Papers by Author: Tian Guo Wang

Abstract: TiAl-based alloy with a composition of Ti-47%Al-3%Cr (mole fraction) was prepared by high-energy ball milling and hot-pressing sintering. The relationship between microstructure and mechanical properties of Ti-47%Al-3%Cr alloy was studied by X-ray diffractometry (XRD), scanning electron microscopy (SEM) and mechanical testing. The results showed that the TiAl-based alloy with high density and uniform microstructure could be obtained by high-energy ball milling and hot-pressing sintering. The compactibility and sintering densification of the element powder could be promoted efficiently by high-energy ball milling. The main phase TiAl and few phases Ti₃Al were observed in the hot pressing sintering bulk samples. In addition, the microstructure changed with ball milling times, as a result, the mechanical properties changed with the microstructure. The finer the microstructure was, the higher the strength at room temperature became. After the element powder was milled for 20 hours and hot-pressing sintered at 1300 °C for 2 hours, TiAl-based alloys were found to have good room temperature mechanical properties with the compressive strength of 2870 Mpa and the relative compressive ratio of 27.3%. Keywords: TiAl-based alloys; hot-pressing sintering; microstructure; mechanical properties

Abstract: The influence of TiO₂ nano powder on the electrical properties and microstructure of TiO₂ ceramics was studied. The results showed that nano additive reduces the size of TiO₂ grain. The results also showed that the breakdown voltage and nonlinear constant of the samples were improved and the dielectric constant was reduced by doping nano-titania. An optimal composition dopant with 6 mol% nano-titania exhibited a low breakdown voltage of 11.3 V/mm, a nonlinear coefficient of 5.5, an ultrahigh relative dielectric constant of 7.11×10⁴ and relatively low loss of 0.28.

Abstract: The nonlinear electrical behavior and dielectric properties of WO3-based ceramics with various La2O3 contents have been investigated. Breakdown voltages Eb of WO3 doped with La2O3 are lower than that of undoped WO3, indicating that the dopant can reduce the breakdown voltage. The dielectric constant of doped samples is higher than that of undoped samples, and the high dielectric constant makes them suitable as capacitor-varistor materials. The theory defects in the crystal lattice was introduced to explain the nonlinear electricial behavior of the La2O3-doped WO3 ceramics. In view of these electrical characteristics, the WO3 ceramic doped with La2O3 is a viable candidate for capacitor-varistor functional devices.

Abstract: TiO₂ varistors doped with 0.1 mol% Ta and different concentrations of Pr³⁺ were obtained by ceramic sintering processing at 1400 °C and their properties were characterized by XRD, SEM, I-V and impedance spectroscopy. The effect of Pr on the microstructure, nonlinear electrical behavior and dielectric properties of the Ta-doped TiO₂ ceramics were investigated. It is found that Pr affects the grain size, electrical properties and the dielectric properties of the TiO₂-based varistors. The samples have the nonlinear coefficients (α) values of (3.0-5.0) with low breakdown voltages (4-30 V/mm). A small quantities of Pr can improve the nonlinear properties of the samples significantly. It was found that an optimal doping composition of 0.5 mol% Pr³⁺ leads to a low breakdown voltage of 9.2 V/mm, a high nonlinear constant of 4.9 and an ultrahigh electrical permittivity of 8.38×10⁴ (at 1 kHz), which is consistent with the highest grain boundary barriers of the ceramics. In view of these electrical characteristics, the TiO₂-0.5 mol% Pr³⁺ ceramic is a viable candidate for capacitor-varistor functional devices. The defects theory was introduced to explain the nonlinear electrical behavior of Pr-doped TiO₂ ceramics.

Abstract: WO₃-based capacitor-varistor ceramics doped with Er₂O₃ were prepared and the microstructures and nonlinear electrical properties were investigated. The results show that there exist second phase Er₁₀W₂O₂₁ on the surface of WO₃ grains. Doping Er₂O₃ in WO₃ ceramic can inhibit the grain growth. A small quantity of Er₂O₃ can significantly improve nonlinear properties of the samples. The permittivity of doped samples was higher than that of the undoped, and the high permittivity makes Er₂O₃-doped WO₃ ceramics be applicable as a kind of capacitor-varistor materials.

Abstract: WO₃-based varistors doped with Y₂O₃ were prepared and the microstructure, nonlinear electrical properties, and dielectric properties were investigated. Breakdown voltages E_b of WO₃ doped with Y₂O₃ are lower than that of undoped WO₃, indicating that the dopant can reduce the breakdown voltage. The dielectric constant of doped samples is higher than that of undoped samples, and the high dielectric constant makes them suitable as capacitor-varistor materials. It was found that the sample of WO₃-0.5 mol% Y₂O₃ sintered at 1150 °C has an optimal nonlinear coefficient of 3.4 and a breakdown voltage of 17.1 V/mm. WO₃ ceramic doped with Y₂O₃ is a new kind of low voltage capacitor-varistor material.

Abstract: TiO₂-based capacitor-varistor ceramics doped with Er₂O₃ were prepared and the microstructures and nonlinear electrical properties were investigated. The results show that there exist second phase Er₂TiO₃ on the surface of TiO₂ grains. The grain size was found to decrease with increasing Er₂O₃ content. The addition of rare earth oxide Er₂O₃ leads to increase the nonlinear coefficient and the breakdown voltage. It was found that the nonlinear coefficient presents a peak of α = 4.5 for the sample doped with 1.1 mol% Er₂O₃, which isconsistent with the highest grain boundary in the composition. In order to illustrate the role of grain boundary barriers for TiO₂-Ta₂O₅-Er₂O₃ varistors, a grian boundary defect barrier model was introduced.

Abstract: The microstructure and nonlinear electrical behavior and dielectric properties of the varistor, which are composed of (Y2O3, Ta2O5)-doped TiO2 ceramics, were investigated for various sintering temperatures. It is assumed that the moderate sintering temperature improves the permitivity of TiO2 ceramics, together with high nonlinear properties. The varistor of 99.6 mol%-0.3 mol%Y2O3-0.1 nol%Ta2O5 composite sintered at 1400 °C has a maximal nonlinear coefficient of α =4.4, a low breakdown voltage of 10.8 V/mm, the ultrahigh electrical permittivity of 7.73× 104 and low tanδ of 0.34. The sintering temperature plays an important an important role on the nonlinear electrical characteristics and dielectric properties of the ceramics through its influences on the microstructure of samples.

Abstract: TiO2 varistors doped with 0.1 mol% Ta and different concentrations of CeO2 were obtained by ceramic sintering processing at 1400 °C. The effect of CeO2 on the nonlinear electrical behavior and dielectric properties of the Ta2O5-doped TiO2 ceramics were investigated. The nonlinear current (I)-voltage (V) characteristics of TiO2 are examined when doped with small quantities (0.1-0.9 mol%) of CeO2. It is found that CeO2 affects the electrical properties and the dielectric properties of the TiO2-based varistors. The samples have the nonlinear coefficients (α) values of (3.0-5.0), breakdown voltages (10-30 V/mm) and ultrahigh dielectric constants which is up to 105. A small quantities of CeO2 can improve the nonlinear properties of the samples significantly. It was found that an optimal doping composition of 99.4 mol% TiO2 - 0.1 mol% Ta2O5 - 0.30 mol% CeO2 was obtained with low breakdown voltage of 14.2 V/mm, high nonlinear constant of 4.5 , an ultrahigh electrical permittivity of 8.381.22×105 (measured at 1 kHz) and low tanδ of 0.32, which is consistent with the highest grain boundary barriers of the ceramics. The theory of defects in the crystal lattice was introduced to explain the nonlinear electrical behavior of the CeO2-doped TiO2-based varistor ceramics.

Abstract: TiO2 ceramics doped with 0.1 mol% Ta2O5 and different concentrations of rare earth oxide Sm2O3 were obtained by sintering at 1450 °C. As a varisor material, the microstructure, the nonlinear electrical behavior and dielectric properties of these ceramics were investigated. SEM and XRD were carried out to study the change of microstructure. The results show that there exist second phase (Sm2Ti2O3) on the surface on the surface of TiO2 grains. The ceramics have nonlinear coefficients of α = 2.0-4.0 and ultrahigh relative dielectric constants which is up to 104. The sample doped with 0.5 mol% Sm2O3 exhibits high nonlinear constant of 3.7, low breakdown voltage of 21.5 v/mm, ultrahigh electrical permittivity of 4.25× 104 and low tanδ of 0.37. It is suggested that the sample doped with 0.5 mol% Sm2O3 forms the most effective boundary barrier layer. The defects theory was introduced to illustrate the nonlinear electrical behavior of TiO2-Ta2O5-Sm2O3 varistor ceramics.