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SYNTHESIS AND PROCESSING: GENERAL METHODS 25 particles increases with the development of an electrostatic repulsive barrier around the particles. From a practical point of view, the conditions corresponding to the upper and the lower ends of this diagram must be avoided to obtain high packing densities in the casts and to minimize drying shrinkages. The middle range is the most suitable for the preparation of high-concentration slips that can be converted to high-packing-density casts with a minimum amount of shrinkage. One effective way of developing sufficient repulsive interaction between particles in concentrated colloidal dispersions without increasing the hydrodynamic radius is through the steric repulsion of polymeric protective coatings. Industrial practice has shown that polyelectrolytes are especially useful in achieving this goal in aqueous systems. The nucleation of particle clusters and their networks as hierarchically clustered structures takes place in all size ranges. The extent of the hierarchy determines the overall packing density of the system. Recent studies have shown that, when suspended particles are first coated with lubricating surfactants, it is possible to form close- packed structures (Figure 8). This promising approach needs to be generalized in its applications to a variety of multiphase systems. To achieve lower sintering temperatures and a minimum amount of grain growth, the size of the voids must be minimized during forming. Some of the most commonly used forming techniques are not suitable for this purpose. Slip-and tape-casting techniques that use filtration as the basic consolidation mechanism fall into this category. In contrast, techniques such as extrusion and injection molding, which result in shear deformation, yield a relatively narrow pore size distribution because of the restructuring of particle clusters during the forming operation. A narrow pore or particle size distribution may not always be the most effective route to densification. Appropriate multimodal size distributions of particles can provide an alternative path toward densification. REACTIVE SPUTTERING Reactive sputtering is a versatile technique capable of synthesizing a broad class of materials. Thin films of virtually any metal (Ag, Ti, Mo, etc.) or elemental dielectric (Si, Ge, etc.) can be deposited by sputtering from a source of the same material using Ar or Kr inert gas. Addition of a reactive gas such as O2, N2, or H2 to the Ar or Kr permits deposition of oxides, nitrides, and hydrides from metallic or elemental dielectric sources. For example, a Si source can be used with Ar, Ar + O2, Ar + N2, or Ar + H2 to deposit thin films of Si, SiO2, Si3N4, and Si:H alloys. Compared to conventional evaporative techniques for depositing thin films, sputtering generally provides improved control over the properties of
SYNTHESIS AND PROCESSING: GENERAL METHODS Figure 8 (a) Fractal clusters of 15-nm gold particles that result in the formation of low-density compacts; (b) the dense packing of the same gold particles after they are first coated with lubricating surfactants. (Reprinted by permission of Elsevier Science Publishing Co., Inc. from Better Ceramics Through Chemistry III, by C. J. Brinker, P. E. Clark, and 26 D. R. Ulrich, eds., Material Research Society Proceedings, 1988.)
