R. A. Fastnacht

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 ∼17 000 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.

Large magnetic hysteresis in a melt‐textured Y‐Ba‐Cu‐O superconductor

Melt‐textured YBa2Cu3O7−δ superconductor with a long and well‐aligned grain structure is shown to exhibit magnetic hysteresis at 77 K which is the largest ever reported for bulk polycrystalline Y‐Ba‐Cu‐O. The large ΔM resulted in a magnet‐like behavior as well as a strong suspension phenomenon. It is shown, contrary to the previous reports, that the suspension behavior observed in Y‐Ba‐Cu‐O is a generic consequence of large grain size, and not due to the presence of Ag oxide or Ag particles. There appears to be no substantial enhancement in flux pinning and Jc by these particles. Comparisons of magnetization behavior in various YBa2Cu3O7−δ samples (polycrystals, silver‐oxide doped, melt‐textured, and single crystals) indicated that for applied fields substantially larger than Hc1 the current flow that gives rise to the observed magnetization is intragranular. The pinning force and hence the critical current is roughly the same [Jc (magn)∼104 A/cm2] within a factor of ∼2 regardless of grain size, grain bou...

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 920 °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.

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.

Low‐resistivity contacts to bulk high Tc superconductors

Convenient methods for obtaining extremely low resistivity contacts to bulk high Tc superconductors ( ρc in the range of 10−11–10−12 Ω cm2) 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.

Fabrication and properties of high‐Tc superconducting wires

The discovery of high‐temperature superconductivity in perovskite‐related oxides has generated an enormous amount of research activity and development effort toward applications. Commercially useful bulk superconductors typically require stabilization using a normal metal cladding for reasons of electrical, thermal, and mechanical protection, and in general need to be fabricated into fine fibers and wound into a solenoid configuration. The YBa2Cu3O7−δ type compound is a ceramic material which is mechanically hard and brittle, and is difficult to fabricate into fine wires. However, this difficulty has been overcome by several fabrication methods such as powder‐in‐tube method (metal‐clad composite), powder‐coating method (metal‐core composite), and molten oxide processing method. The effect of various processing steps on the structure and superconducting properties such as transition temperature and critical current density will be discussed. Some experiments to raise the high field critical current through...

Transport measurement of 32 K superconductivity in the Ba‐K‐Bi‐O system

We have successfully measured the electrical resistance of Ba‐K‐Bi‐O as a function of temperature and magnetic field, and thus confirmed the existence of superconductivity in the compound by transport measurement. The results from dense, black‐colored samples prepared from a starting composition with excess potassium (Ba0.6K1.2BiOx) indicate Tc (onset)∼32 K and Tc (R=0)∼22 K with a slightly negative temperature dependence of normal state resistance [R(300 K)/R(33 K)∼0.8]. The strong field dependence of the superconducting transition (500–1000 Oe/K) indicates the likelihood of a relatively low Hc in Ba‐K‐Bi‐O.

Superconducting Properties of YBa 2 Cu 3 O 7-δ Doped with Various Metals and Oxides

The interactions of YBa 2 Cu 3 O 7-δ type high T c superconductors with other metals and oxides are of significant technical importance because of the need for i) proper stabilizing normal metal for composite superconductor wire, ii) nonreactive crucible materials for melt processing or crystal growth, and iii) suitable nonpoisonous substrate materials for thin film/thick film superconducting devices. For these reasons, and also for the purpose of exploring possible improvements in T c , J c and mechanical properties, the effects of various metal and oxide additions (1–40% by weight) have been investigated. It is shown that many of the elements in the periodic table deteriorate the superconducting properties to a various degree ranging from a broadened transition or reduced T c to a complete elimination of the superconducting behavior. However, silver, gold and cadmium were relatively benign or slightly improved the properties. These benign materials have potential for practical application in superconducting composites.

Properties of high Tc superconductors with modified compositions (abstract)

The recent discovery of high Tc superconductor materials such as Ba2YCu3O9−x stimulated worldwide interest in the subject. The interactions of the superconducting oxides with other oxides and metals are of considerable technical importance because of the need for (i) proper stabilizer for superconductor‐normal metal composite wire, (ii) nonreactive crucible materials for crystal growth, and (iii) suitable substrate materials for thin‐film/thick‐film superconductor devices. In this paper, the effects of various metal and oxide additions (up to 10%–50% by weight) on superconducting transition properties have been investigated using magnetic susceptibility and resistivity measurement as a function of temperature. Fine particles of metals and oxides were intimately mixed with the Ba2YCu3O9−x powder, pressed into pellets, and then sintered at 900–1000 °C followed by slow cooling in oxygen atmosphere to achieve chemical homogeneity, desirable crystal structure and oxygen stoichiometry. Some of the oxides and me...