Multi-Phasic Ceramic Composites Made by Sol-Gel Technique
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MULTI-PHASIC CERAMIC COMPOSITES MADE BY SOL-GEL TECHNIQUE RUSTUM ROY, S. KOMARNENI AND D.M. ROY Materials Research Laboratory, The Pennsylvania State versity, University Park, PA 16802
Uni-
ABSTRACT Instead of aiming to prepare homogeneous gels and xerogels, this paper reports on work done to prepare deliberately diphasic materials. This has been achieved by three different paths: (1) mixing 2 sols; (2) mixing 1 sol with 1 solution; and (3) post formation diffusion of either one or two solutions. By the last named process we have made Si0 2 , mullite and alumina based composites, with silver halides, BaSO4 , CdS, etc., as the dispersed phase. The crystal size can be confined to the initial pores by rapid diffusion giving rise to extremely fine second phases in the submicron range. Subsequent reduction of appropriate metallic salts can be used to give finely dispersed metals (e.g. Cu, Ni) in essentially any xerogel matrix. The open porosity makes these metal atoms very accessible. By the first two processes we have made both single phase and di-phasic gels of the same composition (prototype: mullite) and shown that though they cannot be distinguished by XRD, SEM, and TEM, by DTA and thermal processing, they are radically different. Such di-phasic gels store more metastable energy than any other solids. INTRODUCTION We developed the sol-gel technique starting in 1948 for two purposes. First, to make ultrahomogeneous glasses and avoid the tedious standard method of the time of making glasses from oxide melts by crushing, remelting and so on several times. Second, Roy and Osborn had embarked on a low-temperature hydrothermal study of the system A1 2 0 3 -Si0 2 -H 2 0 [1], and the glass forming region in the system A12 0 3 -Si0 2 was limited to -0-25% Other starting materials such as silica glass + y-A12 0 3 or boehA1 2 0 3 . mite reacted to give corundum so early that equilibrium was unattainable. One needed a new method to make noncrystalline phases over a much wider compositional range. Ewell and Insley had already introduced the coprecipitated gels made from Na 2 SiO3 and AI(N0 3 ) 3 , but even after tedious electrodialysis these always contained substantial (-1%) Na+. The use of alkoxide precursors specifically tetraethoxysilane and aluminum isopropoxide proved to be a key to the new generalizable sol-gel process. In the next several years we used the new sol-gel method to make several hundreds of compositions as the homogeneous minimally structurally biased starting materials and making homogeneous glasses [1] for phase equilibrium studies both dry and 'wet', in dozens of binary, ternary and quaternary systems involving all the major ceramic oxides [2-6]. The solution-mixing and sol-gel route was first utilized technologically to make nuclear fuel ceramic pellets by ingeniously shaping the final product while it was a gel at Oak Ridge and Harwell [7]. Scaling up of homogeneous glass making was done at Owens-Illinois [8]. In these studies and in the revival of interest in the sol-gel process consequent Mat.
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