Tsuneo Kuroda

Reversible strain dependence of critical current in 100 a class coated conductors

The strain dependence of the critical current was studied for YBCO and DyBCO coated conductors with different buffer layers on Hastelloy substrates. A maximum of I/sub c/ was observed for both the YBCO and DyBCO tapes, however the sign of the strain at the I/sub c/ peak was opposite for the two superconductors. A reversible variation of I/sub c/ with applied strain was found and the reversible strain limit was observed to depend on the buffer layer. For the IBAD-CeO/sub 2//YSZ buffered YBCO tapes, I/sub c/ recovers reversibly when the applied strain is reduced starting from 0.30%. For those with an ISD-MgO buffer layer the irreversible degradation starts at a strain less than 0.22%. The reason for this difference is discussed based on microscopic observations. Quenching occurred during V-I measurements after the applied strain exceeded 0.30%, which is close to the yield strain of the composite tape.

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.

Effect of SiC nanoparticle addition on the critical current density of MgB2 tapes fabricated from MgH2, B and MgB2 powder mixtures

The effects of the addition of SiC nanoparticles were compared for MgB2 tapes prepared from two different starting powders with respect to the improvement in the critical current, Jc, at 4.2 K and 10 T. The starting powder was prepared by blending premixed MgH2 and B(MgH2+2B) and premixed (MgH2+2B) and MgB2 (30% MgB2) in a ratio of 7:3. All tapes were fabricated by the in situ powder-in-tube method. The use of 30% MgB2 as standard powder yields higher Jc values than MgH2+2B in the case of the pure MgB2 tape; however, the highest Jc (1.8 × 104 A cm−2) was obtained for the tape from MgH2+2B with 10 mol% SiC additive. The tape from 30% MgB2 contained regions with Si-free MgB2 grains, while Si compounds were distributed homogeneously across the tape prepared from MgH2+2B. Furthermore, the use of 30% MgB2 instead of MgH2+2B was found to reduce the effect of carbon substitution of the SiC additive. These data indicate that the technique, which enhances Jc–B characteristics of the pure MgB2 tape, does not promise further improvement through the addition of nanosized impurity particles. It is concluded that we should improve the fabrication technique of the MgB2 tape by keeping (or improving) the overall reactivity between the standard powder and nanosized impurity particles.

Critical-current degradation in multifilamentary Nb/sub 3/Al wires from transverse compressive and axial tensile stress

The effect of transverse compressive stress and axial tensile stress on the critical current of multifilamentary Nb/sub 3/Al superconducting wires has been measured. Compared with commercial Nb/sub 3/Sn, Nb/sub 3/Al exhibits a relatively small sensitivity to both axial and transverse stress. For a given degradation of critical current at 9 T, Nb/sub 3/Al will tolerate about twice as much axial stress as Nb/sub 3/Sn and about five times the transverse stress. The elastic modulus of Nb/sub 3/Al, 169+or-20 GPa, was measured for the first time at cryogenic temperatures and found comparable to that of Nb/sub 3/Sn. A comparison between the effect of axial and transverse stress showed that Nb/sub 3/Al, like Nb/sub 3/Sn, is more sensitive to transverse stress than axial stress. For a given level of critical-current degradation, the transverse stress tolerance of Nb/sub 3/Al is about half the axial stress tolerance. The favorable electromechanical characteristics of Nb/sub 3/Al, compared with Nb/sub 3/Sn, may allow increased operating limits for the next generation of large high-field superconducting magnets.<<ETX>>

Analysis of critical current distribution of bent Bi2223 composite tapes by unifying parameter approach and its application to the description of average critical current–bending strain relation near the average irreversible strain

When the critical current of bent Bi2223 composite tape is measured for many specimens, the critical current value is different from specimen to specimen, being distributed from low to high values. We need to characterize the distribution of critical current at various applied strains and also to predict the distribution of irreversible strain for reliability and safe design. The present work attempted to describe them from the statistical viewpoint. In the analysis, the unifying parameter approach proposed recently by the authors was employed, in which the difference in damage evolution and critical current among the specimens was correlated with the distribution of the unifying parameter that refers to the tensile damage strain of Bi2223 filaments in composite tapes. By application of this approach to the measured critical current values under various bending strains for three Bi2223 composite tapes from different suppliers (VAM1, VAM2 and VAM3 used as the common test samples in the round robin test of VAMAS/TWA16), the unifying parameters were estimated and were formulated by the Weibull distribution function. With the formulated distribution function of the unifying parameter, the distributions of the critical current at various strains, the distribution of the irreversible bending strain and the average critical current?bending strain relation near the average irreversible bending strain could be described satisfactorily.

Complementary analysis of the results of a bending strain effect round robin-test on the critical currents of Ag alloy-sheathed Bi-2223 tapes

As a result of the first round robin-test (RRT) on the bending strain effect in Ag alloy-sheathed Bi-2223 superconducting tapes, it has been found that the scatter in the critical current data reported from 12 participant laboratories is not very large; the coefficient of variation (COV) is, for example, less than 6% at a bending strain of 0.6%. However, the bending strain limit, which is practically a more important parameter and is defined as a strain where the critical current degrades by 10%, has been found much scattered (17% in COV) in the first RRT. To clarify the origins of data scatter and finally reduce it, complementary analysis and tests on the effects of thermal stress relating to the sequences of the bending of the specimen and critical current measurements have been carried out. Additionally, it has been found that the sample inhomogeneity in mechanical properties contributes to some part of data scatter.

Field and temperature dependences of critical current in direct-heated Nb-tube processed Nb/sub 3/Al wires

High-field performance of Nb-tube processed Nb/sub 3/Al conductors is improved and allows these conductors for use above 20 T, when continuous direct-heating is applied before final annealing in the fabrication process. In the present study we prepared 0.74 mm/spl phi/ Nb/sub 3/Al wires containing 121/spl times/121 7-core elementary Nb/Al-Mg composites by this modified method and measured their critical current, magnetization and susceptibility as a function of field and temperature. From these measurements, we have found that the volume pinning force, F/sub p/ of these wires simply follows the Kramers scaling law, i.e. F/sub p//spl prop/B/sub c2//sup 2.5/b/sup 0.5/(1-b)/sup 2/ w here b=B/B/sub c2/, for the entire range of temperature measured, 4.2/spl sim/16 K. Since A15 phase grains formed are very fine, the most probable pinning source seems to be grain boundaries which are dense compared to other possible sources such as interfaces between Nb-Al alloy phase and Nb/sub 3/Al filaments.

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>>

Influence of test methods on critical current degradation of Bi-2223/Ag superconductor tapes by bending strain

The results of two test methods were compared among three laboratories to determine a standard measurement method of critical current (Ic) as a function of bending strain for Ag-sheathed Bi-2223 superconductors. The VAMAS round-robin-test method (RRT) and the bending-rig method developed by Goldacker were used. The Ic degradation started with less bending strain for RRT than for bending-rig. Average irreversible strains (eirr) were 0.30% for RRT and 0.37% for bending-rig. Another test identified parameters that affected the results. A modified RRT method, with a current connection between the sample and the electrode, was used to avoid some thermal stresses of the test procedure. The eirr values increased to the level of the bending-rig, but the modified RRT Ic degradation rate with bending strain was higher. The stress states during sample bending differed between these methods. The shear stress was examined as a source of the Ic degradation rate differences with strain in terms of the crack propagation and delamination defects of oxide filaments from the Ag sheath.

Critical-current degradation in Nb3Al wires due to axial and transverse stress

Abstract Effects of axial and transverse stress on the critical current of Nb-tube processed multifilamentary Nb 3 Al wires have been studied. The degradations of the critical current due to axial or transverse stress for these Nb 3 Al wires are much smaller than those for Nb 3 Sn wires. The microstructures and stress states of the Nb 3 Al wires have been investigated to understand the stress dependences of the critical current through the microscopic observations and the elastic theory. From an engineering standpoint, the importance of mechanical strength of the matrix material where superconducting filaments are embedded has been found especially for transverse stress effects.