Pulsed electron beam annealing of a15 tape superconductors

Abstract

Thick films of Nb 3Ge and Nb 3Sn on niobium and Hastelloy B tape substrates were e-beam annealed using single 300 ns pulses and deposited energy densities, E p, between 0.5 and 5 J/cm 2. In most cases the energy density thresholds causing melting, E pt agreed reasonably with numerical simulation data: The melted and resolidified layers were microcrystalline A15's when E p E pt > 1.1 and coarse-crystalline A15 at E p E pt ⩾ 1.4 . Melting and resolidification resulted in film crazing/cracking due to the differential thermal contraction. The extent of film damage depended upon the film-substrate mismatch. The critical current density, j c, was accordingly reduced. In the absence of crazing/cracking at E p E pt ⩾ 1 an indication of j c increase in Nb 3Sn was obtained. This increase was attributed to the formation of additional flux pinning centers such as grain boundaries and/or second-phase precipitates.