Carbon doping of MgB2 by toluene and malic-acid-in-toluene

The decomposition of malic acid (C₄H₆O₅) in the presence of Mg and B was studied using Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA) which revealed that malic acid reacted with Mg but not B. Also, the addition of toluene (C₇H₈) to dissolve malic acid followed by subsequent drying resulted in no reaction with Mg, indicating that the malic acid had decomposed during the dissolution/drying stage. The total carbon contributed by toluene versus a toluene/5 wt.% malic acid mixture was measured using a LECO CS600 carbon analyzer. The toluene sample contained ～0.4 wt.% C while the toluene/malic acid mixture had ～1.5 wt.% C, demonstrating that the toluene contributed a significant amount of carbon to the final product. Resistivity measurements on powder-in-tube MgB₂ monofilamentary wires established that the toluene/malic acid doped sample had the highest B_c2. However, the toluene-only sample had the highest transport J_c over most of the magnetic field range (0–9 T), equaled only by that of toluene/malic-acid sample in fields above 9 T.