Filamentary niobium--tin superconductors fabricated by a powder metallurgy approach

Procedures are described for making superconducting tapes and wires containing filments of the A-15 compound, Nb3Sn, by powder metallurgy techniques. The Nb3Sn filaments, typically 1-5 m thick, are formed by a short heat treatment. The effects of process variables on the microstructure of the tapes and wires were investigated. Microstructural aspects were examined by optical metallography and transmission electron microscopy. A pulsed field technique was used to determine the critical current density, J/sub c/, at 4.20K as a function of transverse magnetic field up to 170 kG. Satisfactory agreement was oserved between these results and data obtained under steady fields of up to 100 kG. The J/sub c/ of the conductors are related to their individual filament morphologies resulting from the different thermo-mechanical histories. The effect of heat treatment conditions on critical current and transition temperature, T/sub c/, is presented. Through process modifications, optimal values or ranges are developed for the metallugical variables in order to maximize the superconducting properties. The inductively measured T/sub c/ of the conductors is about 17.9 +- 0.10. The values of J/sub c/ for the wires computed on the basis of Nb3Sn and niobium matrix are 2.3 x 105 A/cm2 at a transverse pulse field of 50 kG, 1.1 x 105 A/cm2 at 100 kG and 7.5 x 104 A/cm2 at 150 kG