  The simultaneous addition of various Carbon based additives (in the present
case B4C + SiC) to Mg and B powders has been introduced as a new concept in
view of enhancing the superconducting parameters Bc2, Birr and Jc values of in
situ Fe/MgB2 wires. A series of Fe sheathed monofilamentary wires of 1.1 mm
diameter with a MgB2 core of 600 microm diameter was prepared with various
B4C:SiC ratios, the relation being 2.5:7.5, 5:5, 7.5:2.5 and 7.5:7.5 (values in
wt. %). After reaction of 1 hour at 760 C, the wire containing 7.5 wt.% B4C and
2.5 wt.% SiC powders exhibited a Jc value of 1x104 A/cm2 at 11.3 T and 4.2 K.
Although only 2.5 wt.% SiC were added, these values are considerably above
those of ternary wires with B4C additions, where the same Jc value is obtained
at 10T. The slope Jc vs. B for the B4C + SiC wires is steeper than for SiC
additives, the Jc values at 4.2 K being superior at fields below 9 T. The
lattice parameters a of the B4C + SiC added wires exhibit lower values than
ternary wires with the same nominal C content, suggesting a higher C content in
the MgB2 phase. The disorder in the MgB2 structure has been characterized as a
partial substitution of B by Carbon. A reduction of the domain sizes in the c
direction as well as in-plane to 250 Angstrom and 124 Angstrom has been
determined from the FWHM and the breath values of the (110) and (002) peaks.
With the simultaneous introduction of B4C + SiC, a strong improvement of Jc and
Birr has been obtained with respect to B4C additions. A further enhancement of
Jc is expected when using different combinations of additives with and without
Carbon, aiming for a further raise of Jc in wires with multiple additives, as a
result of the combination of different mechanisms.
