  The size effect of the raw B powder on the MgB2 phase formation has been
studied by the technique of in-situ high temperature resistivity (HT-rT)
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 C and 490 C, 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. On the other hand, the upper limit of the
sintering temperature, TN, above which the sample loses superconductivity, is
below 750 C for NanoB-MgB2, much lower than 980 C 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 C < TN, an interesting, irreversible
transformation in the crystal structure of the MgB2 phase was observed with the
sample going through the stages of initial sintering at 750 C, then
re-sintering at 650 C in an Mg-rich environment after the processes of
regrinding and pressing. Possible explanation of the observed properties is
discussed.
