Mechanism Of High-temperature Oxidation Research Articles

Experimental and modeling study of the effect of ethanol and MTBE on the oxidation of PRF90 in low pressure premixed laminar flame

Two premixed laminar flames of PRF90 (Primary Reference Fuel, iso-octane:n-heptane is 9:1) blended with ethanol and MTBE (Methyl Tert-Butyl Ether) respectively were studied, using molecular-beam sampling technology combining with tunable synchrotron VUV (Vacuum Ultra-Violet) photoionization mass spectrometry. Mole fraction profiles of species including reactants, intermediates and products were derived from the measurement. A detailed mechanism for high temperature oxidation under low pressure of the fuels mentioned above was developed based on USC-Mech II and each fuel sub-mechanism. The modeling result shows that the radical pool is affected by the change of additives and this leads to a slight change of PRF (Primary Reference Fuel) consumption paths. Both experimental and modeling results show that species with carbon number more than four and species with high unsaturation degree are not affected by the change of additives, while alkadienes are a little higher and acetaldehyde is lower in PRF90/MTBE flame, comparing with those in the PRF90/ethanol flame. In addition, the experimental result shows methanol and other carbonyl compounds including formaldehyde, acetone and butanone have higher concentrations in PRF90/MTBE flame.

정전 분무법을 이용하여 제조된 Fe-Cr-Al 분말 다공체 금속의 고온 산화에 미치는 소결 온도의 영향

A new manufacturing process of Fe-Cr-Al powder porous metal was attempted. First, ultra-fine fecralloy powders were produced by using the submerged electric wire explosion process. Evenly distributed colloid (0.05~0.5% powders) was dispersed on PU (Polyurethane) foam through the electrospray process. And then degreasing and sintering processes were conduced. In order to examine the effect of sintering temperature in process, pre-samples were sintered for two hours at temperatures of <TEX>$1350^{\circ}C$</TEX>, <TEX>$1400^{\circ}C$</TEX>, <TEX>$1450^{\circ}C$</TEX>, and <TEX>$1500^{\circ}C$</TEX>, respectively, in <TEX>$H_2$</TEX> atmospheres. A 24-hour TGA (thermo gravimetric analysis) test was conducted at <TEX>$1000^{\circ}C$</TEX> in a 79% <TEX>$N_2$</TEX>+21% <TEX>$O_2$</TEX> to investigate the high temperature oxidation behavior of powder porous metal. The results of the high temperature oxidation tests showed that oxidation resistance increased with increasing sintering temperature (2.57% oxidation weight gain at <TEX>$1500^{\circ}C$</TEX> sintered specimen). The high temperature oxidation mechanism of newly manufactured Fe-Cr-Al powder porous metal was also discussed.

ReaxFF Molecular Dynamics Simulations of Oxidation of Toluene at High Temperatures

Aromatic hydrocarbon fuels, such as toluene, are important components in real jet fuels. In this work, reactive molecular dynamics (MD) simulations employing the ReaxFF reactive force field have been performed to study the high-temperature oxidation mechanisms of toluene at different temperatures and densities with equivalence ratios ranging from 0.5 to 2.0. From the ReaxFF MD simulations, we have found that the initiation consumption of toluene is mainly through three ways, (1) the hydrogen abstraction reactions by oxygen molecules or other small radicals to form the benzyl radical, (2) the cleavage of the C-H bond to form benzyl and hydrogen radicals, and (3) the cleavage of the C-C bond to form phenyl and methyl radicals. These basic reaction mechanisms are in good agreement with available chemical kinetic models. The temperatures and densities have composite effects on toluene oxidation; concerning the effect of the equivalence ratio, the oxidation reaction rate is found to decrease with the increasing of equivalence ratio. The analysis of the initiation reaction of toluene shows that the hydrogen abstraction reaction dominates the initial reaction stage at low equivalence ratio (0.5-1.0), while the contribution from the pyrolysis reaction increases significantly as the equivalence ratio increases to 2.0. The apparent activation energies, E(a), for combustion of toluene extracted from ReaxFF MD simulations are consistent with experimental results.

The mechanism of high temperature oxidation of a SmCo-based magnetic alloy

The oxidation of a Sm(CobalFe0.22Cu0.08Zr0.02)7.5 magnetic alloy in air at 500, 600 and 700°C has been studied. The temperature increase greatly promotes the external and internal oxidation. The external scale consists of a thin outer layer of CuO and a thicker inner layer of (Cox, Fe1−x)Fe2O4 with distribution of α-Fe2O3. Samarium oxide is not seen in the scale but it is mainly internal oxidation product. Breakaway oxidation is observed at higher temperatures. The mechanisms of external and internal oxidation as well as breakaway oxidation are presented, based on detailed compositional and microstructural characterization of the oxidized zone of the alloy.

A Review of Oxidation on Steel Surfaces in the Context of Fire Investigations

During the course of a fire and subsequent exposure to the environment, iron and low-carbon steels oxidize by two mechanisms: high temperature oxidation and atmospheric corrosion. Of particular interest to fire investigators are oxide properties and distribution that could be of use to better understand important characteristics of the fire such as the location the fire originated, the direction the fire traveled or even temperature versus time characteristics. This could be particularly valuable in cases where burn damage to combustible material, which is known to be an important indicator of fire origin, is so extensive that little if any material remains after the fire. However, there is little data in the literature that specifically addresses the utility of oxide properties in the context of fire investigations. In this paper, we review the literature associated with both the high-temperature oxidation and atmospheric corrosion mechanisms in the context of a fire and post-fire environment. This review illustrates that both mechanisms are very complex and coupled to an equally complex and often ill-defined combustion environment associated with fires. As a result, the contribution of oxide characteristics to fire investigations is currently limited to identifying regions at the fire scene that reached temperatures sufficient to damage the protective coatings or to form wustite (FeO), which occurs at about 570°C. While oxide thickness developed during high-temperature oxidation may be related to the fire environment, its significance in fire investigations will likely depend on numerous confounding factors, which could include complex lattice defect formation processes, a complex and variable combustion environment, the dependency on both exposure time and exposure temperature, and the mass and surface area of the steel in question. Although variations in oxide color are often observed after a fire, no clear physical link between color variations and important characteristics of the fire has yet to be demonstrated by data currently available in the literature. Instead, oxide color is dependent on oxide thickness, the type of oxides present, natural variations of color for the same oxide, and the type and concentration of combustion products or other contaminants in the lattice structure. Language: en

Microstructural Characterisations and High Temperature Oxidation Studies of Nanostructured Co–Al Coatings Deposited on Superalloy

The microstructural characterizations and high temperature oxidation of magnetron sputtered Co–Al coatings on the superalloy substrate have been studied in the present work. FE-SEM/EDS and XRD were used to characterize the morphology and formation of different phases in the coatings, respectively. Thermo gravimetric technique was used to investigate the oxidation of the coatings, in air at 900°C. The growth kinetics of the oxide layers was predicted using weight change of the coated sample measured during oxidation. It was found that the oxidation rate of Co–Al coated superalloy was lower than that uncoated superalloy due to the formation of continuous dense, adherent and protective oxide scale such as CoO, Al2O3, Cr2O3, CoCr2O4, and CoAl2O4 over the surface of the coatings exposed to air at high temperature oxidation, 900°C. The microstructural features and phases of the oxidised coatings were used to elucidate the mechanism of high temperature oxidation.

The electronic theory study on high-temperature oxidation mechanism of TiAl alloy

In order to reveal the physical nature of high temperature oxidation of titanium-aluminous alloy from the electronic level, the atom embedded energy, affinity energy, binding energy and other electronic structure parameters are calculated by using recursive method combining with Castep, and the alloy oxidation mechanism is explored. The results show that there is a larger oxygen solubility in titanium and oxygen atoms can aggregate into titanium matrix near surface, and gradually spread into the deep matrix. Oxygen and titanium have a strong affinity to form a titanium oxide film. Aluminum can form clusters with mutual attraction between aluminum atoms in titanium matrix. Titanium atoms in aluminum clusters are mutually repulsive and form chemical compounds with aluminum atoms. Because of the closing affinity energy between aluminum and titanium with oxygen, the preferential oxidation of aluminum cannot occur, but titanium oxide and aluminum oxide form. The binding energy of Al2O3 is slightly lower than that of TiO2, therefore Al2O3 is more stable. Aluminum in TiO2 has a greater solubility, which can replace titanium to form more stable oxide Al2O3.

High-temperature oxidation behaviors of Fe-Cr-Al bulk and powder-sintered materials

In this study, cast bulk specimens and powder sintered specimens were produced from a Fe-22% Cr-5.8% Al alloy, which has been drawing industrial attention for its outstanding high-thermal and oxidation resistance as well as for its applicability as a filter and support material. The high-temperature oxidation behaviors of the specimens were compared and analyzed. Two types of specimens were subjected to 24-hour isothermal oxidation (temperatures of 900 °C, 1000 °C, and 1100 °C and a gas atmosphere of 79% N2+21% O2) to understand their high-temperature oxidation properties. The oxidized specimens were examined via X-ray, SEM, FE-SEM, and EPMA for microstructure and oxide analyses. The results of the oxidation tests showed that both bulk and powder-sintered specimens gained weight as temperature increased, with the powder sintered specimens gaining far more weight than the bulk specimens. As the temperature increased, the bulk specimens showed sequential formation of Al2O3, Cr2O3, and Fe3O4 oxide layers. The stable Al2O3 on the surface of the bulk specimens was found to play a role in arresting the rapid growth of oxide layers. In the powder-sintered specimens, however, the internal diffusion of O2 easily took place along the powder interfaces. As a result, far more oxidation occurred in the powder-sintered specimens than in the bulk specimens. Cracks and disintegration of the oxide layers were also found to have occurred as the temperature increased. The high-temperature oxidation mechanisms of bulk and powder-sintered materials in relation to temperature increase are further discussed.

Review of Several Studies on High Temperature Oxidation Behaviour and Mechanism of Austenitic Stainless Steels

Three studies on the oxidation behaviour of austenitic stainless steels were described in the present paper. (1) High temperature oxidation behaviour and its mechanism in austenitic stainless steels with high silicon: Sulfur contained as impurity in steel showed a harmful influence to the oxidation resistance of 19Cr-13Ni-3.5Si stainless steels. It was found that the abnormal oxidation was caused from the surroundings of MnS inclusions. (2) Effect of a small addition of yttrium on high temperature oxidation resistance of Si-containing austenitic stain less steels: The oxidation resistance of 19Cr-10Ni-1.5Si steels was improved remarkably even with only 0.01%Y addition, which is the same concentration as added for de-oxygenation. Y was enriched at the grain boundary of oxide scale and metal-oxide interface. It was suggested that Y-containing steels shoed good oxidation resistance, because the enriched Y at the grain boundary and metal-oxide interface prevented the diffusion of iron and oxygen ions through the oxide scale. (3) Effect of grain size on the oxidation behaviour of austenitic stainless steels: Type 304, 316 and 310 steels with finer grain size showed better oxidation resistance than those with coarser grain size at 850°C. The oxide scale of steels with coarser grain size easily spalled during the cooling process.

On kinetic mechanisms of n-decane oxidation

A kinetic model is built to describe ignition and pyrolysis of n-decane. The model involves 1021 reversible reactions and 144 species and includes both the high-temperature and low-temperature oxidation mechanisms. The model is tested against experimental data on the ignition delay time, changes in the concentration of OH radicals behind the reflected shock wave, and evolution of various species during n-decane pyrolysis in a flow reactor in a wide range of temperatures (T = 650-1640 K), pressures (p = 1–80 atm), and equivalence ratios (ϕ = 0.5–3). This kinetic model ensures a more accurate description of experimental data on the ignition delay time than other known kinetic models, especially at low initial temperatures.

G080022 燃料構造に起因する高温酸化特性が熱発生速度へ及ぼす影響([G08002]動力エネルギーシステム部門一般セッション(2):動カエネルギーシステムの高度化2)

G080022 燃料構造に起因する高温酸化特性が熱発生速度へ及ぼす影響([G08002]動力エネルギーシステム部門一般セッション(2):動カエネルギーシステムの高度化2)

A Reduced Mechanism for High-Temperature Oxidation of Biodiesel Surrogates

A skeletal mechanism with 118 species and 837 reactions was developed from a detailed LLNL mechanism that consisted of 3329 species and 10806 reactions for a tricomponent surrogate mixture, consisting of methyl decanoate, methy-9-decenoate, and n-heptane, which is suitable for combustion modeling of biodiesel derived from various feedstocks. The method of directed relation graph (DRG) for skeletal mechanism reduction was improved for mechanisms with large numbers of isomers. The improved DRG together with isomer lumping and DRG-aided sensitivity analysis (DRGASA) were subsequently applied to obtain a minimal skeletal mechanism from the detailed mechanism for the given error tolerance. The reduction was performed within a parameter range of pressure from 1 to 100 atm, equivalence ratio from 0.5 to 2, and temperature higher than 1000 K in autoignition and perfect stirred reactors (PSR). Although reduced in size almost by a factor of 30, the skeletal mechanism features high accuracy for high-temperature applications both in predicting the global system parameters, such as ignition delay and extinction time, and detailed profiles of species concentrations. Furthermore, numerical simulations of jet stirred reactors were compared with experimental measurements for rapeseed oil methyl esters. The temperature and species profiles in one-dimensional atmospheric counterflow diffusion flames were well predicted as well compared with experimental data in the literature.

A consistent chemical mechanism for oxidation of substituted aromatic species

Computational studies of combustion in engines are typically performed by modeling the real fuel as a surrogate mixture of various hydrocarbons. Aromatic species are crucial components in these surrogate mixtures. In this work, a consistent chemical mechanism to predict the high temperature combustion characteristics of toluene, styrene, ethylbenzene, 1,3-dimethylbenzene ( m-xylene), and 1-methylnaphthalene is presented. The present work builds on a detailed chemical mechanism for high temperature oxidation of smaller hydrocarbons developed by Blanquart et al. [Combust. Flame 156 (2009) 588–607]. The base mechanism has been validated extensively in the previous work and is now extended to include reactions of various substituted aromatic compounds. The reactions representing oxidation of the aromatic species are taken from the literature or are derived from those of the lower aromatics or the corresponding alkane species. The chemical mechanism is validated against plug flow reactor data, ignition delay times, species profiles measured in shock tube experiments, and laminar burning velocities. The combustion characteristics predicted by the chemical model compare well with those available from experiments for the different aromatic species under consideration.

High-temperature (to 1670°C) air oxidation of AlN–Si3N4 and AlN–Si3N4–(Ni–Cr–Al) ceramic materials

The kinetics and mechanism of high-temperature air oxidation of powders and compact materials with different compositions in the AlN–Si3N4 and AlN–Si3N4–(Ni–Cr–Al) systems are studied using nonisothermal (up to 1500°C) and isothermal (1350 and 1670°C) thermogravimetry, DTA, XRD, petrography, and EMPA. During heating, Al4O4–x N x and Si2N2O oxynitrides, α-Al2O3 and α-SiO2 (crystobalite) oxides, and mullite-based Al2O3 ∙ SiO2 solid solution of the boundary composition form during the first oxidation stage, and 3Al2O3 ∙ 2SiO2 mullite forms at higher temperatures (>1350°C) in the upper scale layer and aluminum-nickel chromite spinel in the intermediate (barrier) scale layer. The AlN–Si3N4 and AlN–Si3N4–(Ni–Cr–Al) ceramics may be regarded as hightemperature composites (HTCs) owing to their high corrosion resistance.

High temperature oxidation mechanism of Nb-Al alloys

The atomic cluster model of the interface between aluminum oxide film and the niobium matrix has been set up with our self-programmed software. By using recursion method, the atom embedding energy, the atomic binding energy and other electronic parameters have been calculated. The high temperature oxidation mechanism of niobium alloys is analysis from the electron levels. Our study shows that， aluminum segregates on the alloy surface through the grain boundary diffusion and combines with oxygen to form dense Al2O3 oxide film which blocks oxygen to diffuse into the niobium matrix. Grain boundary and rare earth elements can increase binding energy between the oxide film and the matrix and increase the bonding strength of the interface to enhance the adhesion between the oxide film and the niobium matrix. Thus, by adding rare earth elements in the alloy or refining the alloy grains, the performance of high temperature oxidation resistance of niobium alloys can be improved.

Electronic theory study on high temperature oxidation mechanism of Nb-Ti-Al alloy

The electronic structure parameters of density of electronic states, atoms embedding energy, affinity energy and cluster energy of Nb alloy have been calculated using the recursive method. The high-temperature oxidation mechanism of Nb alloy was investigated. Studies show that the oxygen adsorption on Nb alloy surface can lower the adsorption energy, so oxygen is easily adsorbed on the alloy surface, and gradually diffuses into the Nb alloy matrix. Oxygen has a high solubility in the Nb alloy matrix because of the negative atom embedding energy, and it has similar density of states to Nb. Because the atom embedding energies of Ti and Al in the alloy matrix are higher than that on the alloy surface, Ti and Al atoms diffuse from the alloy matrix to the alloy surface and segregate on the alloy surface, ultimately making the Nb surface rich in Ti and Al. The clustering energy calculation shows that Ti and Al atom tend to gather on their own area, forming Ti and Al atom clusters respectively. Oxygen can interact with Nb, Ti and Al because of the negative affinity energy with Nb, Ti and Al on Nb alloy surface, to generate the Nb2O5, TiO2 and Al2O3 mixed oxide film which has protective effect on Nb alloy.

Electronic structure and high-temperature oxidation behavior of Nb alloy

In order to reveal the physical nature of high temperature oxidation of Nb alloy in electron level, the density of states, the atomic embedded energy, the atomic affinity energy and other electronic structure parameters of Nb alloy are calculated by using the recursive method. The high-temperature oxidation mechanism of Nb alloy is investigated. The results show that the diffusion rate and the solid solubility of oxygen are high in the Nb alloy, so oxygen can easily react with Nb to form oxides, which makes the oxidation resistance of Nb alloy poor at high temperatures. The calculated results of atomic embedded energy show that the stabilities of alloying elements (Ti, Si, Cr) are lower in the matrix than in the surface of Nb alloy, so they diffuse easily in to the surface of Nb alloy to form a surface layer enrichment with Ti, Si, Cr. Alloying elements Nb, Ti, Si, Cr have negative affinity energies to oxygen in alloy surface layer, thereby forming the corresponding oxide film, which has a protective effect for Nb alloy.

High Temperature Oxidation Mechanism of Ti&lt;SUB&gt;3&lt;/SUB&gt;SiC&lt;SUB&gt;2&lt;/SUB&gt;-64vol%SiC Ceramics

采用热等静压原位合成了高致密的Ti3SiC2-64vol%SiC复相陶瓷.通过热重实验研究其在1100-1450℃中空气气氛的高温氧化行为和机理.研究显示，复相陶瓷的等温动力学曲线遵循抛物线型氧化或抛物线型直线型氧化规律.SiC （64vol%）的引入显著提高了Ti3SiC2-SiC材料的抗氧化能力.XRD及SEM-EDS分析显示，氧化膜由外层金红石型TiO2和非晶态SiO2组成，过渡层为TiO2与SiO2混合物.高温下（1400℃），非晶态SiO2的形成改变了TiO2膜的生长形态，形成致密TiO2膜，有效阻碍了氧的扩散.长时间氧化其抛物线速率常数比在1200℃下氧化低一个数量级.材料在1400℃下的抗氧化性能明显优于在1200℃下的抗氧化性能.

Influence of the structure of higher paraffin hydrocarbons and their derivatives on the mechanism of high-temperature liquid-phase oxidation

The specifics of oxidation kinetics at 160°C for structurally different hydrocarbons (normal and isobranched chains) and some of their functional derivatives were studied. A comparative analysis of products of the high-temperature oxidation of hydrocarbons was made and changes in the phase composition were studied using squalane and hexadecane as examples. It was shown that the spatial structure of a hydrocarbon subjected to oxidation makes a substantial contribution to the formation of reversed micelles. The kinetic behavior of squalane in oxidation and the solubilization of methyl orange by its oxidation products support the assumption that polyhydroperoxides form via the intramolecular 1,3-hydrogen abstraction mechanism.

High-temperature and electrochemical oxidation of transition metal silicides

High-temperature and electrochemical oxidation of transition metal silicides, which are widely used in microelectronics as ohmic contacts and protective coatings for high-temperature alloys, are discussed in this review. The process of oxide film formation during annealing or anodizing is extremely important for both applications of silicides. It is discussed for three disilicides: MoSi2, WSi2, and TiSi2. It has been shown that different types of oxide films may form on the disilicide surface depending on the thermodynamic (formation enthalpies of silicides and associated oxides) and kinetic (diffusion coefficients of silicon in silicide and oxygen diffusivity in oxide) factors: onephase (MoSi2), mixed (WSi2), and multilayer (TiSi2) films. The “silicide pest” phenomenon is discussed in terms of thermodynamic, kinetic, and structural factors influencing the pesting. Analysis of electrochemical oxidation mechanisms for silicides in various electrolyte media reveals numerous similarities between anodic and high-temperature oxidation mechanisms. It is shown that slow silicon transport during anodic treatment at room temperature leads to the formation of multiphase mixed oxide films under electrochemical polarization.