Field and temperature dependencies of critical current on industrial Nb3Sn strands

The increasing need for high field magnetic devices has focused attention on filamentary Nb₃Sn conductors, whose critical data are superior to NbTi conductors. To choose the suitable operating parameters and to determine the stability margin of magnet systems, it is very important to know the effect of temperature and magnetic field on the superconducting properties, especially on the critical current. Up to now, for design calculation, the so-called “Summers model” was assessed theoretically on experimental data obtained by Spencer et al., (The temperature and magnetic field dependence of superconducting critical current densities of multiinflammatory Nb₃Sn and NbTi composite wires. IEEE Trans Mag, Mag-15 (1979) 76) and Suenaga et al., Superconducting critical-current densities of commercial multifilamentary Nb₃Sn(Ti) wires made by the bronze process. Cryogenics (1985) 25, 123). Apart these very useful preliminary experimental data, very little has been done on the very different industrial strands which are now produced in the industry. Industrial Nb₃Sn strands are generally tested and checked only at 4.2 K and their operating design temperature is often very different, sometimes around 6 K. It is now urgent to validate the model and to confirm that the data taken up to now in the design calculations are conservative.     

A practical in situ wire: a composite wire with many in situ processed Nb3Sn cores of an internal diffusion type

As a superconducting in situ wire for practical use, we propose a new type of composite wire with fine cores consisting of in situ processed wires of an internal diffusion type. These Nb₃Sn wires have merit in a simple fabrication process and a high stability compared with ordinary multifilamentary Nb₃Sn wires. Expected properties of the new type of Nb₃Sn wires were estimated based on a series of experimental results for a single in situ Nb₃Sn wire used as a fine core. A quantitative reliability of the new design estimation was examined by comparing the theoretical values with observed data on the electromagnetic properties of a test wire.     

New Fabrication Process of Cu–Nb Composite for Internal Reinforcement of Nb3Sn Wires

We developed Cu–20vol%Nb wires, for the reinforcement and stabilizing of Nb₃Sn wires, using a new Nb rod process. The electrical and mechanical properties of the CuNb wires prepared by different processes were measured to assess their suitability as reinforcing stabilizers for Nb₃Sn. All wires were heat-treated at 670 ^°C, which is the temperature required for formation of Nb₃Sn. After heat treatment, the mechanical properties of Nb-rod-processed CuNb were superior to those of in-situ-processed CuNb wires. The residual resistance ratio of the Nb-rod-processed CuNb was 64, and its magnetoresistivity was 0.16 μΩ?cm at 4.2 K and 15 T. These properties indicate that the new CuNb wire is suitable as a reinforcing stabilizer for using a high field, wide bore, superconducting magnet, such as the 20 T superconducting magnet with 400 mm room temperature bore being planned in Japan.     

Fabrication of Nb3Sn superconductors by the solid-liquid diffusion method using Sn rich CuSn alloy

Superconducting composite wires having thick Nb₃Sn layers (? 20 μm) and high current carrying capacities were fabricated by the diffusion reaction between Nb (solid) and Sn rich CuSn alloy (liquid): the solid-liquid diffusion method. Composite wires with a fine inner core of Cu 12 at % Sn alloy surrounded by Nb were produced by cold drawing and heat treated at about 700°C. The Sn rich intermetallic compounds which formed initially were transformed to Nb₃Sn in 50 ～ 100 h, as the Cu concentration in the CuSn alloy core increased due to the consumption of Sn. The process produced thick Nb₃Sn layers, in comparison with the bronze method, because of the high Sn content in CuSn alloy core. The mechanism of enhanced Nb₃Sn formation by Cu was also studied, and it was clarified that the Cu in CuSn alloy lowers the activity of Sn so that the formation of Sn poor intermetallic compounds Nb₃Sn becomes advantageous in the diffusion reaction as compared with other Sn rich compounds.     

Tensile strain dependence of critical current of RHQ-Nb3Al wires

Nb₃Al superconducting wires produced by rapid heating and quenching (RHQ) have been developed for high-field accelerator magnets. It is known that critical currents of A15 superconductors (e.g., Nb₃Al and Nb₃Sn) have a dependence on stress/strain. It is thus important to clarify the stress/strain behavior of the RHQ-Nb₃Al wires for the development of high-field accelerator magnets. We recently started experimentally investigating the strain dependence of the critical current of the RHQ-Nb₃Al wires with a Ta interfilament by measuring their critical currents under longitudinal tensile strains. To evaluate the effect of residual strain in Nb₃Al filaments induced by thermal contraction of the materials in the wire, neutron diffraction measurements were performed at room temperature.     

Guidelines for LTS magnet design based on transient stability

Stabilities of low critical temperature superconducting (LTS) magnets and their designs are studied and discussed. There are two contradictory necessities; those are low cost and high performance, in the other words, high magnetic field and large current density. Especially, the maximum magnetic fields of the latest high performance Nb₃Sn magnets are around 20 T. Mentioned necessities result in the small stability margins. Needless to say, the superconducting magnet must produce its nominal field reliably. Therefore, maintaining adequate stability margin, the magnet design to draw out the high potential of the superconductor is required. The transient stability of the superconducting magnet is determined by the relationship between mechanical disturbance energy and stability margin. The minimum quench energy (MQE) is one of the index of stability margin and it is defined as the minimum energy to trigger quenching of a superconductor. MQE should be beyond any possible disturbance energy during the operation. It is difficult to identify the mechanical disturbance energy quantitatively. On the contrary, MQE had been evaluated precisely by means of our developed resistive carbon paste heater (CPH). At the same time, we can predict MQE by numerical simulations. Because the magnet comes to quench if the mechanical disturbance exceeds the MQE, the disturbance energies are suspected to be equivalent to MQEs during the magnet-training. When we achieved somewhat larger MQE, we may exclude numbers of training quenches.In this paper, we discuss the guidelines of LTS magnet design from the standpoint of MQE. We represent some case studies for various superconducting magnets and/or some different winding methods.     

Critical current of react-and-jacket processed Nb3Sn conductor

A new type of large-scale Nb₃Sn conductor was developed that has an aluminum-alloy jacket to support an electromagnetic force. The manufacturing process of the conductor has a unique feature, in which the jacketing process is performed after a reaction heat treatment of the Nb₃Sn cable. This enables the conductor to have a high critical current, because the thermal strain of the Nb₃Sn filaments is decreased. Critical current measurements using a short conductor sample confirmed the expected high performance.     

The study on the critical temperature, composition, and microstresses in a Nb3Sn layer

Monofilamentary Nb₃Sn wires of large diameter with niobium tube, which were obtained by the method of solid-phase diffusion, are well suited for the study of the distribution of the critical temperature T_c in Nb₃Sn layers. Three regions with different gradients of Sn and Nb concentration and different Cu content can be distinguished in Nb₃Sn layer. In the central part of the layer, the Sn content comprises 24.5 at.% and the gradient of Sn concentration is negligibly small. Measurements on specimens of 1.2 mm in diameter with a slit cut along the cylinder generatrix showed that the critical temperature of the Nb₃Sn region adjacent to Cu(Sn) bronze is lower than the critical temperature of the central part of the layer. Fluctuations of T_c in the central part of the layer exceed the change of T_c related to the gradient of the Sn concentration, which is very small. These fluctuations spread both the R(T) curve and the high-temperature part of the temperature transition registered by the inductive method.     

Self-healing effect in strained multifilamentary Nb3Sn conductors

Multifilamentary Nb₃Sn conductors can be strained much more without damage than bare Nb₃Sn. The reason is a compression of the Nb₃Sn filaments which acts as a mechanical reserve. It is caused by a stronger thermal contraction of the bronze in the composite. By overloading the conductor at low temperature the compression of the Nb₃Sn disappears. Afterwards the conductor is subject to damage upon further loading. Measurements of the critical current under strain showed that at least part of the compressive strain can be regained by warming the conductor to room temperature. Analysis of the measured low temperature stress-strain diagram revealed that plastic deformation of the bronze during warming up is the cause of this self-healing.     

The breaking strain of Nb3Sn in a multifilamentary superconductor

Room temperature tensile testing has been carried out on a number of multifilamentary Nb₃Sn superconductivity wires. Metallographic studies of the tested wires suggest that filament cracking does not occur until strains approaching 1% have been reached. This conclusion has been confirmed for one of the wires by acoustic emission studies.     

Microstructure,composition and critical current density of superconducting Nb3Sn wires

The production of superconducting Nb₃Sn multifilamentary wires with optimized critical currents requires a detailed knowledge of various processes, involving both the mechanical deformation and the reaction kinetics. The physical properties of the Nb₃Sn phase are briefly reviewed, the precise knowledge of their variation as a function of the Sn content being essential for the optimization of the critical current density, J_c. The variation of the transition temperature T_c, the upper critical field, B_c2(0), and the normal state electric resistivity ρ_o, as a function of the Sn content, β, in the binary system Nb_1?βSn_β is carefully analyzed. The effect of Ta, Ti and Ta + Ti additives to Nb₃Sn on the physical properties is discussed in detail. Low temperature specific heat measurements are introduced for determining the T_c distribution inside Nb₃Sn filaments while avoiding shielding effects.The microstructure of the superconducting phase in Bronze Route and Internal Sn Diffusion processed wires is studied, taking into account the unique microstructure of Bronze Route filaments, comprising an equiaxed and a columnar grain region, their areas being comparable. The Sn content increases gradually, from 18 to 22 at.% in columnar and from 22 to 25 at.% in equiaxed grains. Taking into account the equiaxed grains only in Bronze Route wires, it is found that the pinning force density F_GB is essentially the same as in the superconducting part of Internal Sn and PIT wires. The lower values of the overall F_GB in Bronze Route wires is due to the presence of columnar grains, with lower T_c and B_c2. The presence of columnar grains also explains the deviation from linearity of the Kramer rule in Bronze Route wires.The mechanism leading to the variation of J_c vs. ε, where ε is the uniaxial applied strain, is correlated to the elastic tetragonal distortion of the A15 phase in the filaments, caused by the matrix precompression or by Lorentz forces. The behavior of J_c (ε) is found to show systematic differences between Bronze Route and Internal Sn processed wires. Possible reasons for the stronger variation of J_c (ε) up to 21 T in Internal Sn wires are discussed.     

Boundary resistance in SC composite wires and cryogenic stability

The measurement of transverse resistivity of NbTi composite wires has shown already the existence of a resistive barrier between SC filaments and the copper matrix. The electric and thermal resistances of this barrier are respectively much higher than those of copper matrix and this barrier is expected to have an influence on the cryogenic stability of composite wires. The transverse and longitudinal resistivities are measured for NbTi composite wires which were heat-treated at different temperatures from 300°C to 600°C. These measurements show that the barrier grows with the heat-treatment temperature. From the experimental results, the effect of the barrier on cryogenic stability is estimated to be negligibly small for the composite wire which is heat-treated under the normal condition. As for Nb₃Sn composite wires, two different structures of composite wires, each of which has a tantalum or niobium diffusion barrier, are studied and the same measurements as on NbTi composite wires are carried out. The results obtained indicate that the transverse resistivity depends appreciably on the structure of composite wires and that the larger transverse resistivity reduces not only the cryogenic stability, but also requires a larger transfer length at a current lead junction.     

Superconducting current carrying capacities of the composite-processed Nb-Hf/Cu-Sn-Ga in high magnetic fields

The Nb₃Sn growth rate, H_c2, and J_c of composite processed Nb₃Sn are substantially increased by the addition of hafnium to the niobium core. Furthermore, simultaneous addition of hafnium to the core and gallium to the matrix significantly increases H_c2 and J_c in high magnetic fields. SEM observations indicate that the addition of hafnium to the core enhances the growth rate of the Nb₃Sn layer without increasing the grain size of Nb₃Sn while the addition of gallium to the matrix causes an increase in coarseness of Nb₃Sn grains. The XMA analysis indicates the presence of small amounts of gallium and hafnium in the Nb₃Sn layer, J_c of the Nb-Hf/Cu-Sn-Ga wire specimens exceeds 1 × 10⁵ A cm^?2 at 17 T, suggesting that multifilamentary Nb₃Sn composite wires capable of generating magnetic fields over 17 T may be feasible.     

Finite element simulations of elasto-plastic processes in Nb3Sn strands

The manufacturing of Nb₃Sn strands, with drawing and annealing of multifilamentary strands followed by a heat treatment at about 900 K to form the Nb₃Sn by reaction of tin and niobium, has the potential to create a complex internal stress system. The strain sensitivity of the Nb₃Sn superconducting properties makes prediction of the internal stresses a necessary step to understanding the performance of Nb₃Sn conductors under the magnetic load conditions experienced in a coil. An elasto-plastic one dimensional finite element model, including temperature dependent stress-strain curves, annealing and manufacturing process stresses, is used to derive the internal stresses of Nb₃Sn strands. The model is benchmarked against a range of experimental data, including stress-strain tensile tests, superconducting critical current-strain tests, and length changes through heat treatment and through a 4 K thermal cycle. The model can predict all the experimental features and shows a number of unexpected conclusions regarding the origin of the Nb₃Sn stresses.     

Development of Nb3Sn superconducting wires for high field magnets at Kobe Steel and JASTEC

Research and development activities and some recent results related to Nb₃Sn superconducting wires in Kobe Steel, Ltd. (KSL) and Japan Superconductor Technology Inc. (JASTEC) are introduced. First, an outline of the activities is described briefly from a historical point of view. Following that, improvements in the characteristics (i.e., critical current density (J_c), n-value and mechanical properties) of bronze-processed Nb₃Sn wires are reviewed. Finally, the status of development for the Ta–Sn powder-in-tube (TS-PIT) process is briefly discussed.     

Room-temperature tensile behaviour of bronze-processed multi-filamentary Nb3Sn superconducting materials

Structure and its relation to fracture behaviour of multi-filamentary Nb₃Sn superconducting composite materials prepared by the bronze method were studied by tensile testing at room temperature. There were two types of fracture mode. Type I showed high elongation, accompanied by apparent plastic deformation of composites as a whole and the Nb₃Sn layer exhibited multiple fracture. Type II showed no apparent plastic deformation and the composites fractured in a brittle manner. Type I occurred when the fraction of the Nb₃Sn layer was small and the drop of load-bearing capacity due to fracture of Nb₃Sn layer could be compensated mainly by strain hardening of ductile constituents of Nb, Cu-Sn and Cu. On the other hand, Type II occurred when the fraction of Nb₃Sn layer was large and the fracture of the Nb₃Sn layer caused fracture of composites as a whole. To describe the tensile strength of composites for both types, a model was proposed, which explained well the experimental results. It was found that the strength of the Nb₃Sn layer decreases with increasing diameter of composites and with increasing annealing temperature and time.     

Growth kinetics of monofilamentary Nb3Sn and V3Ga synthesized by solid-state diffusion

Studies of growth kinetics of Nb₃Sn and V₃Ga formation have been carried for mono-filamentary composites of niobium and vandium filaments embedded in bronze wires containing varying concentrations of tin and gallium, respectively. The samples are diffusion reacted at different temperatures and for different lengths of time and the thickness and the microstructure of the resulting A-15 layer are investigated using optical and scanning electron microscopy techniques. The results are discussed in the light of the analytical model previously proposed by the present authors and it is shown that while the rate controlling step for the formation of Nb₃Sn is diffusion of tin through the bronze matrix, for V₃Ga it is the diffusion of gallium through the grain boundaries of the compound layer. The data are used to calculate the activation energies for Nb₃Sn and V₃Ga formation.     

The mechanism and kinetics of growth of the superconducting compound Nb3Sn

A study has been made of the mechanism and kinetics of formation of Nb₃Sn from the elemental components. The Nb₃Sn forms partly by diffusion and partly by a solution/ deposition mechanism which depends on thermal gradient mass transfer. The effect of this is to modify the growth equation to x = kt ^0.36 over the temperature range 950 to 1150° C. The temperature dependence of these two processes, given by the difference between the activation energies for diffusion and solution, is –9.7 kcal/g atom (–0.42 eV/atom) so that the thickness of the Nb₃Sn layer produced in any given time decreases with increasing temperature.Various experimental factors are discussed in terms of their influence on the rate of growth of the layer.     

Strain characteristics of Nb3Sn multifilamentary wires with CuNb reinforcing stabilizer

Bronze processed multifilamentary Nb₃Sn superconducting wires, with a CuNb reinforcing stabilizer instead of the conventional Cu stabilizer, were fabricated. The mechanical properties and the strain dependence of the critical current I_c were evaluated at 4.2 K and a magnetic field of 15 T. A remarkable increase in the yield stress (70%) and the plastic flow stress as compared to the values for the wire with Cu stabilizer was observed. The strain for the peak I_c was also increased by 0.2%. I_c on unloading was reversible within the strain range of 1.5%. The strain sensitivity of I_c in the CuNb/Nb₃Sn wire was almost the same as that of the Cu/Nb₃Sn wire. A decrease in the wire diameter from 0.8 to 0.5 mm resulted in a slight increase in the yield stress of the CuNb/Nb₃Sn wire, but no change in the strain dependence of I_c. An increase in the heat treatment temperature from 700 to 750°C resulted in a decrease in the flow stress of 15%, but no change in the strain dependence of I_c. A marked change in the morphology of the Nb filament in the CuNb reinforcing stabilizer was evidenced during heat treatment.     

Critical data of type II superconductors under hydrostatic pressure

Thin films of Nb₃Sn, Nb₃Ga, and NbN deposited on sapphire substrates by different preparation techniques and diffusion wires were measured under hydrostatic pressures up to 15 kbar. The results on critical current and upper critical induction under pressure can be described by scaling laws similar to those already known for temperature as parameter. The anisotropies of the critical currents and fields depend on the microstructure of the samples. The pinning mechanisms are not changed under hydrostatic pressure, so that the pinning forces in these superconductors can be compared to existing pinning theories.Supported by the Deutsche Forschungsgemeinschaft.