Theoretical Studies On The Effect Of Substituted Mg On The Structure And Property Of Mg₂Ni

Nowadays,the mineral energy resources are used most widely,such as the coal,the petroleum oil,which is assigned to the non-regenerated energy resources.With the developing of the industry and the increasing of the population,these energy resources are disappearing very fast.And the environment is polluted seriously by using the mineral energy resources.The hydrogen is a green ideal secondary energy resources,which combustion products is just water.So 21^st century is called the times of hydrogen resource.But the hydrogen is easy to leak out and explode,so it is difficult to store and transport.So,many researchers are focus on the development of the hydrogen storage materials.A first-principle based on density functional theory and plane wave pseudo-potential (PWP) method were used to research the A₂B-type Mg-based hydrogen storage materials Mg₂Ni.The PW91 function of the generalized gradient approximation (GGA) was employed for the exchange correlation of electrons and the plane wave energy-cutoff is 380ev in the calculation.The crystal structure,electron density distribution,density of states(DOS) and the enthalpy of hydrides formation of Mg₂Ni and low temperature Mg₂NiH₄ hydrides were studied.We also investigated the Mg₂Ni alloy doped by Al and Ag atoms respectively in order to modify the rate of hydrogen evolution reaction of the alloy,and discussed the changes of crystal structure,electron density distribution,density of states(DOS) and the enthalpy of hydrides formation, that was used to assess the results of the modification.The dissertation consists of three parts as follows.Part one,gives the theoretical studies on the properties of Mg₂Ni and LT-Mg₂NiH₄ regarding the crystal construction,the density of states(DOS),the enthalpy changes of hydrogenation process.As a result,Ni-Ni shows strong interaction in the direction of axis a in the crystal cell of Mg₂Ni.Moreover,the strong interaction between H and Ni atoms was discovered in LT-Mg₂NiH₄.And due to the strong interaction mentioned above between H and Ni atoms,[NiH₄]structure unit was formed within LT-Mg₂NiH₄, which leads us to Jasen's calculation result.The enthalpy changes of hydrogenation process from Mg₂Ni to LT-Mg₂NiH₄ were also calculated,the calculation result is -65.07kJ/mol(H₂) which is coincident with the experimental observation perfectly, the relative error is only 0.656%.Part two and part three shows the studies on the modified properties of Al-doped Mg₂Ni and Ag-doped Mg₂Ni respectively.Both of Al and Ag possess relatively high electronegativity.Furthermore,these two chapters compared the properties of both undoped and Al/Ag-doped Mg₂Ni alloy and its undoped and Al/Ag-doped hydrides LT-Mg₂NiH₄ in respect to the crystal construction,the density of state(DOS) and the enthalpy changes of hydrogenation reaction.We fond that both Al-Ni and Ag-Ni showed strong interaction,which leads to the interaction of Ni-H be weakened. Because of the high electronegativity,the outer electrons structure of Ni was changed that weaken the interaction of Ni-H.Then[NiH₃]with tripod-like structure was formed,which led to the decreasing of hydrogen storage capacity.Therefore,the feasibility of modifying the properties of Mg₂Ni alloy was proved by using dopants possessing relatively high electronnegativity.The novel conclusions and ideas of this work are listed as follows:1.The crystal structure,electron density distribution,density of states(DOS) and the enthalpy of hydrides formation of Mg₂Ni and low temperature phase of Mg₂NiH₄ were calculated in theory,and we also found that[NiH₄]structure unit was formed within LT-Mg₂NiH₄ crystal cell,which led to the high stability of LT-Mg₂NiH₄.The reason of difficulty of the dehydrogenation was explained in theory.2.It explained that the properties changes of Al-doped and Ag-doped Mg₂Ni by the calculating in theory.We also shows the new properties of the new alloy of Al-doped and Ag-doped Mg₂Ni in this paper.3.The crystal structure,electron density distribution,density of states(DOS) and the enthalpy change of hydrides formation of Al-dope and Ag-dope Mg₂Ni was calculated, and the possibility,of Al-doping and Ag-doping to modification of Mg₂Ni was proved in theory.