Study On High-performance Lead-free BaTiO₃-based Ceramics With Positive Temperature Coefficient Of Resistivity

Because of the controllable room-temperature resistivity,Curie temperature and the positive temperature coefficient of resistivity?PTCR?,BaTiO₃-based PTCR ceramics have been widely used in the devices for over-current protection,self-controlled heater and temperature detection.However,because of the high toxicity of lead oxide in the conventional Pb-containing high Curie temperature BaTiO₃-based PTCR ceramics,it is of much significance to prepare lead-free BaTiO₃-based PTCR ceramics with high Curie temperature,low room-temperature resistivity and better PTC effect.Therefore,this dissertation focused on the fabrication of high performance lead-free BaTiO₃-based PTCR ceramics.This work mainly focused on the improvement of the Curie temperature and the PTCR value of BaTiO₃ based semiconducting ceramics by compositing them with new peak shift materails(Bi_0.5M_0.5)TiO₃?BMT,M=Li,Na,K,Rb?,and with donor and acceptor dopants.By analyzing the composition,structure,morphology and electrical properties of the obtained samples,the following results can be obtained.?1?Lead-free high Curie temperature BaTiO₃-based PTCR ceramics were obtained by the addition of BMT?M=Li,Na,K,Rb?into Ba_0.97TiO₃?BT?.The Curie temperatures of 0.912BT-0.088 BMT were enhanced which displayed a tendency to increase first and then decrease while the M was changed from Li to Rb.Meanwhile,the room temperature and the ratio of maximum to minimum resistivity decreased first and then increased,the samples with?Bi0.5Na0.5?TiO₃?BNT?had the lowest room temperature resistivity,the sample with?Bi0.5Li0.5?TiO₃?BLT?had the best PTCR performance.?2?The samples 0.912BT-0.088 BNT together with Ta⁵⁺ or Sm³⁺ ions as the donor dopants were prepared.It was revealed that the room temperature resistivity of the as-prepared samples with proper amount of Ta⁵⁺ or Sm³⁺ ions could be decreased.With 0.003?mole ratio?of Ta⁵⁺ ions and 0.004?mole ratio?of Sm³⁺ ions,the room temperature resistivity of the obtained samples reached their minimum of about 200 and 100 ?·cm,respectively.The Curie temperature of the samples raised first and then decreased with increasing amount of Ta⁵⁺ ions,presenting the highest value up to175 °C when the mole ratio is 0.003,while the Curie temperature of the samples decreased with increasing amount of Sm³⁺ ions.Moreover,after the incorporation of Ta⁵⁺ or Sm³⁺ ions,the PTCR value decreased first and then increased,and the PTCR value reached to their maximums when the mole ratio is 0.005.?3?The samples 0.912BT-0.088 BNT together with optimum amount of Ta⁵⁺ or Sm³⁺ ions as the donor dopant and increasing amount of MnO₂ as the acceptor dopants were prepared.It was revealed that the PTCR of the as-prepared samples with proper amount of MnO₂ was enhanced.The PTCR value of and Curie temperature of0.912BT-0.088BNT-0.003Ta2O5-0.0004MnO₂ both reached their maximums,which were 6.14 orders magnitude and 158 oC,respectively.And both the Curie temperature and PTCR value of 0.912BT-0.088BNT-0.004Sm2O₃-0.0002MnO₂ reached their maximums of 5.64 orders of magnitute and 155 oC,respectively.The room temperature resistivity of the as-prepared samples increased somewhat with the addition of MnO₂.