Current-voltage characteristics of donor-doped BaTiO3semiconducting ceramics

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Abstract
Current-voltage measurements have been made on barium titanate ceramics with a positive temperature coefficient of resistance (PTCR) to investigate the nature of the current transport processes. The specimens were made from commercial BaTiO3 which was doped with varying amounts of the donor impurity, Ho, added as Ho2O3. Samples with donor dopant concentrations between 0.2 and 0.5 at.% and showing a typical PTCR effect, as characterised by a low room temperature resistivity (<or approximately=30 Omega cm) and a large PTCR jump (about three orders of magnitude), as well as heavily doped ones (up to 0.8 at.% Ho) with considerably modified PTCR characteristics, were investigated. The measurements were performed above the ferroelectric transition temperature of the materials to avoid complications arising from the spontaneous polarisation, which tends to compensate the trapped negative charge at the grain surfaces. Less heavily doped samples (<0.4 at.% Ho) appeared to follow the Heywang model up to approximately 0.25 V per grain boundary, provided diffusion rather than thermionic emission theory was used. But in the more heavily doped specimens the voltage range within which the Heywang model was found to apply was considerably reduced.