S. A. Freeman

Modeling and numerical simulations of microwave-induced ionic transport

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.

Novel method for measuring intense microwave radiation effects on ionic transport in ceramic materials

Many experimenters over the past few years have observed rate enhancements when using microwave processing compared to conventional processing. The results have remained somewhat controversial because the driving forces are not constant throughout the experiments and because of the possibility of inaccurate temperature measurement. This paper describes a different experimental approach that avoids the pitfalls of the previous experiments. Specifically, the alternative approach involves fixed driving forces and short duration experiments. Reliable and accurate knowledge of the sample temperature is ensured by restricting the amount of microwave heating to negligible levels and using conventional ohmic heating to control temperature. This approach has allowed us to confirm the presence of a ‘‘microwave effect’’ on kinetic processes in ionic ceramic materials. With this configuration, many further experiments are possible with different materials and conditions.

Microwave Radiation Effects on Ionic Current in Ionic Crystalline Solids

We describe experiments to test for microwave enhancement of bulk vacancy diffusion in NaCl, bulk interstitial diffusion in AgCl, and grain boundary diffusion in NaCl. Experimental results for bulk vacancy diffusion in NaCl indicate that intrinsic vacancy mobility is not enhanced by microwave field intensities typical of microwave sintering experiments. Nevertheless, a microwave effect is observed with features consistent with a recent theory for microwave-enhanced driving forces.

High-Frequency Field Effects on Ionic Defect Concentrations and Solid-State Diffusion Processes

We describe computer simulations of a microwave-induced driving force for ionic transport. The simulations are based on a model which predicts rectification of ionic fluxes at interfaces and a resulting depletion or accumulation of defects near the interface. Some details of the model are discussed, results of the simulations are presented, and the impact of these effects on sintering and diffusion is discussed.

Statistical Comparative Analyses of Engineering Properties of Microwave and Conventionally Sintered Alumina

Processing conditions such as temperature, soak time, and heating rate affect the final density of conventionally-sintered and microwave-sintered ceramics. Of additional importance is the question of whether microwave-sintered ceramics display intrinsically superior macroscopic engineering properties compared with conventionally-sintered control specimens. An analysis using the Yates algorithm indicates that the processing condition which has the largest impact on the density of the specimen is the heating method (microwave vs. conventional). The microwave-sintered specimens resulted in higher densities and higher fracture strengths. However, it was determined that the higher fracture strengths were due to the higher sintered densities rather than a significantly different microstructure.

Nonthermal effects on microwave radiation on ionic diffusion in ionic crystalline ceramics

Summary form only given, as follows. Numerous claims have been made of observations that microwave heating of ceramics promotes faster processing or solid state reactions (e.g., sintering, bonding, tracer ion diffusion) than conventional furnace heating. These claims are controversial due to the absence of a verifiable theoretical explanation as well as questions on the interpretation of sample temperature measurements obtained in the presence of microwave fields or during microwave heating. A recent experimental investigation has clearly established that intense microwave fields can yield enhanced ion (or vacancy) diffusion in ionic crystalline ceramics. The results are in qualitative agreement with a model in which the microwave fields exert a ponderomotive or radiation pressure acting near the surfaces of NaCl crystals or grains. It is believed that this phenomenon explains the observations of microwave-enhanced tracer ion diffusion in higher temperature oxide ceramics and glasses. Future work will explore the role of this phenomenon in grain boundary diffusion and sintering during microwave processing of ceramics.