Origin of the Bc2enhancement in ternary Nb-Sn phases

Nb-Sn-Ga alloys were chosen as a model system in order to investigate the influence of third element additions on the normal and superconducting properties of alloyed Nb3Sn phases. With respect to compositions formed by the bronze process (with Ga added to the Cu Sn matrix) alloys with concentrations close to the Sn-rich side of the A15 phase boundary were prepared. The analyses based on the GLAG theory shows that inspite of a decrease in the density of states at the Fermi level with increasing Ga content the upper critical field, Bc2, is increased due to a large enhancement of the residual resistivity. However, Ga destabilizes the stoichiometric composition of the A15 phase and the critical temperature decreases for higher Ga contents. Therefore, Bc2maximized at 1 to 1.5at% Ga for reaction temperatures at 700°C. High critical fields up to 31.5 T (at T = 4.2K) have been achieved for higher reaction temperatures, which shift the phase boundary of the A15 phase field towards stoichiometry.