~~~~~~ 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. 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