INFLUENCE OF CUBIC PEROVSKITE PHASE CONTENT ON THE POSITIVE TEMPERATURE-COEFFICIENT OF RESISTANCE OF N-TYPE BATIO3 CERAMICS

The resistivity-temperature as well as the current-voltage (I-V) relations in donor-doped polycrystalline BaTiO3 (tetragonal) ceramics are tremendously modified by the presence of cubic perovskites such as BaSnO3, BaZrO3, SrTiO3 and BaTiO3 in minor quantities. The minor cubic phase contents arise out of the compositional inhomogeneity in large industrial batches of titanate solid solutions or in chemically prepared BaTiO3 powders or because of the grinding of raw materials for extended periods of time. Maintaining the same processing parameters, chemical compositional factors and microstructural contributions, the shape of the positive temperature coefficient of resistance (PTCR) curves are found to be completely altered by the addition of less than 10% cubic phase. The resistivity continues to increase not only across the Curie point T(C) but also at higher temperatures, without any apparent maximum even above 500 K. Thus the PTCR region is broadened and the ratio of resistivities across T(C) has decreased. This is in sharp contrast with the characteristics of homogeneous solid solutions having the same chemical compositions wherein rho-max/rho-min is over six orders of magnitude, with PTCR occurring in a narrow temperature range. The I-V curves of the mixed-phase ceramics show stable current-limiting characteristics at higher field strength without any current maximum. The electron paramagnetic resonance spectra indicate the contribution of the disorder component induced by the minor phase. The vibronic activation of acceptor states arising mostly from barium vacancies located at the grain boundary layers takes place over a broader temperature range, as a result of the disorder contribution and the consequent spread of energy values.