Permittivity-temperature behaviour of donor-doped positive temperature coefficient of resistance BaTiO₃ceramics

Abstract

Positive temperature coefficient of resistance (PTCR) BaTiO3 ceramics were prepared from commercially available BaTiO3 powder, to which a suitable quantity of donor dopant was added. The samples were sintered at 1320 degrees C for 20 min and, while cooling to room temperature, annealed at 1220 degrees C for a further 20 min. The apparent relative permittivity, epsilon 'app, was measured at a frequency of 30 kHz between ambient and approximately=360 degrees C. Plots of 1/ epsilon 'app versus temperature were found to follow the Curie-Weiss law only within a small temperature range ( approximately 60 degrees C) above the ferroelectric transition temperature. At higher temperatures 1/ epsilon 'app rose more slowly with temperature, giving a second region with smaller slope. This behaviour can be explained on the basis of the well known Heywang model, following narrowing of the grain boundary barrier layers as a result of the depopulation of the surface states at high temperatures.