EFFECTS OF ANOMALOUS PERMITTIVITY ON THE MICROWAVE-HEATING OF ZINC-OXIDE

Highly nonuniform heating has been observed in zinc oxide (ZnO) powder compacts exposed to 2.45 GHz microwaves in oxygen deficient atmospher es such as pure nitrogen or argon. This phenomenon manifests as a loca lized zone of rapid heating which propagates outward from the sample c ore, and is documented by real-time surface and core temperature measu rements performed during the microwave exposure. Measurements of the c omplex permittivity, epsilon'', during heating of identical ZnO sample s in a conventional furnace and in a nitrogen atmosphere, demonstrated that epsilon'' experiences at least one significant maximum between 2 00 and 500 degrees C. Mass spectrometry results indicate that the peak s in epsilon'' correlate well with the rate of desorption of chemisorb ed water from the surface of the ZnO powder. It was also noted that th e nonuniform heating does not manifest when the microwave exposure is performed in air. Similarly, the anomalous peaks in epsilon'' are almo st completely suppressed during heating in air. It is well known that oxygen adsorbs strongly to the surface of ZnO in the temperature range from room temperature to 300 degrees C, and that this adsorption resu lts in a drastic decrease in the electrical conductivity and, thus, in epsilon''. It is proposed, therefore, that the effect of water desorp tion upon the complex permittivity may be, in effect, counterbalanced by the adsorption oxygen from the atmosphere. The effect of this behav ior may be significant during microwave processing, where nonuniform p ower absorption can result in extremely localized heating. (C) 1998 Am erican Institute of Physics.