Complex Impedance Analyses of High Tc BaTiO3-based Pb-free PTCR Ceramics

A distinct positive temperature coefficient of resistance (PTCR) effect has been observed in Bi_0.5Na_0.5TiO₃ (BNT)-added BaTiO₃ (BT) ceramics sintered in N₂ flow with a low O₂ concentration . With the addition of 10 mol.% BNT, the samples exhibited resistivity jumps of above 10 at 189degC. The BNT-added BT ceramics was also analyzed using an a.c. complex impedance spectroscopy to reveal the origin of the PTCR effect. An electrically inhomogeneous microstructure of the grain interiors, surface layers surrounding the gains and grain boundaries were deduced by the complex impedance analyses. The observed d.c. resistivity increase was mainly due to the increase in grain boundary resistivity. The surface layer appeared to slightly contribute to the PTCR effect in the BNT-added BT ceramics. The grain interior also exhibited a resistivity anomaly around T_s. But, the resistivity of gain interior decreased with temperature higher than T_s.