Controlled mechano-chemical synthesis of nanostructured ternary complex hydride Mg2FeH6 under low-energy impact mode with and without pre-milling

The nanostructured ternary complex hydride Mg2FeH6 has been successfully synthesized in the present work by controlled reactive mechanical alloying (CRMA) of elemental Mg and Fe powders under low-energy impact mode in the magneto-mill Uni-Ball-Mill 5. The average nanograin (crystallite) size Of Mg2FeH6 is within the range of similar to2-6 nm as estimated by XRD. The highest yield of Mg2FeH6, Close to similar to34 wt.% as determined by XRD and DSC, is observed after 270 h of direct (no pre-milling) CRMA. The formation of MgO during CRMA is identified as the primary factor that greatly hinders the initial formation Of Mg2FeH6. Pre-milling of elemental Mg-Fe mixture can accelerate formation of Mg2FeH6 but the pre-milling time has not yet been optimized in the present work. The persistent presence of unreacted Fe even after CRMA for as long as 370 h is observed in all powders synthesized in this work. DSC hydrogen desorption temperatures obtained in the present work are much lower than those reported by other researchers for the Mg-Fe-H system. These desorption temperatures (onset and peak) are close to or substantially lower than 300degreesC irrespective of whether the desorbing hydride is either beta-MgH2 or Mg2FeH6 although it must be noted that the lowest DSC desorption peak temperature is associated with the hydrogen desorption from Mg2FeH6. Hydrogen desorption peak temperatures determined by thermally programmed desorption (TPD) are, on average, at least 50degreesC lower than those determined by DSC. Within the range of similar to1 to similar to10 nm nanograin (crystallite) size of beta-MgH2 and Mg2FeH6 no systematic correlation is observed between the DSC and TPD desorption peak temperatures of both hydrides and their nanograin sizes. (C) 2004 Elsevier B.V. All rights reserved.