Evolution of core connectivity in MgB 2 wires and tapes during PIT processing

Abstract

Critical current density, J c, ac susceptibility, χ, and the core microhardness in Ni/MgB 2 wires and tapes were measured in as-deformed state at the various stages of the powder-in-tube process. We found that during a drawing process J c reached a peak at some strain value, followed by rather steep degradation down to zero level with further strain growth. It was shown that the data of electrical and magnetic measurements correlated with the core microhardness during the deformation processes. χ vs. T measurements showed that J c degradation was resulted from the deterioration of a core connectivity, that was confirmed by microhardness measurements. It was revealed that rolling the as-drawn wires restored a core connectivity and thus caused drastic J c, growth. This effect was explained by differences in powder flow between drawing and rolling processes. XRD examination of MgB 2 cores showed that rolling the Ni/MgB 2 tapes resulted in gradual growth of the core c-axis texturing with the tape thickness reduction, though texture degree remained relatively low (21% at maximum).