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SYNTHESIS AND PROCESSING: GENERAL METHODS 21 supercritical regime is illustrated in Figure 6. A supercritical fluid behaves as a high-density gas with liquid-like density. At moderate temperature and pressure, such fluids have high solubility for other materials and exhibit no surface tension. Figure 6 Supercritical fluid characteristics (R. A. Wagner, private communication). The technique offers several opportunities for the processing of submicron-level particles, structures, and coatings. One recent example demonstrated this potential (Wagner, 1988). Current oxidation protection for carbon-carbon composites relies on outer coatings of silicon carbide ceramic and glassy sealers. Thermal expansion mismatch between the silicon carbide and the composite results in coating cracks and oxidation at temperatures below 800°C. Using supercritical fluid impregnation, both oxidation resistance and bend strength of the coated carbon-carbon composite were substantially improved. There is a broad range of applications for this approach (Smith et al., 1987). These include powder preparation, surface modification, thin film deposition, and, as shown by the above example, composite impregnation. LASER PYROLYSIS Laser-initiated gas phase synthesis methods have been developed for making powders and thin films of ceramic materials. Powders have been synthesized with crossflow and counterflow gas stream-laser beam configurations and with both static and flowing gases (Cannon et al., 1982).
