Preparation and high-field superconducting properties of vapor-deposited Nb3Sn alloys

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Abstract
Nb3Sn alloyed with Si, Ti, V, Ta, Bi, or Mo has been grown from the vapor phase on stainless‐steel (Hastelloy) ribbon substrates. Compared to a value of 185 kOe observed for unalloyed Nb3Sn, 1.3% Si increased Hc2 to 225 kOe, while 0.002% Bi or 0.05% V raised Hc2 to about 215 kOe. Whereas 0.0006% Ti or 3.7% Ta did not affect Hc2 appreciably, 0.4% Mo decreased Hc2 to 140 kOe. Diffusion of Ni from the stainless‐steel substrate into the Nb3Sn layer appears to be responsible for the observed Hc2 value of 196 kOe for the unalloyed (but carbon doped) ribbon since the same Nb3Sn layer on a Nb substrate has an Hc2 value of 225 kOe. The effect of the Ni outdiffusion on Hc2 can be reduced by reducing the Nb3Sn growth temperature or by increasing the Nb3Sn layer thickness. Hc2 does not appear to correlate directly with Tc since Tc is about 15.5°K for both the Si‐ and Mo‐alloyed Nb3Sn, as well as for the unalloyed Nb3Sn deposited on a Nb substrate, while Hc2 ranges from 140 to 225 kOe. The intermediate phase region between the Nb3Sn layer and the stainless‐steel substrate consists of a rich variety of phases including NbNi, NbNi3, Ni3Sn, Ni3Sn2, and MoNi. Alloying Nb3Sn with Bi, Ti, V, and Mo reduces Jc by a factor of 2 to 4 compared to the unalloyed sample. Ta does not significantly affect Jc and Si significantly increases Jc above 120 kOe. CO2 or C2H6 additions can increase Jc for the alloy ribbons by up to a factor of 8, but not to values greater than observed for unalloyed Nb3Sn doped with carbon. Si alloys doped with carbon are an exception and have higher Jc values above 120 kOe than the carbon‐doped unalloyed ribbon.