Advanced Materials Research Vols. 415-417

Abstract: A new system, Bi_1.3Yb_0.7O₃-BaTiO₃ was found as a perovskite dielectrics with some advantageous properties. The structure was identified by X-ray diffraction method and SEM was also employed to observe the surface and fracture morphologies. The studies indicate that the relative dielectric constant initially increases and then decreases slightly, the dielectric loss initially decreases and finally close to a stable value. The experimental results reveal that the grain size became smaller initially, then increase after the sample E and finally the sample F had the maximum grain size. Experiments show the system Bi_1.3Yb_0.7O₃-BaTiO₃ can significantly improve the thermal stability of the BaTiO₃ ceramics. Electric hysteresis loop show that the system is relaxor ferroelectric. The samples doped with 20% BiYbO₃ calcined at 750°C for 2h and then sintered at 1280°C for 2h exhibited attractive properties, including high relative dielectric constant (>1000), low dielectric loss (<2.5%), low temperature coefficient of capacitor (<±15%) over a wide temperature range from －55°C to +180°C, satisfying EIA X8R specifications.

Abstract: In the present study, various quantities of Bi₂O₃ were added into 0.98(Na_0.5Bi_0.5)TiO₃ -0.02Ba(Sn_0.08Ti_0.92)O₃ (0.98NBT-0.02BST) ceramics. High-density samples were obtained through the addition of Bi₂O₃ into 0.98NBT-0.02BST ceramic. It was found that 0.98NBT-0.02BST with the addition of 0~3.0 wt.% Bi₂O₃ exhibit relatively good piezoelectric properties. For 0.98NBT-0.02BST ceramic with the addition of 2 wt.% Bi₂O₃, the electromechanical coupling coefficients of the planar mode kp and the thickness mode kt reach 0.12 and 0.61, respectively, at the sintering of 1100oC for 3 h. The ratio of thickness coupling coefficient to planar coupling coefficient is 5.1. It is obvious that 0.98NBT-0.02BST solid solution ceramic by adding low quantities of Bi₂O₃ is one of the promising lead-free ceramics for high frequency electromechanical transducer applications.

Abstract: Abstract. The wheelchair comfort is a subjective evaluation of users, which is influenced by wheelchair itself, environmental factors and physiological factors. It is difficult to make evaluation about wheelchair comfort by a general approach. In this essay, Analytic Hierarchy Process, based on a scientific decision-making idea which combines qualitative analysis with quantitative analysis, was used to establish the evaluation system about wheelchair comfort indicator, and the weight of each indicator was determined. For this reason, the evaluation of wheelchair comfort is more scientific and reasonable.

Abstract: Fused quartz granule was used as raw material, and CeO₂ nanopowder was used as additive with dosages of 1wt %, 2 wt % and 3 wt %. Fused quartz ceramic materials were fabricated in reduction atmosphere at 1300 °C, 1350 °C and 1400 °C for 1 h, respectively. The properties of samples including apparent porosity, bending strength and thermal expansion ratio were measured. The results show that the sample with 1 wt % CeO₂ nanopowder has the lowest apparent porosity, the highest bending strength and the lowest thermal expansion ratio. The XRD and SEM results show that through the CeO₂ nanopowder adding, the diffraction peak intensity of quartz phase in the fused quartz ceramics decrease, and the microstructure of the samples become more compact. It can be deduced that CeO₂ nanopowder plays the excellent role as the crystallization inhibitor and sintering assistant to fused quartz materials sintered at various temperatures.

Abstract: Perovskite-based (1-x) (Na_0.5Bi_0.5)TiO₃-x NaNbO₃ [(1-x) NBT-x NN, x = 0.1, 0.2, 0.3 and 0.4] ceramics were sintered at 1080°C. Dielectric characteristics, crystalline structures, and Raman investigations were carried out on (1-x) (Na_0.5Bi_0.5)TiO₃-x NaNbO₃ ceramics. X-ray diffraction (XRD) patterns showed that NaNbO₃ ceramic would form a solid solution with (Na_0.5Bi_0.5)TiO₃ ceramic, and and unknown or second phases were not observable as well. NaNbO₃ ceramic diffused into the crystalline structure of (Na_0.5Bi_0.5)TiO₃ ceramic and (1-x) NBT_-x NN ceramics still revealed a rhombohedral structure. The temperature-dielectric constant curves showed that as NN content increased, the temperature to reveal the maximum dielectric constant (T_m) was raised, the depolarization temperature (T_d) was shifted to lower value, and the dielectric constant at Tm and the loss tangent at T_d gradually decreased. The Raman bands at 770 and 830 cm^-1 were attributed to the existence of the oxygen vacancies. In this study, the relaxor-type ferroelectric properties of NBT ceramic had been improved as NN ceramic was added.

Abstract: To obtain ZnO arrester with high voltage gradient and small size, through the optimizing foundamental formula of arrester and changing sintering temperature and holding time, the electrical properties and microstructure of varistors were studied. The density of varistors was determined by using the mass - volume method , voltage gradient and leakage current of ZnO arrester were measured with Ⅱ Surge Arrester Tester DC parameters, microstructure of varistor ceramics were studied by means XRD and SEM. The experimental results show that with increasement of the sintering temperature, the density of varistors increases, and the voltage gradient continues to decrease and leakage current almost keeps unchangable. As the holding time increases, while as the voltage gradient continues to decrease, and leakage current almost unchanges.

Abstract: Fused quartz granule was used as raw material, and La2O3 nanopowder was used as additive with dosages of 1wt %, 2 wt % and 3 wt %. Fused quartz ceramic materials were fabricated in reduction atmosphere at 1300 °C, 1350 °C and 1400 °C for 1 h, respectively. The properties of samples including apparent porosity, bending strength and thermal expansion ratio were measured. The results show that the sample with 2 wt % La₂O₃ nanopowder has the lowest apparent porosity and the highest bending strength, and sample added 1% nano-La₂O₃ has the best inhibiting effect on crystallization of fused quartz. The samples with La₂O₃ nanopowder adding have lower thermal expansion ratio than blank sample. The XRD and SEM results show that the diffraction peak intensity of quartz phase decrease and the microstructure of the samples become more compact, after the La₂O₃ nanopowder was added into fused quartz ceramics. It indicated that La₂O₃ nanopowder has excellent inhibiting effect on crystallization of fused quartz and facilitating effect on sintering of fused quartz ceramics.

Abstract: The effect of La₂O₃ addition on the microstructure and electrical properties of Mn_0.75Ni_1.25CuO_4-xLa₂O₃ (0≤x≤0.3) was studied. The crystal structure, phase compositions and morphology were analyzed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS). The sintered ceramic bodies was typical polycrystalline, with cubic spinel structure of copper manganese oxide, rocksalt structure of nickel copper oxide, along with monoclinic phase of copper oxide and orthorhombic perovskite structure of lanthanum manganese oxide. The obtained ρ_77K, B_77K/90K constant and activation energy of the thermistors were in the range of 3.88-9.29 Ω cm, 245-261 K, and 0.0211–0.0225 eV, respectively. This means that the electrical properties can be adjusted to desired values, depending on the La content. So these prepared thermistors were intended to be used under low temperature conditions.

Abstract: The microstructure of V-Nb inoculants after melt spinning was investigated by performing TEM examination and to the sample steels, the prior austenite grain sizes and fracture morphology were investigated by SEM observation. The results show that the V-Nb inoculants obtained is amorphous and the sample steel modified by V-Nb inoculants which make it possible to be a large amount of nucleation centers existing in the form of (V, Nb)C and achieve the effect of grain refinement in the solidification stages of liquid steel has been found to have the finest prior austenite grain sizes and the average prior austenite grain sizes of steel without addition of V and Nb(Steel A), steel microalloyed with V and Nb(Steel B) and steel modified by V-Nb inoculants(Steel C) are respectively 30μm, 20μm and 10μm. Tensile behavior of samples was studied under tempering at 400°C for 30 min after quenching, tensile results show that the fracture of steel modified by V-Nb inoculants appears dimples which represent ductile fracture. However the tensile fracture of V-Nb microalloyed steel is mixture of quasi-cleavages and dimples. Under tempering at 550°C for 30 min, Steel B and Steel C both show a fractograph with cleavage fracture.

Abstract: In this paper, the effects of rolling and annealing on the high speed impact deformation behavior of the Mg-3%Li-1%Ce alloy were investigated by using the Hopkinson compressive bar. The results showed that the microstructures of the as-rolled alloy consisted mainly of deformed grains, with some secondary Mg₁₂Ce particles in grain interior or on grain boundary. After annealing at 350 °C for one hour, static recrystallization happened and grains were significantly refined. For the annealed alloy, the impact stress-strain behavior is non-sensitive to strain rate in the strain rate range of 1000~1750 s^-1. Under the strain rate of 2250 s^-1, the dynamic behavior demonstrated negative strain rate effect. The occurrence of cracking was supposed to be the main reason for the strain rate softening effect.