Hydriding/dehydriding behavior of Mg2CoH5 produced by reactive mechanical milling

Complex Mg2CoH5 hydride was obtained by a combined procedure that included a milling stage of a 2Mg-Co Mixture under argon followed by reactive mechanical alloying (RMA) under hydrogen, both at room temperature. During RMA. MgH2 is produced at short milling times (10h) and Mg2CoH5 (50 wt%) after 90 h. Improvement in the yield and the formation times could be associated with both refinement of microstructure and enhancement of intermixing of Mg-Co during pre-milling stage. DSC Studies of Mg2CoH5 phase produced by RMA show that the starting decomposition temperature is about 205 degrees C. Absorption and desorption PCIs were determined under static (300 degrees C) and dynamic (230-330 degrees C) conditions. An important hysteresis and two plateaus were observed and correlated with formation/decomposition of Mg2CoH5 (high-pressure plateau) and Mg6Co2H11 (low-pressure plateau) hydrides. For comparing hydrogen sorption Kinetics, Mg2CoH5 (65 wt%) was also obtained by a sintering method at 410 degrees C and 6.0 MPa of hydrogen pressure. Absorption was very fast in the temperature range of 150-350 degrees C, independently of synthesis procedure. However, desorption curves showed a better behavior for RMA powders. MgCo was observed after decomposition of Mg2CoH5 under particular thermal treatments, while MgCo2 phase was not detected. The results of this study reinforce the idea that kinetics factors related with atomic mobility play a key role in the formation of Mg-Co intermetallics. (C) 2007 Elsevier B.V. All rights reserved.