Reaction kinetics of phase formation in Nb-Al powder metallurgy processed wire

The sequence of phases formed in the low-temperature reaction of niobium and aluminium to form the A15 superconductor, Nb₃Al, was studied in high areal reduction powder metallurgy processed wire samples. The reaction path observed was the same as that reported previously for multilayer Nb/Al thin films. The reaction proceeds without the formation of the sigma phase, Nb₂Al resulting in a metastable A15 of higher Al concentration. The extent of this metastable extension is estimated to be 22-24 at.% Al, based on the resistive T_c of 17.0 K observed. The reaction kinetics of the wire samples were examined by scanning heat flux calorimetry, and the activation energy of A15 formation was found to be 3.1 eV. The formation of NbAl₃ and dissolution of Al into the Nb lattice were both observed as precursor reactions with much lower activation energies