Controlled mechano-chemical synthesis and properties of nanostructured hydrides in the Mg-Al-H and Mg-B-H systems

Abstract

The present work reports a study of mechano-chemical synthesis (MCS) and mechano-chemical activation synthesis (MCAS) of nanostructured hydrides in the Mg-H, Mg-Al-H and Mg-B-H systems by controlled reactive mechanical alloying/milling (CRMA/CRMM) in the magneto-mill Uni-Ball-Mill 5. Regardless of the hydride systems, the morphologies of milled Mg-H, Mg-Al-H and Mg-B-H powders after a prolonged milling time can be characterized by dramatic particle size refinement and high tendency to form agglomerates. In the Mg-Al-H system, no successful synthesis of magnesium alanate has been achieved by MCS of the nanostructured magnesium alanate using four starting stoichiometric Mg-2Al mixtures. It is hypothesized that Al(Mg) solid solution in the initial stage (~10h) of CRMA and free Al(s) decomposed from solid solution as the milling time increases the initial stage inhibit the reaction to form magnesium alanate. In contrast to an unsuccessful synthesis in MCS process, a successful synthesis of the magnesium alanate and 2NaCl mixture by MCAS has been achieved. DSC and TGA analysis show that the decomposition of magnesium alanate occurs in a two-step reaction at the temperature ranges of 125-180 and 225-340°C. In the Mg-B-H system, when the Mg-2B mixture is made with the oxidized amorphous boron containing B2O3 then after a prolonged MCS time (200h), only nanometric γ- and β- magnesium hydrides are formed. In contrast, oxide-free amorphous boron in the original Mg-2B mixture prompts the formation of a resulting mixture of nanometric MgB2 and an amorphous phase containing hydrogen. Further annealing of the milled Mg-2B mixtures at ~100-400ºC under ~4-4.3 MPa of hydrogen for 20-100h does not result in the formation of ternary magnesium alanate. Alternatively, a powder mixture of 2NaBH4 and MgCl2 is used as a starting material to synthesize Mg(BH4)2 hydride. Amorphous Mg(BH4)2 phase might have been synthesized after MCAS process. However, the formation of Na(Mg)BH4 solid solution might prevent the synthesis of a large amount of Mg(BH4)2 hydride. Once the solid solution is formed, the amount of Mg will be insufficient to form a large amount of Mg(BH4)2 hydride.