PTCR effect of Ba-excess barium titanate ceramics

The effect of Ba-excess in modified BaTiO3 was investigated with respect to the microstructure and electrical properties. Y2O 3 additions between 0.18--0.3mol%, along with a variance in the Ba/Ti ratio from 0.987 to 1.02 were studied. It was found that excess behavior, i.e., a much higher eutectic temperature, small grain size and electrical insulating behavior with a resistivity near 1011 ohm·cm. Through addition of SiO2, these materials were found to exhibit semiconducting behavior with low (100--103 O-cm) room temperature resistivity. This transition was determined to be a result of SiO2 induced liquid phase sintering which aids in donor dispersion. A low sintering temperature was necessary in order to avoid development of the high cation concentrations to compensate conduction mechanisms. MnO was used as an acceptor dopant in these materials to enhance the magnitude of the PTCR rise, from 2 to 6 orders. The current-voltage and current switch characteristics of materials with various compositions and processing parameters were investigated. It was found that Ba-excess materials were able to withstand higher electrical voltage than the Ti-excess materials. Through structural observation, it was found that the domain configuration in Ba-excess materials is a function of Ba/Ti ratio and SiO2 concentration, which is consistent to the material electrical resistivity, Randomly oriented domain sets within individual grains exist in samples is developed during the sintering of Ba-excess BaTiO3 while the domain alignment is uniformly directional through each grain as SiO2 concentration is increased to 2mol%. In this study, a secondary thermal treatment (450--650°C/10--30 minutes) of previously processed PTCR samples was discovered and developed. This treatment can decrease the rhoRT, corresponding to enhancement in the PTCR behavior, as there is essentially no change in the maximum resistivity. Characterizations of this effect indicate that it originates from internal stress relief within the grain boundaries. The effect is modeled with respect to the enhanced spontaneous polarization for surface state compensation. The defect dipoles (electric dipoles or elastic dipoles) are proposed to exist near grain boundary and provide a driving force for realignment of spontaneous polarization. The well-known Heywang-Jonker model was able to be revised with consideration of the impact of the internal stress on the spontaneous polarization and its compensation to the grain boundary potential barrier. In study, a relationship of the effective spontaneous polarization and internal stress is proposed and defined.