G. W. Kammlott

High critical currents in Y‐Ba‐Cu‐O superconductors

Melt‐textured growth of polycrystalline YBa2Cu3O7−δ superconductor using directional solidification created an essentially 100% dense structure consisting of long, needle‐ or plate‐shaped crystals preferentially aligned parallel to the a‐b conduction plane. The new microstructure, which completely replaces the previous granular and random structure in the sintered precursor, exhibits dramatically improved transport Jc values at 77 K of ∼17u2009000 A/cm2 in zero field and ∼4000 A/cm2 at H=1 T (as compared to ∼500 and ∼1 A/cm2, respectively, for the as‐sintered structure), with the severe field dependence of Jc (‘‘weak‐link’’ problem) no longer evident in the new melt‐textured material. The improvement in Jc is attributed to the combined effects of densification, alignment of crystals, and formation of cleaner grain boundaries. Microstructure and distribution of various phases present in the melt‐textured material are discussed in relation to the superconducting properties.

Superconductivity in the Bi‐Sr‐Ca‐Cu‐O compounds with noble metal additions

The Bi‐Sr‐Ca‐Cu‐O superconductors have been doped with various noble metals and their superconducting properties have been investigated. The resistivity and magnetic susceptibility measurements on sintered Bi4Sr3Ca2Cu4O16+x containing 20% by weight of Au, Ag, or Pt‐group metals indicate that Au and the Pt‐group metals significantly suppress or eliminate the superconducting transition in Bi‐Sr‐Ca‐Cu‐O. Only Ag is found to be benign, maintaining both the 115 and 85 K transitions in the compound. This nonpoisoning behavior of silver is of significant technical importance because of the need for a proper stabilizing normal metal for composite superconductor wire, nonreactive crucible materials for melt processing or crystal growth, and suitable nonpoisonous substrates or barriers for thin‐ or thick‐film superconducting devices.

Enhanced flux pinning by phase decomposition in Y-Ba-Cu-O

Significantly improved flux pinning has been achieved in bulk YBa2Cu3O7−δ superconductor (‘‘123’’ compound) containing fine‐scale defects (<∼50 A thick). The measured Jc intragrain of ∼105 A/cm2 at 77 K, H=0.9 T is about ten times higher than the typical values for bulk Y‐Ba‐Cu‐O. The improved structure was produced by rapid decomposition at 920u2009°C of the YBa2Cu4O8 (‘‘124’’) precursor. This new and simple processing route could lead to a commercially viable processing technique for flux‐pinning enhancement in bulk Y‐Ba‐Cu‐O.

Synthesis and properties of the YBa2Cu4O8 superconductor

Abstract A new simple synthesis route was used to produce the ‘124’ superconductor YBa 2 Cu 4 O 8 . The method utilizes YBa 2 Cu 3 O 7 powder (‘123’) as a precursor, which is then converted to the 124 superconductor by reaction with a stoichiometric amount of CuO through normal grinding and sintering, without the need for high oxygen pressure processing. Sintered bars of the 124 superconductor exhibited a T c of ∼ 75 K in resistivity and AC magnetic susceptibility measurements. Transport critical current density was measured to be ∼ 150 A/cm 2 at 60 K, and showed a strong field dependence. This behavior, in combination with a relatively high J c (magn.) of 4 × 10 4 A/cm 2 at 60 K and H = 0.9 T, is indicative of Josephson weak links at grain boundaries, which is similarly observed in the 123 phase. It is also noted that the intragrain J c in the twin-free 124 superconductor is about the same as that in the twinned 123 superconductor at ∼ 15 degrees below their respective T c .

Fabrication of dense Ba2YCu3O7−δ superconductor wire by molten oxide processing

Fabrication of high Tc ceramic superconductors by an oxide melting method in place of a conventional sintering method has been attempted. Using three different processes, i.e., melt drawing, melt spinning, or preform‐wire melting, it is demonstrated that the Ba2YCu3O7−δ type superconductors can successfully be fabricated into a desired geometry such as wire and ribbon. Tc’s for R=0 were about 92 K. The density of the melt‐processed compound was measured to be as high as 6.2 g/cm3, or ∼98% of the theoretical density 6.3 g/cm3 as compared to the value of 80–85% density for sintered samples. The increased density is likely to be responsible for the noted improvements in fracture resistance and in the Jc value of the melt‐processed compound.

The thermal conductivity of chemical‐vapor‐deposited diamond films on silicon

The thermal conductivity of chemical‐vapor‐deposited diamond films on silicon is measured for the case of heat flow parallel to the plane of the film. A new technique uses thin‐film heaters and thermometers on a portion of the film which is made to be free standing by etching away the substrate. Effects of thermal radiation are carefully avoided by choosing the length scale properly. Data for several films yield thermal conductivities in the range 2–6 W/cmu2009°C. This is comparable to copper (4 W/cmu2009°C) and is in a range that would be useful as a thin‐film dielectric material, provided that the interface thermal resistance can be minimized. The conductivity varies inversely with the growth rate and the Raman linewidth.

Grain-growth enhancement in the YBa2Cu3O7−δ superconductor by silver-oxide doping

Abstract The addition of silver oxide has been found to accelerate the decomposition and melting of the Y-Ba-Cu-O superconductor. The doped sample exhibits, after sintering near 980°C, morphology of very large, stacked-plate grains for the YBa 2 Cu 3 O 7−δ phase and the presence of decomposition products. By contrast, doping with metallic silver does not noticeably induce such an effect. The enhanced grain growth in the Ag 2 O-doped superconductor is most likely to be the main cause of the previously reported increase in magnetization hysteresis and the superconductor suspension effect at 77 K. Beneficial effects of the addition of metallic silver such as enhanced oxygen diffusion and extremely low contact resistance are also discussed.

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.

Low‐resistivity contacts to bulk high Tc superconductors

Convenient methods for obtaining extremely low resistivity contacts to bulk high Tc superconductors (u2009ρc in the range of 10−11–10−12 Ωu2009cm2) are described. Three different configurations of silver contact metal in Y‐Ba‐Cu‐O have been employed, i.e., embedded Ag wire, embedded Ag particles, and selectively patterned Ag clad on superconductor wire. In all three cases, the low‐resistivity metallic contacts are formed in situ during the sintering or melt processing of the superconductor, thus eliminating the need for separate steps of contact preparation such as vacuum deposition of contact metal and additional heat treatment. The distribution and morphology of the silver contacts will be discussed. The measured contact resistivities in the present work are the lowest reported for the high Tc superconductors, and these methods may serve as a useful basis for important contact technologies needed for bulk superconductor applications.

Recovery of 90 K superconductivity in transition‐metal‐doped Y‐Ba‐Cu‐O

Most of the transition metal elements incorporated into the lattice of YBa2Cu3O7−δ are known to severely suppress the superconducting transition temperature. In this work, we show that an essentially complete recovery of the 90 K superconductivity can be achieved by partial melt processing of the material doped with Zr, Ta, Ti, Pt, Rh, and Re, which is attributed to second phase particles draining the dopant elements away from the matrix. Such a full recovery of Tc is not obtained in the case of Nb, V, Ni, Fe, Co, Pd, and Ru. The results are of technological interest in view of the need for nonpoisonous, high melting point substrate, clad or core materials desirable for melt processing of Y‐Ba‐Cu‐O.