Closely Spaced Fine Filament Multifilamentary NbTi Strands

Abstract

A series of papers^1–8 showing the advantages of close spacing and matrix alloying for the development of high J_c, fine filament, NbTi materials which have low electrical coupling have appeared in the last seven years. In order to achieve the highest J_c’s, it has been shown that close spacing has many advantages^1–3. This, however, leads to proximity coupling which has to be overcome by the addition of alloying elements to the matrix between the filaments^4–11. Of the three alloying materials normally used for this purpose, Ni, Si and Mn, the most effective is Mn, which operates by a spin flip scattering mechanism whereas Ni and Si produce decoupling by less effective resistive scattering. Ni and Si, however, harden the matrix more than does the small amount of Mn, [0.5wt.%], which has been used in most of the past work on the reduction of proximity coupling. This hardening allows the filaments to be separated to a greater extent than is possible in the case of a pure copper matrix without a significant increase in filament sausaging and a resultant J_c decrease. Silicon also has one additional advantage over the other alloying elements in that it reduces the formation of compounds on the surface of the filaments^8–11, thus it may obviate the necessity for a Nb barrier layer⁸ and thus allow an even greater increase in J_c.