Simultaneous Addition of B4 C+SiC to MgB 2 Wires and Consequences for Jc and Birr

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 B_c2, B_irr and J_c 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 mum 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 760degC, the wire containing 7.5 wt.% B₄C and 2.5 wt.% SiC powders exhibited a Jc value of 1 times 10⁴ 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 J_c value is obtained at 10 T. The slope J_c vs. B for the B₄C + 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 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 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 B₄C + SiC, a strong improvement of J_c and B_irr has been obtained with respect to B₄C 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 d- ifferent mechanisms.