Ronald H. Arendt

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.

Thermal expansion measurements using neutron diffraction of Bi2CaSr2Cu2Ox

Abstract The differential thermal expansion of Bi 2 CaSr 2 Cu 2 O x , Bi-(2122), has been measured using high-temperature neutron powder diffraction in air, 0.980% oxygen in nitrogen, and pure argon. Thermal expansion in the c -axis direction is greater by a factor of about 1.4 than that in either the a - or b -axis direction for all sample environments. The neutron results for Bi-(2122) in air are compared with results from similar X-ray powder diffraction measurements.

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>>

An alternate preparation for (Bi, Pb)2Ca2Sr2Cu3Oz

Abstract An alternative synthetic route for obtaining (Bi, Pb) 2 Ca 2 Sr 2 Cu 3 O z (2223), by using Bi 2 CaSr 2 Cu 2 O z , (2122), as a starting material is described. The advantages of this route include lower processing temperatures, shorter reaction times, and improved phase purity over conventional synthetic routes.

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.

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.

Flux pinning and flux creep in uranium-doped (Bi, Pb)-Sr-Ca-Cu-O superconducting powders after thermal-neutron irradiation

Fission fragment damage was introduced into uranium-doped powdered Bi/sub 1.7/Pb/sub 0.3/Sr/sub 2/Ca/sub 2/Cu/sub 3/ O/sub y/ by irradiation with thermal neutrons. Measurements of magnetic hysteresis and flux creep were made using a vibrating sample magnetometer. Magnetic hysteresis and intragranular critical current densities, obtained from the magnetic hysteresis using the critical state model, show an increase upon irradiation of a factor of 70 at 50 K and 0.8 T. Flux creep data, interpreted as nonlinear relations between pinning energy and magnetization, as temperature-dependent pinning energies, or as distributions of pinning energies, show significant increases in pinning energy upon irradiation. The irreversibility line is found to shift to higher magnetic fields upon irradiation. It is concluded that the increase in flux creep and decrease in critical current density at higher temperatures appear to limit the potential utility of this family of oxide superconductors to the lower-temperature regime.

High-temperature neutron diffraction study of the melting behavior of Bi2CaSr2Cu2Ox

High‐temperature neutron powder diffraction has been used to study the melting, and subsequent reformation, behavior of Bi2CaSr2Cu2Ox (2122), the 80 K transition temperature oxide superconductor. It was found that the (2122) substantially reforms upon return to a temperature just below the peritectic melting point of the material. This finding implies that difficulties encountered in melt processing this material may be due largely to gross segregation of the constituents while the material is partially molten, as opposed to slow formation kinetics for this species.

Flux creep after thermal neutron irradiation of uranium-doped oriented compacts of YBa2Cu3Ox

Fission fragment damage was introduced by thermal neutron irradiation into aligned, sintered powder samples of YBa2Cu3Ox with uranium additions of 0.08 wt % UO2. This resulted in an increase in the intragranular critical current and a decrease in flux creep. Flux creep of the magnetization M exhibited simple log time dependence and was characterized by absolute and normalized slopes as functions of magnetic field and temperature. Before irradiation, the slopes were strongly field dependent and showed peaks as a function of temperature. After irradiation the slopes were much less field dependent and the peaks were suppressed. Both the magnitude and the field dependence of the creep were improved. Apparent pinning potentials at 0.8 T increased by a factor of about 2 on irradiation, for temperatures from 10 to 35 K. These increases in the apparent pinning potentials are attributed to the damage caused by the travel of the fission fragments through the crystal.