A transmission electron microscopy study of bronze-processed Nb3Sn and (Nb,Ta)3Sn multifilamentary superconducting wire

The morphology, grain size and composition of A15 diffusion layers produced on heat-treating Nb-bronze and (Nb, Ta)-bronze multifilamentary composites over the temperature range 650 to 800 C have been investigated and compared using transmission electron microscopy with energy dispersive spectroscopy. A characteristic two-fold layer structure of columnar and equiaxed grains has been confirmed in both cases at all temperatures while tantulum was shown to be incorporated into the A15 phase and to retard grain growth. A model for evolution of the microstructure is proposed and discussed.     

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.     

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.     

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.     

Influence of disordering by low-temperature neutron irradiation on the superconducting transition temperature of Nb3Sn

The superconducting transition temperatureT _c of a Nb₃Sn diffusion wire has been studied as a function of low-temperature neutron irradiation (T=10 K).T _c is observed to be essentially constant until the fast neutron dose t (for neutron energiesE>0.1 MeV) exceeds a value of 10¹⁸ n/cm²; after this valueT _c decreases linearly with t up to the maximum applied dose of 1.05×10¹⁹ n/cm². Comparison with theory and other experimental data indicates that radiation-induced disorder is the primary mechanism for the observedT _c changes.Work supported by the Bundesministerium für Forschung und Technologie and by the Deutsche Forschungsgemeinschaft.     

A study of the microscopic deformation behavior of Nb3Sn composite superconducting tape using the acoustic emission technique

Nb₃Sn-based composite superconducting tapes have been widely used because of their excellent properties such as high critical current density, low AC loss and high critical temperature. However, one of the disadvantages of Nb₃Sn-based composite superconducting tape is that the Nb₃Sn compound often exhibits multiple cracking owing to its intrinsic brittleness when subjected to mechanical loading such as bending, winding, and operation. Such cracking eventually causes severe degradation of the critical current density. Therefore, it is very important to understand the microscopic deformation behavior of Nb₃Sn-based composite superconducting tape under mechanical loading.In this study, the microscopic deformation behavior and the fracture mechanism of Nb₃Sn-based composite superconducting tape were investigated using acoustic emission (AE) technique at room temperature. The tensile behavior of Nb₃Sn-based composite superconducting tape was analyzed using the AE parameters including amplitude, duration time, and event count, which are representative of various deformation parameters such as elasticity, yielding, and cracking. The results show that the AE technique is very effective for evaluating the deformation behavior of Nb₃Sn-based composite superconducting tape.     

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.     

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.     

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.     

Nb3Al superconducting tape prepared by CO2 laser beam irradiation

A15 Nb₃AI tape superconductors have been prepared by high power and continuous wave CO₂ laser irradiation onto moving NbAl composite tapes made by the powder method. The irradiated area was melted and then immediately resolidified, which resulted in a dendritic microstructure consisting of Nb dendrites embedded in a Nb₃AI matrix. Irradiated only specimens showed critical temperatures, T_c, >16.0 K and subsequent heat treatment at 750°C enhanced T_c to 18.6 K. For laser irradiated and then heat treated specimens, critical currents, I_c, were little decreased, even at >20 T, compared with specimens which were only heat treated. A maximum critical current density, J_c, for the reacted area of 4.8 × 10⁴ A cm⁻² was obtained at 23 T and 4.2 K.     

Processes to produce superconducting Nb3Sn powders from Nb-Sn oxide

The superconducting compound Nb₃Sn was produced by the reduction of the oxides or hydroxides of niobium and tin. The procedure consists of the following three steps; (i) preparation of the mixed oxides or hydroxides, (ii) direct alloy reduction, and (iii) homogenizing heat treatment of the reduced metal powders. For mixing the two oxides, two methods were tried: the simultaneous precipitation from the aqueous solution and the solidification of the molten oxides. These mixed oxides were reduced by ca!ciunn or magnesium vapour in the temperature range 973 to 1373 K. After calcium or magnesium oxide was removed by leaching the fine compound powder had an A-15 type crystal structure, but showed a relatively low superconducting critical temperature (T _c). The isothermal annealing improvedT _c to 18.0 K.     

The microstructure of Nb3Sn in modified jelly roll superconducting composites

Transmission electron microscopy (TEM) has been used to characterize the microstructure of the Nb₃Sn layers developed during heat treatment of two superconducting wires, with and without 0.8 wt% titanium addition to the niobium, manufactured by the modified jelly roll (MJR) process. The composites in the as-received state are shown to contain pre-reacted layers formed during fabrication anneals, while heat treatments over the range 650 to 750° C yield a two-fold layer structure of columnar and equiaxed grains. Examples of both transverse and longitudinal TEM micrographs are given. The addition of 0.8 wt% titanium to the niobium before fabrication leads to coarsening of the equiaxed grains after identical reaction times. The results are discussed in terms of a recently proposed model for the development of microstructure in A15 multifilamentary composites.     

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.     

The low temperature enhancement of the critical field for a Nb3Sn superconducting magnet

The maximum quenching field for a Nb₃Sn superconducting magnet was increased from 137 kG to above 150 kG by cooling the magnet from 4.2 K to 1.9 K. A method of magnetic field calibration was employed using the Raman spin-flip signal in ZnSe:In.     

The superconducting properties and structure of Nb3Ga

The influence of composition and heat-treatment on the superconducting transition temperature, T _c, of Nb₃Ga has been studied. A complementary structural investigation of the alloys was carried out using X-ray diffraction to determine the degree of long-range order and the proportions of the phases present. It was found that the greater the deviation from stoichiometry, and the more disordered the Nb₃Ga, the lower were the T _c values. The superconducting transition temperature was maximized by producing a non equilibrium structure of essentially fully ordered, near-stoichiometric Nb₃Ga.