In situ formation process and mechanism of bulk MgB₂ before Mg melting

Abstract

Differential thermal analysis, as the main means of measurement, was used to prepare bulk MgB₂ samples and monitor the sintering reaction process. Combined with microstructure observation by scanning electron microscopy and x-ray diffraction analysis, the formation process of MgB₂ phase at the temperature before Mg melting was summarized. Additionally, a new kinetic analysis (a variant on the Flynn–Wall–Ozawa) method under nonisothermal conditions was used to determine that the reaction between Mg and B powders involves random nucleation followed by an instantaneous growth of nuclei (Avrami–Erofeev equation, n = 2), which can properly explain the in situ formation process of bulk MgB₂ at the temperature before Mg melting. The value of activation energy E and the function of conversion f(α) are obtained independently, and thereby the determination of mechanism function is not affected by the value of E. The values of E decrease from 175.418 to 160.395 kJ mol⁻¹ with the increase of the conversion degrees (α) from 0.1 to 0.8. However, as the conversion degrees approach 0.9, the value of E increases to 222.647 kJ mol⁻¹, and the corresponding pre-exponential factor A is about three orders of magnitude larger than the previous ones.