Oxide Growth Mode Research Articles

Nano-porosity effects on corrosion rate of Zr alloys using nanoscale microscopy coupled to machine learning

Porosity throughout the zirconium oxide films plays a crucial role in the corrosion behavior and oxidation kinetics of Zr alloys, as it likely provides pathways for oxidizing and hydriding species through the oxide. In addition, it is known that substrate texture can affect the corrosion rate, although the detailed mechanisms are unclear. In this work, Zircaloy-4 was oxidized for up to 120 days at 260 and 360 °C in autoclave. Nano porosity was characterized by transmission electron microscope and precisely quantified by both manual counting as well as a newly developed, grayscale-value-difference-based, machine learning method. The oxide/substrate crystal structure, including suboxide and oxide phases, was investigated by ASTAR and directly correlated to the local pore density. The pore density shows a complex evolution as functions of oxide and metal grain orientations, oxide depth, temperature, and exposure time. Corrosion rate, substrate texture, and oxide texture are shown to affect pore density throughout the pre-transition zirconium oxide films. Two oxide growth modes (lattice-match and stress-driven) were studied, and the latter one was found to have much better resistance to corrosion and pore formation. The development of ML-based method enables a high-throughput measurement of nanopores in ZrO2, enabling rapid and systematic quantification of pore density in multiple oxides formed at different exposure times, temperatures, and substrate orientations.

Strain-modulated initial oxidation of Al(1-x)Nix alloy surface

Despite of extensive studies about surface oxidation of metals and alloys, the impact of external loadings on nanoscale initial oxidation remains relatively unclear. Here, we demonstrate that surface prestrains and dynamic loadings can effectively modulate the initial oxidation of Al(1-x)Nix alloys. We find that prestrains can not only suppress or promote outward and inward ionic diffusions, but change the oxygen Ehrlich-Schwöbel (ES) barrier across oxide islands. The former affects the oxidation kinetics while the latter leads to a oxide growth mode transtion, which comprehensively determines the evolution of oxide nanostructure. In addition, we propose a theoretical chemo-mechanical coupling model to predict the internal stress evolution and the stress relaxation in Al0.7Ni0.3 oxidation. Finally, the study of Al and Al0.7Ni0.3 oxidations under dynamic strains shows that alloying nickel in aluminum effectively enhances the oxidation resistance under cyclic loadings by intensifying the chemo-mechanical coupling effect. Findings in this work may shed the light on the modulation of surface oxidation through applying mechanical loadings, and is helpful to the materials design and protection.

Atomic Scale Mechanisms of Multimode Oxide Growth on Nickel-Chromium Alloy: Direct In Situ Observation of the Initial Oxide Nucleation and Growth.

The initial growth mode of oxide on alloy plays a decisive role in the development of protective oxide scales on metals and alloys, which is critical for their functionality for high temperature applications. However, the atomistic mechanisms dictating that the oxide growth remain elusive due to the lack of direct in situ observation of the initial oxide nucleation and growth at atomic-scale. Herein, we employed environmental transmission electron microscopy and the first-principles calculations to elucidate the initial atomic process of nickel-chromium (Ni-Cr) alloy oxidation. We directly revealed three different oxide growth modes of initial NiO islands on Ni-Cr alloy upon oxidation by O2, which result in distinct crystallography and morphology. The multimode oxide growth leads to irregular-shaped oxides, which is shown to be sensitive to the local mass transport. This localization of oxide growth mode is also demonstrated by the identified vigorous competence in oxide growth and thus shown to be kinetically controlled. The concept exemplified here provides insights into the oxide formation and has significant implications in other material and chemical processes involving oxygen gas, such as corrosion, heterogeneous catalysis, and ionic conduction.

Oxidation and microstructure evolution of cobalt aluminide coatings on directionally solidified superalloys during long term exposure at 1000°C

Using low pressure chemical vapour deposition, cobalt aluminide coatings were deposited onto directionally solidified Ni based superalloys (DZ466 and DS GTD111). Isothermal oxidation testing of the coated specimens was performed at 1000°C under atmospheric pressure for times up to 2500 h. Oxidation and interdiffusion behaviour of the coating superalloy systems was examined. It was observed that the oxide growth mode of both coating systems conformed to a staged parabolic law, with differences in the transient stage related to the growth of θ-Al2O3. Coating degradation and formation of chromium containing precipitates occurred during exposure because of severe interdiffusion between the coating and superalloy. The composition of the superalloy substrate played an important role in the microstructure evolution of the aluminide coating, and the corresponding mechanism is discussed.

Real-time Analysis of Initial Oxidation Process on Si(001) by Means of Surface Differential Reflectance Spectroscopy and Reflectance Difference Spectroscopy

We introduce quantitative investigation of initial oxidation process on Si(001)-(2×1) by means of linear optical spectroscopic methods, namely, surface differential reflectance spectroscopy and reflectance difference spectroscopy. Our recent results obtained with these real-time measurement techniques revealed that the transition between two different oxide growth modes, Langmuir-type adsorption and two-dimensional island growth, could be identified. The activation energies were estimated from Arrhenius plots of the oxidation periods for the growth of an oxide monolayer. Our results suggest that a finite activation energy exists for monolayer oxide formation on Si(001)-(2×1) at high temperatures.

Amorphous versus crystalline state for ultrathin Al2O3 overgrowths on Al substrates

The thermodynamic and kinetic background of the stability of ultrathin (<3nm) amorphous Al2O3 overgrowths on Al{111}, Al{100}, and Al{110} substrates was investigated by thermal oxidation of the bare substrates in pure oxygen gas for oxidation times up to 6000s in the temperature range of T=350–650K. The microstructural evolutions of the developing oxide films were analyzed by angle-resolved x-ray photoelectron spectroscopy, low energy electron diffraction, and high-resolution transmission electron microscopy. For sufficiently small thicknesses, stable amorphous Al2O3 films form on all substrates. The critical thickness values beyond which a crystalline state for the Al2O3 film is thermodynamically preferred can be reliably calculated provided that a layer-by-layer mode of oxide-film growth occurs. With increasing temperature, a transition from a layer by layer to an island-by-layer type of oxide growth mode occurs and, consequently (tensile), growth strain in a crystalline Al2O3 overgrowth can be more relaxed by lateral extension of the oxide islands, which results in a reduction of the critical thickness for the amorphous-to-crystalline transition. Oxygen incorporation in the metal substrate at the onset of oxidation, in particular, at higher temperatures and for the less densely packed surfaces of the metal substrate, can provide a mechanism for easy nucleation of crystallization at the metal/oxide interface of the amorphous oxide film of thickness larger than the critical thickness.

Simulation of kinetic oscillations in the CO+O 2/Pt reaction on the nm scale

Using the Monte Carlo technique, we simulate kinetic oscillations in CO oxidation occurring on nm-sized supported Pt particles via the conventional Langmuir–Hinshelwood mechanism and surface oxide formation and removal. Incorporating cooperative effects into the oxide growth mode, we demonstrate that the oscillations may be accompanied by remarkable segregation between oxide and CO. During oscillations, the maximum CO coverage may be appreciably smaller that that at saturation. These findings are in line with recent transient Fourier transform infrared spectroscopy measurements (Fanson, P.T., Delgass, W.N., Lauterbach, I., J. Catal. 204 (2001) 35) indicating that during oscillations on Pt/SiO 2 the CO coverage is relatively low and the shift of the stretching frequency of linearly bonded CO is nearly negligible.

Surface and Interfacial Topography of Oxides on Si(111) With Ultra-Low Atomic Step Density

ABSTRACTAtomic force microscopy has been used to study the morphology of the oxide surface and the Si-SiO2 interface after oxidation of Si(111) surfaces that are either totally free of atomic steps or have well characterized low step density. The step-free areas were formed by thermally processing a patterned Si surface in which flat areas are enclosed by a square array of ridges; flow of the atomic steps into the surrounding ridge barriers produces a regular array of step-free areas each of which can be up to ∼50µm×50µm. Arrays of widely spaced steps (e.g. 5µm) can also be produced in the step-free areas. AFM scans of the same areas were taken prior to (dry) oxidation, after oxidation, and after chemical removal of the oxide. It was found that at an oxide thickness in the 5-13nm range, the initial step structure of the underlying Si substrates is translated through the oxide to the surface after oxidation with the oxide surface being somewhat rougher than the initial substrate. Furthermore, the initial step morphology of the substrate remains at the Si-SiO2 interface after etching away the oxide by HF. The interface roughness is less than that of the oxide surface. The results suggest that the initial oxidation of silicon proceeds in a ‘layer by layer’ manner and not through a preferential step-flow oxide growth mode.

Growth of ZrO2 Corrosion Film on Zr-2.5%Nb Pressure Tube Alloy

Abstract The microstructure of Zr-2.5wt%Nb pressure tubes used in CANDU reactors consists α-Zr grains with a [110] texture in the radial direction and surrounded by a thin layer of metastable β-Zr. Information on oxide formed on β-regions have been reported. For the predominantly monoclinic ZrO2 (a=0.514, b=0.521, c=0.531nm, β=99.2°) formed α-Zr, two types of grain morphology and texture have been observed. In some areas of the oxide, columnar grains with a fibre texture exhibit no specific orientation relationship with the metal; such regions are not the subject here. Results presented here address a textured growth mode which is sensitive to the metal orientation. Plan-view foils of oxide on the pressure tube radial-normal surface formed by a treatment of 24 hours in steam were prepared by ion-beam thinning and examined in a JEOL 2010 TEM. Selected area diffraction (SAD) and convergent beam diffraction (CBD) patterns showed that oxide growth mode was dominated by a [001] growth and consisted of a combination of orientation related [001] growth and mixed [001]/[100] growth.

X-ray photoelectron spectroscopy and electron spin resonance studies on O 2 and N 2O plasma oxidation of silicon

O 2 and N 2O plasma oxidation of (111) and (100)Si on the anode is investigated in detail mainly using X-ray photoelectron spectroscopy (XPS) and electron spin resonance (ESR). The oxide growth rate estimated using ellipsometry is discussed. ESR observation reveals that at the SiO 2-(111)Si interface formed by plasma oxidation there are Si dangling bonds similar to those in amorphous Si in addition to P b centres. The P b centre density at the SiO 2-(111)Si interface formed by N 2O plasma oxidation is comparable with that formed by thermal oxidation. It is also suggested that the oxide growth mode on Si in the initial stage is two-dimensional growth.

Comparative study of the oxide growth mode on GaAs(111) and (1̄1̄1̄) surfaces: Implications for direct oxide electron beam writing

The growth mode of oxide films (<3 nm thick) obtained by electron beam oxidation is investigated on a wide range of polar GaAs{111} surfaces. Information is derived from a quantitative analysis of the Auger intensities for different oxidation models. Oxidation is always found to progress by a layer-by-layer mechanism with separate Ga and As oxide phases. The Ga-oxide growth is nearly insensitive to the starting surface, whereas a clear dependence is observed in the evolution of the As signal. Only a small fraction of the As atoms initially contained in each monolayer (ML) of GaAs{111} remains embedded in the Ga oxide; this fraction is zero, one-sixth or one-third of 1 As ML, for each oxide layer depending on the perfection of the initial surface. Therefore, this oxidation model supports previous indications that the elemental As often detected at the oxide/GaAs interface could not be formed during the oxidation process, but rather it is due to anion inclusions in the bulk which segregate at the interface.

(111)Cu2O growth modes on (111)Cu surfaces

Cuprous oxide growth modes on smooth, flat (111)Cu surfaces at 350°C were investigated by in situ UHV-RHEED and TEM techniques. Oxygen partial pressures used in this work ranged from 10-4 to 10-7 Torr. At 10-7 Torr of oxygen, the initial Cu2O overgrowth consists of a molecular 3 X 3 hexagonal superlattice having a coincidence boundary at the Cu2O/Cu interface. This monolayer type of growth is strained approximately to the coincidence lattice misfit of -1.22%. At 10-5 Torr the original oxide superlattice rapidly increases in thickness to a thereotically estimated value of approximately five monolayers. Further growth occurs by the formation of significantly less strained lamellas having more or less flat tops and coincidence lattice misfit dislocations between the lamellas and the substrate. Under other growth conditions, three-dimensional, ball-like or pyramidal islands were observed to form.