Oxidation Front Research Articles

Ge concentrations in pile-up layers of sub-100-nm SiGe films for nano-structuring by thermal oxidation

The data and analysis presented herein aims to facilitate the design and manufacture of SiGe based nanostructures and devices by describing the enhancement of Ge concentration in sub-100-nm thin films of SiGe by dry thermal oxidation. Thin films of SiGe were restructured by using thermal oxidation induced self-organization of Si and Ge atoms to create a layer of enhanced Ge concentration. The dry thermal oxidations were carried out at temperatures between 800 °C and 1000 °C. The influence of temperature on the Ge content at the oxidation front, as measured by x-ray diffraction, is examined and supported by simulation results. A model for determination of the Ge content in the pile-up layer is presented along with appropriate values for the activation energy and pre-exponential constant for diffusion of Si in Si1-XGeX. This model may also be used for determination of the diffusivity of Si in Si1-XGeX by fitting the model results to the measured Ge concentration in the pile-up layer. It is observed that the Ge content at the oxidation front is a function of temperature and varies linearly between 64% at 800 °C and 36% at 1000 °C. However, the Ge content is largely independent of oxide thickness and the Ge content in the initial SiGe layer. When the Ge concentration at the oxidation front is considered, the experimental results presented here indicate that the oxidation rates of SiGe closely match those of Si and provide evidence that the presence of Ge in very thin films of SiGe does not lead to enhanced or retarded oxidation rates as compared to Si.

Magnetic properties of loess deposits in Australia and their environmental significance

There are vast dry areas in Australia, but few loess sections have been reported. Mackenzie's Waterholes Creek (MWC) profile in New South Wales was the deepest and first dated loess profile. In this paper, various measurements of magnetic parameters from MWC were carried out and comparison was made with those of Luochuan profile, which lies in the central Chinese Loess Plateau (CLP). The types of magnetic minerals are similar to the CLP: magnetite, maghemite, limonite and hematite. However, the magnetic properties of the Australian aeolian sediment are quite different. Frequency-dependent magnetic susceptibility (χfd%) is rather high, while values of magnetic susceptibility (χ) are low. The relative content of hard (paramagnetic) – magnetic components is much higher than CLP, whereas the content of maghemite and hematite is much less. Furthermore hard magnetic minerals increase with depth, while soft maghemite decrease with depth. Maghemite is more sensitive than magnetite under pedogenic condition of waterlogged environments. It increases along with enhancement of magnetic susceptibility in Luochuan section; however, decreases in MWC profile. This very likely indicates that the pedogenesic processes in MWC took place under an environment in which maghemite/magnetite were unstable. The modern annual precipitation in the MWC area is 924 mm, which is about 300 mm higher than that in Luochuan. That extra moisture may lead MWC to exceed the stable range for maghemite/magnetite. Those minerals therefore were gradually converted to more stable goethite, causing magnetic susceptibility to decrease. Fe and Mn nodules are abundant in the lower part of section, implying mildly reducing conditions that allowed Fe and Mn to be mobilised and enriched in near the oxidation front, indicating that under waterlogged and reducing conditions wind-blown maghemite/magnetite were transformed to iron hydroxides, such as goethite/limonite, or other paramagnetic minerals, resulting in reduced magnetic susceptibility. The susceptibility from most of the section shows a generally positive relationship to the loess particle size, and seems to fit the Alaskan ‘wind vigor’ model. However, Fe chemical weathering and movement in MWC suggests that the precondition for the ‘wind vigor’ model cannot be satisfied and that the susceptibility variation can only be explained by pedogenesis.

Local Equilibrium and Nodule Formation on Al‐TRIP‐Aided Steel Surfaces During Intercritical Annealing

AbstractSeveral advanced high strength steels were intercritically annealed at a dew point of −5°C. Afterwards, surface morphology was investigated by scanning and transmission electron microscopy (TEM). In Al alloyed TRIP‐aided steels, nodules of metallic Fe containing traces of Mn could be proven by TEM analysis. To clarify the mechanism of origin, several further annealing trails were done and finally a new component of the occurring heterogeneous gas/metal reaction during intercritical annealing was postulated. It is concluded that neither the diffusion mechanism of the alloying elements nor the stress gradient between stress‐free surface region of the internal oxidation front nor the oxygen partial pressure dominates alone the selective oxidation process. It is suggested that the reactivity of the considered surface, its local surface chemistry and the local thermodynamic equilibrium should be taken into account in greater detail. Preferred dissociation points of absorbed water vapor could lead to a local increased oxygen partial pressure. With this, a nanoscaled oxidation/reduction process could be initiated.

High temperature oxidation behavior of chromium-rich alloys containing high carbides fractions. Part I: Nickel-base alloys

Six wear-resistant alloys based on nickel, containing 30 wt.% Cr and from 2.5 to 5.0 wt.% C, were elaborated by foundry and subjected to oxidation by air at 1000, 1100, and 1200 °C, for evaluating the oxidation behavior of hard bulk alloys. Their microstructures are rich in chromium carbides, eutectic, or pro-eutectic, and they also contain graphite for the highest carbon contents of interest. All the studied alloys obviously display a chromia-forming behavior, despite the initial low chromium content in the matrix. During oxidation, carbides disappear over an increasing distance from the oxidation front, with the consequence of the enrichment of the neighbor matrix in chromium. The minimal chromium content on the surface after oxidation decreases when the alloy is richer in carbon and increases with the oxidation temperature. The carbide-free zone tends to be deeper when the oxidation temperature increases, and also when the alloy's carbon content increases, in contrast with low-C Ni-30Cr alloys previously studied. The disappearance of carbides means a carbon loss as gaseous oxidized species. This probably disturbs the oxide scales growing on the external surface and may influence the oxidation behavior of the alloys. When present, graphite does not deteriorate dramatically the oxidation resistance of the alloys. The hardness of the alloys are lowered by the exposures to high temperature and by the presence of graphite.

On the coupled growth of oxide phases during internal oxidation of Ag–Sn–Bi alloys

AbstractInternal oxidation experiments using two Ag–Sn–Bi alloys with Bi contents of 2.6 wt.% and 5.0 wt.% were carried out. The presence and spatial distribution of Bi2O3, SnO2and Bi2Sn2O7in the oxidized microstructure was characterized using a field emission gun secondary electron microscope, energy dispersive X-ray analysis and X-ray diffraction techniques. It is shown that the addition of Bi prevents both external oxidation and internal oxide bands in Ag–Sn alloys with Sn content above 4 wt.%. Conditions are defined under which these alloys develop instabilities at the internal oxidation front. A result of the unstable growth of the reaction front is a dendritic morphology consisting of different oxide phases. Coupled growth of Bi2Sn2O7 and SnO2 inside the dendrites determines the local arrangement and distribution of the phases.

An Anomalous Internal Oxidation of Dilute Cu-Ni Alloys at 800 °C under 1 atm O2

AbstractThe oxidation of six Cu-

Model-based analysis of photoinitiated coating degradation under artificial exposure conditions

Coating degradation mechanisms of thermoset coatings exposed to ultraviolet radiation and humidity at constant temperature are investigated. The essential processes, photoinitiated oxidation reactions, intrafilm oxygen permeability, water absorption and diffusion, reduction of crosslink density, and development of a thin surface oxidation zone are quantified and a mathematical model for degrading coatings developed. Front-tracking techniques are used to determine the rate of movement of the oxidation and ablation fronts, the positions of which define the extension of the surface oxidation zone. Three previous and independent experimental investigations with two-component, densely crosslinked, epoxy–amine model coatings were selected for verification of the mathematical model. Simulations can match and explain transient mass loss and coating thickness reduction data and are in agreement with infrared measurements of carbonyl groups formed in the surface zone. The thickness of the stable surface oxidation zone, which is established after an initial ablation lag time, is estimated by the model to 0.5–2 μm in good agreement with previous measurements. Simulated concentration profiles of active groups, oxygen, and radicals in the stable surface oxidation zone are presented and analyzed. The mathematical model can be used for obtaining a quantitative insight into the degradation of thermoset coatings and has potential, after further development, to complete commercial coatings and dynamic exposure conditions, to become a supplementing tool for predicting in-service coating behavior based on accelerated laboratory measurements.

Simulation of pyrite oxidation in fresh mine tailings under near-neutral conditions

Sulphidic residual products from ore processing may produce acid rock drainage, when exposed to oxygen and water. Predictions of the magnitude of ARD and sulphide oxidation rates are of great importance in mine planning because they can be used to minimize or eliminate ARD and the associated economic and environmental costs. To address the lack of field data of sulphide oxidation rate in fresh sulphide-rich tailings under near-neutral conditions, determination and simulation of the rate was performed in pilot-scale at Kristineberg, northern Sweden. The quality of the drainage water was monitored, along with oxygen and carbon dioxide concentrations. The chemical composition of the solid tailings was also determined. The field data were compared to predictions from simulations of pyrite oxidation using a 1-D numerical model. The simulations' estimates of the amount of Fe and S released over a seven year period (52 kg and 178 kg, respectively) were in reasonably good agreement with those obtained by analysing the tailings (34 kg and 155 kg, respectively). The discrepancy is probably due to the formation of secondary precipitates such as iron hydroxides and gypsum; which are not accounted for in the model. The observed mass transport of Fe and S (0.05 and 1.0 kg per year, respectively) was much lower than expected on the basis of the simulations and the core data. Neutralization reactions involving carbonates in the tailings result in a near-neutral pH at all depths except at the oxidation front (pH < 5), indicating that the dissolution of carbonates was too slow for the acid to be neutralized, which instead neutralized deeper down in the tailings. This was also indicated by the reduced abundance of solid Ca at greater depths and the high levels of carbon dioxide both of which are consistent with the dissolution of carbonates. It could be concluded that the near-neutral pH in the tailings has no decreasing effect on the rate of sulphide oxidation, but does reduce the concentrations of dissolved elements in the drainage water due to the formation of secondary minerals. This means that sulphide oxidation rates may be underestimated if determined from drainage alone.

Nanostructured layers of anodic aluminum oxide on insulating substrates

A method for creation of porous alumina layers on insulating substrates is proposed. The anodic oxidation of thin aluminum films is performed in two stages: formation of a thin dense oxide layer and its local breakdown followed by the final oxidation of the film with a propagation of the oxidation front from a breakdown region to periphery. Aluminum oxide layers exhibit quasi-ordered structure with the pore diameter ranging from 10 to 100 nm and depending on conditions of electrochemical oxidation. The method provides the aluminum film oxidation right to the insulating substrate (i.e., the absence of residual metal phase inclusions on the interface) and can be used for the creation of matrix and composite structures for semiconductor microelectronics.

Microscopical investigation of steamside oxide on X20CrMoV121 superheater tubes

X20CrMoV121 is a 12%Cr martensitic steel which has been used in power plants in Europe for many decades. Superheater tubes exposed for various durations up to 135,000 hours in power plants in Denmark at steam temperatures varying from 450 to 575°C were investigated. Light optical and scanning electron microscopy was used to investigate steamside oxide morphologies. At all temperatures there is a double layered oxide, however, the inner:outer oxide thickness is not always equal. At the lower steam temperature range of <500°C, there is an internal oxidation zone at the oxidation front indicating that chromium is less mobile at these temperatures. At a higher steam temperature range of 540 – 575°C the inner oxide consists of chromium rich and chromium poor oxide running parallel to the oxidation front indicating that the chromium is more mobile within the steel. Both types of morphology have been observed in the laboratory, however the internal oxidation is observed up to 600°C and the chromium rich oxide striation are observed at 700°C.

Modeling the effect of stratification on cemented layer formation in sulfide-bearing mine tailings

Abstract Reactive transport simulations have been applied to investigate possible effects of stratification on the potential of sulfide-bearing mine tailings to form protective cemented layers. The simulations are based on characteristic strata found at a German tailings site, including sulfide-enriched heavy mineral layers, mica-enriched silt layers, and homogeneously mixed layers. The simulated secondary phases (jarosite, gypsum, amorphous ferric arsenate, amorphous Fe hydroxide, alunite, amorphous silica, and kaolinite) are similar to those observed in the field. Using scanning electron microscope analyses of cemented layers, it has been observed that the pore area becomes disconnected if the porosity is decreased to values below 15%, which would indicate a strong decrease in permeability. Stratification was found to play a crucial role in cemented layer formation. Cemented layers are absent or insignificant in systems with a homogeneous distribution of Fe-bearing sulfides. They are extensively developed in systems with (a) an arsenopyrite-rich layer or (b) a mica-enriched layer situated immediately below an Fe-sulfide enriched layer. The modeling results have clearly demonstrated that the key processes operating in scenario (a) are very different from the key processes in scenario (b). In scenario (a), the oxidation of arsenopyrite is followed by the precipitation of amorphous ferric arsenate, which can be solely responsible for significant pore reduction. In scenario (b), the presence of a large amount of reactive aluminosilicates (e.g. biotite and Ca-bearing plagioclase) immediately below the Fe-sulfide rich layer appears to be crucial. Key processes are extensive formation of Acid Rock Drainage (ARD) followed by enhanced (pH-driven) weathering of aluminosilicates, resulting in the accumulation of secondary phases directly below the Fe-sulfide rich layer. In both scenarios, a cemented layer is formed that effectively retards the further downward movement of the oxidation front. The presented details on the role of stratification in the formation of cemented layers could be considered in the construction of mining heaps as a possible measure to stimulate natural attenuation.

Internal Oxidation of Ni(Al) Solid Solution Zone Prepared by Pack Cementation of Pure Ni

Pure Ni was aluminized with an Ni3Al, NaCl and Al2O3pack powder mixture at 1100°C for 12 h. The Ni(Al) solid solution formed in aluminized Ni was internally oxidized at 1000°C for 2 to 12 h with a Co/CoO buffer. The internal oxidation zone consisted of 2 regions: granular precipitate zone near the surface and rod-like zone near the oxidation front. The growth of internal oxidation zone deviated from the parabolic law due to the non-constant Al profile developed by the aluminizing process. The granular zone grows linearly from the surface of the alloy.

Marcasite in Black Shales--a Mineral Proxy for Oxygenated Bottom Waters and Intermittent Oxidation of Carbonaceous Muds

Abstract Black shales in the geologic record have long been considered to reflect deposition under anoxic or even euxinic conditions. Although bottom-water anoxia favors deposition of black shales, certain biological indicators, such as benthic fossils and bioturbation, suggest that bottom-water conditions may not have been as persistently anoxic as commonly presumed. Examination of twelve marine carbonaceous shale-bearing units, ranging in age from Middle Proterozoic to Cretaceous, suggests that in addition to paleontological clues, early diagenetic marcasite formed in uncompacted surficial sediments is a reliable indicator of oxygen-bearing bottom waters. Marcasite requires low pH values to form, and oxidation of already existing sedimentary pyrite provides a plausible chemical scenario to achieve that condition. In the studied examples, degraded pyrite framboids and corrosion features on pyrite grains indicate partial pyrite destruction in surface sediments, and suggest that intermittent reoxidation of earlier formed iron sulfides may indeed have generated the low pH values required for marcasite formation. Thus, observations of textures spatially associated with secondary marcasite growth are consistent with a scenario whereby oxygen in overlying sea water drove downward migration of oxidation fronts. Pervasive marcasite in black shales from a wide range of occurrences suggests multiple reoxygenation events during the deposition of these shale units and calls for more dynamic ocean circulation than previously assumed. The latter consideration may, for example, prove important for those instances where a black shale is associated with an oceanic anoxic event (OAE), because these are commonly presumed to have been deposited under fully anoxic conditions.

Observation of overstrain in the coalescence zone of AlAs/AlOx oxidation fronts

Strain from oxidation-induced volume shrinkage is studied by micro-photoluminescence. An InGaAs/GaAs quantum well (QW) placed at the vicinity of the selectively oxidized AlAs layer is used to probe the spatial distribution of the strain with a resolution of 1 µm. A QW wavelength shift of 1 nm imputed to the embedded strain is observed in agreement with finite element calculations. With this method, an overstrained zone is highlighted where the counter-propagative oxidation fronts merge.

Spatial and Temporal Analysis of the Microbial Community in the Tailings of a Pb-Zn Mine Generating Acidic Drainage

Analysis of spatial and temporal variations in the microbial community in the abandoned tailings impoundment of a Pb-Zn mine revealed distinct microbial populations associated with the different oxidation stages of the tailings. Although Acidithiobacillus ferrooxidans and Leptospirillum spp. were consistently present in the acidic tailings, acidophilic archaea, mostly Ferroplasma acidiphilum, were predominant in the oxidized zones and the oxidation front, indicating their importance to generation of acid mine drainage.

Geothermal surface alteration of basalts, Krýsuvík Iceland—Alteration mineralogy, water chemistry and the effects of acid supply on the alteration process

The geothermal surface alteration of basalts and associated water chemistry at Krýsuvík, SW Iceland were studied. The geothermal area was characterised with zones of intensive surface alteration, steam vents, mud pots and hot springs. The steam-heated geothermal surface waters had pH values between 1.69 and 7.67 and total dissolved solid (TDS) concentrations between 154 and 6660 ppm, with Cl and SO 4 concentration decreasing and increasing with decreasing pH, respectively. Alteration mineral assemblages observed were strongly associated with the surface intensity. In areas of most intensive activity the basaltic rocks were altered to amorphous silica, anatase and pyrite with a crust of native sulphur at the surface. With decreased activity, kaolinite became important, as well as iron oxyhydroxides and oxides. On the flanks of the area montmorillonite was the predominant alteration product. Based on these observations the surface geothermal activity was divided into three groups: (1) high activity areas with active steam vents and mud pots and intensive acid leaching, (2) medium activity areas where the ground is hot, steam vents and mud pots are uncommon and the surface alteration is less intensive and (3) low activity areas on the margins of the surface geothermal activity. The primary factors influencing the steam-heated acid sulphate alteration of basalts included the redox state (oxidation front), supply of acids and pH, and the extent of reaction. The formation of iron- and sulphur-containing minerals and the respective elemental mobility depended on the redox conditions with pyrite formation under reduced conditions and goethite and/or hematite under oxidised conditions. At low pH, Ca, Mg, K and Na were mobile and leached out, whereas Fe, Ti and Al and to a large degree Si were retained in the alteration product. At higher pH values > 5 the mobility of Ca, Mg, K and Na was reduced due to the formation of clays.

Influence of laminate lay-up on oxidation and damage growth: Isothermal aging

The thermo-oxidative behavior of a composite is significantly different from that of the constituents as the composite microstructure, including the fiber/matrix interphase/interface, architecture and ply lay-up introduce anisotropy in the diffusion and oxidation behavior. In this work, light microscopy and scanning electron microscopy techniques are used to characterize the oxidative process in laminated carbon fiber-reinforced polyimide composites. Four different composites are considered, namely, unidirectional [0] 16T, quasi-isotropic [0/±45/90] 2S, cross-ply [0/90] 4S, and angle-ply [±45] 2S laminates. The observed anisotropy in composite oxidation is explained by carefully monitoring the development and growth of damage through the use of fluorescence imaging using dye impregnation. It is shown that the oxidation behavior of a laminate is strongly dependent on the ply stacking sequence, while alternative pathways for transport of oxygen into the interior of the composite are fiber–matrix debonds and matrix cracks that propagate with the oxidation front.

Role of Forming Gas Annealing Characteristics on the Selective Surface Oxidation of Fe-Mn-Si-Cr Model Alloy

To understand the role of annealing conditions on the segregation and selective oxidation phenomena, Fe 2Mn 2Si 0.8Cr (all in wt.%) model alloy was investigated. The samples were annealed at 820°C in N2-5%H2 forming gas atmospheres over a wide range of dew pints (-80 to 0°C) with dwelling time of 1 to 5 minutes. At all dew points, the strong chemical interaction between Mn and Si causes the formation of manganese silicates. External oxidation of Mn, Si and Cr were decreased at the higher dew points. Increasing the dwelling time shows an extended oxidation front i.e. pronounced external/internal oxidation of Mn, Si and Cr. Basically, the present work addresses the two stages of oxidation front.

Chalcopyrite formation through the metathesis of pyrrhotite with aqueous copper

The heterogeneous chemical environment which develops in the heap leaching of some pyrrhotite-containing copper ores can promote covellite and chalcopyrite formation particularly in acid-depleted regions of a heap. In such circumstances, copper recovery will be delayed until the acid and oxidation fronts move through the bed of ore and these secondary copper sulfides are re-leached. The transition from pyrrhotite to chalcopyrite most probably follows the sequence, pyrrhotite to copper-pyrrhotite to unnamed mineral CuFe 3S 4 to isocubanite to chalcopyrite, with a major structural expansion occurring prior to CuFe 3S 4. The mechanism is one in which copper is incorporated into pyrrhotite, which maintains its NiAs-type structure up to a stability limit, above which the structure rearranges to a chalcopyrite-like structure followed by isomorphic substitution of copper for iron. The structural rearrangement proceeds with significant expansion in one of the hexagonal axis directions and contractions in the other directions. Depending on the orientation, this expansion induces different levels of strain in the product chalcopyrite. The level of strain subsequently impacts on the rate of chalcopyrite metathesis to covellite. The depth of chalcopyrite formation into the pyrrhotite varies with pyrrhotite orientation.

Temperature and gas composition dependence of internal oxidation kinetics of an Fe10%Cr alloy in water vapour containing environments

AbstractRecently it was proposed, that the hampered formation of external protective chromia scales on FeCr‐alloys in water vapour containing, low‐pO2 gases is correlated with enhanced internal oxidation of chromium. In the present study the internal oxidation kinetics of Fe10Cr (in mass%) during isothermal oxidation in ArH2H2O mixtures at temperatures in the range 800–1050 °C has been investigated. It was found that the tendency for Cr to become internally oxidized decreased with decreasing temperature. At the higher test temperatures the internal oxide precipitates consisted of Fe/Cr‐spinel. With decreasing temperature the precipitates near the oxidation front gradually exhibited increasing amounts of chromia. At 900 °C the oxidation morphology in the ArH2 base gas mixture changed from exclusive internal oxidation of Cr at a water vapour content of 2% towards a combined internal Cr oxidation and external Fe‐oxide formation at higher water vapour partial pressures.