Characterization of aluminosilicate (mullite) precursors prepared by a mechanochemical process
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Characterization of aluminosilicate (mullite) precursors prepared by a mechanochemical process J. Temuujin Institute of Chemistry of the Mongolian Academy of Sciences, Ulaanbaatar 51, Mongolia
K. Okada Department of Inorganic Materials, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152, Japan
K. J. D. MacKenzie New Zealand Institute for Industrial Research and Development, Lower Hutt, New Zealand (Received 8 August 1997; accepted 11 October 1997)
Aluminosilicate precursors were prepared by mechanochemical treatment of gibbsitesilica gel mixtures. The effect of grinding on their structure and thermal behavior has been examined by 27 Al and 29 Si MAS NMR, x-ray diffraction (XRD), differential thermal analysis-thermogravimetry (DTA-TG), and Fourier transform infrared (FTIR). After 8 h grinding, the hydrated alumina was completely changed to an amorphous phase which showed a new exothermic DTA peak at about 980 ±C due to the formation of g –Al2 O3 or spinel phase. This behavior was related to changes in the Al and Si environments, as deduced from the MAS NMR spectra. With increased grinding time, some 4-coordinated Al appears, together with an Al resonance at about 30 ppm. Simultaneously, a new 29 Si resonance appears at about 290 ppm, indicating a greater degree of homogeneity in the ground samples. Mullite crystallizes at 1200 ±C from samples ground for 8–20 h, its XRD intensity increasing with increased milling times, in agreement with the NMR, DTA, and FTIR data. Changes in the Al and Si environments during heat treatment, as reflected by the NMR spectra, are also reported.
I. INTRODUCTION
The use of oxides and hydroxides for mullite synthesis is one possible method for preparing mullite ceramics. In particular, the thermal reactions between gibbsite and silicic acid1 and aluminum oxide with quartz or silicic acid2 have been examined and mullite formation was observed at high temperatures. The solid-state reactions between aluminous and siliceous compounds during heating depend on the starting materials and their preparation methods. The reaction between alumina and silica is a diffusion-controlled process, in which the rate of mullite formation increases as the particle size of the sample mixture decreases.3 Grinding is an important technique in ceramics science because it not only enhances the rate of solidstate reaction by decreasing the particle size and increasing the concentration of structural defects, but it also introduces the possibility of forming new compounds by mechanochemical reaction. In particular, mechanochemical treatment of hydroxides or oxide hydrogels has now become a useful method for obtaining fine ceramic powder,4,5 by exploiting the possibilities of mechanochemical reactions to rapidly produce homogeneous precursors. Previous studies have determined the influence of mechanochemical treatment on the structure of aluminum hydroxides and reported high rates 2184
http://journals.cambridge.org
J. Mater. Res., Vol. 13, No. 8
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