1.5 - Evaporation Methods

Abstract

This chapter discusses evaporation methods that can be used to make ultrafine particles (UFPs). In evaporation methods, a substance is heated and vaporized. The vapor is then cooled by some method and condensed into UFPs. A representative method is evaporation in an inert gas atmosphere and is referred to as gas evaporation method. One of the evaporation methods is the SiC UFPs that are formed by evaporating silicon in argon gas containing methane. There are three steps involved in the processes of evaporation and condensation of a gas: (1) evaporation and supersaturation, which involves fluid mechanics including diffusion and thermal conduction, (2) vapor growth that resembles cloud and snow formation from supersaturated water vapor, and (3) fusion growth, which involves Brownian motion within an aerosol until collision occurs, after which it involves crystal growth including the surface energy of fusion. Experimentally, the parameters controlling the particle diameter are the temperature of the evaporation source and the type and pressure of the atmospheric gas. Impurities also affect the diameter and morphology of UFPs. The chapter describes the large scale evaporation equipment used in laboratory. This equipment is intended to make 10 to 100 g of high purity material, using an electric arc as the heating source. For easy-to-evaporate materials such as silver, direct resistance heating was used. UFPs made with this equipment were not uniform. Electron micrographs of the various UFPs represent typical UFPs that were produced.