Crystallization of In-Situ SiC-Mullite Composites from Multicomponent Alumina-Silica Gels
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CRYSTALLIZATION OF IN-SITU SiC-MULLITE COMPOSITES FROM MULTICOMPONENT ALUMINA-SILICA GELS S. Jagota, A. Parvizi-Majidi Center for Composite Materials and the Department of Mechanical Engineering, University of Delaware, Newark, Delaware 19716. ABSTRACT Silica and alumina gels were mixed to make a hybrid of a mullite precursor gel and a silicon carbide precursor gel. The crystallization of these above 1400 °C, to form in-situ composites of silicon carbide whiskers in a matrix of mullite, cristobalite and a glassy phase, was studied for different processing conditions. INTRODUCTION The sol-gel route to the synthesis of crystalline ceramics offers a means of control over the final microstructure by suitable manipulation of the precursor structure during the chemical processing step. This can be an attractive way to synthesize ceramic-ceramic composites where the starting precursor materials are chosen and mixed so that, on crystallizing at high temperature, a reinforcing whisker phase and a glass-ceramic matrix form in-situ. In the alumia-silica gel system, it has been seen that the precursors chosen, and the conditions under which they are mixed and gelled influence the phases formed, the temperature of crystallization and the final morphology attained by the crystals [1-8]. Yoldas and coworkers [1-3] have studied the ultrastructure of alumina-silica gels and its correlation to the high temperature crystallization processes. The precursors used were alkoxides of aluminum and silicon. Two types of gels were identified: (i) polymeric, with an intimate bonding of Al and Si, and (ii) colloidal, with coarser mixing. For the right stochiometry of alumina and silica, mullite forms directly from the amorphous polymeric gel at 980'C; wheras in the colloidal gels a spinel phase forms first and mullite forms between 1300-1400 'C. Pouxviel and Boilot [4] studied the chemical and kinetic features of the reactions between various precursors in this system. Wei and Halloran [5] have looked at the phase transformations in colloidal aluminosilicate gels. Okada and Otsuka[6] have characterized the formation of an intermediate spinel phase in the crystallization of mullite from diphasic alumina silica gels made from silicon alkoxide and aluminum nitrate under different hydrolysis conditions. Chakravorty and Ghosh[7] studied the formation of the spinel phase in and aluminum nitrate derived alumina system with varying amounts of colloidal silica. Messing and Huling[8] used the lower mullite crystallization temperature in polymeric gels compared to colloidal gels to seed colloidal gels with polymeric gels. By this technique, they synthesized in-situ seeded hybrid gels with a lower crystallization temperature and a finer mullite grain size than in unseeded colloidal gels.
Mat. Res. Soc. Symp. Proc. Vol. 180. 01990 Materials Research Society
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In this study, we draw upon this background to synthesize a multi-component Precursor Gel gel, which, on crystallization, will form an sic Mulit in-situ composite of silicon carbide whiskers Pre
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