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Effect of grinding on synthesis of MgAl2O4 spinel from a powder mixture of Mg(OH)2 and Al(OH)3
Powder Technology 113 Ž2000. 109–113 www.elsevier.comrlocaterpowtec
Effect of grinding on synthesis of MgAl 2 O4 spinel from a powder mixture of Mg žOH/ 2 and Al žOH/ 3 Wantae Kim, Fumio Saito ) Institute for AdÕanced Materials Processing, Tohoku UniÕersity, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan Received 5 July 1999; received in revised form 13 September 1999; accepted 6 December 1999
Abstract Dry grinding of the powder mixture composed of magnesium hydroxide and gibbsite using a planetary ball mill followed by its heating was conducted to prepare MgAl 2 O4 spinel. Both the starting materials are easily converted to amorphous phase by intensive grinding within 60 min. The monolithic MgAl 2 O4 spinel phase can be obtained from the mixtures ground over 15 min by calcination at 9008C. The reactivity of the mixtures ground over 30 min is not improved. Prolonged grinding promotes the aggregation of ground fine particles, but the aggregates deflocculate into fragments after calcination. q 2000 Elsevier Science B.V. All rights reserved. Keywords: MgAl 2 O4 spinel; Mechano chemical treatment; Planetary ball mill; Magnesium hydroxide; Gibbsite
1. Introduction Much attention has been paid on the synthesis of MgAl 2 O4 spinel, although naturally occurring as an accessory mineral in basic igneous rock and contact metamorphic limestone, with conspicuous thermal, electrical and optical properties demanded on the special performances in advanced industrial applications for several decades. Generally, MgAl 2 O4 spinel has been manufactured by solid-state reaction of Mg 2q- and Al 3q-bearing compounds at high temperature. In order to obtain the monolithic spinel phase with high reactivity and chemical homogeneity, various techniques through co-precipitation, spraypyrolysis, sol–gel and freeze-drying have been investigated w1–5x. Although the spinel powders with chemical homogeneity can be synthesized, it is believed that high calcination temperature degrades the densification significantly. To overcome this, mechano chemical route could be acceptable as one of the alternative methods for spinel synthesis. Recently, the mechano chemical synthesis and structural modification of the functional materials with spinel structure have been reported w6,7x, however, little attempts have been made on the preparation of MgAl 2 O4
) Corresponding author. Tel.: q81-22-217-5135; fax: q81-22-2175211. E-mail address: [email protected] ŽF. Saito..
spinel from the constituent powder mixture by mechanochemical route. The main purpose of this study is, therefore, to synthesize MgAl 2 O4 spinel from the mixture of magnesium hydroxide and gibbsite activated by dry grinding using a planetary ball mill. This work involves the direct conversion of the starting materials into MgAl 2 O4 phase by heating with an aid of grinding. The thermal behavior of the ground mixtures was also investigated by monitoring thermogravimetric ŽTG. and differential thermal analyses ŽDTA.. 2. Experimental Magnesium hydroxide ŽMgŽOH. 2 , Wako, Japan. having mean particle size of 2.9 mm and gibbsite ŽAlŽOH. 3 , Sumitomo Chemical, Japan. having mean particle size of 25.8 mm were used in this work. These two materials were weighed in molar ratio of 1.0, coinciding the stoichiometric valence of MgAl 2 O4 and mixed thoroughly using an agate mortar with a pestle in acetone prior to grinding. The mixture prepared prior to grinding is called unground mixture in this experiment. A planetary ball mill ŽFritsch Pulverisette-7. with a pair of stainless steel pots of 50 cm3 inner volume containing seven stainless steel balls ŽB: 15 mm. was used for the grinding of the mixtures. The mixture Ž4.0 g. was put in the mill pot and ground at 790 rpm in rotational speed of the mill. The duration of
0032-5910r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 2 - 5 9 1 0 Ž 0 0 . 0 0 2 0 8 - 4
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W. Kim, F. Saito r Powder Technology 113 (2000) 109–113
grinding was varied from 5 to 240 min. The grinding was suspended for 15 min after every 15-min grinding to avoid excess temperature increase inside the mill pots during prolonged grinding. The ground mixtures were calcined at 600–12008C for 1 h in an atmospheric condition Žheating rate: 88Crmin.. X-ray diffraction ŽXRD. analysis was carried out with Cu-K a radiation to investigate the phase and crystallinity of the ground and calcined mixtures ŽRAD-B System, Rigaku.. The mixtures were also subjected to TG–DTA simultaneously in air at the heating rate of 108Crmin ŽTAS-200, Rigaku.. Mean particle sizes of the ground and calcined mixtures were measured by a particle size analyzer ŽLaser Micron Sizer, LMS-30, Seishin.. The morphology of the mixtures was observed by a scanning electron microscope ŽSEM, SEM-4100, Jeol..
3. Results and discussion Fig. 1 shows the XRD patterns of the mixtures ground for various duration of time. The peak intensity of the starting materials reduced markedly except the broad main peaks after 30 min grinding. Further grinding promotes the amorphization of the starting materials. Accordingly, the lattice structures of the starting materials disordered completely at 60 min. After 120 min grinding, very weak peaks of boehmite ŽAlOŽOH.. were detected and the peaks became markedly as the grinding continued for 240 min,
Fig. 1. XRD patterns of the mixtures ground for various duration of time.
Fig. 2. TG–DTA curves of the mixtures ground for various duration of time.
which implied that the dehydration of AlŽOH. 3 occurred by grinding. However, the mechano chemical reaction between MgŽOH. 2 and AlŽOH. 3 has not occurred under the grinding condition employed. Fig. 2 shows TG–DTA curves of the mixtures ground for various duration of time. The curve of the unground mixture was virtually the same as the mixture ground for 5 min. Three main endothermic peaks at around 3008C, 4008C and 5308C were observed in the DTA curve of the mixture ground for 5 min. These peaks corresponded to the dehydration of MgŽOH. 2 Ž4008C. and AlŽOH. 3 Ž3008C and 5308C., respectively. After 15 min grinding, the peak at 3008C and 4008C decreased and shifted slightly to the lower temperature side, in addition to the disappearance of the other peak at 5308C. Instead, a new broad endothermic peak at around 1708C and two exothermic peaks at 7808C and 8508C appeared. The peak intensity at 1708C increasing with the prolonged grinding strongly suggested the structural alteration of the AlŽOH. 3 into amorphous andror loosely bonded hydroxyl structure by grinding w8x. The peak at 7808C coincides with the crystallization of MgAl 2 O4 and the other at 8508C is considered as the crystallization of the ground product from MgŽOH. 2 to MgO. After 60 min grinding, the only peak at 7808C is detected with the disappearance of the peak at 8508C due to the progress of structural disordering of MgO by grinding. Despite the changes in DTA profiles, no significant difference in the total weight loss values of the ground mixtures was observed and the weight loss values were almost the same as the theoretical water content of the starting materials Ž38.8 wt.%..
W. Kim, F. Saito r Powder Technology 113 (2000) 109–113
In order to clarify the thermal behavior of the ground mixtures, the mixtures were calcined at 9008C for 1 h. The XRD patterns are shown in Fig. 3. The pattern of the unground mixture is also shown as a reference. The peaks of MgO and x- or k-Al 2 O 3 were observed mainly in the unground mixture. It is revealed that MgO was attributed to the dehydration of MgŽOH. 2 and the conversion of AlŽOH. 3 to x- or k-Al 2 O 3 phase was made by heating operation w9,10x. In the case of the calcined mixture after grinding for 5 min, though small peak of crystalline MgO is observed, the peaks of MgAl 2 O4 are markedly detected. Monolithic MgAl 2 O4 phase was obtained by the calcination of mixtures ground for more than 15 min. However, no distinct differences were observed in the XRD profiles. Fig. 4 shows the XRD patterns of the 15 min ground mixtures calcined at different temperatures. The original peaks of MgŽOH. 2 and AlŽOH. 3 disappeared and the broad peaks of x-Al 2 O 3 appeared in the mixture calcined at 6008C. With the increment of calcination temperature up to 8008C, the peaks of x-Al 2 O 3 reduced and monolithic MgAl 2 O4 phase was obtained at 9008C. According to the results from Shiono et al. w11x showing that MgAl 2 O4 phase was obtained by the calcination of co-precipitated heterogeneous alkoxides with fine MgO up to 12008C and Nakagawa et al. w12x indicating that nearly perfect MgAl 2 O4 powder was prepared by the calcination of freeze-dried magnesium and aluminum sulfate between 11008C and 13008C. The present result showed the extremely lower synthesis temperature than those presented
Fig. 3. XRD patterns of the ground mixtures calcined at 9008C for 1 h.
111
Fig. 4. XRD patterns of the 15 min ground mixtures calcined at different temperatures.
in these reports. It is also noticeable from Fig. 4 that the peaks of MgAl 2 O4 were detected in the mixture calcined at 6508C. This finding implies that the starting materials suffer the size reduction as well as the structural deformation, readily forming new surfaces activated by the mechanical stresses of friction and compression. The repetitive phenomenon enables the activation of particles in the grinding circuit favorable for the solid-state reaction of the components forming the MgAl 2 O4 phase.
Fig. 5. Mean particle size of the ground mixtures calcined at 9008C as a function of grinding time.
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Fig. 6. SEM photographs of the ground mixtures Žunground ŽA., 15 min ŽB., 60 min ŽC.. and the mixtures calcined at 9008C Ž15 min ŽD., 60 min ŽE., 120 min ŽF...
Fig. 5 shows the mean particle size of the ground mixtures and the mixtures calcined at 9008C as a function of grinding time. In the case of the ground mixtures, the mean particle size reduced sharply to 4.1 mm in the early stage of grinding within 15 min. After 30 min of grinding, the mean particle size increased slightly to 5.6 mm and then kept the steady value, in which no further variation was observed in the prolonged grinding. It can be explained by the mechano-chemical behavior in terms of the state of aggregation, where the fine particles are adhered by weak aggregation force on the surfaces of them. In the SEM photographs shown in Fig. 6, the starting mixture of hexagonal plate-like AlŽOH. 3 and MgŽOH. 2 ŽFig. 6ŽA.. were finely ground at 15 min but aggregated after grinding for 15 min ŽFig. 6ŽB... The aggregation of ground fine particles is promoted when the grinding prolongs for 60
min ŽFig. 6ŽC... On the contrary, the increment of particle size was not observed for the calcined powders, where equilibrium particle size kept 3.8 mm in the calcined mixtures. As shown in Fig. 6ŽD–F., the agglomerates were changed into fine fragments by dehydroxylation of adhered fine particles during calcination.
4. Conclusion Dry grinding of the powder mixture of magnesium hydroxide and gibbsite using a planetary ball mill followed by its heating was conducted to prepare MgAl 2 O4 spinel. The results obtained from the present experiment are summarized as follows.
W. Kim, F. Saito r Powder Technology 113 (2000) 109–113
1. Dry grinding of the mixture of MgŽOH. 2 and AlŽOH. 3 enabled the formation of amorphous phase of the starting materials within 60 min. Crystallization of MgAl 2 O4 from the ground mixtures was detected at 7808C when the mixtures were ground over 15 min. 2. The monolithic MgAl 2 O4 phase could be formed from the mixtures ground over 15 min by calcination at 9008C for 1 h. However, the calcined mixtures ground over 30 min did not show distinct differences in the XRD profile. 3. The reactivity of the mixtures ground over 30 min was not improved. Prolonged grinding promotes the aggregation, but the aggregates deflocculated into fine particles after calcination.
Acknowledgements One of the authors ŽW. Kim. wishes to thank the Ministry of Education, Science and Culture of Japan ŽMonbusho. for the financial support provided through the
113
scholarship. The authors are grateful to Mr. Mitsuo Tanjo, Sumitomo Chemical, for kindly supplying the gibbsite sample and offering invaluable comments in this work. References w1x S. Hokazono, K. Manako, A. Kato, Br. Ceram. Trans. J. 91 Ž1992. 77. w2x S. Kanzaki, T. Nishida, N. Otsuka, Yogyo Kyokaishi 91 Ž4. Ž1983. 166. w3x M. Sugiura, O. Kamikaito, Yogyo Kyokaishi 92 Ž11. Ž1984. 605. w4x C. Wang, L. Lin, S. Yang, J. Am. Ceram. Soc. 75 Ž8. Ž1992. 2240. w5x T. Shiono, K. Miyamoto, T. Nishida, Mater. Sci. Res. Int. 2 Ž1. Ž1996. 61. w6x M.V. Zdujic, O.B. Milosevic, Mater. Lett. 13 Ž1992. 125. w7x V. Sepelak, K. Tkacova, V.V. Boldyrev, U. Steinike, Mater. Sci. Forum 228 Ž1996. 783. w8x K. Sugiyama, J.M. Filio, F. Saito, Y. Waseda, Mineral. J. 17 Ž1994. 28. w9x T. Tsuchida, N. Ichikawa, React. Solids 7 Ž1989. 207. w10x Y. Arai, T. Yasue, H. Miyake, Nippon Kagaku Kaishi 3 Ž1972. 547. w11x T. Shiono, Y. Kadoyama, T. Nishida, J. Soc. Mater. Sci., Jpn. 47 Ž6. Ž1998. 576. w12x Z. Nakagawa, K. Hamano, M. Sakaguchi, S. Kanzaki, Yogyo Kyokaishi 90 Ž6. Ž1982. 313.
Effect of grinding on synthesis of MgAl 2 O4 spinel from a powder mixture of Mg žOH/ 2 and Al žOH/ 3 Wantae Kim, Fumio Saito ) Institute for AdÕanced Materials Processing, Tohoku UniÕersity, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan Received 5 July 1999; received in revised form 13 September 1999; accepted 6 December 1999
Abstract Dry grinding of the powder mixture composed of magnesium hydroxide and gibbsite using a planetary ball mill followed by its heating was conducted to prepare MgAl 2 O4 spinel. Both the starting materials are easily converted to amorphous phase by intensive grinding within 60 min. The monolithic MgAl 2 O4 spinel phase can be obtained from the mixtures ground over 15 min by calcination at 9008C. The reactivity of the mixtures ground over 30 min is not improved. Prolonged grinding promotes the aggregation of ground fine particles, but the aggregates deflocculate into fragments after calcination. q 2000 Elsevier Science B.V. All rights reserved. Keywords: MgAl 2 O4 spinel; Mechano chemical treatment; Planetary ball mill; Magnesium hydroxide; Gibbsite
1. Introduction Much attention has been paid on the synthesis of MgAl 2 O4 spinel, although naturally occurring as an accessory mineral in basic igneous rock and contact metamorphic limestone, with conspicuous thermal, electrical and optical properties demanded on the special performances in advanced industrial applications for several decades. Generally, MgAl 2 O4 spinel has been manufactured by solid-state reaction of Mg 2q- and Al 3q-bearing compounds at high temperature. In order to obtain the monolithic spinel phase with high reactivity and chemical homogeneity, various techniques through co-precipitation, spraypyrolysis, sol–gel and freeze-drying have been investigated w1–5x. Although the spinel powders with chemical homogeneity can be synthesized, it is believed that high calcination temperature degrades the densification significantly. To overcome this, mechano chemical route could be acceptable as one of the alternative methods for spinel synthesis. Recently, the mechano chemical synthesis and structural modification of the functional materials with spinel structure have been reported w6,7x, however, little attempts have been made on the preparation of MgAl 2 O4
) Corresponding author. Tel.: q81-22-217-5135; fax: q81-22-2175211. E-mail address: [email protected] ŽF. Saito..
spinel from the constituent powder mixture by mechanochemical route. The main purpose of this study is, therefore, to synthesize MgAl 2 O4 spinel from the mixture of magnesium hydroxide and gibbsite activated by dry grinding using a planetary ball mill. This work involves the direct conversion of the starting materials into MgAl 2 O4 phase by heating with an aid of grinding. The thermal behavior of the ground mixtures was also investigated by monitoring thermogravimetric ŽTG. and differential thermal analyses ŽDTA.. 2. Experimental Magnesium hydroxide ŽMgŽOH. 2 , Wako, Japan. having mean particle size of 2.9 mm and gibbsite ŽAlŽOH. 3 , Sumitomo Chemical, Japan. having mean particle size of 25.8 mm were used in this work. These two materials were weighed in molar ratio of 1.0, coinciding the stoichiometric valence of MgAl 2 O4 and mixed thoroughly using an agate mortar with a pestle in acetone prior to grinding. The mixture prepared prior to grinding is called unground mixture in this experiment. A planetary ball mill ŽFritsch Pulverisette-7. with a pair of stainless steel pots of 50 cm3 inner volume containing seven stainless steel balls ŽB: 15 mm. was used for the grinding of the mixtures. The mixture Ž4.0 g. was put in the mill pot and ground at 790 rpm in rotational speed of the mill. The duration of
0032-5910r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 3 2 - 5 9 1 0 Ž 0 0 . 0 0 2 0 8 - 4
110
W. Kim, F. Saito r Powder Technology 113 (2000) 109–113
grinding was varied from 5 to 240 min. The grinding was suspended for 15 min after every 15-min grinding to avoid excess temperature increase inside the mill pots during prolonged grinding. The ground mixtures were calcined at 600–12008C for 1 h in an atmospheric condition Žheating rate: 88Crmin.. X-ray diffraction ŽXRD. analysis was carried out with Cu-K a radiation to investigate the phase and crystallinity of the ground and calcined mixtures ŽRAD-B System, Rigaku.. The mixtures were also subjected to TG–DTA simultaneously in air at the heating rate of 108Crmin ŽTAS-200, Rigaku.. Mean particle sizes of the ground and calcined mixtures were measured by a particle size analyzer ŽLaser Micron Sizer, LMS-30, Seishin.. The morphology of the mixtures was observed by a scanning electron microscope ŽSEM, SEM-4100, Jeol..
3. Results and discussion Fig. 1 shows the XRD patterns of the mixtures ground for various duration of time. The peak intensity of the starting materials reduced markedly except the broad main peaks after 30 min grinding. Further grinding promotes the amorphization of the starting materials. Accordingly, the lattice structures of the starting materials disordered completely at 60 min. After 120 min grinding, very weak peaks of boehmite ŽAlOŽOH.. were detected and the peaks became markedly as the grinding continued for 240 min,
Fig. 1. XRD patterns of the mixtures ground for various duration of time.
Fig. 2. TG–DTA curves of the mixtures ground for various duration of time.
which implied that the dehydration of AlŽOH. 3 occurred by grinding. However, the mechano chemical reaction between MgŽOH. 2 and AlŽOH. 3 has not occurred under the grinding condition employed. Fig. 2 shows TG–DTA curves of the mixtures ground for various duration of time. The curve of the unground mixture was virtually the same as the mixture ground for 5 min. Three main endothermic peaks at around 3008C, 4008C and 5308C were observed in the DTA curve of the mixture ground for 5 min. These peaks corresponded to the dehydration of MgŽOH. 2 Ž4008C. and AlŽOH. 3 Ž3008C and 5308C., respectively. After 15 min grinding, the peak at 3008C and 4008C decreased and shifted slightly to the lower temperature side, in addition to the disappearance of the other peak at 5308C. Instead, a new broad endothermic peak at around 1708C and two exothermic peaks at 7808C and 8508C appeared. The peak intensity at 1708C increasing with the prolonged grinding strongly suggested the structural alteration of the AlŽOH. 3 into amorphous andror loosely bonded hydroxyl structure by grinding w8x. The peak at 7808C coincides with the crystallization of MgAl 2 O4 and the other at 8508C is considered as the crystallization of the ground product from MgŽOH. 2 to MgO. After 60 min grinding, the only peak at 7808C is detected with the disappearance of the peak at 8508C due to the progress of structural disordering of MgO by grinding. Despite the changes in DTA profiles, no significant difference in the total weight loss values of the ground mixtures was observed and the weight loss values were almost the same as the theoretical water content of the starting materials Ž38.8 wt.%..
W. Kim, F. Saito r Powder Technology 113 (2000) 109–113
In order to clarify the thermal behavior of the ground mixtures, the mixtures were calcined at 9008C for 1 h. The XRD patterns are shown in Fig. 3. The pattern of the unground mixture is also shown as a reference. The peaks of MgO and x- or k-Al 2 O 3 were observed mainly in the unground mixture. It is revealed that MgO was attributed to the dehydration of MgŽOH. 2 and the conversion of AlŽOH. 3 to x- or k-Al 2 O 3 phase was made by heating operation w9,10x. In the case of the calcined mixture after grinding for 5 min, though small peak of crystalline MgO is observed, the peaks of MgAl 2 O4 are markedly detected. Monolithic MgAl 2 O4 phase was obtained by the calcination of mixtures ground for more than 15 min. However, no distinct differences were observed in the XRD profiles. Fig. 4 shows the XRD patterns of the 15 min ground mixtures calcined at different temperatures. The original peaks of MgŽOH. 2 and AlŽOH. 3 disappeared and the broad peaks of x-Al 2 O 3 appeared in the mixture calcined at 6008C. With the increment of calcination temperature up to 8008C, the peaks of x-Al 2 O 3 reduced and monolithic MgAl 2 O4 phase was obtained at 9008C. According to the results from Shiono et al. w11x showing that MgAl 2 O4 phase was obtained by the calcination of co-precipitated heterogeneous alkoxides with fine MgO up to 12008C and Nakagawa et al. w12x indicating that nearly perfect MgAl 2 O4 powder was prepared by the calcination of freeze-dried magnesium and aluminum sulfate between 11008C and 13008C. The present result showed the extremely lower synthesis temperature than those presented
Fig. 3. XRD patterns of the ground mixtures calcined at 9008C for 1 h.
111
Fig. 4. XRD patterns of the 15 min ground mixtures calcined at different temperatures.
in these reports. It is also noticeable from Fig. 4 that the peaks of MgAl 2 O4 were detected in the mixture calcined at 6508C. This finding implies that the starting materials suffer the size reduction as well as the structural deformation, readily forming new surfaces activated by the mechanical stresses of friction and compression. The repetitive phenomenon enables the activation of particles in the grinding circuit favorable for the solid-state reaction of the components forming the MgAl 2 O4 phase.
Fig. 5. Mean particle size of the ground mixtures calcined at 9008C as a function of grinding time.
112
W. Kim, F. Saito r Powder Technology 113 (2000) 109–113
Fig. 6. SEM photographs of the ground mixtures Žunground ŽA., 15 min ŽB., 60 min ŽC.. and the mixtures calcined at 9008C Ž15 min ŽD., 60 min ŽE., 120 min ŽF...
Fig. 5 shows the mean particle size of the ground mixtures and the mixtures calcined at 9008C as a function of grinding time. In the case of the ground mixtures, the mean particle size reduced sharply to 4.1 mm in the early stage of grinding within 15 min. After 30 min of grinding, the mean particle size increased slightly to 5.6 mm and then kept the steady value, in which no further variation was observed in the prolonged grinding. It can be explained by the mechano-chemical behavior in terms of the state of aggregation, where the fine particles are adhered by weak aggregation force on the surfaces of them. In the SEM photographs shown in Fig. 6, the starting mixture of hexagonal plate-like AlŽOH. 3 and MgŽOH. 2 ŽFig. 6ŽA.. were finely ground at 15 min but aggregated after grinding for 15 min ŽFig. 6ŽB... The aggregation of ground fine particles is promoted when the grinding prolongs for 60
min ŽFig. 6ŽC... On the contrary, the increment of particle size was not observed for the calcined powders, where equilibrium particle size kept 3.8 mm in the calcined mixtures. As shown in Fig. 6ŽD–F., the agglomerates were changed into fine fragments by dehydroxylation of adhered fine particles during calcination.
4. Conclusion Dry grinding of the powder mixture of magnesium hydroxide and gibbsite using a planetary ball mill followed by its heating was conducted to prepare MgAl 2 O4 spinel. The results obtained from the present experiment are summarized as follows.
W. Kim, F. Saito r Powder Technology 113 (2000) 109–113
1. Dry grinding of the mixture of MgŽOH. 2 and AlŽOH. 3 enabled the formation of amorphous phase of the starting materials within 60 min. Crystallization of MgAl 2 O4 from the ground mixtures was detected at 7808C when the mixtures were ground over 15 min. 2. The monolithic MgAl 2 O4 phase could be formed from the mixtures ground over 15 min by calcination at 9008C for 1 h. However, the calcined mixtures ground over 30 min did not show distinct differences in the XRD profile. 3. The reactivity of the mixtures ground over 30 min was not improved. Prolonged grinding promotes the aggregation, but the aggregates deflocculated into fine particles after calcination.
Acknowledgements One of the authors ŽW. Kim. wishes to thank the Ministry of Education, Science and Culture of Japan ŽMonbusho. for the financial support provided through the
113
scholarship. The authors are grateful to Mr. Mitsuo Tanjo, Sumitomo Chemical, for kindly supplying the gibbsite sample and offering invaluable comments in this work. References w1x S. Hokazono, K. Manako, A. Kato, Br. Ceram. Trans. J. 91 Ž1992. 77. w2x S. Kanzaki, T. Nishida, N. Otsuka, Yogyo Kyokaishi 91 Ž4. Ž1983. 166. w3x M. Sugiura, O. Kamikaito, Yogyo Kyokaishi 92 Ž11. Ž1984. 605. w4x C. Wang, L. Lin, S. Yang, J. Am. Ceram. Soc. 75 Ž8. Ž1992. 2240. w5x T. Shiono, K. Miyamoto, T. Nishida, Mater. Sci. Res. Int. 2 Ž1. Ž1996. 61. w6x M.V. Zdujic, O.B. Milosevic, Mater. Lett. 13 Ž1992. 125. w7x V. Sepelak, K. Tkacova, V.V. Boldyrev, U. Steinike, Mater. Sci. Forum 228 Ž1996. 783. w8x K. Sugiyama, J.M. Filio, F. Saito, Y. Waseda, Mineral. J. 17 Ž1994. 28. w9x T. Tsuchida, N. Ichikawa, React. Solids 7 Ž1989. 207. w10x Y. Arai, T. Yasue, H. Miyake, Nippon Kagaku Kaishi 3 Ž1972. 547. w11x T. Shiono, Y. Kadoyama, T. Nishida, J. Soc. Mater. Sci., Jpn. 47 Ž6. Ž1998. 576. w12x Z. Nakagawa, K. Hamano, M. Sakaguchi, S. Kanzaki, Yogyo Kyokaishi 90 Ž6. Ž1982. 313.