Superconducting Nb3Sn diffusion layers

In the present work an experimental apparatus for the production of Nb₃Sn superconducting ribbon is described. The fabrication method for these ribbons takes advantage of the diffusion process of tin into niobium. We give also some details of the technique by which the superconducting properties of Nb₃Sn have been tested. A number of preliminary experimental results are discussed with the purpose of pointing out the main fabrication parameters which influence the superconducting properties. Finally future developments of this research program are outlined.     

Effects of the IVa element addition on the composite-processed superconducting Nb3Sn

The effect of the IVa element addition to the niobium core and that of gallium addition to the matrix on the composition, growth rate and superconducting properties of the composite-processed Nb₃Sn have been studied. The IVa elements added to the niobium core enhance the growth rate of Nb₃Sn, and prevent the grain coarsening of Nb₃Sn. A much larger amount of titanium is incorporated into Nb₃Sn than zirconium or hafnium. T_c shows a slight maximum against the IVa element concentration in the niobium core. J_c at high magnetic fields is more significantly increased by titanium addition than zirconium or hafnium additions. The gallium substitution for tin in the matrix is effective for increasing T_c and J_c in high fields, except for the specimen with Nb-Ti alloy core. The simultaneous addition of hafnium and gallium is most effective for the enhancement of J_c in high fields.     

Heat Treatment of Two-Turn Nb3Sn Superconducting Coil

A heat treatment system was built at High Magnetic Field Laboratory, Chinese Academy of Sciences (CHMFL) for the heat treatment of large-scale Nb₃Sn superconducting coils and conductors. In order to evaluate the manufacturing technique of the joints for the Nb₃Sn CICC, a two-turn Nb₃Sn superconducting coil with two superconducting joints had been developed. A two-turn coil together with several Nb₃Sn witness samples was heat-treated. Experimental and finite elemental analysis was also discussed. The results of the measurements met the requirements of the magnet design.     

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.     

Zero-Current Deposition of Superconducting Nb3Sn Coatings from Molten Salts

Superconducting Nb–Sn coatings are produced by zero-current Sn deposition on Nb at 800–1070 K from KCl + NaCl and CsCl + KCl + NaCl eutectic melts containing SnCl₂. Their phase composition and superconducting properties are studied. This approach offers the possibility of producing single-phase Nb₃Sn coatings perfect enough to be used as working layers of superconducting microwave systems.     

Effect of Heat Treatment on the Structure and Properties of Superconducting Nb and Nb3Sn Electrodeposits

The effect of vacuum heat treatment (900–1600°C) on the structure and properties of superconducting Nb and Nb₃Sn coatings produced via electrolysis of molten salts is studied. The results demonstrate that the primary recrystallization of the coatings is characterized by a nonuniform nucleation of new grains: nuclei appear first in a defect-rich layer near the coating–substrate interface. The presence of the substrate during heat treatment prevents complete relaxation of the lattice strain and dislocations in the coatings, without significantly influencing the macroscopic stress. The relaxation of lattice strain and the reduction in dislocation density during recrystallization bring the coatings closer to equilibrium, thereby changing their superconducting properties.     

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.     

Superconducting properties of Nb3Ge thin films prepared by coevaporation

Nb₃Ge films with thicknesses between 60 and 800 nm were deposited onto heated sapphire substrates by thermal coevaporation. Stoichiometric Nb₃Ge samples crystallize in a single-phase A15 structure with a lattice parameter of 5.14 ». Critical current densities and upper critical magnetic inductions were measured as a function of temperature, film thickness, and magnetic field orientation. The critical currents and fields show maxima for parallel field orientation. This is due to the random orientation of the crystallites. With decreasing thickness an increasing second maximum for normal field orientation is observed. This behavior can be explained by additional precipitates caused by internal stress.Dedicated to Professor Dr. B. T. Matthias on the occasion of his 60th birthday.Research supported by Deutsche Forschungsgemeinschaft.     

Superconducting properties of Nb3Ge thin films prepared by cosputtering

Nb₃Ge films with thicknesses between 0.5 and 1.5 m have been deposited on heated sapphire substrates by dc sputtering in a pure argon atmosphere. With a maximum superconducting onset temperature T _c= 22.7 K, these high-T _csamples crystallize in a single-phase A15 structure with lattice parameters down to 5.14 ». Critical current densities and upper critical magnetic inductions have been measured as a function of magnetic field orientation, temperature, and film thickness. The anisotropies of the critical currents and fields depend on the microstructure of the samples and show maxima for the field orientation normal to the sample surface in those films that exhibit columnar growth of the crystallites normal to the substrate. The temperature, field, and angular dependences of the pinning forces in these films are compared to existing pinning theories.Dedicated to Prof. Dr. B. T. Matthias on the occasion of his 60th birthday.Supported by Deutsche Forschungsgemeinschaft.     

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.     

Strength and elongation of multifilamentary Nb3Sn superconducting composite materials with small amounts of Nb3Sn compound

In order to describe the tensile strength and elongation to failure of multifilamentary Nb₃Sn superconducting composite materials with small amounts of Nb₃Sn showing multiple fracture, approximate calculation methods are proposed. In the proposed calculation method, the concept of shear-lag analysis and the plastic instability approach for metallic composites are employed. The experimental results are fairly well described by the present calculation methods.     

Simulation of transport properties in Nb3Sn strand under bending

Superconducting performance of a large-scale Nb₃Sn cable-in-conduit conductor (CICC) is degraded by periodic bending of strands subjected to a distributed transverse electromagnetic force during operation. The current transport in a single strand depends mainly on the bending strain and transverse resistivity. In particular, in the case of high-level strain and/or crack occurring among the filaments in the strain-sensitive Nb₃Sn strand, these two parameters are required for better understanding of the critical current I_c degradation of a single strand. We use finite element method to simulate transport properties of a single Nb₃Sn strand under bending. The simulation allows treating a wider range of transverse resistivity of strand, compared with our previous analytical method (Cryogenic, 58, 2013). Also, the present simulation incorporates the change of the area of strand cross section due to filament fracture into the boundary of the current transport, rather than simply imposes the condition of vanishing current on the filament fracture region as in the previous analytical method. We show the current/field profiles in the strand for various bending loads and transverse resistivities, as well as the I_c degradation of several types of strands under bending.     

Modelling the effect of twisting on electro-mechanical properties of Nb3Sn conductors

The influence of external loading on the superconducting filaments of Nb₃Sn wires depends on the wire structure, for example, on twisting. Electro-mechanical behaviour of wires can be studied with finite element models. Three-dimensional models, whose numerical solution is heavy, are generally needed for twisted wires. However, this paper presents a two-dimensional model for twisted conductors that can be used under certain loading situations. Computational tests showed that twisting influences the superconducting properties significantly and must be modelled only if the ratio of the twist pitch to the distance between the outermost filament and the wire axis is smaller than 25.     

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.     

The effects of neutron irradiation damage on the superconducting properties of Nb3Sn

Specimens of Nb₃Sn have been irradiated by fast neutrons at 70°C to doses in the range 3 × 10²¹ neutrons m^?2 to 9 × 10²³ neutrons m^?2. Their critical temperatures were depressed linearly with dose, to less than 4.2 K after about 3 × 10²³ neutrons m^?2. The critical temperature recovered to their initial values in anneals of 2 hours at 900°C, and in 64 hours at 750°C.The critical current can be enhanced by low neutron doses, particularly at high fields, but is always depressed by higher doses.The observations are shown to be consistent with a qualitative model, and in the light of this the likely consequences of irradiation at operating temperature are considered.     

Effect of irradiation on the mechanical properties of commercially produced Nb3Sn tapes

The effect of neutron irradiation on the mechanical properties of commercially produced Nb₃Sn tapes has been investigated using scanning electron microscopy, sound velocity measurements, tensile and three-point bend tests. The elastic moduli show an increase on irradiation which is independent of dose in the range 4×10²¹ to 4×10²³ neutrons m^–2. The majority of the Nb₃Sn tapes show no evidence of significant amounts of plastic deformation prior to failure, which occurs by the intergranular fracture of the Nb₃Sn layers followed immediately by ductile overload failures of the niobium core. The latter changes to a more brittle failure on irradiation and in tapes containing ZrO₂ particles. The fracture stress decreases for doses up to 10²³ neutrons m^–2 but increases at higher doses. Irradiation reduces the critical stress intensity factorK _c, butK _c and the fracture stress are increased in tapes containing ZrO₂. These results are discussed in terms of various micro-structural features and previously determined radiation damage.     

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.