The difference in microwave loss mechanism between nano- and micron-sized Ag particles

Abstract

Separated E/H-field heating in microwave cavities enables us to examine the microwave loss mechanisms. We prepared mixtures of SiO2 particles (having almost no microwave loss) and Ag particles with micrometer- and nano-sizes. Their average impedance and dielectric constants were measured. Electric conductivity exhibited percolation behavior at a specific threshold of Ag fraction. When the mixtures were heated in the separated fields, H-field heating was dominant in all the fraction ranges for the micrometer-sized Ag mixture. However, E-field heating was dominant below the percolation threshold for the nano-sized Ag mixture, and above the threshold, the H-field became dominant. This change in heating rates indicates the change in the energy loss mechanism. According to the size dependence of the induction current loss in a metallic sphere, the loss becomes less effective below the size of skin depth, namely, in the nano-size region. The permittivity of nano-sized Ag particles was estimated by an approximated Lindhard’s equation, which was input into the Maxwell–Garnett mixing model for predicting the effective complex permittivity of the mixture. It was possible to interpret the measured permittivity dependence on the Ag fraction.