Development of a Nb3Sn multifilamentary composite conductor for a.c. use

A Nb₃Sn composite conductor with ≈ 10 000 submicron diameter filaments has been manufactured using the external diffusion process. A.c. losses were greatly reduced by the use of a fine filament size (0.53 μm, design value), a tight twist pitch (0.87 mm) and a small wire diameter (0.153 mm) with a bronze matrix. In an a.c. field with a frequency of 50 Hz and amplitude of 2.0 T, the hysteresis loss and the coupling current loss were observed to be 465 kW m⁻³ and 26 kW m⁻³, respectively. A triplex conductor was constructed by cabling three strands at a twisting pitch of 3 mm, and a small coil was wound from this cable (i.d. 11 mm, o.d. 33 mm, axial length 19 mm). With d.c. the coil generated a field of 1.3 T at the critical current, l_c of 37.4 A. When the coil was operated at 50 Hz, with an exciting current of I_c, the observed loss averaged over the windings was 240 kW m⁻³. The quenching current for 50 Hz operation was 53 A at a maximum field of 1.8 T. This was considerably higher than the critical values under d.c. conditions. Preliminary studies have shown that, if this conductor is used in superconducting armature windings of rotating machines, economical benefits are obtained compared with the use of conventional armatures.     

Continuous Wrapping Tantalum Barriers for Internal-Tin Nb3Sn Multi-Filamentary Wires

A process continuous wrapping tantalum barrier has been developing and investigated in this paper. By eliminating the need for inserting expensive tantalum tube, barrier is applied to unlimited piece length prior restack. A tantalum barrier with 20% overlap was wrapped onto subelements. Then 18-filament Nb₃Sn plus 1 copper core restack billet was successfully drawn down to wire with 0.84-mm diameter. The longitudinal and cross-sectional images revealed most of barriers were continuous and intact but some disrupted. So, we are still working on the optimization of manufacturing process.     

Dendritic flux avalanches in superconducting Nb3Sn films

The penetration of magnetic flux into a thin superconducting film of Nb₃Sn with critical temperature 17.8 K and critical current density 6 MA/cm² was visualized using magneto-optical imaging. Below 8 K an avalanche-like flux penetration in form of big and branching dendritic structures was observed in response to increasing perpendicular applied field. When a growing dendritic branch meets a linear defect in the film, several scenarios were observed: the branch can turn and propagate along the defect, continue propagation right through it, or “tunnel” along a flux-filled defect and continue growth from its other end. The avalanches manifest themselves in numerous small and random jumps found in the magnetization curve.     

Critical current of react-and-jacket processed Nb3Sn conductor

A new type of large-scale Nb₃Sn conductor was developed that has an aluminum-alloy jacket to support an electromagnetic force. The manufacturing process of the conductor has a unique feature, in which the jacketing process is performed after a reaction heat treatment of the Nb₃Sn cable. This enables the conductor to have a high critical current, because the thermal strain of the Nb₃Sn filaments is decreased. Critical current measurements using a short conductor sample confirmed the expected high performance.     

Magneto-structural analysis of the fermilab TQC Nb3Sn high gradient quadrupole end region

The Fermilab TQC magnets are Nb₃Sn technological quadrupoles based on the collar-yoke-skin mechanical structure. These magnets, with an aperture of 90 mm, have a design gradient in excess of 200 T/m. In operation the conductor is subjected to forces which tend to pull it away from the poles and endparts to which it is bonded. Given the implications of bond failure for quench initiation, it is of interest to simulate the behavior of these interfaces. The ANSYS general purpose finite element program is used to perform both the magnetic and structural analyses. Interface elements between bonded parts are monitored during assembly, cool down, and excitation, and the birth–death capability of the program is applied to remove from the solution those portions of the interface which experience a tensile stress in excess of a presumed bond failure stress. The cracking of previously bonded interfaces can be tracked graphically over the range of operation. Emphasis will be placed on the details of the magnetic simulation, the implementation of various interface conditions, and the effects (and shortcomings) of material property models.     

Continuous Wrapping to Prepare Intact Tantalum Barriers for Internal Tin Nb3Sn Multifilamentary Strand

The capability of the cost-effective process of continuous wrapping to prepare tantalum barrier for Nb₃Sn multifilamentary wire is investigated in this paper. By eliminating the length limit of inserting a subelement into the expensive tantalum tube, tantalum sheet is wrapped with 20% overlap onto subelement. Then an assembly of 19 restacks was drawn down to the final diameter with 0.8 mm. Two batches of such 19 filamentary assemblies were packed into a copper tube with round and hexangular restacks, respectively. The result has manifested that this continuous wrapping technique can be used to get an intact and continuous tantalum barrier.     

Is Nb3Si a 25 K superconductor?

Nb₃Si is predicted to have the highest superconducting critical temperature of all known A15 compounds, but it can only be prepared by non-equilibrium methods and resulting T_c values are disappointing. Experimental results on Nb₃Si prepared by a variety of methods are reviewed. By analogy with Nb₃Ge it is argued that T_c for stoichiometric fully ordered A15 Nb₃Si should be 25 K.     

Analysis of magnetization, AC loss, and deff for various internal-Sn based Nb3Sn multifilamentary strands with and without subelement splitting

Magnetization, AC loss, χ_DC and d_eff were measured for several designs of rod-in-tube based internal-Sn Nb₃Sn type superconducting multifilamentary strands. Two kinds of subelement geometries were used in strand construction. The first had the standard annular Nb/Cu ring surrounding a Sn source; the second was similar but included an internal split intended to reduce magnetization and loss. Strands with 18 and 36 subelements were measured, at strand diameters of 0.5-0.8 mm. Optical, SEM, and EDS measurements were performed on these samples; average radii are reported and physical barrier integrity is found to be good. The magnetizations of these structures were analyzed in terms of a d_eff parameter, in this case calculated for annular structures. Analytical and numerical results of these calculations are presented. It was found that in general annular structures should be expected to have d_eff values somewhat larger than the subelement diameter; the value of this enhancement is reported. Also, the effect of subelement splitting on d_eff and magnetization was calculated. The results of these calculations are compared to the experimentally measured results. Reductions in d_eff due to subelement splitting are compared to direct, low-field susceptibility measurements. Magnetization values are seen to be nearly uniformly lower in the split subelement strands, and this leads in some but not all cases to significantly lower d_eff values. Possible reasons for these discrepancies are discussed.     

Field and temperature dependencies of critical current on industrial Nb3Sn strands

The increasing need for high field magnetic devices has focused attention on filamentary Nb₃Sn conductors, whose critical data are superior to NbTi conductors. To choose the suitable operating parameters and to determine the stability margin of magnet systems, it is very important to know the effect of temperature and magnetic field on the superconducting properties, especially on the critical current. Up to now, for design calculation, the so-called “Summers model” was assessed theoretically on experimental data obtained by Spencer et al., (The temperature and magnetic field dependence of superconducting critical current densities of multiinflammatory Nb₃Sn and NbTi composite wires. IEEE Trans Mag, Mag-15 (1979) 76) and Suenaga et al., Superconducting critical-current densities of commercial multifilamentary Nb₃Sn(Ti) wires made by the bronze process. Cryogenics (1985) 25, 123). Apart these very useful preliminary experimental data, very little has been done on the very different industrial strands which are now produced in the industry. Industrial Nb₃Sn strands are generally tested and checked only at 4.2 K and their operating design temperature is often very different, sometimes around 6 K. It is now urgent to validate the model and to confirm that the data taken up to now in the design calculations are conservative.     

AC loss properties of some Bi:2212/Ag Rutherford cables and a comparison with those of cables wound with NbTi and Nb3Sn

Calorimetric measurements of AC loss have been performed on Rutherford cables wound with NbTi, Nb₃Sn, and Bi:2212/Ag strands, respectively. For the NbTi cables, various strand coatings had been applied, while for the Nb₃Sn and Bi:2212/Ag cables the strand surfaces were just bare Cu or Ag, respectively. Most of the cables contained resistive cores: ribbons of kapton or titanium (NbTi cables), stainless steel (NbTi and Nb₃Sn cables), and nichrome-80 (Bi:2212/Ag cables). In all cases the cores were found to lead to a strong suppression of the face-on (field normal to the broad cable face) coupling current loss; to such an extent that even the Bi:2212/Ag cables, which would otherwise be severely cross-sintered, evinced acceptable coupling loss. For most of the cables side-by-side interstrand contact resistances have been calculated. An `effective interstrand contact resistance', R<sub>⊥,eff</sub>, has been defined, and an expression devised to enable its value in different-size cables to be converted to that of a `standard reference cable', R_{⊥,eff.,ref.}, for the purpose of intercomparison.     

Multiscale analysis of the influence of the triplet helicoidal geometry on the strain state of a Nb3Sn based strand for ITER coils

A theoretical model of a beam of unidirectional composites—based on the homogenisation theory and a refined kinematical hypothesis—is used for the analysis of the influence of the helicoidal geometry of a superconducting strand triplet on its strain state. The triplet is the first cabling stage of the superconducting cables used to wind the coils of ITER fusion reactor. The multiscale modelling strategy is presented, for which a finite element code has been developed. A triplet of Nb₃Sn based strands subjected to an axial stretch is analysed, and the resulting complete 3D strain state in the Nb₃Sn filament is recovered. An “extra” strain is found due to the helicoidal geometry of the triplet. Discussion of the results concludes the paper.     

Superconducting properties of internal-tin route Nb3Sn wires with radially arranged filaments : Development that realizes high Jc and low hysteresis loss

An internal-tin route Nb₃Sn superconducting wire that has both remarkably low hysteresis loss (Q_h) and high critical current density (J_c) was developed according to a new design idea. The wire was constructed by arranging the filaments in a radial layout, enlarging the outer filaments along the radial direction, narrowing the filament spacing in the radial direction intentionally and enlarging the filament spacing in tangential direction. Thus, the electromagnetic coupling among the filaments in tangential direction due to the bridging and/or proximity effect was suppressed without decreasing the volume fraction of Nb. As a result, excellent properties such as J_c(12 T) = 1.15 × 10³ A/mm² and Q_h = 301 mJ/cm³ (for 1 cycle of B = ±3 T) were obtained. We also evaluated the transition temperature (T_c) and upper critical field (B_c2) of the wire. The values for T_c and B_c2 were 17.3 K and 24.1 T, respectively, which were much better than those of usual internal-tin route wires. Moreover, electron probe micro-analyses confirmed that the good T_c and B_c2 were the result of the qualitative improvement of the Nb₃Sn compound based on the effects of arranging the Nb filaments radially, increasing the ratio of Sn-to-Nb and shortening the diffusion length for Sn. This wire is promising for use with conduction-cooled high-field magnets, in which there is a need to decrease the load of the cryocooler, and also for the strands of fusion coils.     

Preparation and characterization of Nb2O5-Al2O3 composite oxide formed by cathodic electroplating and anodizing

Al foil was coated with niobium oxide by cathodic electroplating and anodized in a neutral boric acid solution to achieve high capacitance in a thin film capacitor. X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) revealed the niobium oxide layer on Al to be a hydroxide-rich amorphous phase. The film was crystalline and had stoichiometric stability after annealing at temperatures up to 600 °C followed by anodizing at 500 V, and the specific capacitance of the Nb₂O₅-Al₂O₃ composite oxide was approximately 27% higher than that of Al₂O₃ without a Nb₂O₅ layer. The capacitance was quite stable to the resonance frequency. Overall, the Nb₂O₅-Al₂O₃ composite oxide film is a suitable material for thin film capacitors.     

Critical current test results of 13 T-46 kA Nb3Al cable-in-conduit conductor

In the framework of ITER-EDA, a 13 T-46 kA Nb₃Al conductor with stainless steel jacket has been developed in order to demonstrate applicability of an Nb₃Al conductor with react-and-wind technique to ITER-TF coils. Using a 3.5 m sample consisting of a pair of conductors with 0% and 0.4% bending strain, the critical current performances of the Nb₃Al conductors were studied to verify that the conductor achieves the expected performance and the bending strain of 0.4% does not originate degradation. The critical currents were measured at background magnetic fields of 7, 9, 10 and 11 T at temperatures from 6 to 9 K. The expected critical currents were evaluated taking into account the variation of the strain in the cross-section due to the bending strain as well as self-field and non-uniform current distribution as results of an imbalance in the joint resistance and inductances. The calculation results indicate that the current distribution is almost uniform and the experimental results showed good agreement with the expected critical currents. Accordingly, we can conclude that the fabrication process of this conductor is appropriate and the react-and-wind technique using the Nb₃Al conductor is applicable to ITER-TF coils. In addition, the critical current of the Nb₃Al conductor is expected to be 108 kA at 13 T and 4.5 K, resulting in a sufficient margin against the nominal current of 46 kA. Furthermore, it was found that the decrease in the critical current by thermal strain can be made small by applying the bending strain to the conductor so as to reduce the compressive strain at higher fields, i.e. inner side of the coil, in the conductor cross-section.     

The effect of Sb2O5 additions on the dielectric properties of Ag(Nb0.8Ta0.2)O3 ceramics

The sintering behavior and dielectric properties for perovskite Ag(Nb_0.8Ta_0.2)O₃ ceramic with Sb₂O₅ doping was explored. A small amount of Sb₂O₅ (2.5 wt.%) led to high densification at temperatures < 1060 °C. The dielectric constant increased and the temperature coefficient decreased with increasing concentration of Sb₂O₅, and the dielectric constant reached 673, combined with a low temperature coefficient of 147 ppm/°C, and dielectric loss of 0.0044 (at 1 MHz) for the sample with 3.5 wt.% Sb₂O₅ sintered at 1080 °C.     

Synthesis of Nb2O5 nanosheets and its electrochemical measurements

In this study, Nb₂O₅ nanosheets were first synthesized using NbO₂ particles as the precursor via a simple hydrothermal route. The synthesized Nb₂O₅ nanosheets are highly crystalline and their thicknesses are found to be ca. 3–5 nm. Based on the experimental results of XRD, SEM and TEM measurements, a possible mechanism for the formation of nanosheets was discussed. An electrode materials made of the product containing Nb₂O₅ nanosheets shows a larger capacity of 355 mAh g⁻¹ at a current density of 0.1 A g⁻¹. Cyclic measurements indicate that such an electrode exhibits a high reversible charge/discharge capacity and cycling stability. This might be attributed to the intrinsic characteristics of Nb₂O₅ nanosheets.     

Preparation and characterization of Cu3Sn nanoparticles via a facile ultrasonic irradiation

Cu₃Sn nanoparticles were prepared with ease by allowing bulk tin to react with copper acetate under ultrasonic irradiation. The microstructure and thermal stability of resultant Cu₃Sn nanoparticles were analyzed by means of transmission electron microscopy (TEM), X-ray diffraction (XRD) and thermogravimetric analysis (TGA). In the meantime, the possible growing mechanism of Cu₃Sn nanoparticles was presented; and the tribological properties of Cu₃Sn nanoparticles as lubricating additives were evaluated. It has been found that as-synthesized hexagonal Cu₃Sn nanoparticles are of spherical shape and have a narrow size distribution and an average diameter of 90 nm. The growth of Cu₃Sn nanoparticles involves three stages of ultrasonic dispersion, reaction and surface modification. Besides, ultrasonic irradiation in combination with surface-capping by oleic acid contributes to prevent on-growing Cu₃Sn nanoparticles from aggregation, making it feasible for Cu₃Sn nanoparticles to be well dispersed in lubricating base stock and significantly increase the antiwear ability and load-carrying capacity of liquid paraffin.     

PrBa2Cu3O7: A new superconductor

PrBa₂Cu₃O₇ superconducts, provided Pr is kept off Ba-sites — experimentally confirming the prediction of the oxygen model and indicating that superconductivity originates in the chains, not in the planes.     

Studies on switching mechanisms in Pd-nanodot embedded Nb2O5 memristors using scanning tunneling microscopy

Current imaging tunneling spectrum obtained from scanning tunneling microscopy has been used to probe the formation and/or rupture of conductive filaments responsible for bipolar switching in Pd nano-dots embedded Nb₂O₅ memristors. Filamentary conduction mechanism has been confirmed by scanning tunneling microscopy study using a Pt-Ir tip that enabled performing electroforming and reset operations at the nanoscale. The back and forth transition between the fully oxidized and metallic sub-oxide states of niobium under applied bias, as observed from X-ray photoelectron spectroscopy, is believed to be the source of bipolar switching in Nb₂O₅ memristors. The incorporation of Pd nanodots in Nb₂O₅ matrix plays a critical role by acting as an oxygen ion reservoir and/or by polarizing a large volume of oxygen vacancies. The formation and/or rupture of the conducting filaments through trapping-detrapping phenomena are found to boost the memristive switching performance.     

Effect of calcination conditions on phase formation and particle size of Zn3Nb2O8 powders synthesized by solid-state reaction

The solid-state mixed oxide method via a rapid vibro-milling technique is explored in the preparation of single-phase Zn₃Nb₂O₈ powders. The formation of the Zn₃Nb₂O₈ phase in the calcined powders has been investigated as a function of calcination conditions by TG-DTA and XRD techniques. Morphology, particle size and chemical composition have been determined via a combination of SEM and EDX techniques. It has been found that the minor phases of unreacted ZnO and Nb₂O₅ precursors and the columbite ZnNb₂O₆ phase tend to form together with the Zn₃Nb₂O₈ phase, depending on calcination conditions. It is seen that optimization of calcination conditions can lead to a single-phase Zn₃Nb₂O₈ in a monoclinic phase.