Papers by Author: Jian Feng Zhu

Abstract: ZnO-based linear resistance ceramics were fabricated by the conventional ceramic method. The effect of calcination of the raw materials on the microstructure and electrical properties was investigated in detail. The results show that the electrical properties, such as resistivity, nonlinear coefficient and resistance temperature coefficient, have been obviously influenced by the calcining process. The optimal samples fabricated with the materials calcinated for twice possess the highest stabilization of resistivity. Meanwhile, the resistance temperature coefficient is improved by 30%, and the nonlinear coefficient of voltage decreases to 1.16, decreased by 20.7%.

Abstract: SnO₂-based varistors were successfully fabricated from the mixed powders, SnO₂, Co₂O₃, Nb₂O₅ and Cr₂O₃. The effects of sintering temperature (1250, 1300, 1350 and 1400 °C) on the microstructure and electrical properties were investigated. The results reveal that the grain size increases with increasing the sintering temperature, and the breakdown electrical filed decreases gradually. When the sintering temperature was 1300 °C, the nonlinear coefficient of the as fabricated SnO₂ based varistors presents the maximum of 27. Meanwhile, the leakage current possesses the minimum of 4.5 µA.

Abstract: Mo modified Ti₂A1C /Al₂O₃ composites were successfully fabricated from an elemental power mixture of Ti, Al, TiC and MoO₃ by an in situ reaction/hot pressing method. The reaction path and effect of the molar ratio of the initial materials on the phase composition were investigated in detail. The as-prepared materials are mainly composed of (Ti_1-xMo_x)₂AlC solid solutions, Al₂O₃, and a small amounts of the Mo rich compounds. It is found that the in situ formed fine Al₂O₃ particles tend to disperse on the matrix grain boundaries. Compared with the monolithic Ti₂A1C, (Ti,Mo)₂AlC/10 wt% Al₂O₃ composite possesses a fine grain sized structure. The Vickers hardness, flexural strength, fracture toughness, and compressive strength of the as composite are 4.75 GPa, 458 MPa, 6.03 MPa·m^1/2, 971 MPa, respectively.

Abstract: The Ti₃AlC₂/Al₂O₃ in situ composites were successfully synthesized from the system of Ti-TiC-Al-TiO₂ by reactive hot pressing at 1350 °C. The effect of TiC content on the phase composition, microstructure and mechanical properties were investigated in detail. The results indicate that the fabricated products possess the highest purity as the TiC contents reduce to 90 % of its theoretical content. This deviation is mainly attributed to the decomposition of Ti₃AlC₂ and vaporization of Al at high temperatures. The effect of TiC content on the fracture toughness, flexural strength, Vickers hardness of Ti₃AlC₂/Al₂O₃ composites was also discussed in detail.

Abstract: Al₂O₃/TiAl composites doped with Cr and V₂O₅ were successfully prepared from Ti, Al, TiO₂, Cr and V₂O₅ by in situ hot-pressing. The synthesis process and the microstructures of as-fabricated samples were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results show that the reaction process could be divided into three stages: firstly, Ti reacted with Al to form TiAl₃, then TiO₂ and V₂O₅ were reduced by Al to form Al₂O₃, and finally TiAl and Ti₃Al occur from the competitive solid-state diffusion reactions. The as-synthesized composites mainly consist of V and Cr doped matrix phases of γ-TiAl and α₂-Ti₃Al and dispersive Al₂O₃ phase. The in situ formed fine Al₂O₃ particles mainly disperse on the boundaries of TiAl. Compression strength shows a peak value of 1096.22 MPa at 7.54 at.% Al₂O₃ content.

Abstract: Al₂O₃/TiAl composites were successfully fabricated by hot-press-assisted exothermic dispersion method with powder mixtures of Ti, Al, TiO₂ and Cr₂O₃ as raw materials. The effect of sintering temperature on the microstructures and mechanical properties of Al₂O₃/TiAl composites has been investigated. The results show that the Rockwell hardness and density of the composites increased with increasing sintering temperature. But the flexural strength and fracture toughness peaked at 825 MPa and 7.29 MPa·m^1/2, respectively, when the sintering temperature reached to1300 °C.

Abstract: ZnO-based linear resistance ceramics were synthesized using Al2O3 doped ZnO-based system as raw materials by sintered at 1340 °C for 3 h. The effects of Al2O3 content ranged from 1 to 15 wt% on the microstructure and electrical properties of the ceramics were investigated in detail. The results show that the electrical properties such as nonlinear coefficient, resistivity and resistance temperature coefficient have been obviously influenced by Al2O3 doping. The optimal samples obtained by doping Al2O3 with 9 wt% have a nonlinear coefficient of 1.3, resistivity of 130 (Ω•cm) and resistance temperature coefficient of -5.4×10-3/ °C.

Abstract: TiAl/Ti₅Si₃ in situ composites were successfully fabricated by reactive hot-press method from powder mixtures of Ti, Al and Si. The influence of the Si addition on the microstructures and mechanical properties of TiAl/Ti₅Si₃ composites was investigated in detail. The results show that an appropriate amount of addition of Si refined the matrix structure obviously due to the in situ formed Ti₅Si₃, and as a result, the flexural strength and fracture toughness of the composites are modified. When the Si content is 1.82 wt %, the flexural strength and the fracture toughness reach the maximum value of 685.67 MPa and 9.02 MPa·m^1/2, respectively. The enhancing mechanism was also discussed.

Abstract: TiAl/Al₂O₃ in situ composites have been prepared by a reactive hot pressing method from Ti, Al and Nb₂O₅ powders. The phase transformation and the mechanism of synthesis were studied by differential scanning calorimeter (DSC) of starting powers and X-ray diffraction (XRD) of samples hot press sintered at different temperatures from 500 °C to 1300 °C. Scanning electron microscopy (SEM) coupled with energy-dispersive spectroscopy (EDS) was utilized to investigate the morphology characteristics of the products. The resultant in situ formed TiAl/Al₂O₃ composites exhibited multiplex structures containing TiAl, Ti₃Al, Al₂O₃ and NbAl₃ phases. Fine Al₂O₃ particles that act as reinforcing phase are dispersed along the interface of the matrix. The formation of TiAl/Al₂O₃ composite involves many transitional stages. Firstly, Ti and Al reacted to form TiAl₃ and Ti₃Al intermediates, then Nb₂O₅ was reduced by Al to form the Al₂O₃, and finally, the competitive solid-state diffusing reactions among Ti₃Al, TiAl and TiAl₃ produced the final matrix phases of the resultant composite.

Abstract: Full dense and highly pure TiAl/Ti5Si3 in situ composite was successfully synthesized by reactive synthesis from the powder mixtures of Ti, Al and Si. The reaction process was investigated in detail by the X-ray diffraction analysis (XRD) and differential scanning calorimetry (DSC). The microstructural characteristics of the TiAl/Ti5Si3 in situ composite were also studied by scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The results show that the as fabricated composite possesses three phases, namely, TiAl, Ti3Al and Ti5Si3. The matrix phases are mainly equiaxed TiAl with a minor lamellar Ti3Al phase. Ti5Si3 particles with size less than 1 μm are distributed uniformly in matrix grains as a reinforcing phase.