. The product of the two fractions provides a scale (called the reactivity index) for describing the A1-SIC reactivity. The reactivity decreased with decreasing infiltration temperature, and was higher for Sic whiskers than Sic particles. This work was supported by the Advanced Research Projects Agency of the US Department of Defense and the Center for Electronic and Electro-Optic Materials of the State University of New York at Buffalo. References 1 R. Warren and C-H. Anderson, Composites, 1984, 15, 101. 2 T. A. Chernyshova and A. V. Reborv, J. Less Common Met., 1986, 117,203. 3 W. C. Moshier, J. S. Ahearn and D. C. Cooke, J. Muter. Sci., 1987, 22, 1154. 4 D. J. Lloyd and I. Jin., Met. Trans. A, 1988, 19, 3107. 5 D. J. Lloyd, H. Lagace, A. McLeod and P. L. Morris, Muter. Sci. Eng. A, 1989,107,73. 6 T. Iseki, T. Kameda and T. Maruyama, J. Mater. Sci., 1984, 19, 1692. 7 K. Kannikeswaren and R. Y. Lin, J. Met., 1987,39, 17. 8 J. C. Viala, P. Fortier and J. Bouix, J. Muter. Sci., 1990,25, 1842. 9 H. Ribes, M. Suery, G. L'Esperance and J. G.Legoux, Met. Trans. A, 1990,21,2489. 10 S-W. Lai and D. D. L. Chung, J. Muter. Sci., 1994,29,3128. 11 S-W. Lai and D. D. L. Chung, J. Muter. Sci., 1994,29,2998. 12 J-M. Chiou and D. D. L. Chung, J. Muter. Sci., 1993,28, 1435. 13 J-M. Chiou and D. D. L. Chung, J. Muter. Sci., 1993,28, 1447. 14 J-M. Chiou and D. D. L. Chung, J. Muter. Sci., 1993,28, 1471. 15 V. C.Nardone, Scriptu Metullurgicu, 1987,21, 1313. Paper 5/04195C; Received 29th June, 1995 J. Mater. Chem., 1996, 6(3), 469-477 477

ISSN:0959-9428    

DOI:10.1039/JM9960600469

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

Interfacial chemistry of adhesive joint failure: an investigation by small area XPS, imaging XPS and TOF-SIMS Stephen J. Davis and John F. Watts* Department of Materials Science and Engineering, University of Surrey, Guildford, Surrey, UK GU2 5XH The strength and mode of failure of adhesively bonded iron substrates has been investigated. Joints were assembled from polished iron and from iron substrates that had been pretreated in a novel manner by the cathodic deposition of yttrium. Joints were exposed to an aqueous environment and a reduction in joint strength with increasing exposure time was noted. Failure surfaces of joints exposed to water for 1200 and 7500 h were examined by small-area XPS, imaging XPS and TOF-SIMS. An interfacial failure was observed, and although all surfaces were rich in nitrogenous species from the curing agent, no evidence of epoxy residues on the interfacial metal failure surfaces was recorded by TOF-SIMS.The metal failure surface at extended exposure times shows that there are two delamination fronts advancing from the exposed edge of the adhesive joint: one is associated with bond cleavage and gives rise to a 'zero-volume debond' and proceeds faster than the other, which is associated with gross separation of the adhesive and substrate, oxide growth on the exposed metal adherend and mass transport within the disbondment crevice. Adhesive bonding of metal components has been practised in the aerospace industry for many years. During this time a variety of pretreatment methods have been devised that provide excellent durability for joints specified for strategic appli- cations, which invariably involve the adhesive bonding of aluminium, titanium and their alloys.Recent years have, however, seen a widespread increase in the use of adhesive bonding for automotive and other high-volume applications which also require very good durability characteristics. Consequently, recent research has concentrated, not on pro- ducing an exceptionally strong bond, but on understanding the manner in which joint strength is reduced on exposure to aggressive environments, and developing pretreatments for steel substrates that will afford improved durability. A key step in understanding the mechanism of adhesive bond failure is the definition of the exact locus of failure.Surface analysis techniques such as X-ray photoelectron spec- troscopy (XPS) have been used for a number of years by several authors to study the exact locus of failure of adhesive bonds and organic coatings,',2 more recently, imaging spectro- scopies have been utilised to investigate the spatial distribution of failure sites.3 The ability to detect very low levels of adhesive, associated with a thin overlayer, has enabled very precise failure mechanisms to be suggested. It has been shown by several authors, for example ref. 4, that the composition of such overlayers, possibly affected by the segregation of minor components, can greatly affect the durability of adhesive bonds. Water ingress in adhesive bonds is known to greatly reduce their performance and has been highlighted as playing the major role in failure of environmentally exposed adhesive as surface energy considerations lead to a preference for a metal-water interface over a metal-polymer interface.* The nature of the weakening effect is, however, unclear.Cathodic disbondment has been identified as a route to failure in the case of steel substrates since 1929,9 however, there is still much debate as to the exact mechanism. Several workers have reported the major methods of failure due to cathodic delamination of organic Surface pre-treatments also play an important role in the performance of adhesive bonds; in the study described in this paper a rare-earth-metal pre-treatment described pre-vi~usly,~~-'~which involves forming a thin layer of yttrium hydroxide on a mild steel or iron adherend prior to bonding, has been considered. For this approach to work successfully it is necessary to form a tenacious coating of cationic yttrium which is not disrupted by environmental attack.Analysis of failed lap shear joints has enabled the exact locus of failure to be determined when such a pre-treatment is employed. In this paper we report the use of XPS (in small area and imaging modes) and time of flight secondary ion mass spec- trometry (TOF-SIMS) to study the failure surfaces of lap shear joints exposed to an aggressive environment (water at 30°C). The aim of the work is to use these surface analysis methods to provide analytical data, from which it is possible to deduce the interfacial chemistry of failure and provide a comprehensive model of the chemistry and electrochemistry that contributes to the observed reduction in joint strength.Methods and Materials Adhesive bonds In this study, single lap shear joints have been used as a method of comparing adhesive joint durability for joints with and without an inorganic adhesion promoter. Although not an ideal geometry offering fully quantifiable results in terms of fracture mechanics, it is suitable, as the failure load can be used in a relative manner to compare sets of samples. The single lap joint has several advantages over more complex joint geometries. To enable good surface analysis of the failed joint surface to be undertaken, it is necessary that the specimen is analysed in the 'as-failed' state.This limits the use of systems such as the double cantilever beam or Boeing wedge tests, as the failed surface is exposed to the testing environment; this results in post-failure attack which will dominate the results and possibly lead to erroneous conclusions being drawn about the actual failure mode. Adherends were made from a pure iron (Goodfellows, 99.5%) 1.02 mm thick sheet which was cut to size using a laboratory guillotine. A schematic of the lap shear joint used is given in Fig. 1. XPS of the as-received iron sheet showed the presence 1.02 mm IRON ADHEREND Fig. 1 Schematic of a lap shear joint J. Muter.Chem., 1996, 6(3), 479-493 479 of manganese at the surface. This is likely to be a result of segregation during the rolling or subsequent closed-coil annealing processes used to produce the thin sheet. The iron adherends were abraded with fine alumina powder prior to bonding to remove the surface-enriched manganese (confirmed using XPS). After abrasion, the iron was ultrasonically cleaned using acetone. A fully formulated two-part amine-cured epoxy adhesive (Ciba Geigy Araldite 2013) was used in this work. This was cured at 40°C for 16 h to ensure a high crosslink density. The glue line thickness was set at 150 pm; this was controlled by using two nylon wires as spacers in the adhesive joint. Nylon was selected as opposed to the more commonly used copper to minimise galvanic corrosion. Two series of lap shear joints were produced, one with the yttrium pretreatment and one control.A set of eight (four of each) were pulled to failure after cure to provide a control. Mechanical testing was performed on a J. J. Lloyd Tensile Tester, operated at 0.5mm min-' cross-head speed at room temperature. The remaining joints were immersed in milli-Q water (resistivity= lo1*Cl cm-') in sealed glass jars at 30 "C. Joints were removed periodically from the test solution, dabbed dry using a tissue and pulled to failure within as brief a time as possible (1-2 min) after being removed from the solution. The load to failure was recorded for each failed joint. Surface Analysis XPS Small-area and imaging XPS were used to study the failed lap shear joints.Standard XPS is an area integrating technique and has a relatively large analysis area (10 mm2). In this work it was essential to be able to analyse different areas within the failure region, thus a higher spatial resolution was necessary. Analysis of the failure regions was carried out using a VG Scientific ESCALAB 220i (BP Research, Sunbury) and a Scienta ESCA 300 spectrometer located at the RUST1 facility at Daresbury Laboratory. Both instruments offer high spatial resolution; however, the spectral resolution of the Scienta instrument was greater than that of the VG Scientific ESCALAB 220i (housed at BP Research, Sunbury). The XPS acquisition parameters used are given in Table 1.The Scienta ESCA 300 instrument was used to produce both point analyses and also energy-distance (E-X) plots. E-X plots enable both chemical and spatial information to be gained from one analysis. They are produced by plotting binding energy us. position, with the spectral intensity being represented as a heat scale on the z-axis (normal to the page). These plots are described in the literat~re'~.'~ and Plate 1 indicates how they are related to a complete spectrum and position on the specimen; the single spectrum of Plate 1 is effectively a hori- zontal line scan across the plane of the detector, the heat scale of the image now being transformed into the intensity scale of the spectrum. A montage of spectra can also be re-created from an E-X plot, as shown in Fig.2. Such analyses were carried out on different regions within the failure surface of 41 Intensity (Slit length/magnification) +aberrations ie 35/10 mm +50pm =3.55 mm 4 ( * ) !(Slit widWmagnification) +aberrationsSCAN AREA ie 80pm +50pm =130pm Plate 1 Creation of an E-X plot Distance 12 3 4 5 6 7 8 9 1C Split the E-X scan up mto a series of spectra Montage of spectra Fig. 2 Creation of a montage from an E-X plot the adhesive joint, thus enabling chemical state changes to be detected as a function of position. The analysis area for the Scienta ESCA 300 instrument is dependent on the mode of operation. For high-transmission analyses, the analysis area is defined by the X-ray footprint (6.0 x0.5 mm).However, for E-X scans an additional lens is added within the electron optics of the analyser chamber which has the effect of increasing the magnification of the transfer lens by a factor of ten. In this high spatial resolution mode the analysis area perpendicular to the hemispherical analyser acceptance aperture slit is given as (slit width/ magnification +spatial resolution), and parallel to the slit as (slit length/magnification +spatial resolution). It has been shownI7 that spherical abberations in the lens are small, and spatial resolution is typically 25 pm. Thus, for an E-X image with a 0.8 mm slit width, the analysis area displayed in the image produced is [3.5 mm +(2 x25 pm)] x [80 pm + (2 x25 pm)], i.e. 3.55 mm x 130 pm. In this work the VG Scientific ESCALAB 220i instrument acquisition parameter X-ray source X-ray power monochromated pass energy/eV survey high resolution take-off angle (to the sample surface)/degrees analysis area 480 J.Muter. Chem., 1996, 6(3),479-493 Table 1 XPS analysis conditions VG ESCLAB 2201 Al-Kr 15 kV, 20 mA, 300 W no 100 40 90 4x lo5 pm2 Scienta ESCA 300 Al-Ka 14 kV, 200 mA, 2.8 kW Yes 300 150 90 main lens =6.0 x0.5 mm E-X scan= 3.55 mm x 130 pm Fig. 3 Analysis regions across the failed lap shear joint after 7500 h in water at 30°C (a) XPS, (b)TOF-SIMS was used to study the failure surfaces ofjoints after environmen- tal exposure for 1200 h. After this time, two different failure regions could be visually identified.Initial results from these failure surfaces suggested that failure was truly interfacial between the adhesive and the adherend. This led to the Scienta ESCA 300 being used in future analyses of specimens exposed for 7500 h in water, as this spectrometer has better spectral resolution and has been used previously to precisely pinpoint the locus of failure in a complex system.'* Using the ESCA 300, XP survey spectra were recorded along a line from the specimen edge towards the central cohesively failed region. Starting from the overlap edge, spectra were recorded every 1 mm until the thick cohesively failed region was reached, this is shown schematically in Fig. 3(u) for the failure surface of a joint exposed to water for 7500 h.These survey spectra were then quantified using the appropriate sensitivity factors. The atomic percentage of each element present was then plotted against position to give elemental compositions across the surface. When displaying these data, the results were duplicated (assuming symmetry) to represent the entire joint overlap. This was undertaken to enable the results to be compared with those from TOF-SIMS in which analyses were acquired across the entire width of the overlap. These montages allow differences in spatial distribution of elements to be observed readily. Results from the VG Scientific ESCALAB 220i instrument are presented with the binding energy increasing from left to right; however, results from the Scienta ESCA 300 have the binding energy decreasing from left to right (increasing kinetic energy).This form of discrepancy arises as there is no univer- sally accepted format for plotting ionisation potentials and different manufacturers have adopted, for essentially historic reasons, either right-handed or left-handed formats. TOF-SIMS TOF-SIMS was utilised to provide further details regarding the organic material remaining on the metal side of the failed Table 2 Experimental conditions for TOF-SIMS analysis primary beam 26 kV Ga69+ ions specimen current/nA 2 pulse width/ns 20 pulse per pixel 32767 number of frames 50 magnification x 500 electron-flood gun/mA 0.1 3.0total ion dose (analysis)/lO" ions cm-2 joint.The high sensitivity combined with the ability to detect fragments of the adhesive system enabled SIMS to be used to fingerprint the adhesive. Analyses were made using a VG Ionex Type 23 mass spectrometer fitted with a Poschenreider analyser and a 30 kV gallium liquid-metal gun. Data were acquired using VG Ionex VGS 5100 software operating under RTll on a PDPll computer. This was then transferred to a PC via a conversion package to enable further processing using spreadsheet software (Borland Quattro Pro 4). Typical analysis conditions are given in Table 2. These parameters result in a static SIMS analysis, i.e. the total ion dose per analysis is well below the suggested threshold value for damage, of lOI3 ions cmP2.l9 All the SIMS data were normalised by dividing the intensity of the mass peak of interest by that of the total counts in the m/z 0-200 region minus gallium counts, i.e.[counts in peak]/ {[total counts m/z 0-2001 -[counts of (Ga6'+ +Ga7'+)]}. This corrects for the reduced total intensity which is observed during analysis and also for the increased signal associated with implanted gallium. Normalising also reduces any effect due to a variation in the specimen current which may occur between different analyses. To perform such normalisation it is necessary to process the raw SIMS data; this is achieved by transferring the intensities of each mass peak to a spreadsheet to facilitate inspection and subsequent normalisation. TOF-SIMS enables any fragments associated with the epoxy which may be present on the apparently metal failure surfaces to be detected.TOF-SIMS point spectra were recorded across both regions within the failure surface (i.e. thick cohesively failed epoxy and apparent metal surfaces) as shown sche- matically in Fig. 3(b).The analysis area for each spectrum was a 400 pm square. This enables plots of peak intensity us. position to be produced, thus allowing spatial differences to be detected easily. Results and Discussion Mechanical testing and observations on failure surfaces Results from the durability investigation are shown in Fig. 4, which shows that the yttrium treatment alone has had little affect on the failure load for these test conditions. The failure Fig.4 Durability results for lap shear joints (2013 adhesive) immersed in water at 30 "C for 1200 and 7500 h: yttrium treated sample us. control J. Muter. Chem., 1996, 6(3), 479-493 481 surfaces of the as-cured joints showed cohesive failure within the epoxy. This was evident to the naked eye as the thick black epoxy could clearly be seen on both surfaces. After 1200 h in water at 30°C the failure surface has two clearly defined regions. There is a thick cohesively failed region in the centre of the overlap surrounded by a shiny metal/smooth epoxy region. This outer region appears to be a result of interfacial failure. After 7500 h in water there are still two main regions of failure; however, as expected, the centre cohesively failed region is much smaller owing to prolonged exposure.Within the outer metal region there appear to be two different areas present as a halo around the centre region. The outer ring (zone 2) is slightly duller than the inner (zone 1). This is likely to be caused by oxidation of the metal surface on exposure to water. The failure surface can be defined by two failure fronts; front 1 is the fastest at 18.5 pm per day while front 2 is slower at 10.2 pm per day. These failure fronts are shown schematically in Fig. 5(u) and (b). To understand the mechanisms involved in the failure process it is necessary to consider the electrochemical activity present within the joint. The outer edges consist of exposed iron and will be anodic, whereas the inner regions (covered with the epoxy) will be relatively cathodic. As the epoxy is displaced (allowing access to the metal surface), this newly exposed region will become anodic and hence the corrosion path will move towards the joint centre.The following reactions will occur at the anode and cathode, respectively. anodic reaction: FetFe’’ +2e- (oxidation) cathodic reaction: 0,+2H,O +4e-+40H-(reduction) There are a number of possible cathodic reactions; however, the above equation is the most likely one in a non-acidic media and has been suggested as the most likely reaction to occur under an organic film permeable to oxygen.” Such reactions will result in environs of the crack tip becoming cathodic and the build up of OH-ions will result in a high pH.If an applied potential was present, this high pH could result in oxide reduction; however, at the free corrosion potential for iron, oxide reduction cannot occur.” This means that the inner shiny region is not cathodically reduced to give a thinner oxide, but rather that the outer region has experienced oxide growth during immersion in water. The extent of delamination (distance from the metal edge to the cohesively failed region) prior to catastrophic failure brought about by mechanical test has been investigated using Fig. 5 Schematics of (a) failed lap shear joint surfaces after 1200 and 7500 h in water at 30 “C;and (b)the disbondment fronts and crack tip 482 J. Muter. Chem., 1996, 6(3), 479-493 Fickian diffusion theory. The diffusion coefficient was deter- mined for the 2013 epoxy in water at 30°C by performing mass-gain experiments.Results indicated that if a critical water concentration is necessary for delaminatioq2, then failure is not controlled by water diffusing through the bulk of the adhesive. This was concluded as the water concentrations at the failure distances were not similar for the two exposure times when determined from modelled water penetration assuming Fickian diff~sion.’~It is likely that failure has occurred owing to enhanced interfacial diffusion of reactive species. Thus, the visual observations indicate that exposure to water brings about apparent interfacial failure between adhesive and substrate; the load-bearing ability of the joint following such exposure is a function of the area of the adhesive-substrate interface that has not been displaced by such electrochemical activity. This IS in agreement with the work of Gledhill and Kinl~ch,,~although they reported extensive corrosion of the steel substrate at the crevice mouth. This is presumably related to the test time and the ease with which the crevice mouth can open.In the present work a mere thickening of the oxide is observed rather than gross material degradation. A Cambridge Instruments Stereoscan 250 scanning electron microscope (SEM) was used to study the failure surface in more detail. Prior to analysis, the failure surfaces were first coated with a thin gold overlayer to prevent excessive sample charging. A 5 kV electron beam was also used during this investigation to minimise charging effects.During analysis, energy dispersive X-ray analysis (EDX) was performed in selected areas to determine the chemical composition of certain features. Despite the gold overlayer, meaningful results could be obtained using a windowed LINK EDX detector. As SEM is a bulk technique compared to more surface-specific analyses such as XPS and TOF-SIMS, care must be taken when assessing SEM micrographs. Results from SEM revealed the presence of many plate-like structures in the thick cohesively failed region; these are shown in Fig. 6(a). EDX was performed to determine their chemical composition. Typical plates have a maximum internal diameter (feret) of about 20 pm and are approximately circular in shape (they are not elliptical).EDX results indicate that the plates are iron- and potassium-rich compared to both the rough cohesively failed epoxy and the smooth epoxy on the epoxy side of the interfacial failed outer region [Fig. 6(b),(c) and (d)]. The smooth nature of these platelets in addition to their high potassium and iron content indicates that they come from a mica filler which has sheared along its potassium-containing cleavage planes, which are only weakly bonded by van der Waals’ forces. This cleavage process accounts for the smooth surface morphology detected by SEM. Mica is added to the adhesive as a filler and is therefore not unexpected at the fracture surface.Analysis with surface specific techniques (XPS) does not indicate the presence of these platelets in the bulk adhesive. This is because they are normally wetted by epoxy adhesive to a thickness greater than the analysis depth for techniques such as XPS (about 5 nm). Small-area XPS of the failed lap shear surfaces Small-area XPS (700 pm) was undertaken (VG Scientific ESCALAB 220i) on the failure surfaces of the 1200 h immersed sample in the outer failed region as well as the central cohesively failed region. The survey spectrum from the central region is presented in Fig. 7 and shows the expected peaks associated with carbon, oxygen and nitrogen (from the amine curing agent). There are also peaks associated with aluminium and silicon; these are likely to arise from additives used in the adhesive, e.g.the filler. Results from the outer region indicate that it has failed interfacially, and this is reinforced by studying the survey spectra and high resolution spectra for the carbon 1s peak. The metal side of the failed samples shows very low tion at 30°C27rather than immersed in water as in the earlier work, it is therefore unlikely that nitrogen was introduced from the test environment. It is probable that nitrogen present on the failure surfaces for the lap shear joints is either from a nitrogen-rich layer within the epoxy adhesive, or from re-adsorbed nitrogen from the adhesive or from nitrogen from the atmosphere which has adsorbed on the high-energy metal surfaces during failure.The nitrogen on the metal failure surface must be present as a discontinuous layer, because if it was present as a uniform overlayer, as an amine or similar adduct, the associated carbon signal detected on the metal failure surfaces would be much higher. The low iron signal on the epoxy side is likely to result from either the transfer of a small amount of iron from the metal substrate on failure or from the mica platelets in the epoxy. For the latter case to be true, the platelets must be at the surface and not covered with too thick an epoxy overlayer. If iron on the failed epoxy surface originates from the adherend oxide, then for the yttrium-treated sample such iron must result from the regions of back-deposited iron formed over the yttrium layer.This back deposition has been reported pre-vi~usly.~’The lack of any yttrium signal on the epoxy side indicated that failure has not occurred within the iron oxide beneath the yttrium layer. In view of the results associated with yttrium it is likely that the iron originates from the mica in the adhesive as the back-deposited regions only occupy a small fraction of the surface. XPS was also carried out on the failure surfaces of bonds after 7500 h exposure. These analyses were performed on the Scienta ESCA 300 spectrometer. XP survey spectra were acquired using the high-transmission mode (6 mm x 0.5 mm) from both the inner (shiny) region on the metal surface and also the outer (dull) region (Fig. 8). The spectrum from the inner region is comparable to that from a clean iron surface with its thin native oxide film.However, this inner region has a much higher chlorine signal than the outer region. The outer region has a reduced iron signal and higher oxygen, carbon and silicon signals. This can be seen in more detail by analysing the line scans presented in the next section. High-resolution core line spectra were also recorded for certain elements to enable chemical state information to be determined. The carbon 1s line acquired on a spectrometer with such high spectral resolution can often be used to determine the exact locus of failure, i.e. determine the presence of any organic overlayer by fitting the C 1s envelope. Such a spectrum is shown in Fig.9 for the ‘metal side’ of a failed joint. Although it is possible to produce an acceptable peak fit, as shown in Fig. 9, the result is still somewhat ambiguous as binding energies of individual components do not match well with those of adhesive and adventitious contamination expected on high energy surfaces. To resolve this uncertainty Plate 1 Ti Rough Cohesively Failed Epoxy Smooth Interfacially Failed Epoxy Si /Ads T’ I (4 0 2 4 6 8 10 12 14 16 18 20 Fig. 6 (a) SEM image of plate structures within the central cohesively failed epoxy region of a joint failed after 7500 h in water at 30°C. EDX results from: (b) plate structure in central cohesively failed region, (c) cohesively failed epoxy, and (d) smooth epoxy in the outer interfacial region.levels of carbon and a very clear iron signal, thus supporting the concept of interfacial failure. For the yttrium treated surface, there is a high yttrium signal and associated attenu-ation of the iron signal by the yttrium layer. The epoxy side, however, shows no yttrium and this confirms that a thin tenacious yttrium film has been deposited as failure has not occurred within the yttrium layer. The background slope following a peak can also offer much information about the hierarchy of species present on a specimen.25 Following the yttrium peaks at 301 and 313eV (Y 3p3I2 and Y 3~”~, respectively) the background has a negative post-peak slope (PPS), and this is indicative of a surface phase.25The nitrogen which is present on both metal surfaces could have originated from a number of sources.It may have arisen from water during the durability test” (no attempt was made to de-aerate the water), or from remnants of the amine curing agent which have segregated to the surface, from leaching of nitrogen-containing species from the adhesive which have re-adsorbed onto the surface26 or from the subsequent exposure of the failed surfaces to air on failure. Nitrogen was not present prior to bonding and this is confirmed by XPS analysis of an alumina-polished iron adherend. However, analysis of failed Boeing wedge test samples using this adhesive system show nitrogen to be present on the ‘metal surface’ even when the samples have been suspended in a controlled environment (75% relative humidity) created using a saturated NaCl solu-TOF-SIMS was used to fingerprint the epoxy adhesive and hence determine the presence of any residue on the ‘metal surface’.XPS line scans from the failed lap shear surfaces The results from quantified small area XPS analyses have been plotted us. position to give a representation of the sample surface across the entire joint overlap, i.e. an XPS line scan. These plots are given in Fig. 10 and 11 for the metal side and the epoxy side of the failed joint (7500 h in water), respectively. The centre of each plot corresponds to the thick cohesively failed epoxy, the data points either side of this central region correspond to the crack tip with the outer regions being the disbonded zone.Results for the carbon and oxygen signals indicate that the thick cohesively failed epoxy is relatively high in carbon and low in oxygen compared to the metal surface. This is to be expected as the clean iron oxide is high in oxygen and relatively free from carbon contamination. The epoxy side J. Muter. Chem., 1996, 6(3), 479-493 483 1 I I 1 1 I 1 1METAL: COWROL EPOXY :CONTROL40 1W-1 1 1 I 0-10-EPOXY :YTTRIUM --s-c 1s I binding energy/eV Fig. 7 Small area XP survey spectra from both the metal and epoxy side of a joint failed after 1200 h in water at 30 "C: yttrium treated sample us. control (VG ESCALAB 220i) shows the inverse of the metal side with a relatively lower carbon signal and higher oxygen signal compared to the smooth failed adhesive, although the differences are much less pronounced.The carbon signal is lower from the central region owing to higher signals from both oxygen and silicon; this indicates that silicon and oxygen are present in the cohesively failed central region. For the metal side, the iron signal is much higher at the crack tip; this is in keeping with the earlier result of Fig. 8 where survey spectra are compared from the inner and outer regions for the sample exposed to water for 7500 h. This indicates that the iron signal in the outer region is somewhat attenuated, and this is probably caused by a thicker oxide as well as other deposits which have formed during exposure to water over a long time.The iron signal on the epoxy side, although overall of much lower intensity, indicates that there is a reduced intensity for the cohesively failed region. The aluminium signal (for the metal side) is highest at the crack tip, with its intensity reducing towards the edges of the overlap. This is expected as the aluminium cations (from the adhesive) have segregated towards the cathode. The chlorine signal is highest at a point slightly away from the cathode. To understand the failure mechanism in more detail it is essential to determine the chemical nature of the elements present on the failure surfaces. To this end, the exact binding energies of both the chlorine and the silicon have been determined. High resolution spectra were recorded for both the C 1s and the C1 2p photoelectron lines.This enabled the 484 J. Mater. Chem., 1996, 6(3), 479-493 exact binding energy of the chlorine species to be determined. Spectra were recorded from both the metal and the epoxy side of the failed joint. The C 1s line was then fitted to determine the position of the C-C/C-H component. This was then given the value of 285.0 eV as a method of charge referencing. The C1 2p doublet was also fitted to determine the position of the C1 2p3/2 component. Its binding energy was then corrected using the previously determined level of charging. Results from analyses of the metal and epoxy side give a C1 2p3,, binding energy of 198.2 eV. This is in keeping with an alkali-metal chloride between an electropositive cation and an electronega- tive anion, i.e.Cl- from KC1 or NaCl which has predominately ionic bonding.28 A similar approach was adopted for silicon found in the outer region on the metal side and the binding energy of the Si 2p peak was determined as 102.2 eV. This is in keeping with a silicone or a silicate and suggests that the silicon in the outer region may have arisen from siloxanes present in the fully formulated adhesive, from contamination due to silanised glassware used during the durability testing or from the silicon (as a silicate) in the laboratory glassware itself. For the chloride ions, segregation away from the cathode is expected as anions present (i.e. chloride ions) are likely to be attracted to the anode (outer surface).This suggests that chloride ions from the adhesive segregate to the interface where they move towards the anode. Their inability to reach the anode is due to the very low volume debond (perhaps a 'zero volume debond') which occurs in the inner region (zone 1). 40' 3!j20. 0 I50i ::f 20 moo aoo 600 400 0 binding energy/eV Fig.8 Small area survey spectra from the inner (a) and outer (b) regions of a failed joint after 7500 h in water at 30 "C: control (Scienta ESCA300) 14t r 2 t 200 204 202 280 270 276 binding energy/eV Fig.9 XP high resolution C 1s spectrum from the metal side of a failed lap shear joint after 7500 h in water at 30 "C This makes movement of ions relatively slow.Their absence in the outer region (zone 2) suggests that transport is the controlling factor and that chloride ions in the outer regions are more mobile than those in the central (shiny) region. Increased mobility is caused by the bond in the outer region 'opening up'; this results in the chloride ions at the edge being displaced into the bulk solution by the movement of water within this more open region. The silicon signal for the metal side shows a slight increase in the central region but a large increase at the joint edge. The central signal is associated with the fillers, etc. in the adhesive; however, the very high signal at the edges is more likely to be due to post-failure contami- nation from the glassware used during the durability test.The far higher level of this contamination on the metal surface is expected as this surface will have a higher surface energy than the epoxy surface. The nitrogen signal from both the metal and the epoxy sides proves to be very interesting. Nitrogen is expected in the adhesive from the amine curing agent (aliphatic polyamine adduct hardener); however, the nitrogen signal is lowest for the cohesively failed epoxy. Both the metal surface and the smooth failed epoxy have a much higher nitrogen signal at the crack tip with the signal dropping (but still higher than for the cohesively failed epoxy) towards the edge of the joint. This suggests that either failure has occurred within a nitrogen-rich layer which has segregated towards the surface and has resulted in a discontinuous overlayer on the iron surface, or that nitrogen has been adsorbed onto the high-energy metal surface on failure.However, with the evidence available here it is not possible to determine the singular source of nitrogen on the metal failure surface. E-X images of the failed lap shear joint surfaces The images suffer from slight abberations across their width resulting from a reduction in counting efficiency in the centre of the channel plate detector. This is demonstrated in Plate 2(a), where a uniform gold band has been analysed.,' If the detector were perfect, the two spectral bands (from the 4f,,, and the 4f,,, photo-peaks) would have uniform intensity across the whole area of the detector. However, despite this slight inhomo- geneity, the results obtained can be used to study the transition between the two regions.E-X plots for C Is, 0 1s and Fe 2p are given in Plate 2 (b)-(d) for a region crossing between zone 1 (shiny) and zone 2 (dull) of a control sample. The C 1s and 0 Is images are not very clear and they remain fairly constant across the scan. The Fe 2p image of Plate2(d) is, however, far more informative. Both the oxide and metal components can be distinguished with a peak separation of about 5 eV. The metallic component reduces in intensity away from the crack tip. The fact that the metallic component does not show on the right of the E-X plot (away from the crack tip) indicates that the reduced intensity is due to oxide growth and is not merely a function of the analyser inhomogeneity.If this were the case, the metallic component would increase on the right-hand side also. To enhance this data, a montage of spectra has been recreated from the E-X plot (Fig. 12). Although quite noisy as a result of dissecting an image, this clearly shows the reduction of the metallic component across the sample surface. This indicates that the iron has a thinner oxide at the crack tip compared to that in the outer region. This result corroborates the visual inspection which clearly distinguishes the two regions as shiny and dull. The outer region (zone 2) is likely to experience oxide growth as it is relatively anodic compared with conditions at the crack tip, and is exposed (by the displacement of the epoxy) to a hostile environment.The as-received iron adherends will have an air- formed oxide typically 2 nm thick. TOF-SIMS spectra from 2013 epoxy adhesive Positive and negative SIMS of a failed polymer region from an adhesive bond was performed to determine the fragmen- tation pattern for the epoxy used in this work. Results from such analyses are given for cohesively failed epoxy [Fig. 13(a)]. Results indicate the presence of many silicon-containing frag- ments (m/z 28+, 43+, 73'); these are associated with siloxane contamination from the adhesive. The high m/z 39+ signal confirms the presence of potassium on the failure surface which has originated from fracture of the mica filler (SEM-EDX confirms the presence of platelet mica structures).The peak at m/z27' may be caused by the presence of aluminium at the cohesive fracture surface of the adhesive; however, this is unlikely as XPS results show a marked reduction of aluminium in the central epoxy region (Fig. 10). The m/z 27+ component is more likely to be caused by C,H,+ from the adhesive. Negative SIMS from the cohesively failed epoxy [Fig. 13( b)] reveals the presence of m/z 16- from oxygen, m/z 17- (hydrox- ide), m/z 12- (carbon), m/z 13- (CH-). There is also a small component due to nitrogen (m/z 14-) and CN- (m/z 26-). Such peaks are all associated with the adhesive system. The J. Mater. Chem., 1996, 6(3), 479-493 485 25.00 0 2 4 6 8 10 12 1 28.004 0 2 4 6 8 10 12 I 14 0.w7 7.00.6 00- 5.00- 4.00- s g 3.00-* 2! L O 8 2.007 2 4 6 8 10 12 ' 3.00 4 0 2 4 6 8 10 12 . 4.OO 0.004 0 2 4 6 8 10 12 2.004 0 2 4 6 0 10 12 ' 4 distance from joint edge/mm Fig. 10 XPS line scans (metal side) of a failed lap shear joint after 7500 h in water at 30 "C x-axis=distance from joint edge (in mm), y-axis = range of intensities for species present (concentration in atom%) (a) Carbon (Is), (b) oxygen (Is), (c) iron (2p),(d) nitrogen (Is), (e) silicon (2p), (f ) chlorine (2p), (g) aluminium (2p) 486 J. Mater. Chem., 1996, 6(3), 479-493 71.001 0 2 4 6 8 10 12 14 6.50 6.oO 5s 6.oo 4.50 1 I0.254 . ib 12 14 4.oo 0 2 4 6 6 0 2 4 6 10 12 ' 1.OO.c 0 2 4 6 8 I0 12 14 distance from joint edge/rnm Fig.11 XPS line scans (epoxy side) of a failed lap shear joint after 7500 h in water at 30 "C: x-axis=distance from joint edge (in mm); y-axis= range of intensities for species present (concentration in atom%). (a) Carbon (Is), (b) oxygen (Is), (c) iron (2p), (d) nitrogen (Is), (e) silicon (2p), (f) chlorine (2p). low intensity of nitrogen-containing species detected by SIMS precludes the nature of the surface nitrogen phase being determined. Spectra recorded for the 2013 adhesive are different to those from bisphenol A (which is the known base epoxy compound) and typical epoxy-related groups (m/z 135+, 191+, 252+ and 269+; 133-, 211- and 283-) are absent from the spectra recorded from the 2013 adhesive.Peaks in this fully formulated adhesive are associated with additives and indicate that these phases are dominant at the surfaces examined and are perhaps associated with a weakened zone within the adhesive which has failed preferentially. SIMS was also performed on the apparently metallic part of the failure region 2mm from the joint edge in zone 2 (see Fig. 3). These spectra are given for positive and negative ions in Fig. 14(u) and (b),respectively. The positive ion spectrum has dominant peaks at m/z 28' and 56+ from silicon and iron, respectively. However, the absence of peaks at m/z 43 and+ 73' shows that silicon is not present as a silicone or a siloxane, as peaks from Si(CH3) and Si(CH,), would be detected.XPS results confirm the presence of silicon at the joint edge and exact binding-energy measurements (Si 2p =102.2 eV) indicate that it is present either as a silicone or a silicate. The certainty of the XPS results (silicon is clearly present as both the Si 2p and Si 2s signals are very clear) confirm the presence of silicon in this region, therefore SIMS peak overlaps or doubly charged iron (mass 56/2) cannot explain the high m/z 28' component. From both the XPS and the SIMS results it is therefore clear that silicon in this outer region is present as an inorganic silicate rather than an organic silicon compound and that it has originated from the glassware used for durability testing. The negative ion spectrum is similar to that from the bulk adhesive; however, for the metal surface the m/z 12- (C-) and m/z 13- (CH-) peaks are smaller indicating a clean surface. The m/z 35- and m/z 37-(35Cl-and 37Cl-) peaks are com- paratively large for the metal region, this is an observation that is considered in more detail below.TOF-SIMSline scans of the failed lap shear surfaces TOF-SIMS was used to determine the exact nature of adhesive failure as the high sensitivity of SIMS allows very low contami- J. Mater. Chem., 1996, 6(3), 479-493 487 Plate 2 XPS E-X plots from the transition between the inner (shiny) and outer (dull) regions in a joint failed after immersion in water at 30°C for 7500 h. (a) Gold test band on silicon wafer, (b) C Is, (c) 0 Is, (d) Fe 2p. nant levels of species to be determined.Results from a series of point analyses across the joint overlap are displayed for mass fragments given in Table 3 as line scans (Fig. 15 and 16) in the same way as the XPS results. These plots allow the surface chemistry across the entire failure region to be examined. The m/z 56+ (56Fe) plot does not correlate with the m/z 28' (28Si) plot. This confirms that the m/z 56' peak is associated with iron and that it is not a double m/z28' (28Si) cluster peak. The iron signal varies across the failed region in a similar manner to the earlier XPS results, and this indicates that both techniques are suitable for analysing failure surfaces. The m/z 28' peak also shows a similar trend to the XPS results; however, the relative intensities are not the same.This is expected as the XPS quantification includes all silicon present (in different chemical states), whereas the SIMS data refers only to the m/z28+ fragment which is either present or is formed from fragmentation of larger fragments. The m/z 23' (Na+) component shows a clear trend with sodium present at the crack tip only. The level of sodium is very low, this is clear as it is not detected with XPS and the SIMS signal is relatively low considering the very high probability of sodium for ionisation. Sodium has segregated to the cathodic crack tip under the influence of the corrosion potential present in the system. The speed of ionic migration has been shown to depend upon the size of the solvation sheath which surrounds the ion in solution.The size of the solvation sheath is controlled by the charge/ionic radius.30 The detection of sodium at the crack 488 J. Muter. Chem., 1996, 6(3), 479-493 tip is expected as it is a small cation (charge/ionic radius= 9.80)31 which can easily migrate to the tip of the cathodic crevice. The m/z 40' fragment (calcium cation) is also intense around the cathodic crack tip. The speed of ion migration is about the same for calcium as for sodium as the charge/ionic radius for calcium ions is 10.15. This demonstrates that the movement of ions within the failure region is dictated by electrochemical considerations. The m/z 17- peak (OH-) confirms the high concentration of hydroxy ions at the crack tip.This is expected as they will be formed by the cathodic reduction of water at the cathode. The m/z35- and 37- data (35Cl- and 37Cl-) both show identical trends. The plots confirm that chlorine is present in the inner region near the crack tip and that it is absent from the outer surface of the joint overlap. This confirms the XPS result and once again raises the question of chloride ion mobility within the failed region. The chlorine profiles and the sodium profiles are not the same. It is likely that chlorine has reached the bond line (from the adhesive or water) and that it then starts to segregate towards the anode (outer region). This explains the displacement (compared to sodium) shown in the position of the chlorine-rich region towards the outer anodic region.The localised chemistry and migration of species within the adhesive bond are shown schematically in Fig. 17. The main aim of the TOF-SIMS investigation was to determine the exact locus of failure, i.e. was there a thin epoxy overlayer? Analysis of the results show that there are many mass peaks associated purely with the central adhesive region, 712 710 708 706 704 7 binding energy/eV Fig. 12 Montage of the Fe 2p peak showing the transition between shiny and dull regions within the failure surface (created from the E-X plot) 8000 'Si/CZH, cohesivelyfailed epoxy 7000 6000 K 5000 4000 3000 2000 -ul C3 1000 ;T& \8 x 0 0 10 20 3 40 50 60 70 80 90 100 al .-E 9000- 7 cohesively failed epoxy 8000- 7000- 6000- 5000-4000]300012000 CH; I IIP ,1111 0 10 20 30 40 50 60 70 80 90 100 masskharge Fig.13 TOF-SIMS spectra from the cohesively failed epoxy region of a failed lap shear joint after 7500 h in water at 30 "C:(a)positive ions; (b)negative ions 3500 (a "SiGH, =Fe metal surface 250030001 1000i 500-C 3 9 O HAr:111 -5 12000 .-(b) 0 metal surfaceC 10000-8000-6000-40001 c SCI 1112ooou0 0 10 20 30 40 50 60 70 80 90 100 masdcharge Fig. 14 TOF-SIMS spectra from the metal region (2 mm from the joint edge) of a failed lap shear joint after 7500 h in water at 30°C: (a)positive ions; (b)negative ions Fig. 16. Peaks at rn/z73+, 77+, 98+, 105+, 107' and 207' amongst others are all attributable to organic material.These mass fragments arise either from the bulk adhesive formulation (amine-cured epoxy) or from minor additions such as silicones/ siloxanes and fillers which are often added to fully formulated adhesives. Unfortunately, the exact content of the adhesive is unknown as it is a fully formulated commercial system. The clear discrimination between the central and outer regions for all of these mass fragments combined with the very high sensitivity of TOF-SIMS confirms that epoxy is not present on the apparently metal surface. This indicates that under the test conditions used here, true interfacial failure has occurred at the iron oxide+poxy interface; however, the presence of nitrogen on both sides of the failed joint (easily detectable by XPS) suggests either that failure has occurred through a surface nitrogen-rich phase or that nitrogen has back-deposited in the failed regions.General discussion The analysis of a failed lap shear joint has been investigated using a multi-technique approach. Failure of joints (both control and yttrium-treated) immersed in water is seen to involve a two-stage process and this can be observed visibly by inspecting the failure surfaces. The yttrium layer, although not enhancing adhesion in this case, has proven to be tenacious to environmental attack by water. Although bulk diffusion is thought to be a controlling parameter in the failure of adhesive joints,22 it has been shown that bulk diffusion theory cannot be used to model the failure of the system reported here.This confirms many previous beliefs that surface/interface diffusion rates are very different to those for bulk materials. This is likely as the surface (or very near surface) will have very different properties to the bulk in terms of crosslink density and porosity. Two clear regions develop on samples tested for 1200h in water, these are associated with bulk cohesively failed epoxy and an apparent interfacially failed region. XPS J. Muter. Chem., 1996, 6(3), 479-493 489 Table 3 Mass fragments analysed using line scans mass/charge ion/fragment 56+ 56Fe+ 28' 28s1+ 23' 'jNa+ 39 39K + + Y + +27' 27Al /CzH3 /CHN + H-Y=C-H 40' 40Ca+ 17 OH 35 c1 26 CN 73 28SiC3H9'+ 77 + 98' + 105' 0 0 II II107' H-SI-0--8-H + y3 207 + of this apparently metal region indicates that failure, if not uniformly interfacial, is pnmarily interfacial with possible small overlayer regions present This failure region appears identical to that identified as zone 1 on the longer exposed test sample After 7500 h in water, three regions appear on samples These are the central cohesively failed epoxy and a ringed outer 'metallic' surface which shows a bright metal surface at the joint centre (zone 1) and a duller region towards the joint edge which is a result of oxide thickening on exposure to water (zone 2), this is confirmed by XPS Analysis of the halo using small-spot analyses (Scienta ESCA 300) showed that chlonne was present in the inner region in fairly high quantities, whereas the outer (dull) region contained no chlorine Exact binding- energy measurements have enabled the chlorine to be dis- tinguished as a chloride ion from a highly ionic salt such as KCl or NaCl The distribution of species across the failure region has been further analysed using spatially resolved techniques and macro line scans have been produced both by XPS and TOF-SIMS The mobility of species can be assessed from these line scans and they allow the structure suggested in Fig 17 to be proposed On exposure, water permeates into the adhesive and diffuses towards the centre of the bond This process is controlled by surface diffusion or diffusion through a modified outer region within the epoxy The actual cause of delamination is still a matter of discussion with several routes being suggested Cathodic disbondment due to the presence of electrochemical potentials and a high pH at the crack tip will result in bond breaking and subsequent failure of the adhesive, this is ident- 490 J Muter Chem , 1996, 6(3), 479-493 ified as zone 1 Once disbonded, the epoxy displaces from the metal at a slower rate to the disbondment (bond breaking) to produce a crevice (zone 2) The disbondment front (just breaks the bonds) occurs at 18 5 pm per day to yield zone 1 whilst the slower delamination front (where the epoxy is physically moved from the metal surface allowing a crevice to form) proceeds at 10 2 pm per day in water at 30 "C which gives the visually identifiable zone 2 The presence of cations at the interface (detected by XPS and SIMS) is known to result in an increased failure rate due to solvated ions moving through the polymer allowing water ready access to the region3' The rate of failure is also dependent upon the cation present with delamination rates being higher for solutions containing potassium ions than lithium, sodium or barium electrolytes of equivalent concen- trations in the order of K, Na, Li and Ba A rapidly solvated ion will allow water to move quickly behind it, whereas a much slower ion (Mg2', charge/radius =27 78) will hinder the movement of water, resulting in slower failure lo In this case small mobile ions are present, therefore water will be able to move through the adhesive at a higher rate This will result in increased electrochemical attack and subsequently faster bond breaking Before separation, the delaminated surfaces merely remain in contact without displacing from one another This results in a very low volume debond (zero volume debond') At a critical point, the delaminated faces separate, allowing bulk solution to fill the crevice thus formed This increased volume allows post-failure oxide growth as well as ionic mobility Oxide growth is evident from both the survey spectra taken from the outer and inner regions and is demonstrated in the Fe 2p,,, E-X scan which shows the metallic component reducing in intensity towards the outside of the joint Increased ionic mobility is apparent from the chlorine signal which shows chloride ions being removed from the crevice region towards the more anodic outer surfaces Towards the centre of the bond (zero volume debond) the surfaces have not separated, thus ions move very slowly in this region The chlorine signal is much higher for this central region indicating that chloride ions are prevented (physically) from reaching the outer anodic surfaces Conclusions A thin yttrium hydroxide layer proves tenacious during expo- sure to water for 7500 h at 30°C However, this pretreatment alone does not improve durability After 1200 h, failure can be attributed to two clearly defined regions A central cohesively failed region and an outer appar- ently interfacial region There is no evidence of epoxy fragments on the metal surface, however, nitrogen is apparent (by XPS) on both failure surfaces The source of the nitrogen is not clear and three possible scenarios exist to account for its presence on the failed metal surfaces It is either present as a discontinu- ous layer resulting from failure within an enriched amine-rich surface phase within the adhesive, from leaching and sub- sequent re-deposition of nitrogen-rich phases from the adhesive or from adsorption of atmospheric nitrogen on failure After 7500 h in water at 30°C there is a ringed structure within the outer metal region This appears dull towards the joint edge and shiny towards the centre of the joint (crack tip) XPS confirms that a thicker oxide has formed in the outer region The inner zone corresponds to the interfacial failure (I e cathodic delamination) of the 1200 h joint Small-area XPS shows the presence of chlorine (as chloride ions) within the inner shiny region No chlorine is detected in the outer region It is proposed that although the entire failure surface has disbonded up to the crack tip, the delamination front (when a physical crack of a given volume is formed) has only opened up to the transition between the two regions It 50.00t I 20.::10.00 0.0040 2 4 6 8 101214 2 4 6 8 10 12 4 4.00d i 4 6 8 ioiirl 0.00 2 4 6 8 io i2 114 I distance from joint edge/mm Fig.15 TOF-SIMS line scans (metal side) of a failed lap shear joint after 7500 h in water at 30 "C: x-axis=distance from joint edge (in mm); y-axis=range of intensities for species present (in counts).m/z=(a) 56' (iron-56), (b) 28+ (silicon), (c) 23+ (sodium), (d) 39+ (potassium), (e) 27+ (aluminium/C,H,+/CHN+), (f) 40' (calcium), (8) 17-(OH), (h) 35-(chlorine-35), (i) 26- (CN-). J. Muter. Chem., 1996, 6(3), 479-493 491 5.00. 4.00. 3.00. 2.00. 1.00. 4 distance from joint edgelmm Fig. 16 TOF-SIMS line scans (metal side) of a failed lap shear joint after 7500 h in water at 30 "C: x-axis =distance from joint edge (in mm); y-axis=range of intensities for species present (in counts). m/z=(a) 73+, (b) 77+, (c) 98+, (d) 105+, (e) 107+, (f) 207'.Fig. 17 Schematic of failure within the lap shear joint is therefore proposed that two failure fronts exist in this system. The faster disbondment front operating at 18.5 pm day-' and the slower at 10.2 pm day-'. The data from XPS and SIMS analyses have been processed to produce line scans across the joint overlap. These plots 492 J. Mater. Chem., 1996,6(3), 479-493 show the spatial distribution of species present within the failure regimes. The presence of cations (sodium and calcium) at the crack tip indicate electrochemical activity whilst the absence of chloride ions in the outer region suggests that this region has better mobility, thus allowing anions to migrate towards the anode. The presence of chloride ions in the cathodic region near the crack tip confirms that this region is of much lower volume, thus preventing easy ionic movement. There is a high silicon signal in the outer region; both XPS binding energy measurements and SIMS analyses of this outer region enable this silicon to be identified not as a silicone but as silicate from the glassware.It is likely that contamination has occurred post-failure after the outer region has opened up, allowing movement of species from the bulk solution to occur. Electrochemical activity detectable from the movement of various ions indicates that the adhesive joint is under attack from cathodic delamination. However, without an applied potential, the pH achieved at the crack tip will not be sufficient to cause oxide reduction.A schematic of the failure region as a result of electrochemical activity prior to mechanical separation is proposed (Fig. 17). This work has shown the benefits of using a range of small- area and imaging spectroscopies including SAXPS, iXPS and TOF-SIMS in a complementary manner to study a complex system. S.J.D. gratefully acknowledges the financial support of both BP Research and the EPSRC for funding this CASE award. Special thanks go to Dr. Graham Beamson (RUSTI), Dr. Len Hazell (BP Research), David Bond and to Andy Brown for their invaluable assistance. References 1 J. F. Watts and J. E. Castle, J. Mater. Sci., 1983, 18,2987. 2 J. F. Watts, Su$. Interface Anal., 1988,12,497. 3 L. P. Haak, M. A. DeBolt, S. L.Kaberline, J. E. de Vries and R. A. Dickie, Surf. Interface Anal., 1993,20, 115. 4 A. M. Taylor, J. F. Watts, J. Bromley-Barratt and G. Beamson, Surf Interface Anal., 1994,21,697. 5 C. Kerr, N. C. Macdonald and S. Orman, Br. Polym. J., 1970,2,67. 6 A. J. Kinloch, Adhesion and Adhesives: Science and Technology, Chapman and Hall, London, 1987, pp. 363-366. 7 J. F. Watts, J. E. Castle and T. J. Hall, J. Mater. Sci. Lett., 1988, 7,176. 8 E. L. Koehler, Corrosion, 1984,40, 5. 9 U. R. Evans, Trans. Electrochem. SOC., 1929,55,243. 10 J. F. Watts, J. Adhesion, 1989, 31, 73. 11 J. M. Atkinson, R. D. Granata, H. Leidheiser Jr and D. G. McBride, IBM J. Res. Dev., 1985,29, 27. 12 J. E. Castle, 53rd International Meeting on Physical Chemistry, Paris, 1995, American Physical Society, in press.13 R. A. Cayless and D. L. Perry, J. Adhesion, 1988,26, 113. 14 R. A. Cayless and L. B. Hazell, Eur. Pat., EP 0331 284 Al, 1989. 15 S. J. Davis, J. F. Watts and L. B. Hazell, Surf Interface Anal., 1994, 21,460. 16 U. Gelius, C. G. Johansson, J. Larsson, P. Munger and G. Vegerfors,J. Electron Spectr., 1990,52, 747. 17 G. Beamson, D. Briggs, S. F. Davies, I. W. Fletcher, D. T. Clark, J. Howard, U. Gelius, B. Wannberg and P. Balzer, Surf. Interface Anal., 1990,15, 541. 18 A. M. Taylor, PhD Thesis, University of Surrey, 1994. 19 D. Briggs and M. J. Hearn, Vacuum, 1986,36,105. 20 R. A. Dickie and A. G. Smith, Chemtech, 1980,10,31. 21 M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, Pergamon Press, Oxford, 1966, pp. 177-182. 22 R. A. Gledhill, A. J. Kinloch and S. J. Shaw, J. Adhesion, 1980,11,3. 23 S. J. Davis and J. F. Watts, unpublished results. 24 R. A. Gledhill and A. J. Kinloch, J. Adhesion, 1974,6, 3 15. 25 J. E. Castle, R. Ke and J. F. Watts, Corrosion Sci., 1990, 30, 771. 26 J. F. Watts, R. A. Blunden and T. J. Hall, SurJ Interface Anal., 1990, 16, 227. 27 C. C. Telford, S. J. Davis and J. F. Watts, unpublished results. 28 Handbook of X-ray Photoelectron Spectroscopy, ed. Jill Chastain, Perkin Elmer Corporation, Minnesota, USA, 1992, pp. 62-63. 29 G. Beamson, personal communication, RUSTI facility, Daresbury Laboratory, 1995. 30 J. F. Watts, J. E. Castle, P. J. Mills and S. A. Heinrich, in Corrosion Protection by Organic Coatings, ed. M. W. Kendig and H. Leidheiser Jr, The Electrochemical Society, 1987, pp. 68-83. 31 R. D. Shannon and C. T. Prewitt, Acta Crystallogr., Sect. B, 1970, 26, 1046. Paper 5/05897J; Received 6th September, 1995 J. Mater. Chem., 1996, 6(3), 479-493 493

ISSN:0959-9428    

DOI:10.1039/JM9960600479

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

A novel preparation of calcia fully stabilised zirconia from molten alkali-metal nitrate Huda Al-Raihani,' Bernard Durand,b.c Fernand Chassagneuxb and Douglas Inman' 'Department of Materials, Imperial College of Science, Technology and Medicine, Prince Consort Road, London, UK SW7 2BP bLaboratoire de Chimie Minerale 3, URA CNRS no. 116, ISIDT, Universitk Claude Bernard Lyon I, 43 Boulevard du 11 Novembre 191 8, 69622, Villeurbanne Ckdex, France 'Laboratoire de Matkriaux Minkraux, URA CNRS no. 428, Ecole Nationale Supkrieure de Chimie de Mulhouse, 3 Rue Alfred Werner, 68093 Mulhouse Cedex, France A novel preparation of cubic calcia stabilised zirconia (CaO-ZrO,) which employs the coprecipitation of the constituents from zirconium oxychloride (Zr01.22C11.56)and calcium nitrate [Ca( NO,),] with eutectic lithium nitrate-potassium nitrate (LiN0,-KNO,) at around 600 "C is described.The optimisation and advantages of the preparation are discussed. Pure and stabilised zirconia (ZrO,) have been the objectives of numerous investigations, initially as refractories' and more recently as solid electrolytes in fuel cells., Pure ZrO, exhibits three polymorphs, monoclinic (m), tetragonal (t) and cubic (c).,-~The polymorphic transformation from monoclinic to tetragonal ZrO, involves a considerable contraction, and vice versa an expansion, creating a sharp volume change of about 9 Yo resulting in substantial weakening and fra~ture.~ The brittleness problem has been partially overcome by the addition of certain oxides such as magnesia (MgO), calcia (CaO), yttria (Y203)or rare-earth-metal oxides (e.g.CeO,), to stabilise the ZrO,, either partially or fully, depending on the quantity of oxide dopant added.' These materials have shown good thermal shock resistance, high degree of toughness, chemical inertness and refractory behaviour and these proper- ties render them useful in many application^.^-^' Calcia fully stabilised zirconia (Ca-FSZ) exhibits excellent ionic conductivity and low thermal conductivity and, therefore, it is being used as a solid electrolyte in galvanic cells, fuel cells, gas sensors; in the near future it may find use as a membrane in water thermolysis for the production of hydrogen." CaO-Zr0, has been produced by several well known methods, ranging from the conventional to the very advanced new routes.These include mixing powders with ball milling and coprecipitation in aqueous solutions followed by decompo- sition, whilst new methods are hydrothermal, spray-drying, freeze-drying and sol-gel methods.', So far, most of the fabrication routes are either expensive or have employed very high temperatures. On the grounds of cost, quality of the powder and time, it is desirable to avoid the necessity for very high temperatures in the fabrication process. Molten salt routes are possible alternatives, where metal oxides can be readily precipitated from oxyanion melts, (e.g. and nitrite^'^), or from simple ionic melts (e.g. chlorides''). This can be achieved by the oxide anion produced either from the dissociation of the melt itself'' or by the addition of alkali-metal bases,,' (e.g.hydroxide, peroxide or carbonate). The preparation of ceramic powders uia low-temperature molten salt routes has just begun to be explored. Lux-Flood acid-base reactions can be applied, e.g. in the precipitation of pure Zr0213,20,21 or coprecipitation of Y203-Zr02,22*23from molten nitrates. The purpose of the present study is to investigate the possibility of coprecipitating CaO-ZrO, via a molten salt route. Experimental Materials Zirconium oxychloride. Commercial hydrated zirconium oxychloride ZrOC1,.8H2O (Aldrich) was used after dehy- dration in an oven at 195 f5 "C for about 48 h. From thermo- gravimetric investigation it was initially thought that this treatment led to the anhydrous oxychloride ZrOC1,.24 In fact, further IR spectroscopy studies have shown that it is quite difficult (almost impossible) to completely dehydrate zirconium oxychloride without losing a small amount of hydrochloric acid which involves the formation of oxychlorides correspond- ing to the formula ZrO,, +x.C12(1-x).25 Chemical analysis of zirconium and chlorine contents corroborated the conclusion that the thermal treatment at 195°C for 48 h led to the oxychloride ZrOl.22Cll.56. Calcium nitrate.Ca( NO,), (BDH general purpose reagent) was used after a thermal treatment similar to that used for zirconium oxychloride. No nitrate decomposition was noticed. Zirconium sulfate.Commercial zirconium sulfate, Zr(S0,)2.4H20 (BDH, AnalaR) was dehydrated according to Bear's method.26 The dehydration was carried out by heating Zr(SO,), in the presence of H2S04 (AnalaR grade, 98%), at temperatures between 350 and 410°C for several hours, in a similar manner to that previously indicated for the preparation of zirconia by reaction of zirconium sulfate with molten alkali- metal ni tra tes.,' Lithium nitratepotassium nitrate eutectic mixture. LiN0,-KNO, eutectic, 132 "C, was also prepared according to the method described in ref. 20. The starting materials were LiN03.3H20 (Fisons general purpose reagent) and KNO, (BDH, AnalaR). Sodium nitrate. NaNO, (BDH, AnalaR), mp 308 "C, was used as received. Procedure Most of the preliminary experiments were carried out in a small diameter furnace with a vertical steel pot, where a chromel-alumel thermocouple was located between the reac- tion tube and the furnace wall.A few reactions were performed in a larger diameter mullite furnace, but with a Pt/Pt-13% J. Muter. Chem., 1996, 6(3), 495-500 495 Rh thermocouple placed within the hot zone outside the furnace All reactions were performed in a Pyrex Quickfit tube joined to a drying tube containing glass wool and silica gel to provide a dry atmosphere Some experiments involved the use of ultra-centrifugation The precipitate-melt mixture was spun for a few minutes in order to remove the unreacted melt, without the use of water washing A special apparatus was designed and used for the filtration of the melt, consisting of an inner tube with a sintered disc and an outer tube in which the filtrate melt could be received All experiments were performed from mixtures of the start- ing metal salts [Ca(N0,)2, ZrO, 22C11 56 and (Li,K)NO,] containing an excess of alkali-metal nitrates compared to stoichiometric ratio Techniques Thermogravimetry.The reactivity of metal salts towards alkali-metal nitrates was investigated with a Stanton-Redcroft TR- 1 thermobalance which revealed mass losses corresponding to various reactions including precipitation, coprecipitation, dehydration and decomposition Ultra-centrifugation. An MSElO centrifuge was modified for high-temperature requirements The apparatus rotates four heated capsules The maximum working temperature could be up to 1000"C at a maximum speed of 2400 rpm X-Ray powder diffractometry.A Philips X-ray diffractometer was employed to identify the insoluble solids, whether unwashed or washed The resulting diffraction patterns were recorded and analysed according to the standard data listed in the JCPDS files27 Chemical analysis. Elemental chemical analysis (Ca, Zr, N, C1, Li, K or Na) was performed by the Micro-analysis Center of Solaize (CNRS, France) Results Preliminary experiments Reaction of Ca (N03)2 with eutectic LiN0,-KN03. Anhydrous Ca( N03)2 and LiN0,-KNO, were premixed thor- oughly before reaction The experiment was programmed to start heating from room temperature (25 "C) with a heating rate of 12"C min-l upto 630 "C, and to maintain this tempera- ture for 30 min No changes were observed below 195 "C, at which tempera- ture the eutectic salt started to melt This is rather higher than the literature value of the mp (132°C) Between 300 and 500"C, Ca(N03)2 was observed to be completely soluble in the melt Above 500 "C, the melt became yellowish, and colour- less O2began to evolve When the temperature reached 540 "C, the release of both brown NOz and colourless O2was intensi- fied and this continued up to around 600"C, when a white precipitate could be seen The precipitate-melt mixture was then cooled in a desiccator, washed with distilled water and finally subjected to XRD analysis The X-ray diffraction pat- terns for the washed product showed several lines correspond- ing to CaO, Ca(OH), and CaCO, (Table 1) It was thought that calcium hydroxide was formed by hydrolysis of calcium oxide during water washing, and calcium carbonate by car- bonation of calcium hydroxide The analysis, by X-ray diffractometry, of the unwashed cooled reaction mixture cor- roborated this assumption Lines for calcium hydroxide and calcium carbonate disappeared from the pattern, which revealed only lines of calcium oxide and of the eutectic LiNO,-KNO, (Table 2) Note that experimental lines of the 496 J Mater Chem ,1996, 6(3), 495-500 Table 1 XRD results of the white precipitate obtained from the reaction of Ca(NO,), with eutectic LiN0,-KNO, CaO Ca(OH), CaCO, washed (JCPDS (JCPDS (JCPDS precipitate 27-775) 4-733) 5-586) d/A I/I, d/A I/I, d/A I/I, djA I/I, 4 90 61 490 74 303 100 300 100 304 100 2 78 36 2 62 15 263 100 2 49 13 2 49 14 2 28 13 2 29 18 2 09 12 2 10 18 191 15 191 60 193 42 191 17 187 15 188 60 187 17 Table 2 XRD results of the unwashed solid obtained from the reaction of Ca(NO,), with eutectic LiN0,-KNO, CaO LlNO3-KNO3 KNO, unwashed (JCPDS eutectic (JCPDS solid 28-775) (experimental) 5-377) d/A Ill, d/A I/I, d/A I/I, d/A I/Io 4 63 9 4 63 13 466 23 4 54 9 4 54 13 458 11 3 75 40 3 75 79 378 100 3 70 26 3 70 56 373 56 3 21 6 321 5 307 15 3 02 100 300 100 302 100 303 55 2 75 17 2 75 35 276 28 2 70 6 2 69 9 271 17 266 41 2 64 29 2 64 47 265 55 2 63 15 2 63 35 263 20 2 40 5 241 7 2 36 6 237 4 2 32 6 233 9 2 18 20 2 18 47 219 41 2 15 13 2 15 12 216 20 2 06 4 2 06 7 207 13 2 04 10 2 04 16 205 18 194 13 194 26 195 24 193 4 193 7 194 6 191 20 191 60 188 20 188 60 eutectic are very close in position and intensity to those of orthorhombic KNO, Reaction of ZrOl 22C11 56 with eutectic LiN03-KN03.56Thermogravimetry of ZrO, 22c11 with the eutectic LiN03-KN03 showed a total mass loss of 488% of the starting ZrO, 22C1, 56 This occurred in two stages (Fig 1) I $1? 40 t Ls) -' Q) Ig 0 -----' : . '' U 100 200 300 400 500 600 700 800 TPC Fig. 1 TG data for the reaction of ZrO, &ll 56 with a LiN0,-KNO, eutectic mixture (1)from 120 to 380 "C, the experimental mass loss was 23.9% (maximum rate at 365 "C); (2) from 380 to 500 "C, the exper- imental mass loss was 24.9% (maximum rate at 450°C).Between 500 and 600"C, there was no change in mass, but this was followed by a sharp mass loss which corresponds to the decomposition of the melt, probably starting just above 600°C to give a yellow melt. X-Ray diffraction identified the reaction product as mainly tetragonal zirconia, with sometimes a small proportion of monoclinic zirconia. Zirconium oxychloride has been reported previously to react with the NaN0,-KNO, mixture in a similar manner.21,24,28 Co-precipitation of CaO-ZrO, from Ca (NO3),, Zr0,.22C1,,, and the eutectic LiNO,-KNO, CaO-ZrO, samples with different proportions of CaO, (4.7, 12.2 and 20.8 mol%) were coprecipitated from the reactions between Ca(NO,),, ZrOl.22Cll.56 and LiNO,-KNO,.For the three reactions, the programme was set to heat the reaction mixture from room temperature to around 630°C with a heating rate of 12 "C rnin-'. The temperature was maintained at 630 "C for 30 min. Four stages could be identified during the furnace runs: (1) from room temperature to 200"C, NO, (light brown fumes) started to evolve. The mass loss revealed by thermogravimetry is negligible. (2) From 200 to 500"C, reaction was established. A continuous release of NOz and 0, was observed simultaneously with the formation of a white precipitate. The thermogravimetric curve (Fig.2), quite similar to the one for the reaction of ZrOl.22Cll.56 alone (Fig. l), indicates a two-step transformation with maximum rates at 350 and 450 "C. (3) From 500 to 600 "C, reaction was complete and a negligible amount of NO, was evolved. During this time, the melt became yellowish in colour. (4) Above 600"C, a further loss was noticed due to the decomposition of the excess of nitrates, producing a NO, release. The precipitate-melt mixture was cooled in a desiccator and both unwashed and washed samples were analysed by XRD. Whatever the ratio Ca(NO,), :Zr01.22C11.56, the unwashed powders were identified as a poorly crystallised cubic phase similar to cubic zirconia (JCPDS 27-997) and also to cubic 85Ca, 15Zro 8501 (JCPDS 26-341) and a well crystallised LiN0,-KNO, mixture.Nitrates were eliminated by washing and the three washed powders contained only cubic calcia stabilised zirconia as shown in Fig. 3 for 12.2 mol% CaO- ZrO,. Investigation of the influence of ultra-centrifugation on the coprecipitation of CaO-ZrO, from Ca( NO3), and Zr01.22C11.56 in molten LiNO3-KNO3 In order to test the efficiency of ultra-centrifugation at high temperature to separate the reaction product from the unreacted nitrate melts, an experiment was performed using '0° i 1 8ok600 70 .-o, 500 i t 0\ I03 400 1 ,-I I 2 300 200 300 400 500 600 700 5 TI'C Fig.2 TG data for the simultaneous reaction of Ca(NO,), and ZrO, ,,C1, 56 with eutectic LiN0,-KNO,. -, Am; ---,AmlAT 10 20 30 40 50 60 70 28ldegrees Fig.3 XRD pattern of washed 12.2 mol% CaO-ZrO, Ca(NO,),, Zr01.22C11.56 and LiN0,-KNO, in the same pro- portions as in the previous section to prepare 20.8 mol% CaO- ZrO,. The heating profile was slightly changed to ensure complete dryness of the starting materials. The mixture was first maintained at 50°C for about 15 min, then heated to 630°C at a rate of 200°C h-' and finally maintained at this temperature for 13h. The precipitate-melt mixture produced from the reaction was divided into three portions and treated as follows: (a) spun/unwashed: this portion was spun in the ultra-centrifuge for 5 min at 800 rpm at a temperature of 300 "C to filter off the melt; (b)spun/washed: this portion was spun in a similar manner, but was then washed with a very small quantity of distilled water and then dried in a desiccator; (c) spun/unwashed and sintered: this portion was also spun in a similar manner, but for a longer time (ca.15 min) without washing. It was then pressed into a pellet and sintered at 1100 "C in a muffle furnace. The XRD patterns of the three samples, as in the previous section are comparable to either cubic zirconia (JCPDS 27-997) or 15 mol% Ca0-85 mol% ZrO,, (JSPDS 26-341), as shown in Fig. 4 for samples (a) and (c). The strongest lines of the LiN0,-KNO, mixture emerge slightly in sample (a). They are detected neither for the washed sample (b),nor for the sintered sample (c). Discussion Ca(NO,), was reported to decompose after heating for 2 h in air at 580"C, giving NO,, 0, and CaO which was confirmed by X-ray powder diffra~tion.,~ In eutectic LiN0,-KNO,, Ca(NO,), was found to be soluble and stable up to 500"C,29 which is in agreement with our work.The fact that Ca(NO,), did not react with LiN03-KN0, at these low temperatures can be explained in terms of the relative polarising powers /I 10 20 30 40 50 60 70 80 2Bldegrees Fig. 4 XRD pattern of CaO-ZrO, samples (a) and (c) prepared from reactions of Ca(NO,), and ZrO, 22Cll 56 with eutectic LiN0,-KNO, using ultra-centrifugation J. Muter. Chem., 1996, 6(3),495-500 497 (acidities) of the Ca2+ and Li+ cations At higher temperatures (above 500 "C), CaO is precipitated according to eqn (1) Ca2++ 2N03- +CaO + 2N0, +to, (1) In this study, CaO has been precipitated between 540 and 600 "C ZrOC1, was reported to react with eutectic NaN03-KN03 mixture (mp 225 "C) and with pure NaNO, (mp 308 "C) giving ZrO, either in the tetragonal or tetragonal + monoclinic forms 23 29 Thermogravimetry of the reaction of ZrO, ,,ell 56 with LiNO3-KNO, showed a total mass loss of 48 8%, which is closer to the value of 507% calculated for eqn (2) which leads to oxygen than to the value of 38 3% calculated for eqn (3) which leads to chlorine ZrOi ,,c1, 56+ 1 56N0,- -+ ZrO, + 1 56N0, + 0 390, + 1 56C1- (2) ZrO, 56 -l-0 78NO3--+ ZrO, + 0 78N0, + 0 39C1, +0 78C1- (3) In our experiments the release of oxygen is more likely to occur than that of chlonne, in agreement with Jebrouni's earlier results 24 This may be related to the concentration of zirconium oxychloride in the molten bath, a low concentration (z e the present experiments) favouring the evolution of oxygen, whereas a high one (I e Jebrouni's experiments) promoting the evolution of chlorine Calcia stabilised zirconia can be obtained by firing, at elevated temperature (> 1000 "C), mixtures of zirconia and calcium oxide or carbonate According to Duwez et ul, cubic CaO-ZrO, is produced with powders containing between 16 and 30molYo CaO, whereas for lower contents cubic and monoclinic phases are produced 30 In 1963, Tien and Subbarao reported that cubic calcia fully stabilised zirconia (Ca-FSZ) is obtained from mixtures at compositions between 12 and 21 mol% CaO Mondal et a1 corroborated this result as in their work the addition of 20mol% CaO to ZrO, favoured the formation of cubic calcia stabilised zirconia 32 For the same ratio, Garvie suggested that the cubic solid solution 20 mol% CaO-ZrO, could be the mixed oxide CaZr,O, 33 The coprecipitation of Ca-FSZ (cubic solid solution after heat treatment of the amorphous phase obtained after drying) by freeze drying has been reported by Roosen and Hausner,,, using Ca(NO,), and ZrOCl, as starting materials The same chemicals were used for the present study but the preparation was in non-aqueous solution The main advantage of freeze- drying is the avoidance of the formation of hard agglomerates The method is, however, rather expensive, compared to the method used in the present work employing molten salts In transformations involving calcium nitrate and zirconium oxychlonde together, Ca(NO,), reacts with molten eutectic LiN0,-KNO, at lower temperatures than when it is alone, 1 e under 500 "C The simultaneous reaction leads to calcia Table 3 Influence of reaction parameters [ratio Ca(N03), ZrO, ,,Cll lines in diffraction patterns of calcia stabilised zirconia unwashed CaO-ZrO, intensity, III, stabilised zirconia, in which calcium oxide is incorporated inside the zirconia lattice On the basis of the literature, the formation of either cubic or monoclinic solid solutions is expected in the range 0-30 mol% CaO-ZrO, and, for the composition 20 mol% CaO-ZrO,, the mixed oxide CaZr,O, should be obtained The existence of domains containing a tetragonal CaO-ZrO, solid solution in Stubican's phase diagram,' 36 leads to the suggestion that this phase could be produced in a metastable state by rapid quenching, from temperatures above 1170°C to room temperature, of melts of CaO and ZrO, In our experi- ments in molten nitrates, it is shown that neither the formation of a monoclinic solid solution even for the lowest content in CaO, nor the precipitation of CaZr,O, for the highest content, take place At first glance, the XRD patterns of all the samples are comparable either to the one of cubic zirconia (JCPDS 27- 997) or to the one of cubic Ca, 15Zro s50185 (JCPDS 26-341) The main difference between cubic and tetragonal zirconia is the splitting of some cubic lines For well crystallised powders, the splitting appears clearly in XRD patterns, allowing an easy identification Thus, it can be concluded undoubtedly that the powder 20 8 mol% CaO-ZrO, separated from the molten salt by ultra-centrifugation, then pressed and sintered [sample (c), previous section] is cubic calcia stabilised zirconia, even if the intensity of lines 220 and 3 11 is a little lower than indicated in JCPDS for cubic ZrOz or cubic Ca, 15Zro 8sOi 85 (Table 3) For finely crystallised powders, the convolution of XRD peaks makes the detection of the splitting more difficult or even impossible and the question arises whether the non-sintered CaO-ZrO, samples, prepared from molten salts, are cubic or tetragonal calcia stabilised zirconia Taking into account the fact that the splitting is never detected, and the results pre- viously published in the literature, it can reasonably be assumed that the 20 8 mol% CaO-ZrO, samples, either unwashed and extracted by washing (see earlier), or extracted by ultra-centrifugation without washing and with further water washing [sample (a) and (b)]are also cubic calcia stabilised zirconia, all the more so because the intensity of the XRD peaks is in fairly good agreement with those for both cubic ZrO, and cubic Ca, i5Zro 850i (Table 3) For samples with a lower85 CaO content, 4 7 and 12 2 mol% CaO-21-0, (see earlier), the situation is more complex Yet, both for unwashed and for washed powders, the increase of calcia content from 47 to 20 8 mol% does not involve any variation in the intensities of the zirconia peaks (Table 3) This can be interpreted on the basis that, whatever the calcium content in the range 4 7-20 8 mol%, the structure of calcia stabilised zirconia co- precipitated from molten alkali-metal nitrates is cubic Moreover, it is noticeable that water washing changes neither the intensities of the XRD peaks, as shown in Table3 for 122mol% CaO-ZrO,, nor the ratio CaO (CaO+ZrO,) determined from chemical analysis, as revealed for the same 56, washing, use of ultra-centrifugation] on the intensity of zirconia XRD ultra-centrifugated unwashed washed unwashed & sintered CaO-ZrO, (12 2 mol%) intensity, I/Z, d/A hkl 4 7 mol% 12 2 mol% 20 8 mol% intensity, I/Z, (4 (4 2 95 111 100 100 100 100 100 100 2 55 200 21 22 20 22 19 21 181 220 51 54 53 54 45 35 155 311 33 35 36 31 27 17 148 222 6 8 7 6 7 4 128 400 5 6 6 4 5 3 498 J Muter Chem, 1996, 6(3), 495-500 samples in Table4.This means that calcium oxide is partly hydrolysed by water when it alone is precipitated, whereas when CaO is incorporated inside the zirconia lattice, forming a solid solution, water washing has no effect. The comparison between both 12.2 mol% CaO-ZrO, samples indicates also that water washing eliminates quite completely the excess of alkali-metal nitrates and the formed alkali-metal chlorides, producing a calcia stabilised zirconia with purity ratios which are satisfactory for further uses.In the same manner, the comparison of 20.8 mol% CaO-ZrO, samples (a) and (c) extracted by ultra-centrifugation (Table 4) reveals that pressing and sintering at 1100 "C eliminates most of the alkali-metal nitrates remaining in the powders after ultra-centrifugation. From chemical analysis (Table4), it appears also that cubic calcia zirconia solid solutions coprecipitated from molten nitrates are characterised by a CaO mol% quite close to that initially introduced into the molten medium ratio Ca(N0,)2 : [Ca(NO,), + ZrOl.22Cll,56}. Sintering does not significantly change this ratio. The last point to discuss is the mechanism of the reaction in the molten nitrate medium.Transformations occurring below 500 "C, the formation of solid solutions CaO-ZrO,, when calcium nitrate and zirconium oxychloride react together, would seem difficult if the starting salts remained in the solid state. They are much more likely to be partially dissolved in the molten medium giving calcium and zirconyl ions, according to eqn. (4) and (5), which react with 0,-ions coming from the dissociation of nitrates, to precipitate the final mixed oxide according to eqn. (6): Ca(N03)2$ Ca2++ 2N03-(4) ZrO(, +x)Clz(l-x) sZrO(, +,),('-,)+ +2(1-x)c1-(5) aCa2++ bZrO(l+,,2(1-X)++ [a+b( 1--)lo2-+ca,Zr,O(,+,,) (6) where Ca,ZrbO(, + 26) is equivalent to ( 100a/a+ b) mol% CaO- ZrO,.Concerning the dissociation of nitrates, Jebrouni et ~1.~' proposed an oxidation-reduction transformation evolving chlorine via eqn. (7): 2NO,-+ 2C1- -+2N02 + 20,-+ C1, (7) But, as explained previously in connection with the reaction of zirconium sulfate with nitrates,,, a dissociation evolving oxygen via eqn. (8) can also be considered: 2N03- +2NO, +02-+0.50, (8) As stated earlier under the present experimental conditions, particularly the low concentrations of calcium nitrate and zirconium oxychloride in molten alkali-metal nitrates, the evolution of oxygen take place. The observed mass losses are in fair agreement with eqn. (9) and corroborate this assumption: aCa(NO,), + bZrOl.22Cll~56+ 1.56bN03- -+Ca,ZrbO(, + 2,) +2(a+0.78b)NO2+0.5(a+0.78b)O2+1.56bC1- (9) As an example, the results of the chemical analysis of sample (a) (Table 4) agree with the formula 20.8 mol% CaO-ZrO, which is equivalent to Cao~21Zro,7gOl 79.Replacing a and b by their values (0.21 and 0.79, respectively) in eqn. (9) gives a calculated mass loss of 56.3% of the starting quantity in calcium nitrate + zirconium oxychloride, which is relatively close to the calculated value, 59.0%. Some Morphological Characterisations of 20.8 mol%CaO-ZrO, Powders The morphology of two 20.8 mol% calcia stabilised zirconia samples was investigated by transmission electron microscopy. Both samples were obtained from the same experiment carried out as described earlier. The first sample, very close to sample 20.8 mol% formed earlier, was extracted from the excess of molten salt by water washing.The second sample, identical to sample (a),was separated by centrifugation at 300 "C. For both powders, only cubic calcia stabilised zirconia is detected from the electron diffraction patterns, as shown on Fig. 5 for the sample extracted by washing. Considering chemi- cal analysis data (Table 4), such a result could be expected for the washed sample but is more surprising for the centrifuged sample. It can be understood by assuming that the interaction of the electron beam with the powder involves an increase of the temperature sufficient to melt the alkali-metal nitrates (mp 132°C). Examination of both powders at low magnification reveals agglomerates with sizes in the range 0.2-2 pm.High magnifi- cation investigations (Fig. 6) show that crystallites are larger in the powder separated by centrifugation (40-50 nm) than in the powder extracted by water washing (10-20nm). The difference is corroborated by the XRD patterns, which exhibit lines broader for the washed powder than for the centrifuged one [full width at half maximum: washed wz3 mm, Fig. 3; centrifuged w x2 mm, Fig. 4(a)]. Conclusions The reaction of zirconium oxychloride with molten eutectic LiN0,-KN03 produces finely divided zirconia at temperatures below 500°C. Calcium oxide can also be precipitated from reaction of calcium nitrate with LiN0,-KNO,, but at tempera- tures above 500°C. The CaO obtained in this way is very unstable to water washing.Fig. 5 ED pattern of 20.8 mol% CaO-ZrO, extracted by washing Table 4 Chemical analysis of CaO-ZrO, samples obtained from reactions of Ca(N03), and ZrO, ,,C1, 56 with eutectic LiN0,-KNO, mass% Zrsample Ca K Li NO3 c1 mol% Ca : (Ca + Zr) 39.1unwashed 12.2 mol% 2.4 11.7 1.5 27.1 3.1 12.3 67.0ed 12.2 mol%wash 4.1 <0.1 <0.1 <0.1 <0.1 12.2 46.0ultra­centrifuged 20.8 mol% 5.3 7.9 1.1 18.6 2.3 20.8 65.6sintered 20.8 mol% 7.5 0.3 <0.1 <0.1 <0.1 20.7 J. Muter. Chem., 1996, 6(3),495-500 499 References 1 A. B. Searle, Refractory Materials: Their Manufacture and Uses, Griffen, London, 1940, p. 210. 2 N. Q. Minh, J. Am. Ceram. SOC., 1993,76,563. 3 J.D. McCullough and K. N. Trueblood, Acta Crystallogr., 1959, 12, 507. 4 D. K. Smith and H. W. Newkirk, Acta Crystallogr., 1965, 18,983. 5 G. Teuffer, Acta Crystallogr., 1962, 15, 1187. 6 D. K. Smith and C. F. Cline, J. Am. Ceram. SOC., 1962,45249. 7 W. D. Kingery, H. K. Bowen and D. R. Uhlmann, Introduction to Ceramics,2nd edn., Wiley, New York, 1976, p. 81. 8 R. C. Garvie, R. H. Hannink and R. T. Pasoe, Nature Phys. Sci., 1975,258,703. 9 D. W. Richerson, Modern Ceramic Engineering, Marcel Dekker, New York and Basel, 1982, p. 376. 10 S. Shinroka, Fine Ceramics, Elsevier, Oxford, 1985, p. xvi. 11 D. Gauthier, G. Olalde and A. Vialaron, Advances in Ceramics, vol. 24B, Science and Technology of Zirconia 111, Am.Ceram. SOC., Ohio, 1986, p. 879. 12 P. Cousin and R. A. Ross, Muter. Sci. Eng. A, 1990,130,119. Fig. 6 TEM images washing, and (b) by of 20.2 mol% CaO-ZrO, extracted (a) by water centrifugation at high temperature 13 14 D. H. Kerridge and J. Cancela Rey, J. Inorg. Nucl. Chem., 1977, 39,405. B. Durand, M. Jebrouni and M. Roubin, Euchem Conference on The reactions at temperatures below 500 "C, of mixtures of calcium nitrate and zirconium oxychloride, with mole ratios Ca: (Ca+Zr) in the range 4-20%, leads to calcia stabilised zirconia. Owing to the incorporation of calcium oxide inside the zirconia lattice, the obtained solid solutions are stable under water washing which allows, after cooling, an easy separation of the insoluble stabilised zirconia from the excess of alkali-metal nitrates and the salts formed during the reaction.Powders with a satisfying degree of purity are thus recovered. At the end of the transformation, the unreacted molten salt can also be separated from the formed stabilised zirconia by ultra-centrifugation at elevated temperature. The major part of the remaining alkali-metal nitrate can then be eliminated 15 16 17 18 19 20 21 22 Molten Salts, Greece, 1990. D. H. Kerridge and A. Y. Khudhari, J. Inorg. Nucl. Chem., 1975, 37, 1893. H. Frounzanfar and D. H. Kerridge, J. Inorg. Nucl. Chem., 1979, 41, 181. S. S. Alomer and D. H. Kerridge, J. Chem. SOC., Dalton Trans., 1978,1589. G. Picard, Euchem Conference on Molten Salts, Greece, 1990. D. H. Kerridge, Molten Salts as Nonaqueous Solvents, ed.J. J. Lagowski, Academic Press, NY, 1978, ch. 5, p. 229. H. Al-Raihani, B. Durand, D. H. Kerridge and D. Inman, J. Muter. Chem., 1994,4,1331. M. Jebrouni, B. Durand and M. Roubm, Ann. Chim. Fr., 1991, 16, 569. M. Jebrouni, B. Durand and M. Roubin, Ann. Chim. Fr., 1992, 17, 143. by natural sintering at 1100°C. Besides avoiding the use of water which can favour the agglomeration of the powders, the greatest interest in ultra-centrifugation is that, in the case of larger scale development, it could permit the recycling of the molten medium. 23 24 25 H. Al-Raihani, B. Durand and D. Inman, J. Muter. Chem., to be submitted. M. Jebrouni, Thesis, Universite Claude Bernard Lyon 1, France, 1990. J. L. Tosan, Thesis, Universite Claude Bernard Lyon 1, France, 1991.The reaction of calcium nitrate and zirconium oxychloride 26 I. J. Bear, Aust. J. Chem., 1966, 19, 357. with molten eutectic LiN03-KN03 proceeds according to a dissolution-precipitation process involving the dissolution of starting salts with the probable formation of complex soluble species whose decomposition induces the precipitation of the obtained solid solution. The solid solutions contain a mole ratio Ca: (Ca+Zr) which is practically identical to the one introduced in the molten medium. Whatever the CaO content, 27 28 29 30 31 32 Joint Committee on Powder Diffraction Standards, Pennsylvania, 1973. B. Durand and M. Roubin, Muter. Sci. For., 1991,73-75, 663. H. Abood, PhD Thesis, University of Southampton, 1984. P. Duwez, F. Ode11 and F. H. Bowen, Jr, J. Am. Cerum. Soc., 1952, 35, 107. T. Y. Tien and E. C. Subbarao, J. Chem. Phys., 1963,39,1041. B. Mondal, A. N. Virkar, A. B. Chattopadhyay and A. Paul, in the investigated range, the reaction in molten nitrates below 500 "Cproduces cubic calcia stabilised zirconia. 33 34 J. Mater. Sci. Lett., 1987,6,7-12, 1395. R. C. Garvie, J. Am. Ceram. SOC., 1968,51,553. A. Roosen and H. Hausner, Ceramic Powders, ed. P. Vincenzini, This work was supported by the Commission of the European Communities in the frame of a twinning contract between the Imperial College of London and the University of Lyon. The 35 36 Elsevier, Amsterdam, 1983, p. 773. V. S. Stubican and J. R. Hellmann, Adv. Ceram., 1981,3,25. V. S. Stubican and S. P. Ray, J. Am. Ceram. SOC., 1977,60,534. authors are indebted to the Commission of European Paper 5/06183K; Received 19th September, 1995 Communities for its financial support. 500 J. Muter. Chem., 1996, 6(3), 495-500

ISSN:0959-9428    

DOI:10.1039/JM9960600495

出版商:RSC    

年代:1996

数据来源: RSC

摘要:

MATERIALS CHEMISTRY COMMUNICATIONS Charge-transfer complex and radical cation salt of a new donor EDT-TTFCl,: unique conductivities and crystal structures Masahiko Iyoda,"" Hironori Suzuki," Shigeru Sasaki," Harukazu Yoshino," Koichi Kikuchi," Kazuya Saito," Isao Ikemoto," Haruo Matsuyama" and Takehiko Morib "Department of Chemistry, Faculty of Science, Tokyo Metropolitan University, Hachioji, Tokyo 192-03, Japan bDepartment of Organic and Polymeric Materials, Tokyo Institute of Technology, O-okayama, Tokyo 152, Japan X-Ray structural analyses of the charge-transfer complex (EDT-TTFCI,), *TCNQF4, 2, and the radical cation salt EDT-TTFC1, -ClO,, 3, revealed an interesting effect of chlorine substitution on the crystal stacking mode; the electrical conductivity of 2 is unusually high, although it has a mixed-s tacking structure. Recently, weak intermolecular interactions have been recog- nized as one of the most important factors for deciding crystal structures.' In particular, the Cl...Cl interaction has been investigated extensively, because the Cl-..Cl npn-bonded con- tacts, which are sometimes very short (3.2-3.6 A),are estimated as ca.3% of a covalent bond., In addition, the substitution of the chlorine atom on a TC donor may give rise to a small dipole moment from the donor moiety to the chlorine substituent, although the electronegativity of chlorine decreases the donor properties, thus causing difficulties in forming molecular conductor^.^ In a previous paper,, we reported the synthesis and crystal structure of the dichlorinated ethylenedithiotetrathiafulvalene (EDT-TTFCl,), 1, which shows a centrosymmetric head-to- tail stacking. Although there is no strong Cl...Cl interaction in the crystals of 1, some intermolecular S..-S,Cl.-.S, Cl..-C and Cl..-Cl contacts can be detected. Thus, these observations prompted us to prepare single crystals of the charge-transfer complexes or radical cation salts of 1.The first oxidation potential (0.68 V), of 1 is a little higher than that (0.52 V) of bisethylenedithiotetrathiafulvalene (BEDT-TTF). Therefore, 1 and 2,3,5,6-tetrafluoro-p-tetra-cyanoquinodimethane (TCNQF,) can be expected to be a suitable pair for making a charge-transfer complex. A warm solution of 1 in benzene was mixed with a warm solution of TCNQF, in benzonitrile and the resulting mixture was allowed to stand at room temperature.The charge-transfer complex 2 was obtained as black crystals. On the other hand, single crystals of the cation radical salts (3) of 1 were obtained by galvanostatic oxidation of a solution of 1 and Bun4NC10, in chlorobenzene-CS, ( 10:1) (Scheme 1). The molecular structure and packing diagram of 2 are shown TCNQF, 1-w 2 l-TCNQF, 3(EDT-TTFC12) Scheme 1 in Fig. 1.tThe crystal packing of 2 shows that two donor and one acceptor molecules are stacked along the a axis to form a mixed-stacking structure, the donor molecules being stacked in the dimeric mode with 'atom-over-atom' overlap. The donor and acceptor molecules are quasiplanar, and the dithiacy- clohexene ring in the donor shows a slight distortion towards a chair conformation.The fa5e-to-face distance between the donor and acceptor is ca. 3.2 A,whereas the distance between the two donors is ca. 3.8 A. The TCNQF, molecules have crystallographic C, symmetry, and the two-fold axis passes through the centre of the six-membered ring. Furthermore, the packing of the molecules in the crystal shows that a pair of EDT-TTFC1, molecules lie on the position of crystallographic symmetry. t X-Ray diffraction data were collected on a Rigaku AFC7R diffractometer with Mo-Kcr (/I=0.71069 A) radiation up to 28 =55.1". The intensities were corrected for Lorentz and polarization effects, and analytical absorption corrections were carried out.The structures were solved by direct methods and refined by full-matrix least-squares analyses using reflections with I >3.00o(Z). Anisotropic thermal par- ameters were used for non-hydrogen atoms. Crystal data for 2 (EDT-TTFCl,)-0._5(TCNQF4),CI4Cl2F2H4N2S6, M, =501.46, $clinic, space group P1, a =12.449( 2), h =13.153( 2), ~=5.722+(9)A, a=93.85( l), p=93.48(1), ~=70.025( lo)', V= 877.9(2)A3, Z=2, D,= 1.897 g~m-~,F(000)=500.00, R=0.081, R,= 0.083, GOF=6.88 for 2829 observed reflections out of 4076 unique reflections. Crystal data for 3 C8C13H404S,, M, =4t2.84, triclinic, spacegroupPl,a=9.362(1), h=10.129(1), c=8.363(1) A, a=90.41(1), 8=94.28(1), :,=74.56(1)", V=762.2(2) A3, Z=2, D,=2.017 g ~m-~, F(000)=462.00, R =0.038, R, =0.036, GOF = 1.96 for 2442 observed reflections out of 3517 unique reflections. Atomic coordinates, bond lengths and angles, and thermal parameters have been deposited at the Cambridge Crystallographic Data Centre.See Information for Authors, Issue No. 1. CI1 fC Fig. 1 Short contacts (in A) between two sulfurs, two chlorines, and sulfur and chlorine in 2 [Cl(l)..-C1(2) 3.705(3); S(l)-..S(2)3.826(3); S(3)*..S(4) 3.852(3); S(5)*..S(6) 3.684(3); C1( 1)4(5) 3.474(3); S(5)*..S(1) 3.410(3); S(l).-.S(3) 3.917(3); S(3)..-S(3) 3.871(4)] J. Muter. Chern., 1996,6(3), 501-503 501 Although there is no intermolecular short distances less than the sum of van der oWaals radii along the a axis (S..-Sdistances are 4.07 and 4.08 A), many intermolecular short contacts are observed between the molecules aligned along the c axis. In addition, there is a Cl;..Cl short contact along the b axis [Cl( l)..-Cl( 1) 3.418(4) A].Thus, the S(l)-..S(5), S(5)-.C1( 1) and Cl(!).-.Cl( 1) distances are 3.410(3), 3.474(3), and 3.418(4) A, respectively, which are much shorter than the>...S, S...C,' and Cl...CI van der Waals distances (S: 1.85 A; C1: 1.80A). Interestingly, the dimeric donor molecules interact along the c axis (Fig. 1); one head-to-tail, and one head-to- head interaction. The S...Snetworks extend along the c axis in the crystal. In contrast, no strong S...Sinteraction exists along the b axis, and the Cl..-Cl interaction may control the packing mode in the crystal.The room-temperature conductivity of 2 is about 1S cm-' which is unusually high in spite of the mixed-stacking struc- ture.' Therefore, we measured the anisotropic conductivities using the method reported by Montgomery.6 The conductivit- ies along the a and c axes at room temperature are 0.28 and 5.6 Scm-' with the activation energies of 83 and 74meV, respectively, These results show that the considerably high conductivity and low activation energy of 2 depend on the S...Snetwork aligned along the c axis. However, the conduc- tivity of the mixed-stacking column along the a axis is also rather high with a low activation energy. In order to estimate the conducting interaction, the overlap integrals of the conduction orbitals have been calculated (Fig.2). The largest interaction is c2 for a head-to-head, side- by-side arrangement, whereas a similar head-to-tail, side-by- side interaction, c4, is about one-third of c2. Along the a axis the donor-donor interaction a2 is large, but the donor-acceptor interaction a1 is only very small, presumably owing to mismatch of the phase between the donor's HOMO and the acceptor's LUMO. Thus, the conductivity along the a axis is prevented by the small interaction of al. As for the interaction along the b axis, there is no overlap integral since there is no S.-.Sinteraction. It may be concluded that the large side-by- side interaction between the adjacent dimeric EDT-TTFCl, molecules permit the mixed-stacking 2 to have a fairly large conductivity.The molecular structure and packing diagram of 3 are shown in Fig. 33 The donor molecules are stacked in a strongly dimeric mode with 'atom-over-atom' overlap: The face-to-face distance between the two donors is cu. 3.5 A which shows a Fig.2 Crystal packing in 2. Overlap integrals ( x lo3)of the conduction orbitals in 2 are: a1 = -0.10; a2= -2.54; cl=0.50; c2= -5.56; ~3= 0.25; ~4 = -1.84 502 J. Muter. Chem., 1996, 6(3), 501-503 CI1 c12 Fig.3 Crystal structure and packing diagram of 3 [S(l)..-S(5) 3.491 (2); S(3)**.S( 5) 3.575( 2); C1(2)*.-S(3) 3.486( 2); S(3)**.S( 2) 3.574(2); C1(2).*.S(5) 3.538(2); S(1)4(4) 3.537(2); S(2)*-*S(3) ~ 3.377(2)A] very close contact between two TTF framework!. The length of the central double bond C(3)-C(4) [ 1.375(5) A] i! compar- able to that of the TTF radical cation7 [ 1.369(4) A] and is fairly elonsated as compared with that of the neutral l4 [1.323(4) A].In a similar manner, the ring double bonds C( 1)7C(2) and C(5)=C(6) are elongated by about 0.01 and 0.04 A, respectively. Thus, the cationic charge is localized at the fulvalene ring containing the ethylenedithio group. In agreement with these results, the perchlorate ion is located on the fulvalene ring containing the ethylenedithio group. This localized cationic charge may cause the donor molecules to be stacked in a head-to-tail dimeric mode. The S(1)--S(4), S(2)-..S(3), and S(5)...C1(2) distanceso in the dimeric structure are 3.537(2), 3.377(2) and 3.538(2) A, which are much less than the S..-Sand S...Cl van der Waals distances.Furthermore, the EDT-TTFC1, molecules interact along the c axis, and many S.-.Scontacts less than the van der Waals distances are observed, as shown in Fig. 3. Although 3 forms as a 1: 1 radical cation salt, the room temperature conductivity of 3is found to be 2.3 x S cm-l. Thus, 3 shows a rather high conductivity as expected for the 1: 1 radical cation salt. These findings suggest that the head- to-tail dimeric contact and the side-by-side s...S interaction in 3 lead this radical cation salt to be a semiconductor. Recently, attention has focused on iodine-bonded n donors in synthetic metals.' However, our results show that chlorine- bonded TTF derivatives may produce a new type of synthetic metal using the Cl...S and Cl...Cl interactions in crystals.We thank Dr. M. Yoshida, Tokyo Metropolitan University for helpful discussions. Financial support by a Grant-in-Aid for Scientific Research on Priority Areas from the Ministry of Education, Science and Culture, Japan (06243 105), is gratefully acknowledged. References A. Gavezzotti, Acc. Chem. Res., 1994, 27, 309; J. A. R. P. Sarma and G. R. Desiraju, Acc. Chem. Res., 1986, 19, 222, and references therein. D. E. Williams and L-Y. Hsu, Acta Crystallog., Sect. A, 1985, 41,296. For the synthesis of chlorinated tetrathiafulvalene, see: M. Jarrgensen and K. Bechgaard, Synthesis, 1989,207; M. R. Bryce and G. Cooke, Synthesis, 1990,263. 4 5 U. Kux, H. Suzuki, S. Sasaki and M. Iyoda, Chem. Lett., 1995,183. For a semiconductor with a mixed-stacking structure, see: R. Kato, H. Kobayashi, A. Kobayashi, T. Naito, M. Tamura, H. Tajima and H. Kuroda, Chem. Lett., 1989,1839. 8 C. Wang, A. Ellern, V. Khodorkovsky, J. Bernstein and J. Y. Becker, J. Chem. SOC., Chem. Commun., 1994, 583; R. Gompper, J. Hock, K. Polborn, E. Dormann and H. Winter, Adu. Muter., 1995, 7, 41; T. Imakubo, H. Sawa and R. Kato, 6 7 H. C.Montgomery, J. Appl. Phys., 1971,42,2971. Crystallogr., Sect. B, 1974,30, 763. T. J. Kistenmacher, T. E. Phillips and D. 0. Cowan, Actu J. Chem. SOC., Chem. Commun., 1995,1097. Communication 5/07442H; Received 13th November, 1995 J. Muter. Chem., 1996, 6(3), 501-503 503

ISSN:0959-9428    

DOI:10.1039/JM9960600501

出版商:RSC    

年代:1996

数据来源: RSC