  Cold high pressure densification (CHPD) is presented as a new way to
substantially enhance the critical current density of in situ MgB2 wires at 4.2
and 20 K at fields between 5 and 14 T. The results on two binary MgB2 wires and
an alloyed wire with 10 wt.% B4C are presented The strongest enhancement was
measured at 20K, where cold densification at 1.85 GPa on a binary Fe/MgB2 wire
raised both Jcpara and Jcperp by more than 300% at 5T, while Birr was enhanced
by 0.7 T. At 4.2K, the enhancement of Jc was smaller, but still reached 53% at
10 T. After applying pressures up to 6.5 GPa, the mass density dm of the
unreacted (B+Mg) mixture inside the filaments reached 96% of the theoretical
density. After reaction under atmospheric pressure, this corresponds to a
highest mass density df in the MgB2 filaments of 73%. After reaction, the
electrical resistance of wires submitted to cold densification was found to
decrease, reflecting an improved connectivity. A quantitative correlation
between filament mass density and the physical properties was established.
Monofilamentary rectangular wires with aspect ratios a/b < 1.25 based on low
energy ball milled powders exhibited very low anisotropy ratios, Gamma =
Jcpara/Jcperp being < 1.4 at 4.2 K and 10T. The present results can be
generalized to alloyed MgB2 wires, as demonstrated on a wire with B4C
additives. Based on the present data, it follows that cold densification has
the potential of further improving the highest Jcpara and Jcperp values
reported so far for in situ MgB2 tapes and wires with SiC and C additives.
Investigations are under work in our laboratory to determine whether the
densification method CHPD can be applied to longer wire or tape lengths.
