Hybrid Gels Designed for Mullite Nucleation and Crystallization Control
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HYBRID GELS DESIGNED FOR MULLITE NUCLEATION AND CRYSTALLIZATION CONTROL JEFFREY C. HULING AND GARY L. MESSING
Department of Materials Science and Engineering, Pennsylvania State University, University Park, PA 16802 ABSTRACT The controlled nucleation of phase transformations by seeding is an established technique for influencing transformation kinetics and sintered microstructures in ceramics. Previous studies have focused on seeding with ultrafine, solid particles having the requisite crystal characteristics for either homo- or heteroepitactic nucleation of the desired phase. Size separation of particulate seed crystals is not an efficient process and thus more recent efforts have concentrated on chemical approaches to nucleating solid phase transformations. Hybrid gels, in which two or more gels are combined to capitalize on the benefits of each, have been reported for the homoepitactic nucleation of mullite. In principle, the molecularly-mixed gel crystallizes to mullite at -1000'C and, in turn, acts to nucleate the colloidal gel component's transformation to mullite at higher temperatures. However, the transformation sequence and kinetics are profoundly affected by the interfacial reaction between the two gels comprising the hybrid. This paper discusses how the physical distribution and chemistry of the gel components can be manipulated for the control of mullite nucleation, crystallization and microstructure development. INTRODUCTION Sol-gel processing provides several distinct routes [1] for synthesizing dense, pure mullite (3A12 0 3 -2SiO2 ). Colloidal or "diphasic" gels [2-7] consist of 10-100 nm particles of boehmite (yAlOOH) or y-A12 0 3 and colloidal silica. Upon heating, the independence of the individual phases is maintained to >1 100 0 C, where viscous flow of the silica promotes densification prior to mullite crystallization [5-7]. More attention, however, has been focused on "molecularly-mixed", "polymeric", or "single phase" gels [3,8-13], which are produced by cohydrolysis or coprecipitation of aluminum and silicon salts and/or alkoxides. Processing of molecularly-mixed gels is intended to maximize the extent of direct alumina-silica bonding. In both cases, crystallization, densification and microstructure are indirectly controlled by changing precursors and processes to influence the scale of alumina-silica mixing. The term "control" is used loosely, however, since characterization of unfired gels is currently insufficient for relating alumina-silica mixing and gel structure to high-temperature behavior. More direct control over transformations has been achieved in oa-alumina [14-16] and several sol-gel silicate systems [17,18] by addition of well characterized crystalline "seed" particles. The crystallites act as substrates for preferred epitactic nucleation. Sufficient concentrations of seeds effectively increase nucleation frequencies, allowing direct control over both transformation kinetics and microstructural development. Despite the straightforward characterization of crystalline seed par
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