Microwave assisted preparation and sintering of Al2O3, ZrO2 and their composites from metalorganics Bhuvaragasamy G. Ravi, Peelamedu D. Ramesh, Navneet Gupta† and Kalya J. Rao* Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012, India Controlled pyrolysis of Al(OBus)3, Zr(OPrn)4 and their mixtures in ethyl acetate induced using microwaves of 2.45 GHz frequency has been carried out.Microwave irradiation yields second-stage precursors for the preparation of respective oxides and their composites. It is observed that the microwave irradiation has a directive influence on the morphology of the ultimate oxide products. Al2O3, ZrO2 and the two composites 90% Al2O3–10% ZrO2 and 90% ZrO2–10% Al2O3 are also found to be sintered to very high densities within 35 min of microwave irradiation by the use of b-SiC as a secondary susceptor.Use of microwaves for the synthesis and processing of ceramic preparation of the composites calculated quantities of the precursors were mixed in ethyl acetate and a uniform solution materials has attracted much attention recently. Microwave methods oVer several advantages such as unique synthetic was made.However for the preparation of pure Al2O3 and ZrO2, the precursors were used directly. Precursors were pathways, rapid heating rates, short processing durations, low power requirements, product uniformity, etc.1–9 We have placed in Borosil glass beakers and in the first stage of the experiment precursors were decomposed by microwave reported rapid and clean methods of synthesis of b-SiC10 AlN,11 MoO2,12 etc.using microwaves. We have also achieved irradiation in the oven for brief periods. Within a few minutes of heating (see later) we observed a rapid increase in the very rapid sintering of ZrO2–CeO2 ceramics using microwave irradiation.13 Metalorganics such as metal alkoxides, amides, viscosity of the precursors leading to formation of a foam-like product in the beakers.Further exposure to microwaves did imides, esters, etc. are generally employed as precursors for preparation of high purity ceramic powders. Use of alkoxides not cause any change in the foam-like products. There was no further increase in temperature or further decomposition. The for the preparation of oxides through sol–gel techniques is widely established.14 We have found that direct pyrolysis of oven was switched oV and the foam-like powder was collected from the beaker.metalorganics by conventional heating is unsuitable for powder production because it gives rise to non-uniform powder mor- Powders were characterized by X-ray diVraction studies (Philips X-ray DiVractometer, model PW 1050/70, Cu-Ka phology.Several other technological problems have been reported to be associated with complete combustion using radiation, l=1.5418 A ° ), thermogravimetry and diVerential thermal analysis (CAHN Instruments, California, USA, heat- conventional heating methods.15 However, when microwave irradiation is employed, very rapid and volumetric heating of ing rate 10 K min-1, in air) and particle size distribution (Micrometer Photosize, SKC 2000) (Table 1).metalorganic liquids is achieved. This can be expected to result in the formation of products of good structural and morpho- In the second stage, the powders were heated to 1673 K (only to 1473 K in the case of Al2O3) in air, with a heating logical uniformity. Microwave pyrolysis is known to be superior to conventional heating as it is self-regulating (micro- rate of 10 K min-1 in a conventional furnace (Thermolyne 46100).X-Ray diVraction studies were also performed on wave absorption characteristics change drastically once the susceptor is chemically altered) and therefore products having powders recovered from various temperatures. Transmission electron microscopy (JEOL-200CX TEM) was used to charac- uniform particle size distribution can be expected to form.Further microwave heating can be used with advantage in the terize the completely crystallized composites. The composite powders were ultrasonically dispersed in acetone medium. A preparation of ceramic composites because of the diVerential susceptibilities (and hence the heating rates) of the diVerent drop of the suspension was placed on the holey carbon film, supported on a copper grid and the bright field images precursors used in their preparations to generate unique morphologies.were recorded. In the third stage, the crystallized powders were subjected There is, however, very little known in the literature with regard to the use of microwaves in the preparation of ceramic to sintering again in the microwave oven by a method described earlier13 using b-SiC as a secondary heater.Sintered products powders and composites starting from organic precursors. In this paper we report our investigation on the unique influence were characterized using scanning electron microscopy (Cambridge Instruments, Stereoscan 360). of microwave irradiation of organic precursors during the preparation of ceramic composites of ZrO2 and Al2O3.We have also examined the sintering of Al2O3–ZrO2 composites by microwave irradiation and using a secondary heater. Results and Discussion Pure alkoxides Experimental We first consider the eVect of microwave irradiation of the The experimental set-up used for microwave heating is an metalorganics. Both Al(OBus)3 (ASB) and Zr(OPrn)4 (ZIP) ordinary kitchen microwave oven (Batliboi Eddy, India) appear to produce fumes when irradiated and become viscous operating at a frequency of 2.45 GHz and a maximum output liquids.This results in the formation of foam-like solids desigpower level of 980 W. We have chosen as organic precursors nated ASB-F and ZIP-F respectively. The products were Al(OBus)3 and Zr(OPrn)4 (Fluka AG, H-9470, Buchs).For the scraped from the beakers and the scraped materials were powdery (very fine particles) and are produced by irradiation after only a few minutes. Combined TG-DTA of the powders † Undergradute Research Scholar under linkages programme. J. Mater. Chem., 1997, 7(10), 2043–2048 2043Table 1 Microwave preparation conditions and the compounds formed particle starting material exposure durationa/min product colour size/mm Al(OBus)3 7+7+7+7 Al2O3 white 0.72 Zr(OPrn)4 7+7 ZrO2 yellowish white 0.41 Al(OBus)3+Zr(OPrn)4+MeCo2Et 7+7+7+7 90Al2O3–10ZrO2 white 0.75 +MeCo2Et Al(OBus)3+Zr(OPrn)4+MeCO2Et 7+7 10Al2O3–90ZrO2 yellowish white 0.73 aThe microwave power (560 W) was briefly interrupted every 7 min to examine the nature of the contents in the beaker. Fig. 1 Combined TG-DTA of microwave obtained powders of (a) Fig. 2 X-Ray diVractogram of ASB-F heated to various temperatures ASB-F and (b) ZIP-F (see text for definition of ASB-F and ZIP-F) (Cu-Ka radiation, l=1.5418 A ° ; #, a-Al2O3; *, c-Al2O3) is shown in Fig. 1. The loss of mass for ASB-F is gradual and most of the mass loss (25%) occurs below 500 K, corresponding to the first fairly large endotherm in the DTA.The remaining part of the mass loss occurs well below 873 K which corresponds to the region of a minor exotherm in DTA. The mass of the residual product is 65% of the original and remains constant up to 1473 K. The DTA suggests that there are two smaller exotherms in this region occurring at 1173 and 1273 K, respectively. For ZIP-F the first endotherm is rather shallow and extends to ca. 493 K and the mass loss is ca. 21%. The remaining mass loss occurs in small shallow steps and no further loss occurs beyond 1173 K. These step-like losses are associated with one large initial and two smaller subsequent exotherms. The first endotherm observed during heating of both ASBF and ZIP-F can be associated with the loss of water (H2O).The total mass loss for ASB-F (35%) is almost equal (34.6%) to the mass loss expected if ASB-F is Al(OH)3. The negligible diVerence may be due to residual organics which are burnt out in the region 473–673 K. However, for ZIP-F the exotherm in this region is large and the final yield is only 67%, which is much less than would be expected if ZIP-F corresponded to Zr(OH)4 (77.4%).Thus, microwave-irradiated material ZIP-F contains a significant proportion of undecomposed organics. It was noted in the IR spectrum (not shown) that there was a significant amount of residual ZIP in the ZIP-F. The spectra Fig. 3 X-Ray diVractogram of ZIP-F heated to various temperatures of both ASB-F and ZIP-F exhibited H2O related absorptions (Cu-Ka radiation, l=1.5418 A° ).(a) 1673 K, (b) 843 K, (c) 743 K, at 3300 and 1600 cm-1. (d) 623 K. We have examined XRD of the initial and heated powders of ASB-F and ZIP-F. Both are amorphous up to 623 K (Fig. 2 and 3). For ASB-F the XRD of the sample heated to 1273 K a-Al2O3 since samples cooled from 1473 K gave XRD typical of a-Al2O3 (Fig. 2) only. Generally, under conventional con- ( just above the second exotherm in DTA of Fig. 1) was found to be crystalline and could be indexed to a mixture of a- and ditions, the transition of c- to a-Al2O3 occurs at 1373 K.16 For ZIP-F, however, the major product was found to be cubic (c) c-Al2O3. A cubic unit cell (a=7.90 A° ) and a trigonal unit cell (a=b=4.758 A ° , c=12.991 A ° ) were used to index the c- and ZrO2 at 843 K [with monoclinic (m) ZrO2 as a minor phase].A cubic unit cell (a=5.09 A ° ) was used to index cubic ZrO2 a-Al2O3 structures respectively. The high-temperature small exotherm around 1373 K observed for alumina can be associ- while a monoclinic unit cell (a=5.146 A ° , b=5.213 A ° , c= 5.311 A ° , b=99.20) was used to index monoclinic ZrO2. ated with a transition of the small amount of c-Al2O3 to 2044 J.Mater. Chem., 1997, 7(10), 2043–2048Conventionally, under atmospheric conditions, ZrO2 the two Al2O3–ZrO2 composites examined were 90Al2O3–10ZrO2 (CA) and 90ZrO2–10Al2O3 (CZ) respectively. undergoes the following transformation sequences before it melts,17 We found that the time required to obtain foam-like powders (second-stage precursors) was 28 min for the alumina-rich mixture (CA-F) and 14 min for the zirconia-rich mixture monoclinic CA 1443 K tetragonal CA 2646 K cubic CA 2953 K liquid (CZ-F).The powders CA-F and CZ-F were then directly heated to A metastable cubic phase of ZrO2 was also observed at 673 K 1673 K and kept at that temperature for 2 h. The X-ray before the formation of stable monoclinic ZrO2 at 1273 K in diVraction patterns of the resulting oxide composites are shown crystallization studies of zirconia gels.18 in Fig. 4. It is evident that, upon heating, CA-F gives a The crystallization appears to start around 700 K, just at composite product which consists of tetragonal ZrO2 particles the beginning of the first of the twin peaked exotherms in the (a tetragonal unit cell with a=b=5.12 A ° and c=5.25 A ° was DTA (Fig. 1). In fact, the XRD at 743 K (middle of the twin used to index tetragonal ZrO2) in an a-Al2O3 matrix while exothermic peaks) reveals incomplete crystallization. Samples CZ-F gives an a-Al2O3 dispersion in monoclinic ZrO2. heated to 1673 K revealed that only monoclinic ZrO2 (Fig. 3) Fig. 5 shows transmission electron micrographs of CA and was present. We are therefore led to believe that the exotherm CZ composites.In the TEM of the CA composite, ZrO2 at 1173 K is associated with a cubic to monoclinic transformparticles (dark) are seen to be finely dispersed in a-Al2O3 ation of ZrO2 while the exotherm in the region 723–773 K matrix. The diameters of ZrO2 particles vary from 3 to 35 nm. could be largely due to burning (oxidation) of either strongly On the other hand in CZ composites only large crystallites of attached organic fragments or oxidation of any residual carbon ZrO2 were seen and Al2O3 particles could not be identified formed during the rapid initial combustion of organics in with certainty.The X-ray evidence from Fig. 4 however clearly insuYcient oxygen. suggests that formation of a ZrO2–Al2O3 solid solution does The above observation suggests that the alkoxides ASB and not occur.We do not consider the possibility that CA-F and ZIP do not decompose in a single step to their respective CZ-F are actually solid solutions which decompose to give oxides by microwave irradiation. The extent of residual rise to composites at higher temperatures because there are no organics in the amorphous powder products ASB-F and ZIPreports of such ready formation of solid solutions during F are also diVerent (higher in ZIP-F) at the stage where thermal decomposition of alkoxide mixtures.microwave interaction almost ceases. Both ASB-F and ZIP-F We therefore note that microwave heating of the alkoxides contain some water and their masses suggest that there is influences the nature of the final products. This is due to enough for formation of Al(OH)3 (AlOOH H2O?) or Zr(OH)4 diVerential microwave susceptibilities and to a certain extent [ZrO (OH)2 H2O? or ZrO2 2H2O?]. In the second stage of diVerences in quantities of undecomposed organics. It has been furnace heating ASB-F and ZIP-F are converted to oxides at observed by Yoshimatsu et al.19 that a fine distribution of higher temperatures. The various steps leading to the formation ZrO2 particles can result by decomposition of a zircanoalumin- of oxides are visualized as follows.ium compound under optimum conditions. The procedure of Yoshimatsu et al.19 and the present procedure can not be Al(OBus)3 CA microwaves [Al(OH)3(AlOOH H2O?)+ compared although the product morphologies are similar.The composite preparation is achieved here starting from homogeneous liquid mixtures of alkoxides and therefore a dehomo- little organic residue] CA 623 K amorphous Al2O3 genization step must be involved in the formation of the observed composite. This we attribute tentatively to the diVer- CA 1273 K [a-Al2O3+c-Al2O3] CA 1473 K a-Al2O3 ences in the microwave characteristics.Microwave sintering Zr(OPrn)4 CA microwaves [Zr(OH)4? (or ZrO (OH)2 H2O Another focus of this work has been to show that ceramics which are normally microwave inert at low temperatures such or ZrO2 2H2O ?)+organic residue] CA 623 K as Al2O3, ZrO2 and ZrO2–Al2O3 can still be sintered remarkably rapidly in microwaves by a technique which employs a amorphous ZrO2 CA 843 K [c-ZrO2+m-ZrO2] CA 1173 K m-ZrO2 Thus ASB-F and ZIP-F are actually second-stage precursors for the formation of oxides.Mixture of alkoxides We have also noted in our experiment that the time required to obtain the dry foam-like product ASB-F was 28 min compared to only 14 min for ZIP-F. We attribute this to the significant diVerence in the microwave susceptibilities of two alkoxides themselves since the decomposition products do not couple to microwaves eVectively.However the diVerence may also arise from diVerent thermal stabilities of the alkoxides. The presence of a high level of remnant organics in ZIP (which decomposes faster) suggests that thermal instability may not be the cause. On both counts we expect microwave decomposition of the alkoxide mixture to produce a mixture of fine amorphous powders since the microwave assisted decompo- Fig. 4 X-Ray diVractogram of 90Al2O3–10ZrO2 (CA) and sition rates are diVerent. The tendency of ZIP to be associated 90ZrO2–10Al2O3 (CZ) composites heat-treated at 1673 K for 2 h (Cuwith a greater proportion of remnant organics may also help Ka radiation, l=1.5418 A° ). A=Al2O3; Z=monoclinic ZrO2; ZT= tetragonal ZrO2.diVerentiation of ASB-F and ZIP-F phases. Compositions of J. Mater. Chem., 1997, 7(10), 2043–2048 2045Pt–Pt13%Rh thermocouple. The maximum temperatures of the pellets were usually attained after ca. 15 min of exposure and are given in Table 2. The densities of the pellets were also measured after 35 min of sintering (Table 2). Pure Al2O3 could be sintered to 95% of its theoretical density in under 35 min using microwaves: this, we believe, is extraordinary.Pure ZrO2 pellets sintered well but developed visible cracks upon cooling due to the well known tetragonal to monoclinic transformation. Pellets of Al2O3–ZrO2 composites also sintered well but to a maximum of only ca. 90% theoretical density. Alumina can be sintered to a high density when a small amount of MgO is added.20 Addition of 3% Y2O3 with zirconia resulted in tetragonal zirconia polycrystals (TZP) having very good mechanical properties.21 The eVect of adding MgO to Al2O3 and Y2O3 to ZrO2 on the microwave sintering was examined.Using sintering durations as above with the addition of 3% MgO as a sintering aid, Al2O3 was found to sinter to 97% of its theoretical density.ZrO2 also sintered to 98% of its theoretical density with the addition of 3% Y2O3. SEM micrographs of sintered Al2O3 (without and with 3% MgO), ZrO2 and the two composites (both with 3% Y2O3) are shown in Fig. 6 and the corresponding densities, sintering temperatures and other relevant data are given in Table 2. The flaky morphology of the sintered high density Al2O3 is similar in both cases, with or without the additives, and agrees well with the microstructures reported in the literature.9 The microstructure of ZrO2 consists of multifaceted particles with virtually complete elimination of enclosed porosity accounting for its high density.The SEM images of the sintered composites are quite similar to the morphology of the major components in the composite in their respective pure states.Particles of the two diVerent phases in the composites, however, are not so clearly distinguished. Fig. 5 TEMs of (a) CA composite (dispersion of ZrO2 particles is seen clearly in the Al2O3 matrix), and(b) CZ composite Conclusions secondary heater. In this ‘hitch-hiking’ approach low-temperature microwave inert materials are embedded in other strongly Two observations of this work are significant to ceramic microwave active materials (secondary heater) and subjected science and both are related to the use of microwaves. First to microwave irradiation.The critical requirement is that the microwave irradiation has a direct influence on the structure material of the secondary heater is chemically unreactive with of the initial powders obtained from metalorganic mixtures the material to be sintered.In the present case Al2O3, ZrO2 used as precursors in the preparation of composites. This and Al2O3–ZrO2 composite samples were first pelletized under eliminates the need to employ the rather slow and tedious 196 MN m-2 pressure to obtain 10 mm diameter pellets of sol–gel route to make composites starting from metalorganics.approximately 3 mm thickness using 1% poly (vinyl alcohol) Also, the metalorganics upon microwave irradiation yield as a binder. These pellets were surrounded by b-SiC powder second-stage precursors for the preparation of ceramic comin a silica crucible (ca. 10g of b-SiC is suYcient) and the posites. Secondly, the rate of sintering of composites in microcrucible was placed inside a microwave oven.In the present waves is fascinating in the novel ‘hitch-hiking’ heating process set-up 35 min was required for complete sintering of the pellets. where use is made of a secondary microwave susceptor. It is Details of the microwave sintering procedure with the use possible to achieve very high densities of products in extremely of the secondary heater has been described earlier.13 The short times.temperature of b-SiC used as a secondary heater reaches initially about 1073 K. The temperature of the pellet also rises, as a consequence, to nearly 1000 K. Around this temperature The authors acknowledge the help of Dr. G. N. Subbanna and Mr. Sam Philip for carrying out electron microscopy experi- the ceramic materials themselves couple well to microwaves.EVorts were made to monitor temperatures of the pellet surface ments. The Authors also thank the European Commission for financial assistance. by quickly interrupting the microwaves and inserting a Table 2 Data related to sinteringa of Al2O3, ZrO2 and their composites by microwave irradiation green sintered relative sample density/g cm-3 density/g cm-3 density(%) Al2O3 2.60 3.78 95 Al2O3+3%MgO 2.58 3.85 97 ZrO2 3.80 5.65 96 ZrO2+3%Y2O3 3.81 5.73 98 (90Al2O3–10ZrO2)+3% Y2O3 2.70 3.89 95 (90ZrO2–10Al2O3)+3%Y2O3 3.56 5.4 96 aMicrowave power=980 W, sintering duration=35 min, maximum temperature=1773 K. 2046 J. Mater. Chem., 1997, 7(10), 2043–2048Fig. 6 SEMs of freshly fractured surfaces of microwave sintered (a) Al2O3, (b) Al2O3 (3% MgO), (c) ZrO2 (3% Y2O3), (d) 90Al2O3–10ZrO2 and (e) 90ZrO2–10Al2O3 composites Materials, ed.W. H. Sutton, M. H. Brooks and I. J. Chabinsky, References Materials Research Society, Pittsburgh, PA, 1988, vol. 124, p. 235. 1 D. R. Baghurst and D. M. P. Mingos, J. Chem. Soc., Chem. 6 E. J. A. Pope, Am. Ceram. Soc. 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