Sang-Soo Oh

The strain effect on critical current in YBCO coated conductors with different stabilizing layers

The tensile strain dependences of the critical current (Ic) in YBa2Cu3O7−δ (YBCO) coated conductors fabricated by using the rolling-assisted biaxially textured Ni–W substrates (RABiTS)–pulsed laser deposition (PLD) method were examined at 77 K and in self magnetic field. Cu and stainless steel layers were used as stabilizers to the YBCO coated conductor, and the effects of stabilizing layers on the strain tolerance of Ic were investigated, compared with the case without a stabilizing layer. The lamination of stabilizer produced an increase in the yield strength and strain tolerance of Ic in coated conductors. All YBCO coated conductors tested showed a reversible strain effect and a peak in the relation between Ic and applied strain. The peak strain of Ic and the irreversible strains for Ic degradation were enhanced when the YBCO coated conductor was laminated with a stabilizing layer. For the case laminated with a stainless steel layer, Ic recovered reversibly until the applied strain reached to about 0.5% and showed its peak at a strain of 0.42%, comparing to the case without a stabilizing layer, which were 0.21% and 0.18%, respectively. It can be predicted that the lamination of a stabilizing layer produced a significant residual compressive strain to the YBCO film during cooling to 77 K, which influenced the axial strain tolerance of YBCO coated conductors. Therefore, the Ic–tensile strain relation in YBCO coated conductors could be explained by a two-stage deformation; stage I is the region where YBCO film behaves elastically and Ic recovers when the stress is released. Stage II is the region where Ic decreases irreversibly attributable to the cracking induced in the YBCO film due to the significant plastic deformation of the substrate or the stabilizing layer.

Tensile damage and its influence on the critical current of Bi2223/Ag superconducting composite tape

We have studied the tensile behaviour of Bi2223 superconducting composite tapes at room temperature, and the influence of the tensile damages introduced at room temperature on the critical current Ic and the n values at 77 K. In the measurement of the Ic and n values, the overall composite with a gauge length 60 mm was divided into six elements with a gauge length of 10 mm in order to find the correlation of the Ic and n values of the overall composite to those of the local elements which constitute the composite. From the measured stress–strain curve of the composite and the calculated residual strain of the Bi2223 filaments, the intrinsic fracture strain of Bi2223 filaments was estimated to be 0.09–0.12%. When the applied strain was lower than the onset strain of the filament damage, the original Ic and n values were retained both in the overall composite and the elements. In this situation, while the overall voltage at the transition from superconductivity to normal conductivity of the composite was the sum of the voltages of the constituent elements, among all elements the overall voltage was affected more by the element with the lower Ic (higher voltage). The damage of the filaments arose first locally, resulting in a reduction of the Ic and n values in the corresponding local element, even though the other elements retained the original Ic and n values. In this situation, the voltage of the overall composite stemmed dominantly from that of the firstly damaged weakest element, and the overall Ic and n values were almost determined by the values of such an element. After the local element was fully damaged, the damage arose also in other elements, resulting in segmentation of the filaments. Thus, the Ic and n values were reduced in all elements. The correlation of Ic between the overall composite and the elements could be described comprehensively for non-damaged and damaged states from the voltage–current relation.

Residual and fracture strains of Bi2223 filaments and their relation to critical current under applied bending and tensile strains in Bi2223/Ag/Ag alloy composite superconductor

Mechanical and electromagnetic stresses are exerted on Bi2223∕Ag∕Ag alloy superconducting composite tapes during fabrication∕winding and operation, which cause reduction in critical current when the Bi2223 filaments are damaged. In the damage process, the thermally induced residual strain and fracture strain of the Bi2223 filaments play a dominant role. The aim of the present work was to propose a comprehensive method for estimation of these strain values and a quantitative description method of the relation of critical current to the applied bending∕tensile strain, and to examine the accuracy of the method in comparison with the experimental results. The residual strain of Bi2223 filaments in the composite tape was measured by the x-ray diffraction method. The measured residual strain value was used for analysis of the load-strain curve, from which the intrinsic fracture strain of filaments was estimated. The relation of critical current to applied bending∕tensile strain was predicted by the proposed cal...

Critical Current Degradation Behavior in YBCO Coated Conductors Under Torsional Strain

The Ic degradation behaviors of a YBCO coated conductor (CC) tape (RABiTS/MOD) was investigated using a sample holder which gives torsional angles to HTS tapes. The Ic degradation in YBCO CC tape under torsional strains occurred gradually which is a characteristic feature under torsion. Uniform torsional deformation was induced in the YBCO CC tape evident from the consistent Ic degradation behavior at each subsection along the longitudinal direction of the tape. Similar with the tension case, the reversible behavior of Ic under torsional loading was found. The irreversible strain, epsivirr.t., was ~0.6%. The critical strain defined by the 95% Ic retention criterion was 1.4% which was located within the irreversible limit. The n-value-thetas behavior in the YBCO CC tape was similar to the Ic/Ic0-thetas behavior.

Ultra-High Performance, High-Temperature Superconducting Wires via Cost-effective, Scalable, Co-evaporation Process

Long-length, high-temperature superconducting (HTS) wires capable of carrying high critical current, Ic, are required for a wide range of applications. Here, we report extremely high performance HTS wires based on 5 μm thick SmBa2Cu3O7 − δ (SmBCO) single layer films on textured metallic templates. SmBCO layer wires over 20 meters long were deposited by a cost-effective, scalable co-evaporation process using a batch-type drum in a dual chamber. All deposition parameters influencing the composition, phase, and texture of the films were optimized via a unique combinatorial method that is broadly applicable for co-evaporation of other promising complex materials containing several cations. Thick SmBCO layers deposited under optimized conditions exhibit excellent cube-on-cube epitaxy. Such excellent structural epitaxy over the entire thickness results in exceptionally high Ic performance, with average Ic over 1,000 A/cm-width for the entire 22 meter long wire and maximum Ic over 1,500 A/cm-width for a short 12 cm long tape. The Ic values reported in this work are the highest values ever reported from any lengths of cuprate-based HTS wire or conductor.

Bending Strain Characteristics of the Transport Property in Lap-Jointed Coated Conductor Tapes

The electro-mechanical properties of lap-jointed ReBCO coated conductor (CC) tapes have been investigated. Different contact configurations considering its geometry were adopted in this study. The configuration where the YBCO layer is in close proximity showed much less joint resistance. In all joint configurations, the electrical resistance decreased as the joint length increased. For HTS tapes, the joint resistance-joint length behavior could be explained by parallel circuit analysis. In easy and hard bending modes, the Ic degradation behaviors in lap-jointed HTS tapes were examined. The critical radius for 95% Ic retention in each bending mode were determined.

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.

A Monte Carlo–shear lag simulation of tensile fracture behaviour of Bi2223 filament

The damage evolution in Bi2223 filaments and its influence on critical current was described by a Monte Carlo–shear lag simulation method. The experimentally observed zigzag crack propagation across aligned Bi2223 grains under tensile strain was effectively modelled by including transverse and longitudinal failure modes for individual grains. From the simulated stress–strain curve, the survival parameter (slope of the stress–strain curve normalized with respect to the original Youngs modulus) was estimated with increasing applied strain. With this parameter combined with the strain sensitivity of the critical current, the measured change of critical current of the composite tape with applied strain could be described well.

Distribution of normalized critical current of bent multifilamentary Bi2223 composite tape

The distribution of the normalized critical transport current (critical current normalized with respect to the original value) of Bi2223/Ag/Ag alloy composite tape under bending strain of 0%–0.833% was studied experimentally and analytically. The experimental results were analyzed by a modeling approach based on the relation of the heterogeneous damage evolution to the distribution of the critical current. The main results are summarized as follows. (1) The measured distribution of the critical current values varying with bending strain was described well by the present approach. (2) When all specimens were damaged at high bending strains (0.338%–0.833% in the present work), the distribution of the critical current of the bent-damaged specimens was expressed by the three-parameter Weibull distribution function, the reason for which was revealed. (3) The distribution of the irreversible strain was estimated, with which the influence of the increase in the fraction of damaged specimens on the variation of c...

Numerical analysis of stability margin and quench behavior of cable-in-conduit NbTi conductors for KSTAR

A numerical model has been proposed to analyze the stability margin and quench characteristics of the cable-in-conduit NbTi conductors for the KSTAR-PF (Korea Superconducting Tokamak Advanced Research) magnets. The dependence of the thermal, hydraulic and electrical properties on the external thermal disturbance was investigated. The algorithm of the program is based on the finite volume method which adopts space discretization and time integration by multi-step Runge-Kutta method to obtain stable numerical solutions. It was confirmed that the disturbance duration can influence the conductor stability and limiting current.