The Effect Of Barium Lead Oxide On Lower Resistivity Of BaTiO₃-based PTCR Ceramics

Because of the very low resistivity of metallic layered barium lead oxide, in this work, BaPbO3 or BP ( Pb3O4+ BaCO3) was added into barium titanate-based PTCR to reduced its resistivity at room temperature. In the experiment, in order to find the best process, four different schemes adding barium lead oxide into BaTiO3 matrix were designed and compared in detail: (1) BaPbO3 synthesized at 880oC was composited with crushed BaTiO3-based PTC ceramics powder (2) The synthesized BaPbO3 (same as scheme 1)was mixed with synthesized BaTiO3 (3) The raw materials BaCO3 and Pb3O4, which can be expected to form barium lead oxide in the sequent firing process, was mixed into the initial materials used for synthesizing BaTiO3, (4) the raw materials BaCO3 and Pb3O4 was added into the synthesized BaTiO3 ,and the barium lead oxide composited by BaCO3 and Pb3O4 can be positioned in the grain boundary by controlling the firing process.The experiments showed that through the fourth scheme ,high-performance PTCR ceramics of lower resistivity and high Rmax/Rmin was successful prepared. From the further experiments about the effects of the amount of BP additives and the ratio of Pb/Ba on the properties, an optimized process was obtained with the amount of BP additives at 1.5mol%, Pb/Ba at 1.3 and sintering temperature at 1140oC for 60 min. By the optimized process , PTCR ceramics of low resistivity (8.Ω?cm) and good magnitude (4×104) was obtained. Meanwhile, the paper studied influences of BP additives on the effects of other additives such as MnO2, BN and AST, which have been proven to exist in the grain boundary. It was shown that, with BP additives, the effect of Mn element on the PTC characteristics was suppressed obviously and the effect of AST became less prominent, while the effect of BN became prominent, which considerably decreased the sintering temperature and the resistivity at room temperature of the PTCR ceramics including BP additive. The phase constituent, microstructure, element valence and the distribution of Pb in the sintered ceramics were analyzed by XRD, SEM, XPS and EDAX. The results showed that the reduction of ceramics' resistivity in scheme 4 was attributed to the metallic layered Ba3Pb2O7 phase, which was newly formed during final firing and existed on the grain surface. Ba3Pb2O7 phase grew epitaxially along BaTiO3 lattice and contact well with BaTiO3, and presented a very high conductivity and played an important role to create oxygen vacancy, which can be compensated by free electron. According to the experiments and analysis, a structural model was proposed, which can explained some phenomena occurred in the experiments very well.