Preparation of Nb3Ge films by chemical transport reaction and their critical properties

Abstract

Niobium-germanium films have been deposited on sapphire substrates at 900 ‡C by a chemical transport reaction method. The highest superconducting transition onset temperature TC,onof 22.4 K is observed for a nearly stoichiometric Nb3Ge film with the A15-type structure (thickness ~ 93.5 Μm). Lattice constants for the Nb3Ge phase formed in the Nb-Ge films with both TC,onabove 22 K and TC,midpointabove 21 K are found to extend from 5.143 to 5.153 ». Deposition rates for the obtained films are in the range of 2-10 Μm/min. Critical current densities for the Nb3Ge film with the highest TC,onvalue are observed to be relatively low (~ 103 A/cm2 at 19 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. Field variations of the pinning forces operating in this film in magnetic fields both parallel to the film surface and perpendicular to the film surface are found to follow closely b1/2(1\s-b)2, to which the pinning force for flux pinning at the surface of normal regions, such as grain boundaries, film surfaces, etc., is proportional, and where b is the reduced magnetic induction (B/BC2).A small increase in JCat low fields is caused by the presence of a small amount of the Nb5Ge3 phase in a Nb3Ge film, and seems attributable to additional flux pinning on Nb5Ge3-phase particles in the film. © 1979 Plenum Publishing Corporation.