Abnormal reoxidation effects in Ba‐excess La‐doped BaTiO 3 ceramics prepared by the reduction‐reoxidation method

Ba‐excess La‐doped BaTiO 3 ceramics have been successfully applied to prepare laminated chip thermistor. Ceramics were firstly fired in a high‐purity N 2 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 (Ba 2 TiO 4 ) at grain boundary, which destroyed conducting pathways between semiconducting BaTiO 3 grains. When reoxidation temperature was up to ~800°C, the insulating Ba 2 TiO 4 could be gradually consumed and then conducting pathways reestablished leading to RT resistivity reduction. To further prove the proposed explanation, the reoxidation effects of TiO 2 ‐added ceramics were also studied, in which no Ba 2 TiO 4 existed and certainly the abnormal RT resistivity reduction disappeared.