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

Faced with more and more serious environmental pollution and energy crisis,the urgent task facing humanity is to develop non-polluting but also able to regenerate new energies.As a new type of clean energy,Hydrogen energy is one of the most important new energies in the 21st century.Low cost hydrogen production,safe and high-efficient storage and transportation,and large scale utilization techniques have been received much attention.Hydrogen storage is the key to hydrogen energy application.Hydrogen storage materials,which are important carriers during hydrogen storage and transport,can absorb and desorb hydrogen reversibly. Magnesium-based hydrogen storage alloys are considered to be one of the most valuable metal hydrogen storage materials,due to their extremely high hydrogen storage capability,a good platform of absorbing and desorbing hydrogen,and rich natural resources.The first-principles calculation method based on density functional theory is used to investigate the A₂B-type magnesium-based hydrogen storage materials Mg₂Ni.The crystal structure,electron density distribution,density of states(DOS),the formation enthalpy of hydrides of Mg₂Ni and the low temperature phase of Mg₂NiH₄ hydrides were investigated.We also studied the Mg₂Ni alloy substituted by 3d elements(Cr,Mn,Fe,Co,Cu) and Sn atom respectively in order to modify the properties of the magnesium-based hydrogen storage materials Mg₂Ni,and discussed the changes of crystal structure,electron density distribution,density of states(DOS) and the enthalpy of hydrides formation,which was used to assess the results of the modification.The thesis consists of five chapters as follows.In the first chapter,the development of the Mg₂Ni-type magnesium-based hydrogen storage materials on modified experimental and the theoretical studies were investigated during recent years.In the second chapter,the density functional theory was described briefly. In the third chapter,the properties of Mg₂Ni alloy and its hydride LT-Mg₂NiH₄ were researched theoretically based on the First Principle.The density of state(DOS), the enthalpy changes of hydrogenation reaction of Mg₂Ni and Mg₂NiH₄ were studied. Regarding to our calculation results,Ni-Ni shows strong interaction in the direction of a axis in crystal cell of Mg₂Ni.Moreover,the strong interaction between H atom and Ni atom was discovered in Mg₂NiH₄.The enthalpy changes of hydrogenation reaction from Mg₂Ni to LT-Mg₂NiH₄ were also calculated in this chapter.Since theoretical calculation is very close to the experimental data,it will play an important guiding role in practical.In the fourth chapter,3d elements(Cr,Mn,Fe,Co,Cu)were used to substitute Ni atoms in the alloy.And we found the stability of the new substitution hydride alloy with Fe,Co increased and a bit of substitution will play an significant anti-powdering role after several cycles.Hydrogen absorption after substitution with Cr,Mn,Cu, M-Ni,Ni-H bond energy has been decreased,so that the balance pressure of the hydrogen absorption reaction increased in comparison with non-substitution.Also at a large extent the energy of the hydrogen desorption reaction has been decreased.It may be the cause that the stability of the hydride decreased.And so the tempreture of hydrogen desorption decreased.In the fifth chapter,we studied the effects of the modification about properties with Sn element substitution for the Ni atoms in Mg₂Ni alloy.Using Sn element substitution whose radius is larger than Ni element,the interaction between Ni-H atoms is weakened and the resistance of hydrogen desorption decreased.With the H₂ gradually adding into the alloy,the balance pressure increased,while the stability of the hydride also decreased.The bond energy between Sn and Mg elements is stronger than Ni and Mg.A bit of substitution will play a significant anti-powdering role after several cycles.It may be the cause that the stability of the hydride decreased.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 Mg₂NiH₄ hydrides were calculated in theory.And we found the strong interaction between Ni and H atoms in the stable hydride.So we explained the excuse of the difficulties about the low temperature hydrogen desorption in theroy.2.After Ni atoms of hydride alloy substituted by Cr,Mn,Cu,Sn,the temperature of desorption decreased.Fe-Ni,Co-Ni interaction between the atoms have been increased,so a bit of substitution will play an significant anti-powdering role after several cycles.It may be the cause that the stability of the hydride decreased.3.The crystal structure,electron density distribution,density of states(DOS) and the enthalpy of hydrides formation of the hydrides of 3d-substituted and Sn-substituted Mg₂Ni was calculated,and the possibility of 3d-substituted and Sn-substituted to modification of Mg₂Ni was proved in theory.