Enhancement of dielectric constant at percolation threshold in CaCu3Ti4O12 ceramic fabricated by both solid state and sol-gel process

We have investigated the large dielectric enhancement at the percolation threshold by introducing metallic RuO$_{2}$ grains into a matrix of CaCu$_{3}$Ti$_{4}$O$_{12}$ (CCTO). The intrinsic response of the pure CCTO samples prepared by solid state and sol-gel processes results in a dielectric constant on the order of 10$^{4}$ and 10$^{3}$ respectively with low loss. Scanning electron microscopy and energy dispersive x-ray spectroscopy indicate that a difference in the thickness of the copper oxide enriched grain boundary is the main reason for the different dielectric properties between these two samples. Introducing RuO$_{2}$ metallic fillers in these CCTO samples yields a sharp increase of the dielectric constant at percolation threshold f$_{\mathrm{c}}$, by a factor of 6 and 3 respectively. The temperature dependence of the dielectric constant shows that the dipolar relaxation plays an important role in enhancing dielectric constant in composite systems.