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•Spherical pores form during the formation reaction of MgB2.•The pore formation is remarkable at the temperature where a liquid forms.•The pore formation accompanies by pellet expansion and weight loss of MgB2.•The critical current density (Jc) of MgB2 depends on a reaction temperature.
The pore formation in in situ processed MgB2 bulk superconductors was systematically examined in terms of a reaction temperature and time. Powder compacts of magnesium (Mg) and boron (B) were heat-treated at a temperature below or above the melt point (m. p., 649°C) of Mg for various time periods. As the reaction time increased at each reaction temperatures, the size of the powder compacts increased and the compact weight was decreased. The de-densification (so-called the pellet expansion) phenomena at the temperature above the m. p. of Mg was more remarkable. The density decrease was attributed to the three main factors of the pore formation, the out-growth of MgB2 plates and the Mg evaporation during the formation reaction of MgB2. The microstructure investigation for the initial formation stage of MgB2 at the temperature below the m. p. showed that the pores begun to form at the Mg/matrix interfaces and the spaces that the Mg powders occupied turned into open space (pores) gradually as a reaction time increased. This result indicates that the pores formation at the solid state (temperatures below the m. p.) is attributed to the large difference in diffusivity between Mg and B. In comparison to the solid state, the pore formation at the liquid state (temperatures above the m. p.) was attributed to the melting of Mg powders and the subsequent rapid movement of the melt due to the capillary force. Due to the faster mass transfer through the liquid, the pore formation at temperatures above the m. p. was much faster than that below the m. p. The critical current density of the MgB2 superconductor prepared at 600°C was higher than that at 900°C owing the higher pellet density and the smaller grain size.