Influence of raw Mg powder shape on behavior of phase formation for Mg-B system

MgB₂ bulks were prepared by using the spherical magnesium powder (Q-Mg) and scrapping magnesium powder (X-Mg) as raw materials. The two kinds of magnesium powder were mixed respectively with amorphous boron powder with a nominal stoichiometry of MgB₂, then pressed into pellets under a pressure of 10 MPa. The pellets were sintered at 650, 750 and 800°C, respectively. XRD results for the sintered samples show that the phase formation rate of Q-Mg series samples is lower than that of X-Mg series samples. SEM observations show that the shape and size of holes are evidently correlated with the shape and size of the raw magnesium powder. DSC results indicate that the solid-solid reaction, Mg melting and liquid-solid reaction during increasing temperature occur in turn for the X-Mg and Q-Mg series samples. However, because of the small surface area, large surface energy and low activity of the surface atom the starting temperature of the solid-solid reaction of Q-Mg sample is 53. 7°C higher than that of X-Mg sample. The results of DSC, XRD and SEM also showed that the starting temperature of the solid-solid reaction increases, the reaction rate between Mg and B decreases at low temperature if the spherical magnesium powder was used as raw materials, but the formation and growth mechanism of MgB₂ crystals are not changed.