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.