Pore formation in in situ processed MgB2 superconductors

MgB2 bulks were prepared by an in situ process which utilizes the reaction between boron and magnesium powder. The reaction time was fixed at 0.5 h and the temperature was changed from 600 °C to 1000 °C. The density decrease due to pore formation and mass (mainly magnesium) loss during the formation reaction of MgB2 was observed in all samples. In addition to the pore formation, a pellet expansion which can be explained by the outgrowth of MgB2 grains was also observed. Two different mechanisms were adopted to explain the pore formation; Kirkendall pores formed at a temperature below the melting point (m.p.) of magnesium by a difference in the diffusivity between magnesium and boron, and the pores formed at a temperature above the m.p. by melting of magnesium and a capillary movement. The density, Tc and Jc results suggest that the current carrying capacity can be improved by a careful control of the process parameters regarding a pore evolution.