IMPEDANCE ANALYSES OF COOLING-RATE EFFECTS ON THE DEPLETION LAYER OF PTCR MATERIALS

Cooling-rate effects on the depletion layer of BaTiO3 positive temperature coefficient of resistance (PTCR) were analysed using d.c. resistivity and a.c. complex impedance spectroscopy. As the cooling rates decreased, the d.c. resistivity of the sample increased. A.c. complex impedance spectra showed that the observed d.c. resistivity increase was mainly due to the grain-boundary resistivity increase. The grain resistivity also exhibited relatively weak PTCR behaviour. The built-in potential and the depletion-layer width were analysed using the a.c. complex impedance data. Slow cooling rate produced higher built-in potential and large depletion-layer width. The relationship between the built-in potential and the depletion-layer width at different temperatures was analysed and used to explain the observed grain-boundary resistivity change. The resistivity decrease in the low-temperature region was due to the depletion-layer width decrease. The resistivity jump over the temperature was due to both the built-in potential and the depletion-layer width changes.