Presentation Information
[WB8-04-INV]In-plane domain control of REBCO coted conductors by applying bending strain and its effects on superconducting properties
*Tatsunori Okada1,2, Daiki Hisatomi1, Ryoma Kawamura1, Itta Eshima1, Kosei Ogawa1, Yuhi Shimamura2, Satoshi Awaji2 (1. Kyushu Inst. of Tech. (Japan), 2. Tohoku Univ. (Japan))
Keywords:
REBCO coated conductors,domain control,bending strain,transport properties
[Purpose]
Since REBCO is an orthorhombic material, an in-plane twin structure comprised of two domains, “A-domain” and “B-domain”, where the a- and b-axis of REBCO are respectively aligned in the direction of applied strain εa. It has been clarified that the critical temperature Tc of A- and B-domains exhibit almost opposite response to εa, which likely underlies the complicated εa dependence of critical current Ic under finite magnetic fields B. To understand the complicated Ic(εa, B) of REBCO CCs, several attempts have been carried out to control the domain structure via heat treatment under compressive/tensile bending or uniaxial tension.
Recently, we found that the domain fraction of REBCO CCs can be modified simply by keeping REBCO CCs bent at room temperature for a few days. This means that the superconducting properties of REBCO CCs wound into coils or cables may differ from those measured on straight short samples. Therefore, it is important to clarify how the bending-induced domain control affects on superconducting properties of REBCO CCs.
[Method]
We (i) controlled in-plane domain fraction of GdBCO CCs without artificial pinning centers (APCs) by applying tensile/compressive bending strain εb; (ii) qualified the domain fraction by x-ray diffraction; and (iii) performed DC transport measurements with B||ab ≦ 15 T. Because the controlled domains tend to relax to their original state once the bending is released, experiments (ii) and (iii) were performed while keeping REBCO CCs bent as in (i).
[Results]
We confirmed that the A-domain fraction fA, which was determined from the (200) and (020) reflections, varies linearly with εb as fA(εb) = (56 ± 3) − (3.5 ± 0.2) × 103εb [%] in −0.45% ≦ εb ≦ +0.45%, demonstrating that the domain structure of REBCO CCs can be controlled within this bending-strain window. Tc exhibits a “Λ”-shaped dependence on fA with a maximum at fA ≃ 58% (εb ≃ −0.14%), while the upper critical field Bc2||ab exhibits a “N”-shaped, non-monotonic dependence on fA. Ic(fA, B||ab) measured at 77.3 K with increasing B shows a crossover from an “N”-shaped to a “V”-shaped fA dependence at around 4 T. This crossover in Ic(fA, B||ab) suggests the change in the dominant vortex-pinning mechanism in high- and low-fA samples (i.e., compressive- and tensile-bent REBCO CCs).
[Conclusions]
We succeeded in controlling the fraction of in-plane domain of GdBCO CCs by applying bending strain at room temperature and measuring superconducting properties. Obtained in-field properties of domain-controlled GdBCO CCs without APC depend on the A-domain fraction complicatedly. We found that there is a possibility that in-field quantities of domian-controlled GdBCO CCs become superior to those in the as-received sample, but further investigations are necessary to understand those behaviors.
At the symposium, we would like to explain those above in detail and, if possible, report on-going investigations on REBCO CCs with APCs.
Since REBCO is an orthorhombic material, an in-plane twin structure comprised of two domains, “A-domain” and “B-domain”, where the a- and b-axis of REBCO are respectively aligned in the direction of applied strain εa. It has been clarified that the critical temperature Tc of A- and B-domains exhibit almost opposite response to εa, which likely underlies the complicated εa dependence of critical current Ic under finite magnetic fields B. To understand the complicated Ic(εa, B) of REBCO CCs, several attempts have been carried out to control the domain structure via heat treatment under compressive/tensile bending or uniaxial tension.
Recently, we found that the domain fraction of REBCO CCs can be modified simply by keeping REBCO CCs bent at room temperature for a few days. This means that the superconducting properties of REBCO CCs wound into coils or cables may differ from those measured on straight short samples. Therefore, it is important to clarify how the bending-induced domain control affects on superconducting properties of REBCO CCs.
[Method]
We (i) controlled in-plane domain fraction of GdBCO CCs without artificial pinning centers (APCs) by applying tensile/compressive bending strain εb; (ii) qualified the domain fraction by x-ray diffraction; and (iii) performed DC transport measurements with B||ab ≦ 15 T. Because the controlled domains tend to relax to their original state once the bending is released, experiments (ii) and (iii) were performed while keeping REBCO CCs bent as in (i).
[Results]
We confirmed that the A-domain fraction fA, which was determined from the (200) and (020) reflections, varies linearly with εb as fA(εb) = (56 ± 3) − (3.5 ± 0.2) × 103εb [%] in −0.45% ≦ εb ≦ +0.45%, demonstrating that the domain structure of REBCO CCs can be controlled within this bending-strain window. Tc exhibits a “Λ”-shaped dependence on fA with a maximum at fA ≃ 58% (εb ≃ −0.14%), while the upper critical field Bc2||ab exhibits a “N”-shaped, non-monotonic dependence on fA. Ic(fA, B||ab) measured at 77.3 K with increasing B shows a crossover from an “N”-shaped to a “V”-shaped fA dependence at around 4 T. This crossover in Ic(fA, B||ab) suggests the change in the dominant vortex-pinning mechanism in high- and low-fA samples (i.e., compressive- and tensile-bent REBCO CCs).
[Conclusions]
We succeeded in controlling the fraction of in-plane domain of GdBCO CCs by applying bending strain at room temperature and measuring superconducting properties. Obtained in-field properties of domain-controlled GdBCO CCs without APC depend on the A-domain fraction complicatedly. We found that there is a possibility that in-field quantities of domian-controlled GdBCO CCs become superior to those in the as-received sample, but further investigations are necessary to understand those behaviors.
At the symposium, we would like to explain those above in detail and, if possible, report on-going investigations on REBCO CCs with APCs.