Papers by Author: Long Hao Qi

Abstract: The high-pure alpha-alumina powders with some agglomerate were treated by bead milling and their sintering behaviors were studied in this paper. During the treatment, the effect of different types and different amounts of dispersants was also investigated. It was shown that optimum dispersant content was 0.5% citric acid and optimum milling time is 6 hours. The sintering property of alumina powder after bead milling was significantly improved. Their relative density after sintering at 1500 degrees can reach 99.3% while the raw powders’ can only reach 90% at same condition.

Abstract: B2O3-doped ZnO-Pr6O11 based varistor ceramics were fabricated by sintering samples at 1350 °Cfor 2h with conventional ceramic processing method. The microstructure and electrical properties of the as-prepared samples were investigated. X-ray diffraction analysis showed that after the addition of B2O3 with the amount designed in this study, no new phase was examined in the detection limit. Through scanning electron microscopy it was found that the doping of B2O3 can promote the growth of ZnO grains, and the sintering of the samples. The result of electrical properties indicated that the nonlinear coefficient and varistor voltage of the samples could be improved to some extent with appropriate doping amount of B2O3, resulting in the highest nonlinear coefficient 6.7, lowest leakage current 329 μA/cm2, and highest varistor voltage 92.4 V/mm, respectively.

Abstract: Ultrafine WC-Ni cemented carbides with addition of SiC whisker (SiCw) were fabricated by spark plasma sintering. The microstructure and mechanical properties of the fabricated cemented carbides were investigated. It was found that the addition of SiC whisker had no obvious influence on the phase compositions of the cemented carbides, but the mean grain size of the cemented carbides decreased as the addition fraction of SiC whisker increased. The fabricated WC-Ni cemented carbides presented the highest hardness when 0.75 wt% SiC whisker was added. However, the addition of SiC whisker was detrimental to the flexural strength of the cemented carbides because of the formation of inhomogeneous microstructure in the WC-Ni cemented carbides.

Abstract: WC-Ni-SiCw alloys were prepared through hot-pressed sintering by using WC, Ni, VC, TaC and SiC nano-whisker as raw materials. The influence of SiC whisker addition on the microstructure and mechanical properties of the as-prepared WC-Ni alloys was investigated. Except WC hard phase and Ni binder phase, no other phase was detected by X-ray diffraction. The observations by scanning electron microscope demonstrated that there were some micropores in the as-prepared materials, and their size and quantity increased with increasing amount of SiCw. Vickers hardness and flexural strength tests indicated that the bulk hardness of WC-Ni-SiCw alloys slightly decreased with increasing amount of SiCw; and the flexural strength increased at first and then decreased with increasing amount of SiCw. The alloys with 0.53 wt% SiCw presented the highest flexural strength of 1700 MPa.

Abstract: TiCN-based cermets with different amounts of SiC nano-whiskers were prepared by spark plasma sintering. The microstructure and mechanical properties of the as-prepared cermets were investigated. X-ray diffraction revealed that there were no SiC peaks detected, turning out some peaks of new carbide and silicate hard phases. Scanning electron microscopy indicated that there were more and more pores in the cermets with increasing amount of SiC whisker added, and the fracture mechanism of the cermets was mainly inter-granular fracture. With increasing addition amount of nano-SiC whisker, the hardness and flexural strength of the cermets increased first and decreased then, presenting the highest hardness (2170 HV) and flexural strength (750 MPa), respectively, when the addition content of nano-whiskers is 2.5 wt%.

Abstract: In this article we present the testing, evaluation of infrared radiation properties of polycrystalline materials especially as cordierite-based glass-ceramics. Researches aim to make a comprehensive and systematic exposition of emissivity definition, test principles and test methods. And on the basis of measurement results of the infrared emissivity of cordierite-based glass-ceramics, the Infrared radiation property of this polycrystalline material was discussed. Research has a positive significance on the development of the infrared radiation heating and drying materials.

Abstract: The electrical properties of ZnO-Pr6O11-based varistors, which composed of (98.5-x)mol% ZnO +0.5mol% Pr6O11 +1.0mol% Co3O4 +xmol% Cr2O3 (x=0.0, 0.25, 0.5, 1.0, 2.0), were investigated as a function of sintering temperature in the range of 1250-1400°C for 2 h. At the same sintering temperature, the nonlinear exponents increased with the increase of Cr2O3 doping amount no more than 0.5mol%, but decreased with more Cr2O3 doped. With the same doping level of Cr2O3, the samples' varistor voltage decreased with the sintering temperature increased; and the samples' nonlinear exponents increased first with the sintering temperature increased to 1300 °C, but decreased when the temperature increased further. Conclusively, the electrical properties of ZnO-Pr6O11-Co3O4- Cr2O3 varistors were strongly affected by the sintering temperature and the doping level of Cr2O3, and the samples, doped with 0.5 mol% Cr2O3 and sintered at 1300 °C, exhibited the optimum nonlinear performance.

Abstract: Through a modified ASTM B611 wet sand rubber rimmed wheel test system (model: MLS-225), the wear resistance of commercially bought TiCN cermets FD22 was studied with water-based slurries of abrasive sands. The applied abrasives were angular sands of carborundum, corundum, and silica with particle size of about 350 μm. During experiments, the rotation speed of the rubber wheel was fixed at 498 rpm, and the load was 225 N. The results show that with increasing concentration of abrasive sands in the slurry, the wear loss of the cermet samples increased. Under the same conditions, when carborundum was used as abrasive, the cermet samples presented the heaviest wear loss and the highest wear rate, but when silica was used as abrasive, the cermet samples presented the slightest wear loss and the lowest wear rate. Under the abrasive concentrations of 5-20 wt% in slurry, the wear rate remained almost unchanged as the sliding distance increased. However, when the concentration was higher than 20 wt%, as the sliding distance increased, the wear rate increased at first and gradually decreased with the sliding distance over 30 km. Through the observations of 3D white-light interfering surface profiler, it was found that the wear mechanisms of TiCN cermet samples involved in extensive plastic deformation, grooves, binder removal and fracture.

Abstract: This paper describes a method of catalyst-assisted pyrolysis of preceramic precursors for the synthesis of Si-based nanostructures. Through tuning the processing conditions, clustered SiOx amorphous nanowires, bundled single-crystalline Si3N4 nanowires, Y-shape and network-like branched single-crystalline Si3N4 nanostructures, aligned single-crystalline Si3N4 nanowires, and so on, have been synthesized. The results reveal that it is a simple and easy-controlling method, and has potential for mass production of nanomateirals of controlled morphology.

Abstract: Based on La2Zr2O7 ceramic for thermal barrier coatings, LaSmZr2O7 ceramic doped with samarium ion was successfully prepared using solid state reaction method. The pellets were sintered at 1600°C for 10 hours in air. The phase structure, thermal conductivity and thermal expansion coefficient of LaSmZr2O7 ceramic and La2Zr2O7 ceramic were measured by XRD, laser-flash device, high-temperature dilatometry, respectively. The results show that the crystal structure of LaSmZr2O7 ceramic is not affected by the doped samarium ion in the zirconium lattice. The thermophysical results show that the thermal conductivity of the LaSmZr2O7 ceramic is lower than that of La2Zr2O7 ceramic, while the thermal expansion coefficient is higher than that of La2Zr2O7 ceramic. These results indicate that LaSmZr2O7 ceramic or Ln2Zr2O7 ceramics doped with other rare earth ions could be candidate materials for future thermal barrier coatings.