Microwave Dielectric Resonance and Negative Permittivity Behavior in Al$_{2}$O$_{3}$-CuO-Cu Nanocomposites

The frequency-dependent microwave (0.1-18 GHz) complex permittivity of nanocomposites based on the Al$_{2}$O$_{3}$/CuO/Cu system is investigated. The composites are formed by solution infusion of copper precursors into a porous Al$_{2}$O$_{3}$ matrix, followed by thermal decomposition to copper oxides and localized formation of CuAl$_{2}$O$_{4}$ spinels, and finally partial reduction by H$_{2}$ firing. The final material has a complicated microstructure and exhibits strong amplitude, relatively narrowband dielectric resonance in the microwave regime at intermediate concentrations ($\sim $15-18{\%} by volume) of Cu. The resonances are superficially similar in structure to plasmon and Reststrahlen resonances typically seen in conductors at far-infrared to optical frequencies, but occurring at much lower frequencies in the composites. This is in contrast to the usual broadband induced-polarization dielectric relaxations observed in standard composites. Large concentrations of copper cause negative permittivity behavior below 6 GHz. Permittivity data, SEM micrographs, and possible explanations will be presented.