The effect of sequential and continuous high-energy impact mode on the mechano-chemical synthesis of nanostructured complex hydride Mg2FeH6

The effect of sequential and continuous high-energy impact mode in the magneto-mill Uni-Ball-Mill 5 on the mechano-chemical synthesis of nanostructured ternary complex hydride Mg₂FeH₆ was studied by controlled reactive mechanical alloying (CRMA). In the sequential mode the milling vial was periodically opened under a protective gas and samples of the milled powder were extracted for microstructural examination whereas during continuous CRMA the vial was never opened up to 270 h duration. MgO was detected by XRD in sequentially milled powders while no MgO was detected in the continuously milled powder. The abundance of the nanostructured ternary complex hydride Mg₂FeH₆, produced during sequential milling, and estimated from DSC reached 44 wt.% after 188 h, and afterwards it slightly decreased to 42 wt.% after 210 and 270 h. In contrast, the DSC yield of Mg₂FeH₆ after continuous CRMA for 270 h was 57 wt.%. Much higher yield after continuous milling is attributed to the absence of MgO. This behavior provides strong evidence that MgO is a primary factor suppressing formation of Mg₂FeH₆. The DSC hydrogen desorption onset temperatures are close to 200 °C while the desorption peak temperatures for all powders are below 300 °C and the desorption process is completed within the range 10–20 min. Within the investigated nanograin size range of 5–13 nm, the DSC desorption onset and peak temperatures of β-MgH₂ and Mg₂FeH₆ do not exhibit any trend with nanograin (crystallite) size of hydrides. TPD hydrogen desorption peaks from the powders containing a single ternary complex hydride Mg₂FeH₆, are very narrow, which indicates the presence of small but well-crystallized hydride particles. Their narrowness provides good evidence that the phase composition, bulk hydrogen distribution and hydride particle size distribution are very homogeneous. The overall amount of hydrogen desorbed in TPD from single-hydride Mg₂FeH₆ powders is somewhat higher than that observed in DSC and TGA desorption.

The powder milled sequentially for 270 h and desorbed in a Sieverts-type apparatus at 250 and 290 °C, yielded about a half of the hydrogen content obtained during DSC and TGA tests. No desorption of hydrogen was detected in a Sieverts-type apparatus at 250 and 290 °C after 128 and 70 min, respectively, from the powder continuously milled for 270 h. The latter easily desorbed 3.13 and 2.83 wt.% hydrogen in DSC and TGA tests, respectively.