1.13 - Quenching of γ-Iron UFPs to Room Temperature

Abstract

In bulk materials, the γ-form of iron cannot be retained at room temperature. However, γ-iron particles could be precipitated in MgO matrices when heat-treated. ultrafineThough a small amount of γ-iron ultrafine particles (UFPs) are present in UFPs of iron made by the evaporation method, the amount of γ-iron was too small for the study of the properties of the material. A sufficient quantity of γ-iron UFPs was obtained and quenched to room temperature so that the properties could be measured. This chapter describes an efficient method for preparing γ-iron UFPs by using microwave plasma processing. Attempts were made to make α-iron smoke by using the gas evaporation method, transforming this to γ-iron by heating, and then quenching the material to room temperature. The chapter presents the electron micrographs of different iron UFPs. With the experimental method, only 10% γ-iron was obtained. Though it is not clear what cooling rate is required to obtain 100% γ-iron, the factors that led to the reduced cooling rate were considered, The first factor is insufficient differential pumping, which induces heating in particles attached to the cooling surface by the bombardment of gas molecules arriving later, and cooling due to adiabatic expansion when the smoke moves out of the heating chamber. Finally, it is possible that γ-iron particles captured at liquid nitrogen temperature become α-iron when they are warmed to room temperature for observation.