Positive temperature coefficient of resistivity effect in highly donor-doped barium titanate

BaTiO3 ceramics doped with different La concentrations (0-12 mol%) were pre pared by sintering under the reducing conditions of a nitrogen atmosphere c ontaining 1% hydrogen. The critical donor concentration that causes blockin g of the exaggerated grain growth was observed to be similar to 10 mol% La. The samples, which were semiconducting after sintering under reducing cond itions, were subsequently reoxidized by annealing in air to induce the posi tive temperature coefficient of resistivity (PTCR) effect. After reoxidatio n at 1150 degreesC a noticeable PTCR effect was observed in the samples dop ed with La concentrations as high as 2.5 mol%. The room-temperature resisti vity after reoxidation was found to increase with increasing donor concentr ation due to an increase in the thickness of the insulating layers at the g rain boundaries. TEM analysis showed that reoxidation of the samples caused precipitation of the Ti-rich compound Ba6Ti17O40 inside the doped BaTiO3-m atrix grains.