Nb3Sn Layer Research Articles

New design of inter-layer splice joint for the TF coils of the DEMO- EUROfusion tokamak

Since the year 2013, the Swiss Plasma Center (SPC) has proposed a Toroidal Field (TF) layout for the DEMO- EUROfusion tokamak, based on a graded winding pack made of layers of Nb3Sn (react-and-wind) conductors. In summer 2015, a new reference baseline is issued for the DEMO- EUROfusion tokamak, leading to an update of the TF coil requirements, e.g. the operating current has been reduced from 80 kA to 63kA. Consequently, the conductor layouts for every graded layer of the TF coil winding pack is re-designed in order to match the new requirements.Each layer of TF coil winding pack has to be connected electrically in series to form the coil. The inter-layer Nb3Sn splice joint design which does not exceed the conductor dimensions is proposed in this paper for the updated 63kA Nb3Sn TF conductor. This proposed joint design allows a continuous winding of TF coil winding pack from layer to layer, housing the joint at the zone of inter-layer transition. Ultimately, the all inter-layer joints should be arranged within the winding pack at the low-field and low mechanically stressed region of D-shaped TF coil.

Effect of Nb3Sn layer structure and morphology on critical current density of multifilamentary superconductors

The microstructure and morphology of superconducting Nb3Sn layers in multifilamentary composites differing in the fabrication route (bronze technology and internal-tin method), the shape of Nb filaments (continuous, coupled and tubular) and in the mode of Ti doping (doping of bronze matrix or Nb filaments) have been studied. Significant factors determining critical current density of these wires are the average grain sizes and the fraction of equiaxed grains in the superconducting layers. The minimal grain sizes are characteristic of the composites with tubular Nb filaments, whereas the maximal fraction of equiaxed grains is obtained in the internal-tin wires.

Microstructure observations on butt joint composed of Nb3Sn CIC conductors

To precisely evaluate a butt joint technology for the JT-60SA CS coils, microstructure observations on the butt joint composed of Nb3Sn CIC conductors were conducted using a FE-SEM. As a sample for the observations, the butt joint sample utilized in the joint resistance measurement was used. During the sample fabrication, the butt joint sample was heated up to about 920K from room temperature for diffusion bonding after heat treatment for Nb3Sn production. Then, the sample was subjected to the cycles of electromagnetic force in the joint measurement.The observation results indicated that Nb3Sn strands and a copper sheet were butted properly at the interface of the butt joint. In addition, there were hairline cracks in the Nb3Sn layers of the strands near the interface. To investigate a cause of the crack initiation, the stresses generated in the butt joint under same conditions were analyzed using a simple model. As a result, the cracks would occur with an axial compressive stress generated by the butt joint fabrication.

Multifilamentary Nb3Sn Wires Fabricated Through Internal Diffusion Process Using Brass Matrix

New multifilamentary Nb 3 Sn wires have been fabricated through an internal diffusion process using brass matrix. Brass has not only good elongation characteristics but also high yield strength more than double compared with Cu. Electron probemicroanalysis results show good interdiffusion between Sn and brass during the heat treatment. Zn remains homogeneously in the matrix after the Nb 3 Sn synthesis, expecting improvement of the mechanical strength of the wire. High mechanical strength would be preferable, particularly for react & wind magnet applications. Moreover, Zn seems effective to accelerate the Nb 3 Sn synthesis. The Nb 3 Sn layer in the brass matrix sample is more than 60% thicker than that in the reference Cu matrix sample. The thicker Nb3Sn layer accounts for the higher critical current density J c of the brass matrix sample. The B c2 of the wire is estimated to be about 25.5 T at 4.2 K. The volumetric ratio of Nb, Sn and Cu-Zn matrix has not yet optimized in terms of .Ic characteristics. The grain morphology of the Nb 3 Sn layer exhibits almost no difference between brass and Cu matrix samples. In order to improve the non-Cu .Ic characteristics, a newly designed wire has been developed.

Study and Manufacture of Nb3Sn Strands by Bronze Route

Three kinds of Nb3Sn strands with different designs were made by bronze route. Studies were carried out on bronzeto-NbTa-volume ratio, filament diameter, annealing, hot isostatic pressure process, diffusion barrier, and heat treatment. It is important for strand designs to achieve a good performance. Moreover, bronze-to-NbTa-volume ratios of 2.4 and 2.8 were applied in the strand designs. Strands with lower bronze ratio means that more Nb3Sn layer fraction formed, while, at the same time, it is provided with a high critical current density (J c ). J c increases with the reduction of filament diameter, when there is the same bronze ratio. In addition, Nb and Ta were used as diffusion barrier materials. A strand that used Nb barrier has several times hysteresis loss to a strand that used Ta barrier. When it comes to the intermediate processes, such as annealing and hot isostatic pressure, they could affect the homogeneous deformation and properties of strands. It is important to keep a low annealing temperature to realize a high J c and n-value. As for the hot isostatic pressure process, it is helpful for uniform cross section. Furthermore, different heat treatments were also carried out, while analysis was also conducted on the microstructure.microstructure.

Morphology and Structure of Diffusion Layers in Nb<sub>3</sub>Sn-Based Superconductors of Different Geometry

The evolution of structure and morphology of superconducting Nb3Sn layers forming under various regimes of the diffusion annealing in multifilamentary Nb/Cu–Sn superconductors of different design is briefly reviewed based on the authors’ (with their coauthors) original studies and the available publications. Different modes of the solid-state diffusion formation of the superconducting phase are compared. Possible mechanisms of the Nb3Sn nucleation upon the diffusion reaction of the Nb filaments and bronze matrixes are discussed. The effects of different doping elements, especially Ti, on the formation of the superconducting phase, its structure and morphology, and, consequently, on the current-carrying capacities of multifilamentary Nb3Sn-based wires are considered. Special attention is paid to the effect of the mode of Ti doping, the content of this alloying element and the regimes of the diffusion annealing on the structure, morphology and resulting performance of the bronze-processed wires with coupled Nb filaments worked out at Bochvar Institute of Inorganic Materials for the creation of International Thermonuclear Experimental Reactor.

Morphology and Structure of Nb<sub>3</sub>Sn Diffusion Layers in Superconductors with Tubular Nb Filaments

The structure and morphology of Nb3Sn layers in superconducting Nb/Cu-Sn composites with ring (tubular) Nb filaments have been studied by transmission (TEM) and scanning (SEM) electron microscopy after various regimes of diffusion annealing. It is demonstrated that the tubular geometry of Nb filaments, in which Sn diffuses from the bronze matrix both from inside and outside, ensures practically complete transformation of Nb into the superconducting Nb3Sn phase. Besides, at certain regimes of the diffusion annealing this geometry enables the improvement of the superconducting layers morphology compared to that of wires with continuous filaments, namely, to obtain wide Nb3Sn layers with fine equiaxed grains and to avoid the formation of columnar grains, which promotes enhanced current-carrying capacities of the wires.

Fabrication and Superconducting Properties of the Bronze-Processed $\mbox{Nb}_{3}\mbox{Sn} $ Multifilamentary Wire Using Cu–Sn–Zn Alloy Matrix

We developed several new Cu-Sn-Zn-(Ti) alloys, which are so-called “CSZ alloy,” for practical bronze-processed Nb 3 Sn wires. Several CSZ alloys were fabricated via the Mizuta method. The Zn was uniformly dissolved in the α phase of the Cu-Sn alloy, and the Zn-Ti compounds were not found in the CSZ alloy. Various 19 Nb-filament CSZ Nb 3 Sn precursor wires were fabricated using CSZ matrices with different Sn and Zn nominal compositions. We also found that the CSZ/Nb composites showed excellent workability at room temperature. In this paper, changes in microstructure and superconducting properties of Nb 3 Sn wires with different CSZ matrices as a function of the heat treatment condition were investigated. We clearly observed that the amount of Zn in the matrix remained constant after the Nb 3 Sn formation. Thicker Nb 3 Sn layers were formed by the Zn addition. T c transitions of about 18 K have been obtained. Finally, we have succeeded in fabricating multifilamentary wires using CSZ matrices through the restacking method. The number of Nb filaments is 7771 with a diameter of 3.4 μm. The effect of the Zn addition on microstructures and superconducting property of the CSZ wires are also reported.

Specific features of the formation of Nb3Sn superconducting layers in multifilamentary composites with ring Nb filaments

The structure and morphology of Nb3Sn layers in Nb/Cu-Sn superconducting composites with ring (tubular) niobium filaments are studied after different regimes of diffusion annealing. It is shown that the application of ring filaments provides the almost complete transformation of niobium into Nb3Sn at all studied regimes of the diffusion annealing. The Nb3Sn layers with the minimal grain size are obtained in the composites after shortened two-stage diffusion annealing at 575°C/100 h + 625°C/50 h. In certain regimes of the diffusion annealing, the use of niobium tubes in the composite instead of solid rods improves the morphology of superconducting layers that consist only of fine equiaxed grains. This allows one to achieve critical the current density of 2200 A/mm2, which is maximum possible for the composites produced by bronze technology.

Fabrication of New Nb3Sn Wires Through Brass Method

New Nb3Sn wires have been fabricated through internal tin method using gold brass (Cu-15%Zn) matrix with central Sn-Ti core and surrounding Nb rods. Gold brass has excellent mechanical properties which yield multifilamentary wires with better configuration than conventional Cu matrix does. Central Sn and Ti diffuse uniformly to the Nb cores. Nb3Sn filaments with nearly stoichiometric composition have been synthesized. Zn does not penetrate into Nb3Sn layers, and remains uniformly in the matrix. The residual Zn in the matrix may improve the mechanical performance of wires, which is a key issue in Nb3Sn wires. Present process may be named brass method like bronze process widely being used.

The Nb3Sn layers formation at diffusion annealing of Ti-doped multifilamentary Nb/Cu–Sn composites

The structure and morphology of superconducting Nb3Sn layers in multifilamentary Nb–Ti/Cu–Sn and Nb/Cu–Sn–Ti composite wires after various regimes of the diffusion annealing have been studied by transmission and scanning electron microscopy. The effect of the doping mode and Ti concentration on the thickness, structure and morphology of Nb3Sn layers is pronounced even after the first (low-temperature) stage of the diffusion annealing. After various regimes of the annealing the wires with doped bronze matrix (Nb/Cu–Sn–0.24Ti) demonstrate wider zones of fine equiaxed Nb3Sn grains with narrow grain size distributions compared to those with Ti-doped Nb filaments, which is of great importance for an attainment of the highest critical current densities Jc.

Grain boundary segregation in a bronze-route Nb3Sn superconducting wire studied by atom probe tomography

Atom probe tomography was used to characterize the A15 phase in a bronze-route Nb3Sn superconducting wire with a bronze matrix composition of Cu–8Sn–0.3Ti (in at.%). We observed depletion of niobium and segregation of Cu and Ti atoms at Nb3Sn grain boundaries. While the Nb depletion is about 15% relative to the grain interior, the average ratio between Cu and Ti excess values is 9 to 2. Segregation extends to a distance d ∼ 9 Å from the point of maximum Cu and Ti concentrations. Such local variation in the stoichiometry at the grain boundary region can be an additional source of flux-pinning in the Nb3Sn phase. Other microstructural parameters, such as the grain size and chemical composition of the Nb3Sn layer, were investigated by electron backscatter diffraction and transmission electron microscopy.

High current superconductors for DEMO

In the assumption that DEMO will be an inductively driven tokamak, the number of load cycles will be in the range of several hundred thousands. The requirements for a new generation of Nb3Sn based high current conductors for DEMO are drafted starting from the output of system code PROCESS. The key objectives include the stability of the DC performance over the lifetime of the machine and the effective use of the Nb3Sn strand properties, for cost and reliability reasons. A preliminary layout of the winding pack and conductors for the toroidal field magnets is presented. To suppress the mechanism of reversible and irreversible degradation, i.e. to preserve in the cabled conductor the high critical current density of the strand, the thermal strain must be insignificant and no space for micro-bending under transverse load must be left in the strand bundle. The “react-and-wind” method is preferred here, with a graded, layer wound magnet, containing both Nb3Sn and NbTi layers. The implications of the conductor choice on the coil design and technology are highlighted. A roadmap is sketched for the development of a full size prototype conductor sample and demonstration of the key technologies.

Microstructure evolution of Nb3Sn superconductors during diffusion treatment by bronze route

The element diffusion process of Nb3Sn superconductors by bronze route was studied using X-ray diffraction, scanning electron microscopy and energy dispersive spectroscopy. The critical current of superconductors was measured by four-point method. The results show that a diffusion layer has formed around the boundaries between the filaments and bronze matrix after 15 h heat treatment. The diffusion layer thickness keeps stable after heat treatment duration of 50∼75 h. The stable and solid Nb3Sn layer is obtained in the sample after 100 h heat treatment. Excessive heat treatment would induce superconductivity degeneration because of superconductor grain coarsening. The characteristics of the element diffusion process were discussed. The diffusion of tin atom is the governing factor in diffusion. In this study, Nb3Sn superconductors with good superconducting property were fabricated successfully at 670 °C after 100 h heat treatment.

Optimization of Bronze Processed ${\hbox {Nb}}_{3}{\hbox {Sn}}$ Strand for ITER at WST

Operated under high magnetic field (>;11 T), the toroidal field (TF) system consists of complex conductors using Nb3Sn as superconducting material. According to the strand requirements for the ITER TF conductor, bronze processed Nb3Sn strands with high superconducting performance have been manufactured using high tin content bronze at WST. The studies on filament diameter, diffusion barrier, sample heat treatment and annealing temperature have been carried out to enhance Nb3Sn strand properties. The results of these investigations have been presented. The result indicates that the different filament diameter has a weak influence on the critical current (Ic) value for the different reaction degree. In the manufacturing process the annealing temperature should be kept below 500°C to realize a high n value. The continuous Nb3Sn layer formation on the internal surface of the barrier causes a considerable increase of hysteresis loss when single Nb barrier is used. There is almost no difference of Ic, hysteresis loss and grain morphology for two heat treatments: ITER heat treatment cycle B and WST heat treatment cycle.

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.

Deposition of superconducting Nb3Sn and high-purity Nb coatings on the rotor of a cryogenic gyroscope

Superconducting Nb3Sn and high-purity niobium layers have been deposited onto a protective copper layer grown on a spherical beryllium substrate from molten salts, and their structure and superconducting properties have been studied. The results demonstrate that such coatings are suitable for use as working layers of the rotor of a cryogenic gyroscope.

Solid-State Diffusion Formation of Nanocrystalline Nb<sub>3</sub>Sn Layers at Two-Staged Annealing of Multifilamentary Nb/Cu-Sn Wires

The structure of Nb3Sn-based, bronze-processed Ti-doped multifilamentary superconducting wires has been studied by TEM and SEM after the first (5750C,100 h) and the second (6500C,100 h) stage of the diffusion annealing. The Nb3Sn layers formation in all the composites proceeds by one and the same mechanism and starts with nucleation of particles and very fine grains of this phase in Nb filaments where Sn diffuses from the bronze matrix. Ti, inserted both in the bronze matrix, or Nb filaments, diffuses into the growing superconducting layer and promotes its more active formation. At the first stage of annealing (5750C, 100 h) Nb3Sn grains have an average size of 40 nm, and at the second stage (6500C, 100 h) they increase by a factor of 1.5 and the grain size distribution gets wider. After the two-stage annealing the amount of the residual niobium is small, and some Nb filaments, especially in doped composites, almost completely transform into Nb3Sn. In the Nb3Sn layers of a zone of columnar grains is adjacent to the residual Nb.

The Study of Nb3Sn Phase Content and Structure Dependence on the Way of Ti Doping in Superconductors Produced by Bronze Route

One of the most effective ways to increase the current-carrying ability of Nb3Sn superconductor is doping of the superconducting phase by Ti, since it leads to Hc2 increase due to the rise of Nb3Sn phase electrical conductivity in normal state, and also to acceleration of the Nb3Sn layer growing during reaction and to critical current density increasing. The investigations of structure and composition of Nb3+xSn superconducting strands by SEM, TEM and EDXS microanalysis have been carried out. The structure and composition of Nb3Sn superconducting phase of the strands after reaction at 575°C 150 h+650°C 200 h have been studied. The dependence of Ti content in Nb3Sn phase from the amount of Ti introduced in Nb filaments or in bronze matrix has been also studied. It was found, that the lowest Ti content corresponds to introduction of it through the matrix doping. It has been established that Nb content in Nb3Sn essentially exceeds the stoichiometric one (3:1). The Nb3Sn lattice parameters have been defined by XRD. It was found, that the lattice parameters in the samples with Ti doped matrix were the closest to the stoichiometric Nb3Sn. The correspondence between grain morphology and composition of superconducting phase and critical current density of studied strands has been established.

Nb3Sn-based superconducting helicoids

A procedure is proposed for the fabrication of superconducting helicoids which includes consecutive electrodeposition of Nb and Nb3Sn layers from molten salts onto the surface of a plane-turn copper tape helix.