Simulation Study Of Hydrogen Adsorption On The α-Al₂O₃（0001） Surface

The alpha-Al₂O₃is a good metal matrix composites reinforced phase which widelyused in the field of materials. It is one of the coating candidate materials for fusion reactorresistance tritium due to its excellent properties as low conductivity, high hardnessproperties, low permeability, structural stability, corrosion resistance, feasibility of theprocess etc. The tritium resistivity of α-Al₂O₃film grown on metal base has beenconfirmed in the experiments. But the resistance mechanism of tritium is not yet clear. Soit is important to study the electronic density of states and hydrogen adsorption which maycontribute to further in-depth understanding of their nature and effects of blocking tritium.In this paper, the surface structure, bonding geometry and electronic structure ofresistance tritium adsorption penetrate materials of α-Al₂O₃（0001） surface have beeninvestigated using density functional theory （DFT） calculations. Meanwhile, We alsoinvestigated the adsorption of H atoms and H2molecules in the clean and defectiveα-Al₂O₃（0001）, and explored the process of adsorption of H atoms and H2molecules inthe α-Al₂O₃coating. The major results obtained are described as follows:（1） The α-Al₂O₃（0001） surface has a greater relaxation and no reconstructionappearing in the hydrogen environment. The O atop site is the energetically mostfavorable adsorption site. Its adsorption energy reaches to6.05eV, and the adsorptionenergy on the α-Al₂O₃（0001） with a pre-adsorbed H atom gets smaller, which indicatesthat α-Al₂O₃has a better blocking performance of hydrogen. H atoms tend to occupy theoriginal location of Al atoms of α-Al₂O₃structure, so that the large surface dipole anddangling bonds of the O-terminated surface can also be compensated by the addition ofhydrogen to the surface. Adsorption of H atoms in α-Al₂O₃（0001） surface is mainlydominated by the coupling and hybridization of H1s orbital and O atom2s,2p orbit.（2） Adsorption of H in α-Al₂O₃（0001） surface reduces the work function. At lowcoverages, the H atoms are more easily absorbed in the O atop site; in the regime of higherH coverages, the H atoms prefer to form atom cluster on the α-Al₂O₃（0001） surface,which prevents the penetration of H.（3） Adsorption of H2molecules on the clean α-Al₂O₃coating surface is physical process, and the adsorption performs as intensive molecular form rather than dissociatedatom form. The adsorption of H2molecules performs in a parallel way while the growthpattern is not, and this can prevent more adsorption of H2molecules on the surface, whichwill contribute to the prevention form H2penetration.（4） After the optimization both O-Al （short） bond and O-Al （long） bond becomeshorter, which result in the downward compression of surface O atoms and the strongeratomic interaction between O atom and adjacent Al atom. There is a repulsion against H2molecules on the O atoms missing α-Al₂O₃（0001） surface. It is not conducive to the H2molecules adsorbed and diffused on the surface of the defect.