Mechanisms for nonthermal effects on ionic mobility during microwave processing of crystalline solids
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Reid F. Cooper Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53706
Ian Dobson Department of Electrical and Computer Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53706 (Received 28 January 1991; accepted 11 September 1991)
Models for nonthermal effects on ionic motion during microwave heating of crystalline solids are considered to explain the anomolous reductions of activation energy for diffusion and the overall faster kinetics noted in microwave sintering experiments and other microwave processing studies. We propose that radiation energy couples into low (microwave) frequency elastic lattice oscillations, generating a nonthermal phonon distribution that enhances ion mobility and thus diffusion rates. Viewed in this manner, it is argued that the effect of the microwaves would not be to reduce the activation energy, but rather to make the use of a Boltzmann thermal model inappropriate for the inference of activation energy from sintering-rate or tracer-diffusion data. A highly simplified linear oscillator lattice model is used to qualitatively explore coupling from microwave photons to lattice oscillations. The linear mechanism possibilities include resonant coupling to weak-bond surface and point defect modes, and nonresonant coupling to zero-frequency displacement modes. Nonlinear mechanisms such as inverse Brillouin scattering are suggested for resonant coupling of electromagnetic and elastic traveling waves in crystalline solids. The models suggest that nonthermal effects should be more pronounced in polycrystalline (rather than single crystal) forms, and at elevated bulk temperatures.
I. INTRODUCTION The unique characteristics of ceramics, ceramic/ refractory composites, and glasses have led to their widespread application in many areas of the consumer and industrial marketplace. Ultimately, it is hoped that these materials will address needs in such advanced applications as high-temperature engine components, structural components for space vehicles/station, shock and radiation-resistant high-voltage insulators, specialty waveguides, structural and engine components for advanced aircraft, etc. Significant improvements in materials and material properties, as well as in cost and ease of manufacture, are needed, however, before many of these applications are realized. To this end, perhaps the greatest improvements are to be found in the technique of sintering, due to the major influence that sintering has in determining critical microscopic and macroscopic properties of the final product, as well as its obvious role as a relatively inexpensive manufacturing process for brittle, refractory solids. Sintering is conventionally accomplished in ohmically heated ovens as a conduction process. That is, heat is deposited on the outside of the object (by radiant, J. Mater. Res., Vol. 7, No. 2, Feb 1992 http://journals.cambridge.org
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conductive, and/or convective heat transfer) and diffuses inward by conduction. Theref
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