Significant enhancement of the superconducting properties of MgB2 by polyvinyl alcohol additives
We report a systematic study of the effects of polymer addition on the lattice parameters, microstructure and superconducting properties of MgB2. Polyvinyl alcohol [-C2H4O-]n (PVA) as a typical polymer was used as an additive to MgB2. It was found that PVA additions have the following features: (1) the polymer can have a very low oxygen (O) content or even none at all, and PVA has a low O content (C:O = 2:1), which reduces the impurities brought into MgB2 from the doping, and (2) PVA decomposes at a temperature of 400-650 degrees C, which means that the reaction occurs in the same temperature range as MgB2 formation, providing highly reactive C, which homogeneously substitutes for B at this low-temperature range. The above considerations significantly enhance the critical current, J(c), the irreversibility field, H-irr, and the upper critical field, H-c2, of MgB2 compared to un-doped samples or those doped with other carbon sources. In this work, suitable PVA doping levels improved both the connectivity and flux pinning, so that the J(c) of PVA-doped MgB2 was improved over the whole field range.