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SYNTHESIS AND PROCESSING: GENERAL METHODS 22 In the crossflow configuration, the laser beam having a Gaussian shaped intensity profile orthogonally intersects the reactant gas stream possessing a parabolic velocity profile. The laser beam enters and exits the cell through KCl windows. The premixed reactant gases, under some conditions diluted with an inert gas, enter through a stainless steel nozzle located below the laser beam. A coaxial stream of Ar is used to entrain the particles in the gas stream. Cell pressures are maintained between 0.08 and 2.0 atm with a mechanical pump and throttling valve. The powder is captured in a microfiber filter located between the reaction cell and vacuum pump. The counterflow geometry, with laser beam and reactant gas streams impinging on each other from opposite directions, has the important advantages of exposing all gas molecules to identical time-temperature histories and absorbing all of the laser energy. However, the achievement of a stable reaction requires that the reaction and gas stream velocities must be equal and opposite to one another. This is readily accomplished once process conditions are defined. Principally, Si, Si3N4, and SiC powders have been made from appropriate combinations of SiH4, NH3, and C2H4 gases. B2H6 has been used to add boron to the Si and SiC powders as a sintering aid. TiB2 has been successfully synthesized from TiCl4 + B2H6 mixtures. Boron powder has been made from both BCl3 and B2H6. TiO2 and Al2O3 have been made from alkoxides and reactants like Al(CH3)3. While most of this process research has been focused on a limited set of compounds and reactants, it is apparent that laser-induced reactions are applicable to a broad range of materials. The Si, Si3N4, and SiC powders all exhibit the same general features and match the idealized characteristics sought. The particles are spherical and uniform in size ranging from about 20 to 300 nm. The Si3N4 and SiC powders are smaller and have a narrower size distribution than the Si powders. For making thin films by the laser-induced CVD process, the laser beam passes parallel to the substrate through an optically absorbing gas that is heated by the laser. Thin films of amorphous hydrogenated silicon ( - Si:H) and Si3N4 have been made by operating under conditions where heterogeneous rather than homogeneous nucleation occurs. The virtually unique combination of high gas temperature and low substrate temperature permits rapid deposition rates and controlled film properties. Resulting films have demonstrated superior electrical, optical, structural, and mechanical properties. COLLOIDAL SYNTHESIS In the shape-forming of ceramics, a desirable goal is to achieve near-net-shape processing of complex monoliths with precise control of structural features at the submicron level. There are three basic steps that are closely linked in the conventional processing sequence: (1) synthesis or selection of
SYNTHESIS AND PROCESSING: GENERAL METHODS 23 the raw materials in powder form, (2) consolidation of powders either with the use of a liquid medium or by dry- pressing techniques, and (3) densification of the powder compacts by sintering. The importance of establishing a strong correlation between these three process steps is well recognized (Aksay, 1988). Colloidal techniques are useful in avoiding problems associated with uncontrolled agglomerate formation during the synthesis of powders and also in obtaining very-low-viscosity powder and fluid systems that are suitable for near-net-shape forming. A desirable goal is to work with systems containing submicron-size particles at solid contents of greater than 70 volume percent and viscosities of less than 1 Pa-s. With the use of polymeric additives and polydisperse particle size systems, this goal can be achieved in the micrometer range. When the particle size is reduced to the nanometer range, it becomes increasingly difficult to accomplish this goal via these conventional methods. However, with the use of lubricating surfactants, it is also possible to form dense (greater than 60 volume percent) nanoscale structures. Processing steps leading to the formation of colloidally consolidated compacts first start with the dispersion of particles in a liquid medium. This dispersion stage is useful for various reasons: (1) when the particle concentration is low, dispersed colloidal suspensions can be used to eliminate flow units larger than a certain size through sedimentation or centrifugal classification; (2) the surface chemistry of the particles can be modified through the adsorption of surfactants; and (3) the mixing of multiphase systems can be achieved at the scale of the primary particle size. Once the desired modifications are achieved, transition from a dispersed to a consolidated structure is accomplished either by increasing the particle-to-particle attraction forces (i.e., flocculation) or by increasing the solid content of the suspension (i.e., forced flocculation). Experimental observations have shown that colloidally consolidated systems always display hierarchically clustered nonequilibrium structures as a result of a nucleation and growth process of particle clusters. The most important consequence of this hierarchical clustering is that, even in monosize particle systems, a monomodal void size distribution is never attained. When the first-generation particle clusters are at a packing density of 75 percent, the packing density of the second- generation clusters drops to an average value of at least 64 percent because of the bimodality of the void size distribution. Since these hierarchically clustered structures signify the formation of nonequilibrium structures, Aksay (1988) has suggested the form of a nonequilibrium phase diagram in V/kT versus particle concentration space (Figure 7), where V denotes the generalized interaction potential, k is the Boltzmann constant, and T is the temperature. The high V/kT region of this diagram outlines the equilibrium transitions observed in highly repulsive systems in electrostatically interacting systems. The onset of fluid-to-solid transition shifts to lower concentrations as the hydrodynamic radius of the
SYNTHESIS AND PROCESSING: GENERAL METHODS 24 Figure 7 Schematic form of the nonequilibrium colloidal phase diagram. In the high V/kT region, the onset of fluid-to-solid transition shifts to lower concentrations because of increasing hydrodynamic radius. A similar trend is observed at the low V/kT region because of the formation of low-density fractal clusters. Body-centered cubic (bcc) packing of particles is observed with organic particles, but ceramic systems have been observed only with face-centered cubic (fcc) packing (Aksay, 1988).
