Superconductors

X. Song (2013). Atomic structure and chemistry of dense nanoprecipitates in MgB2 superconductor, Ceramic International, 39, 4229-4305.

Ceramic MgB 2 superconductor has great potential for applications in the devices. Doping MgB2 with carbon or SiC has been successful in improving critical current density Jc, the upper critical field Hc2 and flux-pinning. However, the presence of nano-precipitates, and their microstructure and chemistry, upon alloying/doping MgB2 grains are largely unexplored. In this paper, the atomic structure and chemistry of 5–10 nm dense nano-precipitates were discovered in purified MgB2 pellets by removing B2O3. The nano-precipitates are platelet-shaped and fully coherent with the MgB2 grains and having the longer axis of the plate perpendicular to the c-axis of MgB2 grains. Systematic conventional and analytical Transmission Electron Microscopy (TEM) was carried out to study the atomic structure and chemistry of those nano-precipitates. The practical experimental approach of optimizing the composition and synthesis heat treatment of MgB2 to induce dense uniformly distributed nano-precipitates is also discussed.

X. Song (2007). (110) facets and dislocation structure of low-angle grain boundaries in YBa2Cu3O7-δ and Y0.7Ca0.3Ba2Cu3O7-δ thin film bicrystals, J. Materials Research, 22, 950-957.

The facet and dislocation structure of 5° and 7° [001]-tilt grain boundaries of YBa2Cu3O7−δ (YBCO) and Y0.7Ca0.3Ba2Cu3O7−δ (YCaBCO) thin film bicrystals were studied. A 24° [001]-tilt YBCO grain boundary was also examined to contrast with the low angle grain boundary faceting behavior. All the low-angle grain boundaries exhibit strong faceting along (100)/(010) and (110) and possess both straight symmetric segments containing equally spaced [100] unit dislocations and step asymmetric segments composed of (110) and (100)/(010) facets. Grain boundaries with a higher degree of meander acquired up to 40% (110) facets. The atomic structure of (110) facets was revealed by the atomic resolution Z-contrast imaging. The (110) facets are dissociated for both the YBCO and YCaBCO grain boundaries. We also found the Ca-doped (110) facets to be more extended along the grain boundary plane, consistent with our earlier finding of a dissociated dislocation core in Ca-doped (100) facets. These 5° and 7° misorientations that we studied are just in the range at which YBCO grain boundaries start to become obstacles to current flow. The above results will be helpful for understanding the current transport across YBCO low-angle grain boundaries.

X. Song, Z. Chen, S. Kim, M. Feldmann, D. Larbalestier, J. Reeves, Y. Xie, V. Selvamanickam (2006). Evidence for Enhanced Flux-pinning by Small, Dense nanoprecipitates in a Sm-doped YBa2Cu3O7-δ coated conductor, Applied Physics Letters, 88, 212508.

About 17vol.% of ∼10nm∼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.

X . Song, G. Daniels, M. Feldmann, A. Guriech, D. Larbalestier (2005). Electromagnetic, atomic structure and chemistry changes induced by Ca-doping of low-angle YBa2Cu3O7-δ grain boundaries, Nature Materials, 4, 470-475.

Practical high-temperature superconductors must be textured to minimize the reduction of the critical current density Jgb at misoriented grain boundaries. Partial substitution of Ca for Y in YBa2Cu3O7–δ has shown significant improvement in Jgb but the mechanisms are still not well understood. Here we report atomic-scale, structural and analytical electron microscopy combined with transport measurements on 7° [001]-tilt Y0.7Ca0.3Ba2Cu3O7–δ and YBa2Cu3O7–δ grain boundaries, where the dislocation cores are well separated. We show that the enhanced carrier density, higher Jgb and weaker superconductivity depression at the Ca-doped boundary result from a strong, non-monotonic Ca segregation and structural rearrangements on a scale of ∼1 nm near the dislocation cores. We propose a model of the formation of Ca 2+ solute atmospheres in the strain and electric fields of the grain boundary and show that Ca doping expands the dislocation cores yet enhances Jgb by improving the screening and local hole concentration.

X. Song, V. Braccini, D. Larbalestier (2004). Inter- and intra-granular nanostructure and possible spinodal decomposition in low resistivity bulk MgB2 with varying critical fields, J. Materials Research, 19 (8), 2245-2255.

Three electromagnetically well-characterized bulk samples with nominal resistivities at 40 K [ρ(40 K)], varying from 1 to 18 μΩcm, were investigated by conventional and high-resolution transmission electron microscopy. Clean, coherent, or semi-coherent grain boundaries and dirty-grain boundaries wetted by amorphous phases were found in all three samples, even though the starting sample A had the very low resistivity of 1 μΩcm at 40 K, characteristic of clean-limit samples. Taking into account its porosity and wetted-grain boundary area, the true resistivity value is about 0.5 μΩcm. Additional samples B and C, prepared by exposing sample A to Mg vapor, showed enhanced ρ(40 K) values of 14 and 18 μΩcm, without noticeable change in either inter- or intra-granular microstructure. Intragranular nanoprecipitates with characteristics of a spinodal of MgB7, with a size of 1–5 nm, were observed in a few areas of samples A and B at high local density; however at too low an overall density to explain the increased resistivities and upper critical fields.

L. Cooley, X. Song, J. Jiang, D. Larbalestier, T. He, K. Regan, and R. Cava (2002). Core pinning by intragranular nanoprecipitates in polycrystalline MgCNi3 , Physical Review B, 65, 214518.

The magnetic properties and nanostructure of polycrystalline MgCNi3, prepared from a batch with overall composition MgC1.5Ni3, were studied by vibrating sample magnetometry and electron microscopy. Very high critical current density, e.g., 1.8MA/cm 2 at 1 T and 4.2 K, is deduced from the magnetic hysteresis and evident subdivision of the sample into 10μm clusters of MgCNi3 grains by excess graphite. The bulk pinning force Fp(H) is comparable to that of other strong flux-pinning superconductors, such as NbN, Nb-Ti, and Nb3Sn, all of which have higher critical temperatures. While Fp(H) indicates the expected grain-boundary pinning mechanism just below Tc≈7.2K, a systematic change to a core-pinning mechanism is indicated by a shift of the Fp(H) curve peak to higher (reduced) field with decreasing temperature. The lack of temperature scaling of Fp(H) suggests the presence of pinning sites at a nanometer scale inside the grains, which are smaller than the diameter of fluxon cores 2ξ(T) at high temperature and become effective when the coherence length ξ(T) approaches the nanostructural scale with decreasing temperature. High-resolution transmission electron microscopy imaging and electron diffraction revealed a substantial volume fraction of cubic and graphite nanoprecipitates comparable to ξ(0)≈5nm in size, consistent with the hypothesis above. Dirty-limit behavior seen in previous studies may thus be tied to electron scattering by the precipitates. To our knowledge, no other fine-grained bulk intermetallic superconductor exhibits a similar change from grain boundary to core pinning with decreasing temperature, suggesting that the arrangement of pinning sites in MgCNi3 is unique. These results also indicate that strong flux pinning might be combined with a technologically useful upper critical field if variants of MgCNi3 with higher Tc can be found.

C. Eom, M. Lee, J. Choi, L. Belenky, X. Song, L. Cooley, M. Naus, S. Patnaik, J. Jiang, M. Rikel, A. Polyanskii, A. Gurevich, X. Cai, S. Bu, S. Babcock, E. Hellstrom, D. Larbalestier, N. Rogado, K. Regan, M. Hayward, T. He, J. Slusky, K. Inumaru, M. Haas, and R. Cava (2001). High critical current density and enhanced irreversibility field in superconducting MgB2 thin films, Nature, 411, 558-560.

The discovery of superconductivity at 39 K in magnesium diboride offers the possibility of a new class of low-cost, high-performance superconducting materials for magnets and electronic applications. This compound has twice the transition temperature of Nb3Sn and four times that of Nb-Ti alloy, and the vital prerequisite of strongly linked current flow has already been demonstrated. One possible drawback, however, is that the magnetic field at which superconductivity is destroyed is modest. Furthermore, the field which limits the range of practical applications-the irreversibility field H*(T)-is approximately 7 T at liquid helium temperature (4.2 K), significantly lower than about 10 T for Nb-Ti and approximately 20 T for Nb3Sn (ref. 7). Here we show that MgB2 thin films that are alloyed with oxygen can exhibit a much steeper temperature dependence of H*(T) than is observed in bulk materials, yielding an H* value at 4.2 K greater than 14 T. In addition, very high critical current densities at 4.2 K are achieved: 1 MA cm -2 at 1 T and 105 A cm -2 at 10 T. These results demonstrate that MgB2 has potential for high-field superconducting applications.

I. J. Thompson, R. Feenstra, D. Christen, A. Gapud, and X. Song (2006). Critical currents of ex situ YBa2Cu3O7-δ thin films on rolling assisted biaxially textured substrates: Thickness, field, and temperature dependencies, Physical Review B 73, 134502.

The critical current density Jc flowing in thin YBa2Cu3O7−δ (YBCO) films of various thicknesses d has been studied magnetometrically, both as a function of applied field H and temperature T, with a central objective to determine the dominant source of vortex pinning in these materials. The films, grown by a BaF2 ex situ process and deposited on buffered rolling assisted biaxially textured substrates (“RABiTS”) substrates of Ni−5%W, have thicknesses d ranging from 28nm to 1.5μm. Isothermal magnetization loops M(H;T) and remanent magnetization M rem(T) in H=0 were measured with H∥c-axis (i.e., normal to film plane). The resulting Jc(d) values (obtained from a modified critical state model) increase with thickness d, peak near d∼120nm, and thereafter decrease as the films get thicker. For a wide range of temperatures and intermediate fields, we find Jc∝H −α with α∼(0.56–0.69) for all materials. This feature can be attributed to pinning by large random defects, which theoretically has power-law exponent α=5∕8. Calculated values for the size and density of defects are comparable with those observed by TEM in the films. As a function of temperature, we find Jc(T,sf)∼[1−(T∕Tc) 2] n with n∼1.2–1.4. This points to “δTc pinning” (pinning that suppresses Tc locally) in these YBCO materials.

Z. Chen, M. Feldamnn, X. Song, D. Larbalestier (2007). Three-dimensional vortex pinning behavior in a Sm-doped YBa2Cu3O7-x coated conductor with a high density of nano-precipitates, Superconductor Science & Technology, 20, S205.

We report on the thickness and angular dependence of the critical current density Jc( H,θ), 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 µm 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 µm. Angular-dependent measurements of J c 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.

S. Kim, A. Gurevich, X. Song, X. Li, W. Zhang, T. Kodenkandath, M. Rupich, T. Holesinger and D. Larbalestier (2006). Mechanisms of weak thickness dependence of the critical current density in strong-pinning ex situ metal–organic-deposition-route YBa2Cu3O7−δ coated conductors, Superconductor Science & Technology, 19, 968.

We report on the thickness dependence of the superconducting characteristics including critical current Ic, critical current density Jc, transition temperature Tc, irreversibility field Hirr, bulk pinning force plot Fp( H), and normal state resistivity curve ρ( T) measured after successive ion milling of ~1 µm thick high- Ic YBa2Cu3O7−x films made by an ex situ metal–organic deposition process on Ni–W rolling-assisted biaxially textured substrates (RABiTSTM). In contrast to many recent data, mostly on in situ pulsed laser deposition (PLD) films, which show strong depression of Jc with increasing film thickness t, our films exhibit only a weak dependence of Jc on t. The two better textured samples had full cross-section average Jc,avg (77 K, 0 T) ~4 MA cm −2 near the buffer layer interface and ~3 MA cm −2 at full thickness, despite significant current blocking due to ~30% porosity in the film. Taking account of the thickness dependence of the porosity, we estimate that the local, vortex-pinning current density is essentially independent of thickness, while accounting for the additional current-blocking effects of grain boundaries leads to local, vortex-pinning Jc values well above 5 MA cm −2. Such high local Jc values are produced by strong three-dimensional vortex pinning which subdivides vortex lines into weakly coupled segments much shorter than the film thickness.

Gurevich, S. Patnaik, V. Braccini, K. H. Kim, C. Meilke, X. Song, L. D. Cooley S. D. Bu, D. M. Kim, J. H. Choi, L. J. Belenky, J. Giencke, M. K. Lee, W. Tian, X.Q. Pan, A. Siri, E. E. Hellstrom, C. B. Eom, D.C. Larbalestier (2004). Very high upper critical fields in MgB2 produced by selective tuning of impurity scattering, Superconductor Science & Technology, 17: 278-286.

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, we show a clear trend of an increase 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 T and T for high resistivity films and 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 opposite to that of single crystals: γ( T) decreases as the temperature decreases, from to. This remarkable Hc2 enhancement and its anomalous temperature dependence are a consequence of the two-gap superconductivity in MgB2, which 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.

L. Cooley, X. Song, D. Larbalestier (2003). Improving flux pinning at high fields in intermetallic compounds: Clues from MgB2 and MgCNi3, IEEE T APPL SUPERCON 13 (2): 3280-3283.

J. Wang, Y. Bugoslavsky, L. Cowey, A. Berenov, A. D. Caplin, L. F. Cohen, J. L. MacManus Driscoll, L. Cooley, X. Song, D. C. Larbalestier (2002). High In-Field Jc's in Bulk MgB2 By A Scaleable, Nano-particle Addition Route, Applied Physics Letters, 81: 2026-2028.

X. Song, S. E. Babcock, C. B. Eom, D. C. Larbalestier, K. A. Regan, R. J. Cava, S. L. Bud’Ko, P. C. Canfield, and D. K. Finnemore (2002). Anisotropic Grain Morphology, Crystallographic Texture and Their Implications for Flux Pinning Mechanisms in MgB2 Pellets, Filaments and Thin Films, Superconductor Science & Technology, 15: 511-518.

S. Patnaik, L. D. Cooley, A. Gurevich, A. A. Polyanskii, J. Jiang, X. Y. Cai, A. A. Squitieri, M. T. Naus, M. K. Lee, J. H. Choi, L. Belenky, S. D. Bu, J. Letteri, X. Song, D. G. Schlom, S. E. Babcock, C. B. Eom, E. E. Hellstrom, and D. C. Larbalestier (2001). Electronic Anisotropy, Magnetic Field-Temperature Phase Diagram and Their Dependence on Resistivity in c-Axis Oriented MgB2 Thin Films, Superconductor Science & Technology, 14: 315-319.

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- T c 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.