The High Temperature Dielectric Relaxation And Energy Store In PbTiO₃-based Ferroelectric Ceramics

Lead titanate (PbTiO3) is one kind of perovskite and has ferroelectricity. The Curie temperature of PbTiO3 is about 490°C. The space group of paraelectric phase is Pm3m, while ferroelectric phase is P4mm. Lattice parameters of PbTiO3 is a= 3.902 A, c= 4.156 A, a= β= γ= 90°, tetragonal distortion degree 1.06, spontaneous polarization 56 μC/cm2. In general, doping or combining with other materials can form a new material, which properties is better than leat titanate.In this thesis, the phases, dielectric properties and ferroelectric switching behavior of strontium lead titanate ceramics (Sr1-xPbxTiO3, SPT) are investigated, firstly. The structural characterization is confirmed by X-ray diffraction which indicated SPT changed from cubic to tetragonal structure with lead content increasing. The dielectric relaxation phenomenon is observed, which is caused by oxygen vacancies. The high dielectric tunability and high figure of merit of ceramics imply that SPT ceramics are promising materials for tunable capacitor applications. A serials of pinched shape hysteresis loops can be found. The pinning effect is caused by oxygen vacancies and can be weakened by rising temperature, lowering frequency, enhancing electric field or reducing oxygen vacancies in samples. Both a qualitative model and a quantitative model are used to explain this phenomenon.Secondly, the properties of high temperature dielectric relaxation and ferroelectric energy storage in lanthanum doped lead titanate ceramics [(Pb1-xLax)Ti1-x/4O3, PLT] are studied. The dielectric relaxation phenomenon is observed in high tempertaure. The Curie temperature and ferroelectric in PLT ceramics decreases with the increasing of lanthanum content, while energy storage increases. When doping 0.32 lanthanum in lead ceramics, the energy density and energy efficiency are 0.182 J/cm2 and 90%, respectively, indicating that PLT ceramics are promising materials for energy storage. Besides, the pyroelectric coefficient of PLT20 is above 300 μC/m2K.Then, cadmium and lanthanum modified lead titanate ceramics with chemical formula (Pb0.85Cd0.05La0.10)Ti0.975O3 were prepared by a sol-gel process. The diffuse dielectric anomaly found in the temperature dependence of dielectric permittivity is a competitive effect of dielectric polarization and electrical conduction caused by the oxygen vacancies, which are extrinsically formed during the sample preparation process. The impedance analysis indicates that the resistance of the sample decreases with the increasing in temperature indicating a negative temperature coefficient of resistance behavior, which is caused by the La3+substitution in Pb2+. The Arrhenius analyses, which are performed on the permittivity data above ferroelectric-paraelectric phase transition temperature and the imaginary part of the impedance, respectively, imply that two different polarization mechanisms are involved in the temperature range of 25 to 600℃. Besides, the ferroelectric and pyroelectric properties of PCdLT ceramic are also investigated. The high pyroelectric coefficient (338.8 μC/m2K) and figure of merits (16.84×10-6 Pa-0.5) make this material a potential candidate for pyroelectric detectors.Finally, the structural and electrical properties of BiNi0.5Mn0.5O3/LaNi0.5Mn0.5O3 (BNM/LNM) and BiNi0.5Mn0.5O3/LaNiO3 (BNM/LNO) heterostructure thin films fabricated on Pt/Ti/SiO2/Si(100) substrates by a chemical solution deposition method were studied. The X-ray diffraction results indicated that polycrystalline hexagonal perovskite structures were formed. It was found that the electrical properties of heterostructure thin films are obviously different from each other. For the Au/BNM/LNM/Pt structure, slightly distorted ferroelectric hysteresis loops were observed and the leakage current density versus applied electric voltage curves at positive and negative biases were asymmetrical. The conduction mechanism was dominated by an ohmic conduction mechanism at positive bias, whereas at negative bias, the conduction mechanism was dominated by the ohmic conduction mechanism, trap-filled-limit (TFL) conduction, and grain boundary-limited behavior. However, for the Au/BNM/LNO/Pt structure, slim hysteresis loops with no distortion were observed and the leakage current is approximately four orders of magnitude lower than that of Au/BNM/LNM/Pt. The conduction mechanisms are controlled mainly by an ohmic conduction mechanism, TFL conduction and grain boundary-limited behavior at positive and negative bias.