Investigation Of Bismuth Contained Barium Titanate-Based Lead-Free PTC Ceramic

With the importance of environmental protection, it is urgert to development lead-free PTC (Positive Temperature Coefficient) ceramcis with low resisitivity room temperature, obvious resistance jump and high Curie temperature. In order to substitute the current harmful lead-contained PTC ceramics, three kind of BaTiO₃-based lead-free PTC ceramic system were investigated in the thesis. We prepared BaTiO₃-Bi_0.5Na_0.5TiO₃, BaTiO₃-Bi_0.5K_0.5TiO₃ and BaTiO₃-Bi_0.5Li_0.5TiO₃ system ceramics by solid state reaction method in air condition, furthermore, BaTiO₃-Bi_0.5Na_0.5TiO₃ and BaTiO₃-Bi_0.5K_0.5TiO₃ system ceramics were studied by reoxidized method. The ceramic samples sintered in reduced atmosphere could obtain low room temperature resistivity, following by reoxidizing in air condition, the grain boundary barrier was re-formed, the ceramics reobtained PTC effect. The performance, characteristics and influencing factors of the ceramic materials were measured by XRD, SEM and the temperature dependence of resistivity, the effects of composition and preparation process on the PTC characteristics were researched.The research results of BaTiO₃-Bi_0.5Na_0.5TiO₃, BaTiO₃-Bi_0.5K_0.5TiO₃ and BaTiO₃-Bi_0.5Li_0.5TiO₃ ceramics sintered in air show that the ceramic sample could obtain good PTC effect (ρmax/ρmin>103), low room temperature resistivity (ρRT ¹03??cm) when the content of dopant was not more than 2 mol%. However, the ceamic samples became hard to be semiconducting when the dopant exceed 2 mol%, theρRT could reach 106??cm. Even while doing with donor as Nb element, theρRT was still high. It was because that BNT, BKT and BLT have the effect to refine the ceramic grain size. The grain size became smaller with the addition of BNT, BKT or BLT. Futhermore, the low cation with samll ionic radius such as Na⁺ and K⁺ would move to grain boundaries at high temperature sintering, then these cations will gather at grain boundaries at the cooling stage. This will cause the number of carriers in ceramics declined. Thus, theρRT increased. The Curie temperature of BaTiO₃ was enhanced by the addition of BNT, BKT and BLT. The Tc was enhanced to ¹50℃by added 2 mol% BNT, BKT or BLT. This indicated that BNT, BKT and BLT could efficiently elevate the Tc at low doping concentration. The shifting efficiency reached ¹5℃/mol%. However, the elevation degree of Tc became slowly when doping with high content of BNT or BKT. The shifting efficiency reduced to 3℃/mol%. Through the reaearch of the changes of BaTiO₃ lattice constant, it was found that the a axis will become shorter with the additon of BNT, BKT or BLT, the length of c axis was almost not alter. The ratio of c/a increased by the addition of Bi-contained dopant.For improving theρRT of BT-BNT and BT-BKT ceramics sintered in air, the reoxidizing method was adopted to prepared the high BNT or BKT concentration ceramics. BT-BNT and BT-BKT system ceramics obtained lowρRT at pure N2 sintering atmosphere. It was because numbers of oxygen vacancies formed in the BaTiO₃ ceramics after sintering in reduced atmosphere. The oxygen vacancies were good carries for electrons, so theρRT was decreased. The reoxidation process could make the oxygen vacancies on the grain boundaries were been compensated by oxygen. It lead the grain barries heigh reformed, thus, the ceramic samples reobtained PTC effect. Through controling the reoxidation temperature and reoxidation time, the BT-BNT and BT-BKT ceramics could obtain good PTC effect (ρmax/ρmin>103), lowρRT (ρRT <10³cm) and high Curie temperature (Tc ¹70℃).