D.S. Shy

Nitrogen-doped graphene nanosheet-supported non-precious iron nitride nanoparticles as an efficient electrocatalyst for oxygen reduction

Nitrogen-doped graphene-supported carbon-containing iron nitride (FeCN/NG) was synthesized by the chemical impregnation of iron and nitrogen-containing precursors in the presence of ammonia under thermal treatment. The resultant graphene-based material acted as an electrode with a much higher electrocatalytic activity in the catalysis via a 4-electron pathway in fuel cells. The results of X-ray diffraction, scanning electron microscopy, Fourier transform infrared and X-ray photoelectron spectroscopy indicated that graphite oxide was successfully reduced to nitrogen-doped graphene. X-Ray absorption spectroscopy further confirmed that carbon was incorporated into iron nitride, demonstrating that Fe–N–C catalytic active sites may be responsible for the oxygen reduction reaction. To the best of our knowledge, this is the first report of the combination of N-doped graphene with non-precious metal for oxygen reduction in fuel cells, and may open up a new possibility for preparing graphene-based nanoassemblies for intensive applications.

Phase evolution of co-precipitated Bi–Pb–Sr–Ca–Cu–O powder

Abstract The evolution of the phase formation in (Bi,Pb)–Sr–Ca–Cu–O superconducting materials prepared by the co-precipitation method has been investigated by X-ray diffraction analysis. Formation of the desired 2223 phase was found to proceed through a complicated reaction route of forming the single, double and triple Cu–O layered superconducting structures in sequence with increasing temperature. The phase with a more Cu–O layered structure was found to grow at the expense of the less Cu–O layered phase. Several intermediate phases of Bi–Sr/Ca–O compounds were found to be closely related to the formation of the superconducting phase with single and double Cu–O layered structures. The investigation of the kinetics of forming the 2223 phase revealed that two mechanisms dominated the reaction rate in the early and late stages of growing the 2223 phase.

Crystal structure of the (Pb, Hg)Sr2(Ca, Y)Cu2O7-δ superconductor

Abstract Bulk superconductivity up to 90 K has been achieved in the Hg containing cuprate (Pb, Hg)Sr 2 (Ca 1- x Y x )Cu 2 O 7- δ when x =0.3, which has the highest T c among the Pb based materials. The crystal structure of the newly discovered Hg containing Pb based 1212 phase has been investigated by Rietveld analysis of X-ray powder diffraction (XRD) data, selected area electron diffraction (SAED) and high-resolution electron microscopy (HREM). We have identified the phase responsible for superconductivity in the so-called 1212 phase with a space group of P4/mmm and lattice constants of a=3.8166(1) A and c=11.9484(4) A for the x =0.5 sample. A disordering of the hole reservoir layer (Pb, Hg) due to the displacement of the oxygen ions and Pb/ Hg ions from their ideal positions of (0.5, 0.5, 0) to (0.39, 0.5, 0) and from (0, 0, 0) to (0.044, 0, 0), respectively, was found in the XRD refinement of the x =0.5 sample. This displacement may have given rise to the appearance of superlattice spots along the [104] ∗ direction with a commensurate modulation factor of 8 in the electron-diffraction pattern viewed down [010]. Moreover, mutual substitutions of cations (≈19% Cu in the Pb/Hg sites and ≈9% Hg in the Ca/Y sites) have been found in the sample.

Superconductivity up to 32 K in a new family of the Hg-containing (Pb, Hg) (Sr, La)2CuO5−δ (1201) system

Abstract A new family of superconductors with a maximum Tc of 32 K were observed in the Hg-containing (Pb0.5Hg0.5) (Sr2−xLax) CuO5−gd (1201) system for the composition range 0.8 ⩽x⩽ 1.0. The chemical composition and crystal structure of this system have been investigated by X-ray powder diffraction including the Rietveld refinement, electron diffraction, and high-resolution transmission electron microscopy. We have identified the phase responsible for the superconductivity to be similar to (Pb, Cu) (Sr, La)2CuO5 or (Ti, Pb) (Sr, La)2CuO5 (the so-called 1201 phase) with a space group of P4/mmm and lattice constants of a=3.7908(2) A and c=8.6915 (6) A for the x=0.8 sample in (Pb0.5Hg0.5 (Sr2−xLax)CuO5−δ. The disordering of the hole reservoir layer of (Pb, Hg)O due to the displacement of the oxygen ion from its ideal position (0.5, 0.5, 0) to (0.33, 0.5, 0) was found by Rietveld refinement for powder X-ray diffraction data of the x=0.8 sample. This displacement may give rise to the appearance of the superlattice spots along the [102]∗ direction with a modulation factor close to 4 in the electron diffraction pattern viewed down [010].

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.

Improvement of phase purity and accelerated formation of the Tl-1223 phase from the stoichiometric compositions (Tl0.6Pb0.2Bi0.2) (Sr2-xBax)Ca2Cu3O9 (x=0.2-0.3)

Abstract We report an efficient and highly reproducible method for the preparation of the almost monophasic Tl-1223 phase by partial substitution of Ba 2+ into Sr 2+ sites in the stoichiometric compositions of (Tl 0.6 Pb 0.2 Bi 0.2 )(Sr 2- x Ba x )Ca 2 Cu 3 O 9 ( x =0.2-0.3). The as-synthesized samples have a superconducting transition temperature around 115 K which can be further increased up to 122 K after post-annealing at 820°C for 20 h in oxygen.

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.

Effect of LiI Amount to Enhance the Electrochemical Performance of Carbon-Coated LiFePO4

We used the wet chemistry method with different amounts of LiI to prepare LiFePO 4 and carbon-coated LiFePO 4 (LiFePO 4 /C) samples. The lithiation process of LiI for preparing the LiFePO 4 is proposed. We found that the amount of LiI greatly affected the purities of products. The LiFePO 4 /C sample showed constant values of current density during potential cycling up to 35 cycles as compared to LiFePO 4 , suggesting that the LiFePO 4 /C has better electrochemical performance. The obtained maximum capacity for LiFePO 4 /C can be approached to the theoretical capacity of 170 mAh/g. Moreover, Brunauer-Emmett-Teller measurements of LiFePO 4 and LiFePO 4 /C showed a surface area of 6.7 and 50 m 2 /g, respectively. It is reasonable to believe that the excellent performance of the compound developed in our work can be attributed to the smaller particle size coated with conductive carbon achieved by the controlling LiI in the sol-gel method.

Effects of powder calcination on the properties of Bi-2223 tape

The effects of powder calcination on the properties of Bi-2223 tape have been investigated. The transport property of the tape was found to depend strongly on the condition for powder calcination. The tape fabricated with powder calcined at lower temperature resulted in a microstructure with a larger grain size and higher transport critical current. The development of the microstructure of the tape in the subsequent heat-treatment process is correlated with the constituents in the starting powder. The magnetization of samples characterized by SQUID indicates that the magnetic property is also correlated to the microstructures of the tape. An anomalous flux disturbance occurring at 70 K was observed on the zero-field cooling curves of all samples.

Crystal structure and superconductivity in the Hg-containing Ba- and Sr-based cuprates

Abstract We have synthesized the Hg-containing Barium-based cuprate of the HgBa 2 Ca 2 Cu 3 O 8+δ (1223) compound with T c up to 135 K without post-annealing by directly mixing from mono-oxide powders of Hg, Ba, Ca, and Cu as starting materials. This is the n = 3 phase of the homologous series HgBa 2 Ca n−1 Cu n O 2n+2+δ . The isotopic strontium-based cuprates, corresponding to the formula HgSr 2 Ca n−1 Cu n O 2n+2+δ have not been synthesized up to now. However, the stabilization of the strontium cuprates with n = 1 and 2 structures can be performed by introducing a second element (e.g. Pb) besides Hg in the hole reservoir layers. Moreover, the chemical doping of Y 3+ into the Ca 2+ sites and La 3+ into the Sr 2+ sites in the strontium cuprates of the n = 1 and n = 2 compounds, respectively, is necessary to induce superconductivity. Bulk superconductivity with T c up to 35 K and 90 K has been achieved in (Pb 0.5 Hg 0.5 )(Sr 1.2 La 0.8 ) CuO 4+δ (n = 1) and (Pb 0.5 Hg 0.5 )Sr 2 (Ca 0.7 Y 0.3 )Cu 2 O 6+δ (n = 1), respectively. The crystal structures of the newly discovered Hg-containing phases have been investigated by selected area electron diffraction (SAED). The appearance of superlattice spots with a commensurate modulation in the SAED patterns was observed in the Hg-containing Sr-based cuprates. However, such a phenomenon was not found in the HgBa 2 Ca 2 Cu 3 O 8_δ phase.