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SYNTHESIS AND PROCESSING: GENERAL METHODS 14 greater the toughness, as would be expected. For a fixed volume fraction of cobalt, the compressive strength or hardness of Co-WC increases linearly with decreasing scale of the bicontinuous structure. Because of the limitations in traditional powder metallurgy processing, it has not yet been possible to extend the range of such measurements below 1 µm particle size. The challenge, therefore, is to devise new processing routes for significantly reducing the scale of bicontinuous metal-ceramic composite structures. A promising approach for achieving this goal is by controlled decomposition of molecular precursors that encompass within their molecular architecture the correct atomic fractions of the elemental species. This has been investigated in the Co-WC system, where the predesigned molecular precursor is a complex water-soluble transition-metal coordination compound (McCandlish and Polizzotti, 1989). An example is Co(en)3WO4 salt, which gives a 50:50 atomic ratio of W and Co. After heat treatment in H2 at 600°C, the resultant structure is a porous aggregate of clusters (order of 10 nm) of the two species. Consolidation at this stage realizes a Co-W nanocomposite whose properties have yet to be explored. If the porous aggregate is subjected to an additional reaction in a controlled C-activity environment, it is possible to access any point along the tie-line connecting 50/50 Co-W to the C corner of the Co-W-C ternary phase diagram. When the activity of the C is set between 0.52 and 1.00 (at a temperature between 800 and 1000°C), this fixes the structure in the two-phase field of Co plus WC. Since this conversion process takes place at 800°C, the material has an ultrafine structure. Powders produced in this way can be consolidated by conventional methods while retaining their submicron-sized microstructure. Examples include rapid liquid-phase sintering, laser glazing, and plasma spraying, where the thermal transient in the liquid phase is so brief that no significant coarsening of the microstructure occurs. This new approach to cermet synthesis is widely applicable and is limited only by the constraints imposed by molecular design. Chemical precursors have already been formulated for the direct synthesis of a wide variety of metal-carbide, metal-boride, and metal-nitride composites. GEL SYNTHESIS The sol-gel processing of mixed organometallic precursors has been used to form ferroelectric and piezoelectric ceramics (Wu et al., 1984). Optically transparent barium titanate, lead titanate, and other perovskites were crystallized that had uniform nanocrystalline structures of preferred orientation. These nanocrystalline ceramics had near single-crystal-like properties, as shown by the dielectric constant-temperature profiles in Figure 4, in contrast to conventional polycrystalline ferroelectrics and piezoelectrics with grain sizes less than 1 µm, which lose their nonlinearity.
SYNTHESIS AND PROCESSING: GENERAL METHODS 15 The sol-gel process refers to a room-temperature chemical route that is used for preparing oxide materials. The process involves initially a homogeneous solution of the appropriate alkoxides (Hench and Ulrich, 1984). Alkoxides are the organometallic precursors for silica, alumina, titania, and zirconia, among others. A catalyst is used to start reactions and control pH. The reactants are first hydrolyzed to make the solution active, followed by condensation polymerization along with further hydrolysis. These reactions increase the molecular weight of the oxide polymer. The pH of the water-alcohol mixture has an influence over the polymerization scheme such that acid-catalyzed solutions remain transparent and base-catalyzed solutions become opaque. Eventually the solution reacts to a point where the molecular structure is no longer reversible. This point is known as the sol-gel transition. The gel is an elastic solid filling the same volume as the solution. When dried, the gel gradually shrinks and transforms to a rigid oxide skeleton. The oxide skeleton has interconnected porosity. The nature of the porosity is determined by the processing steps. As a result, the porosity can be tailored in terms of size, shape, and volume. This controlled porosity can be exploited in many ways. One way is by infiltration of a second phase to form submicron-scale composites. Hypercritical evacuation of the solvent from the pores results in aerogels, and natural evaporation results in xerogels (Klein, 1987). Xerogels may have water and alcohol in the pores or solvent substituted in various proportions with a drying control chemical agents (DCCA). In aerogels the pore size is on the scale of 10 to 50 nm, approaching sizes of those pores in samples prepared by colloidal techniques. In xerogels, the pore size is on the scale of 2 to 5 nm. Aerogels dry much faster than xerogels. This is significant in forming monolithic shapes without cracks. In fibers and thin films, drying is less of a problem. DCCAs make it possible to optimize gelation, aging, and drying of gels to produce large-scale, fully dried monolithic gels (Ulrich, 1988a). They also make it possible to control the size and shape of the pore distribution. Addition of a basic DCCA such as formamide produces a large sol-gel network with uniformly large pores (see Figure 2). An acidic DCCA, such as oxalic acid, in contrast, results in a somewhat smaller-scale network after gelation, but also with a narrow distribution of pores. The DCCAs minimize differential drying stresses by minimizing differential rates of evaporation and ensure a uniform thickness of the solid network that must resist the drying stress. Achieving a uniform scale of structure at gelation also results in uniform growth of the network during aging, which thereby increases the strength of the gel and its ability to resist drying stresses.
SYNTHESIS AND PROCESSING: GENERAL METHODS 16 Figure 2 Control of sol-gel processing with organic acid DCCAs (Hench and Ulrich, 1986). Recently, the sol-gel process has been investigated widely for the synthesis of a variety of glasses and crystalline materials suitable for ceramic matrix composites (Hench and Ulrich, 1986). In all cases the advantages of the process are purity and low-temperature processing (see Figure 3). By far the most common system is that composed of tetraethoxysilane-water-alcohol. Various studies have elucidated the influence of the ratios of the starting species, the catalyst, and other additives on the structure of the final gel or ceramic. Depending on these conditions, powders, fibers, films, and monolithic pieces of transparent or opaque oxide can be produced (Figure 4). Figure 3 Densification microstructures for SiO2 gels. (Reprinted by permission of the publishers, Butterworth & Co. (Publishers) Ltd.©, from Polymer 28, 533, D. Ulrich, 1987)
