A phenomenological model of conduction in Ho-doped barium titanate sensor

Abstract

PTCR effect in doped barium titanate (BaTiO3) sensors originates from the grain boundary resistance and is attributed to the existence of potential barriers at these boundaries. Height of the potential barrier at the grain boundary is a function of the dielectric constant which is strongly temperature dependent. The height of the barriers between the grains has been investigated in semiconducting BaTiO3 samples prepared by doping with 0.3mol% Ho2O3 using the conventional ceramic methods. The results show a Positive Temperature Coefficient of Resistance (PTCR) for temperatures above the Curie temperature (TC) followed by a region of Negative Temperature Coefficient of Resistance (NTCR). The dielectric properties of the samples were studied in the temperature interval of 300–570K using capacitance measurements at 1.2kHz from which the permittivity and the barrier height were determined. It was found that the barrier height increases linearly in a narrow range of temperature where the PTCR effect is observed beyond which it continues to increase slowly. The barrier height does not become constant but varies from 0.06eV to 0.18eV over a temperature interval of 410–570K. A phenomenological model based on Thermionic Emission (TE) and Thermally Assisted Field Emission (TFE) mechanism has been proposed to account for the NTCR behavior.