Different mechanisms between reactions of soot with gaseous and adsorbed NO2

The reactions of soot with gaseous and adsor- bed NO2 were tested over the K/MgA1O catalyst. After the reaction intermediates were identified by combination of in situ IR characterization and first-principles calculation, the different mechanisms were elucidated. It was found that the reactivity of adsorbed NOa is lower than that of the gas form. The adsorbed NOe reacts with soot in the form of nitrates, leading to the observation of two IR bands at 2,234 and 2,110 cm-1, which are ascribed to the vibration fre- quencies of cyanates on K sites and cyanides on the MgAlO support, respectively. On the contrary, the isocy- anates were confirmed as intermediates in the reaction of soot with gaseous NO2. Because the adsorbed NOe species （i.e., nitrates） are restricted by the electrostatic field of K＋, the cyanates are produced and readily cracked into cya- nides, which transfer to the MgAlO support. The gaseous NO2 favours the production of isocyanates due to their higher stability. The weaker reactivity of adsorbed NO2 at lower temperatures can be attributed to the restriction of the electrostatic field of K＋.     

A first-principles study of displacive β to ω transition in Ti-V alloys

The ground state properties of β and ω phases in Ti-(0–30 at%)V alloys were calculated, and subsequently thermodynamics and energy barriers of the displacive β to ω transition were investigated by first-principles. The results show that the lattice parameters of β and ω phases decreases with increasing V content in Ti-V alloys. The principal lattice strains for the β to ω transition are highly compositional dependent, and the volume variation decreases with increasing V content. The mechanical stability of the ω phase increases initially at the V content around 10 at% and then decreases with increasing V content. Based on the quasiharmonic Debye model, a metastable diffusionless phase diagram has been established, showing that the ω phase is thermodynamically more stable than the β phase at room temperature, anticipating a spontaneous transition from β to ω phases in Ti-V alloys. The calculations of energy pathways indicate that there is an energy barrier during the displacive βto ω transition in Ti-V alloys at temperatures from 100 to 500 K, but not at 0 K.     

Au-Ag合金纳米管填充碳纳米管特性的第一性原理研究

基于密度泛函理论框架下的第一性原理计算,研究了Au-Ag合金纳米管同轴填充不同线径锯齿型(n, 0)碳纳米管所形成复合系统的稳定性、电子特性和力学性能.结果表明,内、外管间距约为4.20?的Au_xAg_(4-x)@(15, 0)复合系统为具有较大填充率的最稳定结构.能带结构分析表明,相对于自由Au-Ag合金纳米管复合系统的量子电导有所增加,电子态密度分析表明复合系统中的传导电子主要来源于内部Au原子和Ag原子的s电子以及外部C原子的p电子.相对于自由金属纳米管而言,碳纳米管的包裹使得金属纳米管的轴向拉伸临界应变和理想强度大大增加,有效地提高了其力学性能.     

Ab-initio investigation for the microscopic thermodynamics and kinetics of martensitic transformation

Martensitic transformation(MT) path in Fe-based alloys remains uncertain in spite of decades of research. Lots of relevant works, which mostly focused on the simplest Bain path, have been completed by the state-of-the-art firstprinciples methods based on density functional theory. In this work, the element effects on microscopic thermodynamics and kinetics of the MT are investigated in terms of two different paths, including the well-known shear dominated Bogers-Burgers path and a recently proposed shuffle dominated path. Adding common elements, e.g. Ni, Mn, Cr, Si and C, into steel could induce different kinetics of the MT, as reflected by the energy barrier, which further determines the physically realistic MT path for different alloys. Furthermore, the addition of 3 d transition metals may adjust the transformation equilibrium between the tetragonal α martensite and the hexagonal ε martensite, as reflected by their inverse effect on the energy barrier for the α MT and the stacking fault energy relating to the ε formation. An expected correlation between the thermodynamic driving force and the kinetic energy barrier for the MT is confirmed in binary Fe–Ni and Fe–Cr systems with changing the concentration of the alloying elements. These two paths for the MT are elaborately compared and discussed to reveal the nature of atomic movement in the MT, dominated by shear or shuffle.     

CrN电子和光学性质的第一性原理研究

本文采用平面波赝势密度泛函理论方法研究了CrN平衡态的结构,电子和光学性质,并与现有的结构数据实验值和理论值进行比较,结果符合较好.计算得到了CrN能带结构、态密度随电子能量的变化关系以及复介电函数、折射率、吸收系数、反射系数和损失系数随光子能量的变化关系,并对其变化特征进行了讨论.     

Periodic continuum solvation model integrated with first-principles calculations for solid surfaces

The structural and reactivity properties of the solute in aqueous solution are vital in chemistry and condensed matter physics. In order to understand the detailed adsorption structures and reaction processes on the solid surfaces or liquid/solid interfaces, the solvation effect should be considered in the realistic first-principle calculation. In this work, a periodic continuum solvation model (PCSM) was implemented into a mixed Gaussian and plane-wave density functional theory (DFT) program, CP2K/Quickstep. The reliability of such approach is carefully examined for several typical systems. The results exhibit that the current application can give the accurate solvation energy and reaction pathway for the clusters compared with the available theoretical and experimental ones. We further extend the application to water adsorption on the periodic slab systems, such as metal and semiconductor surface systems. The results reveal that both the adsorption structures and reaction processes are significantly affected by the solvation effect.     

First-principles Study of the Au surfactant on the growth of Zn vacancies in ZnO nanostructures

Influence of Au surfactant on the growth of Zn atom vacancies in ZnO nanostructures has been investigated by using first-principles slab calculations based on density functional theory.The adsorption of Au atoms on the Zn-terminated (0001)polar surface with a (2×2)surface unit cell is studied by using a standard supercell model.It is found that (1)the binding energies of Au atoms on (0001)-Zn increase and the most stable position of the Au atom is invariable; (2)on the (0001)surface,the preferred sites for Zn atom vacancy are on the first layer of Zn atoms; (3)Under the Au surfactant,the Zn atom vacancies become more difficult to form.     

高压下Si的结构转变与弹性性质的第一性原理计算

利用基于密度泛函的第一性原理,计算了在压力作用下Si的结构相变和弹性性质。晶胞总能的计算结果表明,Si在11.7 GPa压力下发生了从立方金刚石结构（Si-Ⅰ）到四方结构（Si-Ⅱ）的转变。能带结构和态密度的计算结果显示,Si-Ⅰ是具有间接带隙的半导体,带隙为0.71 eV,Si-Ⅱ呈现金属的能带结构特性;Si-Ⅰ和Si-Ⅱ费米面附近的能带结构主要来自于2p电子的贡献。高压下Si-Ⅰ的弹性系数计算结果表明,弹性系数C11,C12和C44均随压力的增加呈现线性增大的规律。     

Insight into the effect of Ge modification on the oxidation of NbSi2 from first-principles calculations

The effect of Ge modification on the oxidation of NbSi₂ was performed by using the first-principles method based on Density Functional Theory (DFT). The O₂ molecule absorption configurations on the ideal NbSi₂ (001) surface with and without Ge substitution was constructed to study the initial oxidation behavior as well as oxygen diffusion in the bulk NbSi₂ cell in the late oxidation stage. Ge substitution was found to decrease the absorption energy of O₂ molecule on the NbSi₂ (001) surface and therefore reduce the oxidation rate in the initial oxidation stage. The further linear synchronous transit (LST) calculation revealed that Ge substitution increased the energy barrier of O atom diffusion in the bulk NbSi₂ cell and decreased the oxidation rate in the late stage.     

First-principles comprehensive study of electronic and mechanical properties of novel uranium hydrides at different pressures

The systematic investigation on the U-H binary system up to 300 GPa has been performed by means of firstprinciples calculations in combination with the variable-composition evolutionary algorithm.Twenty uranium hydrides,including UH_x(x=1,2,3,4,5,6,7,8,9,17),U_2 H_y(y=3,5,7,13,15,17),U_3 H_8,U_3 H_(10),U4 H5 and U_4 H_9 have been found under different pressures.Besides thirteen compounds that are in nice agreement with previously reported structural search,new uranium hydrides in terms of the derived convex hull of the formation enthalpies have also been found in the present investigation.These seven novel compounds including CmcmUH_3, Fmmm-UH_4,R3 m-U_2 H_7,Pm-U_2 H_(15), Cm-U_3 H_8,C_2-U_4 H_9 and C2/m-U_4 H_5, are found to be dynamically and thermodynamically stable,and their stable pressure range has been determined.At last,the electronic and mechanical properties of these seven novel structures have further been calculated.