Evolution of a reactive surface via subsurface defect dynamics

We find that the topography and composition of a reactive surface can evolve during epitaxy via motion of point and line defects within the material. We observe the response of a NiAl surface to an Al atom flux with low-energy electron microscopy. Initially, new NiAl layers grow as Al atoms exchange with bulk Ni atoms. When the surface is critically enriched in Al, condensation occurs at dislocations. They dissociate, move linearly, and leave tracks of altered composition and new atomic steps. We show how these dynamics depend on the identity and quantity of point defects near the surface.     

Oxidation and thermal stability of nickel deposited on a thin alumina support

The activity of supported Ni catalysts can easily be affected by high temperatures and oxidative atmosphere. In order to study the origin of this behaviour, we performed systematic LEED and XPS investigations on a model system prepared by physical vapour deposition of Ni on a thin epitaxial alumina film on NiAl(1 1 0). Annealing experiments showed that deposited Ni is thermally stable up to ∼370 K. Further annealing seems to lead first to changes in the island structure and then, at temperatures above ∼480 K, to diffusion of Ni through the alumina film. Analysis of the complex oxidation behaviour revealed that there is a competition between oxidation of the Ni deposit and of the NiAl support. The latter process is probably enabled by a solid state reaction at the interface which means that Ni catalyses the further oxidation of the support. While the surface of the deposited Ni clusters is easily oxidised, the further oxidation processes of the Ni particles and the support were found to be diffusion-controlled.     

NiAl化合物表面成分的准标度关系与偏离放大效应

研究金属间化合物NiAl的表面结构和成分对于理解其抗氧化机制、断裂过程等非常重要. 采用巨正则系综Monte Carlo方法对NiAl内部和(110)表面层的原子分布、点缺陷浓度、长程序参数等进行了模拟计算,发现1273 K温度的NiAl化合物在富Al段表面层和内部成分比例之间存在准标度关系,合金成分比例对化学计量比的偏离在表面层被放大,放大系数超过30倍. 关键词： 金属间化合物 NiAl 表面 准标度关系     

Local structure of NiAl compounds investigated by extended X‐ray absorption fine‐structure spectroscopy

The local structures of pure NiAl and Ti‐, Co‐doped NiAl compounds have been obtained utilizing extended X‐ray absorption fine‐structure (EXAFS) spectroscopy. The results provide experimental evidence that Ni antisite defects exist in the Ni‐rich NiAl compounds. The site preference of Ti and Co has been confirmed. Ti occupies the Al sublattice, while Co occupies the Ni sublattice. The structure parameters obtained by EXAFS were consistent with the X‐ray diffraction results. Owing to the precipitation of α‐Cr, the local structure of NiAl‐Cr has not been obtained, making the site preference of Cr unclear.     

Investigation by X-ray photoelectron spectroscopy of the transient oxidation of NiAl

X-ray photoelectron spectroscopy (XPS) has been used to study the oxidation of NiAl in oxygen at atmospheric pressure. Prior to oxidation, the native oxide scale on the specimen was removed by ion sputtering and the specimens were (pre-)heated in vacuum before exposure to oxygen. At low oxidation temperatures (<750 K) scales consisted of Al₂O₃ and NiAl₂O₄, with a thin surface layer of NiO, but at higher temperatures were of Al₂O₃, apart from about 0.5 at % Ni. The Ni content in the latter case was constant throughout the scale and did not increase dramatically near the alloy/oxide interface. In the experimental conditions used in this study, initial formation of NiO and NiAl₂O₄ seems to be avoided at the higher oxidation temperatures ( > 750 K).     

Long-range effect of ion irradiation on Cu surface segregation in a CuNi system

Ni films were deposited on one side of single crystal Cu substrate discs of 1.0 and 1.5 mm thickness. These discs were irradiated on the Cu side with argon ions. Evidence for enhanced Cu segregation at the Ni surface was found for both thicknesses. This effect decreased with increasing distance between the diffusion zone and the irradiated surface. Slow positron annihilation results indicate lower vacancy-like defects at the subsurface layer after Ar irradiation on the other surface of Cu disks. Such long-range effect is here interpreted on the basis of a particular type of mobile discrete breather called quodon.     

X-ray photoelectron spectroscopic studies on phase identification and quantification of nickel aluminides

Core-level XPS spectra for clean surfaces of Ni₃Al, NiAl, and NiAl₃ alloys were studied. The clean surfaces were obtained by fracturing in the ultra-high vacuum chamber. The positive chemical shifts of Ni 2p_3/2 peak for NiAl and NiAl₃ from Ni metal were 0.2 and 1.0 eV, respectively. The negative shift for Al 2p peak and the positive shift for Ni 3p peaks increased with the decreasing concentration of the corresponding elements. The peak position of the bulk plasmon loss peak for Al 2s peak shifted toward higher energy side, and further, the intensity ratio decreased with the decrease in aluminum concentration. Both the peak intensity ratios of Al 2p to Ni 3p determined by factor analysis and convenient separation are proportional to the atomic ratio of aluminum to nickel. The results indicate that the intensity ratio of Al 2p to Ni 3p determined by these two methods can be applied to the quantification for the surface of the nickel-aluminum alloys.     

Growth of Ni-Al alloys on Ni(1 1 1), from Al deposits of various thicknesses: (II) Formation of NiAl over a Ni3Al interfacial layer

This paper describes the second part of a study devoted to the growth of thin Ni-Al alloys after deposition of Al on Ni(1 1 1). In the previous paper [S. Le Pévédic, D. Schmaus, C. Cohen, Surf. Sci. 600 (2006) 565] we have described the results obtained for ultra-thin Al deposits, leading, after annealing at 750 K, to an epitaxial layer of Ni₃Al(1 1 1). In the present paper we show that this regime is only observed for Al deposits smaller than 8 × 10¹⁵ Al/cm² and we describe the results obtained for Al deposits exceeding this critical thickness, up to 200 × 10¹⁵ Al/cm². Al deposition was performed at low temperature (around 130 K) and the alloying process was followed in situ during subsequent annealing, by Auger electron spectroscopy, low energy electron diffraction and ion beam analysis-channeling measurements, in an ultra-high vacuum chamber connected to a Van de Graaff accelerator. We evidence the formation, after annealing at 750 K, of a crystallographically and chemically well-ordered NiAl(1 1 0) layer (whose thickness depends on the deposited Al amount), over a Ni₃Al “interfacial” layer (whose thickness—about 18 (1 1 1) planes—is independent of the deposited Al amount). The NiAl overlayer is composed of three variants, at 120° from each other in the surface plane, in relation with the respective symmetries of NiAl(1 1 0) and Ni₃Al(1 1 1). The NiAl layer is relaxed (the lattice parameters of cc-B2 NiAl and fcc-L1₂ Ni₃Al differ markedly), and we have determined its epitaxial relationship. In the case of the thickest alloyed layer formed the results concerning the structure of the NiAl layer have been confirmed and refined by ex situ X-ray diffraction and information on its grain size has been obtained by ex situ Atomic Force Microscopy. The kinetics of the alloying process is complex. It corresponds to an heterogeneous growth leading, above the thin Ni₃Al interfacial layer, to a mixture of Al and NiAl over the whole Al film, up to the surface. The atomic diffusion is very limited in the NiAl phase that forms, and thus the progressive enrichment in Ni of the Al film, i.e. of the mean Ni concentration, becomes slower and slower. As a consequence, alloying is observed to take place in a very broad temperature range between 300 K and 700 K. For annealing temperatures above 800 K, the alloyed layer is decomposed, Al atoms diffusing in the bulk of the substrate.     

Surface structure and energy of B2 type intermetallic compound NiAl

The surface structure and energies for 22 surfaces of NiAl, an ordered intermetallic compound of B2 structure, have been studied by using embedded atom method. The results show that, for alternating Ni and Al surfaces with odd numbers of the sum of their three Miller indices, the energy difference between the Ni terminated surface and Al terminated surface increase linearly with increasing the interlayer distance. So from surface energy minimization, the Al terminated surface is favorable for each alternating Ni and Al surface. This is in agreement with experimental results. However, the energy of the (1 1 0) surface belonged to the other kind of the surface consisted of stoichiometric atomic layers and with even numbers of the sum of their three Miller indices, is the lowest in all two kinds of the surfaces. Therefore the (1 1 0) texture of NiAl appears mostly in the experiments.     

NiAl中空位迁移机制的计算机模拟

运用分子动力学方法采用F-S多体势函数从原子尺度上模拟了NiAl金属间化合物中单空位的迁移运动行为，认为空位随成分的变化而采取不同的迁移方式：成分在理想化合比附近空位迁移主要以六步循环方式进行，其中V_Al主要以直型［100］六步循环方式迁移，V_Ni以［110］型六步循环方式占优势；当成分偏离时在富Ni一侧空位迁移则以ASB方式占很大的优势.计算所得NiAl金属间化合物中单空位迁移激活能与实验值相符，从微观上合理地解释了NiAl金属间化合物淬火实验中较高淬火温度对 关键词：     

Temperature and spatial dependence of the magnetization near a surface of a ferromagnet

The temperature and spatial dependence of the magnetization in a semi-infinite, single tight-binding band model ferromagnet is calculated. It is assumed that the spatial dependence of the magnetization arises primarily because of the downward shift of the ‘atomic’ energy level in the surface layer. This shift was found to occur in Ni in a previous calculation. The effects of the surface are approximated by considering a plane of defects in an otherwise translationally invariant metal. The surface layer is furthermore assumed to be paramagnetic as was previously suggested to be the case for Ni. It is demonstrated that the range of the magnetic disturbance arising from the surface increases with temperature and that the deviation in the magnetization of a given layer from the bulk magnetization is a damped oscillatory function of the distance of the layer from the surface. while the present theory assumes a highly over simplified model for the band structure and it is, therefore, not reasonable to apply these calculations directly to real metals, the results found here are qualitatively consistent with experiments on Ni.     

Effect of P impurity on NiAlΣ5 grain boundary from first-principles study

First-principles calculations based on the density functional theory(DFT) and ultra-soft pseudopotential are employed to study the atomic configuration and charge density of impurity P in Ni Al Σ5 grain boundary(GB). The negative segregation energy of a P atom proves that a P atom can easily segregate in the Ni Al GB. The atomic configuration and formation energy of the P atom in the Ni Al GB demonstrate that the P atom tends to occupy an interstitial site or substitute a Al atom depending on the Ni/Al atoms ratio. The P atom is preferable to staying in the Ni-rich environment in the Ni Al GB forming P–Ni bonds. Both of the charge density and the deformation charge imply that a P atom is more likely to bond with Ni atoms rather than with Al atoms. The density of states further exhibits the interactions between P atom and Ni atom, and the orbital electrons of P, Ni and Al atoms all contribute to P–Ni bonds in the Ni Al GB. It is worth noting that the P–Ni covalent bonds might embrittle the Ni Al GB and weakens the plasticity of the Ni Al intermetallics.     

Angular distributions of atoms sputtered from NiAl{1 1 0} and Ni{1 0 0}

Using the laser post-ionization surface analysis technique, I have for the first time studied angular distributions of Ni and Al atoms sputtered from NiAl{1 1 0}. Emission angular distributions from Ni{1 0 0} have also been measured. I have observed preferential emissions of Ni and Al atoms along 〈1 1 1〉 and 〈1 0 0〉crystallographic directions for NiAl{1 1 0} and of Ni atoms along 〈1 1 0〉 and 〈1 0 0〉 directions for Ni{1 0 0}. The observed preferential ejections can be explained in terms of the theory of focusing-collision sequences. Because of the difference in surface binding energy between Al and Ni atoms, preferential ejection angles of Ni atoms are slightly different from those of Al atoms along the 〈1 1 1〉 ejections. For NiAl, the 〈1 1 1〉 preferential ejections were less prominent than the 〈1 0 0〉 preferential ejections and this can be related to the low efficiency of momentum transfer in Ni-Al collision sequences along 〈1 1 1〉 lattice directions. The low efficiency of momentum transfer due to the mass mismatch can also be responsible for the experimental observation that the preferential ejections in the alloy were less prominent than those in the Ni metal.     

Electronic structure of gold nanoclusters on oxidized Ni(755) surface

The electronic energy structure of gold nanoclusters grown on oxidized single-crystal stepped surface Ni(755) is studied. It is shown that oxidation of the stepped Ni(755) surface results in the formation of a well-ordered continuous structure O(2 × 2) similar to that grown on a flat Ni(111) single-crystal surface. Evaporation of gold on such a surface leads to the formation of gold nanoclusters of a size determined by the size of the terraces on the Ni(755) surface. A comparison of the photoelectron spectra of the Au 4f _{5/2, 7/2} core levels in clusters grown on clean and oxidized Ni(755) surfaces reveals that the spectra obtained for a gold cluster system on an oxidized Ni(755) surface contain not only the spectral components characteristic of metallic gold but also additional components of Au^+δ. It is assumed that additional components for gold clusters on the oxidized Ni(755) surface originate from partial oxidation of gold atoms with the participation of defects inherent in the stepped relief of the nickel substrate.     

Sulfur at nickel-alumina interfaces: Molecular orbital theory

An atom superposition and electron delocalization molecular orbital study has been made of surface atomic layer relaxations in Ni(110) and Ni(111), the binding of p(2 × 2)S to Ni(111), S on Ni(100), and the binding of A1₂O₃ to clean Ni(111) and p(2 × 2)S covered Ni(111). Surface Ni atom relaxations for three-layer thick cluster models are calculated to be close to the experimental results and S heights of 1.68 Å on Ni(111) and 1.40 Å on Ni(100) are also in close agreement with experimental determinations, which indicates cluster models are suitable for determining surface properties. Al³⁺ in the basal plane of Al₂O₃ are predicted to support a low-lying surface state band in the O2p---A13s3p band gap. These orbitals are found to participate in strong Al---Ni bonds and Al---S bonds of intermediate strength at the interfaces with Ni(111) and p(2 × 2) S-covered Ni(111). Should such bonds form, they are expected to be too few in number to lead to strong adhesion. The strongest Al₂O₃---Ni bonding is predicted to occur when the Ni surface is oxidized. In this case, though Ni---O bonds are relatively weak, their number is high. It is concluded from the structure models that are studied that the segregation of impurity S in Ni to the interface will markedly decrease the adhesion of protective Al₂O₃ films that grow on NiAl based alloys.     

Surface oxidation investigation of Ni36Fe32Cr14P12B6 glass using angle resolved XPS

Native oxide and in-situ prepared, dry oxides of Ni₃₆Fe₃₂Cr₁₄P₁₂B₆ metallic glass have been investigated using angle resolved X-ray photoelectron spectroscopy (XPS or ESCA). The core-level binding energies of the various constituents of clean and oxidized samples have been determined accurately. A qualitative as well as quantitative estimation of elements in the outermost surface layers with and without oxidation is given by comparing XPS results obtained at normal and grazing emission angles. Stepwise oxidation leads to growing thickness of the surface oxide layer and to identification of different oxide species. The maximum thickness of the in-situ prepared oxide was determined as 3.5 nm compared to 4.5 nm for the native oxide. The sequence of oxidation is found to be Cr, Fe, B, P and Ni, but only some of the P and Ni atoms in the surface region are oxidized. The oxidation reaction induces diffusion of the constituents in the surface region as monitored by the change of relative intensities of the various peaks. For instance, P and especially Ni are strongly depleted in the oxide layer whereas Fe, Cr, and especially B are enriched. Differences between native and dry oxide have been observed and are discussed. The main difference is the abundance of carbon and oxygen containing species other than oxides in the native layer. Ar⁺ sputtering of the dry oxide layer leads to stochiometric changes in the surface region which are due to preferential sputtering.     

铜表面抗氧化性能与其覆盖石墨烯的层数依赖关系研究

We studied the oxidation resistance of graphene-coated Cu surface and its layer dependence by directly growing monolayer graphene with different multilayer structures coexisted, di-minishing the influence induced by residue and transfer technology. It is found that the Cu surface coated with the monolayer graphene demonstrate tremendous difference in oxidation pattern and oxidation rate, compared to that coated with the bilayer graphene, which is considered to be originated from the strain-induced linear oxidation channel in monolayer graphene and the intersection of easily-oxidized directions in each layer of bilayer graphene, respectively. We reveal that the defects on the graphene basal plane but not the boundaries are the main oxidation channel for Cu surface under graphene protection. Our finding indi-cates that compared to putting forth efforts to improve the quality of monolayer graphene by reducing defects, depositing multilayer graphene directly on metal is a simple and effective way to enhance the oxidation resistance of graphene-coated metals.     

Influence of surface orientation and defects on early-stage oxidation and ultrathin oxide growth on pure copper

We investigate oxidation and oxide growth on single-crystal copper surfaces using reactive molecular dynamics simulation. The kinetics of surface oxide growth are strongly correlated with the microstructure of the metal substrates. Simulating oxide layer growth along the (100), (110), and (111) orientations of crystalline copper, oxidation characteristics are investigated at temperatures of 300?K and 600?K. The oxidation kinetics are found to strongly depend on the surface orientation, ambient temperature, and surface defects. The effect of surface morphology on oxidation characteristics is analyzed by comparing oxygen adsorption on various sites and the structure factor. The surface oxide formed on (100) retains the initial crystal structure in the 300–600?K range. The (100) surface shows the highest oxidation rate at both temperature conditions but saturates, facilitating oxygen adsorption on hollow sites. The oxidation kinetics of the (100) orientation are found to be not significantly affected by surface defects. (110) shows modest oxidation at 300?K but the highest oxidation is observed at 600?K. By surface disorder and reconstruction, the oxide layer is produced continuously. The (111) surface is sensitive to ambient temperature and surface defects, showing that surface reconstruction is a key element for further oxidation. The charge distribution of oxidized Cu atoms indicates multiple groups of stoichiometric oxides, while the fraction of CuO-like characteristics increases significantly on the (110) and (111) orientations at higher temperature (600?K). The energetics and mechanisms of oxidation on Cu metal substrates at the nanoscale are discussed in detail, and comparisons with available experimental and other theoretical studies are presented wherever possible.     

3d过渡金属在NiAl中的占位及对键合性质的影响

通过赝势平面波方法系统地研究了3d过渡金属元素在B2-NiAl中的占位以及对键合性质的影响.通过形成能得出Sc,Ti,V 和Zn元素优先取代NiAl中的Al位,而Cr,Mn,Fe,Co和Cu优先取代Ni位.通过分析晶格常数变化量、电荷聚居数、交叠聚居数以及价电荷密度分布, 讨论了晶格畸变和键合性质的变化.结果表明: 取代Al的Sc,Ti,V和Zn元素掺杂使NiAl中晶格发生畸变,这对NiAl键合性质的变化起着重要作用,这些掺杂元素与第一近邻Ni原子产生强烈的排斥作用,形成反键,同时它们之间发生较大的电荷转 关键词： NiAl金属间化合物 3d过渡金属 第一性原理 键合性质     

Effects of alloying elements on the electronic structure and ductility of NiAl compounds investigated by X-ray absorption fine structure

The Ni L_3,2-edge spectra of the pure Ni, pure NiAl and alloying-element-doped NiAl compounds were obtained by synchrotron radiation X-ray absorption fine structure (XAFS). Due to orbital hybridization effect, directional covalent-type bonds formed and decreased the ductility during forming NiAl. Combining the XAFS spectra analysis and electronegativity comparison, the effects of alloying elements on the electronic structure and then the ductility of the NiAl compounds were obtained. The results showed that Cr, Co, Mo, Ru and W doping could be beneficial to the ductility by both weakening the directional bonds along the <111> direction and enhancing the d–d interactions of the transition metals–Ni atom pair, namely by the transition from covalent bonds to metallic bonds which was beneficial for dislocation to migrate. The results agreed well with the available experimental data and other theoretical results, proving that the model linking the electronic structure and ductility is reliable and can be used as guidance for alloy design.