Controlled mechano-chemical synthesis and properties of nanostructured hydrides in the magnesium-aluminum-hydrogen and magnesium-boron-hydrogen

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. Structural and desorption properties of the milled powders are examined by X-ray diffraction (XRD), scanning electron microscopy (SEM), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA) and desorption test in a Sieverts-type apparatus.; 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.; The effect of nanograin size and particle size on the DSC desorption temperature of synthesized MgH2 hydride has been studied. A profound effect of the particle size of synthesized MgH2 hydride on its hydrogen desorption DSC temperature has been found. The nanograin (crystallite) size of MgH2 does not seem to have apparent effect on the DSC desorption temperature. The role of both the particle size and the duality of hydride phase in the desorption process is discussed. The nanostructured MgH 2 obtained by CRMM releases hydrogen at ∼332°C, which is ∼60°C lower as compared to the non-milled commercial MgH2 (Tego MagnanRTM) prepared by current manufacturing practices. In the Mg-Al-H system, no successful synthesis of Mg(AlH4) 2 has been achieved by MCS of the nanostructured Mg(AlH4) 2 complex hydride using four starting stoichiometric Mg-2Al mixtures. It is hypothesized that Al(Mg) solid solution in the iv 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 Mg(AlH4) 2.; After heating the milled Mg-Al-H powder, the MgH2 hydride in nanometric mixture with Al which does not react with Mg and hydrogen to form Mg(AlH4)2 in the milled Mg-Al-H exhibits a hydrogen desorption temperature at the same range to those of the nanometric MgH 2 with no Al, produced by CRMM.; In contrast to an unsuccessful synthesis in MCS process, a successful synthesis of the Mg(AlH4)2 and 2NaCl mixture by MCAS has been achieved. DSC and TGA analysis show that the decomposition of Mg(AlH 4)2 occurs in a two-step reaction at the temperature ranges of 125-180 and 225-340°C. The MgH2 in nanometric mixture with Al which is obtained after the decomposition of Mg(AlH4)2 releases hydrogen at ∼270°C, which is ∼80°C less than that of the nanometric MgH2 with no Al, produced by CRMM. 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 gamma- and beta-MgH2 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 ∼100-400°C under ∼4-4.3 MPa of hydrogen for 20-100h does not result in the formation of ternary Mg(BH4)2.; After heating the long-term milled Mg-B-H powder prepared using oxide-free amorphous boron, the MgH2 in the nanometric mixture with MgB 2 does not exhibit lowered desorption temperature, as compared to nanometric MgH2 existing without the presence of second phase. Alternatively, a powder mixture of 2NaBH4 and MgCl2 is used as a starting material to synthesize Mg(BH4)2 hydride. Amorphous Mg(BH 4)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.