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  • 1985-1989  (27)
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Year
  • 1
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 53 (1988), S. 2105-2107 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Correlations between observed defect structures and the orientation of planes involved in grain boundary formation in single phase, nearly ideal density Ba2YCu3O7−x are presented. Three types of grain boundary structures are observed: (1) coherent boundaries with only a low-energy network of dislocations at the interface, which are associated with boundaries not involving a basal plane face, (2) semicoherent boundaries surrounded by a 50–1500 A(ring) wide band of dislocation loops on the c-axis side of the basal-plane-faced boundary, and (3) incoherent boundaries where severe local strain has produced substantial plastic deformation and generation of small voids, again associated with basal-plane-faced boundaries. These results are interpreted using a model for local stress based on the known highly anisotropic thermal contraction of the material during cooling from sintering temperatures.
    Type of Medium: Electronic Resource
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  • 2
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 52 (1988), S. 2074-2076 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: 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.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 52 (1988), S. 1910-1912 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We report transport and magnetic measurements of critical current densities in single crystals of high Tc superconductors in the Bi-Sr-Ca-Cu-O system. Transport measurements of Jc were made using a pulse method, while a vibrating sample magnetometer was used to measure M-H loops of oriented crystals in fields up to 15 kOe. Values for transport Jc in zero applied field are higher than those of Ba2YCu3O7 single crystals at comparable temperatures. Also we observe a strong magnetic field dependence for Jc at temperatures above ∼20 K.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 55 (1989), S. 283-285 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We have extended Bean's critical state model to explicitly include anisotropic critical currents. Measurements at 30 K of the critical currents parallel to the Cu-O planes but with vortex motion either parallel or across twin boundaries show twin boundaries are probably not an important cause of vortex pinning. In the critical state, current flow perpendicular to the Cu-O planes is about 30 times smaller than flow parallel to these planes.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 54 (1989), S. 2605-2607 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: 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.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 54 (1989), S. 763-765 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We have measured the resistance below Tc of single crystals of the high-temperature superconductors Ba2YCu3O7 and Bi2.2Sr2Ca0.8Cu2O8+δ in magnetic fields up to 12 T. The resistive transition of both compounds is dominated by intrinsic dissipation which is thermally activated, resulting in an exponential temperature dependence of the resistivity well below Tc. The dissipation is significantly larger and of different character in the Bi-Cu compound than in Ba2YCu3O7. The relation between the activated behavior and the depinning critical current is discussed.
    Type of Medium: Electronic Resource
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  • 7
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 54 (1989), S. 584-586 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: 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 boundary configuration, presence of second phase particles, or increased dislocation and twin densities.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 51 (1987), S. 542-544 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Thin films of Fe or Co and rare-earth metals, such as Tb and Gd, are being developed as high density, mass memory storage media in magneto-optic devices. It is shown that a 10-nm duplex film consisting of a metal that is passivated in air, such as Cr or Al, and a diffusion barrier, such as Nb, will protect these alloys against oxidation and corrosion at temperatures up to at least 200 °C. The duplex film is better than a single passivated metal layer, which may interdiffuse with the alloy, or than a dielectric, which may be reduced by the reactive rare-earth element.
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 51 (1987), S. 203-204 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: 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 drawn into fine fibers and wound into a magnet configuration. The recent discovery of high-TC superconductor materials such as Ba2YCu3O7 stimulated worldwide interest in the subject, however, with much concern about fabricability of such brittle ceramic materials into desirable fine wire geometry. In this letter, we report preliminary success in the fabrication of fine-wire, composite superconductors consisting of a high-conductivity normal metal shell such as Ag or Cu/Ni/Au and a superconducting core of Ba2YCu3O7 oxide. The wire is would into a coil, and then heat treated to produce the desired chemistry in a dense structure. The resistivity of the composite wire is measured to be zero at ≈90 K (in zero field) with a zero-field critical density of ≈175 A/cm2. Microscopy and x-ray analysis show that the superconducting core is continuous, and retains phase composition after wire drawing and heat treatment. These results demonstrate the feasibility of producing superconducting ceramic composite wire, although the practical applications must await the resolution of many practical problems and reliability issues.
    Type of Medium: Electronic Resource
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  • 10
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 53 (1988), S. 2223-2225 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: We have measured the critical currrent density Jc as a function of temperature for a Bi-Sr-Ca-Cu-O sample containing a substantial fraction of a phase with Tc≈110 K and for a Ba2YCu3O7 sample (TTc≈92 K). The behavior of Jc was deduced from magnetization measurements. For both samples the magnetization at fixed temperature and field decayed logarithmically with time. The values of Jc so derived, allowing for the decay to occur, went to zero for temperatures above 35 K for the Bi-Sr-Ca-Cu-O ceramic and to zero above 70 K for the Ba2YCu3O7 sample. These values are substantially lower than those deduced from dynamic M-H loops.
    Type of Medium: Electronic Resource
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