We attempted to identify the predominant flux pinning center in in-situ-prepared polycrystalline MgB2 bulk superconductors. From microstructural observations, we succeeded in fabricating MgB2 bulks with systematically controlled grain sizes. It was found that grain size of the starting boron powder, nominal composition of Mg against B, heating temperature and holding time are the four grain-size determinants of MgB2 bulks. The grain size of B powder was found to determine the initial size of the MgB2 grains, while the heating temperature and nominal composition of Mg against B were found to determine the grain growth rate. In all series of samples, MgB2 bulks with smaller grain sizes exhibited higher irreversibility fields and higher pinning forces. We observed a strong relationship between the inverse of mean grain sizes and maximum pinning forces, indicating that grain boundary is the predominant pinning center in undoped MgB2 bulks. Moreover, carbon doping was found to increase the elementary pinning force at the grain boundary. This suggests that it is essential to develop a method to dope carbon into MgB2, without decreasing the density of the grain boundaries.
