Dielectric and Electrical Properties of CaCu3Ti4O12-based Composites with Insulating Oxide Coating

Abstract

The perovskite oxide CaCu3Ti4O12 (CCTO) has recently attracted particular attentions in the field of dielectric materials due to the extraordinary high dielectric constants of 104-105 in the temperature region of 100-500 K below 1 MHz. This dielectric property makes CCTO ceramics as a promising material in the area of micro-electronic techniques. Nevertheless, a relatively high dielectric loss and a large leakage current in CCTO ceramics are not desirable for the practical applications. According to previous reports, the origin of the giant dielectric response in CCTO ceramics can be explained by an internal barrier layer capacitance (IBLC) model

CCTO ceramics are composed of semiconducting grains with insulating grain boundaries. Therefore, the dielectric loss in CCTO ceramics is closely related to the insulation of the barrier layers, and the conductance of the barriers leads to the leakage currents. Thus, in this study, we tried to overcome the fatal problems by fabricating CCTO-based composites with insulating materials of low dielectric loss and negligibly small leakage current. The major results are as the following. First, the effects of the BaTiO3 (BTO) additive on the electrical properties of CCTO polycrystalline ceramics by the conventional solid state reaction were systematically investigated. Compared with CCTO ceramics (εr ~ 77,000 and tanδ ~ 0.11 at 1 kHz), CCTO-BTO composites show significantly reduced dielectric losses although their dielectric constants are depressed approximately one order of magnitude (εr ~ 4,200 and tanδ ~ 0.07 at 1 kHz for 5 mol% BTO-added CCTO composites). In addition, the leakage currents of these composites are greatly reduced at the applied voltage above ~50 V/cm. A large reduction in the dielectric losses and the leakage currents is attributed to the secondary phases at CCTO grain boundary. Second, in order to reduce a high dielectric loss and a large leakage current in CCTO ceramics, sol-gel coating of the perovskite oxides such as BaTiO3, (Ba,Sr)TiO3 (BSTO) and CaTiO3 (CTO) was performed on CCTO particles. CCTO composites with non-uniform BTO and BSTO coating exhibit significantly reduced dielectric losses and leakage currents while the dielectric losses and leakage currents in CCTO-CTO composites are not improved. Especially, the sample using 5 mol% BTO-coated CCTO powder exhibits the most improved properties of the dielectric loss (tanδ ~0.04 up to 100 kHz) and the leakage current (less than 1% up to DC field of ~ 1 kV/cm) although a large suppression in its dielectric constant (εr ~ 6,000 up to 100 kHz) is unavoidable. This improvement is mainly attributed to the increase in the activation energy of grain boundary due to a relatively uniform distribution of the second phases at CCTO grain boundary. Finally, in order to fabricate CCTO-based composites with the core-shell structure, TiO2-coated CCTO powder was prepared by sol-gel coating, and then the coating layer of TiO2 was converted into BTO phase by the hydrothermal reaction at 200°C for 4 h in air. CCTO-TiO2 composites using uniformly TiO2-coated CCTO powder show higher dielectric constants of ~100,000 in 102-107 Hz compared with that of CCTO ceramics, which are the highest values among CCTO-based composites. Furthermore, their dielectric losses are greatly reduced to ~0.05 at 1 kHz, and their leakage currents are also decreased in the applied DC field, which are attributed to the improved insulation of grain boundary. However, the improvement of dielectric and electrical properties was unobtainable from CCTO composites with BTO coating layer due to their relatively low sintered densities. In conclusion, although CCTO composites with BTO additive or with perovskite oxide coating including BTO and BSTO are very effective for reducing the dielectric losses and leakage currents of CCTO ceramics, a serious reduction in the dielectric constants are unavoidable since it is impossible to obtain a uniformly coated layer on the surfaces of CCTO particles by the sol-gel process, and further BTO and BSTO chemically react with CCTO to form the second phases above 950˚C in air. On the other hand, to overcome the fatal problems without degradation in the dielectric constants, TiO2 coating on CCTO particles by the sol-gel process is very effective since TiO2 can be uniformly coated on CCTO particles and also it is chemically compatible with CCTO. In addition, a uniform coating of BTO is possible by the two step coating processes composed of a uniform coating of TiO2 by the sol-gel process and subsequent reaction with Ba hydroxide by the hydrothermal process. Thus, TiO2- and BTO-coated CCTO powder are considered to be good candidates for high performance MLCC devices at relatively low frequency regime (< 1 MHz) which can replace pure BTO or BSTO powder.