Preparation of Nb sub 3Ge Films by Chemical Transport Reaction and Their Critical Properties

Niobium--germanium films were deposited on sapphire substrates at by a chemical transport reaction method. The highest superconducting transition onset temp. T sub C,on of 22.4 deg K is observed for a nearly stoichiometric Nb sub 3Ge film with the A15-type structure approx 93.5 mu m). Lattice constants for the Nb sub 3Ge phase formed in Nb--Ge films with both T sub C,on > 22 deg K and T sub C,midpoint > 21 deg K are found to extend from 5.143 to Critical current densities for the Nb sub 3Ge film with the highest T sub C,on value are observed to be relatively low ( approx 10 exp 3 A/cm exp 2 at 19 deg K at self-field). This is due to the coarse grain structure of the film or the low density of effectual pinning centers in the film. A small increase in J sub C at low fields is caused the presence of a small amount of the Nb sub 5Ge sub 3 phase in a film, and seems attributable to additional flux pinning on Nb sub 5Ge sub 3-phase particles in the film.25 refs.--AA