Superconductivity of copper containing small amounts of niobium

Superconductivity of copper containing small amounts of niobium has been investigated by measuring the electrical resistivity, superconducting volume fraction and by metallographic studies. Small amounts of niobium added to copper has a drastic effect on the low temperature resistivity of the alloys, The annealed alloy Cu_99.5Nb_0.5 shows zero resistance at a current density of 200 A cm^–2 below 3K. The estimated superconducting volume fraction of this alloy at 2K is about fifty times the physical volume fraction of the Nb in the alloy.When more Nb is added these effects unexpectedly become much smaller than those observed in the dilute alloys (< 1.5 at. % Nb). Metallographic results indicate that in all the Cu-Nb alloys studied there are two distinct types of Nb particles in the Cu matrix. The large particles (average size 10 m) randomly distributed in the alloy are probably formed at high temperature when the bulk of the alloy is still in the liquid state. The small Nb particles (size 1 m, interparticle distances < 0.2 m) probably form through a solid state precipitation. It has been found that the large precipitates are more abundant in the alloy containing more than 1.5 at. % Nb, than in alloys containing less than 1.5 at. % Nb. The observed superconducting properties of alloys Cu_99.8Nb_0.2 and Cu_99.5Nb_0.5 have been attributed to the proximity effect of the small Nb particles whose interparticle distances are compatible with the coherence length in the Cu matrix. The very wide superconducting transition shown in both the resistivity and the inductance measurements suggested a distribution in the Nb particle sizes as well as in the interparticle distances.     

High-temperature-tolerable superconducting Nb-alloy and its application to Pb- and Cd-free superconducting joints between NbTi and Nb3Sn wires

For more than 30 years, Pb–Bi alloy and Wood's metal (50% Bi, 26.7% Pb, 13.3% Sn, and 10% Cd) have been used as representative superconducting solder intermedia to establish superconducting joints between NbTi and Nb₃Sn wires in high-field nuclear magnetic resonance magnet systems. However, the use of Pb and Cd has been severely restricted by environmental regulations, such as the Restriction of Hazardous Substances Directive. Herein, a novel method of forming a superconducting joint between NbTi and Nb₃Sn wires without Pb and Cd has been proposed. This approach is based on metallurgical bonding processes using a superconducting Nb-alloy intermedium, whose fine microstructure is maintained even after exposure to temperatures higher than 650 °C. Further, fine crystal defects become sources of magnetic flux pinning centers. Among transition elements close to Nb, Hf is considered the most suitable additive for realizing high-temperature-tolerable (HTT) superconducting Nb-alloy intermedia. Utilizing the HTT characteristic of Nb–Hf, a superconducting joint between Nb₃Sn filaments and one end of the Nb–Hf alloy core was created by forming a superconducting Nb₃Sn layer at the interface through a chemical reaction. The other end of the Nb–Hf alloy core was cold-pressed with NbTi filaments, to connect their active new surfaces to each other in order to create a superconducting joint. Ultimately, a superconducting joint between NbTi and Nb₃Sn wires was realized with a high critical magnetic field (B_c2) of more than 1 T. The formation of the superconducting joint was confirmed by current decay measurements. This method of forming a superconducting joint is promising for application in environmentally friendly nuclear magnetic resonance magnet systems.

Graphical abstract

     

Production of superconducting Cu-Nb-Sn alloy by hot extrusion of rapidly solidified powder

A Cu_92.5Nb₅Sn_2.5 alloy has been rapidly quenched into a powder by ultrasonic gas atomization. The structure of the powder consists of finely dispersed Nb particles in a Cu-Sn matrix. The powder has been consolidated by hot extrusion at 650°C. The extrusion does not change the phases present in the material appreciably but annealing for 100 h at 650°C causes a transformation of the Nb particles to Nb₃Sn. The extruded material shows superconductivity. The critical temperatureT _c, of the asquenched alloys is 8.0 K but increases to 15.6 K after annealing for 100 h at 923 K. The upper critical magnetic field, \(H_{C_2 } \) , and critical current density,J _c, were 3.4 kOe at 4.2 K and 7.4×10⁵ Am^?2 at zero applied field and 4.2 K, respectively. The appearance of superconductivity is attributed to the proximity effect of the Nb₃Sn phase particles which are sufficiently well distributed to satisfy the leak distance and the critical particle size.     

Precipitation hardening and recrystallization in Cu-4% to 7% Ni-3% Al alloys

The effects of microstructure on mechanical properties in three cold-worked Cu-4% to 7% Ni-3% Al alloys have been investigated by changing ageing time at 500 °C. Hardness and strength in the Cu-7% Ni-3% Al and Cu-5.5% Ni-3% Al alloys increase with ageing time and have maximum values at an ageing time of 10³–10⁴s at 500 °C, then decrease. During ageing of Cu-7% Ni-3% Al at 500 °C, the coherent Ni₃Al phase was first precipitated out and later incoherent NiAl phase was formed. Ni₃Al formed during the initial stage of ageing is likely to be a transient phase. The increases in hardness and strength are due to the precipitation of coherent Ni₃Al phase. Coherent Ni₃Al particles are effective in increasing the strength and retarding the recrystallization process. On the other hand, the hardness and strength in the Cu-4% Ni-3% Al alloy gradually declined with ageing time. Only incoherent NiAl phase was formed during ageing at 500 °C. Decreases in hardness and strength in the Cu-4% Ni-3% Al alloy are attributed to softening during recovery and recrystallization, because incoherent NiAl particles have an insufficient effect to increase the strength.     

Superconducting properties and microstructure of crystallized Hf-Nb-Si and Hf-V-Si amorphous alloys

Amorphous Hf-Nb-Si and Hf-V-Si alloys have been produced by rapidly quenching the melts using a melt-spinning technique. The silicon content in the amorphous alloys was limited to 14 to 20 at% and the niobium or vanadium content was limited to 0 to 45 at% and 0 to 35 at%, respectively. These amorphous alloys did not show any superconducting transition down to liquid helium temperature (4.2 K). However, a transition was detected above 4.2 K after inducing crystallization in these alloys by annealing at appropriate temperatures. The highest superconducting transition temperatures, T _c, attained were 8.9 K for the Hf₄₅Nb₄₀Si₁₅ alloy annealed for 1 h at 1273 K and 6.7 K for the Hf₅₀V₃₅Si₁₅ alloy annealed for 1 h at 1173 K. The upper critical magnetic field, H _c2, at 4.2 K and the critical current density, J _c, at zero applied field and 4.2 K were about 5.1×10⁶A m^–1 (6.4 T) and more than 1×10⁴ A cm^–2 for the Hf₄₅Nb₄₀Si₁₅ alloy and more than 8.0×10⁶ A m^–1 (10 T) and 5×10³ A cm^–2 for the Hf₅₀V₃₅Si₁₅ alloy. Detailed transmission electron microscopic studies of the annealed structure of these amorphous alloys established that, after crystallization, these alloys contain a body-centred cubic -Hf(Nb) solid solution and body-centred tetragonal Nb₃Si phases in the Hf₄₅Nb₄₀Si₁₅ alloy and hexagonal Hf₅Si₃, face-centred cubic HfV₂ and cubic V₃Si phases in the Hf₅₀V₃₅Si₁₅ alloy. Since Nb₃Si and Hf₅Si₃ are not superconducting above 4.2 K, it has been concluded that superconductivity in these crystallized alloys is due to the precipitation of -Hf(Nb) solid solution in the Hf-Nb-Si alloys and to the precipitation of HfV₂ and V₃Si compounds in the Hf-V-Si alloys.     

Preparation of Cu-Nb alloys for multifilamentaryin situ superconducting wire

A study has been made of two techniques, chill casting and consumable arc melting, for preparing ingots of Cu-Nb alloy for production of multifilamentary Nb₃Sn superconducting wire. It was found that Y₂O₃, ThO₂ and graphite all make excellent crucible materials for melting Cu-Nb alloys at 1850° C. Some difficulty was found with Nb segregation in chill-cast 5 cm diameter ingots. The consumable arc casting technique was shown to produce a uniform Nb dendrite distribution with little macrosegregation in 5 cm diameter castings and is regarded as having excellent potential for scale-up production of uniform Cu-Nb ingots of 25 to 30 cm diameter.     

The morphology and grain size of Nb3Sn filaments in in situ prepared multifilamentary Nb3Sn-Cu composite wire

Multifilamentary Nb₃Sn-Cu wire was prepared from chill-cast Cu-20 wt% Nb ingots by drawing 12.5 mm ingots to 0.15 mm diameter wire, plating with 9 vol% Sn and reacting at temperatures ranging from 450 to 725° C. The morphology and grain size of the as-cast Nb filaments and the Nb₃Sn filaments formed on heat treatment were characterized by TEM and SEM studies, and the Sn diffusion process was evaluated using electron microprobe analysis. The as-drawn Nb filaments had a cross-section typically 50 Å by 2500 Å and were generally convoluted in the wide direction. Tin diffusion produced Nb₃Sn filaments with an essentially equi-axed cross-section having diameters of around 800 Å for the 550° reaction temperature and grains extending across the diameter. It was shown that the change in cross-sectional shape is produced largely by coarsening of the thin Nb filaments prior to diffusion of Sn to form the Nb₃Sn. Optimum J _c values at both 1 and 8 T were found to occur for diffusion temperatures of 550° C and times of 3 to 6 days where the grain size was 750 to 800 Å.     

The role of silicon in the formation of the (Al5Cu6Mg2) σ phase in Al-Cu-Mg alloys

The role of silicon in the precipitation of the phase (Al₅Cu₆Mg₂) has been investigated through comparative studies on Al-3.63Cu-1.67Mg (wt%) and Al-3.63Cu-1.67Mg-0.5Si alloys. Both alloys were extensively examined after solution treating at 525°C for 2.5 h followed by ageing at 265°C for times up to 650 h. Limited studies were also undertaken on both alloys after ageing at 200 and 305°C. Precipitation of was observed in Al-3.62%Cu-1.66%Mg-0.5%Si for all ageing conditions studied but was absent in Si-free Al-3.62%Cu-1.66%Mg. In addition, S and phases were observed in both alloys. The volume fraction of phase in the Si containing alloy was substantially reduced by a pre-age stretch followed by ageing for 24 h at 265°C with S being the dominant precipitate type. The volume fraction of phase in the Si containing alloy was lower after ageing 24 h at 200°C than after 24 h at 265 and 305°C. Peak hardness was higher for the Si free alloy on ageing at 200 and 265°C, but the Si free alloy softened more rapidly, reflecting the more rapid coarsening kinetics of S compared with .     

Upper critical field measurements on A15 NbGe superconducting films

Measurements of the upper critical fieldB _c2 (T) have been made on a range of high-transition-temperature A15 niobium germanium samples prepared by dc sputtering with varying additions of oxygen and differing sputtering conditions. An overview is presented on the formation of Nb₃Ge with particular reference to lattice parameter, composition, and normal state residual resistivity. The problem of sample inhomogeneity and its effect onB _c2 (T) measurements is discussed. Correlations of are made withT _cand resistivity, and the results compared with previous studies of neutron-irradiated and coevaporated material. Existing theories of are summarized and the problem of comparison of theory and experiment is discussed. An analysis is presented that does not depend directly on assumptions about the Fermi surface structure. The data indicate thatB _c2 has a fundamental dependence onT _cand resistivity that is similar to that found in Nb₃Sn. There is evidence that the mean Fermi velocity for clean Nb₃Ge is about twice the value for Nb₃Sn; the mean Fermi surface area is also about twice the value for Nb₃Sn. This suggests that Nb₃Ge is more strongly coupled than Nb₃Sn.     

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.     

The properties of surface diffusion processed V3Ga tape

The rapid growth of superconducting magnet technology is leading to many applications of superconductivity in science and industry. At present, the most widely used practical superconductors are NbTi alloys and Nb₃Sn compounds. Surface diffusion processed V₃Ga tape possesses an outstanding current carrying capacity in high magnetic fields. This paper is concerned with the properties of the V₃Ga tape which is produced by us on a commercial scale.     

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.     

Effect of Nb addition on the glass forming ability and mechanical properties in the Ni-Zr-Al-Hf-Nb alloys

The effect of substituting Nb for Zr on the glass forming ability (GFA) and mechanical properties of Ni₆₀ Al₆Hf₇ (x = 0 -- 14) alloys was investigated. The substitution of Zr with Nb of 0 -- 14 at% improved the GFA. When increasing the Nb content x from 0 -- 14, the glass transition temperature, T_g, and the crystallization temperature, , of melt-spun Ni₆₀ Al₆Hf₇ (x = 0 -- 14) alloys increased from 833 and 863 K to 877 and 914 K, respectively. The Ni₆₀ Al₆Hf₇ (x = 8, 10, 12 and 14) alloys exhibited high (>0.615), (>0.4) values and a wide supercooled liquid region, (= – T_g) (>36 K) enabling the fabrication of bulk metallic glass (BMG) with a diameter above 1 mm. As such, Ni₆₀Zr₁₇Al₆Hf₇Nb₁₀ and Ni₆₀Zr₁₅Al₆Hf₇Nb₁₂ alloys with a maximum diameter of 2 mm could be fabricated by injection casting. These bulk amorphous alloys also exhibited good mechanical properties, where the true ultimate compressive strength, strain and Vickers hardness (H_v) were approximately 3.0 GPa, 2.08% and 713, respectively, for the amorphous Ni₆₀Zr₁₇Al₆Hf₇Nb₁₀ alloy, and 3.1 GPa, 2.22% and 687, respectively, for the amorphous Ni₆₀Zr₁₅Al₆Hf₇Nb₁₂ alloy.     

Microstructure and superconductivity in annealed Cu-Nb-(Ti,Zr, Hf) ternary amorphous alloys obtained by liquid quenching

Melt-quenched Cu-Nb-(Ti, Zr, Hf) ternary alloys have been found to be amorphous possessing high strength and good bend ductility. The niobium content in the amorphous alloys was limited to less than 35 at % and the titanium, zirconium or hafnium contents from 25 to 50 at %. The Cu₄₀Nb₃₀(Ti, Hf)₃₀ alloys showed a superconducting transition above the liquid helium temperature (4.2 K) after annealing at appropriate temperatures. The highest transition temperatures attained were 5.6 K for the Cu₄₀Nb₃₀Ti₃₀ alloy annealed for 1 h at 873 K and 8.4 K for the Cu₄₀Nb₃₀Hf₃₀ alloy annealed for 1 h at 1073 K. In addition, these alloys exhibited upper critical magnetic fields of 1.8 to 2.3×10⁶ Am^–1 at 4.2 K and critical current densities of 2×10³ to 1×10⁴ A cm^–2 at zero applied field and 4.2 K. Since the structure of the superconducting samples consisted of ordered phases based on a b c c lattice with a lattice parameter of 0.31 nm, it was concluded that the superconductivity in the Cu₄₀Nb₃₀Ti₃₀ and Cu₄₀Nb₃₀Hf₃₀ alloys was due to the precipitation of the metastable ordered b c c phases.     

Reaction diffusion in the Nb-Ge system

Reaction diffusion in the Nb-Ge system was studied in the temperature range 1243 to 1723 K for diffusion couples of (pure solid Nb)-(pure liquid Ge) and (pure solid Nb)-(Ge-37.5wt % Nb liquid alloy). Growth of the NbGe₂, Nb₃Ge₂, Nb₅Ge₃ and Nb₃Ge layers was observed, and the growth rates of all except the Nb₃Ge layer were found to conform to the parabolic law. Growth of the Nb₃Ge layer was observed only along the grain boundaries in the Nb₅Ge₃ layer. Interdiffusion coefficients \(\tilde D\) in the NbGe₂, Nb₃Ge₂ and Nb₅Ge₃ phases were determined by Heumann's method, and the temperature dependence of these was expressed by the Arrhenius equations as follows: $$\tilde D_{{\text{NbGe}}_{\text{2}} } = (6.40_{ - 1.66}^{ + 2.25} \times 10^{ - 6} exp [ - (161 \pm 4) kJ mol^{ - 1} {\text{/RT] m}}^{{\text{2 }}} \sec ^{ - 1} $$ $$\tilde D_{{\text{Nb}}_{\text{3}} {\text{Ge}}_{\text{2}} } = (2.27_{ - 0.60}^{ + 0.82} \times 10^{ - 4} exp [ - (282 \pm 4) kJ mol^{ - 1} {\text{/RT] m}}^{{\text{2 }}} \sec ^{ - 1} $$ and $$\tilde D_{{\text{Nb}}_{\text{5}} {\text{Ge}}_{\text{3}} } = (6.28_{ - 1.93}^{ + 2.78} \times 10^{ - 5} exp [ - (238 \pm 5) kJ mol^{ - 1} {\text{/RT] m}}^{{\text{2 }}} \sec ^{ - 1} $$ In addition to the binary Nb-Ge system, the reaction diffusion of (pure solid Nb)-(Cu-13 wt % Ge liquid alloy) couples was also studied. In this case, only growth of the Nb₅Ge₃ layer containing negligible copper content was observed.     

Criteria for Selecting Substrates for Nb<Subscript>3</Subscript>Sn Electrodeposition

We studied the effect of substrate material on the microstructure and properties of Nb₃Sn coatings produced by electrochemical coreduction of Nb and Sn ions in the cathodic zone in molten salts. The results demonstrate that continuous superconducting Nb₃Sn coatings can be produced on molybdenum, tantalum, niobium, copper, nickel, Invar and the 60% Ni + 22% Mo + 12% Fe alloy. The coatings grown on molybdenum, tantalum, and niobium have the highest purity. To ensure good superconducting properties of electrodeposited coatings, preference should be given, all other factors being the same, to substrate materials with thermal expansion coefficients close to or smaller than that of Nb₃Sn.     

Precipitation hardening of Cu-4Ti-1Cd alloy

Precipitation hardening of Cu-4Ti-1Cd alloy has been studied using hardness measurements and transmission electron microscopy. This alloy exhibited hardness of 238 Hv in solution treated (ST) condition and attained peak hardness of 318 Hv after ageing at 450°C for 40 h. Electrical conductivity of Cu-4Ti-1Cd alloy increased from 5.7 %IACS (International Annealed Copper standard) in ST condition to 8.9 %IACS on ageing at 450°C for 16 h. This alloy exhibited markedly higher yield strength (751 MPa in the peak-aged condition) compared to Cu-4.5Ti alloy but the increase in UTS due to cadmium addition was less significant. The higher yield strength of ternary alloy in peak aged condition is due to the solid solution strengthening of cadmium as well as the presence of -Cu₄Ti precipitate. On over-ageing the alloy showed a decrease in hardness as a result of formation of equilibrium precipitate, -Cu₃Ti. Optical microscopy reveals single phase with equiaxed grains in solution treated condition. A coherent, metastable phase -Cu₄Ti is responsible for high strength and hardness in peak aged condition. The over-ageing in this alloy shows the formation of cellular structure at the grain boundaries of the matrix phase.     

TEM observatiion of a Cu-Mg age-hardenable alloy

The precipitation processes in Cu-Be, Cu-Co, Cu-Fe alloys have been thoroughly investigated; however, much less attention has been paid to studying the Cu-Mg system. In this work the decomposition of Cu-3.5 wt% Mg alloy during ageing was investigated by means of transmission electron microscopy (TEM). The microstructure of Cu-3.5 wt% Mg alloy aged at 340° C is characterized by the presence of fine dispersed coherent precipitates. On continued ageing the coherent precipitates disappear and a new transition phase with oblate octahedron morphology grows. At temperatures above 34O° C the equilibrium phase is formed by discontinuous precipitation. Ageing of Cu-3.5 wt% Mg alloy at temperatures above 450° C results in the formation of the equilibrium Cu₂Mg phase.     

Effect of Nb,Mo, V contents on oxidation resistance of Ti3Al based alloys

In the present paper, the effect of the contents of Nb, Mo, V on the oxidation properties (700°C, in air) of Ti₃Al based alloys has been studied. It has been shown that the alloys were oxidized rapidly as exposed at 700°C in the air. After 100 h exposure, oxygen-affected alloy surface layer of about 10 thickness has been formed on account of the poor protection of the oxide film. An addition of (11–13%)¹ Nb enhanced the oxidation resistance. The addition of Mo and V in the Ti₃Al–Nb system alloy reduced the oxidation resistance significantly.     

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.