Influence of Strand Design, Boron Type, and Carbon Doping Method on the Transport Properties of Powder-in-Tube hbox MgB 2 - rm X hbox C rm X Strands

The transport properties of a number of hbox MgB 2 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 + monel, monel + glidcop, and Nb + 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 hbox BCl 3 . 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 hbox CH 4 into the plasma flame. Critical current densities J c 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 J c values, e.g., 1.1 10 5 Unknown character hbox A / cm 2 at 7 T, 4.5 10 4 at 10 T, and 2.2 10 4 Unknown character hbox A / cm 2 at 12 T. The n -values are given for all strands at 5 and 10 T, and for a certain set of strands, the magnetic field dependencies of the n -values and the influence of C doping is presented. Finally, we demonstrate that, over a wide range of B , log ( J c ) linearly decreases with B with a slope - alpha such that the J c ( B ) of any strand can be parameterized in terms of alpha and its zero-field intercept J c ( B = 0 ) .