Effects of B 4 C doping on critical current properties of MgB 2 superconductor

The relationships between microstructures, crystallinity and critical current properties for B 4 C doped MgB 2 bulks, MgB 2−5 x (B 4 C) x with x = 0, 0.04, 0.1, 0.2 and 0.4, were systematically studied. Decreases of T c and the a -axis length and deterioration of the crystallinity of MgB 2 due to the carbon substitution occurred upon B 4 C doping. Substantially improved J c was observed in the B 4 C doped MgB 2 bulks especially at 5 K under high magnetic fields. Strengthened flux pinning at grain boundaries by carbon substitution was considered to contribute to the improvement of J c . Furthermore, the reactivity of B 4 C with magnesium and boron was found to be much higher than that of graphite. These findings suggested that B 4 C is a promising carbon source dopant for MgB 2 materials with excellent J c properties, particularly under magnetic fields.