Takumi Suzuki

High-performance irreversibility field and flux pinning force density in BaHfO3-doped GdBa2Cu3Oy tape prepared by pulsed laser deposition

The high irreversibility field Bi and strong flux pinning force were obtained for BaHfO3-doped GdBa2Cu3Oy tapes on a metallic Hastelloy substrate with an ion beam-assisted deposition MgO buffer layer. At 77.3 K, Bi of 15.8 T and the maximum flux pinning force density of 23.5 GN/m3 for B ∥ c were achieved for the 1.5 vol % BaHfO3-doped GdBa2Cu3Oy tape, which are the worlds highest recorded values for REBa2Cu3Oy-coated conductors on metallic substrates (RE = rare earth). From the flux pinning analysis, the c-axis correlated pinning by well-aligned BaHfO3 nanorods plays an important role in achieving the high irreversibility field and flux pinning force density performance.

Relation Between the Crystal Axis and the Strain Dependence of Critical Current Under Tensile Strain for GdBCO Coated Conductors

The strain and the critical current of the GdBCO coated conductor were investigated in order to understand the mechanism of the strain effect for a coated conductor. The internal strain behaviors of the longitudinal, width, and thickness directions were measured by the X-rays of the synchrotron radiation. From these results, the residual strains of a, b, and c-axes for the GdBCO crystal were estimated. In addition, the strain dependence of the critical current and the n-value at 4.2 and 77 K were measured under the tensile strain. The relationship between superconducting properties and the triaxial strain in the GdBCO crystal of the coated conductors is discussed.

Strain Dependence of Superconducting Properties for GdBCO Coated Conductor in High Field Under Tensile Load

The strain dependence of T_c and I_c for the GdBCO coated conductor was measured at 77 and 4.2 K in magnetic fields. T_c decreased with increasing applied tensile strain. For the I_c measurements, the magnetic field was applied in the direction parallel to the c-axis (perpendicular to the tape surface). I_c in fields was degraded with increasing strain, and the strain dependence of I_c increased with increasing magnetic field. The ratio of the I_c degradation at 4.2 K was smaller than that at 77 K. These results mean that the T_c degradation is one of the reasons for I_c degradation at 77 K at least. Further, in order to understand the strain effect at 4.2 K, the internal strain of the GdBCO coated conductor under tensile strain was measured using synchrotron radiation. It was found that the strain behavior for the a-axis is different from that for the b-axis. This result suggests that the strain dependence of I_c for GdBCO coated conductors under external tensile strain is related with the T_c degradation behavior of the crystal.

Strain-controlled critical temperature in REBa2Cu3Oy-coated conductors

Recently, we succeeded in detwinning REBa2Cu3O7 (RE123, RE = rare-earth elements)-coated conductors by annealing under an external uniaxial strain. Using the untwinned RE123 tapes, the uniaxial-strain dependencies of the critical temperature Tc along the a and b crystal axes were investigated over a wide strain region from compression to tension. We found that the strain dependencies of Tc for the a and b axes obey a power law but exhibit opposite slopes. In particular, the maximum value of Tc is obtained when the CuO2 plane becomes a square, and its lattice constant is close to 0.385 nm. It is suggested that a tetragonal structure with a ≈ 0.385 nm is the optimum condition for a high critical temperature in high-Tc cuprates.

Applied Strain Effect on Superconducting Properties for Detwinned (Y, Gd)BCO Coated Conductors

We succeeded in the complete detwin of (Y, Gd)BCO coated conductors using a newly developed in-situ tensile apparatus at high temperature. The intensities of (020) and (200) peaks in a longitudinal and a transverse directions of the tape measured by XRD were similar to each other for usual (Y, Gd)BCO coated conductors before annealing. However, the (200) peak disappears for the longitudinal direction after annealing under the tensile stress. It means that the coated conductors are detwinned by annealing under the tensile stress. We found that Tc linearly decreases with increasing strain for the B-domain (b-axis). This behavior is similar with a uniaxial-pressure effect in (Y, Gd)BCO single crystals. However, the strain dependence of Tc for the A-domain (a-axis) behaves as a power-law function, whereas the uniaxial-pressure dependency of Tc for the a-axis is reported to be linear. In addition, the strain dependencies of Jc are similar with those of Tc.

Control of residual strain and twin boundary by annealing under strain

In REBa2Cu3Oy (RE123, RE=rare earth element) coated conductors, the coexistence of a- and b-axes domains related to a twin structure is one of the origins of the complicated critical current density property and its strain dependence. Hence, control of the twin structure is a very important issue. In the case of Y123 single crystals, detwinning is widely carried out by oxygen annealing under uniaxial stress along the a/b axis. In this study, commercial RE123 superconducting tapes were oxygen-annealed under external strains, similar to the detwinning process of Y123 single crystals. We found that the volume fraction ratio of the a-?and b-axis domains changes with the applied external strains during annealing, indicating a change of twin structure. In addition, the residual strains, which are strains induced from the thermal contraction difference between RE123 and the substrate, increase with compression annealing and decrease with tension annealing for RE123 tapes with [100]/[010] orientation along the tape axis. However, those phenomena by strain annealing are not obtained for the [110] oriented RE123 tapes. The strain annealing method that we propose here is very useful to control the internal residual strain and twin structures in an RE123 tape with a [100]/[010] orientation along the tape axis.

Superconducting and Mechanical Properties of Impregnated REBCO Pancake Coils Under Large Hoop Stress

We performed hoop stress tests of REBCO multilayer pancake coils impregnated by epoxy resin. The mechanical deformation and electric field-current properties were measured under the large hoop stress. The maximum hoop stress of about 530 MPa per Hastelloy substrate, calculated from the BJR relation, was applied in the background magnetic field of 8 T. Because the hoop stress level is much smaller than the mechanical tolerance of the GdBCO tape, the coil performance was limited by the angular dependence of critical current in this test. Furthermore, the hoop stress test under the large electromagnetic stresses was also carried out for the other (Y, Gd)BCO coil. The (Y, Gd)BCO epoxy impregnated coil was operated without any degradation even in the huge hoop stress over 1300 MPa. The mechanical deformation of the coil is analysed on the basis of the measured strains. We confirmed the large strain about 0.3%-0.5% because of the hoop stress over 1300 MPa at Iop = 460 A and B = 13.5 T but no degradation of the coils. However, it is suggested that the coil deformation is very complicated under the large electromagnetic stress.

Improvement of

GdBa2Cu3Oy coated conductors were annealed under external strain (strain-annealing). The residual strain after annealing was evaluated using synchrotron radiation. We found that the residual strain is changed by the strain-annealing. The volume fraction of the a-axis domains along the longitudinal direction is increased by compression-annealing and decreased by tension-annealing. In addition, the Jc value of the tension-annealed sample increases drastically below 70 K, although the Jc value is smaller than that of the as-received sample at 77.3 K for B//c. The peak of the angular dependent Jc at θ = 0°, which originates from the c-axis correlated pinning, for the strain-annealed sample is much smaller than that of the as-received sample. It is considered that the shrinking of the c-axis correlated pinning because of the reduction of the twin structure and the increase of carrier doping may be related to the change in Jc properties.