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 nickel-based amorphous alloys with finely dispersed lead and lead-bismuth particles and their superconducting properties

The application of the melt-quenching technique to Ni-Si-B-Pb, Ni-P-B-Pb, Ni-Si-B-Pb-Bi and Ni-P-B-Pb-Bi alloys containing immiscible elements such as lead and bismuth has been tried and it has been found to result in the formation of a new type of material consisting of fine fcc Pb or hcp (Pb-Bi) + bct X(Pb-Bi) particles dispersed uniformly in the nickelbased amorphous matrix. The particle size and interparticle distance were 1 to 3 and 1 to 4 µm, respectively, for the lead phase, and less than 0.2 to 0.5 µm and 0.2 to 1.0 µm for the Pb-Bi phase. The uniform dispersion of such fine particles into the amorphous matrix was achieved in the composition range below about 6at% Pb and 7at% (Pb + Bi). Additionally, these amorphous alloys have been found to exhibit a superconductivity by the proximity effect of f c c Pb or (Pb-Bi) superconducting particles. The transition temperatureT _c was in the range 6.8 to 7.5 K for the Ni-Si(or P)-B-Pb alloys and 8.6 to 8.8 K for the Ni-Si (or P)-B-Pb-Bi alloys. The upper critical fieldH _c2 and the critical current densityJ _c for (Ni_0.8 P_0.1 B_0.1)₉₅ Pb₃ Bi₂ at 4.2 K were, respectively, about 1.6T and of the order of 7 X 10⁷ A m^–2 at zero applied field. Melt quenching of amorphous phase-forming alloys containing an immiscible element has thus been demonstrated, enabling us to produce amorphous composite materials exhibiting unique and useful characteristics which cannot be obtained in homogeneous amorphous alloys.     

Non-equilibrium crystalline superconductors in Zr-Si binary alloys rapidly quenched from melts

The new non-equilibrium superconductor with a b c c structure has been found in rapidly quenched Zr-Si alloys. The silicon content in the b c c alloys was limited to the narrow range between 8 and 11 at%. The b c c alloys showed a superconducting transition whose temperature increased from 3.20 to 3.84 K with decreasing silicon content. The upper critical magnetic field and the critical current density for Zr₉₂Si₈ alloy were of the order of 3.58 × 10⁶ Am^–1 at 2.0 K and 3.3 × 10^–2 Am^–2 at 2.42 K in the absence of an applied field. The upper critical field gradient at the transition temperature and the electrical resistivity at 4.2 K were about — 1.82 × 10^–1 Am^–1 K^–1 and about 150 cm. The Ginzburg-Landau parameter and coherence length _GL (0) were estimated to be about 65 and 6.3 nm, respectively, from these experimental values by using the Ginzburg-Landau-Abrikosov-Gorkov theory and hence it is concluded that the present b c c alloys are extremely soft type-II superconductors with a high degree of dirtiness.     

Microstructure and superconducting properties of rapidly-quenched copper-based alloys containing immiscible lead and bismuth elements

Copper-based superconducting alloys including finely dispersed f c c lead or h c p (Pb- Bi) particles in f c c copper matrix have been obtained by rapid quenching (Cu-M)_100-x Pb_x and (Cu-M)_100-x (Pb_0.6Bi_{0.4 x} (M = aluminium, silicon or tin;x < 10 at%) alloys containing immiscible elements such as lead and bismuth. The particle size and interparticle distance were about 30 to 130 nm and 20 to 200 nm for had particles and about 30 to 60 nm and 30 to 150 nm for (Pb- Bi) particles. The transition temperature,T _c, was in the range of 3.2 to 5.5 K for the Cu-M-Pb alloys and 6.2 to 6.3 K for the Cu-M-Pb-Bi alloys. Critical magnetic field,H _c2, and critical current density,J _c, for the later alloys were 0.47 to 0.93T at 4.2 K and 1.1 × 10⁵ to 2.7 × 10⁵ Am^–2 at zero applied field and 4.21 K. The mechanism of the appearance of such a soft-type superconductivity for the rapidly quenched copper-based alloys was discussed, and inferred to be due to the formation of a percolation path of a superconducting lead or Pb-Bi phase along the grain boundaries, sub-boundaries and/or tangled dislocations where the lead or Pb-Bi phase precipitated preferentially, rather than the proximity effect based on lead or Pb-Bi particles.     

Superconducting properties of amorphous Zr-Ge binary alloys

A new type of refractory metal-metalloid amorphous alloys exhibiting superconductivity has been found in a binary Zr-Ge system by a modified melt-spinning technique. Specimens are in the form of continuous ribbons 1 to 2 mm wide and 0.02 to 0.03 mm thick. The germanium content in the amorphous alloys is limited to the range of 13 to 21 at%. These amorphous alloys are so ductile that no cracks are observed even after closely contacted bending test. The Vickers hardness and crystallization temperature increase from 435 to 530 DPN and from 628 to 707 K, respectively, with germanium content, and the tensile fracture strength is about 1460 MPa. Furthermore, the amorphous alloys exhibit a superconducting transition which occurs very sharply. The superconducting transition temperature (T _c) increases with decreasing germanium content and reaches a maximum value of 2.88 K for Zr₈₇Ge₁₃. The upper critical magnetic field for Zr₈₇Ge₁₃ alloy was of the order of 21.8 kOe at 2.0 K and the critical current density for Zr₈₅Ge₁₅ alloy was about 175 A cm^–2 at 1.70 K in the absence of an applied field. The upper critical field gradient atT _c and the electrical resistivity at 4.2 K increase significantly from 24.6 to 31.5 kOe K^–1 and from 235 to 310cm, respectively, with the amount of germanium. The Ginzburg-Landau (GL) parameter and the GL coherence length §_GL (0) were estimated to be 72 to 111 and about 7.9 nm, respectively, from these experimental values by using the Ginzburg-Landau-Abrikosov-Gorkov (GLAG) theory and hence it is concluded that the Zr-Ge amorphous alloys are extremely soft type-II superconductor with high degree of dirtiness which possesses theT _c values higher than zirconium metal, in addition to high strength combined with good ductility.     

Structural features of certain Ni-Al-Ta and Ni-Al-Hf alloys containing the γ′- and β -phases

An investigation has been made of the structure of certain alloys of the Ni-Al-Ta and Ni-Al-Hf systems. The compositions chosen for investigation were: Ni-28.75 at% Al-2.5 at% Ta; Ni-26.75 at% Al-2.5 at% Ta; Ni-20.0 at% Al-7.5 at% Ta; Ni-20.0 at% Al-7.5 at% Hf; Ni-22.5 at% Al-5.0 at% Hf. The Ni-Al-Ta alloys consisted of +-phases (or-phase transformed during rapid cooling to a martensitic product). Two types of morphology were observed in the transformed-phase, one consisting of parallel sided plates and the other of acicular shaped plates; the plates contained fine twins on {1 0 1} b c t planes. The as-cast Ni-Al-Nf alloys consisted of the -phase, together with eutectic regions of-phase and (Ni, Al)₇Hf₂; both lamellar and discontinuous eutectic morphologies were observed. Extensive solid solution of tantalum and hafnium in the -phase occurred.     

Kinetics of reactive diffusion between Ta and Cu–9.3Sn–0.3Ti alloy

Tantalum is used as a diffusion barrier in the superconducting Nb₃Sn composite-wire manufactured by the bronze method. In order to examine the consumption behavior of the Ta barrier during annealing in the bronze method, the kinetics of the reactive diffusion between Ta and a bronze was experimentally observed using sandwich diffusion couples composed of Ta and a Cu–9.3Sn–0.3Ti alloy. The (Cu–Sn–Ti)/Ta/(Cu–Sn–Ti) diffusion couples were isothermally annealed at temperatures of T = 973–1053 K for various times up to t = 1462 h. Owing to annealing, Ta₉Sn is formed as a uniform layer at the initial (Cu–Sn–Ti)/Ta interface in the diffusion couple, and gradually grows mainly toward Ta. The mean thickness of the Ta₉Sn layer is proportional to a power function of the annealing time. However, the exponent of the power function is equal to unity at t < t _c but smaller than 0.5 at t > t _c. Thus, the transition of the rate-controlling process for the growth of Ta₉Sn occurs at t = t _c. The critical annealing time t _c takes values of 1.83 × 10⁶, 4.63 × 10⁵, and 5.98 × 10⁵ s at T = 973, 1023, and 1053 K, respectively. The growth of Ta₉Sn is controlled by the interface reaction at the migrating Ta₉Sn/Ta interface in the early stages with t < t _c but by the volume and boundary diffusion across the Ta₉Sn layer in the late stages with t > t _c. Due to the transition of the rate-controlling process, the growth rate is always much smaller for Ta₉Sn than for Nb₃Sn. As a result, Ta works as an effective barrier against the diffusion of Sn from the bronze to the Cu stabilizer in the superconducting Nb₃Sn composite-wire.     

New Phases in the Mg–Ta–H2 System at High Quasi-hydrostatic Pressures

The Mg–Ta–H₂ system was investigated at quasi-hydrostatic pressures from 4 to 5 GPa and temperatures from 900 to 1000°C. X-ray diffraction studies indicate the formation of three new hydrides: (Ta,Mg)H _x (solid solution of magnesium hydride in tantalum hydride), (Mg,Ta)H₂ (solid solution of tantalum hydride in -MgH₂), and Mg _x Ta _y H _z .     

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.     

Experiment and theory on the magnetic susceptibility of Bi-Sb alloys

The magnetic susceptibility of Bi _1–x Sb _x alloys is investigated for the concentration range 0 x22 at%, the temperature range 4.2–270 K, and magnetic fields up to 60 kOe. The experimental results are compared with the theoretical dependences of susceptibility on the band parameters, temperature, and magnetic field.     

Structural evolution of a Ta-filament during hot-wire chemical vapour deposition of Silicon investigated by electron backscatter diffraction

We report on the application of electron backscatter diffraction to investigate the structural changes of a tantalum filament operated at typical hot-wire chemical vapour deposition conditions for the synthesis of hydrogenated nanocrystalline silicon. Various tantalum-silicides, identified by electron backscatter diffraction, form preferentially along the length of the filament. The filament has a recrystallized Ta inner core and a TaSi₂ layer encapsulated with a Si layer at the cooler ends. The αTa₅Si₃, metastable Ta₅Si₃ and Ta₂Si phases formed in addition to recrystallized Ta and TaSi₂ at the centre regions. Cracks and porosity were prevalent throughout the length of the filament. The microstructural evolution of the aged tantalum filament can be ascribed to the thermal gradient along the filament length, recrystallization of Ta and the variation of silicon content within the filament.     

Elastic and superconducting properties of supersaturated Al-Si and Al-Ge solid-solution alloys treated under a 5.4 GPa pressure

Supersaturated Al-Si and Al-Ge solid solutions having up to a 10 at% solute concentration were treated at 5.4 GPa. The variations in the elastic moduli, superconducting transition temperature T _c and other physical properties were obtained. Using the resonance method the moduli of the solid solutions in both alloy systems decreased with increasing solute concentration, in contrast with those of the two-phase states which increased. The maximum value of T _c was determined as 6.6 K for an Al-15 at% Si solid solution. It was confirmed that the dependence of T _c on the valence electron concentration was remarkably greater than that of other non-transition and noble metal alloys. Using low-temperature specific-heat experiments as a basis, the superconducting properties were discussed in terms of the electronic specific heat coefficient and the Debye temperature in comparison with other metallic superconductors.     

Specific heat measurements in superconducting and normal state Ta-Nb-V ternary solid solutions

Specific heat measurements between 2 and 6 K on three bulk isoelectronic Ta _x Nb _y V_1–x–y bcc solid solutions have been performed above and below their superconducting transition temperatures in and out of a 46-kOe field. The behavior of T _c with alloying is well predicted by Miedema's charge-transfer method, which uses measured _D, but which has no adjustable parameters. Miedema's model also predicts correctly. Hopfield's method for T _c fails in these alloys, and probably over the entire V-Nb-Ta system except very near the Nb-Ta line.Work supported in part by a grant from the Ohio University Research Committee No. 503.     

Multifilamentary superconducting (NbTa)-Sn Composite wire by solid-liquid reaction for possible application above 20 tesla

Nb alloyed with Ta was employed in fabricating multifilamentary composite wires of (NbTa)-Sn using the liquid-infiltration process. The superconducting A15 phase was formed with subsequent heat treatments at 800–950°C by the solid-liquid reaction. High inductive T_c's of 18.2K with sharp transition width (<0.3K) and high overall J_c's of ～1.6 × 10⁴ A/cm² at 20T and 4.2K were obtained. It was found that 2 wt.% Ta in the Nb was sufficient in the enhancement of the overall J_c at the high fields and in increasing the H_c2 (4.2K) to 25T.     

Rutherford backscattering investigation of thermally oxidized tantalum on silicon

The Rutherford backscattering technique utilizing 2 MeV He⁺ ions was used for studying Ta and thermally grown Ta oxide films on Si substrates. Significant impurity effects were observed for the as-deposited Ta films and are attributed to gettering during deposition. Partially oxidized Ta films exhibit a surface Ta₂O₅ layer with substantial oxygen incorporation in the underlying Ta film. In contrast with anodic Ta₂O₅ films on tantalum, there is no sharp boundary between Ta and Ta₂O₅. Tantalum oxide films on silicon are, to a first approximation, stoichiometric. Their apparent density, as determined from the areal density of Ta atoms, increases with thickness (from 4.7 to 7.3 g cm^-3) as do their refractive indices. This supports the contention that incorporation of silicon is responsible for these effects and that they are not merely due to a change in stoichiometry.     

The influence of composition on the formation and stability of Ni-Si-B metallic glasses

The influence of composition on the formation and stability of Ni-based glassy alloys containing Si and B has been investigated systematically. Depending on the SiB ratio certain compositions, with total metalloid contents in the range 17 to 49 at% (a wider range than has hitherto been reported for metal-metalloid glasses), have been vitrified by melt-spinning to an average thickness of 17 m. For metalloid concentrations greater than 36 to 40 at% the amorphous phase is brittle in the as-quenched state. The highest crystallization temperatures, T _x, occur at 32 to 38 at% metalloid, the actual value of T _x again depending on the SiB ratio. It is shown that, over a wide composition range, the glass-forming boundary corresponds closely with a value for the reduced crystallization temperature isometric of 0.52. This value corresponds to a critical cooling rate for glass formation of about 10⁶ K sec^–1, predicted from kinetic theories and assuming that T _x is a good estimate of the glass transition temperature, T _g. This agrees quite closely with the cooling rate for approximately 17 m thick tape predicted from thermal studies of the melt-spinning process. Hence, T _x/T _liq usefully describes the glass-forming ability (GFA) for much of the composition range studied, although preliminary results suggest that near the centre of the glass-forming range the GFA may be over-estimated. Substitution of Si by other metalloid elements from Groups IIIb–Vb in Ni₇₈Si₁₀B₁₂ glassy alloy generally decreases the thermal-stability.     

Microstructure and superconducting properties of melt-quenched insoluble Al-Pb and Al-Pb-Bi alloys

In order to obtain aluminium-based superconducting alloys including finely dispersed lead or Pb-Bi particles, the application of the melt-quenching technique has been tried for Al-Pb, Al-Si-Pb and Al-Si-Pb-Bi alloys containing immiscible elements such as lead and bismuth. It has been found to result in the preparation of superconducting materials consisting of fcc Pb or h c p (Pb-Bi) particles dispersed finely and densely in the aluminium-based matrix in each composition range below about 2 at % Pb for Al-Pb alloys and 5 at % Pb or (Pb + Bi) for (Al_0.9 Si_0.1)_100–x Pb _x and (Al_0.9Si_0.1)_100–x (Pb_0.6Bi_0.4) _x alloys. The particle size and interparticle distance were 40 nm and 40 to 100 nm, respectively, within the grains, and 100 nm and below 30 nm, respectively, at the grain boundaries for the lead phase in Al₉₈ Pb₂ alloy. Particle size was 15 to 60 nm and interpartide distance 30 to 60 nm for the Pb- Bi phase in (Al_0.9 Si_0.1)₉₅(Pb_0.6 Bi_0.4)₅ Transition temperature,T _c was 4.16 K for Al₉₈Pb₂, 3.94K for (Al_0.9Si_0.1)₉₅ Pb₅ and 7.75 K for (Al_0.9Si_0.1)₉₅(Pb_0.6Bi_0.4)₅. The upper critical magnetic field,H _c2, and critical current density,J _c, for (Al_0.9Si_0.1)₉₅(Pb_0.6Bi_0.4)₅ were 0.22 T at 4.2 K and 1.67 × 10⁷ Am^–2atzeroappliedheld and 4.2 K. The appearance of the superconductivity for the aluminium -based alloys was interpreted as due to the formation of superconducting percolation path along the tangled dislocations, sub-boundaries and/or grain boundaries where Pb and Pb-Bi phases precipitated preferentially.     

Nonequilibrium phases in rapidly quenched Co-C-Si and Ni-C-Si alloys

It has been found that the rapid quenching of Co-C-Si and Ni-C-Si alloys results in the formation of an amorphous phase in the range above 10 at% C and 12 to 23 at% Si in the Co-C-Si system, and a nonequilibrium ordered b c c phase with a lattice parameter of 0.2744 nm in the range above 4 at% C and 15 to 21 at% Si in the Ni-C-Si system. Since the interaction between cobalt and carbon is repulsive, the glass formation in the high metalloid concentration range in the Co-C-Si system is thought to be attributed to a strongly attractive interaction between metalloid atoms (carbon and silicon). Crystallization temperature and Vickers hardness of the amorphous alloys are in the range of 671 to 708 K and 833 to 942 diamond pyramid number (DPN) respectively. Furthermore, the amorphous alloys exhibit a soft ferromagnetism and the Curie temperature, saturated magnetization under an applied field of 100 Oe, coercive force and permeability at 1 kHz are 395 to 432 K, 4.33 to 5.50 kG, 0.031 to 0.210 Oe and 31 000, respectively, in the as-quenched state. The effective permeability of (Co_0.94Fe_0.06)_67.5C_12.5Si₂₀ amorphous alloy is higher than that of Co₆₇Fe₄Si₁₉B₁₀ amorphous alloy with zero magnetostrain at frequencies above 200 kHz. Accordingly, the Co-C-Si amorphous alloys newly found in the present work are very attractive as a soft ferromagnetic material with good characteristics in the high frequency range.     

Effects of the Ta content on the microstructure and electrical property of reactively sputtered TaxZr1−xN thin films

The Ta_xZr_1−xN films were prepared by reactive magnetron sputtering and the concentration of zirconium and tantalum was regulated by controlling the power to the sputtering guns. The effects of the Ta content on the microstructure, composition and electrical properties of Ta_xZr_1−xN films were investigated by x-ray diffraction, field-emission electron probe micro-analyzer, atomic force microscopy, x-ray photoelectron spectroscopy, four point probe and Hall-effect measurements. Results indicated that the Ta_xZr_1−xN films with different Ta contents were crystallized in NaCl-type structure. However, the lattice constant of Ta_xZr_1−xN films decreased with the increase of the Ta content due to the smaller ionic radius of Ta⁵⁺ comparing with that of Zr⁴⁺. The decreasing lattice constant of Ta_xZr_1−xN films with the Ta content evidenced the successful substitution of Zr with Ta. The electrical resistivity of Ta_xZr_1−xN films showed a minimum value of 78 μΩ cm at Ta content of 3.5 at.% and then increased with the increase of Ta content. Hall measurements indicated that the electrical conduction of films was essentially due to electrons (n-type). And the increase of carrier density and mobility at a Ta content of 3.5 at.%, caused by the extra d valence electron of Ta and the less electron scattering of grain boundaries, was responsible for the further decreasing of resistivity from pure ZrN_x films.