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Electrical and thermal stability of high-temperature superconducting (HTS) wires∕tapes are essential in applications involving efficient production, distribution, and storage of electrical energy. We have developed a conductive buffer layer structure composed of bilayer La0.7Sr0.3MnO3∕Ir on a textured Ni–W alloy metal tape to functionally shunt the HTS layer to the underlying substrate. The key feature is the Ir layer, which serves as a barrier to both inward diffusion of oxygen and outward diffusion of metal substrate cations during fabrication. Electrical and microstructural property characterizations of YBa2Cu3O7-δ films on short prototype conductors demonstrate self-field critical current density values, Jc, exceeding 2×106A∕cm2 at 77K and excellent electrical coupling to the underlying metal substrate, with no unwanted insulating oxide interfaces. Implementing this approach in power technologies would significantly increase the engineering current density of the conductor and reduce overall process c...

In YBa2Cu3O7−δ (YBCO) thick films, the superconducting critical current density (Jc) decreases with increasing film thickness (t). The mechanisms responsible for this Jc–t behavior remain unclear. To probe the correlation between the film microstructure and Jc-thickness behavior, we have deposited YBCO thick films up to 3.0μm in thickness on flat and surface-miscut (100) SrTiO3 substrates with 5°, 10°, and 15° vicinal angles. The microstructures of the YBCO films were found to evolve differently on flat and miscut substrates, resulting in different Jc–t behaviors. Surprisingly the small miscut angles of 5°–10° were favorable to obtain higher Jc and smaller Jc reduction at larger film thickness.

The development of YBa2Cu3O7−δ (YBCO)-based coated conductors for electric power applications will require electrical and thermal stabilization of the high-temperature superconducting (HTS) coating. In addition, nonmagnetic tape substrates are an important factor in order to reduce the ferromagnetic hysteresis energy loss in ac applications. We report progress toward a conductive buffer layer architecture on biaxially textured nonmagnetic Cu tapes to electrically couple the HTS layer to the underlying metal substrate. A protective Ni overlayer, followed by a single buffer layer of La0.7Sr0.3MnO3, was employed to avoid Cu diffusion and to improve oxidation resistance of the substrate. Property characterizations of YBCO films on short prototype samples revealed self-field critical current density (Jc) values exceeding 2×106 A/cm2 at 77 K and good electrical connectivity. Magnetic hysteretic loss due to Ni overlayer was also investigated.

Coated conductor applications in power technologies require stabilization of the high-temperature superconducting (HTS) layers against thermal runaway. Conductive La0.7Sr0.3MnO3 (LSMO) has been epitaxially grown on biaxially textured Ni substrates as a single buffer layer. The subsequent epitaxial growth of YBa2Cu3O7−δ (YBCO) coatings by pulsed laser deposition yielded self-field critical current densities (Jc) of 0.5×106 A/cm2 at 77 K, and provided good electrical connectivity over the entire structure (HTS+conductive-buffer+metal substrate). Property characterizations of YBCO/LSMO/Ni architecture revealed excellent crystallographic and morphological properties. These results have demonstrated that LSMO, used as a single, conductive buffer layer, may offer potential for use in fully stabilized YBCO coated conductors.

Superconducting MgB(2) films with Tc = 38.6 K were prepared using a precursor-deposition, ex-situ post-processing approach. Precursor films of boron, ~0.5 micrometer thick, were deposited onto Al(2)O(3) (102) substrates by e-beam evaporation; a post-anneal at 890 deg C in the presence of bulk MgB(2) and Mg metal produced highly crystalline MgB(2) films. X-ray diffraction indicated that the films exhibit some degree of c-axis alignment, but are randomly oriented in-plane. Transport current measurements of the superconducting properties show high values of the critical current density and yield an irreversibility line that exceeds that determined by magnetic measurements on bulk polycrystalline materials., PDF file with 10 pages total, including 4 figures

Superconducting HgBa2CaCu2O6 (Hg-1212) films have been coated on biaxially textured Ni substrates buffered with CeO2/YSZ/CeO2 trilayers. A two-step cation-exchange process was employed for Hg-1212 coatings, whereby epitaxial Tl2Ba2CaCu2O8 precursor films grown in a fast temperature ramping annealing process were converted to Hg-1212 via diffusive Hg–Tl cation exchange. This minimizes chemical diffusion/reaction of Hg with the substrates and allows Hg-1212 films to inherit the epitaxy and smooth surface morphology of the Tl2Ba2CaCu2O8 precursor films. The superconducting transition temperature (Tc) of these Hg-1212 coated conductors is up to 124 K. Self-field critical current densities (Jc) of 2.2×106 A/cm2 at 77 K have been obtained, with Jc values of 0.7×106 A/cm2 achieved at 100 K. These results show Hg-1212 coated conductors may hold promise for electrical power applications above 77 K.

The nucleation of (001)-oriented yttria-stabilized zirconia (YSZ) directly on the (001) Ni surface is realized via nucleation on an oxygen-terminated nickel surface using pulsed-laser deposition. Under conditions where the nickel surface is either oxygen free or substantially covered with NiO, a mixed orientation of YSZ occurs. The epitaxial YSZ layer grown on a biaxially textured Ni(001) surface was used as a single buffer layer for a high temperature superconducting coated conductor architecture, yielding superconducting YBa2Cu3O7 films with high critical current densities, Jc. This architecture eliminates the necessity for a multilayer buffer architecture, since high Jc superconducting films are achieved with no intermediate buffer layer between the (001) YSZ and the biaxially textured metal.

Second generation, high-temperature superconducting wires are based on buffered, metallic tape substrates of near single crystal texture. Strong alignment of adjacent grains was found to be necessary from previous work that suggested large angle, YBa2Cu3O7−δ [001]-tilt boundaries reduce Jc exponentially with increasing misorientation angle (θ). We pursue the low-θ regime by evaluating single grain boundaries (GB) and biaxially aligned polycrystalline films utilizing both the rolling-assisted biaxially textured substrates and ion-beam assisted deposition coated conductor architectures. Analysis concludes that an exponential dependence on Jc is applicable for θ≳4°, where the spacing between the periodic disordered regions along the GB become smaller than a coherence length.

Vortex pinning in textured Bi2Sr2CaCu2Ox/Ag superconducting tapes was enhanced by irradiation with 0.8 GeV protons, creating randomly oriented columnar defects. Measurements of the current density J versus time show that thermally-activated current decay was greatly decreased, compared with unirradiated tapes. However, temperature-independent quantum tunneling of vortices considerably limits J and its temporal stability, for a significant fraction of the finite-Jc region.

We have determined the stability line in the 1/T−log[P(O2)] phase space for the synthesis of Nd1+xBa2−xCu3Oy (NdBCO) films. A systematic study of Tc, Jc, and ρ(T) dependence on oxygen partial pressure and temperature for the deposition of thin NdBCO films grown by pulsed-laser deposition was performed. The conditions for optimal NdBCO film growth were determined by varying oxygen partial pressure from 0.02 to 400 mTorr, and substrate temperature between 730 and 800 °C. The results show that the best NdBCO films are obtained at oxygen pressures in the range of 0.2–1.2 mTorr, depending on the substrate temperature. This is more than two orders-of-magnitude lower than the correspondent oxygen pressure appropriate for YBa2Cu3O7−δ film growth.

Superconducting YBa2Cu3O7/CeO2/YSZ/CeO2 multilayer structures were grown on rolled-textured (001)Ni using pulsed-laser deposition. Critical current densities greater than 1 MA/cm2 were obtained for relatively thick YBa2Cu3O7 films. The compressive and tensile bend strain tolerance of critical currents for the YBa2Cu3O7 films deposited on these rolling-assisted biaxially textured substrates was also determined. These conductors retained up to 80% of their unstrained critical currents for applied compressive bend diameters as small as 1.5 cm and tensile bend diameters of 3.2 cm. The degradation of Jc is caused by the formation and propagation of transverse cracks. The results also suggest a correlation between bend-strain tolerance for these coated conductors and total oxide layer thickness.

YBa2Cu3O7−δ films grown on rolling-assisted biaxially textured substrates carry critical current densities 105–106 A/cm2 at 77 K and low applied magnetic fields. In the low-field and low-current regime, ac transport current studies show hysteresis energy loss (per cycle and per unit length) roughly the value expected for a superconductor of elliptic cross section. The critical current Ic was deduced from observed dc and dynamic current-voltage relations. The power loss rises sharply as I0 (the maximum current in each cycle) is raised above Ic. In the present configuration, ferromagnetic hysteresis of the Ni substrate contributes little or no loss.

In several Tl- and Hg-based high-Tc superconducting materials, vortex-pinning defects were formed by irradiation with 0.8 GeV protons. The protons cause heavy constituent nuclei (Tl, Hg, Bi, …) to fission, which generates randomly oriented tracks. These columnar defects lead to significant enhancements in the current-conducting properties. We investigated bulk materials and thin films, irradiated with or without Pb or Au “amplifier foil” overlayers, which increase the number of column-creating fission fragments.

A method to obtain long lengths of flexible, biaxially oriented substrates with smooth, chemically compatible surfaces for epitaxial growth of high‐temperature superconductors is reported. The technique uses well established, industrially scalable, thermomechanical processes to impart a strong biaxial texture to a base metal. This is followed by vapor deposition of epitaxial buffer layers (metal and/or ceramic) to yield chemically compatible surfaces. Epitaxial YBa2Cu3Ox films grown on such substrates have critical current densities exceeding 105 A/cm2 at 77 K in zero field and have field dependencies similar to epitaxial films on single crystal ceramic substrates. Deposited conductors made using this technique offer a potential route for the fabrication of long lengths of high‐Jc wire capable of carrying high currents in high magnetic fields and at elevated temperatures.

The conversion of semiconductive SrCuO2 ‘‘infinite layer’’ epitaxial films into a metallic and superconducting (Tc≂30 K) oxycarbonate phase related to Sr2CuO2(CO3) has been induced by thermal processing in oxidizing ambient containing a small amount of CO2. Ion beam analysis using the resonant 12C(p,p)12C elastic backscattering cross section for protons incident at 1.73 MeV confirms the presence of substitutional C in the converted perovskite lattice. Comparison of the threshold CO2 partial pressure with thermodynamic stability data for SrCO3 suggests that the conversion is initiated by the high affinity of Sr(O) for CO2.

This work establishes the stability at elevated temperatures of columnar defects, artificially formed in the Bi‐based cupric oxide superconductor Bi2Sr2Ca1Cu2O8 for enhanced vortex pinning. Isochronal anneals, conducted in air, led to losses of critical current density in two stages. The defects were relatively stable up to ∼550 °C, where second stage annealing began; above this, the pinning diminished rapidly. The recrystallization and loss of columnar defects were corroborated by transmission electron microscopy.

A pulsed‐current technique was used to measure the extended I–V characteristics of a wide variety of prototype high‐temperature‐superconductor (HTS) leads. It was found that the average resistivity rises with J(≳Jc) more gradually than in conventional superconductors—often remaining very small compared to silver, for values of J(≫Jc) that are high enough to be practically useful. This observation, combined with the low thermal conductivity (∼50 times smaller than Ag), should extend the utility of HTS leads to the dissipative regime where J/Jc≫1.

Thick films of the high‐temperature superconductor Tl1Ba2Ca2Cu3Oy with high current density Jc have been deposited on polycrystalline Ag substrates by a spin‐coating method. The spin‐coated precursor films were annealed in a two‐zone Tl reaction furnace. Film thicknesses ranged from 10 to 30 μm, leading to good total current capabilities. The films have highly‐textured c‐orientation as evidenced by rocking curves with FWHM of 4° for the (006) peak. Scanning electron micrographs revealed a dense, plate‐like layered structure and almost no reaction between the film and the Ag substrate. Values of Jc (77 K) up to 2.5×104 A/cm2 in zero field and more than 1000 A/cm2 in a 1 tesla field (H∥c axis).

Superconductivity with Tc≂25–45 K has been induced in films based on the ‘‘infinite layer’’ structure though high temperature annealing in an oxygen ambient. The films had compositions Srn+1CunO2n+1+δ (n=3–9) and were previously grown by laser molecular beam epitaxy by periodic insertion of SrO defect layers into the SrCuO2 infinite layer matrix. A marked dependence on the cooling rate at the end of the postgrowth anneals indicates that preservation of a high temperature oxygen sublattice forms the key to enhancing the carrier density in these films, opposite to the situation with YBa2Cu3O7−δ .

Pulsed‐laser ablation has been used to grow epitaxial films of Bi2Sr2CaCu2O8−x (Bi‐2212) and Bi2Sr2CuO6−x (Bi‐2201) on (001) MgO with entirely in situ processing. The films’ layer‐stacking sequence, microstructure, and superconducting Tc are highly sensitive to the growth temperature and deposition rate. Pure Bi‐2212 films exist over only a narrow temperature range. Pure Bi‐2201 films appear at higher growth temperature, while lower growth temperature enhances the Bi‐2223 phase, though it is mixed with Bi‐2212. The phase homogeneity, in‐plane connections among grains, and superconducting Tc of Bi‐2212 grown at a given temperature are greatly improved by reducing the deposition rate. Epitaxial Bi‐2212 films grown at 740 °C and 0.5 Hz (∼0.05 nm/s) have Tc0 (R=0)=71 K, with Jc∼8×105 A/cm2 at 50 K and Jc∼5×106 A/cm2 at 4.2 K.

Spray pyrolyzed T1(1223) films deposited on polycrystalline YSZ substrates are characterized before and after heavy ion irradiation. A factor of 2 decrease in zero field critical current is observed. However, significantly improved critical current is found at fields above 1 T, where intragranular effects dominate. The irreversibility line at 5 T is shifted by ∼20 K to higher temperatures. Scaling of the data before and after irradiation and at different temperatures is modeled by an expression which interpolates between single vortex pinning and collective creep.

Large enhancements in the critical current density Jc were produced in single crystals of the high‐temperature superconductor Bi2Sr2Ca1Cu2O8 by irradiation with high energy Sn ions. In addition, the irreversibility line was moved to considerably higher magnetic fields. In contrast with analogous studies on Y1Ba2Cu3O7, there was little, if any, selective pinning when the magnetizing field was applied parallel to the linear, ion‐damage‐produced tracks.

A self‐consistent critical current model in the Josephson mixed state is proposed for a series of c‐oriented, polycrystalline and for a series of epitaxial triaxially oriented YBCO thin films. The flux pinning activation energies were experimentally determined from electrical transport measurements over a wide range of temperatures and were found to behave quite differently for the two types of granular film. With these activation energies applied to a superconductor‐normal metal‐superconductor weak‐link system, thermally activated flux motion is shown to reproduce the experimentally measured critical current densities.

We investigated the stabilization of magnetic flux in a high‐temperature superconductor (a proton‐irradiated Y1Ba2Cu3O7 crystal), by operating with subcritical current density J. Using the thermal history to obtain an induced current density J≤Jc, we observed a drastically reduced relaxation rate dM/dt (M=magnetization), after ‘‘flux creep annealing.’’ The results show that the field gradient ∼J∼M determined the relaxation rate, independent of the sample’s H‐T history, in agreement with recent theory.

The structure of YBa2Cu3Ox thin films and YBa2Cu3Ox/PrBa2Cu3Ox superlattice films grown on (00L) SrTiO3 substrates has been studied using x‐ray diffraction and transmission electron microscopy. The films consist of four symmetry‐equivalent domains related by mutually perpendicular (hh0) and (hh0) twin boundaries. One twin orientation is often highly favored over the other. We have correlated this in‐plane symmetry breaking with small miscuts of the substrate surface towards the [HH0] direction and propose that interfacial steps act as nucleation sites for twins. Thus, by controlling the surface normal, a technique is described for producing films containing aligned twin boundaries.

We report the growth on polycrystalline yttria‐stabilized zirconia substrates of YBa2Cu3O7−x films with Jc (77 K)=11 000 A/cm2 and Jc (4.2 K)=122 000 A/cm2 by pulsed laser ablation. These Jc values are among the highest reported for YBa2Cu3O7−x on any polycrystalline substrate and approach the intrinsic upper limit for films with large‐angle grain boundaries, as indicated by recent bicrystal experiments. We find that the substrate temperature during film growth is most important in obtaining high Jc polycrystalline films. Although the magnetic field dependence of Jc indicates the presence of weak links, the behavior of Jc (4.2 K, H) suggests that a percolative path consisting of low‐angle grain boundaries exists in the films, resulting in Jc (4.2 K, 60 kOe)=4100 A/cm2.

We performed high-field magnetotransport and magnetization measurements on a single crystal of the 122-phase iron pnictide Ba(Fe1-xCox)2As2. Unlike the HTS cuprates and 1111-phase oxypnictides, Ba(Fe1-xCox)2As2 showed practically no broadening of the resistive transitions under magnetic fields up to 45 T. The mass anisotropy gamma = Hc2ab/Hc2c deduced from the slopes of the upper critical field dHc2ab/dT = 4.9T/K and dHc2c/dT = 2.5T/K decreases from ~2 near Tc, to ~1.5 at lower temperatures. We observed the irreversibility field close to Hc2, and a rather unusual symmetric volume pinning force curve Fp(H) suggestive of strong pinning nano-structure., Comment: 12 pages, 5 figures

High-temperature superconductors (HTS) are finding use as high - current wires for potential application in power devices. The dependence of their basal-plane critical current properties on orientation in a magnetic field can pose important design problems. Here we report material and operating parameter conditions where prototype HTS tape conductors exhibit critical current characteristics that are essentially field-orientation independent. The novel phenomenon is observed for specific magnetic field intensities that depend on the operating temperature, and in materials having strong flux pinning by extended nanoscale structures aligned roughly along the HTS crystalline c-axis. The effect can be described by a simple model for the field dependence of critical current density, generalized for anisotropic electronic response. This description may provide insight into means to fine-tune the material properties for nearly isotropic performance characteristics at a preferred field and temperature., 7 pages, 6 figures

The conduction of supercurrents in a short-segment state-of-the-art coated conductor prototype has been studied over ∼8 decades of dissipation. A combination of conventional transport methods, magnetometry in a swept magnetic field, and “flux creep” measurements was used to obtain current density J versus electric field E characteristics over a span E∼10−5–10−13V∕cm. The inductive measurements allow facile exploration of current conduction over a wide range of temperatures, magnetic fields, and dissipation levels where future applications are envisioned.

The new rare-earth arsenate superconductors are layered, low carrier density compounds with many similarities to the high-Tc cuprates. An important question is whether they also exhibit weak-coupling across randomly oriented grain-boundaries. In this work we show considerable evidence for such weak-coupling by study of the dependence of magnetization in bulk and powdered samples. Bulk sample magnetization curves show very little hysteresis while remanent magnetization shows almost no sample size dependence, even after powdering. We conclude that these samples exhibit substantial electromagnetic granularity on a scale approximating the grain size, though we cannot yet determine whether this is intrinsic or extrinsic., Comment: 3 pages, 6 figures

We have grown thin YBa2Cu3O7−x films on single‐crystal (001) KTaO3 substrates by means of coevaporation of Y, BaF2, and Cu, followed by an ex situ anneal in wet oxygen. The films are epitaxial in three dimensions, consisting of grains with either the c or the a axis of the YBa2Cu3O7−x unit cell perpendicular to the substrate. Films with a composition close to the 1:2:3 cation ratio exhibit sharp superconducting transitions at 90 K and critical current densities in excess of 0.3 MA/cm2 in zero magnetic field at 77 K. In Cu‐rich films, the Y2Ba4Cu8O16–x phase with a Tc of 80 K is formed in grains with the c axis perpendicular to the substrate.