Research On The Over-current Protective PTC Ceramics With Low Resistivity

For its unique resistivity-temperature characteristics and voltage-current characteristics, BaTiO₃-based PTC ceramics are widely used to protect electrical circuits in the fields of household appliances, automotive electronics, mobile communications, aerospace. The purpose of this work is to prepare PTC ceramics with low room temperature resistivity, high ratio of lg（Rmax/Rmin） and high withstanding voltage, which was used as overcurrent protection devices.In this dissertation, BaTiO₃-based PTC ceramics was prepared by solid state reaction process and the effect of donor doping, acceptor doping, Ba/Ti mole ratio and Al₂O₃ on the microstructure and electrical properties was investigated. The low room temperature resistivity, high performance, over-current protective samples with a room temperature resistivity of 27.61Ω?cm, the ratio of lg（Rmax/Rmin） of 5.05, high withstanding voltage of 370.7V/mm was obtained by adjusting the composition.First of all, the effect of Y₂O₃ doping, Sm₂O₃ doping on the properties of BaTiO₃-based PTCR ceramics was studied. The results indicated that when the mole fraction of Y₂O₃ or Sm₂O₃ was 0.5¹.0mol%, with the increasing mole fraction of donor dopants, the average grain size was decreased and homogeneous, the room temperature resistivity exhibited a U-shaped trend, which was decreased first and then increased. Y₂O₃ showed a wide low resistivity doped range between 0.6⁰.8mol%, while Sm₂O₃ showed relatively narrow range.After determining the suitable donor dopants, the effect of Mn（NO₃）2 doping on the properties of BaTiO₃-based PTCR ceramics was studied. The results indicated that when the mole fraction of Mn（NO₃）2 was 0.02⁰.06mol%, with the increasing mole fraction of Mn（NO₃）2, the average grain size was increased. The room temperature resistivity, the resistance temperature coefficient and the withstanding voltage were increased. But when the mole fraction of Mn（NO₃）2 was more than 0.05mol%, the room temperature resistivity was increased remarkably.The effect of Ba/Ti mole ratio on the properties of BaTiO₃-based PTCR ceramics was also studied. The results indicated that when the mole fraction of excess Ti O2 was 0.2⁰.6mol%, with the increasing mole fraction of TiO2, the average grain size was decreased and homogeneous, the room temperature resistivity and the resistance temperature coefficient were increased.The effect of Al₂O₃ on the properties of BaTiO₃-based PTCR ceramics was also studied. The results indicated that when the mole fraction of Al₂O₃ was low, the average grain size was increased, the room temperature resistivity was decreased. But when Al₂O₃ was excessive, the resistance temperature coefficient and the withstanding voltage were also decreased.