Absorption characteristics of single-layer ceramics under oblique incident microwave irradiation

The absorption characteristics of single-layer ceramics under oblique incident microwave irradiation were investigated by evaluating the effect of microwave incident angle (θi) on reflection losses (RL) for both transverse electric (TE) and transverse magnetic (TM) polarizations. The materials investigated were a high-loss SiC layer and a low-loss Al2O3 layer with impedance matching thicknesses of 0.0054 and 0.923 m, respectively. The reflection losses of the ceramic layers over the incident angle ranging from 0° to 90° were determined using equations derived from transmission line theory. From the RL patterns obtained, SiC exhibits a much smaller minimum reflection loss (RLmin) at a higher θi for TM polarization compared with the equivalent for TE polarization. This difference is believed to be a result of the effect of Brewster׳s angle. For Al2O3 at room temperature, there is a negligible difference of RLmin between TE and TM polarizations due to the low dielectric loss of Al2O3. When temperature increases to 1379 °C, the Al2O3RLmin values for TE polarization show frequent fluctuations in the entire temperature range, indicating the difficulty in predicting absorption in low-loss materials for TE polarization during microwave heating. For TM polarization, reflection loss of Al2O3 varies monotonically with θi as the temperature is increased that is shown by a decreasing RLmin value when the corresponding incident angle increases. These results indicate that TM-polarized microwaves help reduce reflection losses of both high- and low-loss ceramics. Power absorption in ceramics throughout the heating process can be improved by modifying the angle of incidence and polarization type of microwaves.