Physicochemical processes in the microwave field

Abstract

Microwave radiation has become an important means of heating in scientific laboratories because of the number of advantages over the conventional heat sources. The extreme acceleration of chemical processes in the microwave field gives the ability to perform a number of syntheses in a very short time and efficient optimization of processes. Together with the capability to conduct processes without solvent (green chemistry), it makes microwave technology attractive for industry also. In the present article, the properties of microwave radiation of interest in physicochemical investigations are shortly described. The mechanisms of microwave heating: a) dipolar polarization, b) conduction and c) interphase polarization are considered as well as quantification of microwave absorption in irradiated samples. The influence of microwaves on the chemical reactions is rationalized through tree effects: thermal, specific and nonthermal. Thermal effects are related with the fast achievement of high temperatures due to the efficient transfer of heat to the reaction mixture. The specific microwave effects are described as thermal effects which are impossible to achieve by classical heating due to the particular mechanisms of microwave heat transfer. Since a number of experiments conducted in the special microwave reactors cannot be explained by thermal and specific effects, the sources of possible non-thermal effects are discussed as well. Although the microwave density showing influence on chemical processes is relatively high, further investigations in this area are justified due to the increasing microwave pollution arising from telecommunication and radar facilities.