Exploring temperature-dependent negative permittivity in zinc oxide multifunctional metamaterials

Abstract

This study investigates the transition from positive to negative permittivity in (Cu + Gd) co-doped zinc oxide multifunctional metamaterials synthesized through a solid-state pressureless technique. At room temperature, the material exhibits a colossal relative permittivity (εr′≥103) and moderate dielectric loss (∼0.7). The AC conductivity measurements and theoretical modeling reveal that the electrical transport mechanism follows the nonoverlapping small polaron tunneling model. A notable transition from highly positive to negative permittivity occurs in the temperature range of 473–673 K while maintaining low dielectric loss across frequencies from 1 kHz to 1 MHz. We attribute this behavior to increased electron scattering and enhanced ion mobility at elevated temperatures, which produce features reminiscent of negative-index materials. Activation energies of 0.63 eV for DC conductivity and 0.98 eV for dielectric relaxation suggest the formation of interstitial defects and polarons. These findings provide a straightforward route to achieving negative permittivity in ceramics and offer insights for designing high-temperature dielectric components.