Superconductivity in dilute Cu-Nb-Sn alloys containing Nb3Sn precipitates

The dilute Cu-Nb-Sn alloys containing small amounts of Nb and Sn less than 1 at % exhibited superconductivity after quenching from the liquid state and ageing. The best superconducting properties ( andJ _c=130 A cm^–2) in a Cu-0.30 at % Nb 0.15 at % Sn alloy were obtained when the sample was aged at 550° C for 384 h. This sample exhibited a structure of fine Nb₃Sn precipitates of 200 to 500Å diameter distributed homogeneously in the Cu matrix, and therefore it was concluded that superconductivity in these alloys resulted from the proximity effect of Nb₃Sn particles. In spite of the similar structure obtained by ageing at 800° C, the Cu-Nb-Sn alloys showed inferior superconducting properties compared to the Cu-0.4 at % Nb alloy and this would be explained qualitatively by the difference in the mean free path in the two alloys.     

Superconductivity and Mössbauer effect of Nb3Sn produced by Sn implantation

We have implanted Sn, Ge, and Si into Nb films. The resulting Nb-Sn compounds and their annealing behavior have been analyzed by the Mössbauer effect and compared to samples obtained by diffusion of Sn into Nb foils. Mössbauer spectra show that Nb₃Sn is obtained just by implantation, but with a T _cof only 5 K. The 925°C annealing temperature necessary to form the A15 structure with long-range order of Nb chains and T _cvalues up to 17.8 K is at least 100 °C higher in implanted samples than in samples prepared by diffusion of Sn into Nb. This is explained in terms of implantation-induced lattice defects. The metastable A15 phases of Nb₃Ge and Nb₃Si could not be formed by Ge or Si implantation, regardless of target or annealing temperature. It is suggested that the high-energy ions only form phases stable at high temperatures and with low T _cvalues.On leave from North Dakota State University, Fargo, North Dakota.     

Preparation of Cu-Nb alloys for multifilamentaryin situ superconducting wire

A study has been made of two techniques, chill casting and consumable arc melting, for preparing ingots of Cu-Nb alloy for production of multifilamentary Nb₃Sn superconducting wire. It was found that Y₂O₃, ThO₂ and graphite all make excellent crucible materials for melting Cu-Nb alloys at 1850° C. Some difficulty was found with Nb segregation in chill-cast 5 cm diameter ingots. The consumable arc casting technique was shown to produce a uniform Nb dendrite distribution with little macrosegregation in 5 cm diameter castings and is regarded as having excellent potential for scale-up production of uniform Cu-Nb ingots of 25 to 30 cm diameter.     

Nb3Sn material development in Russia

In the USSR and later in Russia, the main activities in technical superconductivity were concentrated in the institutes that belonged to the Ministry of Atomic Energy (Minatom). The development of new technologies shortly transferred to the large-scale industrial production of NbTi and Nb₃Sn superconductors in early 1970s. Two main technologies for multifilamentary Nb₃Sn strands were under investigation during that time – bronze-process and internal tin method. More than 25 ton of Nb₃Sn bronze-processed strands were produced for the fabrication of 90 ton of conductors for application in the magnet system of first in the world fusion facility (tokamak T-15) with magnet system based on the intermetallic compound. The characteristics of these strands and conductors have been briefly described. The requirements for the Nb₃Sn strands constantly increased and the main R&D on the enhancement of critical current density have been reviewed. For bronze-processed strands the increase of the tin content in large ingots was the crucial factor. The artificial doping of niobium filaments by niobium–titanium alloy was invented, which enabled to improve the workability of Nb₃Sn strands, with enhanced critical current density in high fields. For internal tin Nb₃Sn strands the main R&D were concentrated on the optimization of the layouts of the strand and on the multistage heat treatment because of the inevitable liquid phase formation which could result in severe distortion of the geometrical arrangement of the filaments and even in destruction of the whole strand. The main results of these investigations have been presented. The corresponding impact of these R&D on the design of bronze-processed and internal tin strands has been analyzed. The quantitative estimations of the grain size were made for bronze-processed and internal tin strands. It was shown that in bronze-processed and internal tin strands subjected to the standard ITER heat treatment characterized by two stages at 575 °C and 650 °C, the variation of Nb₃Sn grain size in the range of 30–300 nm could be observed. The correlations of microstructure and superconducting properties have been discussed. The ITER connected activities in Russia on the development of Nb₃Sn strands, which met the HP-II specification, have been outlined. The results of the ITER Model Coil Program have shown a degradation of the critical current of large cable-in-conduit conductors (CICC) built with Nb₃Sn strands. For this reason, the investigation on the strain dependence of critical current density in Nb₃Sn strands of different designs is of high interest and priority. The R&D on development of bronze-processed and internal tin Nb₃Sn strands with enhanced, by the nanostructured Cu–Nb material, mechanical strength have been reviewed.     

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.     

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.     

Microstructural factors important for the development of high critical current density Nb3Sn strand

Nb₃Sn is the primary candidate for the next generation of accelerator magnets as well as for NMR and other applications that require magnetic fields between 11 and 20 T. Since 1999 the layer critical current density available in long length accelerator quality strand has almost doubled. The microstructural and microchemical factors that are important for high critical current density Nb₃Sn are reviewed. The highest critical current density strands have a Nb₃Sn layer that minimizes chemical and microstructural inhomogeneities and has a high fraction of the layer close to stoichiometric Sn content. Only the internal Sn process has yielded critical current densities beyond 3000 A/mm² at 12 T (4.2 K) and only with interfilamentary Cu thicknesses that are too low to separate the filaments after the final reaction heat treatment. The result of the reaction heat treatment is to produce a continuous ring of Nb₃Sn from hundreds of Nb or Nb-alloy filaments and thus a major ongoing challenge of Nb₃Sn conductor design is to reduce the effective filament diameter to acceptable levels for intended applications. Recent successful attempts to reduce the cost of alloying the Nb₃Sn for high field application are also examined and the potential for future improvements discussed.     

Growth of Nb3Sn in multifilamentary bronze composites

Growth of Nb₃Sn layers in multifilamentary composites has been investigated and their superconducting critical temperatures are measured using both resistive and inductive techniques. The growth parameters are discussed in the light of the analytical models of Reddi et al. Results show that for the composites studied, the rate controlling step for Nb₃Sn growth is diffusion of tin through grain boundaries of Nb₃Sn with the time exponent n determined by both the initial grain size and grain growth. T _c measurements show that for composites with a higher filament number, the width of superconducting transition is broader with no significant change in the onset T _c.     

Fabrication of sections of a Nb3Sn-based rigid superconducting cable

Using Nb₃Sn layers deposited onto the outer and inner surfaces of copper tubes 30 and 50 mm in diameter, we have fabricated sections of a rigid superconducting coaxial cable up to 1 m in length. The highest current-carrying capacity of the cable at 4.2 K was 800–850 A/mm, which corresponded to a critical current density of (5.0–5.5) × 10¹⁰ A/m² in the Nb₃Sn layer.     

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.     

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.     

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.     

Tin supply and microstructure development during reaction of an external tin process Nb3sn multifilamentary composite

The class of high tin multifilamentary Nb₃Sn superconducting composites depends on the diffusion of tin from a high tin reservoir to the niobium filaments where the superconducting A15 phase grows by solid state reaction. In particular, external tin composites are fully fabricated as niobium filaments in a copper matrix and the wire is subsequently coated with tin prior to reaction. In the work reported a detailed study is made of tin diffusion and microstructure development during the reaction of a 1369 filament external tin composite wire consisting of 37 × 37 bundled filaments of 3.5 m diameter. During the initial low-temperature anneal stage the formation and evolution of copper-tin intermetallic phases is followed. High-temperature reaction anneals were then carried out at 755 and 588° C. The rapid conversion of -phase to -Cu is accompanied by diffusion of tin towards the centre of the composite and the growth of Nb₃Sn A15 phase layers on the niobium filaments. The variation of tin composition and layer thickness is reported for different stages of reaction. In addition, the average composition of the A15 layer is measured as a function of the radial position of the filament, and the tin concentration gradient is measured within the A15 layer for the outermost filaments of the composite. The results show clearly the very strong dependence of the A15 layer growth rate on the tin concentration in the matrix. As a result, for an isothermal anneal at 755° C the layer growth is highy non-uniform across the composite. Furthermore it can be seen that additional inhomogeneity of the layer composition arises as a consequence of the bundled geometry of the composite. An important practical observation is that when the layer thickness approaches the filament radius the A15 compound is still far from stoichiometry particularly for filaments near the centre of the composite. The results overall emphasize the need for a detailed understanding of tin supply and compound growth before optimum heat-treatment procedures can be prescribed for a particular external tin design. A comparison of anneals at 755 and 588° C indicates that the relative rates of tin diffusion and A15 layer growth changes strongly with temperature; this suggests that a combination of anneals at different temperatures might be necessary for the optimization of the superconducting properties of high tin composites.     

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.     

Improving the Thermal Conductivity of Niobium in Nb3Sn/Nb/Cu Structures by Solid-State Refining

Experimental data are presented which demonstrate that the thermal conductivity of Nb in Nb₃Sn/Nb/Cu structures can be raised by solid-state refining in the range 1200–1500 K, using titanium and zirconium electrolytic coatings, plates, or powders as impurity sinks. The removal of oxygen from the Nb layer increases its thermal conductivity by a factor of 3–4, without impairing the performance parameters of the superconducting Nb₃Sn layer.     

High-temperature-tolerable superconducting Nb-alloy and its application to Pb- and Cd-free superconducting joints between NbTi and Nb3Sn wires

For more than 30 years, Pb–Bi alloy and Wood's metal (50% Bi, 26.7% Pb, 13.3% Sn, and 10% Cd) have been used as representative superconducting solder intermedia to establish superconducting joints between NbTi and Nb₃Sn wires in high-field nuclear magnetic resonance magnet systems. However, the use of Pb and Cd has been severely restricted by environmental regulations, such as the Restriction of Hazardous Substances Directive. Herein, a novel method of forming a superconducting joint between NbTi and Nb₃Sn wires without Pb and Cd has been proposed. This approach is based on metallurgical bonding processes using a superconducting Nb-alloy intermedium, whose fine microstructure is maintained even after exposure to temperatures higher than 650 °C. Further, fine crystal defects become sources of magnetic flux pinning centers. Among transition elements close to Nb, Hf is considered the most suitable additive for realizing high-temperature-tolerable (HTT) superconducting Nb-alloy intermedia. Utilizing the HTT characteristic of Nb–Hf, a superconducting joint between Nb₃Sn filaments and one end of the Nb–Hf alloy core was created by forming a superconducting Nb₃Sn layer at the interface through a chemical reaction. The other end of the Nb–Hf alloy core was cold-pressed with NbTi filaments, to connect their active new surfaces to each other in order to create a superconducting joint. Ultimately, a superconducting joint between NbTi and Nb₃Sn wires was realized with a high critical magnetic field (B_c2) of more than 1 T. The formation of the superconducting joint was confirmed by current decay measurements. This method of forming a superconducting joint is promising for application in environmentally friendly nuclear magnetic resonance magnet systems.

Graphical abstract

     

A transmission electron microscopy investigation of filamentary superconducting composites

The microstructure of the Nb₃Sn grains in commercially produced superconducting filamentary composites has been studied using transmission electron microscopy. The mean grain size increased with annealing time and temperature while the degree of columnar growth decreased at higher temperatures. Correlation of grain size and superconducting properties showed that the maximum pinning force was obtained for a grain size of about 800 Å.     

Filament contact within situ Nb3Sn superconducting wire

Experimental evidence is presented which indicates that the Nb₃Sn filaments withinin situ prepared Nb₃Sn-Cu superconducting wire are welded together at point contacts along a thin annular region which borders the surface of the original tin supply. For wires with tin added from an external plated layer the welded annular region lies near the outside diameter just below the original tin plate, whereas for wires with the tin added from an internal core the annular region lies along the original core hole. This welded region is expected to increase the a.c. loss characteristics of these wires. A possible cause of the welded annular region and methods for its elimination are discussed.     

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.     

Nb–Al diffusion reaction in high Nb containing TiAl porous alloys

High Nb containing TiAl porous alloys were synthesised by powder metallurgy (PM). In order to reveal reaction mechanism of Nb in preparation of the porous alloys, Nb–Al diffusion reaction was investigated using diffusion couples at relatively low temperatures of 600–800°C. The porous Nb–Al diffusion layer was identified as NbAl₃ phase and the thickness of diffusion layer indicated that the Nb–Al diffusion mainly occurred at 800°C. In addition, the pore diameter distribution indicated that Nb–Al diffusion also contributed to the increase in pore diameter. According to these results, the diffusion reaction model was established for high Nb containing TiAl porous alloys.