Abstract
Current–voltage characteristics of MgB 2 composite wires made of ex situ process using Cu, Fe and stainless steel (SS) sheaths have been measured at 4.2 K in the self-field and external magnetic field between 2 and 4 T. It was found that the used mode of deformation (drawing, rotary swaging and two-axial rolling) influences the slope of I–V curve due to affected grain connectivity. Final heat treatment at 950 °C/0.5 h improves grain connectivity apparently, which results in I–V characteristic with very high n -exponents. The n -exponents of multi-core wires measured in external field are apparently lower due to a current sharing among the non-uniform filaments surrounded by high resistance matrix.
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