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.     

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.     

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.     

Superconducting and metallurgical properties of bronze processed Nb3Sn wires with Ni and Zn additions and comparison with Ta and Ti additions

Critical current density measurements up to 23 T of nineteen core Nb₃Sn wires with simultaneous addition of Ni to the core and Zn to the Cu-Sn bronze matrix have revealed a considerable increase of J_c at fields above 11 T. For a wire with the composition Nb-0.6 wt% Ni/Cu-10 wt% Sn-3 wt% Zn, reacted at 750°C for 64 h, J_c in the layer was determined to 1.3 × 10⁵A cm^?2 at 14 T and to 4 × 10⁴A cm^?2 at 19 T. Comparison with Ta and Ti core-alloyed Nb₃Sn wires, also performed in the present study, shows very similar results in J_c and J_c vs. ε up to the highest fields.Composition profiles for Sn and for Ni, Ta and Ti additives in the A15 layers were studied by Auger spectroscopy. For Sn a concentration gradient across the layer (from ≈ 25 at% to ≈ 22 at% Sn) was observed, with the highest Sn content occurring at the interface with the bronze. The presence of the additives in the layer was detected as well by Auger analysis as by X-ray diffractometry.     

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.     

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.     

Influence of third elements on growth of Nb3Sn compounds and on global pinning force

The crystal growth of Nb₃Sn by the bronze method has been investigated by using diffusion couples consisting of niobium and bronze with the addition of third elements. When the specimens were annealed at temperatures between 973 and 1073 K, the time-dependence of layer thickness was represented approximately by the function ofd=kt ^m . The time exponent changed from 1/3 to unity depending on the annealing condition as well as on the nature of the third element. By the addition of titanium, hafnium, zirconium, indium and galium to the bronze, the growth rate of the compound layer increases. Faster layer growth corresponds to a larger time exponent. The following three processes are suggested to be important for controlling layer growth: diffusivity of tin atoms through grain boundaries in the compound layer, diffusivity of tin atoms through the matrix of the compound, and the rate of the chemical reaction to form the compound. Essentially these combined processes control the overall rate of layer growth. The grain size is found to be the most effective structural parameter to affect directly the maximum global pinning force. The critical current at a magnetic field of 5T can be scaled by both the layer thickness and the inverse grain size.     

Microstructure and superconducting current ofIn situ Nb3Sn superconducting wire

The diffusion of elemental tin and the morphological change of niobium filaments inin situ Nb₃Sn superconducting composite wires and their influences on critical current were studied. When the amount of tin plated on the samples was high, the diffusion of elemental tin was enhanced. The critical current increased with increasing tin concentration but the increase became sluggish at high tin contents. The niobium filaments were initially ribbon-like but they became rod-like and then sausage-like after annealing treatment. Such a morphological change acted to reduce superconducting current capacity. When the amount of niobium was low, the filaments spheroidized by high-temperature and long-term annealing, resulting in serious reduction in critical current and upper critical magnetic field. High niobium contents led to high critical current and high upper critical magnetic field due to retainment of continuity of the filaments after annealing, effective proximity effect and a high amount of Nb₃Sn formed in comparison with low niobium content amount. The titanium addition raised the upper critical magnetic field, resulting in improvement in critical current at high magnetic fields.     

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.     

A review of mechanical behaviour and stress effects in hard superconductors

The mechanical properties of type II superconducting materials are reviewed as well as the effect of stress on the superconducting properties of these materials. The bcc alloys Nb-Ti and Nb-Zr exhibit good strength and extensive ductility at room temperature. Mechanical tests on these alloys at 4.2 K revealed serrated stress-strain curves, non-linear elastic effects, and reduced ductility. The non-linear behaviour is probably due to twinning and de-twinning or a reversible stress-induced martensitic transformation. The brittle A-15 compound superconductors, such as Nb₃Sn and V₃ Ga, exhibit unusual elastic properties and structural instabilities at cryogenic temperatures. p]Multifilamentary composites consisting of superconducting filaments in a normal metal matrix are normally used for superconducting devices. The mechanical properties of alloy and compound composites, tapes, as well as composites of niobium carbonitride chemically vapour deposited on high strength carbon fibres are presented. Hysteretic stress-strain behaviour in the metal matrix composites produces significant heat generation, an effect which may lead to degradation in performance of high field magnets. Measurements of the critical current density, J_c, under stress in a magnetic field are reported. Modest stress-reversible degradation in J_c is observed in Nb-Ti composites while more serious degradation is found in Nb₃Sn sample.The importance of mechanical behaviour on device performance is discussed.     

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.     

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.     

The microstructure and mechanical properties of Nb3Sn filamentary superconducting composites

The deformation processes in filamentary superconducting composites at both room temperature and 4.2 K have been studied using transmission and scanning electron microscopy. In all the composites, the filaments consisted of a central core of unreacted niobium surrounded by a reacted layer of Nb₃Sn. The Nb₃Sn failed in an intergranular manner without any prior dislocation activity and the radial cracks formed in the Nb₃Sn layer during deformation were stopped at the niobium core. The observed variations in ductility, fracture stress and secondary modulus between the different composites were accounted for quantitatively by the presence of the niobium cores.     

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.     

Improved superconducting properties of the composite-processed V3Ga wires with changed configuration

Single core V₃Ga superconducting wires have been fabricated into two configurations following the composite process. The first configuration is a normal one in which the composite consists of a pure vanadium core in a Cu-Ga (19 at%) matrix. In the second configuration the same matrix has been used as a core inside a pure vanadium sleeve. Wires have also been fabricated in these two configurations by adding gallium and magnesium elements to pure vanadium and Cu-Ga matrix respectively. A simultaneous addition of gallium to vanadium and magnesium to the Cu-Ga (19 at %) matrix has been found to raise the critical current density, J_c of the V₃Ga appreciably in conformity with earlier results reported on the composite processed V₃Ga tapes using identical contents of additional elements. The configuration-2 leads to a more uniform V₃Ga layer formation and a faster growth rate thus increasing the overall critical current density still further. A value of J_c of ～ 1.6 × 10⁶ at 4.2 K and 9T has been obtained for single core wires prepared in configuration-2 and using magnesium and gallium additional elements.     

Comparison of projected critical currents in PbMo6S8 and Nb3Ge

Critical current densities, J_c, of sputtered Chevrel phase PbMo₆S₈ films have been measured as a function of field to 19 T at several temperatures. The pinning forces were found to obey a scaling law. Assuming an effective upper critical field B_c2 = 45 T, an effective critical temperature T_c = 13 K, together with empirical estimates of numerical factors in the scaling law, we estimate the upper limits for critical current density J_c in PbMo₆S₈. Comparisons are made to estimates of J_c for Nb₃Ge at 4.2 K. A crossover of J_c 10⁸ A m^?2) vs B is found for B of the order of 25 to 30 T. Below this point, Nb₃Ge is projected to have a higher critical current density. Thus practical use of Chevrel materials in high field magnets only appears to be competitive with Nb₃Ge for fields above 25 to 30 T.     

Critical data of some A15 type superconductors in transverse fields up to 230 kOe

Critical field curves and quenching curves for the A15 (-W) type compounds Nb₃Sn, V₃Si, and V₃Ga have been measured. The critical field curves have been extrapolated to zero temperature, and the following zero upper critical fields have been found: 245 kOe for Nb₃Sn, 235 kOe for V₃Si, and 208 kOe for V₃Ga. These values lie between the upper and lower limits given by theories for critical fields not taking into account spin-orbit scattering. Critical currents for Nb₃Sn samples with small admixtures of zirconium are higher than for corresponding samples without zirconium. Special samples of Nb₃Sn multiwires 522 have still high critical currents in the high-field region up to 230 kOe.On leave of absence from the University of Giessen.Supported by the U.S. Air Force Office of Scientific Research.     

Microstructure and superconductivity in annealed Cu-Nb-(Ti,Zr, Hf) ternary amorphous alloys obtained by liquid quenching

Melt-quenched Cu-Nb-(Ti, Zr, Hf) ternary alloys have been found to be amorphous possessing high strength and good bend ductility. The niobium content in the amorphous alloys was limited to less than 35 at % and the titanium, zirconium or hafnium contents from 25 to 50 at %. The Cu₄₀Nb₃₀(Ti, Hf)₃₀ alloys showed a superconducting transition above the liquid helium temperature (4.2 K) after annealing at appropriate temperatures. The highest transition temperatures attained were 5.6 K for the Cu₄₀Nb₃₀Ti₃₀ alloy annealed for 1 h at 873 K and 8.4 K for the Cu₄₀Nb₃₀Hf₃₀ alloy annealed for 1 h at 1073 K. In addition, these alloys exhibited upper critical magnetic fields of 1.8 to 2.3×10⁶ Am^–1 at 4.2 K and critical current densities of 2×10³ to 1×10⁴ A cm^–2 at zero applied field and 4.2 K. Since the structure of the superconducting samples consisted of ordered phases based on a b c c lattice with a lattice parameter of 0.31 nm, it was concluded that the superconductivity in the Cu₄₀Nb₃₀Ti₃₀ and Cu₄₀Nb₃₀Hf₃₀ alloys was due to the precipitation of the metastable ordered b c c phases.     

Some inherent limitations of the alloy process for superconducting Nb3Sn

The Alloy Process for making multifilamentary superconducting Nb₃Sn is critically examined. It is shown the process itself has certain inherent limitations to realize a reasonably high overall J_c. Further, the microstructure of the material produced by this process is unfavourable for high J_c and it is shown that at low currents avalanche like process may occur driving all superconducting filaments suddenly normal. Attention is drawn to other processes which seem free from the limitations discussed.     

Heat treatment of Nb3Sn conductors

Magnet coils made out of Nb₃Sn superconductors usually are manufactured by the wind- and react-technique. Due to the brittleness of the A15 material the superconductive layer is formed only after the winding of the magnet. This is done by a heat treatment in which Sn diffuses via a matrix into Nb filaments and the superconducting layer is formed. Depending on the exact temperature and time of the heat treatment, the physical properties of the superconductor such as critical current density J_c, upper critical field B_c2, critical temperature T_c and n-value can be varied over a wide range. This is because the diffusion process determines the grain size distribution, the thickness of the superconductive layer as well as the Sn distribution within the layer.This article will provide a review of the investigations concerning different aspects of heat treatment over recent years.