電子ビーム照射により作製したNb<SUB>3</SUB>AlおよびNb<SUB>3</SUB>(Al, Ge) 複合テープの組織と超電導特性

Abstract

Nb3Al and Nb3(Al, Ge) composite tapes with excellent superconducting properties were fabricated by the continuous electron beam irradiation method. Nb-25 at%Al and Nb-20 at%Al-5 at%Ge tapes were prepared by the powder metallurgy process using Nb-sheath. Electron beam irradiation was applied to the tape surface in a vacuum at a tape velocity of 6 m/min. The acceleration voltage, current and diameter of the electron beam were 20 kV, 5-30 mA and 0.5-3 mm, respectively. As the power density was high and the irradiation time was short, the tapes were heated and cooled much faster than a tape heat-treated by a conventional method. As a result stoichiometric A-15Nb3Al and Nb3(Al, Ge) compounds were formed without any excess grain coarsening. Tc of the as-irradiated tape was 16-18 K, which was increased by ∼2 K by a subsequent annealing at 973 K for 360 ks (100 h). The maximum Tc(onset) values obtained were 18.5 K for the Nb-Al tape and 20.2 K for the Nb-Al-Ge tape. Jc increases with increasing power density until the melting occurs and then rapidly decreases at higher power densities, due to the decrease of A-15 volume fraction. The Nb-Al and Nb-Al-Ge tapes processed under proper conditions show excellent Jc-H properties. Jc exceeds 108 A/m2 in the fields up to 25 Tesla for Nb-Al and up to 30 Tesla for Nb-Al-Ge. Jc of Nb-Al-Ge is the largest one so far obtained in the superconducting materials at ∼30 T. The large Jc values in the fields, as well as the easy scale-up procedure, indicate that the continuous electron beam irradiation method is promising for the fabrication of advanced superconductors, e.g. Nb3Al and Nb3(Al, Ge) capable of generating fields over 20 Tesla.