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Chemical synthesis and thermal evolution of MgAl2O4 spinel precursor prepared from industrial gibbsite and magnesia powder
September 2002
Materials Letters 56 (2002) 97 – 101 www.elsevier.com/locate/matlet
Chemical synthesis and thermal evolution of MgAl2O4 spinel precursor prepared from industrial gibbsite and magnesia powder H. Revero´n a,*, D. Gutie´rrez-Camposa,1, R.M. Rodrı´guez b,2, J.C. Bonassin b,2 a
Department of Materials Science, Universidad Simo´n Bolı´var, Caracas 1080-A, Venezuela b Department of Chemistry, Universidad Metropolitana, Caracas, Venezuela Received 20 August 2001; accepted 21 November 2001
Abstract A chemical route, using industrial raw materials, was developed for the synthesis of MgAl2O4 spinel precursor. The asobtained powder was then calcined in air, up to 1200 jC for a period of 1 h. It was found that spinel single phase is formed at several hundred degrees lower than temperatures reported for the conventional powder preparation methods. Moreover, at 500 jC the degree of crystallinity is higher than that reported by other chemical processes at this temperature. The calcined powder reaches 83% of relative density at 1200 jC, indicating reactivity and good sintering behaviour. Scanning electron microscopy images revealed that the product consisted of fine spherical particles of magnesium aluminate spinel. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Co-precipitation; Gibbsite; Magnesia; Spinel; MgAl2O4; Chemical synthesis
1. Introduction Magnesium aluminate spinel, which is the only stable compound in the MgO – Al2O3 system [1], has long been considered an important ceramic material. Many studies have reported its properties, applications and different processing methods [2 –9]. As a ceramic material, MgAl2O4 has mainly been used for structural applications, like refractory linings, due to its superior mechanical, chemical and thermal properties [10 – 18]. Nowadays, new potential uses have been reported, *
Corresponding author. Tel.: +58-212-906-41-70; fax: +58212-906-39-32. E-mail addresses: [email protected] (H. Revero´n), [email protected] (D. Gutie´rrez-Campos), [email protected] (R.M. Rodrı´guez). 1 Tel.: +58-212-906-41-70; fax: +58-212-906-39-32. 2 Tel.: +58-212-241-48-33; fax: +58-212-241-75-95.
including: ceramic ultra filtration membranes, electro-insulators and optical materials [19 – 22]. On the other hand, it is well known that structure and properties of materials may be tailored via processing control. Preparing a fine and agglomerate/aggregate-free ceramic powder is the first, and perhaps the most important, step in obtaining a sintered ceramic of desirable microstructure, and therefore performance. Pure magnesium aluminate spinel has been prepared, in commercial processes, by solidstate reaction of high purity magnesia and alumina powders, heated for extended period of times at temperatures exceeding 1800 jC [23]. However, polycrystalline spinel ceramics prepared by conventional powder processing are difficult to sinter to full density at relatively low temperature. Low sintering temperatures for ultra-fine powders have been observed in a number of studies [24,25]. Various chem-
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X ( 0 2 ) 0 0 4 2 5 - 1
98
H. Revero´n et al. / Materials Letters 56 (2002) 97–101
istry-based novel approaches have been engineered to improve spinel powders sinterability, including sol – gel, freeze – drying, spray drying, complexation and co-precipitation [26 – 29]. Nevertheless, the quality of the final material depends mainly on the properties of the starting powder, such as purity and chemical homogeneity. In this way, the objective of the present work is to obtain a spinel precursor by a chemical route, using industrial raw materials, like gibbsite from a Bayer process and a refractory-grade deadburned magnesia. In addition, thermal evolution and microstructural features of the spinel precursor are presented.
2. Experimental procedures Prior to the preparation of the spinel precursor, the industrial gibbsite (Al(OH)3) was characterized by Xray diffraction (XRD), employing a PW 2525 Phillips diffractometer with Cu-Ka radiation, in order to identify the existing phases. Also, the Al(OH)3 powder was washed twice in de-ionized water at 70 jC, to remove sodium impurities introduced during the Bayer process. The sodium content, before and after washing, was measured by atomic absorption, with a Perkin Elmer 31,000 spectrophotometer. The MgO refractory-grade powder was ground in a ball mill, in which particle size was reduced to less than 38 Am in diameter. This raw material was also analyzed by XRD, with the same equipment and conditions used for the gibbsite powder. The synthesis of the spinel precursor was carried out following a co-precipitation route, schematically shown in Fig. 1. First, the gibbsite powder was dissolved in HCl/HNO3 solution and refluxing was performed for 5 h at 80 jC. Details of gibbsite dissolution procedure have been reported elsewhere [30]. MgO powder was added in a 2:1 Al:Mg molar ratio, and further refluxing was performed for 2 h, to make the mixed solution homogeneous. The co-precipitation for the spinel precursor was carried out in a stirred reactor, at 25 jC. The pH of the reaction media was kept at values between 8.5 and 9.0, by controlled addition of ammonia hydroxide solution. A precipitate was formed instantaneously, which was filtered off and washed with de-ionized water. Then, the precipitate was dried at 90 jC for 24 hours.
Fig. 1. Procedure for preparing the magnesium aluminate spinel precursor.
As precipitated and after heat treatment, powders were characterized. The spinel precursor was calcined for 1 h at temperatures ranging from 400 to 1200 jC (heating rate of 5 jC/min). The crystalline phases of the MgAl2O4 precursor and calcined powders were studied via X-ray diffraction (XRD) analysis, using CuKa radiation. The spinel precursor and the calcined powders were also analyzed by FTIR. The IR spectrum was obtained using a Nicolet-STIR spectrophotometer, at room temperature. The density of heattreated samples was evaluated by means of a Gay-Lussac pycnometer, with distilled water at 18 jC. The morphology of the calcined powder at 1200 jC was determined by SEM (JEOL JSM T300).
3. Results and discussion Since the gibbsite used is a byproduct of the Bayer process, it was important to reduce impurity levels,
H. Revero´n et al. / Materials Letters 56 (2002) 97–101
like sodium content. Table 1 shows the sodium impurity levels of the as-received and washed gibbsite powders. It can be seen that near 40% of sodium content was removed during washing. The XRD spectra of the washed Al(OH)3 raw material (dried at 110 jC), indicate only diffraction peaks corresponding to gibbsite phase. Also, the XRD patterns of magnesia powder showed a well-crystallized periclase single phase. Fig. 2 shows a typical pattern of the precursor. According to XRD analysis, it was concluded that the precursor was amorphous in nature. However, during its synthesis, ammonium chloride (NH4Cl) and ammonium nitrate (NH4NO3) crystallized and characteristic peaks for both compounds were observed in XRD diffractometry experiment. Samples of the precursor were calcined for 1 h at various temperatures. XRD for the different temperatures are collected in Fig. 3. The results indicate that the MgAl2O4 spinel phase develops with increasing the calcination temperature. As the temperature is increased, the diffusion of the different constituents increases, allowing the ions to form the spinel structure much easier. It is also observed in Fig. 3 that the diffraction intensities of the NH4Cl and NH4NO3 peaks decreased with heat treatment, and they completely disappeared at 500 jC, due to salt decomposition. At this temperature, the XRD diagram evidences a partially crystallized spineltype solid solution with broad lines. This formation temperature is several hundred degrees lower than that reported for the conventional solidstate reaction method. Moreover, degree of crystallinity at this stage is higher than that reported at 500 jC using other chemical routes [31,32]. Further increases in the calcining temperature (800 –1200 jC) sharp the diffraction peaks of spinel, as a result of the increase in the amount and crystallinity of the spinel phase. IR spectra of the as-obtained spinel precursor and powders calcined for 1 h at 500, 800 and 1200 jC are shown in Fig. 4. In the spectrum for the dried precursor, the absorption bands at 820, 1380 and
99
Fig. 2. XRD pattern of the precursor. Only NH4NO3 and NH4Cl crystalline phases show up.
1630 cm 1 may correspond to NO3 , NH4 + and N –H bending indicating the absorption of NH4NO3 and NH4Cl. Fig. 4 confirms that the decomposition of ammonia compounds took place during heat treatment, and all the absorption peaks of these compounds disappeared at 500 jC. The patterns of powders at 800 and 1000 jC
Table 1 Sodium impurity removed by washing in Al(OH)3 powder As-received Washed concentration
2200 ppm 1300 ppm
0.30% 0.18%
Fig. 3. XRD patterns of powders calcined at different temperatures.
100
H. Revero´n et al. / Materials Letters 56 (2002) 97–101
Fig. 6. SEM micrograph of precursor powder calcined at 1200 jC.
Fig. 4. IR spectra of precursor and powders calcined at different temperatures.
show absorption bands at < 1000 cm 1 with confirm the formation of oxide. To evaluate the sinterability of the spinel powders that have been prepared in the present work, the density of calcined powders was evaluated. Fig. 5
gives a plot of the relative density of polycrystalline spinel as a function of the calcining temperature. The powder reaches 83% of the spinel theoretical density (3.60 g/cm3) at 1200 jC. This sintering behaviour can be considered to be a reflection of the high reactivity of calcined powders. This explains the desire on the part of many engineering ceramic manufacturers for very fine starting powders, since the higher the surface area of the powder the greater the driving force for densification. Fig. 6 shows a SEM micrograph of precursor powder calcined at 1200 jC, which consists of small almost spherical particles. The mean colloidal particle size lies in the range of 0.2– 0.5 Am forming aggregates.
4. Conclusion
Fig. 5. Effect of calcining temperature on the relative density of MgAl2O4 powders.
MgAl2O4 spinel precursor was prepared by dissolution of industrial gibbsite, from a Bayer process, and a refractory-grade dead-burned magnesia in HCl/ HNO3 solution, followed by a precipitation at basic pH. The method described is satisfactory for production of fine magnesium aluminate spinel powders at relatively low temperature. The spinel precursor was amorphous but NH4NO3 and NH4Cl were crystallized during its synthesis. However, these salts are found to be removed under thermal treatment of powders above 500 jC in air. At this temperature, the precursor was converted in
H. Revero´n et al. / Materials Letters 56 (2002) 97–101
a poorly crystallized MgAl2O4 spinel single phase. Well-crystallized powders were obtained by heat treatment from 800 jC. As the temperature increased, the nanometer-size synthesized particles started to sinter together, forming aggregates. The relative density of MgAl2O4 powders was around 83% at 1200 jC.
Acknowledgements The authors acknowledge the contribution of Dr. A. Ruı´z and Dr. M. Ve´lez for their helpful comments and their stimulating discussions about this work. The support given by the Polymer Laboratory of U.S.B for IR experiments is very much appreciated.
References [1] L. Vollweiler, H. Jost, H. Hausner, Key Eng. Mater. 132 – 136 (1997) 1814. [2] T.E. Mitchell, J. Am. Ceram. Soc. 82 (1999) 3305. [3] C. Ting, H. Lu, J. Am. Ceram. Soc. 82 (1999) 841. [4] C. Ting, H. Lu, J. Am. Ceram. Soc. 83 (2000) 1592. [5] T. Shiono, H. Ishitomi, Y. Okamoto, T. Nishida, J. Am. Ceram. Soc. 83 (2000) 645. [6] K. Jacob, K. Jayadevan, J. Am. Ceram. Soc. 81 (1998) 209. [7] J. Shou-Yong, L. Li-Bin, H. Ning-Kang, Z. Jin, L. Yong, Mater. Sci. Lett. 19 (2000) 225. [8] M.A. Sainz, A. Caballero, Key Eng. Mater. 132 – 136 (1997) 864. [9] C. Aksel, R.W. Davidge, P.D. Warren, F.L. Riley, Key Eng. Mater. 132 – 136 (1997) 1774. [10] D. Gutie´rrez-Campos, J.I. Dı´az, R.M. Rodrı´guez, Ceramica 45 (1999) 93.
101
[11] R. Sarkar, K. Das, S.K. Das, G. Banerjee, Proc. UNITECR ’97 (1997) 1053. [12] Z. Nakagawa, N. Enomoto, I. Yi, K. Asano, Proc. UNITECR ’95 (1995) 379. [13] K. Hong Hwang, K. Dong Oh, R.C. Bradt, Proc. UNITECR ’97 (1997) 1575. [14] K.A. and S. Satoshi, Proc. UNITECR ’95 (1995) 530. [15] L. Yongquan, Proc. UNITECR ’95 (1995) 602. [16] M. Rigaud, C. Xing, V. Kovacˇ, Key Eng. Mater. 132 – 136 (1997) 1818. [17] Z. Nakagawa, T. Itoh, N. Enomoto, Proc. PacRim2 (1996). [18] A. Cisar, W.W. Henslee, G.W. Strother, Adv. Ceram. 13 (1985) 411. [19] K. Pflanz, N. Stroh, R. Riedel, Key Eng. Mater. 150 (1998) 135. [20] A. Larbot, G. Philip, M. Persin, L. Cot, Key Eng. Mater. 132 – 136 (1997) 1719. [21] I.V. Uvarova, I.B. Vynnyk, V.S. Zenkov, M.M. Syrotyuk, Key Eng. Mater. 132 – 136 (1997) 240. [22] D. Janackovic, V. Jokanovic, L. Kostic-Gvozdenovic, R. Cirjakovic, I. Petrovic-Prelevic, D. Uskokovic, Key Eng. Mater. 132 – 136 (1997) 197. [23] M. Wang, M. Muhammed, Mater. Sci. Forum 235 – 238 (1997) 241. [24] H. Hahn, J. Logas, J. Averback, J. Mater. Res. 5 (1990) 609. [25] Y.C. Zhou, M.N. Rahaman, J. Mater. Res. 8 (1993) 1680. [26] O. Yamaguchi, H. Omaki, K. Shimizu, J. Jpn. Soc. Powder Powder Metall. 22 (1975) 202. [27] M. Sugiura, O. Kamigaito, Yogyo Kyokaishi 92 (1984) 605. [28] C.T. Wang, L.S. Lin, S.J. Yang, J. Am. Ceram. Soc. 75 (1992) 2240. [29] S. Hokazono, K. Manako, A. Kato, Br. Ceram. Trans. J. 91 (1992) 77. [30] J. Dı´az, Master thesis, U.S.B., Venezuela (1998). [31] T. Shiono, K. Shiono, K. Miyamoto, G. Pezzotti, J. Am. Ceram. Soc. 83 (2000) 235. [32] V. Montouillout, D. Massiot, A. Douyn, J.P. Coutures, J. Am. Ceram. Soc. 82 (1999).
Materials Letters 56 (2002) 97 – 101 www.elsevier.com/locate/matlet
Chemical synthesis and thermal evolution of MgAl2O4 spinel precursor prepared from industrial gibbsite and magnesia powder H. Revero´n a,*, D. Gutie´rrez-Camposa,1, R.M. Rodrı´guez b,2, J.C. Bonassin b,2 a
Department of Materials Science, Universidad Simo´n Bolı´var, Caracas 1080-A, Venezuela b Department of Chemistry, Universidad Metropolitana, Caracas, Venezuela Received 20 August 2001; accepted 21 November 2001
Abstract A chemical route, using industrial raw materials, was developed for the synthesis of MgAl2O4 spinel precursor. The asobtained powder was then calcined in air, up to 1200 jC for a period of 1 h. It was found that spinel single phase is formed at several hundred degrees lower than temperatures reported for the conventional powder preparation methods. Moreover, at 500 jC the degree of crystallinity is higher than that reported by other chemical processes at this temperature. The calcined powder reaches 83% of relative density at 1200 jC, indicating reactivity and good sintering behaviour. Scanning electron microscopy images revealed that the product consisted of fine spherical particles of magnesium aluminate spinel. D 2002 Elsevier Science B.V. All rights reserved. Keywords: Co-precipitation; Gibbsite; Magnesia; Spinel; MgAl2O4; Chemical synthesis
1. Introduction Magnesium aluminate spinel, which is the only stable compound in the MgO – Al2O3 system [1], has long been considered an important ceramic material. Many studies have reported its properties, applications and different processing methods [2 –9]. As a ceramic material, MgAl2O4 has mainly been used for structural applications, like refractory linings, due to its superior mechanical, chemical and thermal properties [10 – 18]. Nowadays, new potential uses have been reported, *
Corresponding author. Tel.: +58-212-906-41-70; fax: +58212-906-39-32. E-mail addresses: [email protected] (H. Revero´n), [email protected] (D. Gutie´rrez-Campos), [email protected] (R.M. Rodrı´guez). 1 Tel.: +58-212-906-41-70; fax: +58-212-906-39-32. 2 Tel.: +58-212-241-48-33; fax: +58-212-241-75-95.
including: ceramic ultra filtration membranes, electro-insulators and optical materials [19 – 22]. On the other hand, it is well known that structure and properties of materials may be tailored via processing control. Preparing a fine and agglomerate/aggregate-free ceramic powder is the first, and perhaps the most important, step in obtaining a sintered ceramic of desirable microstructure, and therefore performance. Pure magnesium aluminate spinel has been prepared, in commercial processes, by solidstate reaction of high purity magnesia and alumina powders, heated for extended period of times at temperatures exceeding 1800 jC [23]. However, polycrystalline spinel ceramics prepared by conventional powder processing are difficult to sinter to full density at relatively low temperature. Low sintering temperatures for ultra-fine powders have been observed in a number of studies [24,25]. Various chem-
0167-577X/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 7 - 5 7 7 X ( 0 2 ) 0 0 4 2 5 - 1
98
H. Revero´n et al. / Materials Letters 56 (2002) 97–101
istry-based novel approaches have been engineered to improve spinel powders sinterability, including sol – gel, freeze – drying, spray drying, complexation and co-precipitation [26 – 29]. Nevertheless, the quality of the final material depends mainly on the properties of the starting powder, such as purity and chemical homogeneity. In this way, the objective of the present work is to obtain a spinel precursor by a chemical route, using industrial raw materials, like gibbsite from a Bayer process and a refractory-grade deadburned magnesia. In addition, thermal evolution and microstructural features of the spinel precursor are presented.
2. Experimental procedures Prior to the preparation of the spinel precursor, the industrial gibbsite (Al(OH)3) was characterized by Xray diffraction (XRD), employing a PW 2525 Phillips diffractometer with Cu-Ka radiation, in order to identify the existing phases. Also, the Al(OH)3 powder was washed twice in de-ionized water at 70 jC, to remove sodium impurities introduced during the Bayer process. The sodium content, before and after washing, was measured by atomic absorption, with a Perkin Elmer 31,000 spectrophotometer. The MgO refractory-grade powder was ground in a ball mill, in which particle size was reduced to less than 38 Am in diameter. This raw material was also analyzed by XRD, with the same equipment and conditions used for the gibbsite powder. The synthesis of the spinel precursor was carried out following a co-precipitation route, schematically shown in Fig. 1. First, the gibbsite powder was dissolved in HCl/HNO3 solution and refluxing was performed for 5 h at 80 jC. Details of gibbsite dissolution procedure have been reported elsewhere [30]. MgO powder was added in a 2:1 Al:Mg molar ratio, and further refluxing was performed for 2 h, to make the mixed solution homogeneous. The co-precipitation for the spinel precursor was carried out in a stirred reactor, at 25 jC. The pH of the reaction media was kept at values between 8.5 and 9.0, by controlled addition of ammonia hydroxide solution. A precipitate was formed instantaneously, which was filtered off and washed with de-ionized water. Then, the precipitate was dried at 90 jC for 24 hours.
Fig. 1. Procedure for preparing the magnesium aluminate spinel precursor.
As precipitated and after heat treatment, powders were characterized. The spinel precursor was calcined for 1 h at temperatures ranging from 400 to 1200 jC (heating rate of 5 jC/min). The crystalline phases of the MgAl2O4 precursor and calcined powders were studied via X-ray diffraction (XRD) analysis, using CuKa radiation. The spinel precursor and the calcined powders were also analyzed by FTIR. The IR spectrum was obtained using a Nicolet-STIR spectrophotometer, at room temperature. The density of heattreated samples was evaluated by means of a Gay-Lussac pycnometer, with distilled water at 18 jC. The morphology of the calcined powder at 1200 jC was determined by SEM (JEOL JSM T300).
3. Results and discussion Since the gibbsite used is a byproduct of the Bayer process, it was important to reduce impurity levels,
H. Revero´n et al. / Materials Letters 56 (2002) 97–101
like sodium content. Table 1 shows the sodium impurity levels of the as-received and washed gibbsite powders. It can be seen that near 40% of sodium content was removed during washing. The XRD spectra of the washed Al(OH)3 raw material (dried at 110 jC), indicate only diffraction peaks corresponding to gibbsite phase. Also, the XRD patterns of magnesia powder showed a well-crystallized periclase single phase. Fig. 2 shows a typical pattern of the precursor. According to XRD analysis, it was concluded that the precursor was amorphous in nature. However, during its synthesis, ammonium chloride (NH4Cl) and ammonium nitrate (NH4NO3) crystallized and characteristic peaks for both compounds were observed in XRD diffractometry experiment. Samples of the precursor were calcined for 1 h at various temperatures. XRD for the different temperatures are collected in Fig. 3. The results indicate that the MgAl2O4 spinel phase develops with increasing the calcination temperature. As the temperature is increased, the diffusion of the different constituents increases, allowing the ions to form the spinel structure much easier. It is also observed in Fig. 3 that the diffraction intensities of the NH4Cl and NH4NO3 peaks decreased with heat treatment, and they completely disappeared at 500 jC, due to salt decomposition. At this temperature, the XRD diagram evidences a partially crystallized spineltype solid solution with broad lines. This formation temperature is several hundred degrees lower than that reported for the conventional solidstate reaction method. Moreover, degree of crystallinity at this stage is higher than that reported at 500 jC using other chemical routes [31,32]. Further increases in the calcining temperature (800 –1200 jC) sharp the diffraction peaks of spinel, as a result of the increase in the amount and crystallinity of the spinel phase. IR spectra of the as-obtained spinel precursor and powders calcined for 1 h at 500, 800 and 1200 jC are shown in Fig. 4. In the spectrum for the dried precursor, the absorption bands at 820, 1380 and
99
Fig. 2. XRD pattern of the precursor. Only NH4NO3 and NH4Cl crystalline phases show up.
1630 cm 1 may correspond to NO3 , NH4 + and N –H bending indicating the absorption of NH4NO3 and NH4Cl. Fig. 4 confirms that the decomposition of ammonia compounds took place during heat treatment, and all the absorption peaks of these compounds disappeared at 500 jC. The patterns of powders at 800 and 1000 jC
Table 1 Sodium impurity removed by washing in Al(OH)3 powder As-received Washed concentration
2200 ppm 1300 ppm
0.30% 0.18%
Fig. 3. XRD patterns of powders calcined at different temperatures.
100
H. Revero´n et al. / Materials Letters 56 (2002) 97–101
Fig. 6. SEM micrograph of precursor powder calcined at 1200 jC.
Fig. 4. IR spectra of precursor and powders calcined at different temperatures.
show absorption bands at < 1000 cm 1 with confirm the formation of oxide. To evaluate the sinterability of the spinel powders that have been prepared in the present work, the density of calcined powders was evaluated. Fig. 5
gives a plot of the relative density of polycrystalline spinel as a function of the calcining temperature. The powder reaches 83% of the spinel theoretical density (3.60 g/cm3) at 1200 jC. This sintering behaviour can be considered to be a reflection of the high reactivity of calcined powders. This explains the desire on the part of many engineering ceramic manufacturers for very fine starting powders, since the higher the surface area of the powder the greater the driving force for densification. Fig. 6 shows a SEM micrograph of precursor powder calcined at 1200 jC, which consists of small almost spherical particles. The mean colloidal particle size lies in the range of 0.2– 0.5 Am forming aggregates.
4. Conclusion
Fig. 5. Effect of calcining temperature on the relative density of MgAl2O4 powders.
MgAl2O4 spinel precursor was prepared by dissolution of industrial gibbsite, from a Bayer process, and a refractory-grade dead-burned magnesia in HCl/ HNO3 solution, followed by a precipitation at basic pH. The method described is satisfactory for production of fine magnesium aluminate spinel powders at relatively low temperature. The spinel precursor was amorphous but NH4NO3 and NH4Cl were crystallized during its synthesis. However, these salts are found to be removed under thermal treatment of powders above 500 jC in air. At this temperature, the precursor was converted in
H. Revero´n et al. / Materials Letters 56 (2002) 97–101
a poorly crystallized MgAl2O4 spinel single phase. Well-crystallized powders were obtained by heat treatment from 800 jC. As the temperature increased, the nanometer-size synthesized particles started to sinter together, forming aggregates. The relative density of MgAl2O4 powders was around 83% at 1200 jC.
Acknowledgements The authors acknowledge the contribution of Dr. A. Ruı´z and Dr. M. Ve´lez for their helpful comments and their stimulating discussions about this work. The support given by the Polymer Laboratory of U.S.B for IR experiments is very much appreciated.
References [1] L. Vollweiler, H. Jost, H. Hausner, Key Eng. Mater. 132 – 136 (1997) 1814. [2] T.E. Mitchell, J. Am. Ceram. Soc. 82 (1999) 3305. [3] C. Ting, H. Lu, J. Am. Ceram. Soc. 82 (1999) 841. [4] C. Ting, H. Lu, J. Am. Ceram. Soc. 83 (2000) 1592. [5] T. Shiono, H. Ishitomi, Y. Okamoto, T. Nishida, J. Am. Ceram. Soc. 83 (2000) 645. [6] K. Jacob, K. Jayadevan, J. Am. Ceram. Soc. 81 (1998) 209. [7] J. Shou-Yong, L. Li-Bin, H. Ning-Kang, Z. Jin, L. Yong, Mater. Sci. Lett. 19 (2000) 225. [8] M.A. Sainz, A. Caballero, Key Eng. Mater. 132 – 136 (1997) 864. [9] C. Aksel, R.W. Davidge, P.D. Warren, F.L. Riley, Key Eng. Mater. 132 – 136 (1997) 1774. [10] D. Gutie´rrez-Campos, J.I. Dı´az, R.M. Rodrı´guez, Ceramica 45 (1999) 93.
101
[11] R. Sarkar, K. Das, S.K. Das, G. Banerjee, Proc. UNITECR ’97 (1997) 1053. [12] Z. Nakagawa, N. Enomoto, I. Yi, K. Asano, Proc. UNITECR ’95 (1995) 379. [13] K. Hong Hwang, K. Dong Oh, R.C. Bradt, Proc. UNITECR ’97 (1997) 1575. [14] K.A. and S. Satoshi, Proc. UNITECR ’95 (1995) 530. [15] L. Yongquan, Proc. UNITECR ’95 (1995) 602. [16] M. Rigaud, C. Xing, V. Kovacˇ, Key Eng. Mater. 132 – 136 (1997) 1818. [17] Z. Nakagawa, T. Itoh, N. Enomoto, Proc. PacRim2 (1996). [18] A. Cisar, W.W. Henslee, G.W. Strother, Adv. Ceram. 13 (1985) 411. [19] K. Pflanz, N. Stroh, R. Riedel, Key Eng. Mater. 150 (1998) 135. [20] A. Larbot, G. Philip, M. Persin, L. Cot, Key Eng. Mater. 132 – 136 (1997) 1719. [21] I.V. Uvarova, I.B. Vynnyk, V.S. Zenkov, M.M. Syrotyuk, Key Eng. Mater. 132 – 136 (1997) 240. [22] D. Janackovic, V. Jokanovic, L. Kostic-Gvozdenovic, R. Cirjakovic, I. Petrovic-Prelevic, D. Uskokovic, Key Eng. Mater. 132 – 136 (1997) 197. [23] M. Wang, M. Muhammed, Mater. Sci. Forum 235 – 238 (1997) 241. [24] H. Hahn, J. Logas, J. Averback, J. Mater. Res. 5 (1990) 609. [25] Y.C. Zhou, M.N. Rahaman, J. Mater. Res. 8 (1993) 1680. [26] O. Yamaguchi, H. Omaki, K. Shimizu, J. Jpn. Soc. Powder Powder Metall. 22 (1975) 202. [27] M. Sugiura, O. Kamigaito, Yogyo Kyokaishi 92 (1984) 605. [28] C.T. Wang, L.S. Lin, S.J. Yang, J. Am. Ceram. Soc. 75 (1992) 2240. [29] S. Hokazono, K. Manako, A. Kato, Br. Ceram. Trans. J. 91 (1992) 77. [30] J. Dı´az, Master thesis, U.S.B., Venezuela (1998). [31] T. Shiono, K. Shiono, K. Miyamoto, G. Pezzotti, J. Am. Ceram. Soc. 83 (2000) 235. [32] V. Montouillout, D. Massiot, A. Douyn, J.P. Coutures, J. Am. Ceram. Soc. 82 (1999).