Powder Production by Supercritical Fluid Techniques

A supercritical fluid is defined as a material above its critical temperature and critical pressure (see Table 2.1). These fluids are characterized by gas-like transport properties and liquid-like densities. They also offer a greatly enhanced solvating capability in comparison with gases. Recently, the use of supercritical fluids has been applied to the generation of ceramic powders. The rapid expansion of a supercritical fluid solution results in the formation of a powder. As the pressure is reduced, the solubility of the solute decreases and supersaturation occurs. Stable 

Matson et al. have used the RESS process to produce silica powders from supercritical water. The solubility of silica in water at 500 °C and 1000 atm is 2600 ppm. It was found that the size of the product depended strongly on the silica concentration in the supercritical fluid prior to expansion. Particle sizes ranged from 0.1 to 0.5 p.m in diameter. The morphology of the particles was found to depend on the nature of the expansion nozzle—an orifice nozzle produced elongated particles, while a capillary nozzle produced spheres. 

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