Oxidation Kinetics of Chemically Vapor-Deposited Silicon Carbide in Wet Oxygen

The oxidation kinetics of chemically vapor-deposited SiC in dry oxygen and wet oxygen ( P _H2O= 0.1 atm) at temperatures between 1200° and 1400°C were monitored using thermogravimetric analysis. It was found that in a clean environment, 10% water vapor enhanced the oxidation kinetics of SiC only very slightly compared to rates found in dry oxygen. Oxidation kinetics were examined in terms of the Deal and Grove model for oxidation of silicon. It was found that in an environment containing even small amounts of impurities, such as high-purity Al₂O₃ reaction tubes containing 200 ppm Na, water vapor enhanced the transport of these impurities to the oxidation sample. Oxidation rates increased under these conditions presumably because of the formation of less protective sodium alumino-silicate scales.     

Determination of Optimum Conditions and the Kinetics of Methanol Oxidation

In this study, the catalytic oxidation of methanol to formaldehyde was investigated in a laboratory‐scale fixed‐bed catalytic reactor, under a large number of different conditions. Iron‐molybdate catalysts supported by silica or alumina with a molybdenium/iron (Mo/Fe) ratio of 1.5, 3 and 5 were studied for the gas phase reaction. In order to obtain the optimum conditions, six different temperatures in the range of 250–375 °C and three different space times of 50.63, 33.75 and 20.25 g/(mol/h) were investigated. After determining the optimum conditions for this reaction, experiments aimed at understanding the reaction kinetics, were carried out. These experiments were performed on the catalyst favoring the formation of formaldehyde, which has a (Mo/Fe) ratio of 5 on a silica support. Seven reaction models derived by the mechanisms cited in the literature were tested to elucidate the kinetics of the reaction and the surface reaction controlling model was found to be the most suitable reaction mechanism.     

Hydrogen Permeability of Sintered Aluminum Oxide

Hydrogen permeability of high-density, high-purity sintered alumina tubes was measures is a function of temperature and pressure using tritium as a tracer. The permeability, ø, at 1200° to 1450°C and hydrogen partial pressure between 2 and 50 kPa is:
φ(H atoms cm cm⁻² s⁻¹ kPa^−0.43) = exp(48.95±0.61)
exp[−318.2±18.8 kJ(g·atom)⁻¹ (RT)⁻¹]
Diffusion coefficients and solubility values deduced from the permeation experiments are consistent with earlier independent measurements. No accelerated permeation due to micro-structural defects or changes during the experiments was observed. Comparison of hydrogen permeability of alumina with the values observed for metals shows that alumina may be a suitable coating material for use as a hydrogen permeation barrier.     

Kinetics of wet oxidation of black liquor over a Pt–Pd–Ce/alumina catalyst

The catalytic oxidation of industrial wastewater from paper and pulp mills has been investigated in a slurry reactor at a temperature range 433–463 K and at pressures from 1.5 to 2.2 MPa. Adding Ce on alumina support promotes the catalytic activity for oxidation of black liquor. Pt–Pd–Ce/alumina catalyst shows a promising activity for wet catalytic oxidation of black liquor. The oxidation reaction over a Pt–Pd–Ce catalyst is characterized by an initial fast reaction step followed by a slow reaction step. The rate of total organic carbon (TOC) reduction was described by first-order kinetics with respect to TOC concentration in black liquor for both initial and later reaction steps. The activation energies were determined to be 54.53 and 50.13 kJ/mol for the initial and later oxidation steps, respectively. Comparison of the data with the generalized kinetic model was also presented.     

酸浸提取煤矸石中氧化铝工艺优化及其动力学

以贵州盘县煤矸石为原料,采用单因素实验和正交实验方法,以浓硫酸为酸浸介质酸浸提取煤矸石中氧化铝,考察了酸浸反应主要影响因素,研究了浸出动力学. 结果表明,最佳酸浸提取条件为：酸矸质量比1.4,反应时间4 h,反应温度170℃. 该条件下氧化铝的浸取率达98.47%. 该酸浸反应过程符合收缩未反应核模型的粒径不变缩核模型,反应受界面化学反应控制,反应动力学方程为1-(1-x)1/3=kt,反应活化能为61.32 kJ/mol.     

Kinetics of the Growth of Spinel on Alumina Using Rutherford Backscattering Spectroscopy

The kinetics of the initial stage of the formation of a nickel aluminate spinel on the surface of alumina substrates has been studied. The NiO layer was formed by evaporating a thin film Ni onto the surface of single-crystal alumina substrates and oxidizing the specimens in air at 700°C. This method gave a thin layer of NiO film that was uniform in thickness on top of the alumina substrates. The alumina surfaces studied include (0001), [11 2 0], [1 1 02], and [1 1 00]. After the initial oxidation, the samples were annealed in air at 1020°C. The interaction of the thin-film and the single-crystal substrates was examined using Rutherford back-scattering spectroscopy. The crystallography of the alumina substrates was found to influence greatly the kinetics of the spinel formation. For example, the rate of the spinel formation reaction is fastest for the lsqb;1 1 02rsqb; and lsqb;1 1 00rsqb; orientations and slowest for the (0001) orientation. The roughness of the surface of the alumina substrate has also been shown to influence directly the rate of the spinel formation. The presence of surface scratches accelerated the spinel formation.     

Characterization of Chemical Vapor-Deposited (CVD) Mullite+CVD Alumina+Plasma-Sprayed Tantalum Oxide Coatings on Silicon Nitride Vanes After an Industrial Gas Turbine Engine Field Test

Silicon nitride ceramic vanes coated with chemical vapor-deposited (CVD) mullite, CVD alumina, and plasma-sprayed tantalum oxide were exposed to field tests in an industrial gas turbine engine. Results varied due to expected non-uniformities in the CVD coating microstructures, but dense CVD mullite/alumina showed excellent stability and protective capacity after 1148 h of engine testing. Surfaces without CVD coatings experienced massive intragranular subsurface oxidation and/or rapid recession of the ceramic substrate due to volatilization of silica species formed by oxidation. These results suggest that thin (<5 μm), dense, high-purity CVD mullite and CVD alumina are viable components for an environmental barrier coating system to protect structural ceramics in combustion environments.     

The Effect of Yttrium on Oxygen Grain-Boundary Transport in Polycrystalline Alumina Measured Using Ni Marker Particles

The grain-boundary transport of oxygen in polycrystalline α-Al₂O₃ (undoped and 500 ppm Y³⁺-doped) was studied in the temperature regime of 1100°–1500°C by monitoring the oxidation of a fine, uniform dispersion of Ni marker particles (0.5 vol%). The annealing treatments were carried out in a high-purity O₂ atmosphere (>99.5%). The Ni particles, which are visibly oxidized to nickel aluminate spinel, were used to determine the depth of oxygen penetration. The thickness of the reaction zone was measured as a function of heat-treatment time and temperature, and a comparison of the oxidation rate constants and activation energies for undoped and Y³⁺-doped alumina was made. The results indicate that the presence of Y³⁺ slows oxygen grain-boundary transport in alumina by a variable factor of from 15 to 3 in the temperature regime of 1100°–1500°C. The values of the activation energy for undoped and Y³⁺-doped alumina were determined to be 430±40 and 497±8 kJ/mol, respectively.     

填料式SO2-4/TiO2-Al2O3-Al固体酸催化合成乙酸乙酯

采用阳极氧化铝模板法(AAO)制备了Al2O3-Al一体型多孔氧化铝载体,采用电沉积技术将TiO2沉积到氧化铝多孔的纳米孔道内,制备了催化精馏用拉西环填料式TiO2-Al2O3-Al复合载体,采用浸渍法制备了SO2-4/TiO2-Al2O3-Al固体酸催化精馏元件,以乙酸乙酯的合成为模型反应考察了所制备固体酸催化剂的酯...     

Investigation of CO oxidation and NO reduction on three-way monolith catalysts with transient response techniques

The kinetics of CO oxidation and NO reduction reactions over alumina and alumina-ceria supported Pt, Rh and bimetallic Pt/Rh catalysts coated on metallic monoliths were investigated using the step response technique at atmospheric pressure and at temperatures 30–350°C. The feed step change experiments from an inert flow to a flow of a reagent (O₂, CO, NO and H₂) showed that the ceria promoted catalysts had higher adsorption capacities, higher reaction rates and promoting effects by preventing the inhibitory effects of reactants, than the alumina supported noble metal catalysts. The effect of ceria was explained with adsorbate spillover from the noble metal sites to ceria. The step change experiments CO/O₂ and O₂/CO also revealed the enhancing effect of ceria. The step change experiments NO/H₂ and H₂/NO gave nitrogen as a main reduction product and N₂O as a by-product. Preadsorption of NO on the catalyst surface decreased the catalyst activity in the reduction of NO with H₂. The CO oxidation transients were modeled with a mechanism which consistent of CO and O₂ adsorption and a surface reaction step. The NO reduction experiments with H₂ revealed the role of N₂O as a surface intermediate in the formation of N₂. The formation of NN bonding was assumed to take place prior to, partly prior to or totally following to the NO bond breakage. High NO coverage favors N₂O formation. Pt was shown to be more efficient than Rh for NO reduction by H₂.     

对二甲苯二次氧化过程中液相反应的研究

通过半连续实验考察了对二甲苯(PX)二次氧化过程中的液相氧化,并对该过程进行了模拟计算。设计和进行了包含PX液相氧化、中断反应以及降温二次液相氧化三个阶段的实验;通过动力学实验测定和数据回归确定了基于芳烃氧化自由基链式反应机理的PX液相氧化动力学模型的参数;采用建立的PX液相氧化机理模型对PX分段氧化过程进行了预测。结果表明,对中断反应后的降温二次液相氧化间歇过程,采用第一阶段末期自由基浓度做初值时模型预测值与实验值符合良好;而假设自由基初值浓度为零时,PX液相氧化动力学模型对二次氧化的液相反应预测效果较差,其原因可能是中断降温后的氧化母液中存在过氧基团会使二次液相氧化反应极快地被再引发启动。     

Activity and Reactivity of Carbon Formed by the Thermal Degradation of Poly(vinyl butyral) in Alumina Pellets

This paper discusses the kinetics of the oxidation of carbon formed by the thermal degradation of poly (vinyl butyral) (PVB) in alumina pellets. The kinetics, studied in a flow reactor using wet nitrogen as the oxidant, followed zero-order kinetics. The carbon was found to be very reactive, and it possibly was a form of Dent carbon. The activity of this carbon compared with graphite in alumina pellets showed that the carbon formed by the thermal degradation of PVB to be 4 to 6 times more active than graphite. The total organic content of the pellets was varied from 0% to 15% and the kinetics was studied in the temperature range of 873 to 1073 K. Activation energies were expressed as a function of the weight percent of carbon in the pellet. Experimental data with two particle sizes of alumina gave different activation energies.     

Dynamic modelling of a gas phase catalytic fixed-bed reactor—I: Experimental apparatus and determination of reaction kinetics

A large scale fixed bed pilot reactor for performing dynamic experiments is described. The reactor system is especially designed to suppress secondary dispersion effects not characteristic for the packed bed itself.As a model reaction the reaction between oxygen (<1%) and hydrogen on a platinum catalyst supported by alumina has been used.Differential reactor experiments disclosed a hysteresis phenomenon in the catalyst activity. The catalyst is generally more active when going from high to low temperatures than vice versa.A global first order reaction rate expression with Arrhenius temperature dependency fits the fixed-bed reactor profiles well but the static gains badly. However by simultaneous estimation of frequency factor and activation energy in several axial segments a much better approximation to the static gains was obtained. This result indicates that the reaction kinetics is more complicated than first assumed. However for dynamic modelling the exact reaction mechanism is not needed.     

Predicting the Grain-Size Distributions in High-Density, High-Purity Alumina Ceramics

This paper discusses the image-analyzer-based grain-size distributions (GSDs) of fully densified ceramics obtained from pressure casting a high-purity alumina powder, develops an algorithm for predicting the GSDs as a function of sintering time and temperature, and compares of the GSDs thus predicted with those experimentally observed. The GSD data for all sintered specimens, when nondimensionalized in terms of the median grain size, reduced to a single self-preserving GSD curve. The median grain was predicted as a function of sintering time and temperature using the classical kinetics equation. The GSDs predicted using the algorithm developed tallied well with those that were experimentally obtained.     

Relating Grain Boundary Complexion to Grain Boundary Kinetics II: Silica-Doped Alumina

This second paper in a series describes the relationship between grain growth kinetics and grain boundary complexions in silica-doped alumina. Dense high-purity silica-doped alumina samples were annealed for various times in the temperature range of 1300° and 1900°C and their grain growth behavior was quantified. Four different grain boundary complexions were observed in silica-doped alumina, all of which enhanced the kinetics relative to the intrinsic undoped alumina. These complexions included a thick crystallized film that was likely amorphous at high temperatures, a thin intergranular film, multilayer adsorption, and a type of boundary that showed no observable film by high-resolution transmission electron microscopy. A generational change in the population of grains occurred at 1500°C where all of the abnormal grains impinged and reestablished a new normal distribution. At higher temperatures a new set of abnormal grains containing different complexions formed in the microstructure. The activation energy of the normal and abnormal grains was approximately the same. The effects of varying dopant concentration were analyzed. The results for silica-doped alumina are compared with previous results for calcia-doped alumina in order to draw some generalized conclusions about the effect of complexions on grain growth.     

Oxidation Behavior of SiC-Whisker-Reinforced Alumina-Zirconia Composites

During high-temperature oxidation in air, SiC-whisker-reinforced Al₂O₃—ZrO₂ composites degrade by the formation of a whisker-depleted mullite-zirconia scale. The reaction kinetics have been studied as a function of time and temperature for composites with whiskers preoxidized for different times. The evolution of the microstructure has been investigated by optical, scanning and transmission electron microscopy. Possible reaction mechanisms have been discussed. A model compatible with our observations on Al₂O₃—ZrO₂—SiC and the results reported in the literature for Al₂O₃—SiC whisker composites is proposed: The oxidation occurs at an internal reaction front. Oxygen diffuses along dislocations and grain boundaries through the mullite scale to react at this front with silicon carbide, thereby forming amorphous silica and graphite. Silica penetrates grain boundaries and further reacts with alumina and zirconia to form mullite and zircon, while the second reaction product, graphite, is oxidized into carbon monoxide when the reaction front moves deeper into the sample.     

Solidification Behavior of Undercooled Liquid Aluminum Oxide

Solidification of aluminum oxide from undercooled melts was investigated in containerless experiments. Specimens were levitated in a gas jet, stabilized with an acoustic positioning device, and melted with cw CO₂ laser beams. Cooling curves were obtained by optical pyrometry when the laser intensity was reduced. The materials examined were high-purity Verneuil sapphire, 99.5% polycrystalline alumina, and oxide materials recovered from the effluent of an aluminum-fueled rocket motor. The degree of undercooling, the apparent temperature behavior during the thermal arrest on solidification, and the structure of the materials formed were different in argon and oxygen atmospheres. Undercooling of the sapphire and alumina materials was 360 ± 10 K in an oxygen atmosphere and approximately 450 K in argon. Melting and solidification of high-purity sapphire resulted in a dendritic and porous polycrystalline material in oxygen. Dense, larger crystals were obtained in argon. Products formed from 99.5% alumina were discolored and the cores were white, indicating impurity segregation effects. More reproducible behavior was observed for the sapphire and 99.5% alumina than for the tungstencontaminated rocket motor effluent materials.     

Influence of codoping with Hf and La on grain-boundary transport in alumina

The role of reactive elements (RE) is an important topic in understanding the oxidation behavior of high-temperature alloys. In this work, the influence of codoping alumina with two different RE elements (500 ppm Hf + 500 ppm La) was studied. The kinetics of oxygen grain-boundary (GB) transport were studied at 1400°C using metallic nickel particles as markers. The results were compared with data obtained on the corresponding singly doped compositions; alumina-500 ppm La, and alumina-500 ppm Hf. The results showed that singly doping with La did not have any benefit compared to undoped alumina, whereas singly doping with Hf resulted in a slowing of transport by a factor of ~7. The behavior of the codoped sample was very similar to that of the singly doped Hf composition. For all the studied compositions, atomic scale characterization using high-angle annular dark-field scanning transmission electron microscopy and atom probe tomography (APT) revealed strong segregation of the dopant ions to the alumina grain boundaries. In the codoped sample, APT revealed evidence of oxygen excess and aluminum depletion at the GB core.     

The oxidation of aromatic organic compounds at a lead dioxide electrode

The direct electrochemical oxidation of some aromatic hydrocarbons has been investigated using the methods of electrode kinetics. The results are used to suggest conditions under which the oxidation reaction could be used for larger scale synthesis of the corresponding aromatic alcohol and aldehyde.     

红柱石基材料中氧化铝原料的应用研究

实验研究了几种氧化铝原料的加入对高纯红柱石基耐火材料性能的影响,采用XRD,SEM等手段分析了氧化铝原料种类及加入量对试样相组成和显微结构的影响.实验结果表明,纯红柱石制品的烧结性能比添加氧化铝细粉的红柱石制品的好,而不同氧化铝细粉对红柱石制品烧结性能的影响与其纯度及活性有关;红柱石颗粒分解产生的SiO2玻璃相难以全部与氧化铝反应形成二次莫来石;在本试验条件下,当氧化铝细粉加入量达到15%左右时,在试样中引起的二次莫来石化效应和烧成膨胀最大.