Yangshan Sun

Upper critical fields and anisotropy of Ca1−xLaxFe2As2 single crystals

Superconductivity of Ca1-xLaxFe2As2 single crystals with various doping level were investigated via electromagnetic measurements for out-plane (H//c) and in-plane (H//ab) directions. Obvious double superconducting transitions, which can survive in magnetic fields up to several Tesla, were observed in the medium-doped (x = 0.13) sample. Two kinds of distinct Hc2 phase diagrams were established for the low superconducting phase with Tc lower than 15 K and the high superconducting phase with Tc of over 40 K, respectively. Both the two kinds of phase diagrams exist in the medium-doped sample. Unusual upward curvature near Tc was observed in Hc2 phase diagrams and analyzed in detail. Temperature dependences of anisotropy for different doping concentrations were obtained and compared. Both superconducting phases manifest extremely large anisotropies, which may originate from the interface or intercalation superconductivity.

Microstructure, mechanical and wear properties of TiC particulate reinforced 2Cr13 steel by in situ reaction and electroslag remelting

Abstract Microstructure, mechanical and wear properties of titanium carbide (TiC) reinforced 2Cr13 steels prepared by in situ reaction and electroslag remelting (ESR) have been investigated. Experimental results show that the technique of ESR can be used as a preferred new process to solve poor castability of 2Cr13 reinforced with TiC synthesised by in situ reaction. Applying the ESR process results in more fine TiC particles distributed uniformly. For the base alloy 2Cr13, addition of TiC causes the increase in both yield and ultimate strengths, but slight decrease in ductility and great decrease in the impact toughness. However, the impact toughness of TiC reinforced 2Cr13 can be improved through the ESR. TiC addition to the matrix 2Cr13 results in a significant improvement on wear resistance, and the wear resistance can be further improved through the ESR process.

An anomalous tail effect in the resistivity transition and weak-link behavior of superconducting NdFeAsO0.88F0.12

Temperature dependent resistivity of the iron-based superconductor NdFeAsO0.88F0.12 was measured under different applied fields and excitation currents. Arrhenius plot shows an anomalous tail effect, which contains obvious two resistivity dropping stages. The first is caused by the normal superconducting transition, and the second is supposed to be related to the weak-link between the grains. A model for the resistivity dropping related to the weak-link behavior is proposed, which is based on the Josephson junctions formed by the impurities in grain boundaries like FeAs, Sm2O3 and cracks together with the adjacent grains. These Josephson junctions can be easily broken by the applied fields and the excitations currents, leading to the anomalous resistivity tail in many polycrystalline iron-based superconductors. The calculated resistivity dropping agrees well with the experimental data, which manifests the correctness of the explanation of the obtained anomalous tail effect.The temperature dependent resistivity of the iron-based superconductor NdFeAsO0.88F0.12 was measured under different applied fields and excitation currents. The Arrhenius plot shows an anomalous tail effect, which contains two obvious stages of decreasing resistivity. The first is caused by the normal superconducting transition, and the second is supposed to be related to the weak link between the grains. A model for the resistivity decrease related to the weak-link behavior is proposed, which is based on the Josephson junctions formed by the impurities in grain boundaries like FeAs, Sm2O3 and cracks together with the adjacent grains. These Josephson junctions can be easily broken by the applied fields and the excitation currents, leading to an anomalous resistivity tail in many polycrystalline iron-based superconductors. The calculated drop in resistivity agrees well with the experimental data, which indicates the correctness of the explanation for the obtained anomalous tail effect.

Microstructures and Mechanical Properties of Mg-2Y-xZn (x=1, 2, 3 at%) Alloys

Abstract The microstructures and the mechanical properties of the Mg-2Y- x Zn ( x = 1, 2, 3 at%) alloys in as-cast, as-annealed and as-extruded conditions have been investigated. The results show that the 18R-LPSO phases are distributed in a continuous network mainly along the grain boundaries in the alloy with 1 at% Zn. With increasing of Zn content, the 18R-LPSO phase and W-phase coexist in Mg-2Y-2Zn alloy but only W-phase remains in Mg-2Y-3Zn alloy. The 14H-lamellas are precipitated from the block-shaped 18R-LPSO phases into the matrix, and W-phases are broken into particles during homogenizing annealing. After extrusion, the LPSO phases and W-particles are rearranged along the direction of extrusion. The tensile tests show that the as-extruded Mg-2Y-1Zn alloy exhibits the supreme UTS of 320 MPa, with an elongation of 11.2% at room temperature.

Microstructure and mechanical properties of ZA62 based magnesium alloys with calcium addition

Abstract As-cast microstructure and mechanical properties of Mg-6Zn-2Al-0.3Mn (ZA62) alloys with calcium addition were investigated. The as-cast microstructure of the base alloy ZA62 consists of the a-Mg matrix and eutectic phase Mg51Zn20. The Mg51Zn20 eutectic was gradually replaced by MgZn phase and Mg32(Al,Zn)49 phase when calcium is added into the base alloy. Further addition of calcium leads to the increase of grain boundary phases and formation of a new quaternary Mg-Zn-Al-Ca eutectic compound. In comparison with the base alloy, the increase of calcium addition to the base alloy results in the reduction of both strength and ductility at ambient temperature, but increase at elevated temperatures due to the thermal stability of Ca-containing phases. At elevated temperatures, the creep resistance of ZA62 based alloys containing calcium is significantly higher than that of AZ91 which is the most commonly used magnesium alloy.

Electronic behavior of superconducting SmFeAsO0.75

High-quality polycrystalline SmFeAsO0.75 was synthesized with a superconducting transition width less than 1 K, and the electronic behavior was systematically studied by transport and specific heat measurements. An obvious superconducting jump was witnessed, together with a very small normalized superconducting jump, ΔC/γnTc ∼ 0.2, which is much smaller than expected from the BCS theory. A strong temperature-dependent Hall coefficient was found and attributed to the partial gapping of the Fermi surface up to the temperature of 160 K, which was predicted and supported by the emergence of the pseudogap. The charge-carrier density as well as the effective mass were also obtained and discussed in detail.

Annealing Effect on Microstructure and Superconductivity of K_{x}Fe_{y}Se_{2} Synthesized by an Easy One-step Method

High-quality superconducting K

Lead-free solders based on the Sn-8Zn-3Bi ternary alloy with additions of In, Nd or La

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