Preparation of Mg2Co alloy by mechanical alloying.: Effects of the synthesis conditions on the hydrogenation characteristics

The influence of the synthesis route of Mg2Co hydrogen-storage alloy has been studied. It is shown here that mechanical alloying (MA) allows one to obtain crystallized products after milling for 100 h. It seems to crystallize in a face centered cubic structure (a = 1.153 nm). However, when the milling is continued beyond 100 h the amorphisation of Mg2Co is observed. After 210 h of milling Mg2Co is obtained with a yield of about 80%. Furthermore, when the product is annealed, the face centered cubic structure is maintained but the intensity ratio of the diffraction peak is changed markedly. When the milling time increase, the grain and crystallite size decrease so that the rate of the hydruration increases. However, at the same time, a passivation mechanism should occur so that the maximum hydrogen storage capacity decreases.