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 interfaces͒, 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.

We describe the results of experiments designed to test for microwave enhancement of vacancy transport processes in NaCl. Experimental results indicate that intrinsic vacancy mobility is not enhanced by microwave fields. Instead, the evidence strongly suggests an enhancement of the driving force for charge transport. The experimental results display features consistent with a recent theory for microwave-enhanced driving forces in ionic conductors. PACS numbers: 66.30.Hs, 68.35.Fx A growing body of experimental data suggests that microwave heating of ceramic materials leads to enhanced diffusion or solid-state reaction rates when compared with conventional heating at the same temperature [1][2][3][4][5][6][7]. If microwave heating is perceived as a purely thermal process (by rapid equilibration of microwave energy to thermal energy of the material), then it is difficult to explain how microwave and conventional furnace heating can result in markedly different reaction rates. The several explanations attempted for these experimental observations fall into one of two classes: (1) nonequilibrium thermodynamics, and (2) "nonthermal" phenomena.First, it has been argued that most temperature diagnostics only measure surface temperature and are therefore unreliable indicators of internal bulk temperatures. With microwave heating of low-loss materials, inverted temperature profiles (interior hotter than surface) occur at steady state as heat is lost from the surface, and it has been proposed that the internal temperatures responsible for the solid-state reactions exceed the measured surface temperatures by tens to hundreds of degrees Celsius. However, in some of the experiments referenced above (such as Ref.[3]), the sample dimensions and microwave absorption rates are both very small and therefore inconsistent with temperature differences of 50 -200'C between the surface and interior.Several nonthermal hypotheses are based on the idea that microwave field disturbances of sufficient magnitude might enhance high-energy, nonthermal "tails" on ion energy or lattice phonon distributions [8,9]. Such effects would appear as an enhancement of ionic mobility or a lessening of the activation energy for ionic motion in such a lattice. However, calculations based on the phonon kinetic (Boltzmann) equation [10] indicate that these phenomena will be negligible for the microwave field intensities present in Refs. [1 -7]. Most recently, Rybakov and Semenov [11] have proposed a model in which the microwave field induces nearsurface oscillatory fluxes of ionic point defects that are rectified, yielding a net "ponderomotive" (time-averaged nonzero) transport of charged defects. In effect, the microwave field induces a nonequilibrium concentration of vacancies in a small region near the surface of the ionic crystal. Mass flow is required to reach this nonequilibrium condition, and because the vacancies are charged, there is also an induced charge How. If these nonlinear forces are large enough, they could explain the observations of micr...

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A numerical model was developed to simulate and study microwave-induced transport in ionic solids. The model is based on continuum equations, is very general, and could be applied to many materials. The assumptions, boundary conditions, initial conditions, and numerical techniques used in the model are described. Results are presented from a study of microwave driven defect transport in sodium chloride. Static, high-frequency, and quasistatic results show that ponderomotive rectification of vacancy fluxes will act to deplete the vacancies in a near-surface region and will continue to pull vacancies to the surface through diffusion kinetics. The ponderomotive driving force for this transport is characterized over a wide range of variable space. The magnitude of the driving force falls right in the range such that it can explain why microwave-enhanced mass transport is observed in some experimental cases but not in others.