Dielectric relaxation of a composite with tungsten nano-layered spherical filler particles

The thickness and conductivity of a surface coating on filler particles in a composite material are critical parameters in controlling the frequency at which dielectric relaxation occurs. In this paper, experimental results are presented for composites formed from tungsten-coated glass microbubbles, embedded in a matrix of paraffin wax. The tungsten coating is of the order of 10 nm thick. There is an outer coating of alumina, a few nm thick, to prevent oxidation of the tungsten and to prevent the formation of conducting pathways in the composite. Dielectric relaxation is observed at around 10 GHz. A remarkable feature of the system is the way in which the relaxation frequency is shifted, by approximately six decades, from the calculated value for a similar composite formed with solid tungsten filler particles (≈10¹⁶ Hz). This shift is attributed to the geometrical confinement of the conductor within a thin shell, and to the reduction in conductivity of the thin tungsten layer when compared with the conductivity of bulk tungsten.