Kozo Osamura

Electrical and Mechanical Properties of DI-BSCCO Type HT Reinforced With Metallic Sheathes

Electrical and mechanical characteristics of Type HT tape, which is standard 4.2 mm wide DI-BSCCO tape reinforced with metallic tapes, have been evaluated. Longitudinal distributions of critical current and n-index in kilometer long Type HT tape has proved uniform from end to end just like the original insert tape, which is Type H tape. Type HT-CA reinforced with 50 mum thick heat-resistant copper alloy is highly balanced tape with high mechanical properties and low splice resistance. Type HT-SS reinforced with 20 mum thick stainless steel has the best mechanical properties, which has been demonstrated under the actual environment in high field magnet, namely the hoop stress load test energizing a one-turn coil in external high magnetic field and liquid helium.

Force free strain exerted on a YBCO layer at 77 K in surround Cu stabilized YBCO coated conductors

The stress/strain behavior of the surround Cu stabilized YBCO coated conductors and its influence on critical current were precisely investigated. The internal strain exerted on the superconducting YBCO layer was determined at 77 K by using a neutron diffraction technique at JAEA. The initial compressive strain decreased during tensile loading and changed to a tensile component at the force free strain (Aff), where the internal uniaxial stress becomes zero in the YBCO layer. The Aff was evaluated to be 0.19–0.21% at 77 K. The critical current measurements were carried out under a uniaxial tensile load at 77 K. The strain dependence revealed a characteristic behavior, where a maximum was observed at 0.035%. Thus it was made clear that the strain at the critical current maximum does not correlate with Aff for YBCO coated conductors.

Reversible strain limit of critical currents and universality of intrinsic strain effect for REBCO-coated conductors

Intensive research work has been carried out in order to develop industrially available HTS REBCO-coated conductors under the NEDO project in Japan. Recently, several groups in the project succeeded in the development of high performance coated conductors. Their characteristic features have been evaluated in terms of mechanical properties and their influence on critical currents. The mechanical properties at RT and 77 K were analyzed on the basis of the rule of mixtures. The force-free strain (Aff) was analytically deduced, which indicates the strain at which the residual stress exerted on the superconducting layer becomes zero. Tensile strain dependence on critical currents could be divided into elastic and brittle regions. The reversible strain limit (Arev) was defined as a strain at which the critical current recovers elastically to the level of 99% Ico. Within the elastic region, the critical current showed a convex strain dependence, which is explained as Ekins intrinsic strain effect. The degradation beyond the reversible strain limit was attributed to a fracture of the superconducting layer. As a whole, the present study made clear quantitatively the tensile strain behavior of critical currents and proposed a reasonable definition for the reversible strain limit.

Improvement of Reversible Strain Limit for Critical Current of DI-BSCCO Due to Lamination Technique

The DI (dynamically innovative)-BSCCO-Bi2223 tapes achieved high critical current as well as high modulus of elasticity. Further the reversible strain limit and the corresponding stress for critical current have been remarkably increased by means of lamination technique. During the course of development, their optimized architecture has been designed based on the principle of the rule of mixture for maximizing the force free strain exerted on the superconducting component. The reversible strain/stress limit (A rev/R rev) was defined as a strain, at which the critical current recovers to the level of 99% I co. Selecting several kinds of laminating materials and changing condition of the fabrication, the excellent Cu alloy-3ply tape with I co of 311 A/cm was realized of which A rev and R rev reached 0.42% and 300 MPa, respectively. Further during the theoretical analysis, the increase of reversible strain limit were made clear to be attributed to the increase of thermally induced residual strain as well as the compensation effect of laminated layers against a local fracture mode.

Development of a cryogenic load frame for a neutron diffractometer

Strain measurements under loading at cryogenic temperatures are much requested for investigation of the stress/strain effect on the critical current for composite superconductors. In order to provide in-situ measurements of the lattice strain, a cryogenic load frame with a GM refrigerator has been developed, which is suitable for the neutron diffraction facility RESA equipped at JRR-3 in JAEA. The lowest temperature of 4.8 K was achieved, while the capacity of the load frame was 10 kN. Using the present cryogenic load frame, plane spacing measurement was performed under loading for two specified samples of 316 stainless steel and engineering Y–Ba–Cu–O (YBCO) coated conductor. The relation between applied stress/strain and lattice strain has been made clear in a wide range of temperatures.

Local strain and its influence on mechanical–electromagnetic properties of twisted and untwisted ITER Nb3Sn strands

It is important to evaluate the local strain exerted on superconducting filaments in Nb3Sn strands, because it influences both superconducting and mechanical properties, in particular for the ITER (International Thermonuclear Experimental Reactor) project. The local strain in the twisted and untwisted Nb3Sn strands was directly measured at room temperature as well as at low temperatures by means of quantum beam techniques. The local strain consists of thermal strain and lattice strain. The latter changes as a function of external strain. The interrelation between the force-free strain and the intrinsic strain showing a maximum critical current was considered on the basis of the present experimental data as well as the recent theory. The thermal strains along both directions parallel and transverse to the strand axis were numerically evaluated. Their evaluated results could explain well the observed values, when To is the recovery temperature of pure Cu. The force-free strain along the axial direction is deduced to be distributed among grains with different crystal orientation with respect to the axial direction. It is suggested that this fact affects the definition of intrinsic strain.

Internal Strain and Mechanical Properties at Low Temperatures of Surround Cu Stabilized YBCO Coated Conductor

The mechanical properties of surround Cu stabilized YBCO coated conductor were assessed at 5.7, 77 and 298 K. The internal strain exerted on the superconducting YBCO layer was directly evaluated under tensile load at low temperatures down to 9.8 K by neutron diffraction techniques. The compressive internal strain, present in the YBCO layer without external load, is the thermally induced residual strain. When the external tensile load was applied, the compressive component of internal strain decreased and changed into tensile. The force-free strain, Aff, was determined as the strain at which the internal strain becomes zero. The estimated from (200) diffraction data depended weakly on temperatures between 298 and 9.8 K. However, the estimated from (020) data decreased prominently with decreasing temperature.

Microtwin Structure and Its Influence on the Mechanical Properties of REBCO Coated Conductors

The elastic properties of REBCO tape (i.e., REBa₂Cu₃ O_7-d, RE = <i>Y</i>, Sm and Gd) have been investigated by means of diffraction techniques using synchrotron radiation. Total local strain <i>A</i>_hkl^l, which consists of thermal strain <i>A</i>_hkl^T and lattice strain <i>A</i>_hkl, was analyzed using a microscopic structure model, with two types of microtwin configuration along the tensile axis, i.e., with the [100] or [110] crystal axis oriented parallel to the longitudinal direction of the tape. Experimental results were: (a) slope <i>dA</i>_h00^l/<i>dA</i> is larger than <i>dA</i>_0k0^l/<i>dA</i> (where <i>A</i> is the external tensile strain), which is attributed to the elastic modulus <i>E</i>_a along the <i>a</i>-axis being smaller than <i>E</i>_b along the <i>b</i>-axis; (b) both <i>dA</i>_h00^l/<i>dA</i> and <i>dA</i>_0k0^l/<i>dA</i> are smaller than unity, but <i>dA</i>₁₁₀^l/<i>dA</i> is almost unity depending on the configuration of microtwin; (c) the thermal strain for different planes is different as <i>A</i>_h00^T ≠ <i>A</i>_0k0^T due to the elastic behavior of the superconducting layer being modified because it is constrained by the substrate and the outer metallic layer; and (d) a large Poisson ratio is attributed to this constraint state. It is suggested that the microtwin structure is critical to understand and model this unusual stress/strain behavior.

In situ strain measurements by neutron diffraction and residual stress estimation in Ag-sheathed Bi2223 superconducting composite tapes

Heretofore, an accurate residual stress in Ag-sheathed superconducting composite tape could not be obtained from a measurement using conventional low energy x-rays due to the large x-ray absorption in Ag phase. Therefore, we developed a method for evaluating the residual strain in the filaments using neutron diffraction and finite element method (FEM) analysis, measuring the mechanical curvature. As for the residual strain of the Ag phase and Bi phase in a high strength type specimen, the Bi phase has a compressive longitudinal strain, and the Ag phase has a tensile strain; these are thermally balanced residual stresses at room temperature that depend on the mismatch of the coefficient of thermal expansion (CTE). On the other hand, there was very little thermally induced stress at the room temperature in the high Ic specimen. The numerical residual strain obtained by the FEM analysis agrees with experimental neutron method results. In a high critical current (Ic) type specimen, the existence of a stress relaxation mechanism during the heat treatment is suggested.

Internal Strain Behavior Exerted on YBCO Layer in the YBCO Coated Conductor

The stress/strain dependence of elastic property of the surround Cu stabilized YBCO coated conductor was precisely investigated by means of white X-ray and pulsed neutrons. Multiple diffraction peaks were observed along axial and lateral directions with the tape axis as a function of tensile load. The following unusual stress/strain behaviors exerted on the YBCO layer were made clear. Pairs of (020)/(200) and (040)/(400) were observed side by side in two rows and their diffraction intensity was almost constant with increasing tensile load. The ratio of diffraction strain to external strain became less than unity. The diffraction elastic constants estimated from (0k0) diffraction peaks were larger than from (h00) peaks. Their observations strongly suggest that the micro twin structure is key nanostructure to understand the microscopic elastic constant and strain obtained from the diffraction experiments.