| Hydrogen storage capacity of magnesium alloys is very high,with its gravimetric capacity up to 7.6 wt%,volumetric density of 110g/L,and the energy density of up to 9MJ/kg.Magnesium is abundant in nature and can be obtained by extraction of rocks and water,which is low cost.So,it is expected to become the future commercial hydrogen storage materials.Experimental results show that there is higher charge density at the interface of Mg and transition metal models,indicating a stronger hydrogen storage capacity at interface.Therefore,it is vital for hydrogen storage materials to study the properties of the interface of Mg with transition metals.In the first chapter,we introduce the general situation of interface and the lattice mismatch at interface,as well as the magnesium materials and its hydrogen storage properties,and the latest researches on the interface of Mg with transition metals are also discussed.In the second chapter,the difficulty of interface modeling,as well as various approaches,including our approach in this thesis,is discussed.Additionally,the first principles method based on density functional theory,the relevant calculation software VASP and the transition state theory are briefly introduced.In the third chapter,using the method of first-principles calculation,we have done the systematic study for the interfacial stability and electronic properties of magnesium and X?X=nickel,palladium,titanium,niobium?,as well as the interface atomic mixing effect of these systems.With minimizing the mismatch of lattice constant,cell area and cell shape,we can calculate the optimal structure of Mg and X interface.For lattice match of the two phases,enlargement cell method and interpolation method are used to get the common lattice constant,while translation of atom layers is considered to find optimal relative position of interfacial atoms.The charge transfer condition at interface is closely connected with the interface atomic interaction.Compared with immiscible materials,interface of Mg and miscible materials is more stable.Through analyzing the partial density of states and charge density difference,we can find the factors influencing the stability of interface intuitively.The result shows that the interfaces of Mg and miscible materials are more stable than immiscible materials.The optimal matching ratios of Mg-Ni,Mg-Pd,Mg-Ti and Mg-Nb interfaces are4:7,3:4,7:9 and 9:4 respectively and the corresponding interfacial energies are-0.01 J/m2,-0.9 J/m2,0.5 J/m2 and 0.9 J/m2.Through cell search and match program,excluding the conditions of a=b,we find that the interfacial energy of Mg-Ti interface with 7:8 matching ratio is lower,0.4 J/m2.In the fourth chapter,we have done the systematic study for the interface atomic mixing effect of these interface models of magnesium and X.We find that the strain effect of interface is affected by the interfacial atomic mixing effect,which can change the strain level and the chemical interaction.As for large mismatch of Mg-Ni interface,16.7%atomic mixing at interface can reduce the strain from-5.52%to-0.1%.Also,25%atomic mixing of large mismatch Mg-Pd interface can lower the interfacial strain from-6.2%to-0.03%.Based on previous studies of the interface of magnesium and other transition metals,we selected Pd,which is miscible with Mg,to form the magnesium-palladium interface,and study the diffusion effect and its barrier.With the assistant of vacancies at interface,the barrier of atom diffusion barrier is about 0.94 eV,while the diffusion coefficient can reach 10-12 m2/s when the temperature is up to 590K. |
PDF Full Text Request