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.     

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.     

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.     

Finite element simulations of elasto-plastic processes in Nb3Sn strands

The manufacturing of Nb₃Sn strands, with drawing and annealing of multifilamentary strands followed by a heat treatment at about 900 K to form the Nb₃Sn by reaction of tin and niobium, has the potential to create a complex internal stress system. The strain sensitivity of the Nb₃Sn superconducting properties makes prediction of the internal stresses a necessary step to understanding the performance of Nb₃Sn conductors under the magnetic load conditions experienced in a coil. An elasto-plastic one dimensional finite element model, including temperature dependent stress-strain curves, annealing and manufacturing process stresses, is used to derive the internal stresses of Nb₃Sn strands. The model is benchmarked against a range of experimental data, including stress-strain tensile tests, superconducting critical current-strain tests, and length changes through heat treatment and through a 4 K thermal cycle. The model can predict all the experimental features and shows a number of unexpected conclusions regarding the origin of the Nb₃Sn stresses.     

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.     

Comparative Thermodynamic Analysis of Nanocrystalline and Amorphous Phase Formation in Nb/Al and Nb/Sn Systems During Mechanical Alloying

Elemental powders of niobium–tin and niobium–aluminum were mechanically alloyed by Spex ball mill, respectively, in order to fabricate disordered nanocrystalline Nb₃Sn and Nb₃Al. The solid solution phase transitions of MA powders before and after heat treatment were characterized using X-ray diffraction (XRD) analysis. The microstructural analysis was performed using scanning electron microscopy (SEM). Results showed that mechanical alloying (MA) of Nb₇₅Sn₂₅ for 28 h led to the formation of the Nb₃Sn intermetallic phase, while mechanical alloying of Nb₇₅Al₂₅ until 41 h did not show formation of the intermetallic phase. A thermodynamic analysis was performed based on the semiempirical theory of Miedema. The theory’s results showed that the intermetallic phase has a minimum Gibbs free energy compared to solid solution and amorphous states in both systems. Therefore, the most stable phase is intermetallic compound. But in case of the Nb/Al, the Nb₃Al intermetallic compound was not formed during milling and needs heat treatment.     

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.     

Criteria for Selecting Substrates for Nb<Subscript>3</Subscript>Sn Electrodeposition

We studied the effect of substrate material on the microstructure and properties of Nb₃Sn coatings produced by electrochemical coreduction of Nb and Sn ions in the cathodic zone in molten salts. The results demonstrate that continuous superconducting Nb₃Sn coatings can be produced on molybdenum, tantalum, niobium, copper, nickel, Invar and the 60% Ni + 22% Mo + 12% Fe alloy. The coatings grown on molybdenum, tantalum, and niobium have the highest purity. To ensure good superconducting properties of electrodeposited coatings, preference should be given, all other factors being the same, to substrate materials with thermal expansion coefficients close to or smaller than that of Nb₃Sn.     

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.     

Structure and properties of Nb<Subscript>3</Subscript>Sn coatings produced by unsteady-current electrocodeposition of Nb and Sn

We have studied the effect of electric-current mode on the structure and characteristics of niobium stannide coatings produced by electrochemical coreduction of niobium and tin ions at the cathode in molten salts. The results demonstrate that single-phase Nb₃Sn coatings with a superconducting transition temperature T _c = 17.3–17.9 K can be obtained using unsteady-current deposition. The coatings produced in galvanostatic mode and by ac deposition at a frequency of 50 Hz have a columnar grain structure. Current-reversal deposition with pulse ratios above 7–9 results in a layered microstructure with layers parallel to the substrate surface, instead of the columnar microstructure, and ensures a considerably higher critical current.     

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.     

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.     

Status of Nb3Sn strand development in Korea

Although there were many research activities for the development of superconducting Nb₃Sn strands, the major one started under KSTAR (Korea Superconducting Tokamak Advanced Research) project in 1996. After the success of a large scale production test of Nb₃Sn strand using the internal tin route, a new mass production facility is under operation since 2004.KAT (Kiswire Advanced Technology Ltd.), an affiliate of Kiswire Ltd., manufactured various types of Nb₃Sn strands using the internal tin process optimized for fusion magnets. For the Nb₃Sn strand of the KSTAR PF coil, each module has ～190 niobium filaments and 19 modules are restacked for the strand production. For the ITER TF strand, there are two types of basic design. One of them has 37and 19 modules with 169–219 niobium filaments in each module. The other has 19 modules with 164–190 niobium filaments in each module. Both the designs satisfy the requirements for ITER TF strand with enough margins. The characterization of the strands is performed by hysteresis loss measurement, RRR (Residual Resistivity Ratio), n-value, and critical current density measurement vs. temperature, magnetic field, and strain. The critical current density of the strands reached around 1100 A/mm² at 12 T and 4.2 K. A well defined quality assurance program helped to produce a high quality strand with a piece length of more than 15 km. KAT has been provided Nb₃Sn strand for KSTAR PF Coil and ready to produce the Nb₃Sn strand for ITER TF coil.In this paper, the design concept, the fabrication procedure and the result of the strand performance test are discussed.     

Experimental work on the niobium-tin constitution diagram and related studies

Three intermetallic phases Nb₃Sn, Nb₆Sn₅ and NbSn₂ are formed in the niobium-tin system. Nb₆Sn₅ and NbSn₂ appear to be stoichiometric with narrow homogeneity range, but Nb₃Sn can exist over a wide composition range from about 73 to 83 at. % niobium although the niobium-rich compositions are not formed on annealing below 1400° C. Nb₆Sn₅ and NbSn₂ form peritectically at 930 and 845° C respectively. The three phases appear to be stable to low temperatures, but Nb₃Sn and Nb₆Sn₅ are slow to form below about 800° C. The solubility of niobium in liquid tin is small at temperatures below 1000° C and the solid solubility of tin in niobium decreases from about 9 at. % tin at the peritectic temperature of Nb₃Sn to about 1 at. % tin at 1495° C and is negligible below 1000° C. An equilibrium diagram is constructed from the present data and from other published information.     

Microstructure and grindability of as-cast Ti–Sn alloys

In this study, the structure, microhardness, and grindability of a series of binary Ti–Sn alloys with tin contents ranging from 1 to 30 wt% were investigated. Commercially pure titanium (c.p. Ti) was used as a control. The experimental results indicated that all the Ti–Sn alloys showed hcp α structure, and the hardness values of the Ti–Sn alloys increased with greater Sn contents, ranging from 246 HV (Ti–1Sn) to 357 HV (Ti–30Sn). Among these Ti–Sn alloys, the alloy with 30 wt% Sn content showed the highest hardness value. The grindability of each metal was found to be largely dependent on the grinding conditions. The addition of Sn to c.p. Ti did contribute to improving the grindability of c.p. Ti. The Ti–Sn alloys with a higher Sn concentration could be ground more readily. The grinding rate of the Ti–20Sn alloy at 1200 m/min was about 2.8 times higher than that of c.p. Ti. Additionally, the grinding ratios of the Ti–10Sn, Ti–20Sn, and Ti–30Sn alloys at 1200 m/min were about 2.8, 2.7, and 3.4 times that of c.p. Ti, respectively. Our research suggests that the Ti–Sn alloys with Sn contents of 10 wt% and greater developed here are good candidates for machining by the CAD/CAM method.     

Growth kinetics studies of multifilamentary Nb3Sn formed by the bronze process

A systematic study of growth kinetics of Nb₃Sn formation is reported for multifilamentary conductors in the bronze process. At all reaction temperatures studied, the rate controlling step is found to be the diffusion of tin through the grain boundaries of the layer. The growth law R = ktⁿ is obeyed and the measured values of the time exponent n are in accordance with growth kinetics models. The reaction rate constant k shows an optimum behaviour for samples annealed at 725°C. The results indicate that growth kinetics plays a significant role in determining T_C, the superconducting critical temperature of the Nb₃Sn layer.     

Preparation of Ti2SnC by solid–liquid reaction synthesis and simultaneous densification method

Polycrystalline Ti₂SnC was prepared by a novel solid–liquid reaction synthesis and simultaneous densification method using elemental Ti, Sn and graphite as starting materials. The method is a one-step process and it provides the advantages of simultaneous synthesis and densification. The microstructure evolution during the high temperature processing was described and the reaction route was described. The reaction path for the formation of Ti₂SnC can be described in the following steps. Sn melted at 230 °C, which provided a favorable liquid circumstance for the reactions between Ti and Sn to form Ti–Sn intermetallic compounds. At high temperatures, TiC_x formed and reacted with Ti–Sn intermetallics at ∼1100 °C to yield ternary carbide Ti₂SnC. The material prepared by this method was dense and showed a layered anisotropic microstructure. The electrical conductivity of polycrystalline Ti₂SnC was metallic and anisotropic due to the anisotropic microstructure. The Vickers hardness was 3.7 GPa and the flexural strength was 313 MPa. Under compression, Ti₂SnC was damage tolerant at room temperature and deformed plastically at above 1000 °C. Electronic Publication     

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.     

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.