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

摘要： 原料硼粉在MgB₂相形成中的作用已经利用原位高温电阻率(HT-ρT)的测量技术进行了研究。MgB₂相形成中的起始转变温度T_onset和相转变完成温度T_PF在完成了的热处理过程中已直接确定了。用纳米硼粉和镁粉制备MgB₂样品(简称为NanoB-MgB₂)时的T_onset和T_PF这2个温度值分别是440和490℃，类似于用微米硼粉和纳米镁粉制备MgB₂样品(简称为MicroB-MgB₂)的情况。这表明MgB₂相形成的温度并不特别依赖于硼粉的尺寸。另外,烧结温度的上极限T_N是750℃, 在此温度下NanoB-MgB₂样品的超导电性将丧失。此温度远低于用微米硼粉和毫米尺寸的镁粉制备的MgB₂样品(简称为DM-MgB₂)的T_N=980℃。把直接在650℃<T_N，烧结的样品与先在750℃下烧结，之后又再次重磨和处理后，并于富镁的环境中在650℃下再烧结处理的样品相比较，观察到了MgB₂的不可逆的晶体结构转变。

关键词: MgB₂超导体, 相形成, 纳米硼粉