Influence of Strand Design, Boron Type, and Carbon Doping Method on the Transport Properties of Powder-in-Tube [Formula Omitted] Strands

The transport properties of a number of [Formula Omitted] strands have been investigated in terms of their response to strand design, starting B powder choice, and the approach to C doping used. The strands had the following various designs: 1) several chemical barriers were introduced, i.e., Fe and Nb; 2) the strands were encased in various outer sheath materials, i.e., monel, Cu [Formula Omitted] monel, monel [Formula Omitted] glidcop, and Nb [Formula Omitted] monel; 3) the filament counts were varied (1, 18, and 36); and 4) the final strand diameter was varied. In addition, for a subset of the strand designs, several B powder and C-dopant types were investigated. In particular, the following two types of amorphous B powder were used: 1) Moissan-based Tangshan boron from Tangshan Weihao Magnesium Powder Company Ltd., Tangshan, Hebei, China, and 2) SMI boron from Specialty Metals Inc., Huntington, WV, USA, which is produced in a plasma torch by the reduction-by-hydrogen of [Formula Omitted]. The following two approaches to C doping were taken: 1) malic-acid treatment, in which C is introduced into the B powder precursor by the moderate temperature drying out a slurry of B mixed in with a malic acid-toluene solution (during which the malic acid decomposes, leaving C as the only solid residue) before the Mg powder is mixed in, and 2) direct C doping of the SMI-produced B by introducing a known percentage of [Formula Omitted] into the plasma flame. Critical current densities [Formula Omitted] were measured on 1.5-m-long samples at 4.2 K in fields of up to 14 T. Of all the strands measured, the strand doped with SMI-C at a nominal 4 mol% C yielded the highest [Formula Omitted] values, e.g., [Formula Omitted] at 7 T, [Formula Omitted] at 10 T, and [Formula Omitted] at 12 T. The [Formula Omitted]-values are given for all strands at 5 and 10 T, and for a certain set of strands, the magnetic field dependencies of the [Formula Omitted] -values and the influence of C doping is presented. Finally, we demonstrate that, over a wide range of [Formula Omitted], [Formula Omitted] linearly decreases with [Formula Omitted] with a slope [Formula Omitted] such that the [Formula Omitted] of any strand can be parameterized in terms of [Formula Omitted] and its zero-field intercept [Formula Omitted].