N. Savvides

Flux creep and transport critical current density in high-Tc superconductors

Abstract The transport critical current density Jc of thin films and bulk ceramic specimens of high-Tc superconductors has been measured as a function of temperature in the range 4.2 K to Tc under self-field conditions. Theoretical models for thermally activated flux creep in homogeneous superconductors, and for current transport across Josephson weak links (S-I-S and S-N-S junctions) are used to analyze the temperature dependence of Jc. Although it is possible to prepare specimens showing weak link behaviour, we find that flux creep determines the critical current density of both oriented films having Jc(0) = 103to 106A/cm2 and bulk specimens having Jc(0) ≥ 103 A/cm2. In these specimens Jc(T) increases approximately linearly with decreasing temperature for t ≤ 0.7 (where t = T/Tc). Theoretical fits to the data are obtained using a pinning energy or barrier potential U(0) = 10kTc- 65–90 meV.

The interaction of Ag with Bi-Pb-Sr-Ca-Cu-O superconductor

Abstract Bi-Pb-Sr-Ca-Cu-O superconductor compounds have been doped with up to 30 wt% Ag, sintered under variable oxygen partial pressure, and characterised in terms of the electrical and crystallographic behaviour. In contrast to previous reports that claim that Ag is the only metal non-poisoning to the superconductivity of Bi-Sr-Ca-Cu-O (BSCCO), it has been found that Ag additions to Bi-Pb-Sr-Ca-Cu-O depress T c and J c drastically and cause a large decrease in lattice parameters when samples are treated in air or pure oxygen. However, the lattice parameters, T c and J c remain unaffected by Ag additions when samples are heat treated in 0.030–0.067 atm oxygen. It is clear that the Ag reacts with and destabilises the superconducting phase when the samples are treated in air or pure oxygen while, when the samples are heat treated in low oxygen partial pressures, the Ag remains as an isolated inert metal phase that improves the weak links between the grains. This discovery clearly shows the feasibility of Ag-clad superconductor wire. For Ag-clad superconductor tape of 0.1 mm 2 cross sectional area heat treated in air, J c was measured to be 54 A/cm 2 . The same specimen sintered in 0.067 atm oxygen showed that the J c increased to 2078 A/cm 2 .

Stabilisation of 110 K superconducting phase in BiSrCaCuO Pb substitution

Abstract The effect of Pb substitution for Bi in the Bi-Sr-Ca-Cu-O systems, together with heat treatment, on the superconducting transition was investigated. Up to 40% Pb substitution, the crystal structure remains essentially unchanged, but the sub-unit cell parameters decrease with an increase of Pb substitution; some extra peaks from the X-ray diffraction patterns could be indexed assuming c=37 A . The Pb substitution tends to stabilise the high- T c phase transition at 110 K and 95 K even for a wide range of heat treatment conditions due to the increase of Ca and Cu concentration in the superconducting phase. Ag addition sharpens all transitions due to the improvement of the connectivity between grains. Higher temperature treatment promotes the 95 K phase transition and raises the zero resistance temperature. Quenching treatment results in the enhancement of the 95 K phase transition for specimens without Pb substitutions; it also causes semiconductor behaviour for Pb-substituted specimens depending upon the optimisation of the levels of Cu 3+ concentration and oxygen deficiency.

Growth and evolution of microstructure of epitaxial YBa2Cu3O7−x ultrathin and thin films on MgO

Abstract The nucleation and early stages of growth, and the evolution of surface microstructure of epitaxial c -axis YBa 2 Cu 3 O 7− x thin films are studied as a function of film thickness and growth temperature. The films were grown in situ on polished MgO (100) substrates by unbalanced DC magnetron sputtering at a growth rate of 3 nm/min. We use a combination of high-resolution scanning electron microscopy (SEM), scanning tunneling microscopy (STM), atomic force microscopy (AFM), X-ray diffraction, and transport-property measurements. The results from ultrathin films, 3 to 50 c ⊥ unit cells (1 c ⊥ unit cell equals 1.17 nm) indicate that epitaxial growth proceeds by the classical screw dislocation mediated three-dimensional island growth mode, i.e. each island grows both vertically and laterally by the incorporation of adatoms at a spirally expanding step emanating from one or more screw dislocations. The initial adatom clustering leads to a high density of nuclei (≈2×10 11 cm −2 ). The density of screw dislocations, ϱ screw , determined from STM images of the film surface, is found to depend on both the thickness of films and the growth temperature. At moderate growth temperatures (720–760°C) the early stage of coalescence of nuclei and island growth is characterised by a high density of screw dislocations, (3–9)×10 9 cm −2 , decreasing to (2–3)×10 9 cm −2 for 200 c ⊥ unit-cells films. Higher growth temperatures result in smooth films with a low density of screw dislocations, e.g.≤0.3×10 9 cm −2 at 850°C for 200 c ⊥ unit-cells films. A correlation is found between high values of ϱ screw and high values of the critical current density. The ultrathin films exhibit suppression of superconductivity and broadening of the resistive transition with decreasing film thickness, so that T c0 =55 K for a 3 c ⊥ unit-cells film.

Biaxially aligned buffer layers of cerium oxide, yttria stabilized zirconia, and their bilayers

Biaxially aligned cerium oxide (CeO2) and yttria stabilized zirconia (YSZ) films were deposited on Ni-based metal (Hastelloy C276) substrates held at room temperature using ion beam assisted (IBAD) magnetron deposition with the ion beam directed at 55° to the normal of the film plane. In addition, we achieved, room-temperature epitaxial growth of CeO2 by bias sputtering to form biaxially aligned CeO2/YSZ bilayers. The crystalline structure and in-plane orientation of films was investigated by x-ray diffraction techniques. Both the IBAD CeO2 and YSZ films, and the CeO2/YSZ bilayers have a (111) pole in the ion beam direction.

Pinning mechanisms in Ag-sheathed Bi(Pb)SrCaCuO tapes

Abstract The pinning properties of Ag-sheathed (Bi,Pb)2Sr2Ca2Cu3O10+y (2223) wires have been studied in comparison with the normal powder, and the powder scraped from the 2223 tape by the measurements of irreversibility lines (IL). The IL for both the tape and the powder scraped from the tape are nearly equally shifted to higher temperatures in comparison with that for the normal powder. This indicates that the enhanced IL is attributable to the defect pinning. TEM studies reveal that the grains within Ag-clad 2223 tape contain high concentrations of dislocations (1010/cm2) and stacking faults that may act as effective pinning sites. The results on the transport and magnetisation Jc suggest that the critical current is controlled by flux pinning at high temperatures, while the weak links limit the critical current at low temperatures.

In situ growth of epitaxial YBa2Cu3O7 thin films by on‐axis unbalanced direct current magnetron sputtering

Superconducting YBa2Cu3O7−x thin films are prepared in situ by on‐axis dc magnetron sputtering from a single stoichiometric target. The magnetron uses an unbalanced magnetic field configuration to eliminate resputtering effects, and high quality thin films on MgO (100) substrates are realized with excellent reproducibility. The target is sputtered in Ar/O2 mixture at a fixed partial pressure ratio pAr/pO2= 15:1. We report on the growth and properties of films deposited (a) as a function of the total sputtering pressure pt (1–100 Pa) at fixed substrate heater temperature Ts=740 °C, and (b) as a function of Ts (600–860 °C) at fixed pt=42 Pa. Epitaxial c‐axis thin films are obtained over a wide range of deposition conditions (Ts≥700 °C, pt≥30 Pa). These films have superconducting transition temperatures Tc=86–90 K, critical current densities Jc77≥106 A cm−2, and resistance ratios R300/R100=2.0–3.1.

Critical current density and flux pinning in silver/superconductor composites and tapes

Abstract Measurements of the transport critical current density, J c , and AC susceptibility, χ = χ ′+ i χ ″, are reported for silver/superconductor composites and silver-sheathed c -axis textures tapes fabricated from the (Bi 1.6 Pb 0.4 )Sr 2 Ca 2 Cu 3 O 10−y compound. The field dependence of the χ″ intergrain loss peaks are used to obtain qualitative information on the pinning force density, and the Bean critical state model is used to determine the intergrain and intragrain critical current densities. It is found that silver enhances flux pinning in the intergranular regions with optimum critical current density occuring at 20–30 vol.% Ag. The temperature dependence of the transport J c of the tapes is controlled by flux creep, and analysis of the experimental data using flux creep theory gives a pinning energy U (0)≅90 meV. Measurements of the DC field dependence of J c show it to be anisotropic, e.g., J c =6.5×10 3 A cm −2 (77 K) in zero field, decreasing with field to ∼ 10 3 A cm −2 for H ⊥ c -axis and to ∼30 A cm −2 for H ‖ c -axis at 0.4 T. Demagnetisation effects are considered and are though to account for most of this anisotropy.

Melt processing of alkali element doped Bi2Sr2CaCu2O8

Abstract The effect of alkali element doping on the superconducting properties of the BiSrCaCuO system has been investigated. It was found that a Tc of 94 K and T0 of 90 K in alkali element doped Bi2Sr2CaCu2O8 were achieved through the use of a mel processing technique. XRD, SEM and EDS examinations showed that the samples did not contain the high-Tc phase Bi2Sr2Ca2Cu3O10. It is argued that the mechanism for the enhancement of the Tc through alkali element doping derives from a fluxing action, which provides a fast diffusion path for Ca and Cu to the superconducting phase, resulting in the desirable stoichiometry of Bi2Sr2CaCu2O8. On the other hand, the substitution of alkali elements in the superconducting phase may give rise to a reducing effect of the oxygen content, leading to the enhancement of the Tc. The latter mechanism may be compounded with the fluxing action. Alkali elements were found to be excellent sintering aids for processing Bi-based superconductors since they enhance the Tc and promote grain growth and grain aligment in the bulk materials.

Strain tolerance of multifilament Bi‐Pb‐Sr‐Ca‐Cu‐O/silver composite superconducting tapes

The bend strain characteristics of Bi‐Pb‐Sr‐Ca‐Cu‐O/silver composite tapes were determined from measurements of the critical current, Ic(77 K), as a function of bend strain (0%–2%) or radius of curvature (∞−10 mm). The powder‐in‐tube method was used to prepare monofilament and multifilament (up to 49 filaments) tapes, 0.2–0.4 mm overall thickness, with 5–20 vol % superconductor. The irreversible strain limit, eirr, for the onset of bend strain damage, increases with the number of filaments from eirr=0.2%–0.3% for monofilament tapes to values up to 1.5% for 49‐filament tapes. Although strains beyond this limit cause irreversible damage in the form of microcracks and fracture of the superconductor filaments, multifilament tapes are exceptionally strain tolerant, with 49‐filament tapes retaining 85%–90% of the initial critical current at strains of 2%.