Kazumune Katagiri

Critical current degradation behaviour in Bi-2223 superconducting tapes under bending and torsion strains

In this paper, we have investigated the Ic degradation behaviours in Ag alloy sheathed Bi-2223 superconducting tapes under bending and torsion strains. In particular, we have examined the homogeneity of Ic degradation along the longitudinal direction of tapes by adopting multiple voltage terminals. The bending modes influenced the Ic degradation behaviour in Bi-2223 tapes. A gradual and consistent decrease of Ic could be observed at all sections, which was a different behaviour from that under tensile loading. Up to eirr, Ic showed a nearly similar degradation at each section, due to a simultaneous initiation of cracks from defects indicating that uniform deformation occurred in tapes with bending. Over eirr, some variations in the Ic degradation occurred due to the difference of crack growth rates at each section. In particular, the behaviour appeared significantly with hard bending. The degradation of Ic in Bi-2223 tapes by torsion occurred gradually when the torsion angle exceeded 150°. However, the Ic degradation by torsion strain was small compared with other types of loading. Also, the Ic degradation behaviour at each section along the longitudinal direction of tapes was gradual and consistent, representing a uniform torsional deformation in the tapes.

Nb/sub 3/Sn multifilamentary wires with CuNb reinforcing stabilizer

A bronze processed multifilamentary Nb/sub 3/Sn superconducting wire with CuNb reinforcing stabilizer (CuNb/Nb/sub 3/Sn), which exhibits a residual resistance ratio of 20 and a magnetoresistance of 0.2 mu Omega -cm at 23 T and 4.2 K, has been developed. The yield stress at 4.2 K, was noticeably improved; a value of 22 kg/mm/sup 2/ was obtained even after heat treatment of Nb/sub 3/Sn formation at 700 degrees C for 200 h. The effect of strain on critical current density, J/sub c/, was described well by using the upper critical field in the prestrain state. It was verified that the CuNb/Nb/sub 3/Sn wire mechanical and superconducting properties need to perform under an enormous electromagnetic force in high magnetic fields.<<ETX>>

Local variations in the critical current degradation of Ag/Bi2223 tape by tensile and bending strains

Using two types of tensile testing device, we have measured the strain dependence of Ic in AgMgNi-sheathed Bi-2223 57 core tapes at 77 K. The Ic degradation onset strains for the specimens with a short gauge length of 17.6 mm varied from 0.40 to 0.78%. In the relatively long samples with seven voltage terminals 5 mm apart along the gauge length of 60 mm, the smallest strain for the initiation of Ic degradation for the sections was 0.26%. The Ic in some other sections, however, began to degrade at larger strains and no degradation was found in another section even at 0.36%, which by far exceeds the strain corresponding to the yield stress of this tape. The local elongation in the first damaged section occupies most of the total elongation in the sample, while the net strain in the latter section is thought to be significantly less than average and no damage occurred there. The bending tests using several mandrels showed considerably consistent Ic degradation behaviour with increasing strain in all the sections, indicating that the strain distribution is uniform in the longitudinal direction.

Room and low temperature direct three-dimensional-strain measurements by neutron diffraction on as-reacted and prebent CuNb∕Nb3Sn wire

We measured directly by neutron diffraction the axial and lateral residual strains for the prebent and the as-reacted CuNb∕Nb3Sn wires at room temperature and at 7K, in order to investigate the change of the residual strain with prebending treatment. In the axial direction of the wire, the residual strain was changed with 0.20% to the tensile side when measured at both temperatures, while in the lateral direction, the change was 0.08% and 0.03% to the compressive side for the measurements at RT and 7K, respectively. From the obtained data, we estimated the deviatoric strain. At 7K, the value is 0.40% for the as-reacted wires and it reduces to 0.19% when the prebending is applied with a strain epb=0.8%. These results suggest that the reduction of the residual strain in the axial direction as well as in the lateral direction, i.e., of the deviatoric strain is responsible for the observed enhancement in the superconducting properties of the prebent Nb3Sn wires. In addition, we succeeded in the quantitative e...

Tensile mechanical properties of (Nd,Eu,Gd)–Ba–Cu–O bulk superconductors at room and liquid nitrogen temperatures

Abstract In order to investigate the mechanical properties of a recently developed high performance melt-processed (Nd,Eu,Gd)Ba 2 Cu 3 O y (NEG-123) single grain bulk superconductor, tensile tests of specimens cut from the bulk have been carried out at room temperature, 293 K, and at liquid nitrogen temperature, 77 K. Tensile stress was applied in a - or b -direction. The maximum value of tensile strength and Youngs modulus at 77 K, 41 MPa and 158 GPa were higher than those at 293 K, 33 MPa and 134 GPa in the NEG-123 sample with 40 mol% (Nd,Eu,Gd) 2 BaCuO 5 (NEG-211) secondary phase particles, 0.5 wt% Pt and 10 wt% Ag 2 O. Tensile strength of the sample added 30 mol% NEG-211 and the sample added 30 mol% Gd 2 BaCuO y instead of 40 mol% NEG-211 that was added to the sample described above have also been investigated.

Mechanical properties of Gd123 bulk superconductors at room temperature

Abstract In order to investigate the mechanical properties of Gd123 single-grain bulk superconductors fabricated using a modified quench and melt growth method, tensile tests in the direction parallel and perpendicular to the c -axis have been carried out at 293 K by using the small specimens cut from bulk superconductors. As for the mechanical properties perpendicular to the c -axis, there was no significant difference between those in the crystal growth direction and those perpendicular to it. While the average value of the Young’s modulus of the bulk sample with 33.0 mol%-Gd211 secondary phase particles, 118 GPa, was higher than that of the bulk sample with 28.7 mol%-Gd211, 111 GPa, the average value of the tensile strength of the former, 36 MPa, was lower than that of the latter, 40 MPa. The tensile strength and the Young’s modulus in the c -axis, 10 MPa and 37 GPa, were quite low compared with those mentioned above. Poisson’s ratio based on the transverse strain in the c -axis, 0.15, was significantly smaller than that perpendicular to it, 0.30. In the specimens with higher length, however, the difference was decreased to some extent. With regard to the anisotropy of the Poisson’s ratio, the effect of a pre-existing micro-crack opening in the c -axis direction was discussed coupled with the constraints at the interfaces between the specimen and the sample holder.

Reliability of mechanical properties for bulk superconductors with resin impregnation

Abstract We have studied the mechanical properties of Y–Ba–Cu–O rods with and without resin impregnation using a three-point bending test. The reliability of resin impregnation was evaluated using the Weibull distribution function. The Weibull coefficient for the resin-impregnated bulk was 16.03 at 77 K. In order to obtain more reliable data, the bending strengths were converted into the tensile strengths by considering a difference in the effective volume. It was confirmed that the tensile strengths of bulk YBCO could be enhanced by more than four times with resin impregnation. It was also confirmed that resin-impregnated Y–Ba–Cu–O exhibited a high Weibull coefficient and thus a high reliability for practical engineering applications.

Vickers hardness properties of structural materials for superconducting magnet at cryogenic temperatures

In order to estimate the tribological properties of structural materials for superconducting magnet at cryogenic temperatures, we investigated Vickers hardness from 4.2 K in liquid helium to 293 K. The clear change of hardness property was obtained around 77 K. We considered that it would be caused by a transformation of the deformation mechanism from dislocation slip at higher temperature to deformation twinning at lower temperature from the thermal activation process. We also considered that the transition temperature changing hardness property was related to the intersection temperature at which slip stress became equal to twinning stress. The change of deformation mechanism was confirmed experimentally by changing the loading rate, and by the TEM observation of the indentation scar.

Enhancement of Critical Current Densities by the Prebending Strain at Room Temperature for Nb3Sn Wires

The influence of a repeated bending strain at room temperature on the critical current densitiy of bronze route Nb3Sn wires was investigated. We found that the critical current Ic is significantly increased by applying a repeated bending load (pre-bending treatment) at room temperature. The maximum enhancement of Ic was approximately twice at 16 T for CuNb-reinforced Nb3Sn wires by a 0.5% prebending strain and the critical current density Jc became about 528 A/mm2 at 4.2 K and 16 T. A comparison of the stress dependent Ic values obtained before and after applying prebending treatments for CuNb/Nb3Sn wires indicates that not only the stress dependence of Ic but also the maximum Ic at the axial strain-free state are improved. The improvement of the stress dependent Ic is in agreement with the prediction made based on the calculation using the uniaxial stress/strain distribution model. Moreover, the increase in the maximum Ic may be related to the release of stress/strain states along the radial direction through prebending treatments.

Stress–strain characteristics and fracture surface morphology of (Sm,Gd)–Ba–Cu–O bulk superconductor

Abstract The tensile strength in a - or b -direction of melt-processed (Sm,Gd)–Ba–Cu–O single grain bulk superconductor without Ag addition have been investigated by using specimens cut from the bulk sample. The maximum value of tensile strength of the specimens annealed in pure O 2 atmosphere was higher than that of the Y– and Sm–Ba–Cu–O bulk superconductor without Ag addition previously reported. The tensile strength was relatively higher in the central region of the bulk. Fracture surface of the specimens with relatively high tensile strength included the large region with the steps perpendicular to the c -direction. Flow markings showing crack propagation were found on the region without steps on the fracture surfaces.