Showing papers by "David K. Christen published in 1996"

High critical current density superconducting tapes by epitaxial deposition of YBa2Cu3Ox thick films on biaxially textured metals

Abstract: A method to obtain long lengths of flexible, biaxially oriented substrates with smooth, chemically compatible surfaces for epitaxial growth of high‐temperature superconductors is reported. The technique uses well established, industrially scalable, thermomechanical processes to impart a strong biaxial texture to a base metal. This is followed by vapor deposition of epitaxial buffer layers (metal and/or ceramic) to yield chemically compatible surfaces. Epitaxial YBa2Cu3Ox films grown on such substrates have critical current densities exceeding 105 A/cm2 at 77 K in zero field and have field dependencies similar to epitaxial films on single crystal ceramic substrates. Deposited conductors made using this technique offer a potential route for the fabrication of long lengths of high‐Jc wire capable of carrying high currents in high magnetic fields and at elevated temperatures.

High critical current density superconducting tapes by epitaxial deposition of YBa2Cu3Ox thick films on biaxially textured metals

Abstract: A method to obtain long lengths of flexible, biaxially oriented substrates with smooth, chemically compatible surfaces for epitaxial growth of high‐temperature superconductors is reported. The technique uses well established, industrially scalable, thermomechanical processes to impart a strong biaxial texture to a base metal. This is followed by vapor deposition of epitaxial buffer layers (metal and/or ceramic) to yield chemically compatible surfaces. Epitaxial YBa2Cu3Ox films grown on such substrates have critical current densities exceeding 105 A/cm2 at 77 K in zero field and have field dependencies similar to epitaxial films on single crystal ceramic substrates. Deposited conductors made using this technique offer a potential route for the fabrication of long lengths of high‐Jc wire capable of carrying high currents in high magnetic fields and at elevated temperatures.

Epitaxial YBa2Cu3O7 on Biaxially Textured Nickel (001): An Approach to Superconducting Tapes with High Critical Current Density

Abstract: In-plane—aligned, c axis—oriented YBa2Cu3O7 (YBCO) films with superconducting critical current densities Jc as high as 700,000 amperes per square centimeter at 77 kelvin have been grown on thermomechanically rolled-textured nickel (001) tapes by pulsed-laser deposition. Epitaxial growth of oxide buffer layers directly on biaxially textured nickel, formed by recrystallization of cold-rolled pure nickel, made possible the growth of YBCO films 1.5 micrometers thick with superconducting properties that are comparable to those observed for epitaxial films on single-crystal oxide substrates. This result represents a viable approach for the production of long superconducting tapes for high-current, high-field applications at 77 kelvin.

Comparative study of the characteristic length scales and fields of Hg-based high-Tc superconductors.

Abstract: The equilibrium magnetization was studied in magnetically aligned Hg-based cuprate superconductors with a crystal structure containing one, two, and three adjacent Cu-O layers. The mixed state magnetization was measured with the magnetic field applied perpendicular to the layers, along with low field Meissner effect and normal state susceptibility studies for volume correction factors. The internally consistent analysis is used to identify systematics as the number {open_quote}{open_quote}{ital n}{close_quote}{close_quote} of Cu-O layers increases. The study reveals a linear increase in the slope of upper critical field {minus}{ital dH}{sub {ital c}2}/{ital dT} with {open_quote}{open_quote}{ital n}{close_quote}{close_quote}. Most importantly, we find a nearly constant carrier doping per Cu-O layer. This result is compared with other studies, e.g., Rietveld analysis of neutron diffractometry, and its implications are discussed. {copyright} {ital 1996 The American Physical Society.}

Percolative current flow in high-Jc, polycrystalline high-Tc superconductors

Abstract: In this article, experiments designed to ascertain the percolative nature of current flow in high critical-current density (Jc) polycrystalline superconductors are reviewed. A direct correlation between the grain-orientation texture with current transport in high-Jc TlB2Ca2Cu3Ox thick films and Bi2Sr2Ca2Cu3Ox powder-in-tube tapes is obtained. Magnetooptical visualization of the current flow in the same regions confirms the percolative nature of current flow. Furthermore, numerical modeling of the current flow based on the observed grain boundary character distribution and the effects of the grain boundary misorientation angle on (Jc) was found to predict the percolative nature of current flow. These studies indicate that in order to increase the (Jc) further, increased fractions of small-angle boundaries are required and, hence, provide a research direction for the next generation of high-temperature superconducting wire.

Composition and phase development of epitaxial superconducting Tl0.78Bi0.22Sr1.6Ba0.4Ca2Cu3O9−δ thin films by laser ablation and post annealing

Abstract: For the formation of epitaxial superconducting Tl 0.78 Bi 0.22 Sr 1.6 Ba 0.4 Ca 2 Cu 3 O 9−δ ((Tl,Bi)-l223) thin films, the dependence of phase development on the chemical state of the source materials, deposition distance from source to substrate, and O 2 pressure during deposition by laser ablation and post annealing was studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). It was found necessary to have thallium in the precursor film and that a reacted source target is better than the unreacted source for obtaining (Tl,Bi)-1223 phase-pure films. Better (Tl,Bi)-1223 films were obtained at a deposition distance of 44 mm from source to substrate than at a distance of 61 mm over a range of O2 pressure from 5.7 to 31.3 mTorr The best films prepared under the optimal conditions exhibited transport J c close to 3 × 10 6 A/cm 2 at 77 K and zero field.

The impact of tailored defects on length scales and current conduction in high-T c superconductors

Abstract: This article surveys the formation and effects of “tailored” defects, having controlled numbers and several different morphologies, in high temperature superconductors. Defects can affect the equilibrium properties, such as the superconducting length scales ξ (the coherence length) and λ (the London penetration depth). Very importantly, defects provide vortex pinning that supports the conduction of a macroscopic current density. The article introduces these topics and illustrates them with specific examples.

Superconductivity in the indium-doped Tl-1223 phase: (Tl0.8In0.2)(Sr0.8Ba0.2)2Ca2Cu3O9−δ

Abstract: We report the synthesis and characterization of a new Tl- and In-based ‘1223’ type cuprate oxide, (Tl 0.8 In 0.2 )(Sr 0.8 Ba 0.2 ) 2 Ca 2 Cu 3 O 9−δ (Tl,In-1223). The as-synthesized Tl,In-1223 compound becomes superconducting at about 80 K. The T c increased to 103 K when the sample was annealed under an argon atmosphere. Magnetization studies on polycrystalline samples of Tl,In-1223 reveal that persistent current densities ( J p ) of the indium-doped compound are about 10 times smaller than those of corresponding Tl,Bi-1223 with the same structure type. Small substitution of In for Tl in Tl-1223 results in a large reduction of T c and J p . A slight change in the electronic configurations of the rock-salt metal sites appears to affect the delicate balance of the metal and oxygen energy levels, which leads to the virtual changes in the superconductivity in Tl,In-1223.

Biaxially oriented metallic tape substrates for high-temperature superconductors

Abstract: We report a practical synthesis route for the production of long length, biaxially-oriented high-temperature superconducting (HTS) deposits on metallic tapes. The technique involves epitaxial vapor deposition of appropriate oxide buffer layers on rolled, thermo-mechanically processed fcc-structure base-metal tape substrates. Short segments of ≈1 μm thick YBa2Cu3O7-δ deposits produced by pulsed laser ablation on CeO2 and yttria-stabilized ZrO2 (YSZ) buffered nickel exhibit epitaxial biaxial alignment and have critical current densities over 106 A/cm2 at 77 K in zero applied magnetic field.

Equilibrium magnetic studies of Hg-based high-Tc superconductors

Abstract: We have studied the equilibrium magnetization of magnetically aligned Hg-based cuprate superconductors containing 1, 2, or 3 adjacent Cu−O layers. As the number of layers increased, the deduced carrier doping per Cu−O layer remained nearly constant, while the slope of the upper critical field—dHc2/dT increased linearly.

Low-cost metal substrates for films with aligned grain structures

Abstract: Polycrystalline metal substrates that possess a significant amount of in-plane and out-of-plane crystallographic texture have recently been developed for high-temperature superconducting film applications. These substrates enable the virtual elimination of large angle grain boundaries in subsequent epitaxial films, having been successfully utilized in various oxide thin film architectures. This paper describes the characteristics of these substrates, and briefly discusses their potential applicability in polycrystalline thin-film photovoltaic applications.

Transport behavior in superconductors at extreme dissipation levels

Abstract: A number of fundamental physical phenomena unfold in the mixed state of superconductor, when subjected to enormous current and power-dissipation levels. A sufficiently large current can destroy the superconducting state itself--the so-called pair-breaking effect. At intermediate current densities, below the onset of pair-breaking, one expects to see the free viscous flow of flux vortices. In the present work a pulsed-current technique was used to explore this dissipative regime of high-{Tc} superconductors, verifying both free flux flow and the pair-breaking effect, as predicted by traditional theories. This paper concentrates on the dissipation and Hall behavior in the free flux flow state.

Defect formation and carrier doping in epitaxial films of the infinite layer compound

Abstract: The correlation between defect formation and carrier doping in epitaxial films of the infinite layer compound SrCuO{sub 2} has been studied via molecular beam epitaxy controlled layer-by-layer growth experiments, chemically resolved scanning transmission electron microscopy, scanning tunneling microscopy, x-ray diffraction, electrical transport measurements, and post-growth oxidation-reduction annealing. Based on the complementary information provided by these experiments, it is concluded that the carrier doping is dominated by the formation of an electron-doped, Sr and O deficient matrix under mildly oxidizing growth conditions. Hole-doping is induced by extended defects containing excess Sr atoms and may lead to superconductivity after high-temperature oxidation.

Dynamics of vortices pinned by columnar defects in YBa2Cu3O7 crystals

Abstract: We have studied vortex dynamics in YBa2Cu3O7 crystals containing columnar defects (oriented ⊥ to Cu−O planes), formed by 1 GeV Au-ions. The decay rate-dlnj/dlnt of persistent-current j has a large peak near T=40 K for H =1 T. Higher fields suppressed the height and position of this peak, which disappeared for H≥Bϕ, the matching field. Analyzing the pinning energy shows regimes with several modes of pinning.

Hysteretic magnetic moment of YBa2Cu3O7−δ: An angular study

Abstract: The hysteretic magnetization of an YBa2Cu3O7−δ crystal containing a high density of Y2BaCuOx precipitates was investigated by two-axis, quasistatic, de magnetometry. As the eff-axis applied magnetic fieldH was increased and the magnetization magnitude, decreased, its orientation approached a direction near the crystallinec axis. The bulk persistent currentsJ p predominately flowed parallel to the Cu−O planes for a wide range of temperature and applied field. Even forH almost perpendicular toc, where the deduced irreversibility line maximized as commonly observed, the resulting magnetization was observed to be within 15° of [001].