John Eric Tkaczyk

The preparation of “1223” Tl-Ca-Ba-Cu-oxide superconducting films via the reaction of silver-containing spray deposited Ca-Ba-Cu-Oxide with thallium oxide vapor

Abstract A process is described for the preparation of superconducting films of “1223” TlxCa2Ba2Cu3Oy (0.65 10 000 A/cm2 was measured at 60 K-2 T with the magnetic field applied parallel to the crystallographic c-axis of the film.

Transport critical currents in spray pyrolyzed films of TlBa2Ca2Cu3Oz on polycrystalline zirconia substrates

Large critical currents with a relatively weak magnetic field dependence are obtained in thick films of TlBa2Ca2Cu3Oz. Transport measurements indicate Jc ≳105 A/cm2 at 77 K, zero field, and Jc≳104 at 60 K in a 2 T field applied along the c‐axis. The observed behavior is attributed to a large degree of uniaxial alignment of platelike grains, and to superior intragranular flux pinning. These results are consistent with recent theories concerning the nature of vortices in highly anisotropic (layered) superconductors and ‘‘brick wall’’models of intergranular current transport.

High critical current densities in YBa2Cu3O7−x films on polycrystalline zirconia

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.

High temperature superconducting current leads for cryogenic applications in moderate magnetic fields

Large YBa/sub 2/Cu/sub 3/O/sub 7- delta / ceramic conductors suitable for bulk current applications have been fabricated using a magnetic technique to generate c-axis grain alignment. Critical currents on the order of several hundred amperes in a background field of 0.1 T have been achieved as a result of the aligned microstructure. The unusual dependence of the critical current on temperature and magnetic field is discussed in the context of anisotropy, self-field, and hysteretic effects.

Y‐Ba‐Cu‐O thin films grown on rigid and flexible polycrystalline yttria‐stabilized zirconia by pulsed laser ablation

In situ growth of highly oriented YBa2Cu3O7−x thin films (200–500 nm in thickness) has been obtained by pulsed KrF (248 nm) laser ablation on both rigid and flexible randomly oriented polycrystalline yttria‐stabilized zirconia substrates. It is shown that c‐axis‐perpendicular YBa2Cu3O7−x films with a mosaic spread of only 1.0° can be grown on these randomly oriented polycrystalline substrates. Superconducting thin films were obtained with Tc(R=0)∼89 K on well‐polished substrates. For the films deposited on the flexible substrates, the superconducting Tc is not degraded by repeated bending of the flexible substrate/film composite over a 2.25‐cm‐radius arc although the normal‐state resistivity increases slightly, suggesting the creation of microcracks. The YBa2Cu3O7−x films grown on rigid polycrystalline yttria‐stabilized zirconia substrates have a critical current density Jc(H=0)∼1400 A/cm2 at 77 K.

Processing of thallium-based superconducting tapes for high current density

Abstract Optimum thermomechanical processing conditions have been developed to achieve a high current density in powder-in-tube (PIT) thallium-based (Tl,Pb)-1223 and (Tl,Bi)-1223 tapes. Critical currents in excess of 25 A corresponding to a current density of 20 000 A/cm 2 have been achieved in these tapes at 77 K. Heat treatment and intermediate deformation sequences, heating rates, and starting precursor phase assemblages have been examined to optimize the current density. A combination of processing conditions that result in incremental densification, less microcracking and liquid formation in the early and final stages of thermomechanical treatment is found to result in the highest current densities.

Local texture and percolative paths for long‐range conduction in high critical current density TlBa2Ca2Cu3O8+x deposits

A possible microstructural origin of the high critical current densities which have been obtained in c‐axis‐aligned, polycrystalline TlBa2Ca2Cu3O8+x deposits has been identified. The results of x‐ray diffraction determinations of basal plane texture of Tl‐1223 deposits prepared by spray pyrolysis are observed to depend on the size of the x‐ray beam. Furthermore, most grain boundaries were found from transmission electron microscopy to have small misorientation angles. It is concluded that although overall the basal plane orientations are nearly random, there is a high degree of local texture indicative of colonies of similarly oriented grains. The spread in a‐axis orientation within a colony is ∼10°–15°. Intercolony conduction, it is suggested, may be enhanced by a percolative network of small‐angle grain boundaries at colony interfaces.

Enhanced transport critical current at high fields after heavy ion irradiation of textured TlBa2Ca2Cu3Oz thick films

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.

The fabrication of high critical current capability bismuth superconductor tape

A processing protocol for the preparation of Ag sheathed tapes of the bismuth superconductor composition Bi 1.7 Pb 0.3 Ca 2.05 Sr 2 Cu 3.05 O z , Bi-(2223), with critical currents densities, J c , exceeding 10,000 A cm −2 at 77 K and 0 T, is presented. The protocol is based on the combined in-situ reaction and sintering of a mixture of Bi 2 CaSr 2 Cu 2 O z , Bi-(2122), with the additional constituents needed to form the Bi-(2223), within the pre-fabricated tape. The conversion to a dense Bi-(2223) ceramic core was accomplished with a series of anneals at ∼830°C, for periods of up to 100 h, separated by uniaxial cold-pressings of the tape between the anneal. The effects on J c of variations in the anneal temperature, annealing time, and the number of pressings are presented.

Superconducting joints formed between powder-in-tube Bi/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub z//Ag tapes

Superconducting joints between Ag-clad, Bi/sub 2/Sr/sub 2/Ca/sub 2/Cu/sub 3/O/sub z/ tapes have been obtained with approximately 1/2 the current capacity of the tapes themselves. The Ag sheath is removed from one side of each tape without significantly disturbing the superconducting core. The exposed superconducting core of the two tapes is brought into contact and pressed so as to again seal the superconductor in a Ag sheath. A reaction anneal is performed to join the two cores together and repair damage associated with the removal of Ag and the pressing. Transport measurements using multiple voltage taps have been found useful in characterizing the critical current variation across the joint.<<ETX>>