Recent Studies on Fundamentals and Application of Microwave Processing of Materials

Abstract

Microwave (MW) application for heating was discovered in 1946 and has been applied in various fields. The detailed historical tracing is presented in the next section. Among the various aplications, it is possible to classify them into two major classes (Agrawal, 2005). In the first class, MW heating applied to drying, cooking foods and to excitation of chemical reactions such as inorganic/organic synthesis. The MW activated chemical reactions has been investigated in the field categorized as “MW Chemistry”. In these applications, the heating temperature is usually low, because most of the aqueous and organic liquids have the boiling point below 500oC. Mainly, the heating is caused by a dielectric relaxation loss due to rotation of molecules, which is as the result of the interaction of the MW electric field with the electric dipole of the molecules. Electric dipoles in liquid or gas experience relatively free rotational motion, comparing with the dipoles in a solid. On the other hand, for the MW heating application to sintering of metal/ceramics, solid state reaction, solid state phase transition (such as vitrification or devitrification) and high temperature reduction reaction, elevated temperature is needed, which often exceeds above 500oC. In these cases, heating mechanism is not limited to dielectric loss but to the other mechanisms of ohmic loss due to eddy current, and the magnetic loss also becomes important. Most of the oxides become electric conductive above 1000oC (Kingery, et al., 1975. From these considerations, understanding the effect of MW magnetic field interaction with materials becomes important (Roy, et al. 2002, Yoshikawa et al. 2006). As will be given in this paper, the magnetic field not only influences the magnetic heating mechanism directly, but also raises the induction current more effectively than in the electric field. As the special characteristics of MW heating, three different heating aspects have been pointed out, namely they are the internal heating, the rapid heating and the selective heating (Bykov, et al., 2001). It is possible to consider the industrial application of MW heating from these aspects. Moreover, it has been reported that the so-called “non-thermal effect” of MW heating exists and enhances the sintering and the reaction kinetics. Although the origin of the non-thermal effect has not necessarily been clarified in the present stage, its phenomena keep providing us motivation to the reseraches on understanding and application of the MW heating.