Modeling the Heat Wave Generated during Microwave Heating of Powdered Zinc Oxide in a Nitrogen Atmosphere

A theoretical investigation of previous experiments on the microwave heating of cylindrical samples of powdered ZnO in nitrogen and air environments has been presented. Unlike heating in air, which showed an exponentially saturating temperature behavior, a large discontinuity in the measured temperature was observed in nitrogen. Energy considerations suggested that the microwave power was mostly absorbed in a narrow, localized region that moved radially outward as a propagating heat wave. It was assumed that this behavior was caused by an anomaly in the temperature dependence of the permittivity, and the present simulations verify this assumption. The phenomenon has been investigated using a newly developed two-dimensional code, and the results are in good agreement with the experimental observations. The calculated width of the microwave absorbing region was ∼1 mm, and the radial velocity of the electromagnetically driven heat wave was inversely proportional to the square of the distance from the center of the sample, with a maximum speed of ∼20 mm/min.