Formation and environmental stability of nanocrystalline and amorphous hydrides in the 2Mg–Fe mixture processed by controlled reactive mechanical alloying (CRMA)

Abstract

The synthesis of the Mg 2FeH 6 complex hydride has been attempted by controlled reactive mechanical alloying (CRMA) of 2Mg–Fe powder mixture under hydrogen using shearing mode in a magneto-mill Uni-Ball-Mill 5. The first batch of 2Mg–Fe powder was initially mechanically pre-alloyed by milling under argon with the addition of small amount of toluene and subsequently reactively milled under hydrogen. The second batch was directly milled under hydrogen. No formation of Mg 2FeH 6 has been observed even after the longest milling duration of 59 h. In the powders reactively alloyed directly under hydrogen some quantity of nanocrystalline β-MgH 2 hydride was formed during the first 10–15 h of milling. However, the X-ray diffraction (XRD) peaks due to β-MgH 2 disappeared after further milling up to 59 h. In addition, the XRD peaks corresponding to the nanocrystalline Mg completely disappeared after 20–24 h of CRMA under hydrogen in both batches of powders while strong but broadened peaks from Fe were still visible. The absorption of hydrogen from the milling vial occurred continuously up to 20–24 h of milling. Energy dispersive spectroscopy (EDS) analysis clearly showed that Mg was still present in the microstructure of powder particles after milling up to 59 h for both batches of 2Mg–Fe powder. These results clearly indicate that amorphous Mg and amorphous hydrides are being formed after prolonged CRMA under shearing mode. It has been shown for the first time that β-MgH 2 undergoes hydrolysis into Mg(OH) 2 when it is exposed for several months to the moisture in the environment. This is associated with abnormally high weight losses (∼10 wt.%) measured by thermogravimetric analysis (TGA) and the appearance of two endothermic differential scanning calorimetry (DSC) peaks related to the desorption of hydrogen from β-MgH 2 and release of water from Mg(OH) 2. Other powders after prolonged milling showed TGA weight losses on the order of 2–4 wt.% and only a single DSC peak, which provides additional strong evidence that amorphous hydrides were formed during CRMA. During DSC tests up to 500 °C the amorphous Mg crystallized into the nanocrystalline Mg phase.