Marlon J. Dedicatoria

Intrinsic strain effect on critical current in Cu-stabilized GdBCO coated conductor tapes with different substrates

The intrinsic strain effect on critical current, Ic in Cu-stabilized GdBCO coated conductor (CC) tapes under bending and uniaxial tension has been investigated. The bending deformation tolerance of Ic in GdBCO CC tapes, fabricated by reactive co-evaporation by deposition and reaction (RCE-DR) with substrate materials of Hastelloy and stainless steel and fabricated by the metal organic chemical vapour deposition (MOCVD) process was evaluated. The intrinsic strain response of Ic under bending was found to be independent of the fabrication process, the substrate material and the geometry of the sample. For samples with a Hastelloy substrate, the intrinsic strain response of Ic/Icmax under bending was well correlated with those under uniaxial tension. However, for samples with a stainless steel substrate, these had a large strain sensitivity for Ic under uniaxial tension even though this showed a much higher irreversible strain limit up to 1.05%.

Comparison of the Bending Strain Effect on Transport Property in Lap- and Butt-Jointed Coated Conductor Tapes

Although long-length superconducting wires are now being developed and manufactured, there is still a need for jointing in coil manufacturing and power cables for power distribution which requires miles in length. Therefore, it is important to develop an easy and practical jointing that has an acceptable electrical and mechanical integrity. Depending upon the applications, jointed coated conductor (CC) tapes will experience different stresses and strains during manufacturing and operation that could affect its transport property. Bending strain characterization is necessary since CC tapes are being wound into coils for magnet applications and other electrical devices. In this study, two joint types soldered by mechanical controlled jointing with uniform pressure application have been adopted. Critical currents of the jointed tapes have been measured at 77 K and self field. In the case of easy bending test, a new test procedure has been adopted considering the non-uniform flexural stiffness between jointed and unjointed parts of the sample. Relationship between the critical current and bending strain in easy bending mode has been investigated.

Critical Current Degradation Behavior in Lap-Jointed Coated Conductor Tapes With IBAD Substrate Under Uniaxial Tension

Considering the expansion of application field of coated conductor (CC) tapes, it is preferred to adopt IBAD substrate which has high yield strength to resolve the strength problem. The electro-mechanical characteristics of the CC tapes will be influenced by the superconductor type and the deposition process adopted during manufacturing. In this study, the Ic degradation behavior of jointed IBAD processed CC tapes under uniaxial tension has been investigated. The Ic degradation behavior of the jointed CC tapes under uniaxial tension has also been compared with those in single tapes. Differently processed YBCO and SmBCO CC tapes with IBAD substrate have been used as samples. The CC tapes jointed by mechanical controlled method were subjected to uniaxial loading. Joint resistances and critical current of lap-jointed specimens have been derived from the I - V curves obtained at each tensile load at 77 K under self-field. In the case of YBCO CC tapes, lap-jointed tape showed a comparable irreversible strain limit with the single tape.

Analysis on Stress/Strain Tolerances of

Critical current, Ic, in REBCO coated conductor (CC) tapes may increase or decrease reversibly within the reversible region with stress/strain but degrade permanently beyond the irreversible limits due to cracking of the superconducting film. Analyzing quantitatively the stress/strain tolerances of Ic is of importance on the possibility to suppress the onset of cracking on the REBCO coating film. In this study, the stress/strain tolerance of GdBCO CC tapes was experimentally measured from Ic -strain measurement test at 77 K. The improvement in the irreversible strain limit in CC tapes was analyzed by calculating the pre-compression induced by additional layers. The increase of about 0.10%-0.15% in irreversible strain limit with additional Cu and brass laminate is much larger compared with the measured 0.03%-0.06% residual strain due to thermal contraction difference among constituent layers. This result showed that the improvement in the εirr cannot be solely explained by the coefficient of thermal expansion difference among constituent layers.

I_{\rm c}

In this study, the reversible response of <i>I</i>_c with uniaxial strain in differently processed GdBCO coated conductor tapes with different substrates has been investigated under self field and magnetic field conditions at 77 K. Higher irreversible strain limit, ε_irr was observed in those samples with stainless steel substrate. Additional brass lamination to the Cu-stabilized samples produced enhancement of both ε_irr and endurable load limit. However, no significant widening of the <i>I</i>_c/<i>I</i>_c0-tensile strain window with brass addition was observed in the case of samples having stainless steel substrate. Furthermore, under magnetic field, the <i>I</i>_c degradation behavior was independent on the kind of substrate material, but varied with the kind of manufacturing process adopted. The GdBCO samples showed different <i>I</i>_c peak positions and degradation behavior depending on the level of magnetic field applied.

in Externally Laminated GdBCO CC Tapes

SmBCO and YBCO coated conductors (CC) showed different critical current, Ic degradation behavior with strain showing different Ic peak position under self field. Strain effect on Ic under magnetic field in REBCO CC tapes was investigated in order to measure and to describe its tolerance and degradation behavior. Katagiri-type tensile testing apparatus was adopted in which magnetic field was applied parallel to the c-axis of coating film using 10 T superconducting magnet at 77 K. Both strain gauge and Nyilas double extensometers were used to measure the strain. In all the REBCO CC tapes investigated, Ic(ε, B) strongly depends on magnetic field and their degradation behavior depends on the kind of HTS film and fabrication technique adopted. Differently with YBCO CC tapes, SmBCO CC tapes showed increasing Ic strain sensitivity with increasing magnetic field at 77 K and the irreversible strain limit was magnetic field independent over the entire field range studied.

Reversible Strain Response of Critical Current in Differently Processed GdBCO Coated Conductor Tapes Under Magnetic Fields

The effects of low cyclic loading on the critical current, I c, under uniaxial and transverse loadings, and bending deformations in GdBCO coated conductor (CC) tapes were evaluated. Under monotonic continuous bending deformation, CC tapes exhibit a high tolerance of I c up to the lowest bending diameter of 12 mm using the Goldacker bending test rig. However, when the CC tape was subjected to alternate tension–compression bending, a lower irreversible bending strain limit was measured. This was also observed when cyclic bending was applied to the CC tapes which showed a significant decrease in I c just after 10 cycles of alternate tension–compression bending at 20 mm bending diameter. Such different I c degradation behavior under different bending deformation procedures gave insight into the proper handling of CC tapes from manufacturing, coiling and up to operating conditions. In the case of uniaxial tension, when electromechanical properties of CC tape were evaluated by repeated loading based on a critical stress level obtained under monotonic loading, I c also did not show significant change in its degradation behavior up to the irreversible stress limit. The GdBCO CC tape adopted can allow cyclic loading up to 100 cycles without significant irreversible degradation below the monotonic irreversible limit. In the case of the transverse cyclic test, with regard to the large scattering of data especially in the tensile direction, a different cyclic loading procedure was established. For 10 repeated loadings, the mechanical and electromechanical properties of the GdBCO CC tapes showed similar values within the reversible range under the monotonic loading. I c degraded abruptly indicating that no delamination occurred at the REBCO film during the subcritical cyclic loading. Different fracture morphologies were observed under cyclic loading depicting branch-like patterns of the remaining REBCO layer on the substrate of the CC tape.

Characteristic Strain Response of

In superconducting cable applications using the HTS tapes, striation of ReBCO coated conductor (CC) is being considered to reduce ac loss. The striation of ReBCO coating film with laser patterned filaments has been tried. In practical applications, the striated ReBCO CC tapes should also exhibit acceptable tolerance to mechanical stress/strains. However, reports on the electromechanical properties of striated CC tapes in bending or torsion mode are limited. In this study, the critical current degradation behavior of the striated Cu stabilized SmBCO CC tapes in pure torsion mode was investigated using a sample holder which gives torsional deformation to CC tapes. The CC tape has been laser scribed to produce multi-filaments. The Ic/Ic0 and n-value-torsional angle (θ) behaviors in the striated SmBCO CC tapes were examined and compared with the case of non-striated ones. According to the location of the coating film strip striated within the cross-section, there existed some difference in the Icf degradation behavior indicating that a non-uniform deformation was induced on each striated strip.

{\rm I}_{\rm c}

In coil manufacturing for magnets and other electrical device applications that require long lengths of tapes, the joining of coated conductor (CC) tapes is still necessary. The joint is a critical part that may affect the performance of a particular device during its operation. Thus, a practical joint method which has a strong adhesion and low contact resistance without critical current Ic degradation should be developed to meet both purposes of electrical and mechanical connections. In this study, a new joint method using ultrasonic welding (UW) was introduced and the parametric tests were performed to produce a good quality of UW joint. The mechanical-controlled solder joint method was also adopted for comparison. The electrical properties such as Ic and specific joint resistance Rsj were measured at 77 K and the results were compared for each method.

in SmBCO Coated Conductor Tapes Under Magnetic Field at 77 K

Development of a coated conductor tape joint with good quality and low joint resistivity, R sj, in terms of transport and mechanical properties, was attempted by direct bonding at the interface of the Cu–Cu stabilizers in overlapped GdBCO CC tapes. In this study, we attempted to achieve a low R sj by introducing hybrid joining, soldering and ultrasonic welding (UW), and its mechanism was analyzed theoretically. Coated conductor tapes were experimentally joined using various methods of soldering, UW, and combinations of the two. As a result, a much lower R sj of about 57 nΩ cm2 was obtained for RCE-DR-processed GdBCO CC tape joints using the hybrid joining method. The mechanical properties of the jointed CC tapes were also evaluated at room temperature and 77 K under self-field. Load–displacement curves of joined CC tapes followed the curve of the single CC tape. Critical current and joint resistance, R j, of hybrid-joined CC tape were retained after double bending at room temperature up to 20 mm bending diameter.