Hydrogen Storage Performance Of Li-Mg-Al Based Alloys

Li, Mg, and A1are hydrogen storage materials with high capacity and their binary hydride are stable under ambient conditions. In this thesis, we try to improve the hydrogen storage performance of the Li-Mg-Al based alloys by exploring the preparation process.1. Sintering and subsequently mechanical alloying methods is used to prepare the17Mg/12A1and17Mg/12Al+10X(X=Li, LiH, LiAlH4) solid solution alloys. We studied the phase structure, hydrogen storage properties and thermodynamic performances by XRD, PCT and DSC/TG. According to the measurements, the structure of sintered sample without ball milling is mainly composed of Mg2Al3and Mg17Al12phase. After20h mechanical ball milling, the phase constitution appears Mg-Al solid solution with FCC structure. The results showed that the hydride kinetics of composites prepared by milling with Li, LiH, LiAlH4decreased, while the re/dehydrogenation cycling stability and thermodynamic performance is improved and the initial dehydrogenation temperature decreased. Especially for the samples milling with LiH, the initial dehydrogenation temperature decreased from320℃to290℃, the dehydrogenation activation energy decreased from194.5kJ/mol to146.3 kJ/mol.2. We prepared the LiH(17Mg/12Al)x(x=0.05,0.10,0.15,0.20) composite materials where x is present for the mass ratio. After20h ball milling, there was Li-Mg-Al solid solution phase appeared. And it transforms into MgH2and Al(LiH) after hydrogen absorption. When x=0.15, the hydrogen storage capacity of the composite reach to2.25wt%. The dehydrogenation activation energies, calculated by Kissinger equation, of the LiH (17Mg/12Al)x(x=0.05,0.1,0.15,0.2) composites are121.3kJ/mol,125.3kJ/mol,94.7kJ/mol and118.8kJ/mol, respectively.3. We studied the effect of milling time on the hydrogen storage properties of Li-Mg-Al solid solution. The initial samples was milled for5h,10h,15h, and20h, respectively, which are mixed the15LiH with the sintered17Mg/12Al under high pressure hydrogen atmosphere. Our results showed that the structure of the sintered17Mg/12Al under hydrogen atmosphere is mainly composed of MgH2and Mg17Al12phase. With the milling time increasing, the initial amount of hydrogen desorption increased, the diffraction peaks of MgH2phase weaken gradually, which indicates MgH2merged into Mg17Al12, and the grain size of the composites are14.3nm,12.1nm,9.4nm and5.1nm, respectively. After20h ball milling, all of the MgH2merged into Mg17Al12. The hydrogenation products of the sample after5h milling are MgH2, LiH and Al, while the hydrogenation products of the sample after20h milling are MgH2and Li-Mg-Al solid solution. The initial dehydrogenation temperature of the sample after20h ball milling decreased by15℃, and the dehydrogenation activation energy reduced from104.5kJ/mol to93.3kJ/mol compared with that of the sample after5h ball milling. Compared to the former experiment, the dehydrogenation activation energy of the composites with ball milling under high pressure hydrogen atmosphere are much lower than that of the composites with ball milling under argon atmosphere.