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

Stabilized four- filament SiC- added in situ MgB2 wires were prepared by the rectangular wire- in- tube (RWIT) technique, using boron powders of different purity (90% and 99%), SiC powders of different agglomerate size and sheaths of different metals. The critical current density at 11 T and 4.2 K improved by 5 - 10 times by using boron powder of higher purity, SiC powder of small agglomerate size, a chemically inert Ti barrier and mechanical support provided by a Monel sheath. The decrease in critical temperature (T-c) and increase in critical current density (J(c)) of wires could be explained in terms of carbon doping. Formation of Mg2Si and B- rich secondary phases, and carbon substitution was studied using EDX chemical mapping in a scanning electron microscope (SEM). The addition of SiC results in the formation of B- rich secondary phases. Our results support the mechanism of carbon doping through the reaction of SiC with Mg to form Mg2Si and release free carbon. However, the extent of effective carbon doping depends on the size of the precursor SiC agglomerates, as large SiC agglomerates are unable to react with Mg. B precursor powder of lower purity and Mg2Si secondary phase formation led to incomplete phase formation of MgB2 and consequently to lowered J(c).