Complex Impedance Spectra of BaTiO_3 Based PTCR Ceramics

Complex impedance spectra of BaTiO3 with boron addition on the electrical characteristics of PTCR were studied. Cole-Cole plot shows a single semicircle in room temperature (T Tc) but shows a overlapping semicircle and skew circle above the Curie temperature (Tc T Tm), which may indicate relaxational processes with multiple defects, such as VBa, Vo etc, from single defect such as VBa by thermal activation in the regions of the grain boundary layer. The grain resistance, grain boundary resistance and relaxational time constant were calculated from the Cole-Cole plot and the barrier height was calculated by the Heywang model. The results show that PTC effect originates from the grain boundary. The grain resistance exhibits NTC, but the grain boundary resistance shows obviously PTC. UP to date, the Heywang model or Heywang-Jonker model is the most accepted model to explain the PTC mechanism by phenomenal explanation.