1.6 - Oxides

Abstract

This chapter discusses several methods to synthesize oxide ultrafine particles (UFPs). One method involves oxide melting followed by evaporation. A second method produces UFPs by heating and evaporating metals and semimetals in an oxygen atmosphere causing oxidation. When a piece of metal is heated in a mixture of inert gas containing oxygen and evaporated, metallic oxide UFPs are obtained. In comparison to wet methods for making UFPs, the gas evaporation method allows for the synthesis of high purity samples because the process is simple and contamination by impurities is minimal. The purity of UFPs depends on that of the metal and the gas atmosphere. The general features of oxide UFPs made by the arc discharge method are that the UFPs are nearly spherical in shape and that they are meta-stable crystal structures, which is due to the growth of the UFPs in nonequilibrium conditions. This method has yielded oxide UFPs of iron, aluminum, titanium, silicon, etc. The chapter discusses UFPs of γ-alumina that were obtained under the conditions such as total gas pressure of 100–400 torr and gas ratio. One aspect of the UFP studies is to examine the anomalies in the crystal structures due to the particle diameter effect. Multiple twinned particles are observed in metallic UFPs as the diameter decreases. Studies indicate that anomalous crystal structures in UFPs arise due to quenching during crystallization because UFPs have large specific surface areas, making them easily affected by cooling.