First-principles calculations based on density functional theory corrected by Hubbard parameter U （DFT＋U） are applied to the study on the co-adsorption of O2 and H2O molecules to a-U（110） surface. The calculation results show that DFT＋U method with Ueff = 1.5 eV can yield the experimental results of lattice constant and elastic modulus of a-uranium bulk well. Of all 7 low index surfaces of a-uranium, the （001） surface is the most stable with lowest surface energy while the （110） surface possesses the strongest activity with the highest surface energy. The adsorptions of O2 and H2O molecules are investigated separated. The O2 dissociates spontaneously in all initial configurations. For the adsorption of H2O molecule, both molecular and dissociative adsorptionsoccur. Through calculations of co-adsorption, it can be confirmed that the inhibition effect of O2 on the corrosion of uranium by water vapor originates from the preferential adsorption mechanism, while the consumption of H atoms by O atoms exerted little influence on the corrosion of uranium.
We performed density functional theory calculations of O2, CO2, and H2O chemisorption on the UN（001） surface using the generalized gradient approximation and PW91 exchangecorrelation functional at non-spin polarized level with the periodic slab model. Chemisorption energies vs. molecular distance from UN（001） surface were optimized for four symmetrical chemisorption sites. The results showed that the bridge parallel, hollow parallel and bridge hydrogen-up adsorption sites were the most stable site for O2, CO2, and H2O molecular with chemisorption energies of 14.48, 4.492, and 5.85 kJ/mol, respectively. From the point of adsorbent （the UN（001） surface）, interaction of O2 with the UN（001） surface was of the maximum magnitude, then CO2 and H2O, indicating that these interactions were associated with structures of the adsorbate. O2 chemisorption caused N atoms on the surface to migrate into the bulk, however CO2 and H2O had a moderate and negligible effect on the surface, respectively. Calculated electronic density of states demonstrated the electronic charge transfer between s, p orbital in chemisorption molecular and U6d, U5f orbital.
We performed density functional theory calculations of H, C, and O chemisorption on the UN(001) and(111) surfaces using the generalized gradient approximation(GGA) and the Hubbard U parameter and revised Perdew-Burke-Ernzerhof(RPBE) exchange-correlation functional at non-spin polarized level with the periodic slab model. Chemisorption energies vs. distance of molecules from UN(001) and UN(111) surfaces have been optimized for four symmetrical chemisorption sites, respectively. The results show that the Hollow, N-top, and Hollow adsorption sites are the most stable sites for H, C, and O atoms with chemisorption energies of 13.06,25.50 and 27.34 kJ/mol for UN(001) surface, respectively. From the point of adsorbent(UN(001) and UN(111)surfaces in this paper), interaction of O with the chemisorbed surface is of the maximum magnitude, then C and H, which are in agreement with electronegativities of individual atoms. For the UN(001) surface, U-N bond lengths change relatively little(< 9%) as a result of H chemisorption, however C and O chemisorptions result in remarkable changes for U-N bond lengths in interlayer(> 10%). Electronic structure calculations indicate that Bridge position is equivalent with Hollow position, and the most stable chemisorption position for H, C,and O atoms are all Bridge(or Hollow) position for the UN(111) surface. Calculated electronic density of states(DOSs) demonstrate electronic charge transfer between s, p orbitals in chemisorbed atoms and U 6d, 5f orbitals.
To reveal the potential aging mechanism for self-irradiation in Pu-Ga alloy,we choose Au-Ag alloy as its substitutional material in terms of its mass density and lattice structure.As a first step for understanding the microscopic behavior of point defects in Au-Ag alloy,we perform a molecular dynamics(MD)simulation on energetics and diffusion of point defects in Au and Ag metal.Our results indicate that the octahedral self-interstitial atom(SIA)is more stable than the tetrahedral SIA.The stability sequence of point defects for He atom in Au/Ag is:substitutional site>octahedral interstitial site>tetrahedral interstitial site.The He-V cluster(Hen Vm,V denotes vacancy)is the most stable at n=m.For the mono-vacancy diffusion,the MD calculation shows that the first nearest neighbour(1 NN)site is the most favorable site on the basis of the nudged elastic band(NEB)calculation,which is in agreement with previous experimental data.There are two peaks for the second nearest neighbour(2 NN)and the third nearest neighbour(3 NN)diffusion curve in octahedral interstitial site for He atom,indicating that the 2 NN and 3 NN diffusion for octahedral SIA would undergo an intermediate defect structure similar to the 1 NN site.The 3 NN diffusion for the tetrahedral SIA and He atom would undergo an intermediate site in analogy to its initial structure.For diffusion of point defects,the vacancy,SIA,He atom and He-V cluster may have an analogous effect on the diffusion velocity in Ag.
Zhi-Yong LiuBin HeXin QuLi-Bo NiuRu-Song LiFei Wang
