Applications of supercritical fluids in powder processing

Abstract

This paper reviews methods of producing particles using supercritical fluids. First it explains the properties of supercritical fluids, phase behaviors of supercritical solutions, and reaction equilibrium and reaction rate in supercritical fluids. Next it explains the principles of new crystallization methods using supercritical fluids. These explanations cover the RESS method, GAS method, retrogressive crystallization, supercritical drying, and hydrothermal synthesis in SCW. The use of rapid expansion of supercritical solutions (RESS) through a nozzle obtains extremely high supersaturation, which brings about rapid nucleation. This leads to the formation of ultrafine particles. Changing the expansion conditions allows one to control particle morphology and size. Using two nozzles makes it possible to coat particles with other materials. Supercritical CO 2 can be used as an antisolvent. This gas anti-solvent (GAS) method is used to recrystallize the solute, which is thermally or chemically unstable. The solubility of a solute in a supercritical fluid is largely dependent on temperature and pressure, and this temperature and pressure dependence differs from one substance to another. Thus, manipulating temperature makes it possible to precipitate only one solute from a mixture solution. This method is called retrogressive crystallization. Supercritical fluid extraction can be used for drying particles. Using this method eliminates capillary force among particles because there is no gas-liquid interface during drying, thereby inhibiting particle coagulation. Hydrothermal synthesis in supercritical water is an attractive method for producing metal oxide fine particles while controlling particle size, morphology, and crystal structure. Since SCW is miscible with O 2, H 2, or CO gas, these gases can be used to control the oxidizing or reducing atmosphere. The reaction equilibrium for hydrothermal reactions varies greatly around the critical point by slightly varying pressure and/or temperature, which brings about change in the morphology of the produced particles. This paper also considers the possibilities of creating new powder processing processes.