Effect of Ti-Sn co-doping on the superconducting properties of RHQT Nb3Al wires

Abstract

Nb₃Al superconducting wire co-doped with Ti and Sn elements was fabricated by rapid heating quenching, and transformation (RHQT) method. Various amounts of Ti and Sn were doped in the precursor wire, including 0.5%Ti 1% Sn, 1%Ti 1% Sn, 2%Ti 1% Sn with different Ti/Sn ratios, and 0.25% Ti 0.5% Sn, 1%Ti 2% Sn, 2% Ti 4% Sn with fixed Ti/Sn ratio. All the wires consist of Nb₃Al A15 superconducting phase and have a uniform microstructure after the RHQT process. Compared to the pure sample, the J_c of Nb₃Al sample doped with 0.5%Ti 1% Sn increased 52 %, reaching the maximum value of 1.1 × 10⁵A/cm²@4.2 K, 12 T. Other two Nb₃Al samples doped with 0.25% Ti 0.5 % Sn and 1% Ti 2% Sn showed J_c improvement of about 50 % at 10 K, 8 T. The chemical composition of Ti and Sn is highly uniform with the atomic ratio approximating 1:2, which might correspond to the Ti₆Sn₅ intermetallic compound based on the Ti-Sn phase diagram. In the Ti and Sn co-doped samples, the peak position of the normalized flux pinning force shifts to a higher normalized field, indicating the contribution of point pinning centers. Therefore, the improvement of J_c in the Ti and Sn co-doped Nb₃Al samples might be attributed to the formation of nano-sized Ti₆Sn₅ particles in the superconducting matrix. During the rapid heating process, the Ti and Sn atoms were dissolved in the Nb(Al)_ss phase, and then precipitated out during the transformation heat treatment to form nano-sized Ti₆Sn₅ particles in the Nb₃Al phase.