ISSN 1002-1027　　CN 11-2952/G2

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

AN Ling1, 3, CHENG Chinping1, WANG Bo2, ZHUANG Chenggang1, LI Xingguo2, ZHOU Zengjun1, FENG Qingrong1, 4

• Received:2005-08-10 Online:2006-05-20 Published:2006-05-20

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, Tonset, and the completion temperature, TPF, of the phase formation are determined directly during the 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.