PNNL

Abstract

The slow hydrothermal growth of single crystals is a well-established technique that has been used for decades in the production of materials such as single crystal quartz. It is however, not a technique that is usually used to generate particles with dimensions in the nanometer size range. In particular, at temperatures and pressures exceeding those at the critical point of pure water (Tc=374 degrees C, Pc=221 bar, pc=0.322g/cm3) the production of nano-particles becomes more difficult in a static cell. At these conditions, the solubilities of ceramics such as SiO2 or Al2O3 are appreciable, and the rates of crystal growth and Ostwald ripening are high. Furthermore, the kinetics of chemical reactions in a single phase are expected to be fast. Hence, nanometersized particles have so far only been obtianed from a supercritical aqueous environment by two types of non-equilibrium processes. The first approach is to establish a short residence time to limit crystal growth by inducing an abrupt homogeneous nucleation of a dissolved solute in the region of an expanding jet.