多晶MgB2相形成中纳米尺寸硼粉的作用

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Tonset和相转变完成温度TPF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Tonset和TPF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TN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TN= 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.

Effect of Nanometer-Sized B Powder on Phase Formation of Polycrystalline MgB2

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, Tonse, and the completion temperature, TPF,of the phase formation are determined directly duringthe ongoing thermal process. These two temperatures, Tonset and TPF 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, TN, 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 ℃＜TN, 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.