Effect of dopant (Nb) concentration on the grain boundary electrical properties of Nb-doped barium titanate
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A series of coarse-grained BaTiO3 specimens with different dopant (Nb) concentrations were prepared by adjusting the oxygen partial pressure during sintering. They were again heat-treated in air, and the behavior of the grain boundary electrical properties with the increase of Nb concentration was investigated under the conditions of the same microstructure and heat treatment. The interface states of the grain boundaries were estimated using the grain boundary R (resistance) and C (capacitance) values at each temperature that were obtained from impedance analysis. An increase in the interface state density at certain energy levels with increasing Nb concentration was verified experimentally. One type of interface state was observed for specimens with low Nb concentrations and another for specimens with high Nb concentrations. It is proposed that the changes in the interface state with increasing Nb concentration are related to the transition of the compensating defect mode and differences in the extent of oxygen adsorption at the grain boundaries.
I. INTRODUCTION
The incorporation of donor-type dopants such as Y, Sb, La, and Nb, etc., into insulating BaTiO3 ceramics makes the specimen a semiconductor showing positive temperature coefficient resistor (PTCR) characteristics. As this property is related to the grain boundaries, extensive studies on the electrical properties of the grain boundaries have been reported. However, the effects of the donor-type dopant concentration on the electrical properties of the grain boundaries and the PTCR characteristics are poorly understood. Because the donor-type dopant concentration has a large influence on the PTCR characteristics, as does the cooling rate, atmosphere, and 3d acceptor elements, etc., a precise examination and understanding of the effect of the donor-type dopant concentration on the electrical properties of the grain boundaries is crucial. When donor-doped BaTiO3 materials are sintered in air, the dopant concentration varies between 0.1 and 0.3 mol%, causing specimens to exhibit semiconducting or PTCR characteristics. If the concentration exceeds 0.4–0.5%, it reverts to an insulator, and grain sizes become very small. Thus, it is quite difficult to investigate experimentally the effect of the donor-type dopant concentration variations on the electrical properties of
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J. Mater. Res., Vol. 18, No. 1, Jan 2003 Downloaded: 15 Mar 2015
the grain boundaries due to the narrow dopant concentration ranges in which the specimen is semiconducting. In addition, as the grain sizes and microstructures change abruptly with increasing donor-type dopant concentration, variations in the electrical properties of the specimens with increasing dopant concentration are mainly due to the effect of microstructural changes rather than to the dopant concentration itself. Previous reports that investigated the effect of the donor-type dopant concentration on the electrical
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