Studies on the composite processed Nb-Hf/Cu-Sn-Ga high-field superconductors

The effect of hafnium addition to the niobium core, and that of gallium addition to the matrix, on the superconducting properties of the composite processed Nb
3
Sn have been studied. The stress effects on the Nb-Hf/Cu-Sn-Ga composites, and the effects of external diffusion of gallium into the Nb-Hf/Cu-Sn composites have been also studied. The composites consisting of pure niobium, Nb-2Hf and Nb-5Hf cores, and Cu-7Sn, Cu- 5Sn-4Ga and Cu-3Sn-9Ga matrices were fabricated into single-core tapes and 19-core wires. The growth rate of the Nb
3
Sn layer is greatly increased by the hafnium addition to the core. The hafnium addition to the core as well as the gallium addition to the matrix increases T
c
by 0.4-0.5 K and H
c2
(4.2k) by 2.0-4.0 T. The simultaneous addition of hafnium to the core and gallium to the matrix increases T
c
by 0.6-0.9 K and H
c2
(4.2K) by 6.0- 7.0 T. J
c
's of over 1 × 10
⁵
A/cm
²
were obtained at 18 T for the 19-core Nb-5Hf/Cu-5Sn-4Ga and Nb-5Hf/Cu-3Sn-9Ga wires reacted at 700°-750°C for 50-100 hr. The Nb-5Hf/ Cu-5Sn-4Ga composites showed appreciably improved strain dependence of I
c
compared with the Nb/Cu-7Sn composites. Eirr, the strain where the irreversible degradation occurs in I
c
, is increased by the addition of hafnium to the core and gallium to the matrix. The Eirr increases with the volume fraction of the unreacted niobium core. The external diffusion of gallium into the Nb-Hf/Cu-Sn composites enhances the Nb
3
Sn layer growth and improves the J
c
of Nb
3
Sn. The most appropriate amount of external gallium addition is about 5 at.%.