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 MgB₂ 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 BCl₃. 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 CH₄ 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 × 10⁵ A/cm² at 7 T. 4.5 × 10⁴ at 10 T, and 2.2 × 10⁴ A/cm² 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 de- onstrate that, over a wide range of B, log(J_c) linearly decreases with B with a slope -α such that the J_c (B) of any strand can be parameterized in terms of α and its zero-field intercept J_c(B = 0).