Kinetics Of Air Oxidation Research Articles

A model for oxidation kinetics in air at room temperature of hydrogen-terminated (1 ) Si

A quantitative model is proposed for the description of the oxidation kinetics in air at room temperature of single crystalline, hydrogen-terminated, (1 0 0) silicon. The theory separates the growth kinetics of the interfacial suboxide from those of the outer stoichiometric oxide. The theory proceeds assuming that the suboxide grows along the surface at the border of oxidized-silicon clusters, while the formation of the stoichiometric oxide takes place on the top of the suboxide at a rate decaying exponentially with the oxide thickness. In these hypotheses the kinetics of suboxide formation are found to depend on the initial concentration of (defective) oxo groups, while the growth of the stoichiometric oxide is described by the Elovich equation both in the short- and long-time limits.

High temperature corrosion of structural materials under gas‐cooled reactor helium

AbstractThe Generation IV International Forum has selected six promising nuclear power systems for further collaborative investigations and development. Among these six concepts, two candidates are Gas Cooled Reactors (GCR), namely the Very High Temperature Reactor (VHTR) and the Gas‐cooled Fast Reactor (GFR). The CEA has launched a R&D program on the metallic materials for application in an innovative GCR. Structural GCR alloys have been extensively studied in the past three decades. Some critical aspects for the steels and nickel base alloys resistance under the service conditions are microstructural stability, creep strength and compatibility with the cooling gas. The coolant, namely helium, proved to contain impurities mainly H2, CO, CH4, N2 and steam in the microbar range that interact with metals at high temperature. Surface scale formation, bulk carburisation and/or decarburisation can occur, depending on the atmosphere characteristics, primarily the effective oxygen partial pressure and carbon activity, on the temperature and on the alloys chemical composition. These structural transformations can notably influence the mechanical properties: carburisation may induce a loss in toughness and ductility whereas decarburisation impedes the creep strength.There is a valuable theoretical as well as practical knowledge on the corrosion of high temperature alloys in the primary circuit of a GCR but this past experience is not sufficient to qualify every component in a future reactor. On the one hand, the material environment could be significantly different from the former GCR's, especially regarding the higher temperature. On the other hand, the materials of interest are partly different. Ni‐Cr‐W alloys, for instance, may offer significant improvement in the maximum operating temperature as far as the mechanical properties are concerned. However, their corrosion resistance toward the GCR atmosphere is still unknown.We describe here our first corrosion tests of Haynes 230, a high strength Ni base alloy containing tungsten, under low oxidising helium at 750 °C or 950 °C. The experiments were carried out in a purposely‐designed device that allows controlling low impurity partial pressure. The test duration was up to 1000 h. The corrosion behaviour was assessed through gravimetry and microscopy. Some specimens were investigated using EDS and DRX. The results under low oxidizing potential are compared to the oxidation kinetics in air. Tentative interpretations are proposed based on published models.

Oxidation of porous nanocrystalline titanium nitride. I. Kinetics

We used the continuous weighing method to study the oxidation kinetics in air for TiN specimens pressed and sintered from nanocrystalline powders with particle size ≤55 nm. Oxidation was carried out at 500–1000 °C for 240 min. By comparing with the oxidizability of compact titanium, we estimated the total reaction surface S of the porous specimens as a function of their oxidation conditions. The mass of absorbed oxygen Δm was calculated from the mass gain ΔP, taking into account the volatile component N2. We have shown that the maximum mass gain Δm at 600 °C is due to reaction of oxygen with the largest reaction surface. Within 120 min, external pores close up, S decreases, and then a continuous oxide layer forms in which diffusion of oxygen is slowed down. At 700–800 °C, the process of closing up of the pores is activated, and S decreases by an order of magnitude compared to 600 °C. After the first 40–50 min, a continuous oxide film forms and virtually no further mass gain occurs. As the temperature increases, the oxidation rate increases. At 900 °C, the reaction surface becomes equal to the external surface of the specimen, but the thickness of the scale increases linearly. We hypothesize that for T > 850 °C, counterdiffusion of titanium ions is superimposed on diffusion of oxygen.

Room temperature oxidation kinetics of Si nanoparticles in air, determined by x-ray photoelectron spectroscopy

The air oxidation kinetics of low coverages of ∼5nm Si nanoparticles, deposited by pulsed excimer laser ablation (KrF, 248 nm) in He, have been characterized by x-ray photoelectron spectroscopy. A simple model, based on the evolution of the Si 2p spectral components during oxidation, has been developed to determine the nanoparticle oxide thickness. It is found that the short-term oxide thickness is greater, and the long-term room-temperature air oxidization rate of these nanoparticles is less, than those reported for bulk a-Si and c-Si. The results are also consistent with an earlier transmission electron microscope observation of the oxidation of larger Si particles at higher temperatures. The greater short-term oxide thickness may be attributed to surface defects on the prepared Si nanoparticles, and lower long-term oxidation rate is due to the nonlinear decrease of oxygen diffusion in spherical systems.

Oxidation processes for alloys in the Ni - Zr system. III. oxidation of NiZr

We used the continuous weighing method to study the oxidation kinetics in air for the alloy NiZr at 500–1000°C. We used x-ray diffraction and metallography for layer-by-layer phase analysis of the scale. We have established that the oxidation kinetics is described by a parabolic equation q2 = Kpτ (where q is the mass gain per unit area of the sample, Kp is the rate constant, τ is the time). The value of Kp periodically decreased on the kinetic isotherms. In the scale, the phase components are distributed over the layers as follows: top layer, cubic and monoclinic ZrO2, NiO; inner layer, monoclinic ZrO2, Ni, and (or) Ni5Zr. At the boundary with the scale, the alloy layer (underscale) is depleted in zirconium. We have established that oxidation of NiZr is accomplished by predominant diffusion of oxygen through the oxygen vacancies in the lattice of monoclinic ZrO2. The decrease in q and Kp as the temperature rises from 600°C to 850°C is explained by a reduced concentration of these vacancies and (or) slowdown of their mobility. For T ≥ 850°C, the oxidation mechanism changes: counterdiffusion of Zr4+ also occurs through interstices in the lattice of monoclinic ZrO2. The outer layer (NiO), saturated by zirconium dioxide, loses any protective properties and diffusion of oxygen is facilitated. For this reason, both q and Kp increase as the temperature rises to 1000°C.

Oxidation Processes for Alloys in the Ni – Zr System. Part 1. Structure of Ni – Zr Alloys. Oxidation Kinetics for Ni7Zr2

We have studied the synthesis conditions and crystal structures for one-phase intermetallic alloys Ni5Zr, Ni7Zr2, and NiZr. We used the continuous weighing method to study the oxidation kinetics in air for the alloy Ni7Zr2 in the range 500-1000°C. We have shown that the oxidation process is described mainly by a parabolic law. The kinetic isotherms for the change in mass suggest that over time, the rate constant for parabolic oxidation Kp decreases due to improvement in the protective properties of the scale. The temperature dependence of the oxidation rate suggests a change in the oxidation mechanism for T > 800°C, due to changes in the defect structure of the ZrO2 lattice.

Kinetics of Wet Air Oxidation of Phenol over a Novel Catalyst

The wet air oxidation kinetics of an aqueous solution of phenol was studied over extrudates of an Al−Fe pillared clay catalyst in the temperature range of 90−150 °C and air pressure range of 0.8−2.5 MPa. The variables studied included reaction temperature, air pressure, solution pH, initial phenol concentration, and catalyst loading. The obtained findings are compared with those obtained using hydrogen peroxide oxidation. A group of kinetic models considering both the power-law and Langmuir−Hinshelwood approaches was evaluated to describe the catalytic kinetics of the phenol disappearance. The nonlinear dependence of the phenol conversion rate on the catalyst concentration was taken into account via the empirical power-law function of this variable. Finally, a kinetic model was discriminated on the basis of equilibrium adsorption of phenol and dissociated oxygen on two distinct types of active sites. The rate-controlling step was assumed to be the surface reaction between adsorbed reactant species. This model was able to predict well the experimental results over the entire range of the variables studied.

Oxidation of Nb2AlC and ( Ti , Nb ) 2AlC in Air

In this paper, we report on the oxidation kinetics in air of the ternary compounds (actual Nb:Al:C atomic ratios 1.95:1.05:0.95) and (actual Ti:Nb:Al:C ratios 0.94:1.06:0.93:0.94). In the 650-800°C temperature range, the oxidation reaction for the former occurs in two steps. First is At longer times and higher temperatures, and react to form for an overall reaction of In the 900-1200°C temperature range, oxidation of also occurs in two steps. First is At longer times and higher temperatures, and react to form for an overall reaction of In all cases, the oxidation occurs by the inward diffusion of oxygen. The formation of substantially enhances the oxidation resistance of relative to that of At 650 and 900°C the oxidation kinetics are subparabolic, with time exponents of and for and respectively. At 700 and 750°C, the oxidation kinetics of are linear, with rates comparable to those for the oxidation of pure Nb. The oxidation kinetics for at temperatures between 1000 and 1100°C start parabolic but become linear at longer times. At 1200°C, the oxidation is parabolic up to about 16 h; beyond that, the oxide layers tend to spall off. At 1200°C, a Nb-aluminide phase is observed at the oxide/material interface after 4 h of oxidation. Further oxidation leads to the formation of an -rich oxide phase at the oxide/substrate. Based on this work, the maximum extended-use temperature for in air is 650°C; for it is 900°C. © 2003 The Electrochemical Society. All rights reserved.

The Oxidation of NdFeB Magnets

The oxidation kinetics in air of a commercial NdFeB magnet have been investigated over the temperature range 335–500°C. The oxide microstructure has been characterized by SEM, XRD and cross-sectional TEM. The results show that the external scale formed consists of an outer layer of Fe2O3 and an inner layer of Fe3O4 but that the principal degradation process is the formation of an extensive zone of internal oxidation. HREM has been used to show that this zone contains NdO particles embedded in an α-Fe matrix. These particles are discrete and very small, approximately 2 nm in diameter, and have an amorphous structure. The α-Fe matrix has a columnar grain structure with a grain width of approximately 100 nm. It is argued that the high rates of internal oxidation arise because the external-oxide layers are not protective at the oxidation temperature, and oxygen penetrates to the zone front by fast diffusion along the columnar α-Fe grain boundaries.

Oxidation and Resulting Mechanical Properties of Ni/8Y2O3-stabilized Zirconia Anode Substrate for Solid-oxide Fuel Cells

Ni/8 mol% Y2O3-stabilized zirconia cermets are used in thin-film electrolyte solid-oxide fuel cells as support substrates. Rapid oxidation of the metallic Ni can cause failure of the substrate and of the whole system. The rate of Ni oxidation in air and in an inert atmosphere containing water vapor was determined as a function of temperature between 500 and 950 °C. A logarithmic rate law describes the oxidation kinetics in air, whereas a linear rate law fits the first branch of the curve of the experimental data in a humidified inert atmosphere. The substrate exhibits no significant mechanical degradation after uniform oxidation under moderate conditions. However, the observed bending of the samples after oxidation in humidified argon, due to the nonuniform oxidation, can cause damage to fuel cell

Oxidation of Tin + 1AlXn ( n = 1 ­ 3 and X = C , N): II. Experimental Results

In this, Part II of a two-part study, the oxidation kinetics in air of the ternary compounds and are reported. For the first two compounds, in the 1000-1100°C temperature range and for short times (≈20 h) the oxidation kinetics are parabolic. The parabolic rate constants are for and for At 900°C, the kinetics are quasi-linear, and up to 100 h the outermost layers that form are almost pure rutile, dense, and protective. For the second pair, at short times (<10 h) the oxidation kinetics are parabolic at all temperatures examined (800-1100°C), but become linear at longer times. The values are for and for In all cases, the scales that form are comprised mainly of a rutile-based solid solution, where and some The oxidation occurs by the inward diffusion of oxygen and the outward diffusion of Al and Ti. The C and N atoms are presumed to also diffuse outward through the oxide layer. At the low oxygen partial pressure side, the ions dissolve in and diffuse through the layer and react with oxygen to form at the high oxygen pressure side. This demixing results in the formation of pores that concentrate along planes, especially at longer times and higher temperatures. These layers of porosity impede the diffusion of Al, but not those of Ti and oxygen, which results in the formation of highly striated scales where three layers, an -rich, a -rich, and a porous layer repeat multiple (>10) times. The presence of oxygen also reduces the decomposition (into and Al) temperatures of and from a to one less than 1100°C. © 2001 The Electrochemical Society. All rights reserved.

Oxidation assisted creep of alloy 600

The influence of oxidation on the creep behaviour of Alloy 600 at 550°C has been investigated by creep tests in air and in high vacuum (10 -2 Pa). Thin strips (200 μm) were chosen to emphasise the role of the surface reaction phenomena. An important effect of the environmental conditions was observed, the steady-state creep rate being 14 times higher in air than in vacuum. Careful examination of the specimens showed that this effect could be attributed to an enhancement of the recovery processes, which are the limiting phenomena in the dislocation creep mechanism evidenced for the samples tested. Therefore a detailed study of the oxide scale and of oxidation kinetics in air and in vacuum was carried out. Results indicated trial under vacuum a thin. protective Cr 2 O 3 layer grew on the surface of the samples, whereas the formation of a non-protective, nickel-rich oxide was evidenced in air. Thus. the oxidation mechanism was found to dramatically influence the creep properties. This effect was modelled in terms of an increase in the effective diffusion coefficient. and related to vacancy injection induced by the cationic, constrained growth of the non-protective oxide scale in air.

Fenómenos de envejecimiento y oxidación a altas temperaturas en tres aceros inoxidables altamente aleados

The oxidation kinetics in air up to 50 h at 1,373 and 1,473 K of three refractory stainless steels (25Cr20Ni, 21Cr11Ni (1) , 21Cr11Ni (2) ) are studied. At 1,373 K, the best results are those of 25Cr20Ni followed by those of 21Cr11Ni (1) , with 0,036 % lanthanides, and the worst behaviour is that of 21Cr11Ni (2) , with 0,010 % lanthanides. A 2 h oxidation treatment at 1,323 K in air produces oxidation layers from 1 to 3 μm in thickness for the three materials. Scanning electron microscopy and energy dispersive microanalysis (SEM-EDX) and X-ray diffraction (XRD) characterisation of the oxidation layers give the same characteristics for the three steels: Mn Cr 2 O 4 spinel type in the outer part, Cr 2 O 3 in the inner one, with a SiO 2 barrier in the oxide-metal interface and a large amount of internal and intergranular precipitates (also Si02) in 21Cr11Ni materials. After ageing tests, at 1,073 K for 400 h, about 10 vol. % of the σ-phase is present in the 25Cr20Ni (AISI 310 S) material, while no a-phase is present in the 21Cr11Ni materials perhaps due to the high nitrogen content. The application of field emission SEM with EDX allows the analysis of σ-phase and carbides, nitrides and carbonitrides present. The steel 25Cr20Ni only forms, in addition to the σ-phase, M 23 C 6 type carbides, while the steel 21Cr11Ni (1) forms these carbides and M 15 N nitrides (M 8 N in the surface) and the steel 21Cr11Ni (2) forms mainly carbonitrides M 6 (CN). The relatively higher nitrogen level in the 21Cr11Ni (1) alloy could explain the different behaviour of both 21Cr11Ni steels.

Wide Angles X-Ray Scattering (W.A.X.S.) and H.R.E.M. Studies on Nanoscale Cobalt and Cobalt Colloids

Monodispersed cobalt nanoparticles have been synthesised by an original method using the decomposition under hydrogen of an organometallic precursor in the presence of a stabilising polymer. Using W.A.X.S. and H.R.E.M. techniques we have characterized the structure of two particle batches, 1.5 and 5nm average size. The finest particles (1.5nm) present a bcc core and a disordered surface whereas the biggest ones (5 nm) are hcp as the bulk structure. We then have followed very precisely the air oxidation kinetics at room and moderate temperatures. Room temperature leads to CoO divalent cobalt oxide whereas oxidation at 130°C drives to the Co 3 O 4 spinel form.

The oxidation kinetics of titanium carbide, nitride, and carbonitride powders in air

Oxidation kinetics in air have been examined for powders of titanium carbide, nitride, and carbonitride, which has revealed effects from the chemical and grain-size compositions, as well as from the type of surface in these compounds. The initial rate is dependent not only on the surface area of the powder and temperature but also on the nature of the material. The oxidation can be divided into three stages, in which the kinetics may be represented respectively by linear, parabolic, and again linear laws. Activation energies have been determined for the first two stages.

Microstructure/Oxidation/Microhardness Correlations in γ-Based And ツ-Based Ai-Ti-Cr Alloys

AbstractThe relationships between alloy microstructure and air oxidation kinetics and alloy microstructure and microhardness in the Al-Ti-Cr system for exposures at 800°C and 1000°C were investigated. The relevant phases were identified as τ (Ll2), γ (Ll0), r-Al2Ti, TiCrAl (laves), and Cr2Al. Protective alumina formation was associated with τ, Al-rich TiCrAl, and γ/TiCrAl mixtures. Brittleness was associated with the TiCrAl phase and τ decomposition to Al2Ti + Cr2Al. It was concluded that two-phase γ + TiCrAl alloys offer the greatest potential for oxidation resistance and room temperature ductility in the Al-Ti-Cr system.

Fully synthetic immunogens. Part I. Kinetic studies on air oxidation of the human IgG1 bis-cysteinyl fragment 225-232

Air oxidation of the human IgG1 heavy chain bis-cysteinyl fragment 225-232 was found to produce in unexpected high yields (90%) the parallel dimer as it occurs in the native immunoglobulin molecule. Kinetics of air oxidation as well as of disproportionation of the antiparallel dimer via thiol disulfide interchange clearly revealed that thermodynamic control of folding, in terms of cysteine pairings, generates in remarkably preferred manner the native parallel structure albeit the small size of the protein fragment. The structural information inherent in the sequence suffices as driving force via short range interactions, to promote the parallel alignment. This local structure may thus play an important role as a chain folding initiation structure in the assembly pathway of immunoglobulins. These findings also simplify the synthetic accessibility of hinge-peptide 225-232/225'-232'-antigenic peptide conjugates as potential fully syntnetic immunogens.

Activation energy of air-oxidized bituminous coals

The kinetics of air oxidation of a series of bituminous coals were investigated by using diffuse reflectance Fourier transform infrared spectrometry. A low-volatile bituminous coal, a medium-volatile bituminous coal, and two high-volatile. A bituminous coals were ground to three particle sizes and oxidized at 150, 200 and 250 o C. Activation energies and rate constants for the air oxidation of these coals were determined by tracking the decrease in the intensity of the aliphatic C-H band centered near 2920 cm −1

High temperature oxidation of hot-pressed aluminium nitride by water vapour

Hot pressed AIN without additives was oxidized et 1100 to 1400°C in dry air, wet air and wet nitrogen gas atmospheres with 1.5 to 20 kPa of water vapour pressure. AIN was oxidized by both air and water vapour, and formed α-Al2O3 film on the surface above 1150°C. The oxidation kinetics in air were parabolic end were promoted by water vapour. On the other hand, the oxidation kinetics in wet nitrogen were linear below 1250°C and parabolic above 1350°C. The oxidation rate in wet nitrogen was much greater than that in wet air. The rate of oxidation increased with increasing temperature until 1350°C, and then decreased. The parabolic rate constant decreased with increasing temperature and increased linearly with increasing water vapour pressure. The linear rate constant at 1150 to 1250° C increased with increasing the temperature with the apparent activation energy of 250 kJ mol−1. The relation between the linear rate constant and water vapour pressure was of the Langmuir type.

Air-oxidation kinetics of hafnium alloys containing platinum and palladium

Air oxidation kinetics for the 650‡ to 980 ‡C (1200‡ to 1800 ‡F) temperature range are presented for seven hafnium-base alloys containing platinum and palladium. The total fraction of platinum and palladium is always below 3 pct by weight.