Nb Filaments Research Articles

The Structural Examination of Fe/(Cu/Nb)/MgB2 Multifilament Wires During Cold Forming Process

In this study, we have successfully produced a Fe-sheathed 6+1 multifilament wire using Cu/Nb/MgB2 monocore wires. The mono filament wire was prepared using Mg+2B powder mixture by powder-in-tube method without any intermediate heat treatment. The powder mixture of the amorphous nano boron (PVZ Nano Boron, purity of 98.5%, particle sizes < 250 nm) and high purity Mg powder (PVZ Mg, purity of 99%, particle size 74µm) were used. The multifilament wire was produced using groove rolling and cold drawing machines. The geometrical form of the filaments was examined using wire pieces taken from the wire at different steps throughout the production process. Finally, the multifilament wires produced in two different diameters of 1.02 mm and 0.82 mm were investigated in terms of filament uniformity, crack formation, surface roughness, and electrical transport properties. The structural examination was done on Nb filaments after the Fe and Cu sheaths were etched using HCl and HNO3 solution. The I – V measurements of the multifilament wires heat treated at 650 °C for 15, 30, 45, 60, and 240 minutes, and 700 °C for 60 minutes were carried out for the applied current up to 1 A at 25 K under various external magnetic field.

Comparative drawability and recrystallization evaluation of Nb4Ta and Nb4Ta1Hf alloys, and the beneficial influence of Hf on developing finer Nb3Sn grain size

Nb3Sn superconducting wires with high critical current density (Jc, 4.2 K) at fields greater than 15 T are needed for next-generation superconducting magnets for high-energy physics and nuclear magnetic resonance applications. It has been demonstrated that the addition of one atomic % Hf can significantly increase the high field Jc of Ta-doped Nb3Sn and offer the potential to meet the Future Circular Collider (FCC) specification of 1500 A/mm2 at 16 T, 4.2 K if the pure Nb filaments in conventional Nb3Sn composites can be replaced by Nb4Ta1Hf alloy. In this paper we demonstrate that a commercially produced Nb4Ta1Hf alloy is ductile and drawable in a Cu matrix to a large true strain of 15 without intermediate annealing. The hardness and work-hardening rates of Nb4Ta1Hf are shown to be higher than Nb and commercial Nb4Ta, but are comparable to ductile Nb47Ti alloy at high strains, which is significant because more than 1000 tons of Nb-Ti rods are co-drawn in Cu each year. Very importantly for the Nb3Sn properties is that just 1 at%Hf shifts the recrystallization curve of Nb4Ta to temperatures above the normal 600 °C −700 °C A15 range of reaction temperatures used for Nb3Sn wires. For the Hf-based 19 filament rod in tube (RIT) conductors of this study, we observe a refined (<100 nm) Nb3Sn grain size, and an improved upper critical field of 24.6 T (4.2 K), ∼1 T higher than that in a corresponding Nb4Ta composite. The demonstration of improved performance and good drawability with commercially produced Nb4Ta1Hf alloy indicates a pathway for Nb3Sn strand development for next-generation Nb3Sn magnets.

Structure and Properties of High-Strength Cu-7.7Nb Composite Wires under Various Steps of Strain and Annealing Modes

Microstructure and mechanical properties of in situ Cu-7.7Nb microcomposite (MC) wires manufactured by cold drawing with intermediate heat treatment (HT) have been studied. The evolution of Nb filaments morphology under various steps of deformation and modes of intermediate HT have been studied by the SEM and TEM methods. According to X-ray analysis, internal microstresses accumulate in the niobium filaments of the drawn MC, leading to a decrease in ductility. After heat treatment, the ductility of the wire increases significantly, since the microstresses in the niobium decrease even at the lowest HT temperature. The strength of the composite decreases under the HT because of negative changes in morphology and interface density of Nb filaments. The &lt;110&gt;Nb texture is stable under the HT up to 800 °C. The Nb filaments morphology and semi-coherent boundaries at Cu/Nb interfaces are restored under the post-HT cold drawing, leading to a sharp increase in the strength of the MC wire. Reducing the niobium concentration to 7.7%Nb relative to the traditional MC with 16–20%Nb and the recovery of the wire ductility under the HT makes it possible to obtain long-scale high-strength microwires with an extremely small diameter of 0.05 mm and high ultimate tensile strength of 1227 MPa.

Effects of the oxygen source configuration on the superconducting properties of internally-oxidized internal-Sn Nb3Sn wires

We successfully manufactured 12-filament rod-in-tube Nb3Sn wires with oxide nanoparticles formed by the internal oxidation method. We employed Nb-7.5 wt%Ta-1 wt%Zr and Nb-7.5 wt%Ta-2 wt% Hf alloys along with oxygen sources (OSs) in two different configurations—in the core of Nb filaments (coreOS) and at the boundary between the filaments and the Cu tube (annularOS)—to assess the influence of the OS layout on the superconducting properties and grain size. The simultaneous presence of the OS and of Hf or Zr reduced the average Nb3Sn grain size to around 50 nm, leading to an enhancement of the layer critical current density (Jc ) up to 3000 A mm−2 at 4.2 K and 16 T for the Hf-annularOS wire. Samples manufactured with an OS show a shift toward higher reduced magnetic fields of the position of the maximum in pinning-force density, this shift being more pronounced when SnO2 is added in the annularOS configuration, and for the Hf-containing samples. This enhanced pinning at higher magnetic field is beneficial for high-field magnet applications. Moreover, we measured a very high upper critical field, reaching 29.3 T at 4.2 K in the Hf-annularOS samples.

A Method for Measuring Interface Roughness from Cross-Sectional Micrographs

Uniform co-deformation of Nb, Ta, and their alloys in a Cu matrix is vital for performance yield for most internal-Sn Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn conductors, however, multiple factors can result in instabilities during extrusion and drawing. A better understanding of the variability can be attained by quantifying the interface roughness, however, there has been no consistent method developed to do this. Here we present a method to derive traditional stylus-based roughness measurements, including root-mean-square and average roughness values from micrographs of curved interfaces by image and signal processing analysis. Deformation creates interface roughness with different wavelengths. The longer wavelength or waviness and shorter wavelength or roughness are separated in this method. We apply the technique to 12 different commercial Nb filaments drawn in a Cu matrix. We find good correspondence between the roughness and initial Nb grain size of the rods. The procedure developed has been made available with the open-source code made available to the community. The procedure developed could be used to optimize the wire architecture for next-generation high-field magnets that use Nb alloys.

A novel Nb superconducting joint with ultralow resistance fabricated by electron beam welding method for superconducting gravimeter

This paper presents a novel Nb superconducting joint with an ultralow resistance of 7.9 × 10−16 Ω, fabricated using the electron beam welding (EBW) method. After the EBW process, the two Nb filaments formed a single joint with a much larger grain size and smaller grain misorientation. More importantly, the resistance of the EBW Nb joint was nearly one magnitude lower than that of most conventional pressing joint. The ultralow resistance is essential for superconducting gravimeters, which require an extremely low drift rate. The EBW Nb joint allowed the superconducting gravimeter to have a much better performance when applied in the field of structural geology, geodesy, microgravity, and metrology. We believe that the EBW method could be one of the most promising joint fabrication methods for achieving maximum stability (less than 1 μgal yr−1).

Microstructural Features in Multicore Cu-Nb Composites.

The study is devoted to heavily drawn multicore Cu–18Nb composites of cylindrical and rectangular shapes. The composites were fabricated by the melt-and-deform method, namely, 600 in situ rods of Cu–18%Nb alloy were assembled in a copper shell and cold-drawn to a diameter of 15.4 mm (e = 10.2) and then rolled into a rectangular shape the size of 3 × 5.8 mm (e = 12.5). The specimens were analyzed from the viewpoints of their microstructure, microhardness, and thermal stability. The methods of SEM, TEM, X-ray analysis, and microhardness measurements were applied. It is demonstrated that, at higher strain, the fiber texture (drawing direction), characteristic of this material, becomes sharper. The distortions of niobium lattice can be observed, namely, the interplanar distance is broadened in longitudinal direction of specimens and compacted in transverse sections. The copper matrix lattice is distorted as well, though its distortions are much less pronounced due to its recrystallization. Evolution of microstructure under annealing consists mainly in the coagulation of ribbon-like Nb filaments and in the vanishing of lattice distortions. The structural changes in Nb filaments start at 300–400 °C, then develop actively at 600 °C and cause considerable decrease of strength at 700–800 °C.

Dislocation and grain structure formation in Nb filaments of different geometry at fabrication of Nb3Sn-based superconducting wires

The structure of Nb filaments of different geometry artificially doped with NbTi alloy in Nb/Cu-Sn composites after various regimes of deformation beginning from the very first stages of fabrication of thin multifilamentary wires has been studied by transmission electron microscopy, optical microscopy and X-ray analysis. The structure of Nb filaments in composites of different designs is compared, and the development of dislocation and grain structure and texture of niobium in monolithic and assembled filaments under deformation with intermediate annealings is analyzed.

Fabrication of New Internal Tin Nb$_{3}$Sn Wire Using Sn-Zn Alloy as Sn Core

We proposed a new configuration for the precursor of Zn-added internal tin (IT) Nb $_3$ Sn wires, where Zn is added to Sn core and Ti is doped to Cu matrix. The aim is to avoid a Sn-Ti compound layer forming at the Nb filament pack, when one dopes Ti to Sn cores, while maintaining Zn effects such as promotion of Nb $_3$ Sn layer formation: Sn-Ti compound layer could be often a factor to yield inhomogeneous Sn and Ti distribution in the Nb filament modules. We here studied the microstructure and the $J_\mathrm{c}$ characteristic of IT wires fabricated using Sn-Zn/Cu-Ti/Nb composite. Use of Sn-Zn core is also effective to increase the hardness of Sn cores about 2 times higher than that of pure Sn core. In Sn-Zn/Cu-Ti/Nb composite samples, the Sn-Ti compound layer was found to be significantly suppressed, compared with Sn-Ti/Cu-Zn/Nb composite samples, which resulted in better Sn and Ti distributions. Zn diffusion rate across the Cu matrix appears to be so fast that Zn can almost diffuse to the other barrier by the heat treatment at 550 $^{\circ }$ C. Therefore, it is expected to gain the similar effect of Zn addition even for the wires that are made using the Sn-Zn/Cu-Ti/Nb composite, to the Sn-Ti/Cu-Zn/Nb composite wires.

Texture of Nb Filaments and Nb3Sn Phase in Technical Superconductors Fabricated by Bronze and Internal-Tin-Source Techniques

The texture of niobium in composite Nb3Sn superconductors prepared by internal-tin-source and bronze techniques, and in a subelement, which is required for fabricating the composite using the internal-tin-source technique, is studied in this work. It is shown that prior to the reaction heat treatment, the structural components of Nb have the (110) drawing texture; after the reaction heat treatment, no preferred orientation of Nb3Sn superconducting compound grains is observed.

Effect of Zn addition and Ti doping position on the diffusion reaction of internal tin Nb3Sn conductors

Addition of Zn to a Cu matrix during Cu–Zn/Sn interdiffusion reactions at 400 °C leads to the formation of a solid ternary Cu–Zn–Sn phase, β-CuZn, at the outermost reaction layer next to the porous ε phase. The use of a brass matrix considerably suppresses void formation and promotes homogeneous outward Sn diffusion in pre-annealing, prior to Nb3Sn formation. There are exactly three paths for Ti doping in the internal tin process, i.e. doping to Sn cores, Nb filaments, and Cu matrix. Ti doping to Sn cores causes a Ti-rich layer formation at the boundary of the Nb filament pack; however, no Ti-rich layers are formed as a result of small Ti doping to the matrix and to Nb filaments. The absence of Ti-rich layers is believed to contribute to a smooth Sn diffusion and suppression of void growth. Atom probe tomography measurements reveal that Ti doping to Sn cores leads to a more inhomogeneous Ti distribution near the grain boundary and a larger variation of the grain boundary thickness than doping to Nb filaments, which may contribute to a better Sn grain boundary diffusion. It is concluded that Ti doping to the matrix, instead of doping to Sn cores, might be more effective in maintaining better growth kinetics of Nb3Sn.

The microstructure of Nb3Sn superconductors differing in the number of copper inserts at various stages of heat treatment

Superconducting materials with a high critical current density in magnetic fields up to 16 T at a temperature of 4.2 K are required to create magnetic systems of high-energy physics devices. We have studied the change of the microstructure of model samples of various designs with an internal tin source, differing in the number of copper separators (three, six and without them) at different stages of heat treatment. The formation of the following intermediate phases was studied: Cu3Sn, Cu6Sn5 and triple Cu-Sn-Nb phase after annealing at 370°C/100 h, by the use of scanning and transmission electron microscopy. The formation of the Nb6Sn5 phase on Nb filaments located close to the tin source and copper separators was studied after heat treatment at a temperature of 665°C from 10 minutes to 25 hours. The results of the analysis led us to understand the kinetics of the Nb3Sn and Nb6Sn5 formation in internal tin superconductors with an increased current-carrying capacity and a low level of hysteresis losses.

Effect of Intermediate Heat Treatment on the Formation and Structure of Superconducting Layers in Multifilamentary Nb&lt;sub&gt;3&lt;/sub&gt;Sn- Based Wires

In the present study, the Nb3Sn-based multifilamentary wires with coupled Nb filaments have been investigated by SEM and TEM after various regimes of intermediate annealing including short high-temperature heat treatments and after two-staged diffusion annealing. The formation of some amount of pre-reacted Nb3Sn layers has been revealed in all the wires studied, and their amount depends on the wire diameter, temperature and duration of the intermediate heat treatment. The structure of final diffusion layers is also affected by the regimes of these preliminary treatments. This research enables the revealing of the optimal heat treatment schedules for the formation of most perfect nanocrystalline structure of superconducting layers ensuring the highest critical current densities.

Effect of Sn Concentration in Bronze Matrix on the Pre-Reaction Formation of Nb3Sn Layers in Bronze-Processed Superconducting Strands of Different Design

The goal of this paper is to study and analyze the effect of Sn concentration (14% or 16%), the mode of Ti-doping and the shape of Nb filaments on the thickness of the “parasitic” layers of prereacted Nb 3Sn grains (formed before the diffusion annealing) in the ITER-type strands and strands for high field applications processed by bronze technology. The use of short (of total duration not more than 1 h) intermediate heat treatments at 500 °С does not allow avoiding the prereaction formation of Nb3Sn in all the samples under study. Sn concentration in bronze decreases by 1.5–2.0% after heat treatments compared to the initial state in all samples. In the samples with Ti-doped bronze matrix and coupled Nb filaments, the average thickness of “parasitic” layers is 112 nm in the case of 14% Sn in bronze and 140 nm for a bronze matrix with 16% Sn. In the sample with 16% Sn in matrix and extended Nb filaments of intricate shape the average thickness of prereacted layers reaches 200 nm. It is concluded that to minimize the parasitic layers formation the strands with high tin bronze matrix should be subjected to short intermediate heat treatments at temperatures lower than 500 °C.

Ongoing Efforts at Internal-Tin Nb3Sn Strand With Higher Jcn and Lower Hysteresis Loss

Western Superconducting Technologies Co. Ltd. (WST) has been keeping in the research and development of internal-tin Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strand for higher performance level, for the future fusion reactor after ITER. Deformation optimization, Cu splits, and Sn spacers were applied to prepare Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strand with higher J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">cn</sub> and lower hysteresis loss, and the effect of these three factors on performance was investigated and discussed. Compared with the Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strand for ITER, strand distortion was obviously reduced by deformation optimization and J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">cn</sub> was improved, but at the same time hysteresis level was increased. To reduce the level of hysteresis loss, Cu splits were used to replace a few Nb filaments in each subelement. Cu splits could break the bridging Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn filament rings and as a sequence hysteresis loss was dramatically reduced along with the reduction of J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">cn</sub> , which is attributed to low reaction degree of Nb filaments in the outermost subelements. Therefore, Sn spacers between the outermost subelements and the Ta barrier were added, and a 37-subelement structure was used to shorten the diffusion path of Sn. The reaction degree of the filaments was enhanced as well as J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">cn</sub> . Finally, it could be concluded that a 37-subelement structure with Cu splits is effective to reduce the level of hysteresis loss and raise the level of J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">cn</sub> .

Enhanced Critical Current Densities in Nb3Sn Superconducting Strands Prepared by Bronze Process

In the development of Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn superconducting strands, the improvement of the non-Cu critical current density (J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> ) has been greatly emphasized. Different Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn strands were manufactured by the bronze route artificially doped with titanium in bronze. The influences of bronze on Nb volume ratio, filament material, filament diameter, and heat treatment were studied. In this paper, bronze-to-Nb volume ratio affected J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> slightly due to the formation of the Sn shortage and similar Nb <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn volume after heat treatment. The study of filament diameter indicates that J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> increases largely as the filament diameter reduces for the full reaction. Different heat treatment times at 575 °C, 650 °C, and 675 °C will result in various performances. According to microstructure images, residual Nb core can be seen in each filament. Bigger average grain size will lead to lower J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> .

The Role of Interfaces in Evolution of Structure and Thermal Stability of Cu-Nb Composite Processed by High-Pressure Torsion

The structure and thermal stability of Cu-18Nb multicore composite fabricated by repeated cold-drawing of in situ melted mixture of Cu and Nb and subjected to high-pressure torsion (HPT) have been studied by SEM, TEM, X-ray analysis and microhardness measurements. In the cold-drawn state ribbon-like Nb filaments the thickness of 30-70 nm are located in Cu-matrix with sharp texture &lt;110&gt;Nb║&lt;111&gt;Cu║drawing axis. The Nb lattice is distorted, the interplanar spacing (110)Nb being extended along the drawing axis and compressed perpendicular to it, which testifies a semi-coherent character of Cu/Nb interfaces. At annealing these distorsions gradually vanish, and coagulation of Nb ribbons starts at 400С, actively develops at 600С and finishes at 800С with the formation of sausage-like filaments with round transverse sections, which is accompanied with about two-fold decreasing of microhardness. Under the HPT the composite structure is considerably refined, and almost equiaxed grains the sizes of 20-30 nm are formed, which gives rise to a dramatic increase of microhardness. The thermal stability of Cu-Nb composite after cold drawing and HPT is appreciably higher than that of pure Nb and Cu nanostructured by severe plastic deformation.

Structure of superconducting layers in bronze-processed and internal-tin Nb3Sn-based wires of various designs

The microstructure of Nb3Sn layers in multifilamentary composites manufactured by bronze technology and the internal-tin (IT) method differing in the strand design has been studied. The IT strands of International Thermonuclear Experimental Reactor (ITER) type and high-Jc type are characterized by both the higher Sn concentration and the higher fraction of equiaxed grains than bronze-route composites. In all the samples under study, the fraction of equiaxed grains correlates with the average concentration of Sn in Nb3Sn layers. The Ti doping of the bronze matrix in the bronze-processed wires results in the reduction in average grain sizes of Nb3Sn grains and in a higher fraction of equiaxed grains with a higher Sn concentration, which leads to a higher non-Cu Jc (calculated as a ratio of critical current Ic to a cross-sectional area of a strand without stabilizing Cu), 997 A/mm2 at 12 T, 4.2 K, compared to a composite of the same design with artificially Ti-doped Nb filaments. The ITER type IT strand demonstrates a somewhat higher fraction of equiaxed Nb3Sn grains with much larger average grain sizes (120 nm). The grain coarsening in this IT strand results in a decrease in Jc compared to other ITER type strands studied. The highest values of Jc are ensured in the high-Jc type of IT wires with 7 extended tin sources by the highest fraction (92%) of equiaxed Nb3Sn grains despite their coarser sizes (92 nm).

Microstructure and Mechanical Properties of Heavily Worked and Recrystallized Copper for Low-Temperature Superconductor Applications

Oxygen-free high-conductivity (OFHC) copper is generally used as a matrix for mechanical and electrical failure support of Nb3Sn superconducting filaments. This matrix provides mechanical stability to the filaments and a path for large currents in the event of an unwanted large temperature rise. Equal channel angular extrusion (ECAE) is an excellent way to increase the strength of bulk copper without an overwhelming degradation of resistivity caused by alloying. A stronger Cu may be beneficial for fabrication of Nb3Sn multifilamentary wires because of its ability to arrest nonuniform deformation of the precursor Nb filaments as the wire is drawn down. For the work reported, 25.4 × 25.4 × 254 mm 3 bars of OFHC Cu were processed by ECAE to plastic strains of 16 following four different ECAE routes, sectioned by wire EDM, and characterized for strength, resistivity, and microstructure. Results indicate routes E and Bc may be the best for strengthening. Postprocessing rolling was found to further increase strength but also result in the increase in resistivity. Additionally, recrystallization was studied for selected routes and it was found to have an inverse relationship with the amount of cold work. The resistivity ratio (R298 K/R77 K) has a visible drop for the most severe deformation cases.

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.