鈴木 匠

Abstract Given their excellent superconducting properties, REBa2Cu3Oy (REBCO)-coated conductors (CCs) are anticipated to be utilized in a variety of magnet applications. To further increase the critical current density Jc of these materials to levels needed for commercial applications, this study employs reel to reel (RTR) pulsed laser deposition (PLD) to fabricate REBCO+BaHfO3 (BHO) CCs. PLD creates BHO nanorods, which serve as flux-pinning defects. The material is subjected to O2+ irradiation to introduce more defects. The irradiation-induced defects serve as flux-pinning centers to the REBCO+BHO-nanorod CCs, increasing Jc along the c axis and over a wide range of magnetic-field angles compared with conventional REBCO+BHO-nanorod CCs. Both nanorods and irradiation-induced defects are demonstrated to be effective pinning centers in this material.

© 2002-2011 IEEE. It has been recognized as a significant issue that the magnetization of RE-123 coated conductors affects the spatial homogeneity and the time variation of the magnetic field of the magnets for MRI, NMR, and accelerators. Therefore, the understanding and the modelling of the magnetization of a coated conductor are crucial for the quantitative estimation and the compensation of its influence on a magnet. On the other hand, the magnetization of the coated conductor has been usually measured and analyzed as a global value; then it is difficult to clarify the local electromagnetic behavior governing such a global performance. Furthermore, such behavior should be investigated under the condition not only with external magnetic field but also with DC transport current as is exposed in a magnet. In this study, the magnetization of a coated conductor was characterized by a spatially-resolved measurement based on the scanning Hall-probe microscopy (SHPM). The magnitude and the time variation of the magnetization were clarified from the visualized magnetization current distribution and its time variation. In particular, they were modeled successfully including the influence of the transport current. Furthermore, taking account of the findings, the experimental results were successfully reconstructed by a numerical analysis based on finite element method (FEM). This will contribute to the quantitative estimation and the compensation of the magnetization problem for the magnets comprising RE-123 coated conductors.

We have investigated the in-field critical current properties in BaZrO3-added (Y, Gd) Ba2Cu 3O7-δ (YGdBCO)-coated conductor fabricated by using a multi-coating trifluoroacetates metal-organic deposition (MOD) method. We compared these properties with EuBCO-coated conductor fabricated by the pulsed laser deposition (PLD) method. The sample using the 30-nm once-coat-layer-thickness donce shows superior in-field Jc down to 4.2 K than that of the previous standard coating using 170-nm-thick layer for each coating. From the analysis of E-J characteristics, an analytical expression for Jc as a function T and B has been derived. It was confirmed that the high Jc region in sample with donce = 30 nm is widely spread in high-temperature and high-magnetic-field region. By comparison with the Jc properties of the BaHfO3 -added PLD CCs, the minimum Jc, which is estimated from magnetic field angle dependence, shows even higher value up to 5 T of magnetic field at 65 K and up to 3 T at 77 K. From these results, the new MOD-YGdBCO process using the thin once-coat-layer-thickness is very promising for the practically important mid field region such as 3 to 5 T.

We investigated the relationship between current capacity and local critical current distribution of a Cu-sheathed multifilamentary RE-123 coated conductor (CC). Patterning multifilamentary structure on CC will be a promising solution for reducing magnetization to assure spatial homogeneity and its temporal stability of magnet applications such as MRI and NMR. On the other hand, it will become more difficult to maintain the current capacity because a smaller defect can block the current flow in a narrower filament. Permitting electrical coupling among the filaments will work for maintaining current capacity because current can avoid such a defect by flowing into the adjacent filament. However, too small interfilamentary resistance will result in long time constant of filament coupling, which will affect spatial homogeneity and its temporal stability of magnet applications. Therefore, to design a multifilamentary CC satisfying the requirement from magnet applications, it is necessary to understand the quantitative impact of interfilamentary resistance of the multifilamentary CC on its current capacity under the influence of spatial variation of local critical currents. In this study, we estimated global critical current of a Cu-sheathed multifilamentary CC as a function of interfilamentary resistance by considering its local critical current distribution in each filament. As a result, it was confirmed that the electrical coupling among the filaments was very effective to improve the current capacity of such a multifilamentary CC especially for a section with spatially inhomogeneous local critical currents. Furthermore, it was also found that local heat generation could be significantly suppressed even for a section with relatively homogeneous local critical currents.

We have investigated flux flow dissipation in typical two kinds of HTS tapes, i.e., a Bi-2223 multi-filamentary tape and a RE-123 coated conductor (CC) from the view point of heat load under over current conditions. Based on systematic measurements on current-voltage characteristics, the nonlinear flux flow dissipation has been described analytically by taking into account current sharing in metallic sheath or stabilization layer. Flux flow dissipation in the RE-123 CC shows much steeper temperature dependence than that of the Bi-2223 tape. As a result, attainable cooling power becomes smaller in the RE-123 CC in comparison with that of Bi-2223 tape even if the same cooling condition. In other word, acceptable temperature rise in the RE-123 CC is small at over current condition, whereas moderate temperature dependence in the Bi-2223 tape allows stable operation even if the bias current exceeds the critical current. Influence of spatial inhomogeneity in the both HTS tapes has also been investigated. Longitudinal variation of local critical current, I-c, and its statistical behavior have been characterized by use of reel-to-reel scanning Hall probe microscopy. It has been found that the flux flow dissipation is possibly localized more than one order higher than that of the average value due to discrete local I-c drops. (C) 2016 Published by Elsevier Ltd.

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.

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 T-c 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 T-c for the a and b axes obey a power law but exhibit opposite slopes. In particular, the maximum value of T-c 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 approximate to 0.385 nm is the optimum condition for a high critical temperature in high-T-c cuprates.

The high irreversibility field B-1 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 77.3K, B-1 of 15.8 T and the maximum flux pinning force density of 23.5 GN/m(3) for B parallel to c were achieved for the 1.5 vol % BaHfO3-doped GdBa2Cu3Oy tape, which are the world's 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. (C) 2015 The Japan Society of Applied Physics

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. © 2013 IOP Publishing Ltd.

The electronic specific heat of as-grown and annealed single-crystals of FeSe1-xTex (0.6 <= x <= 1) has been investigated. It has been found that annealed single-crystals with x = 0.6-0.9 exhibit bulk superconductivity with a clear specific-heat jump at the superconducting (SC) transition temperature, T-c. Both 2 Delta(0)/k(B)T(c) [Delta 0: the SC gap at 0 K estimated using the single-band BCS s-wave model] and Delta C/(gamma(n) - gamma(0))T-c [Delta C: the specific-heat jump at T-c, gamma(n): the electronic specific-heat coefficient in the normal state, gamma(0): the residual electronic specific-heat coefficient at 0 K in the SC state] are largest in the well-annealed single-crystal with x = 0.7, i.e., 4.29 and 2.76, respectively, indicating that the superconductivity is of the strong coupling. The thermodynamic critical field has also been estimated. gamma(n) has been found to be one order of magnitude larger than those estimated from the band calculations and increases with increasing x at x = 0.6-0.9, which is surmised to be due to the increase in the electronic effective mass, namely, the enhancement of the electron correlation. It has been found that there remains a finite value of gamma(0) in the SC state even in the well-annealed single-crystals with x = 0.8-0.9, suggesting an inhomogeneous electronic state in real space and/or momentum space.

Single crystals of FeSe1-xTex (0.5 <= x <= 1) have been grown by the Bridgman method. After annealing them at 400 degrees C for 100 h in vacuum, single crystals of x = 0.5-0.9 have exhibited bulk superconductivity. Anisotropic properties of the electrical resistivity and upper critical field, H-c2, have been investigated for the single-crystal FeSe1-xTex with x = 0.6. It has been found that the in-plane resistivity, rho(ab), shows a metallic temperature-dependence, while the out-of-plane resistivity, rho(c), shows a broad maximum around 100 K. The resistivity ratio, rho(c)/rho(ab), is 44 and 70 at 290 K and just above the superconducting transition temperature, T-c, respectively. The anisotropic parameter, gamma H-c2(parallel to)/H-c2(perpendicular to) (superscripts parallel to and perpendicular to indicate field directions parallel and perpendicular to the ab-plane, respectively.), is estimated as 2.7 just below T-c.