Papers by Author: Fen Wang

Abstract: ZnO nanorods were prepared by heat treating of aqueous zinc acetate treated with organic of PVA. The products were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscopy (TEM). The results indicate that the as synthesized ZnO are hexagonal wurtzite nanorods with the maximum aspect ratio of 10:1 (100 nm in diameter and about 1 μm in length). The morphology of nanorods was formed by the regulation of appropriate organic under a pH value of 8.5. The growth mechanism of ZnO is proposed that the nanocrystals grow along a preferred direction in a growth tunnel provided by organics.

Abstract: ZnO varistors were synthesized with ZnO powders and intensity milled composite additives as raw materials. The effect of milling time of the composite additives on the grain boundary phase distribution as well as the electrical properties was investigated in detail. The results show that ball milling composite additives have a significant effect on the ZnO varistors. The samples derived from ball milled composite additives possess the smaller size and more uniform distribution of the second phases, which improved the electrical properties obviously. The optimal ZnO varistor samples were obtained by ball milling 15 h for the composite additives, which possess average ZnO crystalline grain size of about 4 µm, the gradient voltage V_1mA of 454 V/mm, the leakage current I_L of 0.12 µA, and the nonlinear coefficient α of 55.

Abstract: Ti(C,N)/Al₂O₃ in situ solid solution composites have been synthesized by mechanically induced self-propagating reactive synthesis (MSR) from the mixture of Ti, Al, C and TiO₂ in nitrogen milling atmosphere. The phase transformation and powder morphology characteristics were studied by using XRD and SEM. Full dense and pure Ti(C,N)/Al₂O₃ composites were fabricated at 1450 °C for 1 h under 10 MPa. The phase composition, microstructure and some mechanical properties of the composites were investigated. The results show that the synthesized Ti(C,N)/Al₂O₃ composites posses a density of 4.66 g/cm³ and a good combination of mechanical properties of hardness of 25.54 GPa and the flexural strength of 404 MPa. The reaction process mechanism was also discussed.

Abstract: High purity ZrSiO₄ powders were prepared using zirconium oxychloride and tetraethoxysilane as raw materials by the microwave hydrothermal (M-H) method, in which NaF was added as mineralizer to promote the crystallization of zircon. The results show that the microwave-hydrothermal method greatly reduces the starting crystallization temperature of ZrSiO₄ powders to 160~200 °C in 30 minutes, the characteristics of the microwave hydrothermally synthesized powders are superior to that synthesized by conventional hydrothermal in stability, purity and morphologies. And the XRD analysis shows that all peaks of the former powders are abnormal broadness except the (200) and (220) peaks, and the crystallization of powders can be improved by adding seed crystals of ZrSiO₄. The ZrSiO₄ particles are disk-like platelets which are monocrystal and have a trend of Single crystal to polycrystalline transition. The synthesis mechanism of ZrSiO₄ by microwave hydrothermal (M-H) process is discussed.

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: ZnO-based varistor ceramics were prepared by sintering ZnO powders with minor additives that were synthesized by standard ball milling, high energy ball milling for 10 h, and pyrogenic decomposition of nitrate. The microscopic structure and composition were analyzed by SEM and XRD respectively, and the electrical properties were investigated by a DC parameter instrument for varistors. Influences of composite additives on composition, microstructure, and electrical properties of ZnO-based varistors were studied. The ZnO varistor with optimum performance is prepared by the pyrogenic decomposition method and it has an average grain size of 5 µm and possesses a varistor voltage of about 561 V/mm, non linear coefficient of 75, and leakage current of 0.45 µA.

Abstract: Al2O3/TiAl in situ composites were successfully prepared by vacuum hot-pressing method with elemental powder mixtures of Ti, Al, TiO2 and Fe2O3. The effect of Fe2O3 addition on the microstructures and mechanical properties of Al2O3/TiAl composites has been investigated. The results show that the Rockwell hardness, flexural strength and fracture toughness of the composites increase with increasing Fe2O3 content. When the Fe2O3 content is 0.84 wt %, the flexural strength and the fracture toughness reach the maximum value of 624 MPa and 6.63 MPa·m1/2, respectively. The improvement of mechanical properties is firstly associated with a more homogeneous and finer microstructure developed by addition of Fe2O3; secondly, it is related to the increase of the ratio of α2-Ti3Al/γ- TiAl matrix phases.

Abstract: The GaOOH Nanocrystal rods were successfully synthesized by microwavehydrothermal method using Ga2O3 ,HNO3 and NH3·H2O as raw materials. Simple heat treatment of GaOOH in the flow of NH3 gas leads to the formation of submicron GaN rods even at 800°C through GaOOH. The resultant GaOOH and GaN nanomaterials were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The growth mechanism of GaOOH and GaN was proposed. The results indicate that the as synthesized GaN were hexagonal nanorods with average aspects ratio of 5:1(diameter 800 nm and length about 4μm). Photoluminescence spectrum shows that a blue emission peak originates at 473.5 nm, indicating that the GaN nanorods displayed luminescence emission in the blue-violet region, which was related to crystal defects, and may be helpful for electro-optical applications of GaN material.

Abstract: Pr6O11 doped ZnO-based varistor powders were prepared by a method of pyrogenic decomposition nitrate, which were together with ZnO powders, made into rounded mass and sintered at different temperatures. The effects of sintering temperatures on the composition, microstructure and electrical properties of Pr6O11 doped ZnO-based varistors were investigated. The results show that the optimum sintering temperature is at 1150 °C with 6 μm ZnO average grain size and the samples possess the advantageous electrical properties: varistor voltage of about 480 V/mm, non linear coefficient reaching 44 and leakage current of 0.7 μA.

Abstract: Effects of rare-earth oxides addition (0.38~1.52 mol% of Sm2O3, Eu2O3 and Er2O3) on the property and microstructure of the hot-pressed (1300°C, 2h, 35 MPa) Al2O3 (12 wt %)/TiAl insitu composites have been investigated. The results show that the doping of rare-earth oxides has a positive effect on both mechanical properties and densities of Al2O3/TiAl composites. Densities enhanced with increasing of rare-earth oxides. The flexural strength and fracture toughness were higher than other samples when the rare-earth oxide content was 0.38 mol %. The matrix grains and Al2O3 particles were significantly refined, and Al2O3 particles evenly distributed in the matrix.