Cu-Sn Bronze Research Articles

The critical current density of filamentary Nb<inf>3</inf>Sn as a function of temperature and magnetic field

We have built and developed an apparatus which permits the reliable evaluation of J c (B,T) for helically wound samples of superconductor. This apparatus is used in conjunction with The Oxford Hybrid Magnet which achieves a maximum field of 15.5T. The temperature range over which measurements can be made is 2 to 30K. Results on filamentary NbTi have already been published. We present here results of a highly detailed characterisation of an Nb 3 Sn filamentary conductor produced by the post-winding, high-temperature reaction of a Cu-Sn bronze - Nb composite which contains high purity copper florets and was developed at A.E.R.E. at Harwell. J c has been measured as a function of temperature, in 0.5K increments, from 2K to 18K, at field levels ranging from 0T to 15T in 0.5T increments. We present also an empirical fit to the data. This was made possible by the near linear behaviour of the function over much of the range of practical interest. We believe these results could be of great interest to the designers of superconducting machines and devices.

Properties of CuSn bronze at 4.2 K

Mechanical properties of bronze at 4.2 K were determined as a function of tin content. The 0.02% offset yield strength varied from 76 MPa for pure copper to 241 MPa for an 8.2 w/o Sn bronze. The work-hardening modulus varied correspondingly from 1.9 GPa to 9.6 GPa. Electrical resistivity at 4.2 K was also measured as a function of tin content and found to vary from 1.27 × 10 −8 ohm-cm (pure copper) to 1.3 × 10 −5 ohm-cm (10.7 w/o Sn bronze).

Radiation-enhanced diffusion growth of Nb3Sn monofilaments: Grain-size effects on critical-current density

2-MeV electron irradiation of Nb cores in Cu-Sn bronze matrixes has produced A15 Nb3Sn layers for irradiation temperatures between 290 and 500 °C. Growth of layers at these temperatures (below those for which Nb3Sn is reacted thermally) results in critical-current densities that are higher than those of thermally produced specimens. Analysis of critical-current–vs–applied-field measurements shows that the increased critical-current density is consistent with smaller grain sizes in the lower-temperature enhanced diffusion grown Nb3Sn. Although this is due in part to reduced grain growth at lower temperatures, the possibility of radiation-enhanced grain nucleation is also considered. This is the first demonstration that enhanced diffusion can be used to grow a layer of material at the interface of two dissimilar metals, the new layer being an entirely different crystal structure from the constituents forming the interface.

Critical-current densities of bronze-processed Nb3(Sn1−xGax) wires up to 23.5 T

Gallium additions to the Cu-Sn bronze matrix in the production of Nb3Sn wires by solid-state diffusion raise Hc2(4.2 K) of the resulting compound to ∼25 T. This increase in Hc2, which is not accompanied by any significant increase in Tc, is associated with the incorporation of gallium into the A-15 compound which has the approximate composition Nb3(Sn0.9Ga0.1). Critical currents in a variety of Ga-containing wires, all reacted at 700 °C, have been measured in fields up to 23.5 T. This is the highest field at which steady-state current measurements have yet been made. The pinning force is found to be closely proportional to h1/2 (1−h)2, where h is the reduced field.

Fabrication and properties of multifilament Nb<inf>3</inf>Sn conductors

Multifilament conductors have been fabricated in several configurations employing various combinations of Cu, Sn, Cu-Sn bronze and Nb. These conductors are in the 8-100 mil size range and contain 400-13,000 filaments. Filament sizes range from 8-100 microns in diameter. Heat treatment studies and their effect upon superconducting properties have been performed for temperatures in the range 600-800°C. Sample evaluation has included measurements of T c , resistivity and J c up to 150kG. Mechanical tests to assess allowable bending diameters and metallurgical studies to determine the distribution of constituent elements subsequent to heat treatment have been made. These results will be discussed and compared in the context of optimizing the properties of these conductors for high field magnet applications.