Formation Of Oxide Islands Research Articles

An investigation on the oxidation behavior of spatters generated during the laser powder bed fusion of 316L stainless steel

The characterization of the oxide islands on the surface of the spatter contributes to a fundamental understanding of the formation and oxidation of spatter in laser powder bed fusion (LPBF). In the present study, the surface and cross-section characteristics of spatters with oxide islands are investigated using high resolution scanning electron microscopy combined with energy dispersive spectroscopy, scanning transmission electron microscopy and selected area electron diffraction. The oxide islands with different configurations on the spatter surface are divided into three main categories: hemispherical, disc-like and film-like. Moreover, the primary component of oxide islands is an amorphous SiO2, which may also contain MnO, MnSiO3, and Mn2SiO4. Furthermore, a transition layer (enriched in Si, Mn, and O) is found at the oxide islands || spatter substrate interface, and the width of which is 4 μm and 20 nm on the spatter surface and within the spatter cross-section, respectively. Besides, the composition and sources of the circular oxide nano-inclusions (50-600 nm) internal to the spatters are also discussed in detail. Eventually, the driving forces affecting the formation of oxide islands on the surface of spatters are summarized, and a schematic diagram of the formation procedure is developed.

Corrosion and Stress Corrosion Cracking Susceptibility of Type 347H Stainless Steel in Supercritical Water

The corrosion resistance and stress corrosion cracking (SCC) susceptibility of Type 347H stainless steel (SS) in supercritical water (SCW) were investigated. The general corrosion behavior was investigated by exposure test and the specimens after testing were characterized utilizing scanning electron microscopy (SEM), Auger and x-ray diffraction analysis, optical microscopy, and energy dispersive spectroscopy (EDS). The results show that with the increase of testing temperature, the corrosion rate of the materials is greatly enhanced. The corrosion process is analyzed and the formation of oxide islands on the surface of the corroded sample is attributed to the higher diffusion rate of Cr along the grain boundary. The effects of temperature and dissolved oxygen on SCC susceptibility were investigated by slow strain rate tensile test. The fractographs were characterized by SEM and the cross-section morphologies were characterized with back-scattered electron imaging, SEM, and EDS. The results indicate that, as temperature is increased, the tensile strength and strain of materials is greatly reduced, while the oxygen content in SCW has a limited effect on the mechanical properties. Intergranular cracking and ductile fracture are the main fracture modes for Type 347H SS tested in SCW, independent of temperature and oxygen content. The implications of the results to the mechanisms of SCC are discussed.

Microstructural characterization of oxide layers in CaO added AZ31 Mg alloy

Using transmission electron microscopy (TEM) we studied the oxidation behaviors of CaO added Mg-3.5Al-1.4Zn (AZ31) alloys as a function of the CaO content and oxidation time, at 500 °C in air. It was shown that the AZ31 alloys with CaO exhibit substantially enhanced oxidation resistance. We identified the mechanism for this behavior: with added CaO the AZ31 alloys form protective oxide layers consisting of inner columnar and outer Ca/Mg rich structures. The thicknesses of the protective layers were observed and found to be correlated with the CaO content. The inner columnar layer grows along the [100]MgO due to the small energy barrier to the diffusion of interstitial oxygen ions in MgO. We observed three distinctive steps during the oxidation reaction: the formation of oxide islands, the dissipation of scale and the growth of a columnar oxide layer, and the formation of a double oxide layer due to Ca reduced from CaO. We clearly captured the microstructural evolution of the three different steps, leading to a detailed description of the formation of the protective oxide layers.

Behavior of aluminium coating by CVD-FBR in steam oxidation at 700°C

Aluminum coatings have been obtained using the Chemical Vapor Deposition of Fluidized Bed Reactor technique (CVD-FBR), on austenitic stainless steels (AISI 304, AISI 316 and AISI 317), in order to evaluate resistance to steam oxidation at 700oC. The coatings were characterized in their morphology/composition and crystalline phases using Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Analysis (EDAX) and X-ray Diffraction (XRD). In addition, thermodynamic simulation was performed using Thermo-Calc® software to make the deposition of the coating on said steels similar to optimal conditions. All specimens -with and without aluminum coating by CVD-FBR- were oxidized at 700oC, in a steam atmosphere of nearly 100% for 1000 h. The aluminum coating prevented the formation of oxide islands or crust, and reduced steel mass gain. Coated AISI steel showed a 25% reduction in mass gain compared to the same steel uncoated and oxidized under the same conditions. AISI 316 and AISI 317 steels underwent loss of mass because the brittle oxide that was formed came loose.

Assembly and structure of Ni/NiO core–shell nanoparticles

In this work it is reported a detailed investigation of the structure in Ni/NiO core–shell nanoparticles (NP). An experimental set-up was realized for the preparation and the study of pre-formed NP films. NiO shell was obtained with controlled dosing of O2 gas in the experimental system. A comparison of HR-TEM experimental images with theoretical simulations shows that the Ni NP core has a regular multitwinned icosahedral structure, composed with single crystal tetrahedra with (111) faces. NiO phase is clearly observed forming islands on the NP surface. In order to better investigate the oxide shell, the exit wave reconstruction method was applied to the images. It was found evidence of oxide island formation with direct or opposite (twinned) stacking on Ni (111) surfaces.

A new view on the relations between tungsten and vanadium in V2O5[sbnd]WO3/TiO2 catalysts for the selective reduction of NO with NH3

The relation between tungsten and vanadium oxide species in V2O5WO3/TiO2 catalysts for the selective catalytic reduction of nitrogen oxides by ammonia was studied with two series of catalysts containing 0.5–5wt.% V2O5 and 0 or 10wt.% WO3 impregnated onto a high surface-area titania hydrate of anatase structure to keep theoretical surface coverages of the transition metal oxides ⩽1. The catalysts, which were investigated by XRD, nitrogen physisorption, Raman and EPR spectroscopy, temperature-programmed reduction (TPR), and catalytic reduction studies, all exhibited the well-known acceleration of the SCR reaction by the tungsten promoter. From the EPR spectra and the TPR profiles, it was concluded that the presence of tungsten induces the formation of a surface oxide phase with intimate mixing between vanadium and tungsten oxide species instead of pronounced surface vanadium oxide island formation, which was found in the absence of tungsten. This suggests that the promotional effect of tungsten may originate from a direct influence on the neighboring vanadium oxide species or from the disruption of too large surface vanadium oxide ensembles.

KCl-Induced High Temperature Corrosion of the Austenitic Stainless Steel 304L – The Influence of SO<sub>2</sub>

The effect of SO2 on the oxidation of alloy 304L in O2+H2O and O2+H2O+KCl environment has been investigated at 600°C. Exposure time was 1-168 hours. The exposed samples were analyzed by SEM/EDX, XRD and IC. In dry O2, a protective and chromium-rich corundum-type oxide forms. In the presence of H2O(g), chromium is volatilized in the form of CrO2(OH)2(g). The corresponding chromium depletion of the protective oxide triggers a partial loss of protective properties resulting in the formation of oxide islands on the alloy grain centers. The oxide islands consist of an outward growing hematite layer and an inward growing FeCrNi spinel layer. By coating the samples with KCl the chromia depletion of the protective oxide dramatically increases due to the formation of K2CrO4. This leads to breakaway corrosion, a rapidly growing scale forming all over the surface. The resulting thick scale has a similar structure as the oxide islands formed in the absence of KCl. The addition of 300 ppm SO2 to the O2+H2O and O2+H2O+KCl environments results in a drastic reduction of corrosion rate. In O2+H2O environment the effect of SO2 is attributed to the formation of a thin sulphate film on the oxide surface that impedes chromium volatilization and decreases the rate of oxygen reduction on the oxide surface. In O2+H2O+KCl environment the corrosion mitigating effect of SO2 is mainly attributed to the rapid conversion of KCl to K2SO4. In contrast to KCl, K2SO4 does not deplete the protective oxide in chromium by forming K2CrO4.

The interaction of O 2 with the surface of polycrystalline gadolinium at the temperature range 300–670 K

Auger-Electron-Spectroscopy (AES) and Direct-Recoils-Spectrometry (DRS) were applied to study the interaction of O 2 with a polycrystalline gadolinium surface, in the temperature range 300–670 K and oxygen pressure up to 2 × 10 − 6 Torr. It has been found that initial uptake of oxygen, at coverage measurable by the techniques used here, results in rapid oxide island formation. The subsurface is believed to be a mixture of oxide particles and oxygen dissolved in the Gd metal, the latter being the mobile species, even at relatively low temperatures. Enhanced inward diffusion of oxygen starts as early as 420 K and dictates the surface oxygen concentration and effective thickness of the forming oxide. The oxygen accumulation rate at the near-surface region, as measured by the O(KLL) AES signal intensity, goes through a maximum as a function of temperature at 420 K. This is a result of the combination of still efficient oxygen chemisorption that increases surface occupation and slow inward diffusion. The thickest oxide, ~ 1.7 nm, is formed at 300 K and its effective thickness was found to decrease with increasing temperature (due to oxygen dissolution into the metal bulk). Diffusion coefficients of the oxygen dissolution into the bulk were evaluated for various temperatures utilizing models for infinitely thin oxide layer and thick oxide layer, respectively. The best fit under our experimental procedure was obtained by the thick layer model, and the coefficients that were calculated are D 0 = 2.2 × 10 − 16 m 2 s − 1 and E a = 46 kJ/ mol.

ActivatedO2dissociation and formation of oxide islands on Be(0001): An atomistic model for metal oxidation

By simulating the dissociation of O2 molecules on the Be(0001) surface using the first-principles molecular dynamics approach, we propose a new atomistic model for the surface oxidation of sp metals. In our model, only the dissociation of the first oxygen molecule needs to overcome an energy barrier, while the subsequent oxygen molecules dissociate barrierlessly around the adsorption area. Consequently, oxide islands form on the metal surface, and grow up in a lateral way. We also discover that the firstly dissociated oxygen atoms are not so mobile on the Be(0001) surface, as on the Al(111) surface. Our atomistic model enlarges the knowledge on metal surface oxidations by perfectly explaining the initial stage during the surface oxidation of Be, and might be applicable to some other sp metal surfaces.

Ultrathin K∕p-Si(001) Schottky diodes as detectors of chemically generated hot charge carriers

Ultrathin potassium layers were deposited on hydrogen passivated Si(001)−1×1 surfaces at a temperature of 120K in the thickness range from submonolayers up to 3nm. They were investigated with Auger spectroscopy, work function, and current-voltage measurements. After the formation of a wetting layer, potassium deposition leads to island growth. The surface hydrogen atoms are removed by the adsorption process. By attaching an electrical contact to the thin film, the current-voltage characteristics of the Schottky diodes could be determined. The analysis yields a homogeneous Schottky barrier height of 0.55eV for K∕p-Si(001) diodes in agreement with the metal induced gap state model. Exposing the diodes to molecular oxygen generates a strong chemicurrent signal which first increases with exposure, passes a maximum, and drops exponentially. The chemicurrent transient is attributed to a nucleation and growth formation of oxide islands and gives strong evidence for the existence of precurser states prior to oxidation.

Kinetic Monte Carlo simulation of an atomistic model for oxide island formation and step pinning during etching by oxygen of vicinal Si(100)

A lattice-gas model is developed to describe the simultaneous oxidation and etching of Si(100) surfaces exposed oxygen. The model incorporates nucleation of oxide islands via conversion of on-surface to back-bonded oxygen, together with an observed transformation in the shapes of just-formed islands from linear to two-dimensional. Model analysis via Kinetic Monte Carlo simulation quantifies oxygen uptake and oxide island nucleation kinetics, including possible enhanced nucleation at step edges. Simulated etching of vicinal Si(100) surfaces reveals that receding steps are pinned by oxide islands and transform into finger-like structures even at higher temperatures where oxide island growth is inhibited.

Further developments in quantitative X-ray photoelectron spectromicroscopy: preliminary results from the study of germanium corrosion

The moisture-induced corrosion of an evaporated germanium film has been observed and studied using quantitative X-ray photoelectron spectromicroscopy. The application of recently reported software for the processing of multi-spectral image datasets (particularly noise reduction) to this real-world problem revealed the formation of oxide islands above the continuous passive oxide layer. The software has been developed to map the thickness of these structures within the XPS sampling depth, with a spatial resolution of a few microns. This exemplifies a characterisation methodology of use in any situation where (non-uniform) surface modification results in chemically shifted components of the same core level (or Auger peak) between the original substrate and new overlayer. Copyright © 2005 John Wiley & Sons, Ltd.

Effects of surface topology on the formation of oxide islands on Cu surfaces

We examined the effects of surface topology on the nucleation and growth of Cu2O oxide islands during the initial oxidation stages of Cu(100) and Cu(110) surfaces by in situ ultrahigh vacuum transmission electron microscopy and ex situ atomic force microscopy. Our observations indicate that nucleation of three dimensional oxide islands on single crystal surfaces is homogenous, surface defects and dislocations play a very limited role as preferential sites for oxide nucleation. On the other hand, grain boundaries are the preferential sites for oxide nucleation and the oxide islands formed along the grain boundaries show a faster growth rate than that on flat Cu surface. The oxidation on the faceted Cu(110) surface results in heterogeneous nucleation of oxide islands in the facet valleys and one-dimensional growth along the intersection direction of the facets.

Reduction of microtrenching and island formation in oxide plasma etching by employing electron beam charge neutralization

During plasma etching of oxide thin-film patterns, nonuniform charge buildup within etching features results in formation of microtrenches. Near the etch endpoint, the underlying film layer adjacent to the feature edges is exposed first, leaving an oxide island in the feature center and potentially causing underlayer damage before the endpoint is reached. Herein, a directional electron flux is added to the plasma ion flux incident on the etching substrate with the goal of minimizing microtrenching and oxide island formation. Scanning electron microscopic images of patterns etched with added electron irradiation show a reduction in microtrenching and oxide island formation as compared to patterns etched under identical conditions without electron irradiation. A computer simulation shows that the added electron irradiation reduces microtrenching by allowing more uniform ion flux to reach the feature bottom.

Modeling of the Transition From Active to Passive Oxidation of Si(100)

ABSTRACTFor a Si(100)2×1 surface exposed to oxygen, there is a transition from etching (“active” oxidation via removal of SiO) to “passive” oxidation (buildup of an oxide film) with decreasing surface temperature. The transition depends sensitively on a competition between SiO desorption, and oxide island formation. We analyze these processes utilizing both ab-initio quantum chemistry studies of key energetics and lattice-gas models for the cooperative behavior.

STM, LEED and AES studies on the oxidation of a CrN overlayer on Fe 0.8Cr 0.2(100)

A combination of Scanning Tunneling Microscopy (STM), Auger Electron Spectroscopy (AES) and Low Energy Electron Diffraction (LEED) was used to study the CrN surface phase grown on a Fe 0.8Cr 0.2(100) substrate. The co-segregation of Cr and N was achieved by annealing the Fe 0.8Cr 0.2(100) crystal to 833 K. LEED data of the annealed surface showed a sharp (1 × 1) pattern corresponding to the segregated CrN phase. STM images showed that the CrN overlayer is rough and inhomogeneous, exhibiting many deviations from ideal regularity. Steps were observed with terrace widths ranging from 5 to 65 nm. The step heights are in the range 0.2–3.5 nm. Most of the steps are oriented preferentially in the 〈100〉 direction. Oxidation of the surface results in disordering of the CrN layer (1 × 1) LEED pattern. STM measurements revealed oxide island formation at saturation coverage. Our Auger data suggest that a Cr oxide overlayer forms over the surface nitrogen. Auger spectra indicated that exposure to O 2 results only in the oxidation of Cr while the underlying Fe is still metallic. Tunneling current-voltage measurements and Auger spectra at different O coverages can be correlated with the surface oxidation behavior of the CrN overlayer. Comparison of the CrN(1 × 1) layer with a N-depleted surface indicates that surface N retards oxidation at 300 K.

Characterization of the surface and interface species formed during the oxidation of aluminum

Abstract The reaction of oxygen with polycrystalline Al has been studied at room temperature using a wide variety of analytical tools (AES, XPS, ARXPS and FA). According to previous studies on Al oxidation, the interaction of oxygen with aluminum occurs in three stages: chemisorption, a fast initial oxidation mechanism with oxygen incorporation and slow growth of a compact oxide film. In the present work it is shown that the application of FA to the Al LVV and O KLL spectra allows the identification of two different chemical species, in addition to Al0, related to chemisorption and oxide formation, respectively. Furthermore, it has been found that ARXPS results are consistent with the formation of a chemisorbed phase consisting of single-layer oxygen islands which grow to completely cover the surface at around 10 L, prior to the formation of oxide islands which represents the first stage of the interaction of oxygen with the substrate. The second stage is characterized by the formation of oxide islands which grow to cover the surface above 1000 L. The third stage is characterized by a slow growth of a compact oxide film beyond 1000 L.

Two-Dimensional Growth and Decomposition of Initial Thermal SiO2 Layer on Si(100)

The growth and decomposition kinetics of initial thermal oxide layers on Si(100) were investigated by in situ ultraviolet photoelectron spectroscopy (UPS). From a time evolution of the correlation between the amount of oxide and the clean area observed, it was found that the initial thermal oxidation as well as the decomposition proceed two-dimensionally with the formation of oxide islands until the clean surface is fully covered with oxides, after which the oxides grow deeply toward the substrate with a very slow oxidation rate. The oxide island thickness was evaluated from the X-ray photoelectron spectra of Si-2p to be 7.1 Å. The two-dimensional oxidation, in which the oxide growth predominantly occurs at the periphery of oxide islands, means that adsorbed oxygen atoms migrate on the clean surface to be captured at the periphery of the oxide islands, resulting in the two-dimensional enlargement of islands, and be desorbed as SiO during the initial thermal oxidation.

ChemInform Abstract: Low‐Temperature Oxidation

Low-temperature oxidation is a reaction, occurring at or below room temperature, between a solid and a gas. It usually involves the combination of oxygen with metals, and it has the greatest commercial impact in the presence of moisture, as in corrosion. Cabrera and Mott put forward a theory of low-temperature oxidation, based on the assumption that cation migration occurs under the influence of a potential built up across the growing oxide film. Recent experimental results require that this theory be expanded to explain recent observations such as anion migration during oxide growth and the transition from the initial chemisorbed monolayer to a bulk, threedimensional oxide. The additional ideas put forward in the present paper may be summarized as follows. Low-temperature oxidation is controlled by the nature of the oxide; whether it is a network former or a modifier. A period of fast, linear oxidation is followed by a slow logarithmic reaction whose rate, in turn, can increase if the oxide film crystallizes to form grain boundaries. The initial fast oxidation is a continuation of the chemisorption process. Place exchange (anions and cations interchanging positions) occurs when the energy due to the image force of an oxygen ion is greater than the bond energy holding the ion in place. A stable film forms when this bond energy is greater than the image force energy. The oxygen ions formed on the oxide surface then set up a potential across the film. This potential provides the driving force for continued reaction. Oxide growth during this later stage is a slow, logarithmic process. A barrier to ion transport exists at the gas-oxide interface in the case of anion migration and at the metal-oxide interface in the case of cation migration. In both cases, the field built up across the oxide lowers the barrier sufficiently so that ion migration can occur. Network modifiers allow cation migration. The reaction rate is sensitive to crystallographic orientation of the metal, but not to oxygen pressure. A constant voltage is maintained across the film, so that the Cabrera-Mott theory explains the logarithmic kinetics. Network-forming oxides allow onion migration. The number of anions, and hence, the rate of reaction, is sensitive to oxygen pressure, but not crystallographic orientation of the metal substrate. Since the potential is a result of the mobile anions, the film tends to grow under constant field. The logarithmic kinetics then must be explained by an increasing activation energy for ion transport, as proposed by Eley and Wilkinson. The logarithmic growth rate can be increased by the presence of water vapor if the water introduces “dangling” bonds into an oxide network structure. Crystallization of the oxide film also increases its rate of growth and results in the formation of oxide islands.

Reduction of oxidized Fe(110) by hydrogen

The reduction of oxidized Fe (110) by hydrogen has been studied by ellipsometry, combined with Auger electron spectroscopy and low-energy electron diffraction (LEED). Oxide island formation occurs after partial reduction of oxidized Fe(110) by hydrogen, as was observed with LEED. At this stage the reaction rate becomes first order in the hydrogen pressure. When compared with the results obtained on Fe(100), it appeared that the reaction probability and the apparent activation energy of the reduction processes are of the same order of magnitude for both surfaces. However, a striking example of plane specific reaction behavior was observed, since the reduction on Fe(100) is incomplete, approximately one monolayer of oxygen remaining on the surface, whereas on Fe(110) the reaction proceeds quantitatively under similar reaction conditions.