The critical current density in industrial Nb 3 Sn and MgB 2 wires is currently optimized by introducing various kinds of additives, either Ta and/or Ti for Nb 3 Sn wires or SiC or C for MgB 2 wires. In the following, several problems linked to the presence of additives in the two classes of compounds are discussed.A reinvestigation of the site occupancy of Ta and Ti additives in Nb 3 Sn wires shows that the Ta atoms occupy the 6c chain sites, while the Ti atoms are located on the cubic 2a sites. It follows that in perfectly ordered A15 compounds A 1−β B β , the relation ρ o versus β exhibits a 'universal' behavior: the effect of the chemical nature of the constituents on ρ o is negligible. The slopes of ρ 0 versus the Ti, Ga and Ni contents in the A15 layer coincide and are much steeper than for the Ta additive, corresponding to the three times higher number of 6c sites with respect to 2a A15 lattice sites.The presence of two grain morphologies, e.g. equiaxial and columnar, is observed in Nb 3 Sn wires produced by the bronze route only. The nonlinearity of the Kramer plot in multifilamentary Nb 3 Sn bronze route wires is explained by the presence of these two different grain types, which have distinctly different Sn contents and sizes. For these wires, the total pinning force can be represented as the superposition of two contributions with different scaling fields.Simultaneous addition of different additives on 'in situ' Fe/MgB 2 wires is presented as an attempt to combine different possible mechanisms influencing J c . The substitution of boron by carbon is known to enhance the value of ρ o and thus of the critical field. In addition, the pinning behavior is expected to be improved by grain boundary effects or nanosize precipitations, caused by the presence of appropriate additives during the MgB 2 phase formation. Since the two mechanisms are independent, their effect on J c is expected to be cumulative. In the present paper, the results on the additive combination B 4 C + LaB 6 in monofilamentary Fe sheathed MgB 2 wires are reported. The data are compared with the additives B 4 C + SiC and show that simultaneous additives could be promising in view of applications at 20 K.

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 µm 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 760°C, the wire containing 7.5 wt.% B 4 C and 2.5 wt.% SiC powders exhibited a J c value of 1x10 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 10T. 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 Å and 124 Å 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.