Effect of particle shape on electrical conductivity and negative permittivity spectra of Cu granular composite materials

Electromagnetic (EM) metamaterials with negative permittivity and permeability can be used to develop advanced EM devices such as perfect EM wave absorbers and frequency selective microwave shields. One of the approaches has been to apply artificial periodic structures such as printed metal patterns or strip lines. However, granular composite materials embedded with metallic and/or magnetic particles can also be utilized to develop EM metamaterials. In this paper, we studied the effect of particle shape on the electrical conductivity and the relative complex permittivity spectra of Cu granular composite materials in the radio-frequency (RF) to the microwave frequency range. Three types of Cu composites containing spherical, flaky, and arborized particles were prepared, and their electrical conductivity and relative complex permittivity were measured. An insulator-to-metal transition was observed at the percolation threshold φc in all composites; the φc of the flaky and arborized particle composites was lower than that of the spherical particle composites. In composites above φc, a negative permittivity spectrum was observed regardless of the particle shape. The negative permittivity spectra of the percolated Cu particle composites were analysed using the Drude model. It was shown that the plasma frequency at which the real part of the permittivity changes from negative to positive depends not only on the particle content but also on the particle shape. Thus, our study facilitates the adjustment of the negative permittivity and enhancement in the electromagnetic properties of EM metamaterials, which can lead to the development of advanced EM devices.