Study Of Nonlinear Electrical Properties In Rare Earth Oxide-Based Functional Ceramics

Rare-earth elements have received a great deal attention due to wide application in luminescent materials, laser materials, permanent-magnet materials, magnetic refrigeration materials, magnetostrictive materials, hydrogen storage materials, high temperature superconductor and other high technology fields. Rare earth elements as doping or composition exist in these functional materials, rare earth-based functional ceramics, especially for rare earth-based electrical functional ceramics, have received comparatively little attention. In this work, the electrical properties of rare earth-based ceramics have been systematically studied, it was found that Pr6O11 and Tb4O7 ceramics have nonlinear electrical properties, which can be used in protection devices for electronic and electrical power systems.In Chapter 1, the category, basic properties and main application of rare earth elements were introduced, the necessary to study rare earth-based electrical functional ceramics was emphasized, the current status of varistors was then described, and finally the research contents of this paper were provided.In Chapter 2, the experimental process and characterization methods used in this paper were introduced. As for experimental process, the conventional ceramic processing techniques were particularly described. As for characterization methods, the analysis process of microstructure, defect energy level and grain boundary barrier parameters were provided.In Chapter 3, the microstructure and electrical properties of pure Pr6O11 ceramics were systematically studied. The ceramic samples began to present nonlinear electrical properties when sintered at 600℃, the density has a significant increase when sintering temperature exceeds 1000℃. Ac impedance analysis indicated that grain boundary high resistance layer present at the grain boundary region. Since there are no segregation phase located at the grain boundary, it was proposed that the grain boundary high resistance layer originates from the grain boundary barriers. The existence of double Schottky-type barriers was affirmed by analyzing the I-V curves measured at different temperature and the corresponding parameters were calculated. Pr6O11 can release oxygen with the increase of ambient temperature and then incorporate oxygen in the process of cooling due to the OSC (oxygen storage capacity) characteristics. It was proposed that the formation of grain boundary barriers can be attributed to the oxygen absorption during the cooling process, which was supported by AES profile analysis. Based on the defect barrier model for ZnO based varistors proposed by Gupta and Carlson, a modified defect barrier model was introduced to explain the formation of the grain boundary barriers, (?) and (?) as the donor defect exist in the depletion layer. Pr6O11 is an n-type semiconductor with oxygen vacancies, which is different from n-type ZnO semiconductor with excess interstitial zinc ions, so Pr6O11 varistors present excellent electrical stability and anti-ageing capacity.In Chapter 4, the densification of Pr6O11 ceramics was studied by liquid phase sintering and substitution reaction. Bi2O3 can improve the sintering process by forming liquid phase segregated to the grain boundary regions, CuO and ZnO can substitute the Pr ions in the lattice, the produced defects can facilitate the densification process by improving the bulk diffusion and grain boundary diffusion. Besides the ionic radius factor, the chemical bonding type should be considered during the substitution reaction.In Chapter 5, the nonlinear electrical properties and conduction mechanism of Tb4O7 varistors were studied. Tb4O7 varistors have low breakdown voltage, and present excellent dielectric properties and energy absorption capability, which is suitable for application in overvoltage protection for microelectronic circuit. According to grain boundary barrier parameters of the samples treated in different atmospheres, it was proposed that the nonlinear electrical properties of Tb4O7 varistors can be attributed to the adsorbed oxygen on the grain surfaces, which is similar to Pr6O11 varistors.In Chapter 6, Tb-based ZnO varistors and ZnO-Pr6O11 mixed oxide varistors were prepared. Similar to Pr6O11, Tb4O7 also play a key role in the formation of nonlinear electrical properties of ZnO varistors. According to the similarity between Tb4O7 and Pr6O11, it was proposed that the nonlinear behavior of Tb-based ZnO varistors should be concerned with the oxygen storage characteristic of Tb4O7, further study is necessary. The prepared ZnO-Pr6O11 mixed oxide varistors have the nonlinear coefficient range from 1.5 to 26.9 and breakdown voltage range from 26 to 829V/mm, so the varistors can satisfy different application by changing the composition of the mixed oxide varistors. In addition, the experimental data of rare earth oxide ceramics with linear electrical properties was also provided.