Progress toward a practical Nb-Ge conductor

Properties of high-T
c
Nb-Ge films deposited by sputtering and by chemical vapor deposition (CVD) have been investigated. Results of sputtering in the presence of controlled levels of O
2
, N
2
, Si, and of reactive sputtering in Ar-GeH
4
, suggest that the high-T
c
A15 phase is impurity- or defect-stabilized. In CVD deposits two tetragonal modifications were found: σ and T2, the latter probably stabilized by C1
2
. High critical current densities, J
c
(H, T) of fine-grained sputtered films are attributed to flux pinning on A15 grain boundaries. In coarse-grained CVD films high self-field J
c
s, 10
⁶
to 10
⁷
A cm
^-2
at T = 4.2 K, are attributed to pinning on dispersed σ-phase. Comparably high J
c
's were also obtained in CVD A15 films doped with impurities. Low field ac losses p (H,T) were correlated with J
c
and coating geometries. The feasibility of fabricating multifilamentary composite conductors by CVD was demonstrated experimentally and a fabrication process for long Nb
3
Ge CVD tapes is being developed.