Non-linear electrical characteristics of a barium-titanate based, positive temperature coefficient ceramic

摘要

Positive temperature coefficient materials are used in temperature sensing applications and as current limiting devices where the large change of resistance over a limited temperature range is exploited. Previous work in this laboratory reported an effect of applied voltage on the form of the resistivity-temperature curves. This was discussed in terms of a possible electric field effect. The presence of such phenomenon would be important in PTC devices used in high current, high field applications. The present work has investigated this effect in barium titanate based materials. By comprehensively monitoring the current-voltage and resistivity-temperature characteristics, it has been shown that, for a particular indicated temperature, an increase in the measuring voltage results in an increase in the resistivity. Once the temperature of the ceramic rises, through a combination of Joule heating and external heating, the resistivity rises sufficiently so that the power dissipated tends to zero. Thereafter the resistivity-temperature curve can follow the same locus. Below the resistivity transition, as a result of Joule heating, the surface (measured) temperature can be significantly lower than the core temperature. This results in the transition occurring at lower apparent temperatures. Above the transition temperature voltage dependent differences were observed which can be attributed to a field effect.