Abstract
Barium titanate semiconducting ceramics with positive temperature coefficients of resistance (PTCR) which were donor doped with holmium have been studied with respect to the effect of holmium concentration on the conductivity in the range of 0.05 to 0.8 at.%. The transition from conductive to insulating behaviour at room temperature for samples with high dopant concentrations (>0.4 at.%) has been investigated; resistivity and dielectric measurements have revealed that this transition is exclusively a grain boundary phenomenon and that existing models do not offer a satisfactory explanation for this effect. The results further revealed that increasing the donor concentration causes significant changes in the maximum resistivity and in the temperature at which it appears, indicating possible changes to the intergranular potential barriers. The increase in the room temperature resistivity at high donor concentration is attributed to the enhancement of the potential at these grain boundary barriers.
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