Papers by Author: Dong Jian Zhou

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: 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: 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.

Abstract: A low-voltage TiO2 capacitor-varistor ceramics doped with Ta2O5 and Nd2O3 was systematically researched. The effect of Nd2O3 on the microstructure, nonlinear electrical properties, and dielectric properties of TiO2-based ceramics was investigated. It was found that an optimal doping composition of 99.20 mol% TiO2-0.10 mol%Ta2O5-0.7 mol% Nd2O3 was obtained with low breakdown voltage of 8.5 v/mm, high nonlinear constant of 4.0, ultrahigh electrical permittivity of 1.07× 105 and low tanδ of 0.39. In view of these electrical characteristics, the ceramics of 99.20 mol% TiO2-0.10 mol%Ta2O5-0.7 mol% Nd2O3 is a viable candidate for capacitor-varistor functional devices. The theory of defects in the crystal lattice was introduced to explain the nonlinear electrical behavior of the Nd2O3-doped TiO2-based varistor ceramics.

Abstract: Ti-6Al-Mo-Fe alloys were prepared by blended elemental powder metallurgy process. In this paper, the mechanical properties and microstructure of Ti-6Al-Mo-Fe alloys sintered from the powders ball-milled for various periods of time were investigated by means of contrition behavior testing, X-ray diffraction, scanning electron microscopy. With an increase in ball milling time form 1h to 10h, the microstructure evolved into a fine β phase within the α matrix. For the Ti-6Al-Mo-Fe alloy, the yield strength and elongation increase as the milling time is prolonged. It was found that the bulk alloys made from the powders ball milled for 6 h exhibited relatively high elongation of 17.8% and high yield strength of 914 MPa.