Abnormal reoxidation effects in Ba‐excess La‐doped BaTiO3 ceramics prepared by the reduction‐reoxidation method

Ba‐excess La‐doped BaTiO₃ ceramics have been successfully applied to prepare laminated chip thermistor. Ceramics were firstly fired in a high‐purity N₂ flow and then reoxidized in air to obtain positive temperature coefficient of resistivity effect. However, an abnormal reduction trend of room‐temperature (RT) resistivity was found to be always beginning at ~800°C during reoxidation. This anomaly was found closely correlated with the insulating second phase (Ba₂TiO₄) at grain boundary, which destroyed conducting pathways between semiconducting BaTiO₃ grains. When reoxidation temperature was up to ~800°C, the insulating Ba₂TiO₄ could be gradually consumed and then conducting pathways reestablished leading to RT resistivity reduction. To further prove the proposed explanation, the reoxidation effects of TiO₂‐added ceramics were also studied, in which no Ba₂TiO₄ existed and certainly the abnormal RT resistivity reduction disappeared.