Giant permittivity and Maxwell-Wagner relaxation in Yb : CaTiO3 ceramics

Yb-modified CaTiO3 ceramics sintered for 24 h at 1450 °C exhibit a giant apparent permittivity of about 10(4) with a remarkable temperature (30-300 K) and frequency (101-106 Hz) stability (maximum variation: ±20%). After a post-annealing treatment of 48 h at 1100 °C, the dielectric response is strongly modified, with a high frequency, low temperature permittivity corresponding to the intrinsic dielectric behaviour of CaTiO3. A step-like rise in the apparent permittivity and a complex dielectric behaviour is observed with increasing temperature. The overall dielectric response of the ceramics can be interpreted in terms of the Maxwell-Wagner interfacial relaxation and modelled using equivalent circuits. The analysis demonstrates that the as-sintered ceramic consists of semiconducting grains (resistivity <10(4) Omega cm at 300 K) and insulating grain boundaries (resistivity ~10(7) Omega cm at 300 K). Partial reoxidation takes place during post-annealing with the formation of oxygen gradients and increased electrical heterogeneity. In general, the dielectric response will be determined by the extent of the reoxidation reaction during cooling or post-annealing treatment below the sintering temperature.