Stabilized in situ rectangular MgB2 wires: the effect of B purity and sheath materials

Abstract

Stabilized four-filament SiC-added insitu MgB2 wires were prepared by the rectangular wire-in-tube (RWIT) technique, usingboron powders of different purity (90% and 99%), SiC powders of differentagglomerate size and sheaths of different metals. The critical current density at 11 Tand 4.2 K improved by 5–10 times by using boron powder of higher purity, SiCpowder of small agglomerate size, a chemically inert Ti barrier and mechanicalsupport provided by a Monel sheath. The decrease in critical temperature(Tc) and increase incritical current density (Jc) of wires could be explained in terms of carbon doping. Formation ofMg2Si and B-rich secondary phases, and carbon substitution was studied using EDXchemical mapping in a scanning electron microscope (SEM). The addition ofSiC results in the formation of B-rich secondary phases. Our results support themechanism of carbon doping through the reaction of SiC with Mg to formMg2Si and release free carbon. However, the extent of effective carbon doping dependson the size of the precursor SiC agglomerates, as large SiC agglomeratesare unable to react with Mg. B precursor powder of lower purity andMg2Si secondary phase formation led to incomplete phase formation ofMgB2 and consequentlyto lowered Jc.