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Kirkendall voids: a detriment to Nb/sub 3/Sn superconductors

Description: Multifilamentary composites with constant Nb/bronze ratio were heat treated for a range of reaction times. With increasing reaction time, the size of diffusion (Kirkendall) voids in the bronze matrix increases, as does the Nb/sub 3/Sn layer thickness. Because of the resultant changes in mechanical and superconducting properties, the strain at onset of degradation of critical current density, J/sub c/, decreases with increasing reaction time. Thus there is a compromise between strain sensitivity and current capacity at zero strain as reaction time is increased. For strains on the order of 0.2 to 1.0%, the increased strain sensitivity of J/sub c/ in specimens which were reacted for longer than approximately 24 hrs offsets the gain in J/sub c/ resulting from increased Nb/sub 3/Sn layer thickness. The diffusion voids are also related to thermal instabilities and crack initiation.
Date: January 1, 1978
Creator: Easton, D.S. & Kroeger, D.M.
Partner: UNT Libraries Government Documents Department

Evidence for microstructural effects under strain in bronze process Nb/sub 3/Sn

Description: The possible effects of externally applied strain on the flux pinning process can be divided into two groups according to whether the significant property changes produced by strain are associated with the crystalline defects which act as flux pinners, or with the bulk superconducting material which carries the supercurrent. The question of whether there is a microstructural component to the strain dependence of Fp by carefully determining whether the bulk property changes which occur can adequately account for all of the variation of Fp with strain. Examination of the results in light of current theories of flux pinning and type-II superconductivity indicates that bulk property changes do not account for all of the change in Fp, suggesting that microstructural effects are also important. (GHT)
Date: January 1, 1980
Creator: Kroeger, D.M.; Easton, D.S.; Koch, C.C. & DasGupta, A.
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

Paths for current flow in polycrystalline high temperature superconductors

Description: Determinations from x-ray and electron microdiffraction studies of the populations and geometrical arrangements of small angle grain boundaries in Bi-2223 and Bi-2212 conductors and Tl-1223 deposits suggest that current flow in these polycrystalline materials is percolative in character. Comparison of measured misorientation angle distributions to calculated distributions suggest that not only texture but also grain boundary energy is important in increasing the number of small angle grain boundaries.
Date: December 1995
Creator: Kroeger, D. M.; Goyal, A. & Specht, E. D.
Partner: UNT Libraries Government Documents Department

Stress effects in a Nb/sub 3/Sn conductor proposed for use in the magnetic fusion energy program

Description: Because of significant interest in multifilamentary Nb/sub 3/Sn conductors with copper stabilization for large magnetic devices, knowledge of stress effects on electrical and mechanical properties of such conductors is important. The conductor studied in this experiment is proposed for use as a basic strand material to form a force-cooled cable for the Large Coil Program at ORNL. Stress-strain curves and stress effects on the critical current of differently reacted conductors are presented.
Date: January 1, 1979
Creator: Specking, W; Easton, D S; Kroeger, D M & Sanger, P A
Partner: UNT Libraries Government Documents Department

Formation of thallium based high temperature superconductors using aerosol flow reacted precursor powders

Description: Highly anisotropic particles of various thallium containing superconductors were grown. The Tl free precursor powders with the nominal compositions Ba{sub 1}Ca{sub 2}Cu{sub 3}Ag{sub 0.37}O{sub 6} and Ba{sub 2}Ca{sub 2}Cu{sub 3}Ag{sub 0.37}O{sub 7} were prepared using an aerosol flow reactor. These precursor powders were post-annealed in 0.1 atm oxygen at 700{degree}C for 4h to reduce the carbon present, and subsequently mixed with Tl{sub 2}O{sub 3} (typical composition of Tl{sub x}; x = 0.6 {minus} 1.0). The Tl-containing powders were heat-treated in semi-sealed gold tubes between 650--890{degree}C with various short and long heat-treatments. X-ray diffraction showed that the Tl{sub 1}Ba{sub 2}Ca{sub 1}Cu{sub 2}O{sub 7} (referred to as Tl-1212) and Tl{sub 1}Ba{sub 2}Ca{sub 2}Cu{sub 3}O{sub 9} (Tl-1223) phases formed below 700{degree}C while Tl{sub 2}Ba{sub 2}Ca{sub 1}Cu{sub 2}O{sub 8} (Tl-2212) formed between 700--830{degree}C, and Tl{sub 2}Ba{sub 2}Ca{sub 2}Cu{sub 3}O{sub 10} (Tl-2223) formed above 830{degree}C. Scanning electron microscopy showed evidence for the presence of high aspect-ratio particles. These highly anisotropic particles may be of interest for the preparation of powder-in-tube and other powder-deposited conductors, for current leads, and for grain alignment studies.
Date: February 1, 1995
Creator: Brynestad, J.; Paranthaman, M.; Goyal, A.; Heatherly, D. E. & Kroeger, D. M.
Partner: UNT Libraries Government Documents Department

Grain boundary studies of high temperature superconducting materials using electron backscatter Kikuchi diffraction

Description: Grain Orientation and gain boundary misorientation distributions in high critical current density, high temperature superconductors were determined using electron backscatter Kikuchi diffraction. It is found that depending on the type of superconductor and the processing method used to fabricate it, there exist different scales of biaxial texture from no biaxial texture, local biaxial texture, to complete biaxial texture. Experimentally obtained grain boundary misorientation distributions (GBMDs) were found to be skewed significantly to low angles in comparison to what is expected on the basis of macroscopic texture alone, suggesting that minimization of energy may be a driving force during the processing of high critical current density materials. In addition, a higher than expected fraction of coincident-site lattice boundaries is observed. Examination of maps of grain boundary misorientations in spatially correlated gains, i.e. the grain boundary mesotexture, suggests the presence percolative paths of high critical current density. A combination of orientation measurements, theoretical modeling of GBMDs and modeling of percolative current flow through an assemblage of gain boundaries is performed to gain an insight into the important microstructural features dictating the transport properties of high temperature superconductors. It is found that maximization of low energy, in particular, low angle boundaries is essential for higher critical currents. The combination of experimental and analytical techniques employed are applicable to other materials where physical properties are dominated by interganular characteristics.
Date: December 1996
Creator: Goyal, A.; Specht, E. D.; Wang, Z. L. & Kroeger, D. M.
Partner: UNT Libraries Government Documents Department

Formation of anisotropic Tl-1212, Tl-2212, Tl-1223 and Tl-2223 particles using aerosol flow reacted powders

Description: Highly anisotropic particles of Tl-1212, Tl-2212, Tl-1223 and Tl-2223 superconductors were grown. The Tl-free precursor powders with the compositions Ba{sub 1}Ca{sub 2}Cu{sub 3}Ag{sub 0.37}O{sub 6} and Ba{sub 2}Ca{sub 2}Cu{sub 3}Ag{sub 0.37}O{sub 7} were prepared using an aerosol flow reactor. These precursor powders were then post-annealed in 0.1 atm oxygen at 700 C for 4h to reduce the carbon present and mixed with Tl{sub 2}O{sub 3} (typical composition of Tl{sub x}; x = 0.6--1.0). The Tl-containing powders were heated in sealed gold tubes between 650--890 C for various times. X-ray diffraction showed that the Tl-2212 and Tl-2223 phases were stable over a wide range of temperatures. Scanning electron microscopy showed evidence for the presence of high aspect-ratio particles. These highly anisotropic particles may be of interest for the preparation of powder-in-tube and other powder deposited conductors, for current leads, and for grain alignment studies.
Date: December 31, 1994
Creator: Paranthaman, M.; Goyal, A.; Heatherly, D.E. & Kroeger, D.M.
Partner: UNT Libraries Government Documents Department

Test of two prototype high-temperature superconducting transmission cables

Description: Two 500-A class prototype high-temperature superconducting cables have been constructed by Southwire Company and tested at Oak Ridge National Laboratory (ORNL). In the first cable, no insulation was used to separate the individual HTS tapes. In the second cable, Kapton tape was used to insulate the HTS tapes between successive layers for the study of AC loss and current distribution. The cables were tested with both DC and AC currents in liquid nitrogen from 77 to 69 K. Both cables achieved DC critical current, I{sub c} greater than 500 A. A calorimetric technique that measures the cable temperature rise under ac currents was used to measure the ac loss of the cables. The un-insulated cable showed a cryoresistive behavior under the 60 Hz AC currents. The insulated cable started to show measurable loss at current where there was corresponding resistive loss.
Date: October 1, 1996
Creator: Lue, J.W.; Lubell, M.S.; Kroeger, D.M.; Martin, P.M.; Demko, J.A.; Jones, E.C. et al.
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

The path for long range conduction in high J{sub c} TlBa{sub 2}Ca{sub 2}Cu{sub 3}O{sub 8+x} spray-pyrolyzed deposits

Description: Grain boundary misorientations and local texture in polycrystalline TlBa{sub 2}Ca{sub 2}Cu{sub 3}O{sub 8+x} deposits prepared by thallination of spray-pyrolyzed precursor deposits on yttria-stabilized zirconia have been determined from transmission electron microscopy, electron backscatter diffraction patterns and x-ray diffraction. The deposits were polycrystalline, had small grains, and excellent c-axis alignment The deposits contained colonies of grains with similar but not identical a-axis orientations. Most grain boundaries within a colony have small misorientation angles and should not be weak links. It is proposed that long range current flow occurs through a percolative network of small angle grain boundaries at colony intersections.
Date: December 31, 1993
Creator: Kroeger, D. M.; Goyal, A.; Specht, E. D.; Wang, Z. L.; Tkaczyk, J. E.; Sutliff, J. A. et al.
Partner: UNT Libraries Government Documents Department

Local texture and grain boundary misorientations in high T{sub C} oxide superconductors

Description: The orientations of hundreds of contiguous grains in high J{sub C} TlBa{sub 2}Ca{sub 2}Cu{sub 3}O{sub x} deposits and (Bi, Pb){sub 2} Sr{sub 2}Ca{sub 2}Cu{sub 3}O{sub y} powder-in-tube tapes have been determined from electron back scatter diffraction patterns (EBSP). The misorientation angles and axes of rotation (angle/axis pairs) for grain boundaries connecting these grains were calculated. For both materials the population of low angle boundaries was found to be much larger than expected from calculations based on the macroscopic texture. The TlBa{sub 2}Ca{sub 2}Cu{sub 3}O{sub x} deposits exhibit pronounced local texture which has been defined by EBSP and x-ray diffraction. Locally grains show significant in-plane (a-axis) alignment even though macroscopically a-axes are random, indicating the presence of colonies of grains with similar a-axis orientations. In (Bi, Pb){sub 2} Sr{sub 2}Ca{sub 2}Cu{sub 3}O{sub x} tapes no local texture was observed. In both materials the existence of connected networks of small angle grain boundaries can be inferred. Coincident site lattice (CSL) grain boundaries are also present in higher than expected numbers. Grain boundary energy thus appears to play a significant role in enhancing the population of potentially strongly-linked boundaries. We propose that long range strongly-linked conduction occurs through a percolative network small angle (and perhaps CSL) grain boundaries.
Date: February 1, 1995
Creator: Kroeger, D. M.; Goyal, A.; Specht, E. D.; Tkaczyk, J. E.; Sutliff, J.; DeLuca, J. A. et al.
Partner: UNT Libraries Government Documents Department

Local texture and strongly linked conduction in spray-pyrolyzed TlBa{sub 2}Ca{sub 2}Cu{sub 3}O{sub 8+x} deposits

Description: Local texture in polycrystalline TlBa{sub 2}Ca{sub 2}Cu{sub 3}O{sub 8+x} deposits has been determined from transmission electron microscopy, electron backscatter diffraction patterns and x-ray diffraction. The small-grained deposits had excellent c-axis alignment and contained colonies of grains with similar but not identical a-axis orientations. Most grain boundaries within a colony have small misorientation angles and should not be weak links. It is proposed that long range conduction utilizes a percolative network of small angle grain boundaries at colony intersections.
Date: December 31, 1993
Creator: Kroeger, D. M.; Goyal, A.; Specht, E. D.; Wang, Z. L.; Tkaczyk, J. E.; Sutliff, J. A. et al.
Partner: UNT Libraries Government Documents Department

Low-cost metal substrates for films with aligned grain structures

Description: 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.
Date: June 1, 1996
Creator: Norton, D.P.; Budai, J.D.; Goyal, A.; Lowndes, D.H.; Kroeger, D.M.; Christen, D.K. et al.
Partner: UNT Libraries Government Documents Department

Pulsed-Laser Deposition of Electronic Oxides: Superconductor and Semiconductor Applications

Description: Over the past decade, pulsed-laser deposition (PLD) has proven to be one of the most versatile and effective methods for obtaining high-quality electronic oxide thin-film materials. Much of this success can be attributed to its initial use in depositing high temperature superconducting materials. However, pulsed-laser deposition is now a leading research tool in the development of various electronic oxide thin-film technologies, In this paper, recent progress in the deposition of oxide materials on dissimilar materials for both superconductor and semiconductor applications is discussed. Recent developments in the synthesis of superconducting wires via epitaxial growth of superconducting oxides on biaxially textured metal tapes is described. In addition, efforts to integrate high-k dielectric oxides on semiconductor surfaces using pulsed-laser deposition are highlighted.
Date: January 24, 2000
Creator: Norton, D. P.; Park, C.; Lee, Y. E.; Budai, J. D.; Chisholm, M. F.; Verebelyi, D. T. et al.
Partner: UNT Libraries Government Documents Department

Cube-textured nickel substrates for high-temperature superconductors

Description: The biaxial textures created in metals by rolling and annealing make them useful substrates for the growth of long lengths of biaxially textured material. The growth of overlayers such as high-temperature superconductors (HTS) require flat substrates with a single, sharp texture. A sharp cube texture is produced in high-purity Ni by rolling and annealing. The authors report the effect of rolling reduction and annealing conditions on the sharpness of the cube texture, the incidence of other orientations, the grain size, and the surface topography. A combination of high reduction, and high temperature annealing in a reducing atmosphere leads to > 99% cube texture, with mosaic of 9.0{degree} about the rolling direction (RD), 6.5{degree} about the transverse direction (TD), and 5.0{degree} about the normal direction (ND).
Date: February 1, 1998
Creator: Specht, E.D.; Goyal, A.; Lee, D.F.; List, F.A.; Kroeger, D.M.; Paranthaman, M. et al.
Partner: UNT Libraries Government Documents Department

Epitaxial Growth of Yb<sub>2</sub>O<sub>3</sub> Buffer Layers on Biaxially Textured-Ni (100) Substrates by Sol-Gel Process

Description: In order to develop an alternative buffer layer architecture using the sol-gel process to produce YBCO (YBa2Cu307+) coated conductors, Yb203 has been chosen as the candidate material. Buffer layers of fi03 were epitaxkdly grown on biaxially textured-Ni (100) substrates by the sol gel process for the first time. The ~03 precursor solution was prepared from an alkoxide sol-gel route in 2-xnetho~ethanol and was deposited on textured-Ni (100) substrates by either spin coating or dip coating methods. The amorphous film was then processed at 1160oC under flowing (96%)MH2(4%) gas mixture for one hour. The fi03 iihn exhibited a strong c-axis orientation on the Ni (100) substrates. The phi and omega scans indicated good in plane and out of plane orientations. The X-ray (222) pde figure showed a cube-on-cube epitaxy. High current YBCO films were grown on the Y&03 sol-gel buffered-Ni substrates.
Date: April 5, 1999
Creator: Beach, D.B.; Chirayil, T.G.; Christen, D.K.; Feenstra, R.; Goyal, A.; Kroeger, D.M. et al.
Partner: UNT Libraries Government Documents Department

Fabrication of High Current YBa<sub>2</sub>Cu<sub>3</sub>O(sub>7-y</sub>) Coated Conductors Using Rolling-Assisted Biaxially Textured Substrates

Description: High critical current YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7-y</sub> (referred to as YBCO) coated conductors were fabricated with a layer sequence of YBCO/YSZ/CeO<sub>2</sub>/Ni. The cube (100) texture in the starting Ni substrates was obtained by cold rolling followed by recrystallization. A thin CeO<sub>2</sub> (Cerium Oxide) layer with a thickness of 100-200 Å was grown epitaxially on the biaxially textured-Ni substrates using an e-beam evaporation technique. This was followed by the growth of a thick (<= 0.77 µm) YSZ (Yttria Stabilized Zirconia) layer using either e-beam evaporation or rf magnetron sputtering. The e-beam CeO<sub>2</sub> film had a dense microstructure. The microstructure of the e-beam YSZ film was porous whereas the sputtered YSZ film was dense. The YBCO films were grown by pulsed laser deposition on both e-beam and sputtered YSZ layers. A transport critical current density of ~ 1 x l0<sup>6</sup> A/cm<sup>2</sup> at 77 K was obtained for ~ 0.8 µm thick YBCO Rims on both YSZ surfaces in zero field. To demonstrate the quality and compatibility of the e-beam CeO<sub>2</sub> layers; YBCO films were also grown on CeO<sub>2</sub>-buffered YSZ (100) single crystal substrates using e-beam co-evaporated Y-BaF<sub>2</sub>-Cu precursors followed by a post-annealing process. A transport critical current density of over 1 x lO<sup>6</sup>A/cm<sup>2</sup> at 77 K was obtained on a ~ 0.3 µm thick YBCO film in zero field.
Date: June 1, 1999
Creator: Christen, D.K.; Feenstra, R.; Kroeger, D.M.; Lee, D.F.; List, F.A.; Martin, P.M. et al.
Partner: UNT Libraries Government Documents Department

Demonstration of High Current Density YBCO Coated Conductors on RE<sub>2</sub>O<sub>3</sub>-Buffered Ni Substrates with Two New Alternative Architectures

Description: In continuation of our effort to develop single buffer layer architectures for YBCO (YBa<sub>2</sub>Cu<sub>3</sub>O<sub>7-<font face="symbol">g</font>) coated tape conductors, we have studied RE<sub>2</sub>O<sub>3</sub> (RE = Y, and rare earths) as candidate materials. Three types of crystal structures including the preferred cubic phase are known for the rare earth oxides. High quality simple cubic RE<sub>2</sub>O<sub>3</sub> buffer layers were grown epitaxiahy on {100}<001> textured Ni substrates using both reactive evaporation and sol-gel processing. Detailed X-ray studies have shown that the Y<sub>2</sub>O<sub>3</sub>, Eu<sub>2</sub>O<sub>3</sub>, Gd<sub>2</sub>O<sub>3</sub>, and Yb<sub>2</sub>O<sub>3</sub> were grown with a single epitaxial orientation. SEM micrographs indicated that both e-beam and sol-gel grown films were dense, continuous and crack free. High J<sub>c</sub> YBCO films were grown on RE<sub>2</sub>O<sub>3</sub>-buffered Ni substrates with sputtered cap layers. Two new alternative buffer layer architectures were developed. A high J<sub>c</sub> of 1.8 MA/cm<sup>2</sup> at 77 K and self-field was obtained on YBCO films with a layer sequence of YBCO (pulsed laser deposition)/Yb<sub>2</sub>O<sub>3</sub> (sputtered)/Y<sub>2</sub>O<sub>3</sub> (e-beam)/Ni. Also, a high J<sub>c</sub> of over 1 MA/cm<sup>2</sup> at 77 K and self-field was obtained on YBCO films with a layer sequence of YBCO (ex-situ BaF<sub>2</sub> process)/CeO<sub>2</sub> (sputtered)YSZ sputtered)/RE<sub>2</sub>O<sub>3</sub> (sol-gel or e-beam)Ni. The performance of sol-gel grown buffers approached the quality of e-beam grown buffers.
Date: July 12, 1999
Creator: Beach, D. B.; Chirayil, T. G.; Christen, D. K.; Cui, X.; Feenstra, R.; Goyal, A. et al.
Partner: UNT Libraries Government Documents Department