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Effect of microwave radiation on the formation of Ce2O(CO3)2·H2O in aqueous solution
Solid State Ionics 151 (2002) 347 – 352 www.elsevier.com/locate/ssi
Effect of microwave radiation on the formation of Ce2O(CO3)2H2O in aqueous solution Yasuro Ikuma a,*, Hideyuki Oosawa a, Eriko Shimada a, Michiyo Kamiya b b
a Kanagawa Institute of Technology, Atsugi, Kanagawa 243-0292, Japan Tokyo Institute of Technology, Nagatsuta, Midori, Yokohama 226-8503, Japan
Received 13 February 2001; accepted 16 June 2001
Abstract Reaction between urea and cerium nitrate with/without microwave radiation was investigated. Decomposition of urea occurred in aqueous solution to form NH4OH when the solution was heated to approximately 80 jC. The decomposition products of urea in aqueous solution reacted with cerium nitrate to form cerium oxycarbonate hydrate powder (Ce2O(CO3)2H2O). When the aqueous solution was heated by hot plate to decompose urea, elongated powder of Ce2O(CO3)2H2O with an aspect ratio of approximately 3 was obtained. For the reaction in which the microwave radiation was used to heat the solution for only 5 min, the resulting Ce2O(CO3)2H2O powder was almost identical to the powder that was formed by hot plate heating. However, when the microwave radiation was used for 15 min, the precipitated powder was spherical in shape. These results indicate that the microwave radiation had an effect on the reaction in the aqueous solution. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Microwave radiation; Urea; Cerium nitrate
1. Introduction The microwave oven is used in our daily life to heat or warm our food. However, microwave radiation can also be used to heat ceramic powder compact so as to be sintered without any other source of heat. A number of studies [1 – 11] have described ceramic powders sintered by microwave radiation, and some researchers [4,5,8,11] have reported that sintering by microwave can result in higher density than conventional heating. However, other studies [12] have suggested that microwave heating results in the same
*
Corresponding author.
density as conventional heating as long as results are compared at the same temperature. Samples heated by microwave radiation have an uneven temperature distribution [12]. Since the surface temperature of a sample heated by microwave is lower than that in the interior of the specimen, the uneven temperature distribution may be responsible [12] for the difference in the sintering behavior of samples heated by microwave compared to those heated by the conventional method. Microwave heating can also be used to prepare ceramic powders in aqueous solution. In the present paper, the effect of microwave radiation on the formation of ceramic powder in aqueous solution was investigated. To this end, a homogeneous precipitation
0167-2738/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 2 7 3 8 ( 0 2 ) 0 0 5 3 8 - 6
348
Y. Ikuma et al. / Solid State Ionics 151 (2002) 347–352
Fig. 1. Temperature profiles of water solution which was heated by microwave radiation or a conventional water bath (Series I experiments).
using urea as one of the reactants [13] was selected to prepare ceramic powder (Ce2O(CO3)2H2O) because the solution of urea has to be heated to f 80 jC in order to initiate the reaction with cerium nitrate in solution, and so the effect of microwave radiation can be tested during this process. The solutions of urea and cerium nitrate were heated by microwave radiation or a conventional water bath and the urea decomposed to form NH4OH, which in turn reacted with cerium nitrate to form Ce2O(CO3)2H2O. The final product was examined to determine whether microwave radiation has any effect on the composition or morphology of the reaction products.
2. Experimental Cerium nitrate (Ce(NO3)36H20, Rare Metallic) and urea (Kanto Chemical) were used as starting materials in the present study. Urea was dissolved into deionized water to form a solution of 0.5– 2 mol dm 3. Cerium nitrate was also dissolved into deionized water to form a solution of 0.02 –0.2 mol dm 3. These two solutions were mixed and then heated by two different methods: microwave heating and conventional water bath heating. In the first set of experiments (Series I), the mixture (400 or 100 ml) of urea and cerium nitrate was heated
Fig. 2. Powder X-ray diffraction patterns of Ce2O(CO3)2H2O prepared by Series I experiments.
Y. Ikuma et al. / Solid State Ionics 151 (2002) 347–352
in a beaker with a glass lid from room temperature to 85 jC by either microwave (Hitachi) or hot-water bath. Temperature increase to 85 jC was performed over 5 min in both heating methods. Since no temperature control system was present in the microwave oven (power output of the oven was always 600 W), we measured the temperature of the mixture as a function of time and tried to follow this temperature profile in the water bath method. Temperature profiles of two heating methods are shown in Fig. 1. Although two curves were not identical, we see that the difference in time to reach particular temperature is less
349
than 60 s. After reaching 85 jC, the mixture for both cases was aged at 85 jC for 60 min in a water bath while the mixture was stirred. The following reaction took place during the heating process: ðNH2 Þ2 CO þ H2 O ! 2NH3 þ CO2
ð1Þ
NH3 þ H2 O ! NH4 OH
ð2Þ
The product reacted with cerium nitrate. In the second set of experiments (Series II), the mixture (400 ml) of urea and cerium nitrate was
Fig. 3. Scanning electron micrographs of Ce2O(CO3)2H2O prepared by Series I experiments. The concentration of cerium nitrate was fixed at 0.02 mol dm 3 in these experiments.
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Y. Ikuma et al. / Solid State Ionics 151 (2002) 347–352
Fig. 4. Powder X-ray diffraction patterns of Ce2O(CO3)2H2O prepared by Series II experiments.
heated from room temperature to 100 jC. In this set of experiments, the solution was heated by either microwave or water bath. Temperature increase to 100 jC was performed over 6 min in both heating methods, and the temperature was maintained at 100 jC for 9 min. After 15 min of heating by either microwave or water bath, the mixture was cooled by switching off the heat source. Finally, the solution in both sets of experiments was filtered and the precipitate was examined by powder X-ray diffraction and scanning electron microscopy.
3. Results and discussion The results of powder X-ray diffraction for the ceramic powder formed by Series I experiments are shown in Fig. 2. For specimens formed by both microwave and water bath heating, the XRD patterns can be identified to be Ce2O(CO3)2H2O. Consequently, the reaction during the homogeneous precipitation can be written as 2Ce3þ þ 2OH þ 2CO2 3 ! Ce2 OðCO3 Þ2 H2 O
sition product of urea, in the solution. The figure reveals no remarkable difference between the results. Akinc and Sordelet [13] studied the reaction between cerium carbonate and urea in nitric acid solution and reported that the reaction products were CeCO3OH. Although most of the peaks of the X-ray diffraction patterns shown in Fig. 2 fit the pattern of CeCO3OH [14], all of the peaks1 observed in this study fit more closely the pattern of Ce2O(CO3)2H2O [15]. Consequently, we concluded that these samples were Ce2O(CO3)2H2O. Scanning electron micrographs of Ce2O(CO3)2 H2O formed by Series I experiments are shown in Fig. 3. These results are for a concentration of cerium nitrate fixed at 0.02 mol dm 3. When cerium nitrate concentration was increased to 0.2 mol dm 3, the morphology of the specimen was similar to that shown in Fig. 3 and so is not shown. In Fig. 3, samples were rod-like and no remarkable difference was observed between the micrographs of samples formed by microwave and those formed by water bath, implying that the microwave, when radiated for only 5 min, does not affect the product shape. The morphology of the samples is consistent with the results reported by Akinc and Sordelet [13].
ð3Þ 1
Cerium oxycarbonate hydrate was formed due to the presence of CO32 ions, which were the decompo-
Comparison was made in the range 2h = 10j – 57j because the data of interplanar spacings only in this range were available [14,15].
Y. Ikuma et al. / Solid State Ionics 151 (2002) 347–352
The situation was different in the products of Series II experiments. The results of X-ray diffraction of specimens formed by Series II experiments are shown in Fig. 4. Both X-ray diffraction patterns can be identified to be Ce2O(CO3)2H2O. The most remarkable difference between these two results is that the height of two peaks (2h = 20j and 30j) in the microwave-heated sample was much smaller than that in the conventionally heated powder. Repeated experiments revealed that this difference in X-ray diffraction pat-
351
terns did not always exist. Consequently, further detailed studies are needed to clarify this difference. The differences between these two samples are very clear when the scanning electron micrographs are examined (Fig. 5). The sample prepared by water bath had an irregular shape and was not uniform in size, whereas the sample prepared by microwave was spherical in shape and was very uniform in size. The experiments were repeated several times in this series of experiments. In all cases, Ce2O(CO3)2H2O of spher-
Fig. 5. Scanning electron micrographs of Ce2O(CO3)2H2O prepared by Series II experiments. The concentrations of cerium nitrate and urea were 0.02 and 1 mol dm 3, respectively.
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Y. Ikuma et al. / Solid State Ionics 151 (2002) 347–352
ical shape was obtained when the solution was heated by microwave for 15 min. Microwave heating can be used to form spherical sample. From the results of the Series I experiments, we know that Ce2O(CO3)2H2O has a tendency to grow in one direction: Ce2O(CO3)2H2O has a preferred direction of growth and this causes the powder of Ce2O (CO3)2H2O to be elongated in shape. Most of the particle coarsening (crystal growth) occurred when the solution was aged at 85 jC on the hot plate. This explains the relative similarity of these specimens. In the Series II experiments, most of the particle coarsening occurred within the last 9 min of the experiment, which lasted 15 min, when the temperature was higher (approximately 100 jC). The solubility of product powder may be higher in the Series II experiments because of higher aging temperature compared to the Series I experiments. Then, the difference in microstructure shown in Fig. 5 can be explained by Ostwald ripening theory [16]. When the solution was heated by water bath in the Series II experiments, although the growth rate of powder was very fast compared to that of the Series I experiments, the time was insufficient for the sample to grow as elongated particles. For the reaction of microwave heating (Series II), the powder was formed during the microwave heating. There are several factors that could be changed by microwave and could influence the coarsening process. Equilibrium solubility and surface energy of particles are less likely to be changed by microwave radiation. Interface mobility and diffusivity in the medium could be enhanced by microwave radiation, leading to fast and uniform growth of particles [16]. This is one way to explain the results shown in Fig. 5. However, we do not have any conclusive experimental evidence to support the growth mechanism of spherical particles. The microwave radiation could have influenced other processes such as chemical reaction, etc. Further study is needed to confirm the mechanism concerning the effect of microwave radiation.
4. Conclusions Microwave radiation had an effect on the formation of Ce 2 O(CO 3 ) 2 H 2 O from aqueous solution of Ce(NO3)3 and urea. The product was spherical in shape when the solution was heated by microwave
for 15 min, whereas the product was rod-shaped when the solution was heated by microwave or water bath for only 5 min and subsequently aged by water bath. Acknowledgements The present study was funded in part by a grant from the Japanese Ministry of Education (no. 12650675). References [1] M.K. Krage, Microwave sintering of ferrites, Ceram. Bull. 60 (11) (1981) 1232 – 1234. [2] T.T. Meek, C.E. Holcombe, N. Dykes, Microwave sintering of some oxide materials using sintering aids, J. Mater. Sci. Lett. 6 (1987) 1060 – 1062. [3] J. Wilson, S.M. Kunz, Microwave sintering of partially stabilized zirconia, J. Am. Ceram. Soc. 71 (1) (1988) C-40 – C-41. [4] L.M. Sheppard, Manufacturing ceramics with microwaves: the potential for economical production, Ceram. Bull. 67 (10) (1988) 1656 – 1661. [5] W.H. Sutton, Microwave processing of ceramic materials, Ceram. Bull. 68 (2) (1989) 376 – 386. [6] M. Aliouat, L. Mazo, G. Desgardin, B. Raveau, Microwave sintering of spinel-type oxides, J. Am. Ceram. Soc. 73 (8) (1990) 2515 – 2518. [7] J.D. Katz, R.D. Blake, Microwave sintering of multiple alumina and composite components, Ceram. Bull. 70 (8) (1991) 1304 – 1308. [8] M.A. Janney, C.L. Calhoun, H.D. Kimrey, Microwave sintering of solid oxide fuel cell materials: I, zirconia – 8 mol% yttria, J. Am. Ceram. Soc. 75 (2) (1992) 341 – 346. [9] Y. Ikuma, K. Takahashi, Rapid-rate sintering of ZnO by microwave heating, J. Ceram. Soc. Jpn. 100 (11) (1992) 1327 – 1331. [10] J.J. Thomas, R.J. Christensen, D.L. Johnson, H.M. Jennings, Nonisothermal microwave processing of reaction-bonded silicon nitride, J. Am. Ceram. Soc. 76 (5) (1993) 1384 – 1386. [11] Y. Ikuma, T. Shigemura, Rapid-rate sintering of anatase-type TiO2 by microwave heating, J. Ceram. Soc. Jpn. 101 (8) (1993) 900 – 904. [12] Y. Ikuma, T. Shigemura, T. Hirose, Temperature profile of anatase – TiO2 powder compact during microwave heating, J. Am. Ceram. Soc. 79 (10) (1996) 2533 – 2538. [13] M. Akinc, D. Sordelet, Preparation of yttrium, lanthanum, cerium, and neodymium basic carbonate particles by homogeneous precipitation, Adv. Ceram. Mater. 2 (3A) (1987) 232 – 238. [14] Joint Committee on Powder Diffraction Standards, File No. 41-13. [15] Joint Committee on Powder Diffraction Standards, File No. 43-604. [16] Y.-M. Chiang, D.P. Birnie III, W.D. Kingery, Physical Ceramics, Wiley, New York, 1997, pp. 388 – 391.
Effect of microwave radiation on the formation of Ce2O(CO3)2H2O in aqueous solution Yasuro Ikuma a,*, Hideyuki Oosawa a, Eriko Shimada a, Michiyo Kamiya b b
a Kanagawa Institute of Technology, Atsugi, Kanagawa 243-0292, Japan Tokyo Institute of Technology, Nagatsuta, Midori, Yokohama 226-8503, Japan
Received 13 February 2001; accepted 16 June 2001
Abstract Reaction between urea and cerium nitrate with/without microwave radiation was investigated. Decomposition of urea occurred in aqueous solution to form NH4OH when the solution was heated to approximately 80 jC. The decomposition products of urea in aqueous solution reacted with cerium nitrate to form cerium oxycarbonate hydrate powder (Ce2O(CO3)2H2O). When the aqueous solution was heated by hot plate to decompose urea, elongated powder of Ce2O(CO3)2H2O with an aspect ratio of approximately 3 was obtained. For the reaction in which the microwave radiation was used to heat the solution for only 5 min, the resulting Ce2O(CO3)2H2O powder was almost identical to the powder that was formed by hot plate heating. However, when the microwave radiation was used for 15 min, the precipitated powder was spherical in shape. These results indicate that the microwave radiation had an effect on the reaction in the aqueous solution. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Microwave radiation; Urea; Cerium nitrate
1. Introduction The microwave oven is used in our daily life to heat or warm our food. However, microwave radiation can also be used to heat ceramic powder compact so as to be sintered without any other source of heat. A number of studies [1 – 11] have described ceramic powders sintered by microwave radiation, and some researchers [4,5,8,11] have reported that sintering by microwave can result in higher density than conventional heating. However, other studies [12] have suggested that microwave heating results in the same
*
Corresponding author.
density as conventional heating as long as results are compared at the same temperature. Samples heated by microwave radiation have an uneven temperature distribution [12]. Since the surface temperature of a sample heated by microwave is lower than that in the interior of the specimen, the uneven temperature distribution may be responsible [12] for the difference in the sintering behavior of samples heated by microwave compared to those heated by the conventional method. Microwave heating can also be used to prepare ceramic powders in aqueous solution. In the present paper, the effect of microwave radiation on the formation of ceramic powder in aqueous solution was investigated. To this end, a homogeneous precipitation
0167-2738/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 2 7 3 8 ( 0 2 ) 0 0 5 3 8 - 6
348
Y. Ikuma et al. / Solid State Ionics 151 (2002) 347–352
Fig. 1. Temperature profiles of water solution which was heated by microwave radiation or a conventional water bath (Series I experiments).
using urea as one of the reactants [13] was selected to prepare ceramic powder (Ce2O(CO3)2H2O) because the solution of urea has to be heated to f 80 jC in order to initiate the reaction with cerium nitrate in solution, and so the effect of microwave radiation can be tested during this process. The solutions of urea and cerium nitrate were heated by microwave radiation or a conventional water bath and the urea decomposed to form NH4OH, which in turn reacted with cerium nitrate to form Ce2O(CO3)2H2O. The final product was examined to determine whether microwave radiation has any effect on the composition or morphology of the reaction products.
2. Experimental Cerium nitrate (Ce(NO3)36H20, Rare Metallic) and urea (Kanto Chemical) were used as starting materials in the present study. Urea was dissolved into deionized water to form a solution of 0.5– 2 mol dm 3. Cerium nitrate was also dissolved into deionized water to form a solution of 0.02 –0.2 mol dm 3. These two solutions were mixed and then heated by two different methods: microwave heating and conventional water bath heating. In the first set of experiments (Series I), the mixture (400 or 100 ml) of urea and cerium nitrate was heated
Fig. 2. Powder X-ray diffraction patterns of Ce2O(CO3)2H2O prepared by Series I experiments.
Y. Ikuma et al. / Solid State Ionics 151 (2002) 347–352
in a beaker with a glass lid from room temperature to 85 jC by either microwave (Hitachi) or hot-water bath. Temperature increase to 85 jC was performed over 5 min in both heating methods. Since no temperature control system was present in the microwave oven (power output of the oven was always 600 W), we measured the temperature of the mixture as a function of time and tried to follow this temperature profile in the water bath method. Temperature profiles of two heating methods are shown in Fig. 1. Although two curves were not identical, we see that the difference in time to reach particular temperature is less
349
than 60 s. After reaching 85 jC, the mixture for both cases was aged at 85 jC for 60 min in a water bath while the mixture was stirred. The following reaction took place during the heating process: ðNH2 Þ2 CO þ H2 O ! 2NH3 þ CO2
ð1Þ
NH3 þ H2 O ! NH4 OH
ð2Þ
The product reacted with cerium nitrate. In the second set of experiments (Series II), the mixture (400 ml) of urea and cerium nitrate was
Fig. 3. Scanning electron micrographs of Ce2O(CO3)2H2O prepared by Series I experiments. The concentration of cerium nitrate was fixed at 0.02 mol dm 3 in these experiments.
350
Y. Ikuma et al. / Solid State Ionics 151 (2002) 347–352
Fig. 4. Powder X-ray diffraction patterns of Ce2O(CO3)2H2O prepared by Series II experiments.
heated from room temperature to 100 jC. In this set of experiments, the solution was heated by either microwave or water bath. Temperature increase to 100 jC was performed over 6 min in both heating methods, and the temperature was maintained at 100 jC for 9 min. After 15 min of heating by either microwave or water bath, the mixture was cooled by switching off the heat source. Finally, the solution in both sets of experiments was filtered and the precipitate was examined by powder X-ray diffraction and scanning electron microscopy.
3. Results and discussion The results of powder X-ray diffraction for the ceramic powder formed by Series I experiments are shown in Fig. 2. For specimens formed by both microwave and water bath heating, the XRD patterns can be identified to be Ce2O(CO3)2H2O. Consequently, the reaction during the homogeneous precipitation can be written as 2Ce3þ þ 2OH þ 2CO2 3 ! Ce2 OðCO3 Þ2 H2 O
sition product of urea, in the solution. The figure reveals no remarkable difference between the results. Akinc and Sordelet [13] studied the reaction between cerium carbonate and urea in nitric acid solution and reported that the reaction products were CeCO3OH. Although most of the peaks of the X-ray diffraction patterns shown in Fig. 2 fit the pattern of CeCO3OH [14], all of the peaks1 observed in this study fit more closely the pattern of Ce2O(CO3)2H2O [15]. Consequently, we concluded that these samples were Ce2O(CO3)2H2O. Scanning electron micrographs of Ce2O(CO3)2 H2O formed by Series I experiments are shown in Fig. 3. These results are for a concentration of cerium nitrate fixed at 0.02 mol dm 3. When cerium nitrate concentration was increased to 0.2 mol dm 3, the morphology of the specimen was similar to that shown in Fig. 3 and so is not shown. In Fig. 3, samples were rod-like and no remarkable difference was observed between the micrographs of samples formed by microwave and those formed by water bath, implying that the microwave, when radiated for only 5 min, does not affect the product shape. The morphology of the samples is consistent with the results reported by Akinc and Sordelet [13].
ð3Þ 1
Cerium oxycarbonate hydrate was formed due to the presence of CO32 ions, which were the decompo-
Comparison was made in the range 2h = 10j – 57j because the data of interplanar spacings only in this range were available [14,15].
Y. Ikuma et al. / Solid State Ionics 151 (2002) 347–352
The situation was different in the products of Series II experiments. The results of X-ray diffraction of specimens formed by Series II experiments are shown in Fig. 4. Both X-ray diffraction patterns can be identified to be Ce2O(CO3)2H2O. The most remarkable difference between these two results is that the height of two peaks (2h = 20j and 30j) in the microwave-heated sample was much smaller than that in the conventionally heated powder. Repeated experiments revealed that this difference in X-ray diffraction pat-
351
terns did not always exist. Consequently, further detailed studies are needed to clarify this difference. The differences between these two samples are very clear when the scanning electron micrographs are examined (Fig. 5). The sample prepared by water bath had an irregular shape and was not uniform in size, whereas the sample prepared by microwave was spherical in shape and was very uniform in size. The experiments were repeated several times in this series of experiments. In all cases, Ce2O(CO3)2H2O of spher-
Fig. 5. Scanning electron micrographs of Ce2O(CO3)2H2O prepared by Series II experiments. The concentrations of cerium nitrate and urea were 0.02 and 1 mol dm 3, respectively.
352
Y. Ikuma et al. / Solid State Ionics 151 (2002) 347–352
ical shape was obtained when the solution was heated by microwave for 15 min. Microwave heating can be used to form spherical sample. From the results of the Series I experiments, we know that Ce2O(CO3)2H2O has a tendency to grow in one direction: Ce2O(CO3)2H2O has a preferred direction of growth and this causes the powder of Ce2O (CO3)2H2O to be elongated in shape. Most of the particle coarsening (crystal growth) occurred when the solution was aged at 85 jC on the hot plate. This explains the relative similarity of these specimens. In the Series II experiments, most of the particle coarsening occurred within the last 9 min of the experiment, which lasted 15 min, when the temperature was higher (approximately 100 jC). The solubility of product powder may be higher in the Series II experiments because of higher aging temperature compared to the Series I experiments. Then, the difference in microstructure shown in Fig. 5 can be explained by Ostwald ripening theory [16]. When the solution was heated by water bath in the Series II experiments, although the growth rate of powder was very fast compared to that of the Series I experiments, the time was insufficient for the sample to grow as elongated particles. For the reaction of microwave heating (Series II), the powder was formed during the microwave heating. There are several factors that could be changed by microwave and could influence the coarsening process. Equilibrium solubility and surface energy of particles are less likely to be changed by microwave radiation. Interface mobility and diffusivity in the medium could be enhanced by microwave radiation, leading to fast and uniform growth of particles [16]. This is one way to explain the results shown in Fig. 5. However, we do not have any conclusive experimental evidence to support the growth mechanism of spherical particles. The microwave radiation could have influenced other processes such as chemical reaction, etc. Further study is needed to confirm the mechanism concerning the effect of microwave radiation.
4. Conclusions Microwave radiation had an effect on the formation of Ce 2 O(CO 3 ) 2 H 2 O from aqueous solution of Ce(NO3)3 and urea. The product was spherical in shape when the solution was heated by microwave
for 15 min, whereas the product was rod-shaped when the solution was heated by microwave or water bath for only 5 min and subsequently aged by water bath. Acknowledgements The present study was funded in part by a grant from the Japanese Ministry of Education (no. 12650675). References [1] M.K. Krage, Microwave sintering of ferrites, Ceram. Bull. 60 (11) (1981) 1232 – 1234. [2] T.T. Meek, C.E. Holcombe, N. Dykes, Microwave sintering of some oxide materials using sintering aids, J. Mater. Sci. Lett. 6 (1987) 1060 – 1062. [3] J. Wilson, S.M. Kunz, Microwave sintering of partially stabilized zirconia, J. Am. Ceram. Soc. 71 (1) (1988) C-40 – C-41. [4] L.M. Sheppard, Manufacturing ceramics with microwaves: the potential for economical production, Ceram. Bull. 67 (10) (1988) 1656 – 1661. [5] W.H. Sutton, Microwave processing of ceramic materials, Ceram. Bull. 68 (2) (1989) 376 – 386. [6] M. Aliouat, L. Mazo, G. Desgardin, B. Raveau, Microwave sintering of spinel-type oxides, J. Am. Ceram. Soc. 73 (8) (1990) 2515 – 2518. [7] J.D. Katz, R.D. Blake, Microwave sintering of multiple alumina and composite components, Ceram. Bull. 70 (8) (1991) 1304 – 1308. [8] M.A. Janney, C.L. Calhoun, H.D. Kimrey, Microwave sintering of solid oxide fuel cell materials: I, zirconia – 8 mol% yttria, J. Am. Ceram. Soc. 75 (2) (1992) 341 – 346. [9] Y. Ikuma, K. Takahashi, Rapid-rate sintering of ZnO by microwave heating, J. Ceram. Soc. Jpn. 100 (11) (1992) 1327 – 1331. [10] J.J. Thomas, R.J. Christensen, D.L. Johnson, H.M. Jennings, Nonisothermal microwave processing of reaction-bonded silicon nitride, J. Am. Ceram. Soc. 76 (5) (1993) 1384 – 1386. [11] Y. Ikuma, T. Shigemura, Rapid-rate sintering of anatase-type TiO2 by microwave heating, J. Ceram. Soc. Jpn. 101 (8) (1993) 900 – 904. [12] Y. Ikuma, T. Shigemura, T. Hirose, Temperature profile of anatase – TiO2 powder compact during microwave heating, J. Am. Ceram. Soc. 79 (10) (1996) 2533 – 2538. [13] M. Akinc, D. Sordelet, Preparation of yttrium, lanthanum, cerium, and neodymium basic carbonate particles by homogeneous precipitation, Adv. Ceram. Mater. 2 (3A) (1987) 232 – 238. [14] Joint Committee on Powder Diffraction Standards, File No. 41-13. [15] Joint Committee on Powder Diffraction Standards, File No. 43-604. [16] Y.-M. Chiang, D.P. Birnie III, W.D. Kingery, Physical Ceramics, Wiley, New York, 1997, pp. 388 – 391.