Composite Superconductors Research Articles

Tensile stress applied to NbTi, Nb3Sn, Bi-2223 and MgB2 composite superconductors at room temperature

Axial tensile stress was applied to four different composite wires containing the superconductors NbTi,Nb3Sn, Bi-2223and MgB2 at room temperature. The basic mechanical properties obtained from the stress-straincharacteristics of these wires are compared and discussed. The wire samples were thentested by transport critical current measurements at 4.2 and 77 K. The results obtainedshow that the critical current degradation of all composite wires occurs at the stressvalue corresponding to the beginning of the plastic deformation. These data haveimportant implications for the technological application of such superconducting wires.

Elastic stiffness model for the critical temperature Tc of Nb 3Sn including strain dependence

A calculation is established for the critical temperature T c of the superconductor Nb 3Sn that includes the dependence on applied mechanical strain. The calculation employs the formalism of strong coupling phonon superconductivity, as usually given in frequency space. The directional nature of strain is included by expressing the equations of strong coupling in wave vector space. The relation between wave number and frequency is provided by the dispersion relations incorporating the effective elastic constants for the symmetry directions of the cubic crystal. An analytical formalism is established in which the elastic constants are derived in a unified way from an assumed strain energy potential function. The form of the strain energy potential is governed by the cubic symmetry. The scalar invariants of the strain tensor under the cubic symmetry group are determined as a set of basis functions for the strain energy potential. In the harmonic approximation, the relation between the strain energy function and the elastic constants determines the harmonic amplitudes of the strain potential from the measured dispersion relations. The electron–phonon coupling characteristic is approximated in a simple analytic form determined by inspection of the experimentally determined tunneling and phonon density of states. The critical temperature is calculated, through the equations of strong coupling, as a sum over the crystal symmetry directions. The anharmonic terms of the strain energy potential are introduced as the source of the strain dependence of the critical temperature. The allowed form of the anharmonic terms is again governed by cubic symmetry. The amplitudes of the anharmonic terms are determined from the strain dependence characteristics of single crystals and composite superconductors. The calculations are found to represent the observed strain dependence well with a strain energy function that contains three scalar invariants of the strain tensor, including the spherical (hydrostatic) strain invariant, and the principle parts of the second and third invariants of the deviatoric strain tensor. The formalism is applied to the analysis of composite conductors. The characteristics of the strain dependence of wire and tape geometries under longitudinal and transverse loads are related to the symmetry of the conductor and direction of applied load. Implications of conductor symmetry and constraint on the measurement of the strain dependent properties are identified.

Transient Stability Analysis of Large Aluminum Stabilized Superconductor by 2D and 3D Finite Element Analysis

Very-large-current composite superconductors are used in SMES coils and fusion applications. These superconductors have large cross-sectional areas of high purity aluminum to improve their stability. Once a normal zone is initiated in such superconductors the current transfers from the superconducting strands to the aluminum stabilizer according to the Maxwell's equations and the temperature distribution. However, the time constant of current diffusion in the aluminum stabilizer is very long as electrical resistivity of aluminum is very low. Therefore, excess Joule heating is generated in a small region of aluminum stabilizer near superconducting strands, and the temperature increases locally. Some 2D numerical analyses have been carried out in order to investigate the transient stability of the superconductor applied to the helical coil of LHD in National Institute for Fusion Science. But, as the performance of computers have improved, huge numerical simulations are new feasible. So we wrote a 3D finite element analysis code ourselves to carry out some now analyses that we compared with 2D analysis.

FEM Analysis of Three Directional Strain States Under Applied Tensile Stress for Various Composite Superconductors

Composite superconductors such as Cu/Nb/sub 3/Sn and Ag/Bi-oxides are subjected to thermally-induced residual strain by other component materials due to the around 1,000 K temperature difference between the reaction temperature and the cryogenic temperatures, and to applied tensile strain by the hoop stress of longitudinal (z) direction during the coil operation. To clarify especially the radial (r) and tangential (/spl theta/) strain behaviors including z-strain, we analyzed elastic-plastically multi-core model by computing of FEM for various composite conductors. As a result, applied tensile stress on the z-direction of the composite conductor gave monotonous decrease of thermally-induced rand /spl theta/-strains, which changed tensile to compressive strain. With increase in the z-strain effective strain showed the minimum value, not zero, at around z-strain-free state. Moreover it was found that all conductors are classified into three types by combination of component materials.

Investigation of mechanical behavior of copper in Nb 3Sn superconducting composite wire

The mechanical properties and the thermal residual stress distribution of copper in Nb 3Sn/Cu composite superconductor were investigated in detail. The stabilizer copper was removed from the composite wire, and the stress–strain behavior of this wire was compared with that of the original composite wire. The subtraction yielded the stress–strain curves of the copper when the Bauschinger effect was taken into account. The tensile test of the composites from which about 30% and 60% of copper was removed suggested the existence of the distribution of the thermal residual stress in the stabilizer copper. When this factor was taken into account, the analytical stress–strain curve agreed well with the experimental stress–strain curve. Thus, the stress–stain behavior of each component was fully understood.

Thermal stability of reinforced Nb 3Sn composite superconductor under cryocooled conditions

Several types of reinforced Nb 3Sn wires have been developed to prevent reduction of superconducting properties by applying a strong electromagnetic force. To fabricate a cryocooled magnet using those reinforced wires, we experimentally measured the minimum quench energy (MQE) under cryocooled conditions of some reinforced Nb 3Sn wires. As a result, it became clear that thermal stability expressed as MQE was controlled by the temperature margin between the temperature of the operating condition and the transition temperature from superconductivity to normal. Using the FEM analysis, it was realized that the cause of the decline in thermal stability for the reinforced wires was the low thermal conductivity of the reinforced materials.

Nonlinear dynamics of the evolution of thermomagnetic perturbations in composite superconductors

Nonlinear dynamics of the thermal and electromagnetic perturbations in composite superconductors is studied.

The effect of processing conditions on the properties of spray dried Nd 1Ba 2Cu 3O y/Ag composite superconductors

The aim of this study is to process Ag-doped Nd 1Ba 2Cu 3O y (NBCO) powders using spray drying and consequent calcinations and sintering. The influence of sintering conditions on the microstructure and superconductivity is investigated using thermal analysis (TG-DTA), X-ray diffractometry (XRD), Scanning electron microscopy (SEM), susceptibility and resistivity measurements. During sintering, a low oxygen partial pressure (1% O 2/Ar) was necessary to avoid Nd for Ba substitution. Sintering was performed at temperatures ⩾990 °C. After annealing in pure oxygen, superconducting material exhibiting Tc=95 K was obtained. The addition of Ag whiskers led to improved grain growth and decreased intergrain contact in the superconducting phase. Negligible structural and morphological interaction between NBCO and Ag was observed.

Preliminary mechanical training of thin coils

Bending with tension of composite superconductors during the coil winding induces a high and very nonuniform normal stress on their cross-sectional areas so that the constituent materials can be in the plastic state inside one or two extensive conductor zones already in the winding process. The removal of the bobbin from the coil results in stress redistribution. Stresses arising in components of a thin solenoidal coil after its winding, removal of the bobbin and subsequent mechanical by means of internal pressure are studied. It is assumed that a multifilamentary composite superconductor is macroscopically homogeneous and its stress-strain diagram is analogous to that of a ductile material following the straight-line law in unloading and reloading whereas interlayer insulation and impregnation materials are linear elastic. The case where one plastic zone develops during the coil winding in the conductor of the rectangular cross section and a new zone does not appear after the bobbin removal operation is considered. The higher the winding pretension, the lower the tensile stress in the conductor after the removal of the bobbin. It is shown that by mechanical training of the coil it is possible to redistribute stresses between its components and decrease the conductor stress.

Novel particulate reinforced tin for tubular Nb/sub 3/Sn multifilamentary superconductors

A novel technique is under development for the production of a tubular core Nb/sub 3/Sn composite superconductor using an inexpensive, easily fabricated core material. Initial experiments on the tin rich core material have included several different combinations of pure Sn, pure Cu, Sn4 wt%Cu, Sn8 wt%Cu, SiO/sub 2/, and Al/sub 2/O/sub 3/ blended powders, which were hydrostatically compacted, and then consolidated by room temperature angular extrusion. The resulting consolidates are characterized by density, hardness, metallography, and compression testing. Of the mixtures tested, the best core material is a blend of rapidly solidified Sn8 wt%Cu powder plus 6.9 weight percent nanosized SiO/sub 2/ particles. This particulate reinforced tin has a flow stress of (124 MPa)18 ksi and sufficient ductility to be conventionally extruded through a reduction ratio of four. A stronger tin alloy core material should be possible by powder metallurgy processing improvements.

Three-directional analysis of thermally-induced strains for Nb/sub 3/Sn and oxide composite superconductors

Composite superconductors like Cu/Nb/sub 3/Sn and Ag/Bi-oxides are subjected to thermally-induced residual strain by other component materials due to the around 1000 K temperature difference between the high temperatures where the superconductor is formed and the cryogenic temperatures where they are operated. To clarify especially the radial (r) and (/spl theta/) tangential strain behaviors, we analyzed elastic-plastically two models, the concentric core model (single-core model) and multi-core model and used two analysis methods, calculation of the force balance equation for the former model and computing of FEM for the former and the latter models. Strains in r and /spl theta/ directions varied with combination of component materials having large and small thermal expansion coefficients; the superconductor sandwiched by high thermal expansion materials is subjected to larger tensile r-strain and larger compressive /spl theta/-strain, as compared with superconducting core only embedded in the matrix. Furthermore it was found that there was a strain distribution by the core location at the inner or the outer. Finally, a better combination of the superconductor with other materials was obtained.

Heat evolution due to pulsed thermal perturbations in superconducting composites with a nonuniformly distributed current

Dissipative processes in superconducting composites under the action of current and pulsed thermal perturbations are studied. Results obtained are compared with the predictions following from the existing theory of thermal stabilization of superconducting composites. It is shown that, if the current is nonuniformly distributed over the composite's cross section and the thermal perturbations are short, intense additional energy dissipation may arise both before and after the occurrence of thermomagnetic instability. Associated Joule heat evolution may substantially exceed the thermal loss predicted by the adopted theory of thermal stabilization throughout the temperature range of the composite. Additional dissipation in composite superconductors narrows the range of stable superconductivity, especially when the current is applied continuously, even if external thermal perturbations are weak.

On the stability of the critical state with inhomogeneous temperature in composite superconductors

The problem of the thermal and magnetic destruction of the critical state in composite superconductors is investigated. The initial distributions of temperature and electromagnetic field are assumed to be essentially inhomogeneous. The limit of the thermomagnetic instability in quasi-stationary approximation is determined. The obtained integral criterion, unlike the analogous criterion for a homogeneous temperature profile, is shown to take into account the influence of any part of the superconductor on the threshold for critical-state instability.

補強型Ｎｂ３Ｓｎ複合超電導線の冷凍機冷却による熱的安定性

補強型Ｎｂ３Ｓｎ複合超電導線の冷凍機冷却による熱的安定性

Dynamics of a magnetic flux jump in a composite superconductor

An attempt is made at direct experimental verification of the theory of thermomagnetic instability in composite superconductors under conditions of external magnetic field or transport current variations. The development of thermomagnetic instability in the form of a magnetic flux jump is experimentally studied in a bulk low-temperature composite niobium-tin superconductor. The liquid-helium-cooled sample representing a compressed tape helix (helicoid) is placed in an external magnetic field orthogonal to the turn plane and varying with a constant rate. For the first time, both the magnetic induction inside the sample and its temperature are simultaneously detected in experiments. The sample overheat preceding the magnetic flux jump is measured to be 0.23 + 0.02 K. This value is found to be independent of the rate of the external magnetic field variation and the value of the jump itself and coincides, within the experimental accuracy, with the temperature parameter of electric field buildup involved in the general exponential I-V characteristic of the composite superconductor, which depends on temperature and magnetic induction.

Growth related microstructure of Bridgman grown YBa2Cu3O7/Y2BaCuO5 single-grain bars

The microstructure of YBa2Cu3O7 (123)/Y2BaCuO5 (211) single-grain bars grown by the Bridgman technique was studied by polarized light optical microscopy and image processing. For the first time, one c- and two a-growth sectors were microstructurally identified and the differences in their microstructure were compared. The subgrain structure of the growth sectors exhibits similar features as in the case of top seeded melt-grown (TSMG) single-grain samples. The realistic shape of the growing interface was drawn on the basis of a detailed microstructure analysis of the growth sectors. Quantitative measurements of the 211 particle volume fraction in the cross-section of the bar revealed higher 211 concentration at the end of the c-growth sector caused by pushing effect. Anisotropy of the 211 particle pushing is responsible for higher 211 concentration in the a-growth sectors than in the c-GS. However, this inhomogeneity of 211 concentration is much lower than that in the TSMG samples. Oscillations of 211 particle concentration developed in the c-growth sector were associated with temperature fluctuations at the growth front. All these findings are original and are essential in understanding superconducting and mechanical behaviour of the YBa2Cu3O7/Y2BaCuO5 composite superconductors prepared by the Bridgman method.

Stability of nonlinear shock waves in type-II superconductors

In this paper, we demonstrate the possibility of the existence of the nonlinear shock wave describing the final stage of the thermomagnetic instability evolution in the resistive state of a superconductor. We also investigate the structure of this wave with the finite amplitude and propagating at a constant velocity. We study the stability of nonlinear shock waves with respect to small thermal and electromagnetic perturbations in hard and composite superconductors. It is shown that only damped perturbations correspond to space-limited solutions.

Time evolution of phase composition and microstructure in the Ag/Bi-2223 composite superconductor heat-treated at specific pO2/temperature set points

The time evolution of the phases present in the ceramic cores of silver-sheathed (Bi, Pb)2Sr2Ca2Cu3Ox (Ag/Bi-2223) multifilament superconducting tapes heat-treated at selected oxygen partial pressure/temperature (pO2/T) set points was investigated using scanning electron microscopy and energy dispersive spectroscopy coupled with computer-based image processing methods. The numerical values for individual phase contents and non-superconducting second phase (NSP) size distributions generated in this way were used to quantify the temporal evolution of composition in the variably treated Ag/Bi-2223 tapes. Results for the three pO2/T set points investigated (21.0% O2/835 °C, 7.5% O2/825 °C and 4.0% O2/815 °C) revealed characteristic patterns of recurring maxima and minima in the time evolution of the NSPs. (Ca, Sr)14Cu24O41 was found to be the stable phase at 21.0% O2/835 °C, possibly evolving as a co-product of the Bi-2223 formation reaction, while (Ca, Sr)2CuO3 and CuO were stable at 4.0% O2/815 °C, presumably as a consequence of competitive secondary reactions proceeding in parallel with Bi-2223 formation. The best-aligned grains were formed in Ag/Bi-2223 tapes treated at 21.0% O2/835 °C, while the best conversion to Bi-2223, together with the least amount of NSP particles >0.5 μm in major dimension, was produced by the 7.5% O2/825 °C treatment. A forward-looking conclusion of the study is that it appears possible to minimize the amount and size of NSP particles during Ag/Bi-2223 heat treatment by using a combinatorial sequence of appropriately timed pO2/T set points.

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.

Joule heat release in a superconducting composite under a transport current charge

The physical peculiarities of temperature dependence of heat release in a composite superconductor induced by varying transport current are analysed. The proposed investigation was based on a model describing the simultaneous thermal and electromagnetic processes inside the composite superconductor with nonlinear current–voltage characteristic correctly. Numerical solutions of the set of Fourier and Maxwell's equations were used. The analytical expression of the heat release in the superconducting composite during the flux-creep regime is found for its stable superconducting state. The results are compared with the well-known formulae describing a current-sharing approximation of heat generation in the superconducting composite. It is shown that for a high rate of current charging and with a low heat transfer coefficient, the Joule heat generation in a composite increases both in the stage of the steady superconducting state and during the development of instabilities. In particular, the value of generated heat release may differ by several orders of magnitude from the amount of heat induced by slower rates of a current charge. In these cases a notable increase of the superconductor temperature is observed during the entire process of current penetration. Therefore, for a high rate of current charge and with low heat transfer coefficients, the value of the heat released in a superconducting composite is notably larger, through the entire charging process, than would be calculated from the existing thermal stability theory.