Simultaneous Addition of B$_{4}$ C$+$SiC to MgB $_{2}$ Wires and Consequences for ${\rm J}_{\rm c}$ and ${\rm B}_{\rm irr}$

The simultaneous addition of various carbon based additives (in the present case B
₄
C + SiC) to Mg and B powders has been introduced as a new concept in view of enhancing the superconducting parameters B
_c2
, B
_irr
and J
_c
values of in situ Fe/MgB
₂
wires. A series of Fe sheathed monofilamentary wires of 1.1 mm diameter with a MgB
₂
core of 600 mum diameter was prepared with various B
₄
C : 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 760degC, the wire containing 7.5 wt.% B
₄
C and 2.5 wt.% SiC powders exhibited a Jc value of 1 times 10
⁴
A/cm
²
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 B
₄
C additions, where the same J
_c
value is obtained at 10 T. The slope J
_c
vs. B for the B
₄
C + SiC wires is steeper than for SiC additives, the J
_c
values at 4.2 K being superior at fields below 9 T. The lattice parameters a of the B
₄
C + SiC added wires exhibit lower values than ternary wires with the same nominal C content, suggesting a higher C content in the MgB
₂
phase. The "disorder" in the MgB
₂
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 Aring and 124 Aring has been determined from the FWHM and the breath values of the (110) and (002) peaks. With the simultaneous introduction of B
₄
C + SiC, a strong improvement of J
_c
and B
_irr
has been obtained with respect to B
₄
C additions. A further enhancement of J
_c
is expected when using different combinations of additives with and without carbon, aiming for a further raise of J
_c
in wires with multiple additives, as a result of the combination of different mechanisms.