Experimental determination of k-factors for grain boundary analysis of alloyed Nb3Sn superconductor wires

In superconductors the critical current densities are limited by the ability of the pinning centers in the material to pin flux lines. In A15-type superconductor wires, such as Nb3Sn, the pinning centers are the boundaries of A15 grains formed during the reaction heat treatment between the Nb and Sn at 650-750°C. The pinning behavior in these wires usually relates the average grain size and average composition of the A15 phase to the pinning force measured for the same wire. Few publications described the variations of the pinning behavior due to changes in the grain and grain-boundary compositions. In order to better understand the pinning behavior in regular A15 wires or in new wires with artificial pinning centers, the concentrations of elements across the grains and at the grain boundaries must be analyzed. These concentrations change the chemical potential of the region and, consequently, the interaction energy between the flux line lattice and the pinning centers. For our superconductor wires, the Nb3Sn grains could contain Nb, Sn, Cu, and Ta or Ti.