Microstructure and superconducting properties of Sn–In and Sn–In–Bi alloys as Pb-free superconducting solders

Abstract

In this work the microstructure and superconducting properties of various compositions of binary Sn–In and ternary Sn–In-Bi alloys have been studied as potential replacements for the Pb-based superconducting solders currently used for superconducting joints between technological low temperature superconducting wires. The influence of chemistry and microstructure on the superconducting properties are investigated. Both the Sn–In β and γ phases are stable over a fairly wide range of chemical composition, with the superconducting properties of each phase improving with increasing solute concentration, but the results show that the In-rich β phase has better superconducting properties than the Sn-rich γ phase. For this reason, in both the binary Sn–In and the ternary Sn–In–Bi systems, the best superconducting properties are achieved in alloys with a high volume fraction of β-phase. The ternary alloys generally show better superconducting properties than the binary alloys, with the highest values of 6.9 K, 0.18 T and 1.3 × 108 A m−2 measured for TC, BC2 (at 4.2 K) and JC (at 4.2 K, 0.01 T) respectively in the Sn35In50Bi15 alloy that consists of a matrix of β-phase with fine fibres of γ and BiIn2. The dependence of the pinning force on the reduced magnetic field exhibits a scaling law behaviour with the maximum at B/BC2 ∼ 0.2 indicating that the dominant mechanism is surface normal pinning in these solders. Rapid solidification in liquid nitrogen results in manipulation of the phase chemistry by both increasing the solute content in the majority phase, changing the fraction of each phase and reducing the scale of the microstructure, leading to significant improvements in superconducting performance. However, the quenched alloys age considerably, even at room temperature, resulting in a deterioration in BC2 and JC values over a few days.