C.E. Klabunde

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

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

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.

Mechanical strength of low-tempepvature-irradiated polyimides: A five-to-tenfold improvement in dose-resistance over epoxies

Neutronics calculations by Engholm show that without additional shielding even the first fusion test reactors such as the Fusion Engineering Device will produce lifetime doses at magnet insulator locations that exceed the radiation tolerance of glass-fabric-filled (gff) epoxies now used. To explore the possible use of an alternative insulator, the mechanical strength of pure and recently available gff polyimides was studied as a function of gamma-ray irradiation at 4.9 K to 100 MGy (10/sup 10/ rads). After a postirradiation anneal at 307/sup 0/K the flexure and compressive strengths of the gff materials measured at 77/sup 0/K were reduced by up to 40% for 100 MGy while the pure material changed little. Testing done at 300/sup 0/K gave similar results, but all stress values were about 40% less. Compared to earlier epoxy studies we find that, overall, the gff polyimides are 5 to 10 times more radiation resistant than comparably prepared gff epoxies.

Debonding of epoxy from glass in irradiated laminates

Abstract Glass fabric filled epoxies irradiated at 4.7 K and examined at room temperature by 20 × stereomicroscopy showed an internal flaw structure which increasingly filled the sample as the γ dose was increased. These flaws were determined to be areas where the plastic had debonded from the glass fibers. The extent of this process correlated well with the dose-dependent loss of mechanical strength. Evidence is reported for a similar mechanism in polyimides although visible flaws have not yet been produced. Possible mechanisms for debonding are suggested. New experiments are also suggested to further clarify the failure mechanism.

Fission neutron damage rates and efficiencies in several metals

Abstract Initial rates of resistivity-measured low-temperature damage production by fission-spectrum fast neutrons have been determined for 14 metals in the same very well characterized irradiation facility. Six of these metals were fcc, five were bcc, and three were hcp. Most were of quite high purity. Observed damage rates, after correction for all known extraneous resistivity-producing effects, were compared with rates predicted by the damage calculation code “RECOIL”, using parameters chosen from the literature. These parameters, effective displacement threshold energy, Ed, and Frenkel-pair resistivity, ρF, were in many cases only best estimates, the further refinement of which may be aided by the present results. Damage efficiencies (measured/predicted rates) follow the same trends by crystal classes as seen in other fast-neutron studies.

Rates of defect production by fission neutrons in metals at 4.7 K

Abstract As part of an interlaboratory program, we have measured the resistivity-damage rates at 4.7 K for the dilute alloys, V-300 ppm Zr, Nb-300 ppm Zr, and Mo-300 ppm Zr, irradiated by virtually unmoderated fission neutrons. In addition, Al, Ni, Cu, and stainless steel have also been measured to provide a broader data base for comparison with other experimental work using a variety of neutron spectra and with defect-production theory. A broad view of the results shows that the ratio of experimentally to theoretically determined production rates (damage efficiencies) for various fast-neutron spectra ranges from about 0.25 to 0.50. On the other hand, for a given element various neutron-energy spectra peaked from 1 to 15 MeV give variations in damage-efficiency values of only 7–30%.

Enhanced transport critical current at high fields after heavy ion irradiation of textured TlBa2Ca2Cu3Oz thick films

Spray pyrolyzed T1(1223) films deposited on polycrystalline YSZ substrates are characterized before and after heavy ion irradiation. A factor of 2 decrease in zero field critical current is observed. However, significantly improved critical current is found at fields above 1 T, where intragranular effects dominate. The irreversibility line at 5 T is shifted by ∼20 K to higher temperatures. Scaling of the data before and after irradiation and at different temperatures is modeled by an expression which interpolates between single vortex pinning and collective creep.

Growth-induced columnar defects in YBa2Cu3O7−x thin films grown on miscut

Abstract Measurements of the critical current density in YBa2Cu3O7−x (YBCO) thin films grown on deliberately miscut (1.6 °) LaAlO3 substrates that also contain a small mosaic spread of subgrain orientations have shown an anomalous angular dependence of Jc. A strong, magnetic field-dependent peak in Jc is observed when the magnetic field is oriented between the c-axis (the film normal direction) and the a-b-plane; in this orientation flux pinning is normally not strong. Transmission electron microscopy (TEM) has been used to determine the defects which are responsible for the flux pinning related to the anomalous Jc peak. [001] columnar defects have been observed to penetrate right through the YBCO films. The diameters of the columnar defect regions are ∼3–5 nm, which is near ideal for producing the observed anomalous pinning and high Jc. Numerous stacking faults involving excess CuO layers have also been observed; these can also serve as effective flux pinners. Small-angle planar boundaries perpendicular to the substrate are rarely seen. Images of the YBCO-substrate interface reveal that some of the columnar growth defects initiate at substrate steps/dislocations. The YBCO unit cells grown on the upper and lower terraces of a substrate step have a relative shift of c/3[001]; thus, the CuO planes are broken within a region less than 10 nm from the substrate, presumably reducing the superconducting order parameter in this region and leading to pinned vortices situated on these sites. Thus, the columnar defects initiated from the interface and the stacking faults are useful for pinning flux through the entire film thickness.

Isochronal recovery of high-energy d-Be neutron damage in Cu, Nb, and Pt from 8 to 400 K☆

Abstract The isochronal recovery of high-energy d-Be neutron damage has been measured in Cu, Nb, and Pt from 8 to 400 K, using changes in the electrical resistivity. High-purity wire specimens were irradiated at 4.2 K with neutrons produced at the Oak Ridge Isochronous Cyclotron by the reaction of 40-MeV deuterons on a thick Be target. The resulting neutron energy spectrum was broadly peaked near 15 MeV. When compared with recent fission neutron results the recovery of d-Be neutron damage is slightly suppressed through stage II for Cu and Nb, but significantly reduced (about a factor of two) in stage I in Pt. This suggests that the primary damage state remains essentially unaltered in lighter elements as the mean neutron energy increases from 2 to 15 MeV, but that substantial configurational changes may occur over the same energy range for heavier elements. The results of recent damage-production measurements and room-temperature irradiations using d-Be neutrons are discussed in terms of the recovery spectra.

Damage production by high‐energy d‐Be neutrons in Cu, Nb, and Pt at 4.2 °K

Electrical resistivity measurements have been used to observe damage production rates for Cu, Nb, and Pt irradiated with high‐energy d‐Be neutrons at 4.2 °K. The neutrons were generated at the Oak Ridge Isochronous Cyclotron by the reaction of 40‐MeV deuterons incident on a thick Be target. The resulting neutron energy spectrum was broadly peaked near 15 MeV. Damage production was proportional to fluence up to 4×1015 n/cm2, and was approximately three times greater than for the same fluence of fission neutrons in these materials. This result is in good agreement with predictions based on damage energy calculations.