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

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.