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Mechanisms for nonthermal effects on ionic mobility during microwave processing of crystalline solids

Published online by Cambridge University Press:  31 January 2011

John H. Booske ,
Reid F. Cooper  and
Ian Dobson
John H. Booske
Affiliation:
Department of Electrical and Computer Engineering, University of Wisconsin–Madison, Madison, Wisconsin 53706
Reid F. Cooper
Affiliation:
Department of Materials Science and Engineering, University of Wisconsin–Madison, Madison, Wisconsin 53706
Ian Dobson
Affiliation:
Department of Electrical and Computer Engineering, University of Wisconsin–Madison, Madison, Wisconsin 53706
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Abstract

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.

Type
Articles
Information
Journal of Materials Research , Volume 7 , Issue 2 , February 1992 , pp. 495 - 501
Copyright
Copyright © Materials Research Society 1992

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