Significant efforts can be found throughout the literature to optimize the current-carrying capacity of Nb3Sn superconducting wires. The achievable transport current density in wires depends on the A15 composition, morphology and strain state. The A15 sections in wires contain, due to compositional inhomogeneities resulting from solid-state diffusion A15 formation reactions, a distribution of superconducting properties. The A15 grain size can be different from wire to wire, and is also not necessarily homogeneous across the A15 regions. Strain is always present in composite wires, and the strain state changes as a result of thermal contraction differences and Lorentz forces in magnet systems. To optimize the transport properties, it is thus required to identify how composition, grain size and strain state influence the superconducting properties. This is not possible accurately in inhomogeneous and spatially complex systems such as wires. This article therefore gives an overview of the available literature on simplified, well-defined (quasi-)homogeneous laboratory samples. After more than 50 years of research on superconductivity in Nb3Sn, a significant amount of results are available, but these are scattered over a multitude of publications. Two reviews exist on the basic properties of A15 materials in general, but no specific review for Nb3Sn is available. This article is intended to provide such an overview. It starts with a basic description of the niobium–tin intermetallic. After that, it maps the influence of Sn content on the electron–phonon interaction strength and on the field–temperature phase boundary. The literature on the influence of Cu, Ti and Ta additions will then be summarized briefly. This is followed by a review of the effects of grain size and strain. The article concludes with a summary of the main results.

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A Review of the Properties of Nb3Sn and Their Variation with A15 Composition, Morphology and Strain State

Superconducting magnets have a variety of industrial, medical and research applications. This review discusses the prospects for MgB2 superconductor for practical magnet applications vis-?-vis the intermetallic low (LTS) and high temperature superconductor (HTS) cuprates. It has high TC (39?K), high JC (105?106?A?cm?2 at 4.2?K and in the self-field) and HC2 (15?20?T at 4.2?K) in wire/tape geometry, with great scope for further improvement in the coming years, making it a promising candidate for practical applications. The superconducting properties of MgB2 differ from those of LTS and HTS in many ways. Besides the unusually high TC, MgB2 has a large coherence length, low anisotropy and transparent grain boundaries. The most important difference between MgB2 and other practical superconductors is that it has two superconducting gaps originating from two different bands. Tuning the scattering rates between the two bands improves the superconducting properties and the practical applicability of MgB2. The different methods of fabrication of MgB2 conductors are described and compared. Fabrication of long length MgB2 conductors is relatively easy and less expensive as compared to HTS and the method allows the use of a variety of sheath materials with suitable barriers or reinforcement. The conductors have much better mechanical properties for practical applications. The critical issues and the challenges to be addressed for realization of MgB2 superconductors as the first choice for high field magnet applications are discussed. At present MgB2 is most suited for 20?25?K operation in fields of 1?2?T.

https://iopscience.iop.org/article/10.1088/0953-2048/20/1/R01/pdf

Prospects for MgB2 superconductors for magnet application

Oxford Instruments, Superconducting Technology (OI-ST) produces Nb/sub 3/Sn wire via several "internal Sn" routes. Recently, 12 T, 4.2 K non-Cu critical current density (J/sub c/) values of /spl sim/2900 A/mm/sup 2/ have been achieved by increasing the Nb and Sn fractions of the filament subelements. Similar conductors for high field use have shown engineering current density (J/sub e/) values of 170 A/mm/sup 2/ at 23.5 T, 1.8 K. OI-ST is also involved with research for the High Energy Physics (HEP) National Conductor Program. Results on composites made entirely by hot extrusion are described. Finally, the present status of Ta-Sn powder-in-tube (PIT) and Nb/sub 3/Al precursor strand development are presented. PIT strands have irreversibility fields over 26 T at 4.2 K, while Nb/sub 3/Al precursor strand has been produced by a route that promotes bonding of the billet components.

High field Nb/sub 3/Sn conductor development at Oxford Superconducting Technology

Comprehensive measurements are reported of the critical current density (JC) of internal-tin and bronze-route Nb3Sn superconducting wires as a function of magnetic field (B?23?T), temperature (4.2?K ?T?12?K) and axial strain (?1.6%??I?0.40%). Electric field?temperature characteristics are shown to be equivalent to the standard electric field?current density characteristics to within an experimental uncertainty of ~20?mK, implying that JC can be described using thermodynamic variables. We report a new universal relation between normalized effective upper critical field (BC2*(0)) and strain that is valid over a large strain range for Nb3Sn wires characterized by high upper critical fields. A power-law relation between BC2*(0,?I) and TC*(?I) (the effective critical temperature) is observed with an exponent of ~2.2 for high-upper-critical-field Nb3Sn compared to the value ?3 for binary Nb3Sn. These data are consistent with microscopic theoretical predictions and suggest that uniaxial strain predominantly affects the phononic rather than the electronic properties of the material. The standard Summers scaling law predicts a weaker strain dependence than is observed. We propose a scaling law for JC(B,T,?I) based on microscopic theory and phenomenological scaling that is sufficiently general to describe materials with different impurity scattering rates and electron?phonon coupling strengths. It parametrizes complete datasets with a typical accuracy of ~4%, and provides reasonable predictions for the JC(B,T,?I) surface from partial datasets.

The scaling law for the strain dependence of the critical current density in Nb3Sn superconducting wires

Microstructure, composition and critical current density of superconducting Nb3Sn wires

Superconducting critical temperatures Tc and magnetic fields Hc2, lattice parameters a0, and chemical compositions were measured for ‘‘bulk’’ layers (∼6 μm or greater) of ‘‘pure’’ and alloyed Nb3Sn which were made by the bronze process. The values of Tc, a0, and the composition of pure Nb3Sn layers were ∼18 K, 0.52900±0.00005 nm, and 25±0.5 at. % Sn, respectively, independent of heat‐treatment temperature (between 650–780 °C) and of the bronze composition, as long as the thickness of the layers was greater than ∼6 μm. Small additions of Ti (∼1 at. %) or Ta (∼3 at. %) slightly increased the value of Tc (by ∼0.2–0.4 K) above that for pure Nb3Sn. However, additions of larger amounts of these elements or addition of other transition elements (V, Zr, and Mo) significantly decreased Tc. Also, small additions of these elements significantly increased Hc2. Specifically, the largest values of Hc2 (∼27 T at 4.2 K) were obtained for Nb3Sn layers containing ∼1.5 and ∼4 at. % of Ti and Ta, respectively, compared with ...

Superconducting critical temperatures, critical magnetic fields, lattice parameters, and chemical compositions of ‘‘bulk’’ pure and alloyed Nb3Sn produced by the bronze process

O. F. Kammerer

Papers

Superconducting critical temperatures, critical magnetic fields, lattice parameters, and chemical compositions of ‘‘bulk’’ pure and alloyed Nb3Sn produced by the bronze process