Closely Spaced Fine Filament Multifilamentary NbTi Strands
A series of papers1–8 showing the advantages of close spacing and matrix alloying for the development of high Jc, fine filament, NbTi materials which have low electrical coupling have appeared in the last seven years. In order to achieve the highest Jc’s, it has been shown that close spacing has many advantages1–3. This, however, leads to proximity coupling which has to be overcome by the addition of alloying elements to the matrix between the filaments4–11. 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 Jc 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 filaments8–11, thus it may obviate the necessity for a Nb barrier layer8 and thus allow an even greater increase in Jc.
KeywordsFilament Size Close Spacing Copper Matrix Fine Filament Precipitation Heat Treatment
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