J. L. Wang

Sulfur-carbon nano-composite as cathode for rechargeable lithium battery based on gel electrolyte

Sulfur–carbon nano-composite with elemental sulfur incorporated in porous carbon was prepared by thermal treatment of a mixture of sulfur and active carbon. The new material was characterized by X-ray diffraction, BET and scanning electron microscopy. The nano-composite, tested at room temperature as cathode in a nonaqueous lithium cell based on PVDF gel electrolyte, exhibited a reversible capacity of 440 mAh g−1 at a current density of 0.3 mA cm−2. The utilization of electrochemically active sulfur was about 90% assuming a complete reaction to the product of Li2S during cycling.

The role of transition metal impurities and oxygen vacancies in the formation of ferromagnetism in Co-doped TiO2

We have investigated the role of transition metal impurities and oxygen vacancies in the formation of ferromagnetism in Co-doped TiO2 using the LSDA +U approach which takes into account strong on-site Coulomb correlations for electronic structure calculations. Several model supercells of rutile TiO2, with a Co2+ ion in high-spin state substituted into the titanium site and with oxygen vacancies, were considered. We found that the exchange interaction of Co magnetic ions is ferromagnetic, but very weak due to the large average impurity?impurity distance. However, it becomes three times stronger when there is a magnetic vacancy nearby. The magnetic moment values are 3??B for Co2+ and ?1??B for vacancy. Our investigation showed that the exchange interaction energy of Co and vacancy moments varies from 330 to 40?meV depending on the distance between them in the supercell. We assume that the strong interaction between Co and vacancy moments should be taken into account to explain the high Curie temperature value in Co-doped TiO2.

Effect of Mn substitution on the volume and magnetic properties of Er2Fe17

The effects of Mn substitution on the magnetic properties of Er2Fe17−xMnx compounds have been investigated by different experimental techniques. An unusual composition dependence of the unit cell volume at room temperature occurs due to a strong magnetovolume effect, which has been confirmed by high temperature linear thermal expansion measurements. From magnetization measurements the composition dependence of the spontaneous magnetization of the 3d-sublattice magnetic moment and of the Curie temperature has been determined. Spin reorientation has been detected for compounds with 3⩽x⩽5.5, as well as remarkable magnetic history effects.

Hopping conduction and low-frequency dielectric relaxation in 5mol% Mn doped (Pb,Sr)TiO3 films

The highly (00l)-oriented 5 mol % Mn doped Pb0.5Sr0.5TiO3 films with remarkable ferroelectric properties (2Pr=27.1 μC/cm2 and 2Ec=55.5 kV/cm) were fabricated on LaNiO3 coated silicon substrates by chemical solution deposition. The ac conductivity and the dielectric response of the films at various temperatures were studied. Hopping conduction accompanied by the dielectric relaxation at low frequencies was observed. The results can be correlated with the thermal-activated short range hopping of localized charge carriers through trap sites separated by potential barriers with different heights, i.e., localized electrons hopping between multivalence Mn sites (the activation energy of this hopping process is ∼0.4 eV), which is also established in terms of the correlated barrier hopping model.

Transition of the polarization switching from extrinsic to intrinsic in the ultrathin polyvinylidene fluoride homopolymer films

Polyvinylidene fluoride homopolymer thin films have been prepared by the Langmuir-Blodgett technique, and their electrical properties have comprehensively been studied. The polyvinylidene fluoride homopolymer films show better ferroelectricity with higher polarization and higher breakdown electric field than that of the poly(vinylidene fluoride-trifluoroethylene) copolymer films. Inspection on the thickness dependence of the coercive field of the polyvinylidene fluoride films suggests an extrinsic polarization switching occurs in the thickness range from 200 to 45 nm, and a non-extrinsic switching is observed in the range between 45 and 11 nm, which is ascribed to the transition range from extrinsic to intrinsic switching.

Effect of Mo content on the structure stability of R3(Fe, Co, Mo)29

Formation of Ce3Fe29−x−yCoxMoy(x=0,3,6,10) compounds has been investigated by means of x-ray diffraction and magnetic measurements. It is found that the required Mo content to stabilize the 3:29 compounds decreases monotonically as Co content increases, totally different from the discovery in the Gd3(Fe1−xCox)29−yCry compounds. Meanwhile, we adopted a lattice inversion method in acquiring the interatomic potentials to rare-earth-transition metal intermetallic compounds, and investigated the substitution behaviors of Mo, Cr, V, and Mn in Fe-based and Co-based 3:29 compounds, respectively, by combining these potentials with computer simulation. The calculated results show that the required Mo content in R3Co29−xMox compounds is lower than that in R3Fe29−xMox compounds, whereas the Cr content in R3Co29−xCrx compounds higher than that in R3Fe29−xCrx compounds, coinciding well with the previous discovery and the experimental results in this work.

Structure and magneto-history behavior of DyNi2Mn

The compound DyNi2Mn with a MgCu2-type structure was synthesized, even though the atom ratio of Dy to transition metal was confirmed to be 1:3, by using the energy dispersive spectrum method. The X-ray diffraction pattern (XRD) could be well indexed with a cubic Laves cell with the lattice parameter a = 7.1406(2) Angstrom and space group Fd3m. The refinement result of the XRD pattern indicates that 73.8% of the 8a sites (Dy sites) are occupied by Dy atoms, 21.4% by Mn atoms, and 4.8% are empty. The compound shows ferrimagnetic coupling of the Dy and Mn moments with an ordering temperature T-C of 94 K. The spontaneous magnetization at 5 K is 6.7mu(B)/f.u. A large coercive field of 0.73 T at 5 K is found for the bulk sample, which can be understood in terms of narrow-domain-wall pinning

Wurtzite-to-tetragonal structure phase transformation and size effect in ZnO nanorods

The deformation of [0001]-oriented ZnO nanorods with hexagonal cross sections under uniaxial tensile loading is analyzed through molecular statistical thermodynamics (MST) simulations. The focus is on the size dependence of mechanical behavior in ZnO nanorods with diameters ranging from 1.95 to 17.5 nm. An irreversible phase transformation from the wurtzite (P6(3)mc space group) structure to a tetragonal structure (P4(2)/mnm space group) occurs during the tensile loading process. Youngs modulus before the transformation demonstrates a size dependence consistent with what is observed in experiments. A stronger size dependence of response is seen after the transformation and is attributed to the polycrystalline nature of the transformed structure. A comparison of the MST and molecular dynamics (MD) methods shows that MST is 60 times faster than MD and yields results consistent with the results of MD.

Surface and electrokinetic properties of Y-TZP suspensions stabilized by polyelectrolytes

Abstract Yttria-doped tetragonal zirconia (Y-TZP, 3 mol% Y 2 O 3 ) aqueous suspensions with NH 4 PAA and NH 4 PMAA polyelectrolytes as dispersants were prepared by the ball mill method. Surface chemistry of Y-TZP particles in aqueous suspensions was evaluated via isothermal adsorption, FTIR spectrum and zeta potential analysis. It was found that there was an optimum amount of dispersants absorbed on Y-TZP particles, which changes their surface properties dramatically in the FTIR spectra. Coated with dispersants, the electrokinetic properties of Y-TZP were obviously altered compared with the naked particles, the isoelectric points (IEP) were shifted to pH 2.1 and 2.3 by NH 4 PAA and NH 4 PMAA, respectively. The rheology of high solids loading Y-TZP suspensions demonstrate dramatic difference with low solids loading suspensions, 47 vol% solids loading suspension showed a viscosity about 600 mPa·s under 20 s −1 shear rate.

Magnetic properties of Sm2(Fe1−xGax)17 (x=0−0.5) compounds and their nitrides

Magnetic properties of Sm2(Fe1−xGax)17 compounds and their nitrides have been studied. Substitution of Ga for Fe leads to an increase in lattice constants. Introduction of nitrogen results in a further increase in lattice constants. Substitution of Ga for Fe causes a dramatic change of the Curie temperature of the Sm2(Fe1−xGax)17 compounds. When x=0.2 the Curie temperature is enhanced by about 200 K. X‐ray‐diffraction patterns of aligned samples of Sm2(Fe1−xGax)17 compounds show that alloys with x=0.15, 0.20, and 0.25 exhibit uniaxial anisotropy at room temperature. The introduction of nitrogen made the samples with x≤0.4 exhibit uniaxial anisotropy at room temperature. The Curie temperature of the nitrides decreases with the Ga concentration. The anisotropy fields of the nitrides derived from the high‐field magnetization. The changes of the magnetic anisotropy, saturation magnetization, and the moment of the Fe atoms in the nitrides and their parent compounds with Ga concentration are discussed.