Microstructures and superconducting properties ofin situ V3Ga composite prepared by external diffusion process

A detailed study of the microstructures and superconducting properties of thein situ V₃Ga prepared by external gallium diffusion has been carried out. The structure of the final reacted layer has been found to be divided into two regions: an outer layer composed of multiple connected V₃Ga globules and an inner layer composed of filamentary V₃Ga surrounded by a Cu-Ga matrix. The particle-like V₃Ga in the outer region has a slightly higherT _c and, hence, higherH _c2 than the filamentary V₃Ga in the inner region. Therefore, it should be recognized that the measuredl _c is a combined contribution from both regions having a different magnetic field dependence ofJ _c.l _c at high magnetic field is primarily contributed by the outer region. Thel _c anisotropy of the tape conductor results from the anisotropic shape of filamentary V₃Ga in the inner region. The highest overallJ _c obtained is 2×10⁵ A cm^–2 at 12.5 T. Further optimization based on the microstructural control, which can be achieved by the appropriate design of the process, may yieldin situ V₃Ga composites having substantially improved properties.     

A 12 T 80 mm bore superconducting V3Ga NbTi magnet system

A V₃Ga-NbTi magnet system for a field of 12 T in 80 mm clear bore is described. The V₃Ga and NbTi magnets are connected in series to one power supply. The differences in the materials (NbTi multifilamentary wire and V₃Ga bronze processed stranded cable) require special measures for protection during a quench. In the V₃Ga magnet no training or degradation was observed. The short sample critical current value was reached in the V₃Ga magnet with a reversible active voltage which made it possible to reduce the current without quench.     

Studies on the formation of V3Ga and V3Si superconducting compounds by a new diffusion process

In the present work, diffusion between vanadium and Cu-20 at.% Ga alloy and that between vanadium and Cu-20 at.% Si alloy were studied. An intermetallic compound, V₃Ga, is formed easily by the selective diffusion of gallium from the Cu-Ga alloy to the vanadium. V₅Si₃ and V₃Si are formed also by the selective diffusion of silicon from the Cu-Si alloy to the vanadium. Copper scarcely dissolves either in V₃Ga, V₅Si₃ or V₃Si. Copper is not effective for enhancing the formation of V₃Si unlike the case of V₃Ga. A large super-conducting critical current density, J _c, is obtained in the V₃Ga formed at temperatures below 700° C while a much smaller J _c is obtained in the V₃Si formed at 800° C. Changes in J _c due to the heat-treatment can be interpreted by the grain growth of the compounds.     

Magnetization of superconducting tape in a perpendicular field

The magnetization of a V₃Ga superconducting tape between two successive flux jumps has been experimentally determined by means of search coil measurement in an increasing field perpendicular to the surface. On the basis of these experimental results, an attempt is made to determine theoretically the current induced region as a function of only one parameter across the width, in order to calculate approximately the magnetization.The influence of the superconductor temperature rise on the magnetization is also calculated. The comparison between measured and calculated magnetization shows good agreement except when the current induction front is very close to the tape edge at the beginning of the application of the field.     

Growth kinetics of monofilamentary Nb3Sn and V3Ga synthesized by solid-state diffusion

Studies of growth kinetics of Nb₃Sn and V₃Ga formation have been carried for mono-filamentary composites of niobium and vandium filaments embedded in bronze wires containing varying concentrations of tin and gallium, respectively. The samples are diffusion reacted at different temperatures and for different lengths of time and the thickness and the microstructure of the resulting A-15 layer are investigated using optical and scanning electron microscopy techniques. The results are discussed in the light of the analytical model previously proposed by the present authors and it is shown that while the rate controlling step for the formation of Nb₃Sn is diffusion of tin through the bronze matrix, for V₃Ga it is the diffusion of gallium through the grain boundaries of the compound layer. The data are used to calculate the activation energies for Nb₃Sn and V₃Ga formation.     

Current-carrying capacities of small V3 Ga pancakes

As an intermediate step leading to the production of V₃Ga tapes long enough for magnets of practical size, we have produced by surface diffusion V₃Ga tapes ranging in length from 5 to 100 meters. These were wound into single pancakes and their current-carrying capacities were measured in external magnetic fields up to 180 kOe. Results indicate that it should be possible in the near future, to construct V₃Ga magnets capable of generating fields as high as 180 kOe, which exceeds by at least 30 kOe the maximum generated by an Nb₃Sn magnet. A further study in the area of stability, however, would be helpful.     

Improved superconducting properties of the composite-processed V3Ga wires with changed configuration

Single core V₃Ga superconducting wires have been fabricated into two configurations following the composite process. The first configuration is a normal one in which the composite consists of a pure vanadium core in a Cu-Ga (19 at%) matrix. In the second configuration the same matrix has been used as a core inside a pure vanadium sleeve. Wires have also been fabricated in these two configurations by adding gallium and magnesium elements to pure vanadium and Cu-Ga matrix respectively. A simultaneous addition of gallium to vanadium and magnesium to the Cu-Ga (19 at %) matrix has been found to raise the critical current density, J_c of the V₃Ga appreciably in conformity with earlier results reported on the composite processed V₃Ga tapes using identical contents of additional elements. The configuration-2 leads to a more uniform V₃Ga layer formation and a faster growth rate thus increasing the overall critical current density still further. A value of J_c of ～ 1.6 × 10⁶ at 4.2 K and 9T has been obtained for single core wires prepared in configuration-2 and using magnesium and gallium additional elements.     

Superconducting properties of A15 V3Ga and Nb3Al by precipitation from supersaturated bcc phase

Structurally multiconnected filamentary A15 V₃Ga and Nb₃Al in their respective bcc phases are formed by heat treating as quenched and deformed supersaturated bcc phases. It was found that T_c of these compounds are ～14 and ～12 K V₃Ga and Nb₃Al, respectively, after appropriate heat treatments. The kinetics and morphology of precipitation of these compounds as well as the critical current density are presented.     

Superconductivity,density-of-states models,and specific heat of A15-type compounds V-Ga and V-Si

Accurate low-temperature specific heat measurements, together with structural investigations, have been performed in V-Ga alloys with 18–32% Ga and V-Si alloys with 20–27% Si. Calculations of the temperature dependence of the normal electronic specific heat are presented for different band models. The comparison with experiment suggests that very low degeneracy temperatures are indeed characteristic of both V₃Ga and V₃Si.This work has been supported by the Fonds National Suisse de la Recherche Scientifique.     

Critical data of some A15 type superconductors in transverse fields up to 230 kOe

Critical field curves and quenching curves for the A15 (-W) type compounds Nb₃Sn, V₃Si, and V₃Ga have been measured. The critical field curves have been extrapolated to zero temperature, and the following zero upper critical fields have been found: 245 kOe for Nb₃Sn, 235 kOe for V₃Si, and 208 kOe for V₃Ga. These values lie between the upper and lower limits given by theories for critical fields not taking into account spin-orbit scattering. Critical currents for Nb₃Sn samples with small admixtures of zirconium are higher than for corresponding samples without zirconium. Special samples of Nb₃Sn multiwires 522 have still high critical currents in the high-field region up to 230 kOe.On leave of absence from the University of Giessen.Supported by the U.S. Air Force Office of Scientific Research.     

Martensitic transformations in V3Ga and Nb3Al

It is shown by low-temperature heat capacity measurements using ac technique that martensitic transformation in small samples of V₃Ga and Nb₃A1 can be prevented by covering them nearly completely with copper plating.     

On the symmetry of phosphorous doped ZnSe

The site symmetry of P doped ZnSe is analysed in detail here, as the recent experiments suggest two possible symmetriesT _d andC _3V. The reduction toC _3V is attributed to the presence of natural impurity, Ga. Our calculations based on molecular model and Green’s functions suggest that the symmetryC _3V is possible with ZnSe : P when Jahn Teller distortion of about ～ 0.2 Å (towards one of Zn atom) is assumed. This has been supported by other experiments.     

Pulsed magnetic field losses and critical current densities of V3 Ga and Nb3Sn multifilament superconductors

Pulsed magnetic field loss measurements have been performed on special V₃Ga filament superconductors, which are known to have very high critical current densities, and on a NB₃Sn filament superconductor.The dependence of the hysteresis losses on the diffusion heat treatment parameters for the V₃Ga layer formation is reported. To analyse the results, the hysteresis losses of hollow superconducting filaments in parallel and perpendicular magnetic fields were calculated.From E.J. Kramer's theory of flux pinning we postulate that, for a given material, upper and lower intrinsic limits of the hysteresis losses are due to the flux line shearing effect. A part of the results was presented at the ICEC 7.     

Projected upper limits to the critical current density of superconducting Nb3Ge, Nb3Sn, and V3Ga

Applying a phenomenological theory of flux pinning developed by Kramer,¹ where the ultimate critical current density (J_c) of a superconductor is determined by plastic shearing of the flux lattice, approximate upper limits are put on J_c for Nb₃Ge, Nb₃Sn, and V₃Ga. At 4.2 K and for magnetic fields H < 100 kG, the J_c of V₃Ga is greater than that of either Nb₃Ge or Nb₃Sn and Nb₃Sn has somewhat higher values than Nb₃Ge. Above 200 kG Nb₃Ge has the highest J_c due to its having the highest upper critical field and at 14 K or above it probably has the largest J_c at all field values.     

Review on the fabrication techniques of A-15 superconductors

This Paper reviews the present state-of-the-art of preparing multifilamentary A-15 superconductors. The most common types, Nb₃Sn and V₃Ga, are presently produced by the so-called bronze process. The highest J_c (overall) = 3.5 × 10⁴ cm⁻² (at 15 T and 4.2 K), obtained for bronze processed Nb₃Sn composites through Ti addition, has pushed the useful limit of this material from 12 to 16 T. Similarly a J_c of 1 × 10⁵ A cm⁻² (at 20 T and 4.2 K) for the A-15 V₃Ga has been attained through elemental additions to the core and the bronze matrix. To circumvent the problem of work-hardening of the bronze, several variations of the bronze process such as the internal tin method, the Nb tube method, the ECN method and jelly roll method have also been upgraded to commercial scale. Composites of Nb₃Sn and V₃Ga have been recently produced successfully on a laboratory scale following the so called in situ technique. These composites not only have a superior J_c value but display improved strain tolerance due to the ultrafine nature of the filaments formed in situ. In situ filamentary A-15 composites with high J_c values have also been produced by following the powder metallurgy technique. The infiltration technique has been found useful for producing high field Nb₃(Al, Ge), Nb₃(Al, Si) and Nb₃Sn composite conductors with high ε_irr. Superior materials such as Nb₃Al, Nb₃Ga and Nb₃(Al,Ge) with high J_c performance have been synthesized using the laser beam technique. Nb₃Ge tapes with T_c = 21 K and J_c = 10⁵ A cm⁻² (at 18 T and 4.2 K) have been successfully produced on a laboratory scale by following the CVD technique. Thus, there are several available options from which to choose a technique for fabricating filamentary composites of ubiquitous Nb₃Sn and V₃Ga. New techniques for fabricating superior materials like Nb₃Al, Nb₃Ga, Nb₃Ge and Nb₃(Al, Ge) also seem to be at an advanced stage of development.     

The thermodynamics of A15 compound formation by diffusion from ternary bronzes

The formation of highT _c Nb₃Al, Nb₃Ga, Nb₃Ge and V₃ (Ga, Si) has been attempted by solid state diffusion from ternary bronzes. None of the desired compounds were formed. The results are explained in terms of the thermodynamics of compound formation by solid state diffusion. Diffusion follows two-phase tie-lines; only those phases which lie on tie-line routes, and which are the most stable as defined by the stability index, are able to form. The addition of the third element to the bronze in general has little significant effect. There is no evidence to indicate that highT _c Nb₃Al, Nb₃Ga or Nb₃Ge can be produced by solid state reaction.Work performed under the auspices of the US Department of Energy.     

Multifilamentary stranded compound superconductor

This paper summarizes our research on newly-developed multifilamentary V₃Ga and Nb₃Sn compound superconductors, including their design, processing, and characteristics such as mechanical and superconducting properties, and coil performance.     

Preparation and microwave dielectric properties of multilayer Li1.075Nb0.625Ti0.45O3 co-fired ceramics with aqueous tape casting

Multilayer Li_1.075Nb_0.625Ti_0.45O₃ (LNT) ceramics for low temperature co-fired ceramic (LTCC) application were prepared by aqueous tape casting. The rheological test indicated that the aqueous slurries exhibited a typical shear-thinning behavior without thixotropy suitable for tape casting process. The green tapes with 2 wt% V₂O₅ addition have satisfactory mechanical properties. Scanning electron microscopy studies revealed that the green tapes have a smooth defect-free surface and that the sintered LNT ceramics have a fine grain microstructure with a uniform grain size. Therefore, aqueous tape casting is suitable for the manufacture of high performance multilayer LNT ceramics. Silver inner-electrode was sintered with LNT tapes at 900 °C, and no reaction been observed between LNT ceramic and sliver layers. The addition of V₂O₅ does not induce much degradation on the microwave dielectric properties. In the case of 2 wt% V₂O₅ addition, the ceramics show good microwave dielectric properties of ε _r  = 65, Q × f = 6,350 GHz. It represents that LNT ceramics could be promising for multilayer LTCC application.     

Dependence of Superconducting Critical Temperature of A15-Type V3Ga on Composition

The composition dependence of the superconducting critical temperature T _c has been investigated for A15-type V₃Ga compounds. The T _c is determined by the specific heat and the magnetic flux transmission ratio through the sample and discussed in terms of the powder X-ray diffraction. The long-range order parameter and the lattice parameter of V₃Ga are evaluated quantitatively by the Rietveld method at each composition, which is evaluated exactly by the electron probe microanalysis (EPMA) measurements. The well-ordered homogeneous alloying structures. V _1–x Ga _x compounds are found to have nearly the same T _c as that for the stoichiometric composition between 0.23<0.25.     

Effects of double-doping on physical properties of V2O3-based alloys

The electrical resistivity of several types of (V_1?x?yM_xM′_y)₂O₃ alloys have been investigated, where M and M′ represent Al, Ga, and 3d transition metal elements. It was found that effects previously obtained by incorporation of M = Al, Cr, may be counteracted by use of M′ = Ti or Ga. On the basis of this information 3-dimensional phase diagrams have been sketched out; several sets of critical points have been encountered. The physical characteristics of (Ti_1?yCr_y)₂O₃ have also been determined; the electrical properties of the alloy y = 0.4 and 0.6 are remarkably similar to those of (V_1?xCr_x)₂O₃ with x = 0.02, 0.03.