Complete absorption of 2.45 GHz microwaves by multiple-conductive-layered system and application in heating technology

This paper discusses the complete absorption of microwaves by conductive plates and its application in heating technology. Numerical calculations were conducted to determine the conditions for complete absorption of 2.45 GHz microwaves propagating through a rectangular waveguide tube with an inner dimension of 5.5×10.9 cm2. Based on the calculated results, we developed a resonating absorber with a three-layered structure of single-crystalline silicon (c-Si) which consisted of a 0.7-cm-thick semi-insulating c-Si substrate (DP) with a resistivity over 1000 Ωcm, a 0.49-cm-thick c-Si substrate (CP1) with 19-22 Ωcm, and a 0.05-cm-thick c-Si substrate (CP2) with 0.001 Ωcm or less. The absorber achieved complete microwave absorption with a measurement accuracy of 2%. Complete absorption was again observed when a half-wavelength (λ/2)-long air gap was introduced between the DP and CP1 layers. A radiation thermometer detected a temperature increase of 75.5 K in CP2 under 500 W microwave irradiation for 10 s, which was higher than the 43 K increase observed without the air gap. This demonstrates that the λ/2-air gap effectively provides thermal isolation between the DP and CP1 layers.