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Anisotropy in the flux-line lattice symmetry of a low-kappa Type-II superconductor

Description: Correlation between the symmetry of the two-dimensional flux-line lattice (FLL) and the real crystal lattice (CL) has been studied in a superconducting niobium sphere by means of small-angle neutron diffraction. A double-perfect silicon-crystal diffractometer enabled precise determination of the three inter-fluxoid distances corresponding to th e FFL basic cell. A systematic study of the anisotropic behavior was made as a function of temperature and magnetic-field amplitude for fields parallel to a few high-symmetry CL axes in the (1 anti 1 0) plane. In addition, at T = 4.30 K progressive deformation of the FLL was studied as the sample was rotated in the (1 anti 1 0) and (100) planes. The FLL was found to be hexagonal only for fields parallel to the three-fold CL axis. Two-fold symmetry prevailed for other CL directions in these planes except near the four-fold axis, where either of two distorted triangular lattices existed, preserving the reflection symmetry in composite, but not individually. This behavior will be discussed in terms of current models for fluxoid-CL interactions; in particular, in terms of the extension of the Takanaka theory done by Delrieu, Roger, and Kahn.
Date: January 1, 1977
Creator: Thorel, P. & Christen, D.K.
Partner: UNT Libraries Government Documents Department

Determination of low-field critical parameters of superconducting niobium by small-angle neutron diffraction

Description: The perfect double-crystal small-angle diffraction technique enables measurement of scattering angles to within 0.3 arc sec. accuracy. At a wavelength of 2.55 A, this provides a resolution of 3 x 10/sup -6/ A/sup -1/ in the scattering vector. This technique has been used to study the anisotropic behavior of the critical parameters B/sub 0/ and H/sub c1/, characteristic of the first-order magnetic phase transition which occurs in low-kappa type-II superconductors. Magnetic fields were applied parallel to several crystal axes of a large single-crystal sphere of pure niobium, resulting in well-defined flux-line lattices (FLL). Measurement of the FLL cell area in the intermediate mixed state field region gives the equilibrium flux density B/sub 0/, which results from an attractive interaction between fluxoids. In addition, field variation of the scattered neutron intensity allows measurement of the transition field between the mixed state and intermediate mixed state. This transition field is related to the lower critical field H/sub c1/ and enables its determination to a precision 0.2%. Data at T = 4.3 K display a small anisotropic effect of about 2% in B/sub 0/ and 1% in H/sub c1/. Although orientation effects of this magnitude are difficult to resolve by bulk measurements, the neutron data are in accord with magnetization data. Observations regarding the temperature dependence of these parameters also will be presented, and comparisons made with current theoretical models.
Date: January 1, 1977
Creator: Christen, D.K.; Spooner, S.; Thorel, P. & Kerchner, H.R.
Partner: UNT Libraries Government Documents Department

Final Report on CRADA ORNL05-0703

Description: The work of this CRADA has been focused on the development of Rolling-Assisted Biaxially Textured Substrate (RABiTS)-based high-temperature superconducting (HTS) coated conductor technology that is in the pre-commercial development stage. Metal-Oxide Technologies, Inc. (MetOx) is a Houston-based small business that is developing and manufacturing second-generation (2G) HTS wire using an all-Metallo-Organic Chemical Vapor Deposition (MOCVD) process, including the buffer layers and HTS coating. Advances toward commercialization were enabled by coordinated interactions that facilitated the synthesis, characterization, and iterative optimization of prototype 2G wire segments.
Date: April 27, 2010
Creator: Christen, D.K.
Partner: UNT Libraries Government Documents Department

Development of Biaxially Textured YBa{sub 2}Cu{sub 3}O{sub 7} Coated Conductors in the U.S.

Description: Two new processes have been under development since 1991 that promise a new, cost-effective way to manufacture flexible, high current density wires made from YBa{sub 2}Cu{sub 3}O{sub 7} (YBCO). The key is to prepare a textured substrate, or ''template,'' on which the YBCO may be deposited as a biaxially aligned thick film. Ion beam assisted deposition (IBAD) of yttria stabilized zirconia or magnesium oxide on alloy tapes enables a final superconducting layer with grain-to-grain, in-plane alignment to within 3-5 degrees. Similar results are achieved on rolling-assisted, biaxially textured substrates (RABiTS) using a variety of oxide layers on textured nickel tapes. The performance of research lengths of prototype wires in strong magnetic fields at 65 K already exceeds that of NbTi and Nb{sub 3}Sn in liquid helium. A scalable, ex-situ process for the YBCO coating has been demonstrated on both types of substrates. Consistent values of critical current density (J{sub c }) greater than 1 x 10{sup 6} A/cm{sup 2} are now obtained on RABiTS, and J{sub c}'s in excess of 2 x 10{sup 6} A/cm{sup 2} have been obtained on both substrates. A nonmagnetic variation of RABiTS (Ni-13% Cr) has also been shown to yield Jc greater than 1.5 x 10{sup 6} A/cm{sup 2} . Six private companies in the U.S. are scaling up YBCO coated conductors for power and physics applications.
Date: November 13, 1999
Creator: Christen, D.K.; Hawsey, R.A. & Kroeger, D.M.
Partner: UNT Libraries Government Documents Department

{open_quotes}Local texture, current flow, and superconductive transport properties of Tl1223 deposits on practical substrates{close_quotes}

Description: Quantitative investigations of the crystal grain orientations and electrical transport properties of high temperature superconducting (HTS)TiBa{sub 2}Ca{sub 2}Cu{sub 3}O{sub 8+x} (Tl1223) deposits on polycrystalline substrates show that current flow comprises percolative networks of strongly-coupled material. Superconductive transport properties on different samples, on the same samples at different widths, and on samples with artificially-induced strong flux pinning defects confirm the nature of current flow, and suggest that these materials may be useful as a new class of HTS conductors.
Date: May 1, 1996
Creator: Christen, D.K.; Specht, E.D. & Goyal, A.
Partner: UNT Libraries Government Documents Department

Fabriacation and properties of high-J{sub c}, biaxially aligned YBa{sub 2}Cu{sub 3}O{sub 7-delta} thick films on metallic tape ...

Description: We report the synthesis and properties of high-J{sub c}, biaxially aligned YBCO films deposited on thermo-mechanically textured Ni tapes. Sharply cube-textured Ni tapes, 125 {mu}m thick, were produced by mechanical rolling followed by recrystallization anneal. Short segments were coated with epitaxial oxide buffer layers, followed by fully aligned YBCO films to thicknesses of 1 to 3 {mu}m. In-plane textures of 7-10{degree} FWHM are achieved, with c-perpendicular alignment to 1{degree} FWHM. Typical zero-field J{sub c} values are in the range 5-9x10{sup 5} A/cm{sup 2} at 77 K, with strong behavior in magnetic fields comparable to that of epitaxial films on single crystal oxides. Assessment of properties necessary for a tape conductor technology are discussed.
Date: March 1, 1997
Creator: Christen, D.K.; Norton, D.P. & Goyal, A.
Partner: UNT Libraries Government Documents Department

Transport properties of Tl1223 deposits for possible conductor applications

Description: We have investigated the microstructure and electrical transport properties of high-temperature superconducting (HTS) TlBa{sub 2}Ca{sub 2}CU{sub 3}0{sub 8{plus}x} (Tl1223) deposits on polycrystalline substrates of YSZ and silver. Samples were formed by deposition of a n-free or TI-poor precursor film, followed by thermal reaction with Tl{sub 2}0 vapor. The resulting deposits have c-axis perpendicular texture with local in-plane alignment, which leads to a colony-type microstructure and improved electrical transport properties. Transport properties on different samples, on the same samples at different widths, and on samples with artificially-induced strong flux pinning defects lead to the conclusion that a relatively small fraction of the material comprises strongly-coupled current paths. Results on 3-20 {mu}m thick deposits on silver indicate that these materials may be useful as a new class of HTS conductors.
Date: December 1, 1995
Creator: Christen, D.K.; He, Q. & Paranthaman, P.
Partner: UNT Libraries Government Documents Department

Experimental determination of the temperature-dependent penetration depth in V/sub 3/Si

Description: Small angle neutron diffraction from the flux-line lattice (FLL) in a high quality, single crystal of superconducting V/sub 3/Si has been used to deduce the low-field penetration depth lambda (T). An absolute determination is possible because the FLL form factor F/sub hk/ is essentially single-valued in the scattering vector magnitude absolute value of K/sub hk/, as well as nearly London-like at low field. We obtain lambda (0) = 102 +- 1 nm, 20% to 30% larger than previous determinations of the London penetration depth lambda/sub L/ (0). The temperature dependence of lambda (T) is found to deviate from that of the BCS theory. An assessment of the data indicates the most prominent source of the discrepancy is due to strong electron-phonon coupling, and we find 2..delta..(0)/k/sub B/T/sub c/ = 3.88 +- 0.07, in reasonable agreement with values found in the literature.
Date: March 1, 1984
Creator: Christen, D.K.; Kerchner, H.R.; Sekula, S.T. & Chang, Y.K.
Partner: UNT Libraries Government Documents Department

Equilibrium properties of the fluxoid lattice in single-crystal niobium

Description: The dimensions and symmetry of the fluxoid lattice in a single-crystal sphere of niobium have been measured by using a double-perfect-crystal small-angle neutron-scattering technique (DCSANS). The bulk magnetization of the same sample has been measured by a field-sweep technique. In addition, the misalignment between the fluxoids and the applied magnetic field was observed by DCSANS. The experimental methods and most of the results are reported elsewhere. The findings are reported here, and the measurements are compared with realistic microscopic theory where it is available.
Date: June 1, 1979
Creator: Kerchner, H.R.; Christen, D.K.; Sekula, S.T. & Thorel, P.
Partner: UNT Libraries Government Documents Department

The influence of N-ion irradiation on superconducting TiN

Description: The influence of disorder on the properties of the B1 phase superconductive compound titanium nitride has been investigated. Following synthesis and characterization of thin films of TiN, the materials were disordered progressively and uniformly by irradiation with nitrogen ions. The superconducting transition temperature T/sub c/ decreased and the low temperature electrical resistivity rho increased as lattice damage was introduced. The dependence of both T/sub c/ and rho on N-ion fluence phi could be described by exponentially saturating functions of phi with similar rate constants. These constants correspond to characteristic damage energy densities of 14 and 12 eV/atom, respectively.
Date: May 1, 1987
Creator: Thompson, J.R.; Ellis, J.T.; Christen, D.K.; Sekula, S.T.; Lewis, J.D. & Williams, J.M.
Partner: UNT Libraries Government Documents Department

Transport behavior in superconductors at extreme dissipation levels

Description: 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.
Date: September 1, 1996
Creator: Kunchur, M. N.; Christen, D. K. & Ivlev, B. I.
Partner: UNT Libraries Government Documents Department

Grain boundary transport properties in YBa{sub 2}Cu{sub 3}O{sub x} coated conductors.

Description: Critical current data obtained as a function of magnetic field on an isolated grain boundary (GB) of a coated conductor and two other types of bicrystal GBs of YBa{sub 2}Cu{sub 3}O{sub x} show a peak in the critical current and an unusual hysteresis. These results provide support for a new mechanism for enhanced GB critical currents, arising from interactions of GB vortices with pinned Abrikosov vortices in the banks of a GB, as suggested by Gurevich and Cooley. A substantial fraction of this enhancement, which can exceed a factor of ten, also occurs upon surpassing the critical current of the grains after zero field cooling. A bulk GB and thin film GBs show qualitatively identical results.
Date: November 2, 2000
Creator: Berghuis, P.; Miller, D. J.; Kim, D. H.; Gray, K. E.; Feenstra, R. & Christen, D. K.
Partner: UNT Libraries Government Documents Department

Flux-line pinning by the grain boundary in niobium bicrystals

Description: Flux-line pinning by the grain boundary in niobium bicrystals was investigated by making four-terminal measurements of the critical current and neutron diffraction measurements of the flux-line bending. The elementary pinning interaction is estimated from the critical current data by using an approximate theory of the current distribution on the grain boundary. The data reported here are mainly for a sample in which the applied magnetic field is parallel to the (111) and the (001) crystal directions in the two grains when it is in the plane of the boundary and perpendicular to the current direction. Evidence is seen of grain boundary faceting and of a flux-flow rectification effect that peaks as a function of temperature below 3/sup 0/K. The scale of the grain-boundary pinning is consistent with the quasiparticle-scattering theory.
Date: November 1, 1985
Creator: Kerchner, H.R.; Christen, D.K.; Lee, E.M.; DasGupta, A.; Cai, B.C. & Chou, Y.T.
Partner: UNT Libraries Government Documents Department

Experimental characterization of the flux-line lattice in superconducting V/sub 3/Si

Description: Several microscopic properties of the flux-line lattice (FLL) in three separate single crystals of V/sub 3/Si have been investigated by means of small-angle neutron diffraction. These low-field FLL characterizations have been correlated with the following material and superconducting properties: (1) the real crystal symmetry parallel to the applied magnetic field; (2) the micro-structure as determined by TEM; (3) magnetic irreversibilities in the mixed state; (4) reversible flux-line motion in ac response; and (5) martensitic structural transformation observed by x-ray diffraction. The three samples, V/sub 3/Si-MP3, -MP4, and -MP5 possessed different defect structures, and this was manifested foremost in the FLL perfection. At low field (B<0.2 T) and T=4.85/sup 0/K, only MP3, which is free of second-phase precipitates, showed a highly resolved FLL. Sample MP5 contains a low density of small (200 A) coherent precipitates, and revealed well-defined FLL Bragg peaks for B greater than or equal to 0.5 T, but a highly mosaic, nearly polycrystalline FLL at lower fields. Sample MP4 contained large (500 to 1000 A) incoherent precipitates, and showed only a polycrystalline FLL at low field. In both MP3 and MP5, distinct anisotropic correlations were observed between the FLL morphology and the real-crystal direction along the applied field. The FLL perfection was strongly dependent on the growth history. The peak width history dependence for two different scattering geometries can be qualitatively modeled by proposed flux-pinning mechanisms. Quantitative comparisons with critical current measurements, however, are not totally reconcilable.
Date: January 1, 1982
Creator: Christen, D.K.; Kerchner, H.R.; Narayan, J.; Chang, Y.K.; Larson, B.C. & Sekula, S.T.
Partner: UNT Libraries Government Documents Department

Fast neutron damage studies of La/sub 1. 85/Sr/sub 0. 15/CuO/sub 4/

Description: The effect of fast neutron damage on the superconducting transition temperature T/sub c/ and the critical current density j/sub c/ of the compound La/sub 1.85/Sr/sub 0.15/CuO/sub 4/ has been investigated. Irradiation to a fluence of 1.3 x 10/sup 18/ n/cm/sup 2/ (E > 0.1 MeV) resulted in a decrease of T/sub c/ of about 3 K while j/sub c/ at 4.2 K and H = 2 T increased by a factor of two to 1.2 x 10/sup 4/ A/cm/sup 2/.
Date: April 1, 1987
Creator: Sekula, S.T.; Christen, D.K.; Kerchner, H.R.; Thompson, J.R.; Boatner, L.A. & Sales, B.C.
Partner: UNT Libraries Government Documents Department

Effect of the colony microstructure on transport critical current of high-J{sub c} Tl-1223 thick films

Description: High J{sub c}, polycrystalline, TlBa{sub 2}Ca{sub 2}Cu{sub 3}O{sub 8+x} thick films on poly-YSZ substrates were examined by x-ray microdiffraction, electron microscopy, and electron backscatter diffraction. The superconducting phase was formed by vapor phase thallination. X-ray phi scans were used to study the local and macroscopic in-plane texture. Electron backscatter kikuchi diffraction was used to discern the grains structure within the colonies. Figure shows results of a limiting path calculation of J{sub c} vs colony mosaic.
Date: June 1, 1995
Creator: Goyal, A.; Specht, E.D.; Kroeger, D.M.; Christen, D.K.; Tkaczyk, J.E.; DeLuca, J.A. et al.
Partner: UNT Libraries Government Documents Department

Investigation of the local superconducting properties at grain boundaries in high-Tc superconductors

Description: Developing an atomic scale study of the structure-property relationships of grain boundaries in high-{Tc} superconductors is essential to understand their current dissipation mechanism and for incorporating these materials into viable devices. Thin YBa{sub 2}Cu{sub 3}O{sub 7{minus}{delta}} films have been deposited by pulsed laser deposition (PLD) on SrTiO{sub 3} symmetric bicrystals. Transport measurements in a magnetic field have been conducted across the grain boundaries through a wide bridge. The data obtained are consistent with microstructural observation in a VG Microscopes HB603 U and a VG HB501 UX dedicated STEM. Of particular interest in the study of high-{Tc} materials is the use of EELS, which can highlight the presence of non-superconducting regions through interpretation of the onset positions and fine-structure (ELNES) of characteristic core-edge features. This preliminary investigation has shown that the study of the grain boundary electronic microstructure in parallel with transport properties study should lead to a better understanding of the transport properties of the superconducting materials. In particular, it has been shown that transport measurements are consistent with a model where the grain boundaries are composed of weak links separated by a very small fraction of much strong links although more boundaries need to be studied to draw definitive conclusions and determine the real nature of these strong links.
Date: February 1, 1998
Creator: Prouteau, C.; Duscher, G.; Browning, N.D.; Pennycook, S.J.; Verebelyi, D.; Christen, D.K. et al.
Partner: UNT Libraries Government Documents Department

Defect formation and carrier doping in epitaxial films of the ``parent`` compound SrCuO{sub 2}: Synthesis of two superconductors descendants

Description: The infinite layer or parent compounds ACuO{sub 2} (A: Ca-Sr-Ba) constitute the simplest copper oxygen perovskites that contain the CuO{sub 2} sheets essential for superconductivity. The stabilization of these basic ``building blocks`` as epitaxial films, therefore, provides alluring opportunities towards the search for new superconducting compounds and elucidation of the underlying mechanisms. In this work, general trends of the defect formation and carrier doping for epitaxial films of the intermediate endmember SrCuO{sub 2} are reviewed. First results are presented from successful attempts to induce hole-doped superconductivity via the processing-controlled incorporation of charge reservoir layers.
Date: April 1, 1995
Creator: Feenstra, R.; Norton, D.P.; Budai, J.D.; Jones, E.C.; Christen, D.K. & Kawai, T.
Partner: UNT Libraries Government Documents Department

Magnetic and Flux Pinning in Hg-Based HTS Thin Films

Description: We have studied the critical current density (J{sub c}) as functions of temperature and magnetic field in epitaxial c-axis oriented Hg-1212 and Hg-1223 thin films. J{sub c} near 2 MA/sq cm at 100K and zero field can be achieved when the weak links between the grains are minimized through epitaxial growth of the film. This results in high irreversibility lines in these films. With the addition of columnar defects, the irreversibility line can be further increased within a certain field range. Moreover, an interesting scaling change with increasing pinning is attributed to an improved pancake vortex coupling along the c-axis of the film.
Date: January 1, 1998
Creator: Kang, B. W.; Ren, Z. F.; Yun, S. H.; Siegal, M. P. & Christen, D. K.
Partner: UNT Libraries Government Documents Department

Growth and transport properties of Y-Ba-Cu-O/Pr-Ba-Cu-O superlattices

Description: The pulsed-laser deposition method has been used to fabricate epitaxial, nonsymmetric M(Y) {times} N(Pr) superlattices in which YBa{sub 2}Cu{sub 3}O{sub 7-x} (YBCO) layers either M = 1,2,3,4,8, or 16 c-axis unit cells thick are separated by insulating PrBa{sub 2}Cu{sub 3}O{sub 7-x} (PBCO) layers N unit cells thick (N = 1 to {approximately}32). The zero-resistance superconducting transition temperature, T{sub c0}, initially decreases rapidly with increasing PBCO layer thickness, but then saturates at T{sub c0} {approximately} 19 K, 54 K, 71 K, or 80 K, or structures containing 1-,2-,3-, or 4-cell-thick YBCO layers, respectively. Critical current density measurements carried out on structures with 16- or 32-cell thick YBCO layers show that the magnitude of J{sub c}(H = 0) {approximately} 1-2 MA/cm{sup 2}, as well as the magnetic field dependence and the anisotropy of J{sub c}(H) all are in good agreement with corresponding measurements on thicker, single-layer YBCO films. Thus, there is no evidence of an enhanced J{sub c}(H) due to the multi-layered structure, for the layer thickness investigated to date. The systematic variation of T{sub c0}, as a function of the YBCO and PBCO layer thickness, is discussed in light of other recent experiments and theoretical model calculations. The superlattices' structural and compositional order are characterized using x-ray diffraction, transmission electron microscopy, and scanning tunneling microscopy, and details of the pulsed-laser deposition process are reported. 42 refs., 7 figs.
Date: January 1, 1990
Creator: Lowndes, D.H.; Norton, D.P.; Budai, J.D.; Christen, D.K.; Klabunde, C.E.; Warmack, R.J. et al.
Partner: UNT Libraries Government Documents Department

Properties of doped and undoped (Ca,Sr)CuO{sub 2} thin films

Description: The authors have studied the transport and structural properties of (Ca,Sr)CuO{sub 2}, Sr{sub 1{minus}y}Nd{sub y}CuO{sub 2}, and Sr{sub 1{minus}x}CuO{sub 2{minus}{delta}} thin films grown by pulsed-laser deposition. Stoichiometric {open_quotes}infinite layer{close_quotes} (Ca,Sr)CuO{sub 2} thin films grown over a large range of growth conditions are insulators, while superconductivity is observed in Sr{sub 1{minus}y}Nd{sub y}CuO{sub 2} films with {Tc}(onset) {approximately}28 K for y = 0.10. A Nd solubility limit of y = 0.10 is observed with the appearance of a new phase with c {approximately} 0.37 nm for y > 0.10. In addition, the transport and structural properties of Sr{sub 1{minus}x}CuO{sub 2{minus}{delta}} thin films grown by pulsed-laser deposition support the contention that the tetragonal phase is capable of accommodating a significant density of alkaline-earth deficiencies up to x {le} 0.3. Resistivity measurements indicate a significant change in the carrier density of the CuO{sub 2} planes as Sr vacancies are introduced. In addition, an enigmatic anomaly in resistivity at 185 K is observed for Sr{sub 0.85}CuO{sub 2{minus}{delta}} thin films. Magnetic measurements on these samples indicate that, although a significant drop in resistivity at 185 K is observed, it is not due to a superconducting transition. Hall measurements, as well as changes in resistivity with film growth conditions, suggest that the majority carriers in these Sr{sub 1{minus}x}CuO{sub 2{minus}{delta}} thin films are electrons even with the Sr-vacancies present.
Date: December 1, 1993
Creator: Norton, D. P.; Chakoumakos, B. C.; Budai, J. D.; Jones, E. C.; Christen, D. K. & Lowndes, D. H.
Partner: UNT Libraries Government Documents Department

Processing and properties of high J{sub c} grain boundaries in melt textured YBa{sub 2}Cu{sub 3}O{sub x}

Description: Conventional polycrystalline HTS possess extremely low transport critical current density, which has been attributed to grain boundary related deficiencies. We have developed a processing method to eliminate secondary phases and to reduce the amount of microcracks at the grain boundaries in melt-textured YBCO superconductors. In this process, a fraction of the liquid phase is removed during melt-texturing which lead to cleaner grain boundaries. These boundaries have been found to carry high currents (3 {times} 10{sup 4}A/cm{sup 2} at self field and 1.5 {times} 10{sup 2}A/cm{sup 2} at 1.5T) at 77 K in spite of relatively high misorientation angles. Also, the weak dependence of J{sub c} on magnetic field indicates that these grain boundaries are strongly coupled. In addition, the linear I-V characteristics of these samples suggest that flux flow along these boundaries may be the J{sub c} limiting mechanism. These results give promise to manufacturing of polycrstalline YBCO superconductor with high current carrying capability.
Date: November 1, 1994
Creator: Salama, K.; Parikh, A.S.; Kunchur, M.N. & Christen, D.K.
Partner: UNT Libraries Government Documents Department