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.     

Microstructural factors important for the development of high critical current density Nb3Sn strand

Nb₃Sn is the primary candidate for the next generation of accelerator magnets as well as for NMR and other applications that require magnetic fields between 11 and 20 T. Since 1999 the layer critical current density available in long length accelerator quality strand has almost doubled. The microstructural and microchemical factors that are important for high critical current density Nb₃Sn are reviewed. The highest critical current density strands have a Nb₃Sn layer that minimizes chemical and microstructural inhomogeneities and has a high fraction of the layer close to stoichiometric Sn content. Only the internal Sn process has yielded critical current densities beyond 3000 A/mm² at 12 T (4.2 K) and only with interfilamentary Cu thicknesses that are too low to separate the filaments after the final reaction heat treatment. The result of the reaction heat treatment is to produce a continuous ring of Nb₃Sn from hundreds of Nb or Nb-alloy filaments and thus a major ongoing challenge of Nb₃Sn conductor design is to reduce the effective filament diameter to acceptable levels for intended applications. Recent successful attempts to reduce the cost of alloying the Nb₃Sn for high field application are also examined and the potential for future improvements discussed.     

Critical current density and stability of Tube Type Nb3Sn conductors

The critical current densities (J_c) and stabilities of Tube Type Nb₃Sn conductors have been measured. The strands had superconducting subelement counts ranging from 192 to 744, and flat-to-flat filament sizes (for 0.7 mm OD wire) of from 35 μm down to 15 μm. These Tube Type conductors had a very simple structure: prior to heat treatment the filaments consist of a Sn core surrounded by a thin Cu tube, itself surrounded by a Nb or Nb alloy tube. Eight different strand types were investigated using various techniques including SEM, residual resistance ratio (RRR), transport J_c, and stability measurement. Most strands were studied at 0.7 mm OD, with one representative at 0.42 mm. The transport measurements were made at 4.2 K in fields up to 14 T. Numerous heat treatment schedules were investigated, with reaction temperatures ranging from 615 °C to 650 °C, and times ranging from 36–500 h. The highest J_cs were seen for the lowest reaction temperatures, with 12 T transport J_c values as high as 2450 A/mm² observed. The RRRs were lower for longer time and higher temperature reactions and ranged from 4 to 180. Strand stability was a strong function of the effective filament diameter, d_eff, and RRR. The most stable strands showed stability currents, J_s, of 8700 A/mm² and 15,300 A/mm² for 0.7 mm OD and 0.42 mm OD conductors, respectively.     

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 920 K 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.     

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.     

Overview of Nb3Sn and V3Ga conductor development in Japan

In the early stage of high-field A15 conductor development in Japan, different type of V₃Ga conductors were fabricated. Then, Ti-doped Nb₃Sn conductors have been developed, and widely used for high-field generation. Increase of Sn concentration in the bronze produces an appreciable progress in the performance of bronze processed (Nb,Ti)₃Sn conductors. New internal Sn processed (Nb,Ti)₃Sn conductors with modified cross-sectional configurations have been produced, which exhibit large J_c in high fields as well as reduced AC loss. Both bronze processed and internal Sn processed (Nb,Ti)₃Sn conductors satisfy recent ITER magnet specifications. As for new type Nb₃Sn conductors, powder core and Jelly Roll processed (Nb,Ta)₃Sn wires with improved high-field performance have been fabricated.     

Status and perspective of the Nb3Al development

Nb₃Al has advantages of better tolerance to strain/stress and a higher critical magnetic field (30 T at 4.2 K) for stoichiometric composition over Nb₃Sn. The rapid-heating, quenching and transformation annealing (RHQT) process enables to form a stoichiometric Nb₃Al with fine grain structures via metastable bcc supersaturated-solid-solution. As a result a large critical current density of Nb₃Al is achieved over the whole range of magnetic fields without trading off the excellent strain tolerance. A long-length of RHQ processing has been established, and a rectangular but Cu stabilized Nb₃Al strand is about be commercially available for NMR uses. Ag or Cu internal stabilization and Cu ion-plating/electroplating techniques have been also developed to enable the stabilized round wire for accelerator and fusion magnets. Successfully energized test coils that were manufactured with a wind-and-react technique have demonstrated that a long piece of Cu stabilized RHQT Nb₃Al wire is really available for practical applications.     

Properties of MgB2 superconductors with regard to space applications

Thin MgB₂ conductors with diameters down to 50 μm have been developed for application as current leads on a satellite and as LH₂ level sensor. The high transition temperature of 39 K makes applications of MgB₂ at temperatures around 20 K possible, significantly above the temperature margin of low temperature superconductors like NbTi or Nb₃Sn. The absence of weak link behaviour and the fact that there is no need to texture MgB₂ to obtain high critical current densities is a crucial advantage compared to high temperature superconductors. We give an overview of the current status of MgB₂ conductor development and show the potential of these wires for space applications.     

Experimental study on a Nb3Al insert coil under high magnetic field

Nb₃Al is one of the most promising superconductors to replace Nb₃Sn in large scale, high field superconducting magnet. Since the complicated conductor manufacturing process, long and stable Nb₃Al conductor is difficult to acquire in a commercial scale. Based on a 70 m length of Nb–Al precursor conductor, we designed and fabricated a Nb₃Al coil. The coil winding, low temperature diffusion heat treatment and epoxy impregnation are described in detail. The finished Nb₃Al coil is tested as an insert in a background magnet. The test is performed at the background field from 7 T to 15 T. The test results are analyzed and presented in this paper.     

XPS and Mössbauer studies on BaSn1−xNbxO3 (x ≤ 0.100)

A few compositions of perovskite oxide BaSn_1?xNb_xO₃ (with x ≤ 0.10) system, prepared by solid state ceramic method, have been studied employing XPS and Mössbauer spectroscopy techniques. Mössbauer spectra of these compositions in the temperature range of 78–300 K reveal that the oxidation state of Sn is +4. In XPS measurements, compositions with x ≤ 0.010 show no evidence of Nb⁵⁺ signal whereas the compositions with x ≥ 0.050 show clear evidence of Nb⁵⁺ signals indicating some unreacted Nb₂O₅ component in the system. This confirms the earlier report where presence of small amount of unreacted Nb₂O₅ was predicted.     

Manufacturing of lightweight low-current Nb3Sn ADR magnets operating at 10 K

Adiabatic demagnetization refrigerators (ADRs) using superconducting magnets are under development for cooling many NASA instruments. Due to higher efficiency of cryocoolers at 10 K, Nb₃Sn magnets operating at 10 K are favored for space ADRs. Further, magnets need to be as light as possible and have low operating currents. This paper discusses technologies for the manufacture of lightweight, low-current Nb₃Sn magnets and reports on testing of a 35 mm bore by 60 mm long magnet. This magnet weighed less than 1 kg and successfully produced 3 T at 11.5 K with an operating current of 8 A.     

Development of a lightweight low-current 10 K 4 T magnet for space-flight ADRs

Future space missions will include detectors and other components cooled to cryogenic temperatures by adiabatic demagnetization refrigerators (ADRs) coupled with mechanical cryocoolers. In such systems the ADRs require lightweight, low-current superconducting magnets. At least one of an ADR’s magnets must operate at the cryocooler’s coldest stage temperature. This temperature should be as high as possible in order to improve operating efficiency and design flexibility. We previously reported the development of a lightweight (1 kg) low-current (8 A) Nb₃Sn magnet which produces a 3 T central field at 10 K. We now report our progress in developing a new 10 K magnet of similar size made with smaller diameter Nb₃Sn wire which will produce a 4 T central field with approximately 5 A.     

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.     

Shear strength and wettability of active Sn3.5Ag4Ti(Ce,Ga) solder on Al2O3 ceramics

The work deals with the study of wettability of Sn3.5Ag4Ti(Ce,Ga) solder on ceramic material of Al₂O₃. The Sn3.5Ag4Ti(Ce,Ga) solder is used for ultrasonic soldering of metallic and ceramic materials. The microstructure of Sn3.5Ag4Ti(Ce,Ga) solder consists of a tin matrix, where non-uniformly distributed constituents of partially dissolved Ti and uniformly distributed fine needles of Ag–Sn phase were observed. The solder was of heterogeneous composition. X-ray diffraction analysis has revealed the presence of following phases: Ag₃Sn, Ti₆Sn₅, Ti₃Sn. For determination of melting point, the Differential scanning calorimetry analysis (DSC) was performed. Wettability of Sn3.5Ag4Ti(Ce,Ga) solder was determined at temperatures 800, 850 and 900 °C in dependence on wetting time. The best wettability of solder Θ = 46° was achieved at 850 °C/43 min. The experiments with high-temperature activation were performed in vacuum of 10^?4 Pa. On the basis of experience attained by measurement of contact angle, the soldered joints of Al₂O₃/Al₂O₃ and Al₂O₃/metal were fabricated in conditions of high-temperature activation in vacuum at temperature 850 °C/10 min. For comparison, also the joints fabricated in the conditions of ultrasonic activation in the air at temperature 280 °C/1 min were applied. The shear strength of joints of Al₂O₃/Al₂O₃ and Al₂O₃/metal fabricated with Sn3.5Ag4Ti(Ce,Ga) solder varied from 17 to 35 MPa. The shear strength of joints fabricated in vacuum is slightly higher than in the case of joints fabricated by use of power ultrasound.     

Magnetic order in tin-doped thiospinels

The tin-doped sulphur-containing compound of composition Fe_1.05Cr_1.90Sn_0.05S₄, in which tin is located on the octahedral sites as Sn(IV) in the spinel-related structure, has been examined by ¹¹⁹Sn Mössbauer spectroscopy. The data complement results obtained by ⁵⁷Fe Mössbauer spectroscopy and show that tin doping increases the magnetic ordering temperature of pure FeCr₂S₄. These results are compared with the data obtained on tin-doped CuCr₂X₄ compounds (X = O, S, Se).     

High temperature strength,fracture toughness and oxidation resistance of Nb–Si–Al–Ti multiphase alloys

Nb–Si–Al–Ti quaternary phase diagram around three-phase region, which consists of niobium solid solution (Nb_ss), Nb₃Al and Nb₅Si₃, is constructed in this study. The three-phase region exists up to titanium content of about 20 mol%. Based on the quaternary phase diagram, three-phase alloys containing Nb_ss from about 50 to 75% in volume are prepared to improve high temperature strength, room temperature fracture toughness and oxidation resistance simultaneously.When microstructure and composition are optimized (Nb_ss+Nb₃Al+Nb₅Si₃) three-phase alloy with the addition of titanium exhibits higher compressive strength than nickel-based superalloys at room temperature to 1573 K. Fracture toughness at room temperature of (Nb_ss+Nb₃Al+Nb₅Si₃) three-phase alloys is increased to over 12 MPa m^1/2 by the addition of titanium without sacrificing high temperature strength. Oxidation resistance of (Nb_ss+Nb₃Al+Nb₅Si₃) three-phase alloys is improved by the addition of titanium.     

Study of annealing time on sol–gel indium tin oxide films on glass

Indium and tin salt-based precursors maintaining In:Sn atomic ratio as 90:10 were utilized for the development of sol–gel dip coated indium tin oxide films (ITO) on SiO₂ coated (∼ 200 nm thickness) soda lime silica glass substrate. The gel films were initially cured in air at ∼ 450 °C to obtain oxide films of physical thickness ∼ 250 nm. These were then annealed in 95% Ar–5% H₂ atmosphere at ∼ 500 °C. The annealing time was varied from 0.5 h to 5 h. Variation of annealing time did not show any considerable change of transmittance in the visible region. Thermal emissivity (ε_d, 0.67–0.79) of the films were evaluated from their hemispherical spectral reflectance. These passed through a minima with increasing annealing time as the reflectivity of the films in the mid-IR passed through a maxima. The microstructure of the films revealed systematic growth of the ITO grains. XRD and XPS studies revealed the presence of both In and Sn metals in addition to the metal oxides. The energy dispersive X-ray (EDX) analysis showed little lowering of tin content in the films with increasing annealing time.     

Thermo-hydraulic analyses associated with a CEA design proposal for a DEMO TF conductor

The future DEMO Toroidal Field (TF) magnets are likely to feature cable-in-conduit conductors (CICC) cooled by forced flow of supercritical helium. Design activities were carried out at CEA to provide a winding pack compatible with DEMO plant requirements. The CEA proposal comprises, for each of the 16 D-shaped windings, 10 double-pancakes (2 × 392 m long) wound in 10 turns. The conductor is a square-shaped Nb₃Sn double channel conductor with a central spiral, carrying a nominal current of 95.5 kA. We present a thermo-hydraulic analyses focused on the central, most critical pancake, where the maximum field is reached, aiming at evaluating the integrity of the proposed conductor design. Both normal and off-normal simulations were performed using detailed electromagnetic and neutron heating load maps as input, and evaluating operational quantities such as the temperature margin in burn conditions, and the hot spot temperature in quench conditions. We assessed the sensitivity of these quantities to some driving parameters, notably mass flow rate and the choice of friction factor correlation for the temperature margin, and quench initiation features for the hot spot temperature. Furthermore, the influence of the casing cooling on the temperature margin is analyzed. The study is carried out using two thermohydraulic models.     

A lightweight low-current 10 K magnet for space-flight ADRs

Future space missions will include detectors and other components cooled to cryogenic temperatures by adiabatic demagnetization refrigerators (ADRs) coupled with mechanical cryocoolers. In such systems the ADRs require lightweight, low-current superconducting magnets. At least one of an ADR’s magnets must operate at the cryocooler’s coldest stage temperature. This temperature should be as high as possible in order to improve operating efficiency and design flexibility. Until now all space-flight compatible magnets have been made with NbTi wire, which has limited their operating temperatures to below about 5 K. We have developed a lightweight (1 Kg) low-current (8 A) Nb₃Sn magnet which produces a 3 T central field at 10 K. We explain the choice of this magnet’s specifications and describe its performance testing.     

New dielectric materials of xSrTiO3–(1 − x)Ca(Mg1/3Nb2/3)O3 ceramic system at microwave frequency

The crystal structures and the microwave dielectric properties of the xSrTiO₃–(1 − x)Ca(Mg_1/3Nb_2/3)O₃ perovskite ceramic system have been investigated. In order to achieve a temperature-stable material, we studied a method of combining a positive temperature coefficient material with a negative one. SrTiO₃ has dielectric properties of dielectric constant ε_r ∼ 205, Q × f value ∼ 4200 GHz and a large positive τ_f value ∼ 1700 ppm/°C. Ca(Mg_1/3Nb_2/3)O₃ possesses high dielectric constant (ε_r ∼ 28), high quality factor (Q × f value ∼ 58,000 at 7 GHz) and negative τ_f value (− 48 ppm/°C). As the x value varies from 0.2 to 0.8, the xSrTiO₃–(1 − x)Ca(Mg_1/3Nb_2/3)O₃ system has the dielectric properties as follows: 40 < ε_r < 123, 4600 < Q × f < 33,400 and − 23 < τ_f < 600. A new microwave dielectric material, 0.3SrTiO₃–0.7Ca(Mg_1/3Nb_2/3)O₃, applicable in microwave devices is suggested and possesses the dielectric properties of a dielectric constant ε_r ∼ 46, a Q × f value ∼ 29,300 GHz (at 6.8 GHz) and a τ_f value ∼− 2 ppm/°C. A near-zero τ_f value can be achieved by adjusting the x value of xSrTiO₃–(1 − x)Ca(Mg_1/3Nb_2/3)O₃ ceramics.