Pressure effects on Jc of In Situ and Ex Situ processed MgB2 wires
The effects of Cold High Pressure Densification ( CHPD) on square shaped MgB2 wires are described. Pressures up to 2 GPa are built up by four hard steel anvils acting simultaneously on the wire, thus resulting in a higher mass density in the MgB2 filaments. The effects of CHPD on the critical current density, Jc, are presented for binary and alloyed mono and multifilamentary MgB2 wires prepared by both in situ and ex situ processing. On short alloyed wires, a pressure of 1.5 GPa enhanced Jc by factors of ~2 and ~4 at 4.2 K/8 T and 20 K/4 T, respectively. An automatic machine was constructed allowing a sequential press/release/advance operation on industrial wire lengths. The Jc enhancements obtained with the short sample device on in situ MgB2 wires were confirmed for lengths exceeding 10m. Using an improved anvil geometry, the wall friction was reduced, thus yielding the same Jc, enhancements, but at considerably lower applied pressures: the same value of Jc,max was already obtained at 0.85 GPA, in contrast to 1.3 GPA for the short sample device. A recent device for hot pressing of long MgB2 wires is briefly discussed.