Yoshitaka Shoji

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.

Mechanical properties of Sm-Ba-Cu-O bulk superconductors at room temperature

Abstract In order to investigate the mechanical properties of Sm–Ba–Cu–O bulk superconductors fabricated using TSMG method such that Sm123:Sm211 in molar ratio was 100− Z : Z ( Z =16.7, 28.6), tensile tests in the direction perpendicular to the c -axis have been carried out at room temperature by using the small specimens cut from the bulk samples. Although, non-linear behavior was seen in a few specimens, stress–strain curves were almost linear until the fracture. The non-linear behavior is presumably ascribed to the opening of pre-existing micro-cracks parallel to the c -axis and the slow propagation of them. The Young’s modulus for both samples was almost equal. Poisson’s ratio based on the transverse strain parallel to the c -axis was smaller than that based on the strain perpendicular to it. One of the possible reasons for it will be the reduced transverse shrinkage by the constraint at the interface between the specimen and the aluminum alloy tensile rod coupled with the opening of the pre-existing micro-cracks perpendicular to the c -axis. Tensile strength for the sample with 28.6 mol% of Sm211 was not higher than that with 16.7 mol%. It is presumably ascribed to the low cohesive force between the Sm211 particles and the matrix. The effect of loading condition with respect to that of the crystal growth on the strength was not clear. No significant difference in the strength was found between the specimens including a growth sector boundary and those without it.

Fractographic observations on Sm-Ba-Cu-O bulk superconductors fractured by tensile tests

The fracture surface morphology of top-seeded melt-grown SmBa 2 Cu 3 O x bulk superconductors after tensile tests in (100) direction at room temperature was observed using SEM. There were voids with and without Ag particles of diameters ranging from 80 to <10 μm on the fracture surface of high density sample, while the size was 300 μm at the largest on the low density sample. Ag particles of 10 to 20 μm had the matching mate on the other side of the fracture surface. In the high density samples, the propagation of the crack was easily traced through the flow markings with steps on the fracture surface, while it was difficult to trace in low density samples.

Mechanical properties of Gd123 bulk superconductor at liquid nitrogen temperature

In order to investigate the mechanical properties of the Gd123 single grain bulk superconductor at low temperature, bending tests were carried out. The bending fracture strength at LNT (liquid nitrogen temperature) scattered from 83 to 120 MPa was higher than that at RT, from 62 to 86 MPa. However, the Youngs modulus at LNT scattered from 109 to 123 GPa was comparable to that at RT, from 110 to 129 GPa. The Weibull coefficient for the bending strength at LNT, 6.5, was almost identical with that at RT, 6.9. The endurance limit for the repeated pulsating bending up to the5 /spl times/ 10/sup 4/ cycles at LNT, 70 MPa, was about 70% of the average value of the monotonic bending strength.

Tensile strength and fracture surface topography of Sm?Ba?Cu?O bulk superconductors

Scanning electron microscopy observations have been made on fracture surfaces induced by tensile tests of Ag-added SmBa2Cu3Oy/Sm2BaCuO5 composite bulk superconductors melt-grown in O2 and 1%O2/Ar atmospheres. It was deduced that the inferior tensile strength in the bulk grown in 1%O2/Ar, as compared with that of the bulk grown in O2, was caused by the larger size and volume fraction of the included voids. The tensile strength of a specimen with a fracture surface consisting of multiple facets was lower than that with a single facet for the bulks grown in both conditions. It was found that a rectangular facet on the fracture surface is a possible origin for the fatal crack and that the secondary phase particles on that area act as anchoring points for initial crack propagation.

Tensile strain/transverse compressive stress dependence of critical current in Ag-sheathed Bi(2212) 7-core superconducting wires☆

Abstract The mechanical properties and tensile strain/transverse compressive stress dependence of the critical current I c in Ag-sheathed Bi(2212) 7-core superconducting wires of different constructions were evaluated at a temperature of 4.2 K and a magnetic field of 14 T. The increase in the volume fraction of the superconductor shifted the stress-strain curves upwards. The tensile strain for I c degradation onset was in the range 0.14–0.27%. This strain depends upon the residual compressive strain and the thickness of the superconductor layer. On the other hand, the transverse compressive stress for I c degradation onset was in the range 9–20 MPa. Wires with a thin Ag-sheath layer showed a remarkably low stress for I c degradation onset. There was no welldefined correlation between the onset stress for degradation of I c caused by the tensile stress and that caused by the transverse compressive stress.

Stress/strain dependence of critical current in Cu-Ag externally reinforced Ag-Zr/Bi-2212 superconducting tapes

Abstract The effects of external reinforcement on the mechanical property and the axial tensile stress/strain dependence of the critical current I c in Bi(2212) superconducting tapes were evaluated at a temperature of 4.2 K and a magnetic field of 14 T. The monocore and 19-core superconducting tapes with three kinds of sheath—Ag, Ag-Zr and Ag-Zr/Ag double layer—were incorporated with a reinforcing Cu-Ag tape. As a result of this reinforcement, the 0.2% proof stress of superconducting tapes increased markedly without significant degradation of the I c . The tolerance limit in strain for I c degradation in the tapes was slightly improved and those of stress were markedly improved. The improvement in the tolerance limit in strain or stress for the degradation was more evident in the tapes with the Ag-Zr sheath, compared with those with a pure Ag sheath.

Tensile strain/transverse compressive stress effects in bronze processed Nb-matrix Nb/sub 3/Sn wires

Mechanical properties and the strain/stress dependence of the critical current I/sub c/ of Nb-matrix Nb/sub 3/Sn multifilamentary wires fabricated through bronze route were evaluated up to a magnetic field of 14 T at 4.2 K. The wire showed a 0.2% proof stress of 550MPa, which is 3.4 times higher than that in the conventional bronze processed Nb/sub 3/Sn wires. The strain sensitivity of I/sub c/ and the reversible strain limit, 0.8%, were almost the same, but the axial tensile strain for the peak I/sub c/, 0.1%, was small compared with those in the conventional wires. On the other hand, the transverse compressive stress sensitivity of I/sub c/ was remarkably low. The irreversible stress where I/sub c/ on unloading no longer recovers to the initial value was larger than 300MPa, which is several times higher than those in conventional wires. Thus, these results show that the wire is highly tolerant to the external stress/strain.<<ETX>>

Deformation and fracture behavior of Sm123 bulk superconductors by compressive loading at room temperature

In order to investigate the compressive mechanical characteristics of Sm123 single-grain bulk superconductors fabricated by TSMG method, compressive tests in the direction parallel and perpendicular to the c-axis have been carried out by using small specimens cut from the bulk samples. Stress-strain curves in the direction perpendicular to the c-axis were almost linear. The Youngs moduli measured were 155-165 GPa. In the case of loading parallel to the c-axis, the curves were exponential in the low stress region. As the stress increased, they tended to a linear relation. The apparent Youngs modulus in this region was 75-110 GPa. These characteristics are mainly ascribed to the closure behavior of pre-existing micro-cracks perpendicular to the c-axis. The average fracture strength in the direction of c-axis, 350-400 MPa, was larger than that perpendicular to it, 235-275 MPa. Specimens in the former were generally broken into great many small pieces while those in the latter were broken into some parts by lateral cracking. The compressive Youngs modulus and the fracture strength for each direction were higher than those evaluated by the tensile loading, respectively. The effects of specimen length on the compressive mechanical properties were also discussed.