Ming-Sheng Zhang

Optical and structral properties of pure and Ce-doped nanocrystals of barium titanate

Abstract Nanocrystals barium titanate (nc-BT) and Ce-doped barium titanate were synthesized by an unusual hydrothermal process. X-ray diffraction and Raman spectroscopy were used to investigate their microstructures, as well as their lattice vibration and photoluminescence (PL) spectra. Particle sizes as small as 20 nm were measured by X-ray diffraction pattern via Scherer equation. The critical size relative to the cubic-tetragonal transition is about 48 nm in nc-BT below which ferroelectricity vanishes. The lattice parameter ratio c/a of is equal to 1.003, much smaller than 1.01 for that of the bulk crystal, leading to a weakening of ferroelectricity. Hydroxyl defects are observed in the Ce-doped nc-BT samples which show a vibrational band at 3300 em t. Good crystallization character and the lattice perfection were characterized by Raman spectra in the nanophase barium titanate. Strong PL spectra centered at 696 and 585 nm were observed in the pure and Ce-doped nc-BT, respectively. PL dependence on temperature and annealing time was examined. A molecule-like recombination and the charge transfer between Ce4+ and Ce 3+ are proposed to elucidate the luminescence process in the nc-BT and Cc-doped nc-BT system.

Phase structures and electrochemical properties of the Laves phase hydrogen storage alloys Zr1−xTix(Ni0.6Mn0.3V0.1Cr0.05)2

Abstract The effects of the partial substitution of Ti for Zr in Zr1−xTix(Ni0.6Mn0.3V0.1Cr0.05)2 alloys are reported in this paper. The main phases C15 and C14 Laves phases, and secondary phase Zr7Ni10 were found. With increasing Ti content, abundance of C15 phase decreases and that of C14 phase increases. Increase of Ti content leads to decrease of the lattice parameters of both C15 and C14 phases. Pressure-composition isotherms show a decrease of the stability of the alloy hydride. Ti substitution for Zr in the alloys is effective to improve the activation and high-rate dischargeability. A critical substitution content of Ti is found at x=0.2. The cycling stability and the high rate dischargeability are deteriorated for the alloy electrodes with high Ti content (x>0.3).

Phase character and structural anomaly of Bi4Ti3O12 nanoparticles prepared by chemical coprecipitation

Abstract Nanoparticles of Bi4Ti3O12 have been prepared by chemical coprecipitation. The phase structure was investigated by X-ray diffraction (XRD) and refined using the Rietveld method. Orthorhombic Bi4Ti3O12 particles were obtained after heating at 700 and 800 °C; however, for the particles calcined at 500–650 °C, the results suggested the presence of a high-temperature tetragonal phase at room temperature. This structural anomaly was further confirmed by differential thermal analysis (DTA).

Hydrogen storage properties of Zr0.8Ti0.2(Ni0.6Mn0.3−xV0.1+xCr0.05)2 (x=0.0,0.05,0.15,0.2) alloys

Abstract The hydrogen storage properties of Zr 0.8 Ti 0.2 ( Ni 0.6 Mn 0.3−x V 0.1+x Cr 0.05 ) 2 (x=0.0,0.05,0.15,0.2) alloys are reported in this paper. The effects of substitution of V for Mn atoms on phase structure and hydrogen storage properties are studied. The main phases were found to be C14 and C15 Laves phase. With increasing V content, abundance of C14 phase increases and that of C15 phase decreases. Pressure-composition isotherms indicate that substitution of V for Mn leads to a decrease of the hydrogen equilibrium pressure and an increase of maximum hydrogen storage capacity. It is found that the amount of solid solution phase (α phase) increases as the V content increases. Hydrogen storage properties also vary with the reaction temperature.

Blue-Light Emission from Nanocrystalline La0.5Na0.5TiO3

Nanocrystalline La0.5Na0.5TiO3 powders were prepared by the stearic acid-gel method, and the grain sizes could be controlled by varying annealing temperatures. The sizes were determined to be from 14 to 50 nm using Scherrers formula. A very strong blue-light emission band around 480 nm was clearly observed in the nano-sized La0.5Na0.5TiO3 with a grain size of 14 nm at room temperature. Also, nano-size effects on the luminescence properties were investigated. These results could be explained with the aid of a self-trapped exciton model.

Lattice vibration and photoinduced light scattering in Co-, Cr- and Fe-doped lithium niobate

Raman scattering and photoinduced light scattering in Co-, Cr- and Fe-doped lithium niobate were examined. The A1(TO) modes appear in E symmetry spectrum of the doped lithium niobate. Their intensities vary with different dopings. In the spectrum of z(yy)x geometry, the properties of the lowest-frequency E(TO) mode of the Cr-doped lithium niobate are different from those of pure, Co- and Fe-doped lithium niobate. The intensity of the A1(TO) mode at 637cm-1 I is decreased in doped lithium niobate compared with the pure crystal. We attribute these properties to both the photorefractive effect which is enhanced by dopants and to the different occupation of the doping ions. A light climbing effect was observed in Co- and Cr-doped lithium niobate for the first time. A higher photodamage threshold and quicker light climbing speed were found in Co- and Cr-doped lithium niobate in comparison with the light climbing effect in the Fe-doped lithium niobate. The results from the photoinduced light scattering experiments were compared with those from a Raman spectroscopic study.

PHOTOINDUCED LIGHT SCATTERING IN CO- AND CR-DOPED LITHIUM NIOBATE

Photoinduced light scattering of Co- and Cr-doped lithium niobate crystals is examined. The light climbing effect is observed in the Co- and Cr-doped lithium niobate crystals. The influences of the incident light wavelength, laser power, sample thickness and doping effects on the light climbing effect are studied. Higher photodamage threshold and quicker light climbing are found in the Cr- and Co-doped lithium niobate, which are attributed to the higher space electrical charge field and both different valence and occupation of the doping ions, respectively.