Deposition of superconducting Nb<Subscript>3</Subscript>Sn 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.     

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.     

Critical-current anisotropy in superconducting Nb<Subscript>3</Subscript>Sn coatings produced by electrocodeposition of Nb and Sn

We report the critical current of superconducting Nb₃Sn coatings produced by electrocodeposition from molten salts and its anisotropy in transverse magnetic fields. The anisotropy is quantified by the ratio of the critical currents measured in magnetic fields parallel and normal to the coating surface, k. The effect of doping with tantalum and nitrogen on the microstructure, critical current, and anisotropy factor k of Nb3Sn coatings is examined. The k of the undoped and tantalum-doped coatings ranges from 0.4 to 1.0, and that of the nitrogen-doped coatings, from 0.4 to 1.4. The critical current anisotropy is interpreted in terms of the microstructure of the coatings.     

Superconducting microwave cavities based on Nb<Subscript>3</Subscript>Sn electrolytic coatings

We demonstrate that Nb₃Sn coatings suitable for use in superconducting microwave devices can be produced by electrocodeposition of Nb and Sn. Superconducting cavities have been fabricated by electrodeposition in a galvanostatic mode using such coatings, and their performance has been evaluated.     

Effect of carbon on the structure and superconducting properties of Nb<Subscript>3</Subscript>Sn coatings produced by electrocodeposition

The effect of carbon doping on the structure and superconducting properties of Nb₃Sn coatings produced by electrocodeposition of Nb and Sn from molten salts was investigated. The results demonstrate that the addition of fine-particle graphite powder to the melt has an insignificant effect on the microstructure, critical current, and superconducting transition temperature T_c of the coatings but reduces the superconducting transition width (T_c0.05). Doping of Nb₃Sn with carbon from the gas phase (Ar + CCl₄) ensures the formation of a fine-grained microstructure and increases the critical current of the material by a factor of 3–3.5 compared to coatings grown in pure argon atmosphere.Translated from Neorganicheskie Materialy, Vol. 40, No. 12, 2004, pp. 1467–1475.Original Russian Text Copyright © 2004 by Kolosov.Presented in part at the II International Conference on Key Issues in Science, Materials Research, and Technology of Carbon, Moscow, 2003.     

Zero-Current Deposition of Superconducting Nb3Sn Coatings from Molten Salts

Superconducting Nb–Sn coatings are produced by zero-current Sn deposition on Nb at 800–1070 K from KCl + NaCl and CsCl + KCl + NaCl eutectic melts containing SnCl₂. Their phase composition and superconducting properties are studied. This approach offers the possibility of producing single-phase Nb₃Sn coatings perfect enough to be used as working layers of superconducting microwave systems.     

Residual stress in Nb<Subscript>3</Subscript>Sn-based composite electrolytic coatings

A combined nondestructive technique utilizing X-ray diffraction strain measurements has been developed for evaluating the residual stress and Poisson’s ratio in Nb₃Sn-based composite electrolytic coatings. The magnitude and nature of the residual stress in niobium stannide on molybdenum, nickel, and copper substrates have been determined.     

Reactions of V<Subscript>2</Subscript>O<Subscript>5</Subscript>, Nb<Subscript>2</Subscript>O<Subscript>5</Subscript>, and Ta<Subscript>2</Subscript>O<Subscript>5</Subscript> with AlN

Reactions of vanadium, niobium, and tantalum pentoxides with aluminum nitride have been studied using X-ray diffraction. At temperatures from 1000 to 1600°C, we have identified various V, Nb, and Ta nitrides. The composition of the niobium and tantalum nitrides depends on the reaction temperature. The tendency toward nitride formation becomes stronger in the order V₂O₅ < Ta₂O₅ < Nb₂O₅.     

Effect of Heat Treatment on the Structure and Properties of Superconducting Nb and Nb3Sn Electrodeposits

The effect of vacuum heat treatment (900–1600°C) on the structure and properties of superconducting Nb and Nb₃Sn coatings produced via electrolysis of molten salts is studied. The results demonstrate that the primary recrystallization of the coatings is characterized by a nonuniform nucleation of new grains: nuclei appear first in a defect-rich layer near the coating–substrate interface. The presence of the substrate during heat treatment prevents complete relaxation of the lattice strain and dislocations in the coatings, without significantly influencing the macroscopic stress. The relaxation of lattice strain and the reduction in dislocation density during recrystallization bring the coatings closer to equilibrium, thereby changing their superconducting properties.     

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.     

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.     

AC losses in multilayer superconducting tapes

ac losses have been investigated experimentally as well as theoretically in tapes having on the surface several normal conducting and/or superconducting layers. The superconducting layers under investigation have been Nb₃Sn on a niobium substrate and Nb₃Ge on a stainless steel substrate. It has been proved that the layered structure of the tapes is well reflected by the stepwise character of the ac losses dependence on the amplitude of the surface magnetic field. The magnetic flux passing through a surperconducting layer or surface barrier into the inside of the tape enhances the losses in the passed barrier or layer.     

Superconducting spherical magnetic shields on the basis of niobium and its compounds

The method of fabrication of superconducting spherical magnetic shields on the basis of super-conducting electrolytic coatings made of niobium, Nb₃Sn, and NbC is developed. These shields can be used for weakening the external magnetic fields with strength up to 800 kA/m and obtaining a magnetic vacuum lower than 0.8 × 10⁻⁵ A/m.     

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.     

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.     

Nb<Subscript>3</Subscript>Sn-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.     

Microstructure development in Nb3Sn(Ti) internal tin superconducting wire

The authors have studied the phase formation sequences in a Nb₃Sn ‘internal tin’ process superconductor. Heat treatments were performed to convert the starting materials of tin, Ti–Sn, copper and niobium, to bronze and Nb₃Sn. Specimens were quenched at different points of the heat treatment, followed by metallography to identify the phases present and X-ray microtomography (XMT) to investigate the void volume and distribution. An unexpected observation of the microstructure development was the uphill diffusion of tin during the Cu–Sn reactive diffusion. Some defects likely to affect the superconducting performance of the wires were observed. Microscopy revealed the presence of a Ti–Sn intermetallic compound displacing the niobium filaments, and XMT revealed the formation of long pores in the longitudinal direction. Two types of pore formation mechanism, in addition to Kirkendall pores, are proposed. The phase and microstructure development suggests that low-temperature heat treatment (below 415 °C) will have significant influence on optimising the final superconducting properties.     

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.     

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.     

Superconducting properties of the composite-processed Nb3Sn superconductor with the Cu-Sn-Zn matrix

Effects of the Zn addition to the Cu-Sn matrix of the Nb₃Sn composite tape have been investigated by measuring the matrix work-hardening behaviour, the rate of Nb₃Sn layer formation and the pertinent superconducting properties. The Zn addition drastically enhances the diffusion rate of Nb₃Sn formation at each Sn level in the matrix examined, leading to sufficient superconducting properties even at a low Sn level of 3.5 at. %; a sample containing 3.5 at. % Sn and 15 at. % Zn in the matrix exhibits a critical temperature and critical current density comparable with those of samples at a Sn level of 7 at. %. The upper critical field obtained for a sample with the 6 at. % Cu-6 at. % Sn-4 at. % Zn matrix beyond 200 kOe. The work-hardening of the composite matrix is found to be essentially a function of Sn level, and insensitive to the Zn addition.