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SYNTHESIS AND PROCESSING: GENERAL METHODS 20 and a thin film of liquid adheres to the filament. Upon exposure to atmospheric moisture, the film gels. The filament may be dipped repeatedly to build up a series of films. At this point, a rigid, continuous shell runs the length of the filament. Further work is in progress to optimize the processing steps so that a crack-free fiber can be collapsed to glass at around 1000°C in containerless conditions. In addition, the volatile host, being flexible, can be used to facilitate conformal coatings on complex shapes. In some cases, the organic polymer is left in the ceramic matrix when the composite is intended for use at low temperatures. The name ceramers has been coined for such materials (Klein, 1987). Infiltrated Composites The third experimental technology with respect to composites involves infiltration (Klein, 1987). In this case the objective is to use sol-gel processing to create a nanoporous matrix that can be infiltrated by a second phase. The second phase is intended to improve the thermal shock resistance of the composite. The product of the sol-gel process is by its very nature two-phase. At the time the oxide polymer condenses, the second phase is the rejected solvent. When the solvent is removed during drying, the second phase is the interconnected porosity. The channels where gas replaces liquid are typically 2 to 20 nm, uniform in cross section, and narrowly distributed in size. In the case of silica, this porosity does not interfere with transmission of visible light despite measured surface areas in the range of 200 to 500 m2/g. Since the porosity in silica is interconnected until temperatures around 800°C, the pore channels provide a means for distributing a second phase with high uniformity. The second phase may be introduced by a solvent-exchange process or by liquid intrusion. The thermal expansion, thermal conductivity, and oxidation behavior at elevated temperatures are the important physical properties of ceramic matrix composites. In the long run, the projected advantages for the sol- gel approach to composite fabrication are the simple processing steps, the flexibility of solution chemistry, the low-temperature treatments, and the small investment in equipment. There is a need for innovative thinking to adapt sol-gel processing to ceramic matrix composite fabrication. SUPERCRITICAL FLUID PROCESSING Dense gases and liquids at conditions above their respective thermodynamic critical points are known as supercritical fluids. The
