J. M. Chen

Origin of thermal degradation of Sr 2-x Si 5 N 8 :Eu x phosphors in air for light-emitting diodes

The orange-red emitting phosphors based on M(2)Si(5)N(8):Eu (M = Sr, Ba) are widely utilized in white light-emitting diodes (WLEDs) because of their improvement of the color rendering index (CRI), which is brilliant for warm white light emission. Nitride-based phosphors are adopted in high-performance applications because of their excellent thermal and chemical stabilities. A series of nitridosilicate phosphor compounds, M(2-x)Si(5)N(8):Eu(x) (M = Sr, Ba), were prepared by solid-state reaction. The thermal degradation in air was only observed in Sr(2-x)Si(5)N(8):Eu(x) with x = 0.10, but it did not appear in Sr(2-x)Si(5)N(8):Eu(x) with x = 0.02 and Ba analogue with x = 0.10. This is an unprecedented investigation to study this phenomenon in the stable nitrides. The crystal structural variation upon heating treatment of these compounds was carried out using the in situ XRD measurements. The valence of Eu ions in these compounds was determined by electron spectroscopy for chemical analysis (ESCA) and X-ray absorption near-edge structure (XANES) spectroscopy. The morphology of these materials was examined by transmission electron microscopy (TEM). Combining all results, it is concluded that the origin of the thermal degradation in Sr(2-x)Si(5)N(8):Eu(x) with x = 0.10 is due to the formation of an amorphous layer on the surface of the nitride phosphor grain during oxidative heating treatment, which results in the oxidation of Eu ions from divalent to trivalent. This study provides a new perspective for the impact of the degradation problem as a consequence of heating processes in luminescent materials.

Effect of Co doping in LiMn2O4

Abstract Lithium transition-metal oxides Li(Mn2−xCox)O4 (0≤x≤0.5) are synthesized by solid state reaction. X-ray and electrochemical data show that the replacement of Mn3+ (d4) ions by Co3+ (d6) in the octahedral framework of the spinel eliminates the local disorder present in the lattice around the [Mn3+O6] octahedra. The capacity loss observed in the undoped Li/LiMn2O4 cell is about 25% after 20 cycles, whereas that for the x=0.1 and 0.2 doped spinel materials is about 0.48 and 1%, respectively. The good capacity retention of Li(Mn2−xCox)O4 (0.1≤x≤0.5) electrode is attributed to the stabilization of the spinel structure by Co doping for Mn ion sites. The chemical substitution of Co3+ for Mn3+ in LiMn2O4 improves the efficiency in maintaining electrochemical capacity over a large number of cycles without sacrificing initial reversible capacity at room temperature.

Band-gap narrowing of TiO2 doped with Ce probed with x-ray absorption spectroscopy

The electronic structure of TiO2 doped with Ce was investigated with x-ray absorption and photoluminescence (PL) spectra. Ce doping narrows the band gap of TiO2, as evidenced by a redshift in the absorption edge in the O K-edge spectrum and PL. Narrowing is ascribed to the formation of an impurity band, composed of Ce 4f (Ce3+/Ce4+) and oxygen defect states, that induces tailing of the conduction band. Band gap narrowing extends the photoactivity of TiO2 to visible light. The impurity band located below the minimum of the conduction band traps excited electrons and suppresses recombination, enhancing the photocatalytic activity.

Gapless band structure of PbPdO2: A combined first principles calculation and experimental study

We present experimental evidence of the gapless band structure of PbPdO2 by combined x-ray photoemission and x-ray absorption spectra complemented with first principles band structure calculations. The electronic structure near the Fermi level of PbPdO2 is mainly composed of O 2p and Pd 4d bands, constructing the conduction path along the Pd-O layer in PbPdO2. Pd deficiency in PbPdO2 causes decreased O 2p-Pd 4d and increased O 2p-Pb 6p hybridizations, thereby inducing a small band gap and hence reducing conductivity. Hall measurements indicate that PbPdO2 is a p-type gapless semiconductor with intrinsic hole carriers transporting in the Pd-O layers.

Structural transformation of LiVOPO4 to Li3V2(PO4)3 with enhanced capacity.

In the present investigation, we report the transformation of alpha-LiVOPO 4 to alpha-Li 3V 2(PO 4) 3, leading to an enhancement of capacity. The alpha-LiVOPO 4 sample was synthesized by a sol-gel method, followed by sintering at 550-650 degrees C in a flow of 5% H 2/Ar. The structural transformation of a triclinic alpha-LiVOPO 4 structure to a monoclinic alpha-Li 3V 2(PO 4) 3 structure was observed at higher sintering temperatures (700-800 degrees C in a flow of 5% H 2/Ar). The alpha-Li 3V 2(PO 4) 3 phase was characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, thermal gravimetric analysis, and X-ray absorption near edge spectrum (XANES) techniques. The valence shift of vanadium ions from +4 to +3 states was observed using in situ XANES experiments at V K-edge. The structural transformation is ascertained by the shape changes in pre-edge and near edge area of X-ray absorption spectrum. It was observed that the capacity was enhanced from 140 mAh/g to 164 mAh/g via structural transformation process of LiVOPO 4 to Li 3V 2(PO 4) 3.

Pb-for-Bi substitution for enhancing thermoelectric characteristics of [(Bi,Pb)2Ba2O4±ω]0.5CoO2

We report strongly enhanced thermoelectric characteristics for a misfit-layered oxide, [Bi2Ba2O4±ω]0.5CoO2, in a wide temperature range, as achieved through substituting up to 20% of Bi by Pb. The Pb substitution kept the thermal conductivity (κ) unchanged but decreased the electrical resistivity (ρ) and increased the Seebeck coefficient (S) simultaneously, such that a three-fold enhancement in the thermoelectric figure of merit, Z (≡S2∕ρκ), was realized. At the same time x-ray absorption near-edge structure data indicated that the valence and spin states of Co are not affected by the Pb-for-Bi substitution.

Crystal and electronic structures of (Ba, Sr)TiO3

The (Ba0.67Sr0.33)TiO3 phase, synthesized by the sol–gel method at 1300°C, is tetragonal (space group: P4 mmm) with a unit cell of a (or b)=3.9724(5) A and c=3.9703(9) A. Based on the refinement data from X-ray powder diffraction of the sample, the crystal structure shows a distortion of the TiO6 octahedron and a buckling within the TiO2 planes which lead to the material with polarizability. Moreover, the electronic structure of (Ba0.67Sr0.33)TiO3 is investigated by the Ti L23-edge X-ray absorption near edge structure (XANES) spectra.

Effect of Co2P on electrochemical performance of Li(Mn0.35Co0.2Fe0.45)PO4/C.

In this paper, we report the synthesis of carbon coated Li(Mn0.35Co 0.2Fe0.45)PO4 and discuss the effect of Co2P formation during the carbothermal reduction process, which enhances the electrochemical performance of cathode material for lithium ion batteries. It was observed that Co2P was favorably formed in 5% H2/Ar than in Ar atmosphere. The conductivity of Li(Mn0.35Co0.2Fe0.45)PO4/C sintered at 600-800 degrees C in 5% H2/Ar is increased as the temperature is increased. The O K-edge X-ray absorption near edge spectrum (XANES) demonstrates that content of hole carriers is increased in Li(Mn0.35Co0.2Fe0.45)PO4/C as the amount of Co2P increased. We also observed that the capacity of Li(Mn0.35Co0.2Fe0.45)PO4/C is increased with sintering temperature, and it exhibited a maximum capacity of 166 mAh/g at 700 degrees C. It was found that the enhancement in the discharge capacity of sintered Li(Mn0.35Co0.2Fe0.45)PO4/C was as a result of its higher electrical conductivity under 5% H2/Ar atmosphere as compared with Ar atmosphere.

Evidence for two distinct superconducting phases in EuBiS

We present a pressure study of the electrical resistivity, AC magnetic susceptibility and powder x-ray diffraction (XRD) of the newly discovered BiS

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