Effects of adding HfO$_{2}$ on the microstructure and dielectric properties of giant dielectric constant ceramic CaCu$_{3}$Ti$_{4}$O$_{12}$

CaCu$_{3}$Ti$_{4}$O$_{12}$ (CCTO), an unusual perovskite-like ceramic, is known for its extraordinarily high ($\sim $10$^{4})$ and relatively frequency independent dielectric constant. It has drawn a lot of attention recently because of its potential applications in microelectronics and microwave devices. In this investigation, HfO$_{2}$ powder was added to a pre-reacted CCTO powder, which was synthesized by a conventional solid-state reaction, at different concentrations from 1 to 70 wt{\%} and the mixture was sintered into disc-shaped ceramic samples. The effects of adding HfO$_{2 }$on the microstructure and dielectric properties of CCTO ceramics were investigated. In general, we found that the dielectric constant tends to increase with HfO$_{2}$ addition up to 8 wt{\%} and then decrease with further addition. Moreover, the dielectric loss was also influenced by the addition of HfO$_{2}$, and a low loss tangent of $\sim $0.035 was obtained. The ac conductivity, impedance, complex dielectric permittivity and electric modulus graphs were used to analyze the data. These observations were explained on the basis of the internal-barrier-layer capacitor model with Maxwell-Wagner relaxations.