First Principles Study On The Oxidation Of Titanium, Aluminum, Titanium And Metal Compounds

The dissertation is devoted to the study of oxygen adsorption on metal Ti(0001) and intermetallic compound TiAl(111) surfaces, as well as alloy effect on the adsorption of oxygen on TiAl(111l) surface from first principles. Our calculated results gain a better understanding of the microscopic mechanism of their oxidation and give a theoretical foundation for the further experiment.In the first chapter, the oxidation behavior of titanium and titanium aluminum by experimental investigations are reviewed. In chapter 2, the basic idea and concept of density functional theory (DFT) is introduced. In the following two chapters, our calculated results for the oxidation of titanium and titanium aluminum are described and discussed respectively.On one side, we investigate oxygen adsorption on Ti(0001) surface from fisrt principles. Two kinds of supercell models (a single and a multiple layers adsorption model) are designed to study the preference of occupation sites for oxygen atoms. In the single layer adsorption model, the oxygen adatoms are restricted in the same adsorption layer. In the multiple layers adsorption model, the adsorbed oxygen atoms take the sites in different adsorption layers at the same time. Our calculated results show that the surface face-centered cubic (fcc) sites are the most favorable sites at 0.25 monolayer (ML), however, the system with a half of the oxygen adatoms taking the fcc sites and the other half taking the octahedral sites between the second and third layer Octa(2,3) is the most stable configuration at 0.50 ML. When the oxygen coverage is larger than 1.0 ML, each of the oxygen atoms prefer separately taking the fcc and octahedral sites such as Octa(2,3), Octa(4,5) and etc. in the alternate layers. It can be found that there are repulsive interactions between the adsorbed oxygen atoms when they take the adjacent hollow sites. Our calculated results of the work functions and the total density of states (TDOS) are in good agreement with experimental results of work function change and ultraviolet photoelectron spectroscopy (UPS). It could be concluded that the multiple layers adsorption model must be taken into account for oxygen adsorption at the Ti(0001) surface.On the other side, we study oxygen adsorption on TiAl(111) surface and alloy effect on the adsorption of oxygen on TiAl(111) from first principles. For oxygen adsorption on TiAl(111) surface, the most favorable site is the adsorption site with more Ti atoms as its nearest neighbors on the surface layer at all coverage and binding energy of per oxygen atom increases with increasing of oxygen coverage. When alloying element Zr is added to TiAl(111) surface, it prefers to occupy surface Ti site. And alloying TiAl(111) with Zr on the surface weaken the binding of oxygen relative to that of pure TiAl(111) surface. When alloying element Nb is added to TiAl(111) surface, it prefers to occupy subsurface Ti site. And alloying TiAl(111) with Nb on the subsurface a little weaken the binding of oxygen relative to that of pure TiAl(111) surface. Our calculated results agree well with experimental results.