Flux pinning properties of undoped and C-doped MgB2 bulks with controlled grain sizes

Abstract The predominant pinning mechanism of undoped and C-doped MgB 2 polycrystals was investigated in relation to their microstructure. Grain sizes of in situ MgB 2 bulks were systematically controlled by changing grain sizes of starting boron powder, nominal composition of Mg against B, and heating conditions. We observed a strong reciprocal relationship between grain sizes and maximum pinning forces, indicating that grain boundary is the predominant pinning centre in MgB 2 bulks. C-doping to MgB 2 was confirmed to enhance maximum pinning forces, especially in the samples with smaller grain sizes. This implies that C-doping strengthened the elementary pinning force at grain boundaries, rather than introducing intragrain precipitates as additional pinning centres.