A New Generation of In Situ ${\rm MgB}_{2}$ Wires With Improved $J_{\rm c}$ and $B_{\rm irr}$ Values Obtained by Cold Densification (CHPD)

Abstract

By means of Cold High Pressure Densification (CHPD), the critical current density, Jc, of binary and alloyed MgB2 wires has been enhanced by more than a factor 2 at 4.2 K and at fields up to 19 T. The relative MgB2 mass density of binary MgB2 wires was enhanced to ~ 54% after applying 2.5 GPa at 300 K before reaction. In C4H6O5 (malic acid) alloyed wires, densification also caused the enhancement of Birr, as a consequence of a slightly enhanced C content, determined by X ray diffraction. Almost isotropic Jc values were obtained for C4H6O5 added wires of 1 × 0.6 mm2 cross section, the values of Jc(4.2 K)=1 × 104 A/cm2 for parallel and perpendicular fields being obtained at 13.8 and 13.4 T, respectively (1 μV/cm criterion). The corresponding values for 20 K were both close to 6.2 T. The value of Birr// at 20 K was 11 T. The positive effects of cold densification on Jc and Birr on MgB2 was also observed on 150 mm long wires alloyed with C4H6O5 (malic acid) or with SiC, by the succession of 6 overlapping pressure steps. This process can be extended to long wire lengths: by means of a newly developed prototype machine with an automatic press/release/advance sequence, a first wire length of 1 m was densified at 1.5 GPa, yielding Jc(4.2 K) = 1 x 104 A/cm2 at 13.1 T. Further improvements are expected after optimization.