High magnetic field properties of CVD-prepared Nb3Ge and Nb3(Ge,X+)

Measurements of the field dependent critical current density, Jc(H), and the upper critical field, Hc2, have been performed on a number of Nb3Ge samples prepared by the chemical vapor deposition (CVD) process. These samples, obtained from up to 20 m lengths of Nb3Ge tape, were deposited on various substrates at temperatures between 835 and 950°C. Flux pinning, provided by the introduction of controlled amounts of second phase precipitate, Nb5Ge3, produced good high field properties with 4.2 K critical current densities on the order of 8.3 × 10⁴A/cm²at 18 T. Measurements of Jc(H) performed on these samples in both liquid helium and hydrogen in fields up to 18.5T have been analyzed in terms of Kramer´s model of flux pinning. Qualitative agreement has been found with the scaling laws predicted by this theory. The effect of ternary additions of Si and Ga on both Hc2and Jc(H) have been investigated as well. Contrary to expectation, the addition of ternaries into the A-15 lattice results in somewhat depressed critical characteristics. The effect of deposition parameters on the high field properties of both Nb3Ge and Nb3(Ge,X) will be discussed.