H原子在Zr(0001)表面吸附的第一性原理研究

结合蒙特卡洛（MC）模拟和第一性原理密度泛函理论（DFT）方法,从Zr-H体系微观结构、吸附概率、吸附能、Mulliken电荷布居数以及电子态密度等方面对H原子在α-Zr(0001)表面的吸附位点和吸附机理等进行计算分析。结果表明:H原子在Zr(0001)表面首先产生物理吸附,然后由物理吸附转变为化学吸附,吸附过程中电荷不断由 Zr(0001)表面原子向H原子转移,最后趋于稳定。另外,稳定吸附后的H原子直接与 Zr(0001)表面最表层原子生成化学键,且主要由H(s)、Zr(s)和Zr(d)轨道的电子态做贡献。综合分析得到H原子在Zr(0001)表面的吸附位点优先级顺序为密排六方间隙位（hcp位）＞面心立方间隙位（fcc位）＞桥位（bridge位）,顶位（top位）不会产生吸附。      

O2在UC(001)表面吸附的第一性原理研究

本文使用GGA+U方法研究了O₂在UC(001)表面的吸附、解离过程。结果表明：U_eff=1.5 eV能很好地再现晶格常数a和内聚能E_coh的实验结果;经过离子弛豫,最表面的两个原子层出现分层现象,各自形成两个亚层;O₂分子的吸附构型对吸附过程影响较大,吸附能在2.21～8.55 eV之间变化。通过Bader电荷、差分电荷和态密度分析,可确定O₂分子的解离活化机理为U的5f/6d电子转移至O₂的π2p和π*2p轨道。     

放射性核素U在针铁矿(010)表面吸附的第一性原理研究

采用基于密度泛函理论的第一性原理研究了有水与无水环境下放射性核素U在针铁矿(010)表面的吸附。结果表明,由于水分子与(010)表面的初始构型不同,其吸附能为-0.56~-0.85eV。同时发现在缺水的情况下,U原子可吸附在针铁矿(010)表面的替代位(T)、八面体间隙位(O)及两个O原子的桥位(B)上。其吸附能为负,大小按吸附T、B、O的顺序依次增大。水对U原子吸附能的影响较明显。当U原子占据替代位和八面体间隙位时,在其附近的水分子可增强U原子在针铁矿(010)表面的吸附,可使U原子在针铁矿(010)表面的吸附能降低1~2eV;当U原子在桥位时,水分子既可增强也可减弱U原子在针铁矿(010)表面的吸附能力,这依赖于U原子与水分子作用后其价态的变化。所有这些吸附均为放热反应。计算结果显示,由于水分子与U原子的吸附引起的表面晶格畸变较小,所有涉及的表面键长变化均不大于0.055nm。所有涉及到的反应过程中均有H、U、Fe原子上的部分电子转移到O原子上。     

XPS/AES研究室温下铀基氮化层的表面氧化

采用俄歇电子能谱仪（AES）以及X射线电子能谱仪（XPS）原位研究了室温下铀基氮化层在纯O₂气氛中的初始氧化过程。原位氧化过程中U的AES微分谱以及U 4f、N 1s、O 1s谱的变化显示,U₂N_3+x氧化形成了UN_xO_y;AES深度剖析结果显示,经18 L以及120 L O₂曝露氧化层与氮化层界面处均出现N的富集,表面形成氧化层-富氮层-氮化层的三明治结构。富氮层U原子的AES微分谱中OPV混合峰的峰位远低于氮化层与氧化层,N 1s谱向低能侧移动, 表明富氮层主要成分为N/U比较氮化层更高的氮化物。推测U₂N_3+x的氧化基于O原子对N原子的置换,被置换出的N原子进入邻近晶格使N/U比增大并阻碍O原子向内的进一步扩散。     

PuGa合金中H吸附行为的第一性原理研究北大核心CSCD

在氢气氛下,PuGa合金会发生氢化腐蚀反应,这会影响材料结构,导致PuGa合金相关性能下降。由于PuGa材料的高辐射性、相关实验技术及条件的制约,难以通过大规模的实验手段对此进行研究。因此,针对H在PuGa合金(111)面及体相内的吸附行为机制进行了第一性原理模拟研究,分析H在PuGa合金中的电子结构及吸附作用过程。研究发现H原子在PuGa体相中倾向吸附于包含Ga原子的四面体间隙,其吸附能为-0.75 eV,而在(111)面上倾向吸附于Pu原子周围,其最大吸附能绝对值为0_(6)2 eV。H在Ga原子邻近吸附位具有较低的扩散能垒,尤其是在表面上其反应能垒小于0.3 eV。本工作从原子尺度理解揭示H在材料表面的吸附行为机理,为降低H对材料结构及性能的影响提供理论指导。     

水蒸气分子在金属钚表面吸附的量子力学计算

采用密度泛函理论（B3LYP）方法，钚原子采用相对论有效原子实势（RECP）SDD基组，氢、氧原子采用aug-cc-pVTZ全电子基组，优化了PuO₂的分子结构，得到了相应的平衡几何构型。同时优化了Pu-H₂O的4个稳定异构体。比较能量发现水分子易于解离，从而与Pu形成更稳定的结构。由于Pu与O原子的电负性相差很大易发生电荷转移，分子的稳定性主要源于Pu-O之间的相互作用。根据电子-振动近似理论，计算了不同温度下金属Pu与H₂O吸附与解离反应的生成热力学函数。计算表明，即使在低温下，H₂O蒸汽分子也无法在金属钚表面形成分子吸附，相反，在金属钚表面水分子的解离可自发进行。     

阳离子有序相Ce2Zr2O7.97的石墨还原制备及其结构表征

采用石墨还原法探索了CeO₂-ZrO₂二元体系在不同还原温度、还原时间条件下的物相组成及结构,借助X射线衍射、红外振动光谱等手段对还原样品进行了物相表征及结构分析。实验结果表明：通过石墨还原能在较低温度（T_red＝1050 ℃）、较短时间（t＝24 h）内合成纯相Ce₂Zr₂O_7.97,较传统先还原（95%Ar+5%H₂,T_red≥1400℃,t＞48 h）后氧化（O₂或空气气氛）工艺更简单经济;Ce₂Zr₂O_7.97相能在1050～1300 ℃、24～72 h还原条件下稳定存在并保留至室温而不发生相分解,其机制可能归因于石墨所形成的弱还原环境;Ce₂Zr₂O_7.97相拥有与烧绿石Ce₂Zr₂O₇相近的有序阳离子亚晶胞结构,但由于其氧空位（O3、O4、O11）被大部分氧离子所填充,导致该相结构对称性降低。本文提出的石墨还原法制备富氧相Ce₂Zr₂O_7.97有望成为一种较传统制备工艺更简单经济的新方法,而Ce₂Zr₂O_7.97相能在还原气氛中稳定存在,可为An₂Zr₂O_7+x固化体的存放环境提供借鉴和参考。     

聚苯胺/TiO2复合材料对U(Ⅵ)吸附性能的研究

采用溶胶-凝胶法合成二氧化钛（TiO₂）,并将苯胺聚合在TiO₂表面制备了聚苯胺（PANI）/TiO₂复合材料（PANI/TiO₂）。使用FT-IR、TGA和XPS表征了制备的TiO₂、PANI和PANI/TiO₂的表面功能基团、热稳定性和表面元素组成。研究了溶液pH值、吸附时间、U(Ⅵ)浓度和温度等因素对TiO₂、PANI和PANI/TiO₂吸附U(Ⅵ)的影响,探讨了3种材料对U(Ⅵ)的吸附动力学、等温线和热力学性质。FT-IR、TGA和XPS表征结果表明,成功制备了PANI/TiO₂复合材料。TiO₂、PANI和PANI/TiO₂吸附U(Ⅵ)的最佳pH值分别为5.0、4.5和5.0;吸附过程均符合Langmuir吸附等温模型和准二级吸附方程,TiO₂、PANI和PANI/TiO₂的单层饱和吸附量分别为11.49、22.41、43.29 mg/g;3种吸附剂对U(Ⅵ)的吸附过程均为自发的吸热过程。同时,PANI/TiO₂具有较好的循环使用性能,第5次使用时,吸附量仅降低了15.4%。     

H原子在γ-U(100)表面吸附与扩散的密度泛函研究

采用密度泛函理论方法研究了H原子在γ-U(100)表面的吸附和扩散。结果表明：H原子在γ-U(100)表面的最佳吸附位依次为穴位、桥位和顶位,吸附能分别为2.696、2.597和2.017 eV;H原子在γ-U中的最佳间隙位为四面体间隙,其次是八面体间隙,吸附能分别为1.534 eV和0.991 eV;H原子在铀表面以及近表面层的不同吸附位之间的扩散需克服不同的势垒,相比于间隙位,H原子更倾向于在γ-U(100)表面吸附聚集。     

稀有气体原子与Pu（100）表面相互作用的第一性原理研究

采用第一性原理研究了稀有气体原子在Pu（100）表面上的吸附。计算结果表明：除He原子外,其他稀有气体原子在桥位处的吸附能均为最大;稀有气体原子在Pu（100）表面吸附后,稀有气体原子失去电荷,而Pu原子得到电荷,稀有气体原子中Xe原子的电荷转移数最大。差分电荷密度的计算结果表明,对位于Pu（100）表面穴位和桥位处的Xe原子,可观察到较明显的电荷再分布现象,这表明Xe原子具有一定极化效应,能与Pu（100）表面发生相互作用,从He到Xe,原子的极化效应越来越明显。     

Cluster size effect on reactive sputtering by fluorine cluster impact using molecular dynamics simulation

The mechanism of high-yield sputtering induced by reactive cluster impact was investigated using molecular dynamics (MD) simulations. Various sizes of fluorine clusters were radiated on clean silicon surface. At an incident energy of 1 eV/atom, F atom and F₂ molecule are only adsorbed on the surface and sputtering of Si atom does not occur. However, fluorine cluster, which consists of more than several tens molecules causes sputtering. In this case, most of Si atoms are sputtered as fluorinated material such as SiF_x. This effect is due to the fact that cluster impact induces high-density particle and energy deposition, which enhances both formation of precursors and desorption of etching products. The deposition of atoms and energy becomes denser as the incident cluster size increases, so that larger clusters have shown higher sputtering yield.     

Molecular dynamics simulation of fluorine ion etching of silicon

Formation of precursors and desorption of etching products by fluorine ion irradiation were studied using molecular dynamics (MD) simulation. When F atoms impact sequentially on a Si substrate, a mixed layer of F and Si atoms is formed on the surface. When the incident energy is below 30 eV, the fluorine coverage reaches steady state after 1.0×10¹⁶ atoms/cm² irradiation. The ratio of F to Si in the mixed layer is about 1:1. At an incident energy of 15 eV, the mixed layer at steady state consists of 4.5 ML of fluorine with a depth of 20 Å and the main etching products are SiF₃ and SiF₄. At 30 eV incident energy, the mixed layer at steady state is 6.5 ML with a depth of 40 Å and the main etching products are SiF₂ and SiF₃. When F atoms were irradiated onto a Si substrate heated at 1000 K, a significant reduction of F coverage was observed due to diffusion of F atoms.     

Density functional theory study of H,C and O chemisorption on UN(001) and(111) surfaces

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.     

First-principle studies of radioactive fission productions Cs/Sr/Ag/I adsorption on chrome-molybdenum steel in Chinese 200 MW HTR-PM

Chrome-molybdenum steel(2·1/4Cr1Mo) is one of the main products of steam generation.The adsorption behaviors of radioactive fission products on2·1/4Cr1Mo surface are critical in the analysis of HTR-PM.Here,the adsorption behavior of cesium,strontium,silver and iodine on 2·1/4Cr1Mo was investigated with first-principle calculations that the Ag and I atoms prefer to be adsorbed at the square hollow site of the face-centered cubic iron cell with a binding energy of about 1 and 3 eV,respectively.In contrast,Cs and Sr atoms are not adsorbed on the surface of the 2·1/4Cr1Mo.These results are again confirmed via analysis of charge density differences and the densities of state.Furthermore,the adsorption rates of these fission products show that only I and Ag have significant adsorption on the metal substrate.These adsorption results explain the amount of adsorbed radionuclides for an evaluation of nuclear safety in HTR-PM.These micro-pictures of the interaction between fission products and materials are a new and useful way to analyze the source term.     

Resonant ion stimulated desorption of O from the TiO2(1 1 0) surface: Effects of oxygenation and ion bombardment

The O⁺ desorption from reduced, oxygenated, and ion-bombarded TiO₂(1 1 0) surfaces has been investigated during He⁺ irradiation. The O⁺ desorption is initiated by creation of an antibonding O 2s core hole state via quasi-resonant charge exchange with the He⁺ 1s state, followed by the intra-atomic Auger decay of the O 2s hole. Upon oxygenation of the reduced TiO₂(1 1 0) surface, the O⁺ yield increases by one order of magnitude. The O₂ molecule is dissociated at the vacancy site of bridging oxygen and the oxygen atoms either fill a vacancy site or chemisorb at a fivefold-coordinated Ti⁴⁺ site as an adatom. The latter is detected with much higher efficiency than the former. The O⁺ yield is increased during He⁺ bombardment of the reduced TiO₂(1 1 0) surface due to formation of lower coordinated oxygen atoms. The oxygen species thus formed by ion bombardment or oxygenation are unstable on the surface and tend to diffuse into bulk vacancy sites or higher coordination surface sites even at room temperature.     

硼氢化物(B2Hn)(n=1~6)电子特性的动力学研究

采用密度泛函(DFT)B3P86方法,结合Dunning的相关一致三重基cc-pVTZ,优化计算硼氢化物 (B₂H_n) (n=1~6)可能的几何构型,得出最稳定结构的几何参数、电子结构和振动频率等参数,给出了最稳定结构的总能量(E_T)、结合能(E_BT)、平均结合能(E_av)、电离势(E_IP)、能隙(E_g)、费米能级(E_F)和氢原子差分吸附能(E_diff)等。结果表明,硼氢化物基态稳定结构的电子态分别为：n为奇数时为双重态2A,n为偶数时为单重态1A。由于B原子属于缺电子原子,能与等电子原子H化合,通过桥键形成多中心键的氢化物,优化计算发现,硼氢化物最稳定结构都存在桥键,且n为奇数的桥键作用比相邻偶数的强。通过分析最稳定结构的电子特性发现,B₂H_n(n=1~6)中B₂H₆的电离势和能隙最大,桥键比端键长,红外光谱强度最大,说明该硼氢化物最稳定,且其氢原子差分吸附能最大,储氢性能相对最好。