John Ryan C. Dizon

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.

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.

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.

Numerical analysis of stress distribution in Cu-stabilized GdBCO CC tapes during anvil tests for the evaluation of transverse delamination strength

Rare-earth–Ba–Cu–O (REBCO) based coated conductors (CCs) are now being used for electric device applications. For coil-based applications such as motors, generators and magnets, the CC tape needs to have robust mechanical strength along both the longitudinal and transverse directions. The CC tape in these coils is subjected to transverse tensile stresses during cool-down and operation, which results in delamination within and between constituent layers. In this study, in order to explain the behaviour observed in the evaluation of c-axis delamination strength in Cu-stabilized GdBCO CC tapes by anvil tests, numerical analysis of the mechanical stress distribution within the CC tape has been performed. The upper anvil size was varied in the analysis to understand the effect of anvil size on stress distribution within the multilayered CC tape, which is closely related to the delamination strength, delamination mode and delamination sites that were experimentally observed. The numerical simulation results showed that, when an anvil size covering the whole tape width was used, the REBCO coating film was subjected to the largest stress, which could result in low mechanical delamination and electromechanical delamination strengths. Meanwhile, when smaller-sized anvils were used, the copper stabilizer layer would experience the largest stress among all the constituent layers of the CC tape, which could result in higher mechanical and electromechanical delamination strengths, as well as high scattering of both of these delamination strengths. As a whole, the numerical simulation results could explain the damage evolution observed in CC tapes tested under transverse tensile stress, as well as the transverse tensile stress response of the critical current, Ic.

Deformation and I

The Ic degradation behaviors of Bi-2223 superconducting tapes under pressurized liquid nitrogen were investigated using a newly developed sample holder which gives a series of bending strains to a sample. Four kinds of commercially available multifilamentary Bi-2223 superconducting tapes were used. At atmospheric pressure, the Ic degradation behavior depended upon the reinforcement adopted, the tapes externally reinforced or densified by over pressure showed better bending strain tolerance than the Ag alloy-sheathed Bi-2223 tapes. But these tapes showed a larger Ic degradation when pressurized to 1 MPa in liquid nitrogen. For all samples, after depressurization to atmospheric pressure from 1 MPa, the Ic was completely recovered to its initial values at atmospheric pressure. When the samples were warmed up to room temperature after being depressurized from 1 MPa, ballooning occurred at some parts of some samples. It was found that the larger degradation of Ic occurred at the regions where significant ballooning occurred

_rm c

Nonuniformity of critical current, Ic is one of the most important factors to be considered for the practical applications of high-temperature superconducting (HTS) REBCO coated conductors. To investigate the variation of Ic due to mechanical strain, local critical currents were measured at various tensile strain values from 0 to 1.2% for brass-laminated GdBCO tapes at 77 K, and analyzed using the Weibull distribution function. Measured local critical currents vary and decrease significantly at strain levels above about 0.8%. The Weibull function with three parameters turned out to be applicable for describing the Ic distribution of REBCO HTS wire for the entire strain range. The statistical minimum Ic calculated from the Weibull distribution function decreased with increased tensile strain. The degree of Ic variation was observed to drastically increase when the applied tensile strain exceeded the irreversible strain of about 0.8%.

Degradation Behavior in Bent Bi-2223 Tapes Under Pressurized Liquid Nitrogen

The evaluation of reliability of HTS tapes is necessary for practical applications such as magnet, power cables, motors, SMES and HTS coils. The bending strain effect on the critical current, Ic, in multifilamentary Bi-2223 HTS tapes have been investigated. Single and double layered tapes (for high current capacity) were subjected to thermal and pressurization cycles at pressurized liquid nitrogen (LN2) up to 1 MPa. After the pressurization test, the subsequent occurrence of damage like ballooning due to the diffusion and expansion of LN2 into the tapes were investigated and compared. The Bi-2223 tapes showed different Ic degradation behavior depending upon the geometry and ratio of reinforcement to superconductor adopted.

Variation of Local Critical Current Due to Mechanical Strain in RCE-REBCO Coated Conductors

The endurance evaluation of HTS conductors for practical applications is necessary. The mechanical properties and the critical current, I/sub c/, of multifilamentary Bi-2223 superconducting tapes, externally reinforced with stainless steel foils, subjected to high cycle fatigue loading in the longitudinal direction were investigated at 77 K. The relations between the applied stress amplitude and the fatigue life (S-N curves) were obtained using the externally reinforced Bi-2223 tapes, and its transport property was evaluated with the increase of repeated cycles at different stress amplitudes. The effect of the stress ratio, R, on the I/sub c/ degradation behavior under fatigue loading was also examined considering the practical application situation of HTS tapes. Microstructure observation was performed in order to understand the I/sub c/ degradation mechanism in fatigued Bi-2223 tapes.

Bending Strain Characteristics of Critical Current in Double-Layered Bi-2223 Superconducting Tapes

2G HTS coated conductor (CC) tapes are now used for electric device applications. The residual stress/strain induced in HTS coating films in CC tapes during cool down influences the critical current, Ic, and should be investigated. In this study, the thermal residual stress/strain induced in the REBCO coating film was calculated from deposition temperature (1073 K) to 77 K by thermo-mechanical analysis and numerical simulation. Additionally, the improvement in irreversible strain limit, εirr., with copper electroplating has been investigated.

Fatigue behavior and its influence on the critical current of externally reinforced Bi-2223 superconducting tape

REBCO-based Coated Conductor (CC) High-Temperature Superconducting (HTS) tapes are now being used for practical device applications such as motors, generators, and power cables. The residual stresses and strains induced in the ReBCO coatings in HTS tapes during deposition and cool down from deposition temperature to operation temperature contributes on its performance, especially the electrical transport property. The stresses occur because of the different thicknesses as well as thermal expansion coefficients of the REBCO film, the buffer layer, and substrate materials. In this study, the residual stress/strain induced in the superconducting coating in CC tapes was calculated from deposition temperature to cryogenic temperature computer simulation using a commercial software. A 2-dimensional analysis has been conducted where the thicknesses of constituent layers (i.e. Silver, REBCO film, and Substrate) were varied in order to understand the induced residual stress/strain with increasing thickness. The results show that with increasing REBCO thickness, the induced compressive residual stress in the REBCO film layer decreased. On the other hand, when the silver and substrate layers were increased, the induced compressive residual stress in the REBCO film layer increased. The data is important for the design of coated conductors for practical device applications. In the future, a 3-dimensional simulation for residual stress analysis would also be important. Keywords - Engineering, Superconductor, Residual Stress/Strain, Mechanical Properties Simulation, Philippines