Evolution of core connectivity in MgB₂ wires and tapes during PIT processing

Critical current density, J_c, ac susceptibility, χ, and the core microhardness in Ni/MgB₂ 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 MgB₂ 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).