Table of contents

One of the important challenges facing the international scientific community at the beginning of the third millennium is how to manage the world's energy resources properly. Superconductivity will provide one of the strategies employed to avoid an energy crisis. Of course the ITER Fusion Tokomak that is to be built in France provides an exciting focus for the whole superconductivity community. In parallel, we can expect that other key technologies for superconductivity such as large capacity transmission cables, energy storage systems, and generators and motors will have a real impact in technologically advanced countries. There is broadly a consensus that the prototype stage for high-current high-field superconducting applications is largely completed, and the required performance has been demonstrated. However, before we move to full industrialization of large-scale superconducting technologies, feasibility studies suggest there are two types of problem that remain. The first is the development of high performance and low cost materials which are fully optimized in terms of critical current, low ac loss and high strength. The second is the establishment of optimal procedures for system design accompanying scale up. As the system design is dependent on material development, there is a critical need to study the key issues for developing high performance superconducting materials. Under the activities of the NEDO Grant Project (Applied Superconductivity), MEM05 was organized by Professor Osamura (Kyoto University), Professor Itoh (NIMS), Professor Hojo (Kyoto University) and Professor Matsumoto (Kyoto University) and held in Kyoto, Japan. The focus for the workshop was the elimination of grain boundary weak links, the creation of strong flux pinning sites, the optimal arrangement of filaments and barriers for reducing ac losses, and the design of high strength strain tolerant composite conductors.

Five subsessions were held at MEM05.

• Mechanical properties of superconductors including the influence of stress and strain on the critical current of practical conductors such as YBCO and ReBCO coated conductors, BiSCCO tapes, MgB₂ wires and Nb₃Sn filamentary conductors. • The intrinsic strain effects on the critical current density in Nb₃Sn YBCO, BiSCCO and MgB₂. • Recent advances in the critical current, mechanical properties and reduction in ac losses of HTS tapes and wires. • The compositional and microstructural dependence of E–J characteristics and its explanation based on flux pinning, grain boundary weak links and other mechanisms. • Standardized test methods: international cooperative research work to establish test methods for assessing the mechano-electromagnetic properties of superconductors based on the activities of IEC/TC90 and VAMAS/TWA-16.

More than 70 researchers attended the MEM05 workshop, coming from more than ten countries. In total, more than 50 presentations were made at the workshop. In this special issue of Superconductor Science and Technology selected papers have been included that are concerned with the comprehensive scientific research subjects mentioned above. The aim of this issue is to provide a snapshot of some of the current state-of-the-art research, and to promote further international research into the mechano-electromagnetic properties of composite superconductors.

The workshop was organized under the activities of the NEDO Grant Project (Applied Superconductivity, 2004EA004) and VAMAS/TWA-16. We wish to thank the following for their contribution to the success of the workshop: AFOSR/AOARD and IEC/TC90-JNC.

We propose a standard method of measurement of the irreversibility field parallel to the c-axis for bulk oxide superconductors at 77.3 K. It is proposed that commercial magnetometers, such as SQUID magnetometers or VSMs (vibrating sample magnetometers), are used for the measurement. Since the irreversibility field depends appreciably on the electric field criterion, the size of the specimen and the time to starting the measurement after setting the external magnetic field are prescribed to fix the electric field criterion. The number of measurement points is proposed as 5–10 within the field range ± 5% around the irreversibility field. It is confirmed that the irreversibility fields of two (Nd, Eu, Gd)-123 (NEG-123) specimens were successfully measured with the proposed standard measurement method. The target COV (coefficient of variation) is 5%.

E–J properties of the high-T_c superconductors are related to the distribution of the flux pinning strength and the local critical current density J_cl. From the previous result of Bi2212 thick films, we found that the distribution of the local J_c is much affected by microstructures, and the local J_c distribution becomes broad and asymmetric for well aligned plate-like Bi2212 grains. These microstructures are similar to the case of practical Ag sheathed Bi2212 tapes. In this study, we performed E–J measurements of practical Ag sheathed Bi2212 tapes at various temperatures and magnetic fields, and the local J_c distribution was investigated.

The dependence of the superconducting layer thickness on the critical current properties was investigated in the range of 0.27–2.0 µm for YBCO coated tapes made by PLD processing on IBAD substrates. The critical current density at low fields in the direction of the c-axis was found to decrease with the increasing thickness d in proportion to d^−1/2 up to 1 µm. This seems to agree with the prediction of the two-dimensional collective pinning of random point pins. However, this dependence did not change over a wide temperature range of 5–60 K and the critical thickness for the two-dimensional pinning was found to be much smaller than 1 µm. This result suggests that the observed thickness dependence does not come from the pinning mechanism but simply from the change of the superconducting layer structure with increasing thickness. The effect of flux creep on the critical current properties at high temperatures is also investigated, and the observed thickness dependences of irreversibility field and n-value are compared with the theoretical predictions of the flux creep-flow model.

The damage evolution in Bi2223 filaments and its influence on critical current was described by a Monte Carlo–shear lag simulation method. The experimentally observed zigzag crack propagation across aligned Bi2223 grains under tensile strain was effectively modelled by including transverse and longitudinal failure modes for individual grains. From the simulated stress–strain curve, the survival parameter (slope of the stress–strain curve normalized with respect to the original Young's modulus) was estimated with increasing applied strain. With this parameter combined with the strain sensitivity of the critical current, the measured change of critical current of the composite tape with applied strain could be described well.

Comprehensive measurements are reported of the critical current density (J_C) of internal-tin and bronze-route Nb₃Sn superconducting wires as a function of magnetic field (B≤23 T), temperature (4.2 K ≤T≤12 K) and axial strain (−1.6%≤ε_I≤0.40%). Electric field–temperature characteristics are shown to be equivalent to the standard electric field–current density characteristics to within an experimental uncertainty of ∼20 mK, implying that J_C can be described using thermodynamic variables. We report a new universal relation between normalized effective upper critical field (B_C2^*(0)) and strain that is valid over a large strain range for Nb₃Sn wires characterized by high upper critical fields. A power-law relation between B_C2^*(0,ε_I) and T_C^*(ε_I) (the effective critical temperature) is observed with an exponent of ∼2.2 for high-upper-critical-field Nb₃Sn compared to the value ≥3 for binary Nb₃Sn. These data are consistent with microscopic theoretical predictions and suggest that uniaxial strain predominantly affects the phononic rather than the electronic properties of the material. The standard Summers scaling law predicts a weaker strain dependence than is observed. We propose a scaling law for J_C(B,T,ε_I) based on microscopic theory and phenomenological scaling that is sufficiently general to describe materials with different impurity scattering rates and electron–phonon coupling strengths. It parametrizes complete datasets with a typical accuracy of ∼4%, and provides reasonable predictions for the J_C(B,T,ε_I) surface from partial datasets.

The reversible axial strain dependence of the critical current of MgB₂ conductors is shown to vary with the temperature and magnetic field. The measured critical temperature and irreversibility field are also found to change reversibly with the axial strain. Combining these effects, we show empirically how the strain dependence of the whole critical surface can be scaled with just three parameters: the strain dependences of its three corner points.

We have found that prebending treatment, which is repeated bending load at room temperature, greatly enhances the critical current, upper critical field and critical temperature of practical Nb₃Sn superconducting wires. In this paper, we focus on the application of the prebending strain effect to practical superconducting coils fabricated by a react-and-wind method.

To demonstrate the prebending strain effect on the react-and-wind coil, we prepared two kinds of CuNb /Nb₃Sn superconducting monolayer coils. For one of the coils, the superconducting wire of 1.0 mm diameter was repeatedly bent by using ten fixed pulleys before the winding process, resulting in a prebending strain value of 1.0%. The final winding diameter of both coils was 200 mm and the number of turns was 49.

In the compressive stress condition, the critical current of the coil with 1.0% prebending strain was 296 A at 11 T, which was larger than that of the coil with 0% prebending strain. These values were much larger than the critical current of the witness sample. The results indicate that the prebending treatment enhanced the coil critical current.

In the case of the hoop stress condition, the coil critical currents were approximately 230 A at 11 T for both coils. The maximum electromagnetic hoop stress was calculated to be 360 MPa. The short sample tensile test results qualitatively explained the critical current deterioration of the coil. Furthermore, the 360 MPa hoop stress did not deteriorate the critical current irreversibly.

Critical current density (J_c) was measured inductively at 77.3 K over large-area YBa₂Cu₃O_7−δ (YBCO) films deposited by pulsed laser deposition (PLD) and its behaviour with increasing YBCO film thickness (d) was investigated. J_c was found to decrease exponentially with the film thickness. In the J_c–d curve two regions can be distinguished: a 'thin-film' region with characteristically high J_c and exhibiting a rapid decrease with thickness, and a 'thick-film' region with characteristically lower J_c and exhibiting a slower decay with thickness. The magnetic-field angular dependence of J_c for several YBCO films with thickness ranging from ∼0.2 to ∼1.6 µm was evaluated. All films exhibited prominent J_c peaks when the applied magnetic field H was oriented along the c-axis of YBCO. The prominent J_c peak at is considered to be due to correlated pinning along extended defects found in dense amounts in the YBCO film: linear defects in the form of screw and edge dislocations, and planar defects possibly in the form of stacking faults. Examination of the microstructure of YBCO revealed a strong dependence of the defect density on film thickness: a higher density of linear defects was found for thinner films whereas planar defects were more abundant for thicker films. The ratio of J_c () to J_c () is generally>1 for films belonging to the thick-film region (typically d≥0.4 µm) and also showed an increase with the density of linear defects. These results suggest that linear defects may be more effective pinning sites than planar defects, or that an overabundance of planar defects may offset the increase of J_c for thicker films. Surprisingly, for films in the thin-film region (d<0.4 µm), the ratio of J_c () to J_c () is ≤1, in spite of a very prominent J_c maximum at as well as a relatively high density of c-axis correlated defects. This deviation suggests that the contribution to J_c in the thin-film region may be attributed principally to random defects, intrinsic pinning, and possibly extended defects parallel to the ab-plane.

Differences in the thermal contraction of the composite materials in a cable in conduit conductor (CICC) for the International Thermonuclear Experimental Reactor (ITER) in combination with electromagnetic charging cause significant axial, transverse and bending strains in the Nb₃Sn layer. These high strain loads degrade the superconducting properties of a CICC. Here we report on the influence of periodic bending load, using different bending wavelengths from 5 to 10 mm on a Nb₃Sn powder-in-tube processed strand. The strand axial tensile stress–strain curve, the critical current versus applied axial strain results, the influence of cyclic loading on the RRR and assessment of the current transfer length from AC loss measurements, required for the analysis, are presented as well.

For the strand under investigation, we find an influence of bending strain on the I_c that corresponds well to the predictions obtained from the applied classical relations, distinguishing ultimate boundaries of high and low interfilament electrical resistance. The reduction versus applied bending strain is similar for all wavelengths and equivalent to the low transverse resistance model, which is consistent with the estimated current transfer length. The cyclic behaviour in terms of critical current and n-value involves a component representing a permanent reduction as well as a factor expressing reversible (elastic) behaviour as a function of the applied load.

The results from the set-up enable a discrimination in performance reduction per specific load type and per strand type. In this paper, we discuss the results of the pure bending tests.

Superconducting and mechanical performance and the strain effect, the influence of mechanical strain on the critical current (I_c), are investigated for a commercially available MgB₂ multifilamentary tape. The conductor was made through the ex situ route and has 14 filaments in nickel matrix with copper stabilizer. I_c was measured as a function of magnetic field (B) at 4.2, 15, 20, and 25 K and we determined the irreversibility fields (B_irr) using I_c–B curves at each temperature. I_c, measured as a function of field angle at 4.2 K and 4 T, shows an anisotropic behaviour. We also measured I_c as a function of temperature in 0–5 T. The stress–strain relation was tested at 296 and 113 K. The strain effect was examined in magnetic fields at 4.2, 15, and 20 K using a U-shape rig made of stainless steel (SUS304). The I_c–strain relation is linear and reversible in the window of external strain between −0.5% (compression) and 0.5% (tension). With increasing strain, large and irreversible degradation occurs after the maximum of I_c (I_cm) at 0.57% tension. Although d (I_c/I_cm)/d (strain), the slope of the linear I_c–strain relation in the reversible regime, depends on both temperature and magnetic field, the relations between d (I_c/I_cm)/d (strain) and B/B_irr fall on a universal line independently of temperature.

We have investigated the influence of post-annealing on the current transport and phase/microstructure evolution in Ag/(Bi, Pb)2223 tapes. After post-annealing, the critical current density was about 1.4 times higher. Also the superconducting transition was greatly sharpened and T_c became much higher, even when a magnetic field was applied. The Rietveld analysis suggested that, before post-annealing, a liquid phase remained in the tape, whose mass fraction was estimated as 9.3% from the sum of the phases that formed during post-annealing. Transmission electron microscopy observations revealed that amorphous layers or lattice disorder areas existed at a large number of the (Bi, Pb)2223 grain boundaries. These insulating layers block the superconducting current paths or produce a Josephson-coupled weak link boundary. Upon post-annealing, phase and structure changes took place at the grain boundaries. The insulating amorphous layer converted into metallic 2212 layers. In addition, a faceted structure was formed at the boundary interface such that the tilt boundary became a superconducting through-path. It is strongly suggested that these two factors are critical for increase of the J_c upon post-annealing.

The relationship between the n-value and critical current (I_C) is investigated for six different ITER-candidate Nb₃Sn wires characterized as a function of magnetic field (B≤28 T), temperature (4.2 K ≤T≤12 K) and intrinsic axial strain (−1%≤ε_I≤+0.4%). For the five wires exhibiting intrinsic behaviour, n(I_C) can be parameterized by a modified power law of the form n = 1+rI_C^s, where s is a constant with a value of 0.41 ± 0.03. The parameter r decreases as the magnitude of the intrinsic strain increases and is a relatively weak function of temperature. For one of the wires, the n-value saturates at high critical currents (low magnetic fields), characteristic of extrinsic filament nonuniformities.

We developed a SQUID biomagnetometer system for detecting the magnetic signals evoked from the human cervix within the scope of non-invasive diagnosis of the function of the cervical spinal cord. The system has two main particular features. One is vector SQUID gradiometers and the other is a cryostat optimized for a reclining subject. The vector SQUID gradiometers can maximize the magnetic information acquired from the limited observation area around the neck and the shape of the new cryostat enables a subject to keep his/her position stably in a relaxed manner during the measurement. As a result, we successfully recorded the evoked magnetic fields corresponding to the neural signal travelling along the spinal cords from the necks of human subjects who were given electric stimuli to their thoracic spinal cords.

The effect of attaching reinforcing materials to first-generation high-temperature superconducting (HTS) wires on retained critical current (I_c) in axial strain conditions was established by axial tensile and compressive strain loading and in situ I_c tests. Tape-shaped BSCCO/Ag composite wires with copper and nickel plated reinforcing layers of different thickness and tapes with solder-laminated stainless steel, invar and molybdenum with different widths, thicknesses and pre-stresses were made and tested. The >95% retained I_c strain window was found to be shifted in a predictable manner based upon the choice of pre-stress, coefficient of thermal expansion (CTE), geometry and assembly method. Higher CTE and pre-stress in the reinforcement shifted the strain window into the tensile side. All reinforcing materials increased the size of the strain window, with higher modulus materials linearly increasing the strain window by up to two-fold compared to the unreinforced BSCCO-Ag composite. A model was developed to calculate the strain window location and size from reinforcement material properties, pre-stress and process variables. The ability to increase the strain window size and to tailor its location enhances the utility of both first- and second-generation HTS wires.

Control and optimization of the residual strain are one of the most important issues in the development of Nb₃Sn superconducting magnets. We found that the repeated bending loads at room temperature change the prestrain of Nb₃Sn wires and result in the enhancement of J_c, B_{c 2} and T_c. We call this repeated bending strain 'prebending strain'. In order to understand the prebending effect, superconducting properties were measured as functions of temperature, field, axial tensile stress and strain. In addition, the three-dimensional strain state was also evaluated by the neutron diffraction. Those obtained results strongly suggest that the prebending process changes the radial residual strain as well as the axial one independently. Hence, it is considered that the prebending effect is effective for the control and optimization of the three-dimensional strain state on the react and wind process.

The recently discovered reversible strain effect in Y–Ba–Cu–O (YBCO) coated conductors contrasts with the general understanding that the effect of strain on the critical-current density J_c in practical high-temperature superconductors is determined only by crack formation in the ceramic component. Instead of having a constant J_c as a function of strain before an irreversible drop when cracks form in the superconductor, J_c in YBCO coated conductors can decrease or increase reversibly with strain over a significant strain range up to an irreversible strain limit. This reversible effect is present in samples fabricated either with rolling-assisted biaxially textured Ni–W substrates or with ion-beam-assisted deposition on Hastalloy substrates. The reversibility of J_c with strain is observed for thin as well as thick YBCO films, and at two very different temperatures (76 and 4 K). The reversible effect is dependent on temperature and magnetic field, thus indicating its intrinsic nature. We also report an enhancement of the irreversible strain limit ε_irr where the reversible strain effect ends and YBCO cracking starts. The value of ε_irr increases from about 0.4% to more than 0.5% when YBCO coated conductors are fabricated with an additional Cu protection layer.

This paper reviews overpressure (OP) processing of Ag-sheathed (Bi,Pb)₂Sr₂Ca₂Cu₃O_x (2223) flat wire. OP processing is a variant of hot isostatic pressing that densifies the 2223 filaments, which increases the critical current density J_c of the wire. OP processing is done at pressures up to 300 bar with a mixed Ar /O₂ atmosphere in which the oxygen partial pressure is set so the 2223 phase is stable. The Ar pressure compresses the wire. The highest reported J_c in OP-processed short wires is 69.6 kA cm⁻² (self-field (SF), 77 K) and 30.8 kA cm⁻² (0.1 T, 77 K). I_c as high as 100 A has been achieved in 1.5 km lengths of OP wire.

Intrinsic strain is well known to influence T_c and j_c of the classical superconductor Nb₃Sn. Similar effects, which can be even more pronounced, have been found in high-T_c superconductors (HTSs). The HTS properties typically depend on hole doping of the CuO₂ planes. This hole concentration can be influenced by doping with substituting atoms or by changing the oxygen content, which can easily be done for RE-Ba₂Cu₃O_x (RE-123) materials. In addition, the hole doping can be massively influenced by ordering effects within the HTS material and in the case of RE-123 by charge redistribution caused by applied strain and/or pressure. In most cases an anisotropic strain sensitivity is found for HTS materials.

Using literature data from doped RE-123 and new data from uniaxial pressure experiments the possible mechanisms for the change of superconducting properties are analyzed. Finally, literature data from other HTS materials are used to show that similar mechanisms occur for these HTS materials too. Targeting the development of actual HTS conductors these properties may be covered by technical problems yet. However, for future HTS cables the problems arising from uniaxial strain effects have to be addressed.

We have measured the field (B) and strain (ε) dependence of the critical current (I_c) in Nb₃Al conductors, prepared by the rapid-heating, quenching and transformation (RHQT) technique. The measurements were made using a modified Walters spring (WASP) at fields up to 21 T, and at 4.2 K. The measurements at the highest field have given a clear field dependence of the I_c versus ε curve, which has enabled us to evaluate the wire's absolute characteristics over wide field and strain ranges. The data were well fitted with empirical scaling laws with specifically given constants for the Nb₃Al wires. The strain sensitivity of the transformed Nb₃Al wire is also compared with a low-temperature diffusion processed wire.

We investigated the intrinsic strain effect on critical current, I_c, and tensile fracture behaviour in the RE-123 coated conductors (RE =  Y, Dy or Sm) with Hastelloy C-276 substrates. It is found that the SmBCO coated conductors exhibit an intrinsic strain effect on I_c which is similar to that in the DyBCO ones. I_c decreases monotonically with increasing applied tensile strain but it can recover reversibly up to the strain of 0.32% when the strain is relieved. Quenching occurred for the coated conductors with Hastelloy C-276 substrate in the narrow strain region of 0.27–0.35% regardless of the buffer layers and superconducting materials. In order to reveal the reason for such quenching, we measured mechanical properties of the Hastelloy C-276 tapes alone and coated conductors at 77 K. The as-received and as-polished Hastelloy tapes show continuous yielding, which results from homogeneous plastic deformation. On the other hand, the Hastelloy tape annealed at 963 K corresponding to the deposition condition for the superconducting layer exhibits discontinuous yielding. In the annealed Hastelloy tapes, occurrence of Lüders bands is observed, indicating that plastic deformation progresses inhomogeneously. Discontinuous yielding is also confirmed in the coated conductor. The yield strains of the coated conductors are almost identical to the strains at quenching. Characteristic fracture behaviour is observed, that transverse crack arrays are initiated from the tape edge and they propagate into the central part. The crack arrays orient to a certain angle to the loading axis, which is similar to that of the Lüders bands. The similarities in the initiation site and the angle to the loading axis between the Lüders bands and the crack arrays and close values of quenching strains to the yielding one strongly suggest that quenching is attributable to the discontinuous yielding in the Hastelloy substrate.

The effects of stress–strain on the critical current, I_c, of ex situ powder-in-tube (PIT)-processed Ni-sheathed MgB₂ tapes and round wires as well as in situ PIT-processed Cu-sheathed wires at 4.2 K in a magnetic field up to 5 T have been studied. The effect of In powder addition on the Ni-sheathed MgB₂ wire was not so clear compared with that in the tape, in which the irreversible strain, ε_irr, for the I_c degradation onset increases significantly by the addition. This is attributed to the difference in the microstructure of the core associated with cold workings. A peak and gradual degradation behaviour of I_c with strain beyond ε_irr was found in the wire, whereas no evident peak and a steep degradation behaviour was found in the tape. As a possible reason, the difference in the triaxial residual stress state at 4.2 K due to the difference in geometry of the cross-section is suspected. The transverse compression tests revealed that I_c of the wire did not degrade up to 270 MPa. Again, the effect of In addition was minimal. The Young's modulus of MgB₂, 31–41 GPa, at room temperature was estimated by a tensile test of Cu sheath wire using a high-accuracy extensometer and the law of mixtures. The tensile strain dependence of I_c in the Cu sheath wire was similar to that in the Ni-sheathed wire, ε_irr being 0.4%. However, the stress corresponding to ε_irr, 50 MPa, was about 1/10 of that for the Ni-sheath wire and the irreversible transverse compressive stress, 150 MPa, was also lower. The effect of bending strain on the I_c in Cu-sheathed wire was compared with that of the tensile strain.

The mesoscopic stress and strain states of Bi2223/Ag/Ag-alloy superconducting composite tapes have been studied both analytically and experimentally under bending deformation. The tapes used in the present study were supplied as the standard samples for the VAMAS round-robin program (classified as VAM1 and VAM3). Detailed tape bending analysis was completed based on a damage-free initial state, and the calculated decrease of critical current, I_c, due to Bi2223 filament fracture was compared to the experimental I_c decrease. The calculated I_c was much lower than that obtained in the experiments for both tapes. Metallography indicated the presence of delamination in as-received as well as bend-tested tapes. The analysis was therefore modified to include delamination and it was completed for the case where delamination occupied the full width of the tape mid-plane. The calculated I_c with delamination was higher than the experimental results for both tapes. Delamination occupying partial width of the mid-plane explained this difference. Finally, the width ratio where delamination exists was calculated by comparing the analytical results with delamination and experimental results. This ratio increased with increasing curvature of the tape.

The tensile strain dependences of the critical current (I_c) in YBa₂Cu₃O_7−δ (YBCO) coated conductors fabricated by using the rolling-assisted biaxially textured Ni–W substrates (RABiTS)–pulsed laser deposition (PLD) method were examined at 77 K and in self magnetic field. Cu and stainless steel layers were used as stabilizers to the YBCO coated conductor, and the effects of stabilizing layers on the strain tolerance of I_c were investigated, compared with the case without a stabilizing layer. The lamination of stabilizer produced an increase in the yield strength and strain tolerance of I_c in coated conductors. All YBCO coated conductors tested showed a reversible strain effect and a peak in the relation between I_c and applied strain. The peak strain of I_c and the irreversible strains for I_c degradation were enhanced when the YBCO coated conductor was laminated with a stabilizing layer. For the case laminated with a stainless steel layer, I_c recovered reversibly until the applied strain reached to about 0.5% and showed its peak at a strain of 0.42%, comparing to the case without a stabilizing layer, which were 0.21% and 0.18%, respectively. It can be predicted that the lamination of a stabilizing layer produced a significant residual compressive strain to the YBCO film during cooling to 77 K, which influenced the axial strain tolerance of YBCO coated conductors. Therefore, the I_c–tensile strain relation in YBCO coated conductors could be explained by a two-stage deformation; stage I is the region where YBCO film behaves elastically and I_c recovers when the stress is released. Stage II is the region where I_c decreases irreversibly attributable to the cracking induced in the YBCO film due to the significant plastic deformation of the substrate or the stabilizing layer.

The current-carrying capability of superconducting wires is degraded by stress. Therefore electromechanical properties are one of the key feedback parameters needed for progress in conductor applications. In this work, uniaxial tensile stresses and bending stresses were applied to Fe /MgB₂ wires at room temperature, followed by measurement of critical current using a transport method at 4.2 K. Basic mechanical properties were calculated from the measured stress–strain characteristics. The irreversible tensile strain at which the critical current density of MgB₂ wire starts to degrade was found to be 0.5%. In addition, the degradation of I_c with decreasing bending diameters was found to be very rapid for wires that were deformed after the heat treatment that forms the MgB₂ compound, while not much degradation of I_c was found for wires that were bent before being annealed. SEM observations confirmed that cracks could be healed by post-annealing.

This paper reports on the relaxation of a trapped magnetic field in a MgB₂ solenoid coil in persistent current (PC) mode operation. For the PC mode test, we fabricated a closed-loop circuit with a small MgB₂ test coil, a persistent current switch (PCS) and some superconducting joints. The MgB₂ test coil was fabricated using a wind-and-react method employing a 96 m long powder-in-tube processed MgB₂ wire. The critical current I_c of the coil reached 88 A at 4.2 K without an external field. The resistance of the fabricated joints between the MgB₂ and NbTi conductors (MgB₂–NbTi) was estimated to be less than 1.0 × 10⁻¹³ Ω. In PC mode with a closed-loop circuit, a magnetic field of 0.68 T was trapped for more than 8 h. These results demonstrate the possible use of the MgB₂ conductor for applications in fields such as MRI and NMR superconducting magnets.

The crucial multi-physics problem of how to extrapolate from the performance of an isolated Nb₃Sn strand measured in the laboratory to the performance of a superconducting coil using multi-strand twisted cables is addressed here. We consider the particular case of the path going from the LMI strand to the international thermonuclear experimental reactor (ITER) toroidal field model coil (TFMC), through its associated Full Size Joint Sample, the TFMC-FSJS. Mechanical, electromagnetic and thermal–hydraulic conditions are simulated using the ANSYS, ENSIC and Mithrandir/M&M codes, respectively. At least in this case, the DC performance of the short sample turns out to be relatively close to (considering error bars) but not fully representative of that of the coil, showing higher (less compressive) effective thermal strain but also higher sensitivity to the electromechanical load.

The results of two test methods were compared among three laboratories to determine a standard measurement method of critical current (I_c) 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 I_c degradation started with less bending strain for RRT than for bending-rig. Average irreversible strains (ε_irr) 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 ε_irr values increased to the level of the bending-rig, but the modified RRT I_c 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 I_c degradation rate differences with strain in terms of the crack propagation and delamination defects of oxide filaments from the Ag sheath.

In the framework of the International Thermonuclear Experimental Reactor (ITER) R&D programme part concerning the Nb₃Sn cable-in-conduit conductor of the Toroidal Field coils, a dedicated programme of action was launched within the European Fusion Technology Programme, designed to improve the investigation of the impact of bending strain on recently developed industrially advanced Nb₃Sn strands. In order to ensure that the ITER TF coils experience relevant mechanical conditions, the Nb₃Sn strands were jacketed inside a 0.2 mm thick stainless steel tube, simulating the mechanical influence of the jacket on the cable strands.

In this paper we will describe in detail the four candidate methods that were investigated for imposing a pure controlled bending strain on the jacketed strands. A common feature of these methods is that they study the reaction of the strand on a heat treatment mandrel and transfer it onto a test mandrel with a different diameter. In practice, the imposed bending strain is limited to 0.5%. Two solutions are considered: a reduction or an increase in diameter of the mandrel. Also, two options are possible for the jacket removal at strand ends for connection to the current leads: before or after the heat treatment. The four options are tested to provide an area of investigation that is as large as possible.

Specific support together with tool design and manufacture will be presented. A general comparison of all options as regards specified criteria is also carried out in order to precisely define an action process of the bending application method developed. We find finally that the best approach is the method in which the radius is increased and the stainless steel jacket is removed before heat treatment.

Nb₃Sn superconductors show a dependence of the critical current and temperature on the strain state of the superconducting material. The basic causes of Nb₃Sn strain effects, primarily differential thermal contraction between elements of the strand, have been known for 30 years, but have received more attention lately as part of a drive to achieve much higher operating current densities and make use of them in practical multistrand cables. The use of the cable-in-conduit (CICC) type of conductors to achieve high current capacity has proved popular, as the conductors offer good local cooling of the strands and distributed electrical contact between strands that is essential to provide stability against the inevitable current non-uniformity that arises with parallel connection of the strands. However, the essential openness of the cable means that the strands have to support local magnetic loads as well as being exposed to the overall magnet strain displacements. Simple structural models are developed based on mechanical measurements on cable-in-conduit conductors which are able to successfully simulate the measured superconducting performance. These suggest that degradation observed in large cables is due to a combination of the repeated bending strain experienced by the strands and filament fracture, which is starting to occur to a significant extent in some large cables. Superconducting performance improvements in strands can only be properly utilized with improved support of the strands in the cable, implying a more ordered structure than in a multistage.

The addition of dysprosium oxide nanoparticles is shown to improve the critical current in perpendicular magnetic fields for second-generation (2G) wire formed by metal–organic deposition (MOD). Typical enhancements in J_c are from 0.17 MA cm⁻² to over 0.33 MA cm⁻² at 77 K and B_perp = 1.5 T. TEM analysis shows that we are introducing (Y,Dy)₂O₃ nanoparticles with dimensions of 10–50 nm. A simple theoretical analysis shows that the maximum pinning effect for additions is expected at excess concentrations of approximately 70% DyO_1.5, i.e. for YBa₂Cu₃O_7−δ+0.7DyO_1.5 if the added nanoparticles are randomly dispersed and a strong pinning model is valid. An interesting feature is that the critical current in parallel field is reduced in these samples. We present evidence that shows this may be due to reduced planar defects in the YBCO.

Fundamental stress versus strain measurements were completed on superconducting Nb₃Sn wires within the framework of IEC/TC90 and VAMAS/TWA16. A key task was the assessment of sensing systems regarding resolution, accuracy, and precision when measuring Young's modulus. Prior to actual Nb₃Sn wire measurements metallic wires, consisting of copper and stainless steel having diameters similar to the Nb₃Sn wire, were extensively investigated with respect to their elastic line properties using different types of extensometers. After these calibration tests Nb₃Sn wire measurements of different companies resulted in several important facts with respect to total size and weight of the used extensometers. The size could be correlated to the initial stage of stress versus strain behaviour. In fact, the effect of wire curls resulting from the production line had a profound effect on Young's modulus measurements. Within this context, the possibility of determining Young's modulus from unloading compliance lines in the plastic regime of the stress–strain curve was considered. The data obtained using this test methodology were discussed under consideration of the composite nature of Nb₃Sn wire. In addition, a non-contacting sensing system based on a double-beam laser extensometer was used to investigate the potential of this new sensing system.

The effect of uniaxial strain on the critical current of 0.8 m long Nb₃Sn wires up to 21 T is studied by the modified Walters spring (WASP). For Nb₃Sn wires, prepared by both the bronze route and the internal Sn diffusion process, the critical current density as a function of the uniaxial strain ε is found to exhibit an asymmetric behaviour on both sides of the strain ε_m, where J_c reaches its maximum. Revisiting earlier x-ray and neutron diffraction measurements on bronze route processed wires between 10 and 600 K, it is shown that the asymmetric behaviour of J_c(ε) on both sides of the strain value ε_m is connected to individual variations of the stress-induced tetragonal lattice parameters a and c.

The present measurements of J_c versus strain for Nb₃Sn wires show stronger strain dependence for wires prepared by the internal Sn diffusion method with respect to those obtained by the bronze route. The reasons for this difference are attributed to the individual details of the filament configuration in both types of wire, for example the different Sn distributions inside the filaments and the very different filament sizes, 4 and 80 µm, respectively.

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