Grain size and its relation to tensile strength of Nb3Sn compound in bronze-processed multi-filamentary superconducting materials

The grain size and room-temperature tensile strength of Nb₃Sn compound were studied over wide ranges of annealing temperature and time using thin and thick multi-filamentary superconducting composite materials. The strength of Nb₃Sn compound in thick specimens was lower than that in thin specimens at any heat treatment. This result was accounted for by the existence of extra-coarse grains in thick specimens. It was found that the strength of Nb₃Sn compound is nearly proportional to the inverse square root of the grain size.     

On pinning of superconducting flux lines by grain boundaries

A simple model is developed which describes the reduction of the mean free path of conduction electrons in metals near a grain boundary. This leads to a decrease of the self-energy of flux lines in a layer which is considerably thicker than the perturbed zone of the boundary itself. The model yields pinning forces which agree, within an order of magnitude, with recent measurements on niobium bicrystals, and with observed values of grain boundary pinning in Nb₃Sn.Work supported by FAPESP, proc. no. 78/1230, and by the Niobium Project of FTI, Lorena SP, Brazil.On sabbatical leave from Instituto de Física da Universidade Estadual de Campinas (UNICAMP).     

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.     

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.     

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.     

Embedding of nanoparticles as flux pinning sites in superconducting samples

Crystallographic orientations of thin films and bulk, melt-textured YBa₂Cu₃O_x high-T_c superconductors with embedded Y₂Ba₄CuMO_x (M = Nb, Zr, Ag) nanoparticles are studied by electron backscatter diffraction. The Y₂BaCuO₅ particles exhibit no preferred orientation but have a strong negative influence on the matrix orientation. In contrast, the nanoparticles are not disturbing the texture of the YBCO matrix. For the bulk samples, depending on the preparation route, a different particle orientation with respect to the matrix is obtained. Untextured M-2411 nanoparticles are formed by solid state reaction during the melt process by adding oxides (Nb₂O₅ or Y₂O₃) to the precursor powder. Pre-formed Y₂Ba₄CuMO_x particles added to the precursor in the form of pre-reacted nanopowder, exhibit a dominant single orientation related to the surrounding YBCO matrix. In the case of thin films prepared using laser ablation with a bulk YBCO sample with embedded nanoparticles as a target, the nanoparticles are transferred into the thin film, but the transferred M-2411 particles are found to be randomly oriented. Nevertheless, these films exhibit increased flux pinning properties.     

The morphology and grain size of Nb3Sn filaments in in situ prepared multifilamentary Nb3Sn-Cu composite wire

Multifilamentary Nb₃Sn-Cu wire was prepared from chill-cast Cu-20 wt% Nb ingots by drawing 12.5 mm ingots to 0.15 mm diameter wire, plating with 9 vol% Sn and reacting at temperatures ranging from 450 to 725° C. The morphology and grain size of the as-cast Nb filaments and the Nb₃Sn filaments formed on heat treatment were characterized by TEM and SEM studies, and the Sn diffusion process was evaluated using electron microprobe analysis. The as-drawn Nb filaments had a cross-section typically 50 Å by 2500 Å and were generally convoluted in the wide direction. Tin diffusion produced Nb₃Sn filaments with an essentially equi-axed cross-section having diameters of around 800 Å for the 550° reaction temperature and grains extending across the diameter. It was shown that the change in cross-sectional shape is produced largely by coarsening of the thin Nb filaments prior to diffusion of Sn to form the Nb₃Sn. Optimum J _c values at both 1 and 8 T were found to occur for diffusion temperatures of 550° C and times of 3 to 6 days where the grain size was 750 to 800 Å.     

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.     

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.     

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.     

Strain scaling law for flux pinning in practical superconductors. Part 1: Basic relationship and application to Nb3Sn conductors

Critical current and flux pinning densities have been determined for a series of Nb₃Sn, V₃Ga, Nb₃Ge, and NbTi conductors as a function of uniaxial tensile strain in magnetic fields ranging from 4 to 19 T. An empirical relationship has been found at 4.2 K that describes these data over the entire range of field under both compressive and tensile strain. The pinning force F has been found to obey a scaling law of the form $\text{F = [B}{}_{\mathrm{c2}}\text{1(?)]}{}^{\mathrm{n}}\text{f(b)}$, where B_c2¹ is the strain-dependent upper-critical field determined from high-field critical-current measurements and f(b) is a function only of the reduced magnetic field $\text{b  B/B}{}_{\mathrm{c2}}\text{1}$. The detailed shape of f(b) depends on the super-conducting material and reaction conditions, but n was found to be nearly constant for a given type of superconductor. For Nb₃Sn conductors n = 1 ± 0.3, for multifilamentary V₃Gan?1.3, for CVD-Nb₃Ge tape n?1.6, and for multifilamentary NbTi n?3.3. The importance of this relationship is that, for these conductors at least, it is possible to measure F at one strain and then immediately be able to predict F (and thus the critical current) at other strain levels simply by scaling the results by [B_c21(?)]ⁿ. Part I of this paper presents the basic uniaxial-strain scaling relationship and focuses on its application to Nb₃Sn conductors. The strain scaling law with n = 1 ± 0.3 was found to hold for all Nb-Sn based conductors examined thus far, including commercial-multifilamentary conductors, extremely fine-filament composites, partially-reacted specimens, ‘insitu’ conductors, and Nb-Hf/Cu-Sn-Ga conductors. The detailed dependence of B_c21 on strain was-found to be nearly universal for highly-reacted commercial Nb₃Sn specimens, greatly simplifying the application of the scaling law to this group of practical superconductors. These results are discussed within the context of flux pinning models and a general scaling relation is proposed which unifies the usual temperature-scaling relation with this strain-scaling relation.     

The superconducting transition temperature and its high-pressure behavior of tetragonal Nb3Sn

The superconducting transition temperatureT _C of the tetragonal phase of a Nb₃Sn single crystal was determined calorimetrically to be 17.78±0.02 K. It is suppressed by the application of hydrostatic pressure at a rate of ?(1.40±0.05)×10^?5 K bar^?1. The previous observations of double superconducting transitions in Nb₃Sn and other A15 compounds are discussed.     

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.     

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.     

Giant magnetostriction and flux jumps in superconducting Nb3Al polycrystalline slab

Transverse and longitudinal magnetostriction of a large polycrystalline slab of Nb₃Al superconductor was investigated by the strain gauge technique. At 4.2 K and in an external magnetic field of several Tesla the vertical width of transverse magnetostriction hysteresis loop was found to be of an order of 10⁻⁴. In a wide range of temperatures and external magnetic fields we observed the phenomenon of giant jumps of both transverse and longitudinal magnetostriction caused by thermomagnetic avalanches. The shapes of the magnetostriction hysteresis loops are also influenced by the phenomenon of the second maximum of the critical current density, called as a peak or fishtail effect. Experimental results can be understood in the framework of the model of the magnetostriction induced by pinning forces.     

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.