Study On The Lead-free High Curie Temperature BaTiO₃ Based Materials

In order to prepare the lead-free PTC ceramics with high Curie temperature (Tc), the BaTiO₃-based PTC materials were studied in this paper. The BT/BNT PTC materials were prepared by compounding BNT with high Tc, and the property and influence factor of the composites were investigated through some test methods, such asρ-T curve measuring,ε-T curve measuring, XRD and so on.The Curie temperature of BNT is 320℃, and it could be solid solutions with BT. It was better for semiconducting by adding the pre-synthesized BNT into the second materials than any other methods and the Tc of the composites could also be improved by this way. However, the addition of BNT had side effects too. The free state Na created by the decomposition of BNT during high temperature sintering had some bad effects on semiconducting. Moreover, the more addition of BNT was, the more serious the effects were. When the addition of BNT was over 1mol%, the original samples could not be semiconducting by air sintering. It was found that the incorporation of A-site substitutes BaCO₃ could efficiently inhibit the decomposition of BNT, which made the air sintering samples semiconducting. The optimum adding quantity of BaCO₃ was 3%. By comparing with some other donor dopants, Y2O₃ was found better for the samples'semiconducting. By investigating the addition of acceptor dopant MnO2, it was also found that the right donor-acceptor ratio was helpful to the improvement of the properties of the PTC composites. When the addition of Y2O₃ was 0.2mol% and the addition of MnO2 was 0.1mol%, the samples'room temperature resistivity is 1142Ω·cm and the resistance jump is 665. The incorporation of BNT could outstandingly improve the samples'Curie temperature. That was because the addition of BNT could strengthen Ti-O bond and enhance the tetragonality of the ceramic crystal lattice. When the addition of BNT is 2mol%, the Curie temperature increased from 120℃to 148℃, which made it possible to prepare the lead-free BaTiO₃-based PTC ceramics with high Tc by BNT instead of PbTiO₃.