关键词: MgB₂超导体, 相形成, 纳米硼粉

Abstract: The size effect of the raw B powder on the MgB₂ phase formation is studied by the technique of in-situ high temperature resistivity (HT-ρT) measurement. The onset temperature, T_onset, and the completion temperature, T_PF, of the phase formation are determined directly during the ongoing thermal process. These two temperatures, T_onset and T_PF of the sample synthesized using nanometer B and Mg powders (NanoB-MgB₂) are 440 and 490℃, respectively, the same as those of the sample using micrometer B and nanometer Mg powders (MicroB-MgB₂). This indicates that the phase formation temperature of MgB₂ do not depend on the B powder size. The upper limit of the sintering temperature, T_N, above which the sample loses superconductivity, is below 750℃ for NanoB-MgB₂, much lower than 980℃ for the MgB₂ prepared using micron-sized B powder and millimeter sized Mg powder (DM-MgB₂). In comparison with the sample directly sintered at 650℃<T_N, an interesting, irreversible transformation in the crystal structure of the MgB₂ phase is observed with the sample going through the stages of initial sintering at 750℃, then re-sintering at 650℃ in an Mg-rich environment after the processes of regrinding and pressing.

Key words: MgB₂ superconductor, phase formation, nanometer B powder