Speaker
Description
Critical current density (Jc) of in-situ powder-in-tube(PIT) processed MgB2 has still to be improved for practical applications. One of the problems of in-situ PIT MgB2 wires is the low mass density of MgB2 core. The density of MgB2 core of in-situ PIT wire fabricated by the conventional technique is as low as ~50%. It is considered that the microstructure of Mg/B powder cores including the density before a heat treatment in in situ PIT processed wires depends on the mechanical deformation process. We found that a wire fabrication by applying a swaging machine is much effective in increasing the density of Mg/B mixed powders and hence the density of MgB2 core. Vickers hardness of MgB2 core after the heat treatment increased from ~120 for groove rolled and drawn wire to ~250 for swaged wire. MgB2 wire fabrication by swaging machine is also effective in obtaining well developed fiber structure of Mg particle. This leads to the fiber structure of MgB2 in the wire. Furthermore, improved uniformity of MgB2 core along the longitudinal direction is obtained by swaging. These three factors much contribute to the increase of Jc values of in situ PIT MgB2 wires. Highest Jc values of 7-filamentary wires fabricated with 4.5%C-coated B powder and by applying only swaging are ~32,000A/cm2 at 4.2K and 10T. These Jc values are about twice as high as those of the wires fabricated with the combination of groove rolling and drawing.
