A Study of Some Inherent Causes for Non-Uniform Microwave Heating

Radio frequency (RF) and microwave heating of dielectric objects is often susceptible to an excessive temperature spread due to uneven energy deposition. The exposure to a nonuniform field is a well-studied cause for this difficulty encountered in numerous applications. There are, however, some less-understood causes, which are inherent in nature in that they persist even in a perfectly uniform field. We present an experimental study on three main inherent causes with rice grains as samples. Of these cause, somewhat less obvious is the polarization charge shielding effect
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. Exposed to the wave electric field, electrical charges in the dielectric object react in a way to partially shield the wave electric field. As a result, the dielectric object's interior electric field can be significantly smaller than the incident electric field, resulting in a much reduced power deposition. Experiments are conducted in an applicator, in which samples are irradiated by a 24 GHz microwave. High radiation uniformity (~99%) and polarization control allow a quantitative examination of each cause. Their individual and collective effects are found to be highly significant. In particular, polarization-charge shielding alone can result in a temperature spread of~ 18.2% for the samples examined. Physical interpretations are given and an effective method for its mitigation is demonstrated.