X.L. Chen

Nonlinear electrical behavior and dielectric properties of (Ca, Ta)-doped TiO2 ceramics

TiO2 ceramics doped with 2.5 mol% Ta and different concentrations of Ca were obtained by sintering at 1300degreesC. As a varistor material, the nonlinear electrical behavior and dielectric properties of these ceramics were investigated. The ceramics have nonlinear coefficients a in the range 2.0-5.5 and high relative dielectric constants. The effects of Ca on the density, nonlinear electrical behavior, resistivity and dielectric properties of the Ta-doped TiO2 ceramics were also studied. The sample doped with 0.75 mol% Ca and 2.5 mol% Ta exhibits the highest nonlinear coefficient and a comparatively lower relative dielectric constant. By analogy to a grain-boundary defect model, the nonlinear electrical behavior of the TiO2 system is explained

Growth and optical characterization of Ga2O3 nanobelts and nanosheets

Monoclinic Ga2O3 nanobelts and nanosheets are obtained by a simple chemical route involved with H2O at 950u200a°C in Ar atmosphere. Electron microscopy observations reveal that the as-synthesized Ga2O3 nanobelts and nanosheets are structurally uniform, single crystalline, and most of them are free of defects and dislocations. The nanobelts are growing along with [001] facets, and the nanosheets are stacked up by (011) facets. The Raman scattering spectrum of Ga2O3 nanostructures shows a 30 cm−1 redshift at high wave numbers in comparison with that of bulk Ga2O3 powder. The photoluminescence spectrum reveals that there exists a stable blue emission band centered at 460 nm, which is mainly attributed to the oxygen vacancies in the Ga2O3 nanostructures.

Aligned silica nanofibres

Large-scale highly aligned silica nanofibres were produced on silicon substrates. The diameters of these nanofibres vary from 50 to 100 nm and the lengths are up to several millimetres. Transmission electron microscopy and selected-area electron diffraction reveal that the fibres are in an amorphous state. The growth of silica nanofibres is probably controlled by a vapour-solid process. Gas-phase SiO seems to play the key role in the synthesis, serving as a transportation medium.

Effect of temperature on nonlinear electrical behavior and dielectric properties of (Ca, Ta)-doped TiO2 ceramics

Polymer electrolytes composed of polyacrylonitrile (PAN), propylene carbonate (PC), and lithium bis(trifluoromethane sulfone imide) [LiTFSI, LiN(CF3SO2)(2)] have been studied by Raman and ac impedance spectroscopy. It is found that the associations of Li+ ions with the polymer and with the solvent are two separate procedures. This makes it possible to study the influences of the polymer and the salt content on the conductivity seperately. Impedance study shows that the conductivity of the electrolyte is strongly dependent on the salt content, and a transition from salt-in-polymer to polymer-in-salt electrolyte is observed when the salt content is very high (at PC: PAN: LiTFSI 5 1: 1: 4 mole ratio). Raman spectroscopic study indicates that the interactions between the lithium ionic species and the polymer are striking after PAN is dissolved in the PC/LiTFSI solution. A new spectral component is observed at 2280 cm(-1) involving the interaction between the polymer and ionic cluster [Lim+TFSIn-] (m > n) (at PC: PAN: LiTFSI = 1:1:3) as well as the 2270 cm(-1) peak due to the nitrile-Li+ interaction. As the interaction between PAN and the ionic clusters and the transition of conduction behavior occur at a similar salt content, it is proposed that PAN plays distinct roles in the transportation of the ionic species in conventional salt-in-polymer electrolyte and in the novel polymer-in-salt electrolytes

Regular arrays of GaN nanorods

Regular arrays of GaN nanorods have been synthesized by reacting gallium and ammonia using Au as a catalyst on a MgO single crystal substrate treated by chemical etching. They were characterized by field emission scanning electron microscopy, energy dispersive X-ray spectroscopy (EDX), selected area electron diffraction (SAED) and X-ray diffraction (XRD). EDX, XRD SAED indicated that the nanorods were wurtzite single crystal GaN. We suggest that regular tactic square shape with steps on the single crystal MgO and discontinuity of the Au film may play an important role during the formation of regular arrays of GaN nanorods.

Structural and superconducting properties of La2CuO4+δ chemically oxidized by NaClO

Abstract A series of La 2 CuO 4+ δ samples chemically oxidized in NaClO aqueous solution at room temperature with different oxidation time of 2–120 h have been prepared. The X-ray powder diffraction analyses indicate that all the oxidized samples are orthorhombic and single phase, and show an enhanced orthorhombic distortion with the oxidation time, compared with that of the as-prepared sample. The magnetic susceptibility measurements indicate that all the oxidized samples are bulk superconductors with an onset transition temperature at 45 K. The excess oxygen contents of the oxidized samples determined by iodometric titration increase with the oxidation time, which versus the oxidation time obeys the law of the second order exponential decay. The grain size observed by scanning electron microscopy has no obvious changes for the sample before and after oxidation. All the results indicate that NaClO is indeed an efficient oxidizing agent for La 2 CuO 4 .

A NOTE ON THE AlB2 TYPE STRUCTURE

In this paper, the crystal structures of a series of diborides are analyzed based on the hard ball model. We show that the packing efficiency has a close relation with the lattice parameter ratio c/a and the closest packing is achieved at c/a = 1.074. For the compounds with c/a 1.074 the packing is loose along the c-direction and the B–B bond is stretched. The structures will be unstable as the c/a value deviates from 1.074. MgB2 has the largest value of c/a among the examined diborides, except for lanthanide diborides and YB2.

Phase relations in the SrO-Ga2O3-B2O3 system

The subsolidus phase relations in the SrO-Ga2O3-B2O3, system were investigated. The system contains 10 binary compounds and two ternary compounds, and can be divided into 15 three-phase regions. The new ternary compound SrGaBO4 has two modifications (alpha- and beta-phases), both of which crystallize in the orthorhombic system but with different space groups

Synthesis, thermal and magnetic properties of new metal iodate: (LiFe1/3)(IO3)2

The density and the surface tension of molten calcium fluoride have been measured in the temperature range from 1690 to 1790K by an improved Archimedian method and a ring depressing technique (J. Crystal Growth 187 (1998) 391), respectively. The ring depressing technique was demonstrated as an effective technique to measure the surface tension in comparison with the conventional ring pulling technique. The density varied with the temperature change corresponding to a linear relationship: p = 3.767 - 6.94 x 10(-4) T (K). The density of the CaF2 melt at the melting point is 2.594 g/cm(3), which is equal to the result obtained by Shiraishi and Watanabe (Bull. Res. Inst. Miner. Dressing Metal, Tohoku Univ. 34 (1978) 1), but the temperature coefficient of the density is different from the results obtained by other investigators. The thermal expansion coefficient of calcium fluoride melt linearly increases with temperature heating. The surface tension of molten calcium fluoride indicates a negative linear relationship as a function of the melt temperature: (T) = 442.4 - 0.0816 x T(K) (mN/m). The surface tension measured using the ring depressing technique is larger than those results obtained by other techniques

Nonlinear electrical and dielectric properties of (Ca, Ta)-doped TiO2 varistors

(Ca, Ta)-doped TiO2 varistors with high nonlinear coefficients were obtained by ceramic sintering process. The nonlinear electrical and dielectric properties of the samples doped with 1.0 mol% Ca and different concentrations of Ta (0.05-2.0 mol%) were investigated. Especially, the effects of Ta dopant on the nonlinear and dielectric properties of the (Ca, Ta)-doped TiO2 varistors were studied in detail. When the concentration of Ta is 0.5 mol%, the sample possesses the highest nonlinear coefficient (16.6). By analogy to a, grain-boundary atomic defect model, the effects of Ta and the nonlinear electrical behavior of the TiO2 system were explained.