This study was concerned with the mechanism of PTCR in $BaTiO_3$ ceramics doped by $Nb^{+5}$. Since the vacancy compensation layer in the grain boundary of n-type doped $BaTiO_3$ ceramics has been known as a major factor for surface state to give PTCR as proposed by J. Daniel et. al.(1978), the dependence of PTCR on such vacancy compensation layer was attempted to be confirmed experimentally in this study.
For the experiment, $BaTiO_3$ ceramics doped by $Nb^{+5}$ were synthesized by using $BaTiO_3$ powders prepared by a chemical coprecipitation method. Quenching at various temperatures, annealing at various temperatures after sintering and the change of density of $BaTiO_3$ ceramic were adopted to induce difference in the thickness of vacancy compensation layer so as to exhibit different degree of PTCR effect each other. The measurement of $Ba^{++}$ concentration in the grain and grain boundary by EDAX as well as the degree of PTCR effect itself were conducted to each specimen in order to evaluate the formation of the vacancy compensation layer.
It was found that ⅰ) either decrease in the temperature for quenching or increase in the temperature for annealing improves the PTCR effect clearly, ⅱ) increase in $Ba^{++}$ concentration in the grain boundary results in the improvement of PTCR effect, ⅲ) decrease in the density of the $BaTiO_3$ in a narrow range exhibits increase in PTCR effect slightly.
It was concluded that all the experimental results gave the evidence for the dependence of PTCR effect on the vacancy compensation layer in the grain boundary which had been induced possibly by the $Ba^{++}$ diffusion by the heat treatment conducted.
