The size effect of raw materials on the phase formation of polycrystalline MgB2

The effect of the size of the B powder and the Mg powder on MgB2 phase formation has been studied by the technique of in situ high-temperature resistivity (HT-ρ(T)) measurement. Two characteristic temperatures indicating the phase formation, the onset point, Tonset, and the completion point, TPF, were directly determined during the continuous heating process. These two temperatures of the samples synthesized using nanometer-size B powder (Nano-B) and micrometer-size B powder (Micro-B) with nanometer-size Mg (Nano-Mg) are almost the same, i.e. Tonset = 713 K and TPF~763 K, respectively, indicating that the phase formation temperature of MgB2 does not primarily depend on the particle size of the B powder. A similar process was then carried out on Mg powders with the same B powder. An exponential relationship between the onset phase formation temperature and the Mg powder size is clearly shown, which can be fully fitted by a modulated diffusion equation and which gives Q = 421 kJ mol−1 describing the grain boundary activation energy of Mg diffusing into B. We also find that the MgB2 crystallites along the [101] direction have the lowest formation energy, revealed by x-ray diffraction (XRD) analysis on raw Mg powders, and the corresponding MgB2 crystallites aligned along [101] will be formed preferentially in the sintering process. This result is useful for the understanding of MgB2 phase formation in the solid reaction process.