We report relationships between the superconducting properties, microstructure and fabrication conditions in high-density MgB2 bulks fabricated using a premix-powder-in-closed-tube (PICT) method. The microstructure of the MgB2 bulks depends on the crystallinity of the initial B powders; use of crystalline B powders reduces the amount of MgB2 and increases re-sidual B and MgO. The heat treatment condition influences the grain sizes of MgB2 as well as the distribution and grain sizes of MgO. A lower temperature and a shorter duration of heat treatment tend to form fine grains of MgB2 and uniform dispersion of fine MgO particles. These microstructural variations influence the distribution of effective flux pinning centers such as grain boundaries and fine MgO particles. Different types of critical current density behavior as a function of magnetic field were meas-ured.
