Mechanically driven crystallization of amorphous MgNi alloy during prolonged milling: applications in Ni–MH batteries

Abstract

The influence of prolonged ball milling on the characteristics of the Mg:Ni (1:1) compound used as hydrogen storage electrode has been studied. Our results show that an amorphous structure is obtained after 10±1(2) h of milling. This optimal period of milling leads to an electrode capacity of 522 mAh/g. Further milling results in a crystallization of amorphous MgNi into nanocrystalline MgNi 2 and Mg 2Ni, which decreases significantly the electrode performance. Experiments carried out at different milling temperatures (20, 60, 100 °C) demonstrate the limited effect of the vial temperature on the crystalline-to-amorphous transition phenomena. In contrast, the milling mode (continuous or discontinuous milling) has a notable influence on the end-product structure. When milling is performed for 10 h followed by a 20-h rest period and milling for 10 h again, material consists essentially of amorphous MgNi which indicates that the crystallization process does not occur in contrast to the case of continuous milling for 20 h. This result indicates that not only the milling duration but also the milling mode have an influence on the end-product structure. Our high energetic milling conditions which accentuate the mechanical deformations during prolonged and continuous milling are believed to be responsible for the mechanically driven crystallization process.