Showing papers by "Thomas Henry Tiefel published in 1991"

Dislocations and Flux Pinning inYBa2Cu3O7-δ

Abstract: Bulk YBa2Cu3O7-δ superconductors, under certain processing conditions such as melt texturing, exhibit a very high dislocation density of 109 to 1010 per square centimeter. In addition, the density of low-angle grain boundaries in such samples can be significantly increased (to less than 700-nanometer spacing) through a dispersion of submicrometer-sized Y2BaCuO5 inclusions. These defect densities are comparable to those in high critical current thin films as revealed through scanning tunneling microscopy, and yet the critical current densities in the bulk materials (at 77 kelvin and a field of 1 tesla for example) remain at a 104 amperes per square centimeter level, about two orders of magnitude lower than in thin films. The results imply that these defect density levels are not significant enough to explain the difference in flux pinning strength between the thin film and bulk materials. The observation of spiral-like growth of the superconductor phase in bulk Y-Ba-Cu-O is also reported.

Microstructure and properties of the Y-Ba-Cu-O superconductor with submicron “211” dispersions

Abstract: A fine-scale dispersion of Y2BaCuO5 (“211”) particles ( ∼8000 A average diam.) within the YBa2Cu3O7-δ supercondu ctor (“123”) has been achieved by local melt-texture processing of off-stoichiometric Y-Ba-Cu-O prepared by the aerosol decomposition technique. The presence of such fine particles significantly reduces the thickness of the superconductor plate to below ∼ 7000 A. The plate thickness almost linearly with 211 particle size. The observed scaling relationship is most likely caused by the 211 inclusions serving as effective nucleation sites for 123 crystallization as well as restricting the plate coarsening below the peritectic temperature. The submicron-sized 211 particles tend to reduce microcracks and segregation of impurity phases at the plate boundaries, however, their effect on flux pinning appears to be insignificant.

Superconducting properties of YBa2Cu3O7−δ with partial rare earth substitution

Abstract: Superconducting properties of the YBa 2 Cu 3 O 7−δ compound with partial rare earth substitution (20 atomic pct substitution for yttrium) have been studied. Among the 14 rare earth elements investigated, La, Ce and Pr caused suppression in T c , while other elements had no effect. Some of the rare earths (Sm, Gd, Eu), when partially substituted for Y, resulted in a slight improvement in intragrain J c (flux pinning) as measured magnetically. Although the rare earth elements have widely different ionic radii, all the (RE) Ba 2 Cu 3 O 7−δ phases have been found to be soluble in YBa 2 Cu 3 O 7−δ with no detectable segregation observed by SEM and energy dispersive X-ray analysis.

High critical currents in c-axis textured Bi-Pb-Sr-Ca-Cu-O superconductor ribbons

Abstract: A highly oriented layer structure was achieved in Bi-Pb-Sr-Ca-Cu-O superconductor ribbons by a combination processing of spray coating on silver foil, cold rolling and partial melting. Excellent transport J c values as high as 230 000 A/cm 2 at 4.2 K, H ⊥ ab ∼8 T have been obtained. Preliminary attempts to further improve the critical currents of the superconductor-on-silver ribbons by proton irradiation have not been successful as both the transport and the apparent magnetization J c 's are found to be monotonically reduced with increased fluence of irradiation. This is in contrast to the results of neutron irradiation on bulk Bi-Pb-Sr-Ca-Cu-O on which a significant increase in J c (magn) is observed. Further studies are needed for better understanding of the irradiation effects in the ribbon samples.

Method of making high Tc superconductor material, and article produced by the method

Abstract: The disclosed method can produce high T c superconductor material e.g., YBa 2 Cu 3 O 7 ) of substantially increased intra-grain critical current density (J' c ), as compared to conventionally produced bulk material of analogous composition. Exemplarily, YBa 2 Cu 3 O 7 pellets produced according to the invention had J' c of about 10 5 A/cm 2 at 77 K. in an applied magnetic field of 0.9 Telsa. The inventive method comprises providing a precursor material whose composition differs from that of the desired superconductor with respect to at least one of the metal constituents of the desired superconductor. It further comprises heating the precursor material above the decomposition temperature (T d ) of the precursor material such that a multiphase material results. The multiphase material comprises, in addition to a majority first phase, a dispersed precipitate phase. The method further comprises cooling the multiphase material to a temperature below T d at a rate such that at least a major portion of the precipitate phase is retained. The first phase differs from the desired superconductor at most with regard to oxygen content, and the heat treatment of the multiphase material is carried out such that the desired superconductor results. Exemplarily, the desired superconductor is YBa 2 Cu 3 O 7 , the precursor material has average composition YBa 2 Cu 4 O z (z˜8), the precipitate phase comprises one or more copper oxides, T d is about 860° C., and the multiphase material is produced at 920° C.

Solubility of nickel in the YBaCuO high Tc superconductor

Abstract: Microstructural and X-ray analyses were carried out on the YBaCuO superconductor compound doped with nickel. It is shown that there is a limited solubility of nickel in YBa2Cu3O7−δ with a resultant T c (midpoint) slightly below liquid nitrogen temperature. Partial melting caused a sharp superconducting transition near 77 K. The maximum solubility of nickel in YBa 2 ( Cu 1−x Ni x ) 3 O 7−δ has been determined by energy-dispersive X-ray analysis to be x ≈ 0.09 . The nickel in excess of the solubility limit is found to be present as isolated nickel oxide particles embedded in the YBaCuO phase. The T c of the nickel-doped YBaCuO can be fine tuned to produce a large change in resistivity near liquid nitrogen temperature, which could be useful for obtaining large signal changes in devices operating at about 77 K.