A New Generation of In Situ MgB₂ Wires With Improved Jc and Bᵢᵣᵣ Values Obtained by Cold Densification (CHPD)

By means of Cold High Pressure Densification (CHPD), the critical current density,Jc, of binary and alloyed MgB2wires has been enhanced by more than a factor 2 at 4.2 K and at fields up to 19 T. The relative MgB2mass density of binary MgB2wires 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 ofBirr, as a consequence of a slightly enhanced C content, determined by X ray diffraction. Almost isotropicJcvalues were obtained for C4H6O5added wires of 1 × 0.6 mm2cross section, the values ofJc(4.2 K)=1 × 104A/cm2for 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 ofBirr//at 20 K was 11 T. The positive effects of cold densification on JcandBirron MgB2was 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 104A/cm2at 13.1 T. Further improvements are expected after optimization.