Xueming Ma

Doping concentration dependence of room-temperature ferromagnetism for Ni-doped ZnO thin films prepared by pulsed-laser deposition

High-quality Ni-doped ZnO thin films of single phase with preferred c-axis growth orientation were formed on Si (100) substrates by pulsed-laser deposition at room temperature. The films exhibited room-temperature ferromagnetic behaviors with saturation magnetic moment per Ni atom of 0.37μB,0.26μB,0.25μB and 0.21μB for the Ni concentration of 1, 3, 5, and 7 at. %, respectively. The decrease of ferromagnetism with doping concentration demonstrates that ferromagnetism observed at room temperature is an intrinsic property of Ni–ZnO thin films, not from any secondary phase.

Nb2O5-carbon core-shell nanocomposite as anode material for lithium ion battery

Abstract Nb 2 O 5 -carbon nanocomposite is synthesized through a facile one-step hydrothermal reaction from sucrose as the carbon source, and studied as an anode material for high-performance lithium ion battery. The structural characterizations reveal that the nanocomposite possesses a core-shell structure with a thin layer of carbon shell homogeneously coated on the Nb 2 O 5 nanocrystals. Such a unique structure enables the composite electrode with a long cycle life by preventing the Nb 2 O 5 from volume change and pulverization during the charge-discharge process. In addition, the carbon shell efficiently improves the rate capability. Even at a current density of 500 mA·g −1 , the composite electrode still exhibits a specific capacity of ∼100 mAh·g −1 . These results suggest the possibility to utilize the Nb 2 O 5 -carbon core-shell composite as a high performance anode material in the practical application of lithium ion battery.

Tetragonal VNb9O24.9-based nanorods: a novel form of lithium battery anode with superior cyclability

Tetragonal VNb9O24.9-based nanorods are synthesized through an aerosol-assisted process. This new form in a V–Nb–O system shows an outstanding cycling stability as well as a moderate rate capability. The electrochemical properties correlate well to the unique susceptibility of the structure to intercalate lithium reversibly.

Microstructure and electrical properties of Y2O3-doped ZnO-based varistor ceramics prepared by high-energy ball milling

Abstract Y2O3-doped ZnO-based varistor ceramics were prepared using high-energy ball milling (HEBM) and low-temperature sintering technique, with voltage-gradient of 1934–2197 V/rhm, non-linear coefficients of 20.8–21.8, leakage currents of 0.59–1.04 μA, and densities of 5.46–5.57 g/cm3. With increasing Y2O3 content, the voltage-gradient increases because of the decrease of ZnO grain size; the non-linear coefficient and the leakage current improve but the density decreases because of more porosity; the donor concentration and density of interface states decrease, whereas the barrier height and width increase because of the acceptor effect of Y2O3 in varistor ceramics.

HIGH POTENTIAL GRADIENT OF ZnO VARISTORS PREPARED BY Y2O3-DOPING AND LOW-TEMPERATURE SINTERING

A high potential gradient of ZnO varistors were fabricated by Y2O3-doping and low-temperature sintering. The value of the potential gradient increased to 2460.5 V/mm with the Y2O3 content of 0.08 mol% and the sintering temperature of 800°C. The effects of Y2O3-doping and sintering temperature on the electrical properties of ZnO varistors were investigated. Under the given experimental conditions, additive Y2O3 exists in the form of Y2O3 phase after sintering at 800°C. High-energy ball-milling in the early period of the experiment induced the grain refinement and realized the sintering formation at the lower temperature of 800°C. Both Y2O3-doping and low-temperature sintering restrained the ZnO grain growth and increased the potential gradient.

High Potential Gradient Of Y2o3-Doped Thick Film Varistors

The electrical and microstructural properties of a series of ZnO-based thick film varistors (TFVs) doped with 0.00, 0.02, 0.04, 0.06, 0.08, and 0.10 mol% Y2O3 were studied. It was found that sample doped with 0.08 mol% Y2O3 showed the highest potential gradient of 3159.4 V/mm with a leakage current of 36.4 μA and a nonlinear exponent of 13.1. The ZnO grain size decreased with increasing Y2O3 content, which was the origin for the increase in potential gradient. Raman spectra results showed that the tensile stress increased linearly with Y2O3 doping. Larger tensile stress was considered to result from the lattice distortion and inevitably influenced the grain boundary characteristics. While the Y2O3 doping concentration was beyond 0.08 mol%, the effect of residual stress on electrical properties was much more remarkable than that of grain size, leading potential gradient to be weakened. As a result, high potential gradient of ZnO-based TFVs could be obtained with Y2O3 doping concentration of 0.08 mol% and qualified as excellent candidates for high voltage varistor application.

INFLUENCE OF SUBSTRATE TEMPERATURE ON TRANSFORMATION OF PREFERRED ORIENTATIONS IN AlN FILMS

Polycrystalline aluminium nitride (AlN) films were deposited on silicon substrates by pulsed laser deposition (PLD) at a substrate temperature in the range room temperature (RT)-800°C. Films grown on Si(111) substrates feature (002) and (110) preferred orientations at substrate temperatures below 400°C and above 600°C, respectively. Films morphology is good enough for surface acoustic wave (SAW) devices. The mechanism for formation and transformation of different preferred orientations in AlN films is discussed.