Xueyan Song

High critical current density and enhanced irreversibility field in superconducting MgB2 thin films

Larbalestier †§ N. Rogado*, K.A. Regan*, M.A. Hayward*, T. He*, J.S. Slusky*, K. Inumaru*, M.K. Haas* and R.J. Cava* † Department of Materials Science and Engineering, Univer-sity of Wisconsin, 1509 University Avenue, Madison, WI 53706 USA § Applied Superconductivity Center, University of Wisconsin, 1500 Engineering Drive, Madison, WI 53706 USA * Department of Chemistry and Princeton Materials Institute, Princeton University, Princeton, NJ 08544 USA

Thin Film Magnesium Boride Superconductor with Very High Critical Current Density and Enhanced Irreversibility Field

Larbalestier †§ N. Rogado*, K.A. Regan*, M.A. Hayward*, T. He*, J.S. Slusky*, K. Inumaru*, M.K. Haas* and R.J. Cava* † Department of Materials Science and Engineering, Univer-sity of Wisconsin, 1509 University Avenue, Madison, WI 53706 USA § Applied Superconductivity Center, University of Wisconsin, 1500 Engineering Drive, Madison, WI 53706 USA * Department of Chemistry and Princeton Materials Institute, Princeton University, Princeton, NJ 08544 USA

Very high upper critical fields in MgB2 produced by selective tuning of impurity scattering

We report a significant enhancement of the upper critical field Hc2 of different MgB2 samples alloyed with nonmagnetic impurities. By studying films and bulk polycrystals with different resistivities ρ ,w e sho wac lear trend of a ni ncrease in Hc2 as ρ increases. One particular high resistivity film had a zero-temperature Hc2(0) well above the Hc2 values of competing non-cuprate superconductors such as Nb3Sn and Nb–Ti. Our high-field transport measurements give record values H ⊥ c2 (0) ≈ 34 T and H || c2 (0) ≈ 49 T for high resistivity films and Hc2(0) ≈ 29 T for untextured bulk polycrystals. The highest Hc2 film also exhibits a significant upward curvature of Hc2(T ) and a temperature dependence of the anisotropy parameter γ( T ) = H || c2 /H ⊥ c2 opposite to that of single crystals: γ( T ) decreases as the temperature decreases, from γ( Tc) ≈ 2t o γ( 0) ≈ 1.5. This remarkable Hc2 enhancement and its anomalous temperature dependence are a consequence of the two-gap superconductivity in MgB2 ,w hich offers special opportunities for further Hc2 increases by tuning of the impurity scattering by selective alloying on Mg and B sites. Our experimental results can be explained by a theory of two-gap superconductivity in the dirty limit. The very high values of Hc2(T ) observed suggest that MgB2 can be made into a versatile, competitive high-field superconductor.

High critical current density and improved irreversibility field in bulk MgB2 made by a scaleable, nanoparticle addition route

Bulk samples of MgB2 were prepared with 5, 10, and 15 wt % Y2O3 nanoparticles, added using a simple solid-state reaction route. Transmission electron microscopy showed a fine nanostructure consisting of ∼3–5 nm YB4 nanoparticles embedded within MgB2 grains of ∼400 nm size. Compared to an undoped control sample, an improvement in the in-field critical current density JC was observed, most notably for 10% doping. At 4.2 K, the lower bound JC value was ∼2×105 A cm−2 at 2 T. At 20 K, the corresponding value was ∼8×104 A cm−2. Irreversibility fields were 11.5 T at 4.2 K and 5.5 T at 20 K.

Electronic anisotropy, magnetic field-temperature phase diagram and their dependence on resistivity in c-axis oriented MgB2 thin films

An important predicted, but so far uncharacterized, property of the new superconductor MgB2 is electronic anisotropy arising from its layered crystal structure. Here we report on three c-axis oriented thin films, showing that the upper critical field anisotropy ratio Hc2?/Hc2? is 1.8 to 2.0, the ratio increasing with higher resistivity. Measurements of the magnetic field-temperature phase diagram show that flux pinning disappears at H*?0.8Hc2?(T) in untextured samples. Hc2?(0) is strongly enhanced by alloying to 39 T for the highest resistivity film, more than twice that seen in bulk samples.

Stabilizing Nanostructured Solid Oxide Fuel Cell Cathode with Atomic Layer Deposition

We demonstrate that the highly active but unstable nanostructured intermediate-temperature solid oxide fuel cell cathode, La0.6Sr0.4CoO3-δ (LSCo), can retain its high oxygen reduction reaction (ORR) activity with exceptional stability for 4000 h at 700 °C by overcoating its surfaces with a conformal layer of nanoscale ZrO2 films through atomic layer deposition (ALD). The benefits from the presence of the nanoscale ALD-ZrO2 overcoats are remarkable: a factor of 19 and 18 reduction in polarization area-specific resistance and degradation rate over the pristine sample, respectively. The unique multifunctionality of the ALD-derived nanoscaled ZrO2 overcoats, that is, possessing porosity for O2 access to LSCo, conducting both electrons and oxide-ions, confining thermal growth of LSCo nanoparticles, and suppressing surface Sr-segregation is deemed the key enabler for the observed stable and active nanostructured cathode.

Evidence for strong flux pinning by small, dense nanoprecipitates in a Sm-doped YBa2Cu3O7−δ coated conductor

About 17vol.% of ∼10nm sized (Y,Sm)2O3 precipitates have been developed in a metal-organic chemical vapor deposition grown Sm-doped YBa2Cu3O7−δ coated conductor. The precipitate spacing of ∼15nm suggests strong vortex-precipitate pinning interactions, which are evidenced by a shift in the peak in the flux pinning force curve, an enhanced irreversibility field exceeding 8T at 77K, and a lack of temperature scaling of the flux pinning force over the temperature range of 65–82K.

Three-dimensional vortex pinning by nano-precipitates in a Sm-doped YBa2Cu3O7−x coated conductor

We report on the thickness and angular dependence of the critical current density Jc(H,?), the irreversibility field Hirr, and the bulk pinning force Fp(H) of a metal-organic chemical vapour deposition (MOCVD) grown YBa2Cu3O7?x (YBCO) coated conductor, which contains ~17?vol% of ~10?nm sized (Y,Sm)2O3 precipitates with an average spacing of ~10?15?nm. Some surface porosity and amorphous second-phase particles on the scale of ~0.5?1??m appear to reduce the current-carrying cross-section, which controls the magnitude of Jc but not the vortex pinning. We observed an enhanced Hirr~9? T at 77?K along the c-axis which, like the shape of Jc(H) and Fp(H), was independent of thickness as the sample was milled down to ~0.16??m. Angular-dependent measurements of Jc showed the usual excess vortex pinning along the c-axis and along the ab-plane, but with a background that could only be fitted with an unusually small anisotropy parameter of 3, which, like the high Hirr and the thickness-independent shape of Fp(H), we ascribe to strong vortex pinning centre interactions. Together, these measurements show very different behaviour from most pulsed-laser-deposited films, which exhibit strong thickness-dependent properties. We ascribe the present different results to the dense array of small, insulating precipitates, which act as strong pinning centres and produce strong three-dimensional (3D) vortex pinning, because their separation of 10?15?nm is always much smaller than the film thickness.

Anisotropic grain morphology, crystallographic texture and their implications for flux pinning mechanisms in MgB2 pellets, filaments and thin films

Grain morphology and crystallographic texture were investigated by electron microscopy in four different polycrystalline forms of superconducting MgB2. The materials included a hot-pressed sintered MgB2 pellet, a pellet reacted in situ from Mg and B, an in situ reacted MgB2 filament and a pulsed-laser-deposited thin film grown on a single crystalline [111] oriented SrTiO3 substrate. Thick plate-shaped grains with an aspect ratio of ~3 and large faces parallel to (0001) planes dominated the microstructure in all four types of sample. The intermediate-sized plate-shaped grains (0.1 μm × 0.3 μm on average) in the electromagnetically most homogeneous parts of the hot-pressed pellets were strongly facetted, but not textured. Large (3–5 μm) plate-shaped grains were seen in the pellet reacted directly from stoichiometric Mg and B. A tendency for parallel alignment of the [0001] axes of the considerably larger grains (~0.25 μm × 1 μm) in the filament was observed near its W core, but degradation of this texture away from the core was apparent. The very small grains (~10 nm) of the thin film possessed a well-defined fibre texture with [0001] parallel to the film normal and no preferred orientation in the plane of the film. Electrical resistivity of the finest grain samples was some 103 times higher than the largest grain sample and their critical current density about one order of magnitude higher. We conclude that, in contrast to the cuprate-based high-Tc superconductors, grain boundaries do not limit the critical current density of polycrystalline MgB2 and indeed act as flux-pinning centres, which enhance the critical current density.

Effect of microstructure on the properties of Zr–Mn–V–Ni AB2 type hydride electrode alloys

The effect of the microstructure for multicomponent ZrMn0.9-xVxNi1.1 (x = 0.1-0.8) hydride electrode alloys on their hydrogen absorbing capacity, activation and high rate discharge capacity was investigated. The electrochemical properties and microstructure changed with the alloys compositions. The microstructure were determined by the combination of transmission electron microscopy (TEM) and X-ray Rietveld analysis. Hexagonal C14 type, cubic C15 type Laves phase and tetragonal body centered Zr9Ni11 coexisted in the alloys of given compositions. The metastable Zr9Ni11 phase was stabilized at room temperature as long range ordered state along [100] and [010] directions. Substructure of each phase and the orientation relations among the existing phases were investigated in detail. Stacking faults in the C15 Laves phase and precipitation of Zr-Ni binaries considerably enhanced the activation and high rate discharge capacity of Laves phase