Overview of Nb3Sn and V3Ga conductor development in Japan

In the early stage of high-field A15 conductor development in Japan, different type of V₃Ga conductors were fabricated. Then, Ti-doped Nb₃Sn conductors have been developed, and widely used for high-field generation. Increase of Sn concentration in the bronze produces an appreciable progress in the performance of bronze processed (Nb,Ti)₃Sn conductors. New internal Sn processed (Nb,Ti)₃Sn conductors with modified cross-sectional configurations have been produced, which exhibit large J_c in high fields as well as reduced AC loss. Both bronze processed and internal Sn processed (Nb,Ti)₃Sn conductors satisfy recent ITER magnet specifications. As for new type Nb₃Sn conductors, powder core and Jelly Roll processed (Nb,Ta)₃Sn wires with improved high-field performance have been fabricated.     

Review on the fabrication techniques of A-15 superconductors

This Paper reviews the present state-of-the-art of preparing multifilamentary A-15 superconductors. The most common types, Nb₃Sn and V₃Ga, are presently produced by the so-called bronze process. The highest J_c (overall) = 3.5 × 10⁴ cm⁻² (at 15 T and 4.2 K), obtained for bronze processed Nb₃Sn composites through Ti addition, has pushed the useful limit of this material from 12 to 16 T. Similarly a J_c of 1 × 10⁵ A cm⁻² (at 20 T and 4.2 K) for the A-15 V₃Ga has been attained through elemental additions to the core and the bronze matrix. To circumvent the problem of work-hardening of the bronze, several variations of the bronze process such as the internal tin method, the Nb tube method, the ECN method and jelly roll method have also been upgraded to commercial scale. Composites of Nb₃Sn and V₃Ga have been recently produced successfully on a laboratory scale following the so called in situ technique. These composites not only have a superior J_c value but display improved strain tolerance due to the ultrafine nature of the filaments formed in situ. In situ filamentary A-15 composites with high J_c values have also been produced by following the powder metallurgy technique. The infiltration technique has been found useful for producing high field Nb₃(Al, Ge), Nb₃(Al, Si) and Nb₃Sn composite conductors with high ε_irr. Superior materials such as Nb₃Al, Nb₃Ga and Nb₃(Al,Ge) with high J_c performance have been synthesized using the laser beam technique. Nb₃Ge tapes with T_c = 21 K and J_c = 10⁵ A cm⁻² (at 18 T and 4.2 K) have been successfully produced on a laboratory scale by following the CVD technique. Thus, there are several available options from which to choose a technique for fabricating filamentary composites of ubiquitous Nb₃Sn and V₃Ga. New techniques for fabricating superior materials like Nb₃Al, Nb₃Ga, Nb₃Ge and Nb₃(Al, Ge) also seem to be at an advanced stage of development.     

Critical current properties of V3Ga with third element additions

Critical current densities of over 1.7 × 10⁶ Acm^?2 at 10T and 4.2K have been achieved in multifilamentary V₃Ga wire produced by a modified bronze technique. Attempts to further improve this value were made by adding a third element, either Ti or Zr, to the V-Ga alloy filaments along with Al to the Cu-Ga matrix. The effects of reaction temperature and layer thickness with the third element additions were qualitatively similar to those measured for wires without third element additions, ie lower reaction temperatures and smaller layer thickness gave higher J_c values. At low reaction temperatures, these composite wires with third element additions exhibited slower growth rates and lower J_c values than those obtained for V-Ga cores in a Cu-Ga matrix, at higher temperatures just the opposite was true. In all cases, however, the J_c values obtained were lower than the best we have been able to achieve using V + Ga filaments and Cu + Ga matrix under optimum conditions.     

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.     

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.     

Kinetics of reactive diffusion between Ta and Cu–9.3Sn–0.3Ti alloy

Tantalum is used as a diffusion barrier in the superconducting Nb₃Sn composite-wire manufactured by the bronze method. In order to examine the consumption behavior of the Ta barrier during annealing in the bronze method, the kinetics of the reactive diffusion between Ta and a bronze was experimentally observed using sandwich diffusion couples composed of Ta and a Cu–9.3Sn–0.3Ti alloy. The (Cu–Sn–Ti)/Ta/(Cu–Sn–Ti) diffusion couples were isothermally annealed at temperatures of T = 973–1053 K for various times up to t = 1462 h. Owing to annealing, Ta₉Sn is formed as a uniform layer at the initial (Cu–Sn–Ti)/Ta interface in the diffusion couple, and gradually grows mainly toward Ta. The mean thickness of the Ta₉Sn layer is proportional to a power function of the annealing time. However, the exponent of the power function is equal to unity at t < t _c but smaller than 0.5 at t > t _c. Thus, the transition of the rate-controlling process for the growth of Ta₉Sn occurs at t = t _c. The critical annealing time t _c takes values of 1.83 × 10⁶, 4.63 × 10⁵, and 5.98 × 10⁵ s at T = 973, 1023, and 1053 K, respectively. The growth of Ta₉Sn is controlled by the interface reaction at the migrating Ta₉Sn/Ta interface in the early stages with t < t _c but by the volume and boundary diffusion across the Ta₉Sn layer in the late stages with t > t _c. Due to the transition of the rate-controlling process, the growth rate is always much smaller for Ta₉Sn than for Nb₃Sn. As a result, Ta works as an effective barrier against the diffusion of Sn from the bronze to the Cu stabilizer in the superconducting Nb₃Sn composite-wire.     

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.     

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.     

Changes ofT c,J c,B c2 and the lattice parameter of the Nb3Sn phase formed at the initial stage of growth in a multifilamentary superconductive wire

Investigations were made of the superconducting transition temperature,T _c, the upper critical flux density,B _c2, and the critical current density,J _c, of Nb₃Sn layers in filamentary wire in a bronze matrix. The lattice parameter,a ₀, andT _c of Nb₃Sn layers in 259-filament wire were determined after removal of the bronze matrix. The microstructure and layer thickness were studied using scanning electron microscopy. The diffusion formation of Nb₃Sn phase at 1023 K was studied until the complete reaction of the niobium filaments. It was found that the Nb₃Sn layer begins to form in the manufacturing process during the intermediate annealing at 793 K, and that there is a considerable degradation of critical parameters due to the non-stoichiometry of the Nb₃Sn phase in layers thinner than 1m.     

Projected upper limits to the critical current density of superconducting Nb3Ge, Nb3Sn, and V3Ga

Applying a phenomenological theory of flux pinning developed by Kramer,¹ where the ultimate critical current density (J_c) of a superconductor is determined by plastic shearing of the flux lattice, approximate upper limits are put on J_c for Nb₃Ge, Nb₃Sn, and V₃Ga. At 4.2 K and for magnetic fields H < 100 kG, the J_c of V₃Ga is greater than that of either Nb₃Ge or Nb₃Sn and Nb₃Sn has somewhat higher values than Nb₃Ge. Above 200 kG Nb₃Ge has the highest J_c due to its having the highest upper critical field and at 14 K or above it probably has the largest J_c at all field values.     

Microstructure and radiation damage of commercially produced Nb3Sn tapes

The microstructure and fast neutron irradiation damage of Nb₃Sn tapes produced by a liquid tin diffusion method have been studied using electron microscopy, X-ray diffraction and secondary ion mass spectrometry.The Nb₃Sn layer consists of an outer region of clusters of Nb₃Sn grains, which have a high oxygen content, and an inner region of smaller equiaxed grains. The rate of growth of the Nb₃Sn layer and the kinetics of grain growth in these commercial tapes are compared with published results for a laboratory system and Nb₃Sn formed by the ‘bronze route’.In Nb₃Sn irradiated at 70°C to doses up to 5.4 10²³ neutrons m^?2 disordered regions and dislocation loops are observed; the latter dissappeared on annealing for short times at temperatures from 300 to 750°C Nb₃Sn tapes irradiated at higher temperatures only show dislocation loops which form pairs on annealing. These results are correlated with previously determined T_c measurements.     

Growth rate of Nb<Subscript>3</Subscript>Sn for reactive diffusion between Nb and Cu–9.3Sn–0.3Ti alloy

In order to examine experimentally the growth behavior of Nb₃Sn during reactive diffusion between Nb and a bronze with the α + β two-phase microstructure, a sandwich (Cu–Sn–Ti)/Nb/(Cu–Sn–Ti) diffusion couple was prepared from pure Nb and a ternary Cu–Sn–Ti alloy with concentrations of 9.3 at.% Sn and 0.3 at.% Ti by a diffusion bonding technique. Here, α is the primary solid-solution phase of Cu with the face-centered cubic structure, and β is the intermediate phase with the body-centered cubic structure. The diffusion couple was isothermally annealed at temperatures between T = 923 and 1,053 K for various times up to 843 h. Owing to annealing, the Nb₃Sn layer is formed along each (Cu–Sn–Ti)/Nb interface in the diffusion couple, and grows mainly into Nb. Hence, the migration of the Nb₃Sn/Nb interface governs the growth of the Nb₃Sn layer. The mean thickness of the Nb₃Sn layer is proportional to a power function of the annealing time. The exponent of the power function is close to unity at T = 923 K, but takes values of 0.8–0.7 at T = 973–1,053 K. Consequently, the interface reaction at the migrating Nb₃Sn/Nb interface is the rate-controlling process for the growth of the Nb₃Sn layer at T = 923 K, and the interdiffusion across the Nb₃Sn layer as well as the interface reaction contributes to the rate-controlling process at T = 973–1,053 K. Except the effect of Ti, the growth rate of the Nb₃Sn layer is predominantly determined by the activity of Sn in the bronze and thus the concentration of Sn in the α phase. As a result, the growth rate is hardly affected by the volume fraction of the β phase, though the final amount of the Nb₃Sn layer may depend on the volume fraction.     

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.     

Superconducting properties of internal-tin route Nb3Sn wires with radially arranged filaments : Development that realizes high Jc and low hysteresis loss

An internal-tin route Nb₃Sn superconducting wire that has both remarkably low hysteresis loss (Q_h) and high critical current density (J_c) was developed according to a new design idea. The wire was constructed by arranging the filaments in a radial layout, enlarging the outer filaments along the radial direction, narrowing the filament spacing in the radial direction intentionally and enlarging the filament spacing in tangential direction. Thus, the electromagnetic coupling among the filaments in tangential direction due to the bridging and/or proximity effect was suppressed without decreasing the volume fraction of Nb. As a result, excellent properties such as J_c(12 T) = 1.15 × 10³ A/mm² and Q_h = 301 mJ/cm³ (for 1 cycle of B = ±3 T) were obtained. We also evaluated the transition temperature (T_c) and upper critical field (B_c2) of the wire. The values for T_c and B_c2 were 17.3 K and 24.1 T, respectively, which were much better than those of usual internal-tin route wires. Moreover, electron probe micro-analyses confirmed that the good T_c and B_c2 were the result of the qualitative improvement of the Nb₃Sn compound based on the effects of arranging the Nb filaments radially, increasing the ratio of Sn-to-Nb and shortening the diffusion length for Sn. This wire is promising for use with conduction-cooled high-field magnets, in which there is a need to decrease the load of the cryocooler, and also for the strands of fusion coils.     

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.     

Effects of H2/Cl2 ratio on CVD synthesis of superconducting Nb3Ge tapes

This Paper investigates the effects of H₂/Cl₂ ratio, R_g, [H₂/Cl₂(Nb,Ge)] in chemical vapour deposition processes on the synthesis and superconducting properties of Nb₃Ge tapes. Critical temperature, T_c, critical current density, J_c, and grain size of A15 Nb₃Ge vary considerably with the gas ratio, R_g. The lattice parameter of A15 Nb₃Ge and its volume ratio to Ge-rich Nb₅Ge₃ phases are also dependent on R_g. The volume ratio of A15 Nb₃Ge to Nb₅Ge₃ increases with increasing R_g, while the grain size of Nb₃Ge considerably decreases. The highest T_c, 19 K, (mid-point) was obtained for Nb₃Ge tape where the A15 Nb₃Ge compound coexisted with a small amount of tetragonal or hexagonal Nb₅Ge₃ compounds. The largest J_c was ≈ 4.5 × 10⁸ A m⁻² at 16 T and 4.2 K.     

Multifilamentary superconducting (NbTa)-Sn Composite wire by solid-liquid reaction for possible application above 20 tesla

Nb alloyed with Ta was employed in fabricating multifilamentary composite wires of (NbTa)-Sn using the liquid-infiltration process. The superconducting A15 phase was formed with subsequent heat treatments at 800–950°C by the solid-liquid reaction. High inductive T_c's of 18.2K with sharp transition width (<0.3K) and high overall J_c's of ～1.6 × 10⁴ A/cm² at 20T and 4.2K were obtained. It was found that 2 wt.% Ta in the Nb was sufficient in the enhancement of the overall J_c at the high fields and in increasing the H_c2 (4.2K) to 25T.     

Growth and structural studies of multifilamentary Nb3Sn formed by solid state diffusion

Growth of Nb₃Sn layer formed in a multifilamentary composite of bronze matrix by solid state diffusion has been investigated and its T_c values are measured after different time periods of diffusion anneal. The grain boundary structures are also systematically studied with scanning electron microscopy. Results obtained beyond reasonable doubt conclude that the rate controlling process for growth of Nb₃Sn is via diffusion through grain boundaries of Nb₃Sn layer, and grain growth significantly affects the layer growth. Both T_c and the transition width ΔT_c change in a regular way as annealing progresses, and the variation of these parameters is related to changes in the grain structure and internal strains of the sample.     

Effect of bronze on the compression of Nb3Sn in multifilamentary conductors

Nb₃Sn in multifilamentary conductors is subject to compressive strain as a result of the relatively small thermal contraction of the filaments as compared to bronze. The critical current l_c is consequently degraded. The critical current increases, when an external tensile stress is applied, and passes through a maximum. The ratio of the maximum critical current to the initial critical current increases with the flux density and reaches a value of two at a flux density of 16 T for technical conductors. The strain _m, at which the l_c maximum is reached, lies between 0.4% and 0.7% for the conductors investigated and depends on the material parameters. For a constant ratio of bronze to filament cross section this strain _m is reduced as the Nb₃Sn layer thickness is increased and can be determined approximately by a graphical method from the stress-strain diagram. _m is to a large extent dependent on the metallurgical properties of bronze, which vary to a considerable extent depending upon the heat treatment.     

Development of niobiumtin conductors at SLE

Superconductors Lips ECN is a recently formed company which produces NbTi and Nb₃Sn multifilament wires as well as superconducting cables. Multifilamentary Nb₃Sn wire, produced by the powder route, is developed commercially. About 30 km of strand material has been produced for the SULTAN 12 T project. These wires showed good performance. Developments are underway to reduce gradually the filament diameter at the same current performance to meet the full requirements for the NET and LHC conductors.