Kenneth Wilbur Lay

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>>

The role of Ag in the preparation of metal sheathed (Bi,Pb)2Ca2Sr2Cu3Oz tapes

High resolution transmission electron microscopy has been used to examine the Ag-superconductor interface of powder-intube fabricated Bi 1.7 Pb 0.3 Ca 2.05 Sr 2 Cu 3.05 O x . No evidence of any chemical interaction between the Ag and the superconductor could be found. The presence of carbonate contaminants is proposed as the mechanism for the reported lowering of the liquidus temperature of the Ag encapsulated superconductor.

Grain-aligned YBCO superconducting current leads for conduction-cooled applications

Grain-aligned YBCO superconductors have been developed for use in a conduction-cooled, cryogen-free superconducting magnet application. A magnetic alignment technique has been used to orient the c-axis of the YBCO grains. Current leads 0.3*1.4*6.4 cm were fabricated and thermally connected between the first and second stage of a Gifford-McMahon cryocooler. The heat conduction down the leads was determined from a load map of the cryocooler. With operating temperatures of 40 K on the first stage and 10 K on the second stage, the measured heat leak was 0.15 W/lead. The YBCO current leads were used to power a conduction-cooled Nb/sub 3/Sn magnet at 10 K to 250 A. Test results for the YBCO leads are reported.<<ETX>>

An alternate preparation for grain aligned structures of (Bi, Pb)2Ca2Sr2Cu3Oz

Abstract A unique process for the preparation of bulk, grain aligned (Bi, Pb) 2 Ca 2 Sr 2 Cu 3 O z (2223), is presented. The process starts with the magnetic alignment of Bi 2 CaSr 2 Cu 2 O z (2122), in a heptane slurry with sufficient particulate PbO, Ca 2 CuO 3 and CuO necessary to form the (2223). Following evaporation of the heptane to physically set the alignment, the compacts are reacted and sintered at high temperature, for long times, to generate the final (2223) product. Critical currents, J c on the order of 700 A cm −2 at 77 K and zero magnetic field have been obtained. The severe degradation of J c with increasing fields suggests that grain alignment alone is not sufficient criterion for achieving large values of J c and that other factors, such as bulk density and connectivity between grains, are of equal or greater importance.

Anisotropic transport properties of Tl(1223) magnetically aligned ceramics and spray pyrolyzed films

Uniaxially textured TlBaSrCa/sub 2/Cu/sub 3/O/sub 9/ bulk ceramics have been fabricated by slip casting in magnetic fields up to 9 Tesla followed by sintering and reduction annealing. The measured transport properties are compared with those of spray pyrolyzed thick films where the degree of <001> texture is controlled through processing. Transport anisotropy is correlated to the degree of texture but not to the magnitude of the critical current density. In fact, although the critical current density of the aligned ceramic has the lowest value and is the most sensitive to small magnetic fields, it is also the most anisotropic with respect to the field direction. These data indicate that <001> texture is necessary but not sufficient for high critical current density in the Tl(1223) system.<<ETX>>