Diffusion in Microwave-Heated Ceramics

Microwave heating is an increasingly important method of chemical activation in both the laboratory and industrial processing. The increased breadth of microwave applications has not, however, been accompanied by a corresponding depth of understanding of microwave heating. The question of what constitutes a “microwave effect” remains unresolved, and that of whether “nonthermal” microwave activation takes place remains controversial. The effects of microwaves in solid materials and in biomolecules, in particular, are incompletely understood. In this paper, a method is described in which the effects of microwave heating may be investigated during the sintering of ceramic materials. By comparison of the diffusion rates parallel with, and perpendicular to, a polarized microwave field, the effects of thermal energy may be separated from those that are directly due to the microwaves. The results suggest that there is an enhanced sintering rate that is due to the presence of the microwave field, which becomes less significant as the sintering time increases. The results are consistent with a “ponderomotive” model that has been suggested previously and may have implications for reduced energy use in solid-state processing and for the rate at which thermally sensitive materials may be processed.