Study Of The NTC Effect In Barium Titanate Based PTC Ceramics

Barium titanate（BT）based PTC ceramic has unique resistance-temperaturecharacteristic.The resistance of PTC ceramic increases several orders of magnitude rapidly in a narrow temperature region near Curie temperature T_C.The typical BT based PTC ceramic exists two characteristic temperatures.One is the lowest resistivity temperature T_ρminmin and another is the highest resistivity temperature T_ρmax.The ceramic exhibits NTC effect at T<T_ρmin,while it shows PTC effect at the temperature between T_ρminmin and T_ρmax.When the temperature is higher than T_ρmax,it shows NTC effect again.At present,most researches about BT based PTC ceramic were focused on the origin of the PTC effect and the tailoring of positive temperature coefficient.Rare study was concentrated on the NTC effect.In such a background we started our researches that concern about the NTC effect in BT based PTC ceramic.Our purpose is to understand the origin of PTC effect based on the studies of NTC effect in PTC ceramic,and try to combine the NTC and PTC effect,which can be expected to develop new type of thermo-sensitive element with NTC/PTC double functions.After a series of researches,we acquire the following results:1.Y doped BT ceramics show NTC effect in low and high temperature regions.Nb doped BT ceramics exhibit NTC effect in high temperature region while no NTC effect in low temperature region can be observed.2.The barrier heights of Y doped BT ceramics are constant values during-55°C¹0°C（orthorhombic）,10°C^T_ρmin（tetragonal）and T_ρmax⁴00°C（cubic）,respectively.The barrier height will be changed at phase transition temperature.The transition from orthorhombic to tetragonal phase drops the potential barrier height while the transition from tetragonal to cubic phase enhances the potential barrier height.3.Partial modification of Pb in Y doped barium titanate ceramics enhances the NTC effect and degenerates the PTC effect at low temperatures.The sample,with the composition of(Ba_0.8464Y_0.0036Pb_0.15)TiO₃+0.1wt%Al₂O₃+0.5wt%SiO₂ and being sintered at 1280℃for 2 h,has the most excellent NTC/PTC comprehensive performance.The room temperature resistivity,rise ratio of resistivity and NTC thermal constant of this sample are 217.8Ωcm,2.99 and 1141 K,respectively.4.Partial modification of Sr in Y doped BT ceramics decreases the NTC effect at high temperatures.A small amount of Sr（<10 mol%）can improve PTC effect,while excess Sr content will produce the opposite result.5.The analyses of complex impedance spectra show that the NTC effect derives from grain boundaries.In low temperature region,the conduction mechanisms of grain boundaries is the small polaron hopping conduction and the hopping type of the small polarons is the Nearest Neighbor Hopping（NNH）.The conduction of grains results from the migration of electrons in the conduction band.In high temperature region,both of the conduction mechanisms of grain and grain boundaries are the small polaron hopping conduction and the hopping types are NNH.The NTC effect is mainly caused by small polarons hopping conduction resulting from the activation of thermal energy.6.The grain boundaries determine the resistance-temperature characteristic of PTC ceramic.We suggest that the barrier height of grain boundaries remains a constant at the temperature range of non-phase transition.But the barrier height will be changed at the phase transition temperature.The expression of barrier height caused by phase transition can be expressed as ΔΦ=ΦO+ΦJ+ΦB，ΦO、ΦJ、ΦB are the potential barriers generated by depolarization,oxygen adsorption layer and glass phase among grain boundaries,respectively.