High Voltage Tin Oxide Varistors And High Temperature Sodium-Potassium Bismuth Titanate Piezoelectric Ceramics

Since 1969, Matsuoka announced the development of ZnO varistors, thorough and extensive investigations have been carried out on this novel varistor material. To the end of 1980's, the applications and theoretical researches of ZnO varistors had been practiced. However, the thermal instability and degradation problem of ZnO varistors have not been resolved because of their multi-phase structure induced by doping more dopants. Therefore, with the research on the improvement of ZnO electrical properties going on, the efforts in searching for new varistor materials have not been interrupted. In 1982, Yan and Rhodes in Bell Lab. reported a new TiO₂ varistor system, and in 1994, Makarov and Trontelj found the WO₃ varistors. In successive researches, it is found that TiO₂ varistors and WO₃ varistors are suited for low voltage applications due to their low breakdown electrical fields, and WO₃ has a serious thermal instability owing to its multi-phase structure at ambient temperature.In 1995, S. A. Pianaro found a new (Co, Nb)-doped SnO₂ varistor material, which possesses high density and good electrical nonlinear properties. Contrary to the multiphase structure of ZnO varistors, SnO₂ varistor material has only a single phase. Due to its high breakdown electrical field, SnO₂ varistors are suited for high voltage applications. In the thesis, it is found that the breakdown electrical field of SnO₂ varistor material can be greatly enhanced with the improvement of nonlinear electrical properties. The enhancement of varistor breakdown electrical field makes it possible to miniaturize the varistor devices, which depressing the fabrication costs by reducing the dosage of raw materials. Thorough studies indicate that the decrease of grain size is the main reason for the enhancement of the breakdown electrical field. In this thesis, the novel high voltage CuO-doped dense SnO₂ varistors are found. The chapter about CuO-doped dense SnO₂ varistors is a key point.The piezoelectric materials are totally different from the varistor materials in nature. The varistor material is a functional material with the performance of field-current nonlinearity, while the piezoelectric material is a functional material with the feature of field-strain linearity. In view of the importances of varistor and piezoelectric materials in application domains, the studies on bismuth layer-structured materials are also conducted. In this thesis, the high Curie temperature bismuth layer-structured ferroelectrics (BLSFs) potassium bismuth titanate (KBT) compound is prepared, and the (LiCe) co-substituted sodium potassium bismuth titanate (NKBT) materials are investigated. We analyze the origin of low piezoelectric activity in detail, and put forward a new idea to improve the piezoelectric activity for bismuth layer-structured compound. The chapter about BLSFs is a key point, and the densification of KBT and high piezoelectric activity of (LiCe) co-substituted NKBT are innovative points of this thesis.Firstly, the development and theoretical evolution of varistors and the character of bismuth layer-structured materials are generally introduced. The most important electrical property of varistor is its nonlinear current-voltage characteristics. The nonlinear coefficient is the most important parameter for a varistor, the higher the nonlinear coefficient, the better its nonlinearity. The breakdown electrical field is another important parameter for a varistor, which determine its application situation.Piezoelectric materials can be polarized not only by exerting an electric field, but also by exerting a mechanical stress. The generation of electrical charge by applying a stress is called direct piezoelectric effect, which was first discovered by the brothers Curie in 1880. The converse piezoelectric effect describes the strain developed in a piezoelectric material when an electric field is applied. The direct piezoelectric effect and the converse piezoelectric effect are both called the piezoelectric effect. The materials having piezoelectric effect are called piezoelectric materials. The Aurivillius phase BLSF materials, possessing high Curie temperature (T_c), have been intensively studied because of their use in piezoelectric devices suitable for use at high frequencies and high temperatures and in non-volatile random-access memories. The Aurivillius phase bismuth layer-structured compounds are comprised of pseudo-perovskite layers interleaved with (Bi₂O₂)²⁺ layers along the c axis of the BLSFs.Because of their loose structure, tin oxide is usually studied as gas sensor materials. In order to obtain good nonlinear electrical properties and low leakage current, the dense SnO₂ ceramics must be prepared. Firstly, the CoO-doped dense SnO₂ varistors are investigated. Though the density is very high, the SnO₂·CoO system does not possess any nonlinear electrical property. The SnO2#Ta2O5 and/or SnO₂·Nb₂O₅ systems have nonlinear electrical properties, but their low density, low nonlinear coefficient and high dielectric loss leave them failed in application. The (Co, Ta) co-doped SnO₂·CoO·Ta₂O₅ (SCT) varistor system possesses high density and good nonlinear electrical properties; therefore, SnO₂·CoO·Ta₂O₅ varistor system is selected as the research object. The introduction of CoO into SnO₂ ceramics improves tin oxide's sinterablity, promotes the mass transport and results in densifying tin oxide. The donor Ta₂O₅ is the key element to make SnO₂ ceramics have nonlinear electrical properties. It is found that the sample with 1.00 mol% Ta₂O₅ has the best nonlinear electrical property and the highest nonlinear coefficient (α = 52) among all samples. The breakdown electrical field E_B is 1412 V mm^-1, indicating this system can be used in high voltage field. Then the effects of Ba, Cu on the electrical properties of SnO₂·CoO·Ta₂O₅ varistors are investigated, respectively. It is found that adding appropriate amount of Ba can improve the nonlinear coefficient and breakdown electrical field, but excess Ba (1.00 mol%) can decrease the density and increase the dielectric loss of SCT system. With the CuO content increasing, the breakdown electrical field decreases, the grain size increases, and dielectric constant increases. The increase of grain size is the main reason for the decrease of breakdown electrical field and the increase of dielectric constant.In this thesis, the effects of rare-earth oxide (La₂O₃, Pr₂O₃, Dy₂O₃, Er₂O₃) on the nonlinear electrical properties of SnO₂·CoO based varistors are also investigated. By scanning electron microscopy (SEM) and transmission electron microscopy (TEM), it is found that rare-earth ions La³⁺, Pr³⁺, Dy³⁺, Er³⁺ with large ionic radius can not dissolve totally into SnO₂ lattices, and easily segregate at the grain boundary. Rare-earth ions residing at SnO₂ grain boundaries hinder their adjacent small SnO₂ grains from conglomerating into large particles, and make the grain size decrease with increasing the rare-earth ion content. With the rare-earth oxide content increasing, the breakdown electrical field increases, and the grain size decreases. The decrease of grain size is the main reason for the increase of breakdown electrical field. It is found that adding appropriate amount of rare-earth oxide can improve the nonlinear electrical properties of SnO₂ varistors.In the research course of SnO₂ varistors, the novel CuO-doped dense SnO₂ varistors are found. Firstly, the effects of CuO on the densification of SnO₂ varistors are investigated. The introduction of Cu²⁺ and/or Cu¹⁺ improves the sinterability, promotes the mass transport, and, as a result, makes the tin oxide densified. Further more, CuO will be transformed into Cu₂O at 1122°C and the melting point of Cu₂O is 1236 °C. The existence of a liquid phase in the Sn-Cu-O₂ system also has a positive effect on densification of tin oxide samples. As mentioned above, copper oxide has a low melting point and high vapor pressure; therefore, it will evaporate when sintered at high temperature. The larger the amount of copper oxide, the greater the evaporation amount of copper oxide. The evaporation of copper oxide has a negative effect on the densification of tin oxide samples. Therefore, firstly the density dramatically increases with increasing CuO content to 0.50 mol%, and then significantly drops with further increasing CuO content. However, the SnO₂·CuO ceramics show ohmic electrical behavior with a very high resistivity of about 2000 MΩ cm and no signs of nonlinearity. In this thesis, the effects of different donors (Nb, Sb, Ta and V) on the nonlinear electrical properties of the SnO₂·CuO ceramic system are also investigated. For the samples doped with Nb or Ta, due to the pentavalent donor modifications, inducing the production of electrons, the varistor samples possess excellent nonlinear electrical properties. As for the samples doped with Sb, when the content of Sb is higher than 0.25 mol%, the samples show signs of conductivity. It is probably the reason for the samples showing conductivity that Sb acts as pentavalent donor at 1300°C and 0.25 mol% Sb maybe provide over more electrons. In the case of the samples doped with vanadium, they are highly resistive, showing absence of nonlinearity. This is due to quadrivalent vanadium is more steady than pentavalent vanadium, as a result, the quadrivalent vanadium does not give any contribution to the electrical nonlinearity of SnO₂ ceramics. According to the impedance diagrams of reactance vs resistance, it is concluded that the samples, whose grain boundary resistivity is much larger than the grain resistivity, has better nonlinear electrical properties. Due to high nonlinear coefficient, high breakdown electrical field, and low leakage current, the novel CuO-doped SnO₂ varistors are suited for high voltage applications.In this thesis, the high Curie temperature bismuth layer-structured potassium bismuth titanate compound is prepared, and the (LiCe) co-substituted sodium potassium bismuth titanate is investigated. We analyze the origin of low piezoelectric activity and put forward a new idea to improve the piezoelectric activity of bismuth layer-structured compound. Due to the very narrow range of sintering temperature, the dense KBT compound has not been obtained in the past several decases. By ordinary firing (OF) method, we successfully prepared the dense KBT compound, ending the history being unable to get a dense KBT compound. The optimal sintering temperature is 1110 °C, and the density of KBT sintered at this temperature is 95.4% of theoretical. The piezoelectric constant d₃₃ is 21.2 pC/N, the Curie temperature T_c is 556 °C, and the mechanical quality factor Q_m is 1602. The A and B-site cations in the perovskite layer are readily replaced by a large number of cations. Because the radius of cations in B-site are close each other and do not play a major structural role in the polarization process for BLSFs, the effect of A-site substitution is more obvious than that of B-site substitution. Also, it is reported that the Curie temperature increases with decreasing the size of A-site cations in bismuth layer-structured compound, so doping small size cation benefits elevating Curie temperature. Further, the vacancy may be considered to make A-site cation decrease or be a cation whose radius is zero. Based on above considerations and the electric charge conservation, A-site (LiCe) co-substitution, and 0.02 mol% vacancies are introduced into the sodium potassium bismuth titanate to prepare the piezoelectric ceramics with high piezoelectric activity and higher curie temperature. Considering the element Ce can improve the resistance and the temperature stability of the piezoelectric ceramics, the element Ce is selected herein. By A-site (LiCe) co-substitution, the piezoelectric constant of the (LiCe) co-substituted sodium potassium bismuth titanate is 25 pC/N. In subsequent composition modification research, the piezoelectric constant has reached 28 pC/N up to now.