Mechanochemical synthesis of amorphous and crystalline magnesium diboride

Abstract

We have studied the phase composition of materials obtained by mechanochemical processing and subsequent heat treatment of mixtures of magnesium and boron powders in the atomic ratio 1 : 2. Differential dissolution, differential scanning calorimetry, and Xra y diffraction data indicate that, during mechanical processing, some of the magnesium reacts with boron to form amorphous magnesium diboride. During annealing of the activated powder mixture, Xray am orphous magnesium diboride forms at 340°C and crys� tallizes at 480°C. As shown by highresolution transmission electron microscopy, the unreacted crystalline magnesium is covered with an amorphous layer consisting of magnesium diboride and boron. The amorphous material obtained by milling contains nuclei of MgB2 crystallites 3-5 nm in size. During subsequent heating of the activated mixture, magnesium and boron react further to form amorphous magnesium diboride and the amorphous phase crystallizes. Heating of mechanically activated mixtures to just below the crystallization temperature allow MgB2 nanoparticles to be produced. The formation of nanocrystalline magnesium diboride nuclei along with the amorphous phase during mechanochemical processing facilitates mecha� nochemical synthesis compared to thermal synthesis.