Microwave Absorption in Percolating Metal-insulator Composites*

We measure several electromagnetic properties of tungsten-Teflon composites as a function of metal volume concentration. The electric (E) and magnetic (H) loss tangents at 2.45 GHz and the dc conductivity each exhibit a percolation transition at a different critical value of the metal volume fraction, $p$. Moreover, the transition behavior depends on the average particle size and size distribution of the metal component. We explain the variation in each case by a schematic model derived from established percolation theory and the distinct response of conducting particles to microwave electric and magnetic fields.