J. O. Willis

Angular-dependent vortex pinning mechanisms in YBa2Cu3O7 coated conductors and thin films

We compare the angular-dependent critical current density (Jc) in YBa2Cu3O7 films deposited on MgO templates grown by ion-beam-assisted deposition (IBAD), and on single-crystal substrates. We identify three angular regimes in which pinning is dominated by different types of correlated and uncorrelated defects. Those regimes are present in all cases, but their extension and characteristics are sample dependent, reflecting differences in texture and defect density. The more defective nature of the films on IBAD turns into an advantage as it results in higher Jc, demonstrating that the performance of the films on single crystals is not an upper limit for the IBAD coated conductors.

Superconductivity above 90 K in magnetic rare earth-barium-copper oxides

Abstract We report measurements of the superconducting and magnetic behavior of GdBa2Cu3Ox. Superconductivity occurs below Tc = 95 and 93 K, respectively, for these compounds with large Meissner effects observed. The Gd-based compound has an upper critical field slope -dH c2 d T greater than 1.2 T/K. Both compounds exhibit Curie-Weiss behavior above Tc with free-ion effective moments and small, negative temperature intercepts. In addition, the Gd-based compound orders antiferromagnetically below 2.24 K with an entropy consistent with the ordering of seven unpaired spins.

Enhancement of transport critical current densities at 75 K in (Bi,Pb)2Sr2Ca2Cu3Oy/Ag tapes by means of fission tracks from irradiation by 0.8 GeV protons

The transport critical current density Jc of oxide‐powder‐in‐tube mono‐ and multifilamentary Bi‐2223/Ag tapes has been determined before and after irradiation by 0.8 GeV protons at fluences up to 7.0×1016 protons/cm2. Proton‐induced fission of the Bi nuclei produced up to 8.6×1013 fissions/cm3, creating long tracks at densities equivalent to matching fields up to 1.1 T. Relative to unirradiated tapes, Jc values at 75 and 64 K show no decrease in self field, indicating no breakdown of intergranular coupling, and show large, dose‐dependent enhancements in magnetic fields oriented parallel to the tape normal.

Novel phase transition in non-antiferromagnetically ordered crystals of La2CuO4

Abstract We report the discovery of a new phase diagram in the magnetic field-temperature plane for single crystals of La 2 CuO 4 that show no sign of antiferromagnetic order. The phase boundary is defined by magneto-resistance and susceptibility measurements. We discuss several possibilities that may lead to the phase diagram observed.

Development of Tl-1223 superconducting tapes

Abstract A superconducting tape based on the TlBa 2 Ca 2 Cu 3 O 9 (Tl-1223) phase sheathed in silver was fabricated using the powder-in-tube method. Optimal conditions to synthesize powder ( T c =110 K ) and anneal composite tapes were examined. Microstructural examination indicates that cracks are easily healed during a 3 h anneal at the relatively low temperature of 775°C. A transport critical current density J c of 6200 A/cm 2 at 75 K was measured on short lengths of the annealed rolled and pressed 0.20 mm thick tape. The J c value drops to about 250 A/cm 2 at 75 K and 1 T due to the presence of weak links; however, a J c of 1000 A/cm 2 at 45 K is maintained constant to above 6 T. Measurements of magnetic hysteresis loops indicate that the Tl-1223 tape exhibits a high degree of flux pinning. Very little texturing of the superconducting core was evident.

Proton radiation damage in superconducting EuBa2Cu3Ox and GdBa2Cu3Ox

Sintered samples of EuBa2Cu3Ox and GdBa2Cu3Ox with x≂7 have been irradiated at 90 °C with 800 MeV protons. The superconducting transition temperature decreases 1.7 K/1017 pcm−2 for both materials. The critical magnetization current density, characterizing intragranular conduction, increases a factor of 3 at zero field and relatively larger factors for finite magnetic fields at 7 and 75 K for fluences on the order of 1017 pcm−2. No and only minor differences between irradiated and unirradiated specimens were seen in calorimetric and transmission electron microscopy analyses, respectively.

The effects of density and size of BaMO3 (M=Zr, Nb, Sn) nanoparticles on the vortex glassy and liquid phase in (Y,Gd)Ba2Cu3Oy coated conductors

We studied the effect of systematically-controlled size (22–83 nm) and density (0.1–13 × 1021 m−3) of strong pinning random BaMO3 (M=Zr, Nb, Sn) nanoparticles (NPs) by engineering of their inclusion in (Y0.77Gd0.23)Ba2Cu3Oy ((Y,Gd)BCO) coated conductors. The critical current density (Jc) gradually increases from that of pure (Y,Gd)BCO to that of 3 wt% BZO with increasing density of NPs for H || c and H || 45°. Moreover, at low/intermediate fields the films with higher densities of isotropic pinning centers show nearly isotropic angular dependence at both 77 and 65 K, indicating that a high density of spherical nanoparticles effectively pins vortices over a broad angular range. We find that the enhancement of Jc depends mainly on the density of the NPs and not on their size. The vortex melting transition (characterized by the critical exponent (s) of the resistive transition) changes, particularly at H || 45° in films with higher densities of NPs. This change is reflected in a decrease of s to a value very close to that observed for H || c, similar to that of a Bose-glass. Thus, the density and morphology of the pinning centers are important factors determining not only Jc but also the character of the solid–liquid transition.

Alternating current losses in YBa2Cu3O7−x coated conductors on technical substrates

We report on measurements of the ac losses of YBa2Cu3O7−x conductors on technical substrates. The samples are taken from conductors 1 m long produced with an ion beam assisted deposition step in the manufacturing process yielding 1 cm wide conductors with critical currents approaching 100 A at 77.4 K. The losses due to combinations of ac transport currents and ac magnetic fields were measured calorimetrically and the results compared to theoretical estimates. While the results of our measurements are similar to the theoretical estimates, there are some important differences in the qualitative behavior.

Isothermal melt processing of Bi-2212 thick films

Abstract An alternative melt-processing technique has been developed for the fabrication of Bi 2 Sr 2 CaCu 2 O y (Bi-2212) thick films at temperatures over 100°C lower than those employed in conventional Bi-2212 melt processing. Isothermal melt processing combines the melting and solidification steps at the same temperature [T.G. Holesinger et al., Appl. Phys. Lett. 63 (1993) 982]. This technique is possible due to the large drop in the solidus temperature with decreasing oxygen partial pressure. High-quality thick films were processed at temperatures as low as 770°C. Such films were found to be well-textured, contain relatively few secondary phases, and possess critical current densities over 1 × 10 5 A/cm 2 at 4 K in self-field.

Determination of the superconducting current path in Bi2223/Ag tapes

We have measured the anisotropic transport properties of Bi2223/Ag tapes with and without the silver sheath. The anisotropy ratio of the resistivity along the rolling plane to the resistivity perpendicular to the rolling plane (∼4–10) is orders of magnitude smaller than single‐crystal anisotropy ratios. Results are consistent with current flow along crystallographic ab planes even for transport normal to the tape plane.