Phase Equilibria And Kinetic Mechanism Of La-Mg-Ni-H System

There are several hydrogen storage alloys in the La-Mg-Ni system. Somedisadvantages have been found in these alloys, for example, slow kinetics and hightemperature for hydriding/dehydriding of Mg-based alloys, and small hydrogencapacity in the La-Ni-based alloys. A great number of studies were carried out inorder to develop alloys in the La-Mg-Ni system with excellent hydrogen storageproperties, but little achievement was obtained due to lack of theoretical guidance.Therefore, the present work focused on theoretical study. The thermodynamicdatabase of La-Mg-Ni-H system was constructed, and PCT statistic thermodynamicmodel and kinetic model were established to provide the theoretical guidance for thehydrogen storage materials design in the La-Mg-Ni system. Present work consists ofthe following three parts:（1） The existence of La₅Ni₁₉was identified by Rietveld refinement method. Thethermodynamic database of La-Mg-Ni-H system was constructed taking into accountof La₅Ni₁₉,（La, Mg）5Ni19, phase transformation of Mg₂NiH₄and phase relationshipsin LaNi₅-H vertical section. The phase equilibrium relationships in the La-Ni,La-Mg-Ni, Mg-Ni-H and La-Ni-H systems were experimentally validated, by themeans of ICP, HR XRD, HP DSC, BSE/EDS. The experimental results showed thatLa₅Ni₁₉existed at the temperature range of room temperature to1276K andthe invariant reaction,L+LaNi₅La₅Ni₁₉, took place at1276K.LaNi₅, MgNi₂and LaMg₂Ni₉could coexist in one sample in thenonequilibrium state. However, MgNi₂would disappear gradually duringannealing treatment, and the final phases were LaNi₅and LaMg₂Ni₉.LaNi₅H7decomposed at343and379K when the pressure was kept under1and3MPa, respectively.（2） The PCT curve was devided into three stages, i.e. α, α+β and β. A statistic thermodynamic model was dedveloped to describe the PCT curves. The modelassumes the equivalent of hydrogen chemical potential in both the real gas and the Hin metal, takes into the account of the amount of H, the contribution of each unit cellto the total energy, the energy of absorbed hydrogen and interaction energy of H-H.The model was used to analyze the PCT curves of La-Mg-Ni hydrogen alloys, whichshown a good agreement with the experimental data.（3） A new diffusion kinetic model was developed on the basis of Chou modeland with the consideration of R_PB.The effects of temperature and pressure on thekinetic process, the shape of the end of the kinetic curve, and the relationshipbetween hydrogen capacity and R_PBwere discussed. The kinetic model was used todescribe the hydriding reaction of La-Mg-Ni alloys, and agreed well with theexperimental data.