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.     

Grain boundary ridge formation during initial high temperature oxidation of Mn/Al TRIP steel

Confocal scanning laser microscopy (CSLM) was used in real-time observation of alloy element oxidation of a Mn/Al TRIP steel in an Ar–O₂ atmosphere. CSLM images reveal a marked role of grain boundaries in the overall initial oxidation kinetics of the alloy, and consequently in the morphology of the initial surface oxide. The oxidation on the alloy surface is dominated by the formation of Mn-rich oxide ridges along grain boundary traces on the surface. Oxide ridge formation kinetics was quantified by measurements on images extracted from real-time recordings of surface oxide evolution. Oxide ridge growth was found to take place at a constant rate. Scanning electron microscopy (SEM) images of the oxidized surfaces showed homogenous oxide ridges along straight grain boundary traces and heterogeneous oxide ridges along non-straight grain boundary traces. A transport mechanism of Mn to the surface is proposed, which relies on grain boundary segregation of Mn and on a relationship between grain boundary diffusivity and grain boundary character. It is suggested that when regarding alloys with significant grain boundary segregation of a solute, separate Wagner balances for internal vs. external oxidation is required for the grain lattices and the grain boundaries, respectively.     

Formation of charged defects during the nitridation of a metal particle

A model explaining and predicting generation of a temporal electric potential during nitridation of a single metal pellet has been developed. The model takes into account the kinetics of defects formation and assumes that the rate of the chemical reaction can be described by the shrinking-core process. The model simulations have shown that time scale of the generated electric potential depends on both the initial nitride shell thickness and heat removal from the particle surface. At thin initial shell and low rate of heat removal the maximum of the surface electric potential is attained before the temperature and surface nitrogen concentration have reached their maximums but after the maximum of nitridation has appeared. Quasi-neutral distributions of metal vacancies and electron holes are formed at the maximum temperature. At thick initial shell and/or high rates of heat removal from the particle surface the potential maximum may be observed much later: after the maximum temperature has been achieved. Correspondingly, non-equilibrium concentrations of the charged defects exist till the end of nitridation. In contrast to oxidation the nitrogen adsorption rate constants (the activation energy and pre-exponent) have negligible effect on the surface potential form and amplitude. At the ignition limit the rate of nitridation is proportional to the power of −1/2 for the ambient nitrogen pressure in the proposed scheme of defects formation. Metal vacancies and electron holes are the main charged defects in nitrides during nitrogen combustion. The nitride formation is limited by transfer of the vacancies in nitride.     

Evolution of reactions on surfaces exhibiting defect structures

This paper considers the time and spatial dependent behavior of an active surface exposed to two reactants which may diffuse on the surface, desorb, and also react. The surface is assumed to contain defect structures corresponding to either inherent lattice faults or foreign material on the surface. A number of case studies are examined corresponding to different assumptions about the desorption and kinetic characteristics of the defect sites. The surface concentration profiles are examined to gain physical insight into the competing surface processes. The net effect of surface defects on chemical production rates was examined by integrating the local production rate over a test region on the surface. Of special interest was the study of the poisoning effect of the product species on the diffusion and reaction of the reactants. The reaction-diffusion equations of the models were solved by the alternating direction collocation technique which shows promise for providing an efficient numerical procedure capable of handling practical large scale problems.     

Oxidation parameters determination of Cu–Al–Ni–Fe shape-memory alloy at high temperatures

In this study, isothermal oxidation behavior of a Cu–Al–Ni–Fe shape-memory alloy between 500 and 900 °C was investigated. Alloy samples were exposed to oxygen by TG/DTA for 1 h at a constant temperature, allowing for calculation of the oxidation constant and activation energy values of the oxidation process. The oxidation constant value increased with temperature, reaching saturation at 800 °C. The effect of oxidation on crystal structure, surface morphology and chemical composition of the Cu–Al–Ni–Fe alloy was determined by X-ray diffractometer (XRD) and scanning electron microscope (SEM)–energy-dispersive X-ray (EDX) analyses. With increasing oxidation temperature, number and intensity of the characteristic 18R martensite phase peaks were reduced while Al₂O₃ phase peaks were increased. In parallel to the XRD results, the same variations were also detected by SEM–EDX measurements.     

Electron spectroscopic study of passive oxide layer formation on Fe-19Cr-18Ni-1Al-TiC austenitic stainless steel

Surface oxidation of a TiC-enriched austenitic stainless steel alloy was investigated at 50 °C by X-ray photoelectron spectroscopy (XPS) and X-ray absorption spectroscopy (XAS). It was found that a passive oxide layer started to form on the alloy surface after 5 L of oxygen exposure. Further oxidation of the alloy was suppressed after 500 L of oxygen exposure when a stable passive layer was formed. It was found also that Ti and Ni did not oxidize and Ti remained in a carbide form during whole oxidation. The oxidation kinetics of different metals were investigated as well.     

The effect of embedded Pb on Cu diffusion on Pb/Cu(1 1 1) surface alloys

We have used scanning tunneling microscopy and low-energy electron microscopy to measure the thermal decay of two-dimensional Cu, Pb-overlayer, and Pb-Cu alloy islands on Pb-Cu(1 1 1) surface alloys. Decay rates covering 6-7 orders of magnitude are accessible by applying the two techniques to the same system. We find that Cu adatom diffusion across the surface alloy is rate-limiting for the decay of both Pb and Pb-Cu islands on the surface alloy and that this rate decreases monotonically with increasing Pb concentration in the alloy. The decrease is attributed to repulsive interactions between Cu adatoms and embedded Pb atoms in the surface alloy. The measured temperature dependences of island decay rates are consistent with first-principles calculations of the Cu binding and diffusion energies related to this “site-blocking” effect.     

Electron spectroscopic study of surface segregation and oxidation of Cu-In and Cu-Au alloys

XPS and AES techniques were employed to study the surface segregation and oxidation of Cu-1at%In alloy. Surface segregation of In has been observed with an enthalpy of segregation of about −34 kJ/mol. The surface oxidation of the Cu-In alloy at 480 K showed first a formation of Cu₂O on the alloy surface. The displacement reaction 3Cu₂O + 2In → 6Cu + In₂O₃ occurred on the alloy surface, on further heating of the oxidized surface in vacuum at 700 K. XPS and UPS techniques were employed to study the oxygen interaction with Cu-5at%Au and Cu-11at%Au alloys. At 300 K, oxygen was dissociatively chemisorbed on the Cu sites of the Cu-11% Au alloy surface, and the oxygen desorbed on heating. TDS study showed that desorption follows second order kinetics with an activation energy of desorption of about 75 kJ/mol.     

High-temperature oxidation behavior of SiO2 protective layer deposited on IN738LC superalloy by combustion CVD (CCVD)

A SiO₂ protective coating was deposited on an IN738LC alloy using CCVD. The physical properties of a SiO₂ protective layer are influenced by the amount of tetraethyl orthosilicate (TEOS, C₈H₂₀O₄Si). Therefore, the SiO₂ protective coating was deposited using different TEOS concentrations and deposition times to optimize the conditions. The deposited coating layer was confirmed to be a SiO₂ layer by SEM, EDX, and ESCA analyses. The oxidation resistance of the alloy was evaluated by thermo gravimetric analysis. The oxidation resistance of the SiO₂ protective coating was highest when the coating was processed at a TEOS concentration of 0.05 mol/l, which is the highest concentration of source material used. The surface roughness of the SiO₂ protective layer also increased with increasing TEOS concentration. The surface roughness of the coating had little effect on the oxidation resistance for a film thickness of approximately 1 μm.     

Absolute composition depth-profiles in surface segregation of PtRh alloys

The absolute composition depth profiles of the first 12 atomic layers of the (001) surface and the first 9 layers of the (111) surface of a PtRh alloy have been measured. As found in earlier studies, the surface is enriched with Pt, the sub-surface layer is depleted of Pt, and the Pt concentration oscillates toward the bulk value. The detailed oscillations are, however, very different for the two surfaces. Using a simple model for the metallic bonds, Monte Carlo simulations are performed to extract a set of bond energy differences of the constituent atom pairs from the experimental data.     

Development of high catalytic activity disordered hydrogen-storage alloys for electrochemical application in nickel–metal hydride batterie

Multi-element, multiphase disordered metal hydride alloys have enabled the widespread commercialization of nickel–metal hydride (NiMH) batteries by allowing high capacity and good kinetics while overcoming the crucial barrier of unstable oxidation/corrosion behavior to obtain long cycle life. Alloy-formula optimization and advanced materials processing have been used to promote a high concentration of active hydrogen-storage sites vital for raising NiMH specific energy. New commercial applications demand fundamentally higher specific power and discharge-rate kinetics. Disorder at the metal/electrolyte interface has enabled a surface oxide with less than 70 ? metallic nickel alloy inclusions suspended within the oxide, which provide exceptional catalytic activity to the metal hydride electrode surface. Received: 14 August 2000 / Accepted: 6 November 2000 / Published online: 9 February 2001     

Investigation of native-oxide growth on HF-treated Si(111) surfaces by measuring the surface-state distribution

The evaluation of the surface state distribution of differently HF-treated Si(111) surfaces during the native-oxide growth in air is investigated by the large-signal field-modulated photovoltage technique. The surface state distribution consisting of intrinsic and extrinsic Si dangling bond defects is directly related to the state of oxidation of the Si surface. It is shown that the kind of HF treatment strongly influences the concentration of extrinsic defects with a lower state of oxidation. Special HF preparations for H termination of the Si(111) surface result in a nearly intrinsic surface state distribution. During the oxidation process three typical phases can be distinguished each characterized by specific defect structures. It was found that native-oxide growth is highly sensitive to the concentration of extrinsic defects directly after HF treatment.     

Surface stoichiometry and the initial oxidation of NiAl(110)

Selective oxidation of the surface of an ordered alloy requires redistribution of the atomic species in the vicinity of the surface. This process can be understood in terms of the formation and movements of point defects in the compound. On the basis of ab initio density-functional calculation we found both the creation of exchange defects near the NiAl surface and segregation of Ni vacancies to the top layer to be extremely favorable in the presence of oxygen. Scenarios for the initial oxidation of NiAl are suggested which demonstrate the appearance of an additional energy barrier on the Ni-rich side compared to the Al-rich side. The expulsion of Ni from the oxide layer as it forms is the driving force for its stability.     

Model for anomalous transport of oxygen in nonstoichiometric perovskites: 1. General formulation of the problem

An attempt to explain the phenomenon of anomalous fast oxygen anion transport previously observed in the course of low temperatures topotactic oxidation of perovskite-related nonstoichiometric compounds has been made. According to the model developed the fast oxygen uptake relates to the high concentration of extended defects which exist in these compounds or can arise during oxidation. Experimentally observed specific kinetics has been described as being a result of fast oxygen transport taking place along the extended defects and followed by slow diffusion into undisturbed areas of the lattice.     

Theory of oxidation/reduction-induced valence transformations of metal ion dopants in oxide crystals mediated by oxide-vacancy diffusion: II. Kinetic analysis

We consider theoretically valence transformations of doping metal ions in oxide crystals induced by oxidation and reduction obtained by changes in the ambient oxygen partial pressure. Three types of oxygen vacancies are assumed to mediate transformations: neutral, singly ionized, and doubly ionized. In the companion part I paper we provide thermodynamic analyses yielding concentration relations among the oxygen vacancy, metal ions, holes and electrons, as functions of the ambient oxygen pressure. In the present companion part II paper we provide time dependent concentration profiles of the various species and reaction rate profiles. The diffusion exhibits a complex behavior; under some conditions, it may be described by a constant diffusivity, and is symmetric with respect to oxidation and reduction. However, under a wide range of conditions, the ionic state changes are highly asymmetric with respect to oxidation and reduction. For example, in the case of a neutral vacancy, a very narrow reaction front may establish during reduction. In the inverse (oxidation) process, however, the different species' profiles are quite smooth.     

铌合金电子结构及其高温氧化行为

为了从电子层面揭示Nb合金高温氧化的物理本质,采用递归法计算了Nb合金的电子态密度、原子镶嵌能、亲和能等电子结构参数,探索Nb合金高温氧化机理.研究表明:氧在Nb中具备较高的扩散速率和溶解度,且氧与Nb较易发生反应,生成氧化物,这使Nb的抗高温氧化性较差.原子镶嵌能的计算结果表明,合金元素Ti,Si,Cr在基体中稳定性较低,易向Nb合金表面扩散,形成富Ti,Si,Cr的表层.合金表层中氧与Nb,Ti,Si,Cr间具有较大亲和性,可以生成相应的氧化物,形成对合金具有保护作用的氧化膜. 关键词： 递归法 高温氧化 Nb合金     

Passivation of defect states in Si-based and GaAs structures

Formation of defect states on semiconductor surfaces, at its interfaces with thin films and in semiconductor volumes is usually predetermined by such parameters as semiconductor growth process, surface treatment procedures, passivation, thin film growth kinetics, etc. This paper presents relation between processes leading to formation of defect states and their passivation in Si and GaAs related semiconductors and structures. Special focus is on oxidation kinetics of yttrium stabilized zirconium/SiO₂/Si and Sm/GaAs structures. Plasma anodic oxidation of yttrium stabilized zirconium based structures reduced size of polycrystalline silicon blocks localised at thin film/Si interface. Samarium deposited before oxidation on GaAs surface led to elimination of EL2 and/or ELO defects in MOS structures. Consequently, results of successful passivation of deep traps of interface region by CN⁻ atomic group using HCN solutions on oxynitride/Si and double oxide layer/Si structures are presented and discussed. By our knowledge, we are presenting for the first time the utilization of X-ray reflectivity method for determination of both density of SiO₂ based multilayer structure and corresponding roughnesses (interfaces and surfaces), respectively.     

A study on the law of oxidation rate in GaAs-based VCSELs

In order to accurately control the oxidation aperture in high power vertical cavity surface emitting lasers (VCSELs), the selective oxidation process is studied with experiments. Selective oxidation experiments are carried out upon the simulate wafer of VCSELs at different temperature. Oxidation products are examined at different oxidation depths of oxidation layer and each component content is analyzed. The results of the experiments are analyzed with the kinetics of thermal diffusion. The analyzed results show that the concentration of oxidant is e exponentially declined with increasing depth of oxidation in high-power VCSELs. The oxidation depth stability and precision can be improved by lowering the oxidation temperature and prolonging the oxidation time.     

Study of the effect of annealing on defects in Fe-Mn-Si-Cr-Ni-C alloy by slow positron beam

FeMnSi shape memory alloys (SMAs) have received much attention as one-way SMAs due to their cost-effectiveness. Variable-energy (0-30 keV) positron beam studies have been carried out on a Fe-Mn-Si-Cr-Ni-C alloy with different degrees of deformation. Doppler broadening profiles of the positron annihilation as a function of incident positron energy were shown to be quite sensitive to defects introduced by deformation. The variation of the nature and the concentration of defects are studied as a function of isochronal annealing temperature. These results are correlated with the data measured with the positron annihilation lifetime spectroscopy (PALS). The positron annihilation results are compared to XRD and optical microscopy (OM).     

Behavior of hydrogen and defects in zirconium under irradiation

The accumulation of hydrogen and defects in the E-125 zirconium alloy (Zr-2.5% Nb) is investigated. The hydrogen concentration is maximum on the surface of zirconium alloy samples after electrolytic hydrogenation. The hydrogen concentration decreases at a depth of about 0.5 μm and then gradually grows with increasing depth. The surface of the zirconium alloy is strengthened and becomes more fragile after hydrogenation. A plastic deformation of the zirconium alloy gives rise to traps with different binding energies of hydrogen. The primary type of traps, the binding energy, and the amount of hydrogen captured by traps depend on the deformation magnitude and the sequence of deformation and hydrogenation processes. High mobility of hydrogen in plastically deformed samples is observed under bombardment of the surface of the zirconium alloy by a helium ion beam with an energy of 2.34 MeV. The variation of the hydrogen concentration in the near-surface region of zirconium under ion bombardment depends on the extent of deformation: upon bombardment by helium ions, the hydrogen concentration in the near-surface region of the metal increases for deformations from 1 to 3% and decreases for deformations of 4 and 5%.