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Gao X.,Kunming University | Gao X.,Key Laboratory of Advance Materials of Yunnan Province | Gao X.,Key Laboratory of Advanced Materials of Precious Nonferrous Metals | Gan G.,Kunming University | And 15 more authors.
Key Engineering Materials | Year: 2012

Double-layered, low-voltage ZnO varistors (including layer A and layer B) have been fabricated by feeding two kinds of ZnO powders into a die using dry extrusion molding. The samples are examined by using energy dispersive X-ray spectroscopy (EDS), electron probe microanalysis (EPMA), scanning electron microscope (SEM) and DC electrical measurements. EDS and EPMA data indicate that doped elements only exists in layer A, the results of SEM indicated that secondary phases are formed at grain boundaries in layer A, not found in layer B. It is found that the electrical properties of low-voltage varistor are improved without reducing thickness and changing energy absorption capabilities. The higher nonlinearity coefficients, the lower breakdown fields and leakage currents of layered structure low-voltage ZnO varistor, as compared to those of ZnO varistor fabricated from the conventional route. The improved current-voltage properties are attributed to the band structure difference in both sides of grains, due to the different ion concentration and species in both sides of grain boundary. Double-layered structure varistor also has more simpler prepared technology than multilayer chip varistor. © (2012) Trans Tech Publications.


Zhang Z.,Kunming University | Zhang Z.,Key Laboratory of Advance Materials of Yunnan Province | Zhang Z.,Key Laboratory of Advanced Materials of Precious Nonferrous Metals | Gan G.,Kunming University | And 15 more authors.
Key Engineering Materials | Year: 2012

To understand the atomic structure and electronic properties of Cr atom adsorbed on polar ZnO surfaces, a theoretical study is carried out by means of first-principles calculations based on density functional theory. The results show different characteristic with the different adsorption sites for Cr. The Cr atom adsorbed at the site on top of O forms a strong ionic bond with O atom, with electrons transferred to O. While for the Cr atom adsorbed at the hcp-hollow site on Zn-terminated surface, it forms a metallic bond with the surface Zn atom, which exhibits a free-electron-like behavior. The adsorbed atoms could not effect more atoms in ZnO due to a strong screening of ZnO to the outside metal, so the character of adsorption surfaces is only decided by the atoms near the surface. © (2012) Trans Tech Publications.


Yan J.,Kunming University of Science and Technology | Yan J.,Key Laboratory of Advance Materials in Rare and Precious and Nonferrous Metals | Yan J.,Key Laboratory of Advance Materials of Yunnan Province | Gan G.,Kunming University of Science and Technology | And 8 more authors.
Yadian Yu Shengguang/Piezoelectrics and Acoustooptics | Year: 2011

Formation mechanism of secondary phase in (La, Nb) co-doped TiO 2 ceramics varistor were investigated. The (La, Nb) co-doped TiO 2 ceramics samples were prepared from anatase TiO 2, Nb 2O5 and La 2O 3 oxide powders by a traditional solid-state sintering method. Microstructure, chemistry composition, crystal structure, thermal etched groove and micrograph of in (La, Nb) co-doped TiO 2 ceramics were tested by SEM, EDS, XRD, AFM and TEM. Formation mechanism of secondary phase was discussed by point defect thermo-dynamical analysis, grain boundary energy and materials structure measurement. The results showed that secondary phase were originated from doping ion La 3+ and Nb 5+ segregation at grain boundaries of in (La, Nab) co-doped TiO 2 ceramics. The driving force of segregation was the elastic strain energy. The segregated ion nucleated at grain surface or grain boundaries interface with higher energy. The nucleation gradually grew to form secondary phase at during high-temperature sintering. The growth of secondary phase was mainly on crystal planes with higher energy to minimize the materials system energy.


Zhang L.,Kunming University of Science and Technology | Zhang L.,Key Laboratory of Advance Materials of Yunnan Province | Zhang L.,Key Laboratory of Advanced Materials of Precious Nonferrous Metals | Du J.,Kunming University of Science and Technology | And 11 more authors.
Advanced Materials Research | Year: 2012

Porous alumina insulation materials were prepared by the combination method of gelcasting and foaming. The effects of the amount of blowing agent, the solid volume, and the sintering temperature on porosity of aluminum oxide ceramic were studied. The result showed that, when the amount of blowing agent is 1wt %, the solid volume is 45 vol % and the sintering temperature is 1500°C, the porous alumina ceramics with high porosity, small pore diameters, evenly distributed, high bending strength and low heat conductance can prepared.


Yang C.-J.,Kunming University of Science and Technology | Yang C.-J.,Key Laboratory of Advance Materials in Rare and Precious and Nonferrous Metals | Yang C.-J.,Key Laboratory of Advance Materials of Yunnan Province | Du J.-H.,Kunming University of Science and Technology | And 11 more authors.
Cailiao Kexue yu Gongyi/Material Science and Technology | Year: 2012

To precisely control the reaction conditions of doped TiO2 powders prepared by coprecipitation-coating process, the ions precipitation reaction equilibrium of Men+(Ni2+/La3+/ Fe3+/Al3+) in NaHCO3-NH3·H2O system was discussed by thermodynamic analysis in this paper. The relation curves between total concentrations of each metal ion and pH value under different total ammonia concentrations cN and total carbon concentrations cC were obtained. Then, the suitable pH of complete precipitation for all metal ions was determined. The results show that when cN is 0.010 mol/L, and cC is 1.000 mol/L, the optimal pH is 9.0 to prepare the doped titanium dioxide powders by coprecipitation-coating process, and the precipitants are NaHCO3 and NH3·H2O.

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