Drawing induced texture and the evolution of superconductive properties with heat treatment time in powder-in-tube in situ processed MgB2 strands

Abstract

Monocore powder-in-tube MgB2 precursor strands were cold-drawn and heat-treated at 600 and 700 °C for times of up to 71 h, and structure–property relationships examined. Drawing induced elongation of the Mg particles led, after heat treatment (HT), to a textured macrostructure consisting of elongated fine polycrystalline MgB2 fibers (or veins) separated by elongated pores. The superconducting transition temperature (Tc), critical current density (Jc) and bulk pinning force density (Fp) were correlated with the macrostructure and grain size. Grain size increased with HT time at both 600 and 700 °C. Critical current density and hence Fp decreased monotonically but not linearly with grain size. Overall, it was observed that at 700 °C the MgB2 reaction was more or less complete after as little as 30 min; at 600 °C full reaction completion did not occur until 72 h into the HT. Transport, Jct(B), was measured in a perpendicular applied field, and the magnetic critical current densities, and , were measured in perpendicular and parallel (axial) applied fields, respectively. Particularly noticeable was the premature drop-off of at fields well below the irreversibility field of Jct(B). This effect is attributed to the fibrous macrostructure and its accompanying anisotropic connectivity. Magnetic measurements with the field directed along the strand axis yielded a critical density, , for current flowing transversely to the strand axis that was less than, and dropped off more rapidly than, Jct(B). In the conventional magnetic measurement, the loop currents that support the magnetization are restricted by the lower of Jct(B) and : in the present case the latter, leading to the premature drop-off of the measured compared to Jct(B) with increasing field. This result is supported by Kramer plots of the and Jct(B) data, which lead to an irreversibility field for transverse current that is very much less than the usual transport-measured longitudinal one, Birr,t.