Effects of rare-earth doping and nano-sized additive on the electrical properties at grain boundaries in the calcium copper titanate system

Abstract

Materials with high permittivity are in demand for developing miniaturized capacitors used in advanced energy storage systems and integrated electronics. The perovskite oxide $CaCu_3 Ti_4 O_{12}$ (CCTO) is a promising material for next-generation capacitors owing to its extremely high dielectric constants of $10^4 - 10^5$ in the temperature range of 100-500 K below 1 MHz. However, a relatively high dielectric loss and large leakage current with low breakdown voltage hinder the use of CCTO in practical applications. Based on the internal barrier layer capacitor (IBLC) model, involving an electrically heterogeneous microstructure consisting of semi-conducting grains and insulating grain boundaries, the drawbacks of CCTO including the dielectric loss and leakage current are closely related to the resistivity of the insulating barrier layers. We tried to overcome the critical problems through dopant engineering of CCTO with rare-earth dysprosium (Dy) ions. Upon doping with one atomic percent of Dy, the donor-acceptor bifunctionality of Dy was identified through impedance spectroscopy, current-voltage (I-V) characteristics, and atomic-scale scanning transmission electron microscopy. The bifunctionality of Dy influences the variation of the resistivity at grain boundaries, thereby improving the dielectric loss. In addition to this dopant, the effect of a nano-sized $BaTiO_3$ (BTO) additive with low dielectric loss and small leakage current on the dielectric responses of CCTO ceramics was investigated by controlling the amounts and sizes of BTO. Because of this additive, the dielectric loss of CCTO-BTO composites decreased with increasing amounts of BTO for a given BTO size. Thus, the present results indicate that rare earth doping and the addition of a nano-sized additive to CCTO ceramics are effective methods for reducing the dielectric loss by dramatically increasing the resistivity of the grain boundaries.