In recent years, a considerable interest has been drawn to microwave heating of powder metals and other electrically conductive materials. In this paper a consistent formulation describing the absorption of microwaves in electrically conductive materials under different microwave heating conditions is developed. A special case when conductive powder particles are surrounded by insulating oxide layers is investigated in detail using the effective-medium approximation. The conditions giving rise to skin effect governed, volumetric, and localized microwave heating are analyzed. Experimental observations of different microwave heating regimes in silicon, iron, and copper powder compacts are in general agreement with the theoretical model.

The possibility of macroscopic mass transfer in an ionic crystal placed in a high-frequency electric field has been studied theoretically. The plastic deformation of the crystal is shown to take place due to the vacancy-drift perturbations caused by space-charge formation in the near-surface layer and by surface permeability asymmetry with respect to the direction of the motion of atoms through it. The estimates prove that the effects considered can be essential in technological processes, e.g., in microwave sintering of ceramics.

A possible explanation of the experimentally observed enhancement of mass transport processes in ionic crystalline solids under the action of high-frequency (HF) electromagnetic fields is suggested. It is based on the ponderomotive effects that occur due to the nonlinear interaction of the HF electric field with the space charge induced by it within the crystal near its surface. Due to the action of the tangential component of the field in the near-surface amorphized layer, where the vacancy mobility is greater than in the bulk, the ponderomotive effects are found to be of sufficient strength to manifest themselves in the experimental conditions.

Numerous observations have been reported in the literature of enhanced mass transport and solid-state reaction rates during microwave heating of a variety of ceramic, glass, and polymer materials. An explanation for these controversial observations has eluded researchers for over a decade. This paper describes a series of recent experimental and theoretical investigations that provide an explanation for these intriguing observations in terms of ponderomotive forces acting on mobile ionic species. The ponderomotive phenomenon, like its conventional-plasma analog, can be described in the continuum model limit by combining the continuity, Poisson’s, and transport equations. However, the solid-state plasma version typically manifests as a result of gradients in mobile charge mobility (e.g., near physical surfaces or interfaces), whereas the conventional plasma ponderomotive transport is typically a consequence of gradients in the radiation field intensity. Both cases can be captured in a single, general, mathematical articulation developed in terms of the mobile particle fluxes.