Interfacial Reaction Kinetics Research Articles

In- situ study of interfacial oxygen transfer in a supported platinum catalyst

Solid state electrochemical cells can be used for the in- situ study of catalysts. The cell is formed using a solid electrolyte such as yttria-stabilised zirconia (YSZ), a very good oxygen- ion conductor above 250°C. The catalyst of interest is deposited in the form of a porous electrode upon the solid electrolyte support and is exposed to reaction conditions. In this work, the closed-circuit operation of such cells has been used to directly investigate the kinetics of interfacial reactions taking place between species adsorbed on the catalyst surface and the support of electrolyte. The system studied was carbon monoxide oxidation over platinum- on- YSZ. It was found that carbon monoxide adsorbed on the platinum surface could react with the YSZ support removing an oxygen species and producing carbon dioxide. Consequently, the rate of carbon dioxide production should be greater over platinum deposited on YSZ than over platinum on a support which will not conduct oxygen ions. Indeed platinum- on- YSZ powder did show a greater rate of carbon dioxide production than platinum- on- alumina powder. Similar interfacial reactions are also expected to occur as metal- support interactions in catalysts such as platinum- on- titania and platinum- on- ceria.

Interfacial reaction kinetics of coated SiC fibers with various titanium alloys

The kinetics of the reaction between the silicon carbide fibers and the titanium-based alloy matrix was investigated at temperatures from 800 to 1000 C for several titanium-based alloys (including Ti-1100 alloy and BETA 21S) and unalloyed Ti, reinforced with coated silicon carbide fiber SCS-6. The reaction zone growth kinetics was studied by exposing vacuum encapsulated samples to temperatures from 700 to 1000 C for times up to 150 hrs, followed by SAM observations of samples which were polished perpendicular to the fiber axis and etched. It was found that the reaction zone growth kinetics of the alpha (hcp) and beta (bcc) phases of unalloyed titanium reacting with SCS-6 fibers exhibited different values of the apparent activation energy and of the preexponential factor. Additions of other metals to Ti was found to slow down the reaction kinetics. Among the alloys studied, the Ti-1100 was the slowest reacting conventional alloy and the Ti-14Al-21Nb (in wt pct) was the slowest overall.

Interfacial reaction kinetics of SiC fiber-reinforced Ti 3Al matrix composites

1. (1) SiC fibers reacted with Ti 3Al + Nb to form multilayer reaction products during consolidation and extended isothermal exposure. Complex carbides and silicides genertated from the reactions between Ti, Nb, Al and SiC appears to be the major components in the reaction zone. The C-rich layer on the surface of a SiC fiber affects the development of the reaction zone and the distribution of the reaction products. 2. (2) The fiber/matrix interfacial reaction is diffusion-controlled with an activation energy of 271.49 KJ/mole and 218.11 KJ/mole for the SCS-6/Ti-24A1-11Nb and Sigma/Ti-24Al-11Nb composites, respectively, at the early stage of reaction. 3. (3) The results of this study indicate that in the SCS-6/Ti-24Al-11Nb composite, the activation energy is higher, the growth rate of reaction zone is slower, and the consumption of C-rich layer is much slower than that in the SCS-6/Ti-6Al-4V composite.

Mathematical models of the kinetics of dynamic thermogravimetry

Mathematical models of the kinetics of nucleation growth and interfacial reaction which are suitable for dynamic thermogravimetry are established by investigating the processes of the thermal decomposition of lead carbonate. The models are represented by ▪ and ▪ The thermal decomposition reactions of lead carbonate are given as 2PbCO 3(s) = PbCO 3·PbO(s) + CO 2 (I) 3(PbCO 3·PbO)(s) = 2(PbCO 3· 2PbO)(s) + CO 2 (II) PbCO 3·2PbO(s) = 3PbO(s) + CO 2 (III) The activation energies of these reactions obtained from the models are 178 60, 208.20 and 232.79 kJ mol −1, respectively The experimental results show that the mathematical models of non-isothermal kinetics are valid, reliable, and practical.

Attenuated Total Reflection FT-IR Spectroscopy to Measure Interfacial Reaction Kinetics at Silica Surfaces

Attenuated total reflection (ATR) FT-IR spectroscopy has been adapted to measure the rates of chemical modification reactions at silica surfaces. In this method, a silicon ATR crystal is oxidized at 1000°C under dry O2 to produce a silicon dioxide layer on the crystal surface, the thickness of which can be measured using ellipsometry. The oxidized silicon layer is then used as a model silica surface in measurements of the rates of chemical modification. A flow cell is filled with a solution of a surface active reagent. ATR infrared spectra are obtained at regular time intervals, where confinement of the intensity to the interface by total internal reflection provides a measure of local changes in concentration of species which adsorb or bind to the surface. The effect of the silicon dioxide layer on the sensitivity of measuring absorbance at the silica-solution interface is investigated. A model reaction study is carried out to determine the binding kinetics diphenylchlorosilane to silica from carbon tetrachloride solution. The technique yields both in situ kinetic and structural information about the reaction of this reagent with silica surfaces.

The effect of Frenkel defect formation on spherical SiO2 precipitate growth in silicon wafers

A general analytic solution is given to the sphere-growth problem which includes (1) oxygen and point defect transport, (2) interface reaction kinetics, (3) surface energy, and (4) strain energy storage. Short, intermediate, and long time behaviors are given. In the long time domain, where the Peclet number for oxygen is constant provided the far field conditions are constant, the growth rate is dominated at low and intermediate temperatures primarily by silicon interstitial formation and somewhat by strain energy storage. At high temperatures (≳1150 °C), these effects become small and the interface begins to approach the thermodynamic equilibrium state. However, at all lower temperatures, the supersaturation of silicon interstitials at the SiO2/Si interface is very large and the interface is far from the thermodynamic equilibrium. Extension of the mathematical approach to precipitates of other shapes is also given.

A Comparative Study of Thin-Film and Bulk Reaction Kinetics and Diffusion Path: the Ir/GaAs System

ABSTRACTControl of the structure and chemistry at the interfaces of compound semiconductors is essential for the commercial use of these materials in electronic and optical technologies. This can only be achieved when the governing thermodynamics and kinetics of interfacial reactions are understood. Based primarily on the experience of metal/Si interactions, however, a prevailing belief was born that thin-film reactions follow a separate set of thermodynamic and kinetic “rules” which are different from bulk reactions. The intent of our work has been to not only characterize metal/GaAs contact reactions but also to rationalize these reactions with equilibrium phase diagrams and bulk metal/GaAs diffusion couple experiments. Through this approach, a better understanding of thin-film and bulk differences has been obtained.The Ir/GaAs system is used as an example. Phase formation and reaction kinetics were studied for 30 nm Ir films on (100) GaAs using TEM, XTEM, and AEM. Bulk diffusion between 0.25 mm thick Ir foil and (100) GaAs wafers was studied with SEM and electron probe microanalysis (EPMA). The diffusion paths and kinetics were the same for thin-film and bulk. The phase sequence Ir/IrGa/IrAs2/GaAs formed for all diffusion couples. Reaction kinetics were parabolic with an activation energy of 3.0 eV for both thin-film and bulk, and the data was colinear in an Arrhenius plot. Reacted layer morphology in both cases was layered. The effects of grain size, crystallographic texturing, and the relative diffusivities of the components on the reaction mechanisms in bulk versus thin-film reactions are considered.

UHV conversion of a 300 kV high-resolution electron microscope

Ultra-high-vacuum high-resolution electron microscopy (UHV-HREM) is a powerful technique for studying the structure of surfaces, and for characterizing the mechanisms and kinetics of surface and interface reactions. It requires an electron microscope capable of atomic resolution, a vacuum of about 10-10torr around the sample, and a range of specimen treatment capabilities. We have replaced the standard specimen chamber of a Philips 430ST high resolution microscope by a special UHV chamber which allows for limited specimen treatment in-situ, and a full range of specimen treatments in a preparation chamber mounted on the side of the microscope column. At 300 kV, the objective lens (Cs= Cc= 1.1mm) of the 430ST has demonstrated a point-to-point resolution of 2.0 Å, and a spatial frequency transfer limit with axial illumination of better than 1.5 Å. A critical specification for the microscope conversion was that this performance should not be compromised even under full UHV operation.

Artificial photosynthesis, highly efficient sensitization of wide band gap semiconductors

The sensitization of semiconductors is a fascinating domain of present research which has found important applications in different areas such as artificial photosynthesis. This lecture discusses first the kinetics of interfacial electron transfer reactions associated with the sensitization process. The advent of colloidal semiconductors combined with ultrafast laser techniques has permitted the unravelling of the dynamics of the charge transfer reactions. In the case of 100 A sized TiO2 particles the electron injection from the excited sensitizer in the conduction band was found to be several orders of magnitude faster than the back reaction. This effect has been exploited by us in the conversion of light into electricity or chemical fuels. Using rough polycrystalline TiO2 layers we have achieved incident photon to current conversion yields of more than 60% in the visible Such high values are unprecedented and constitute a breakthrough in the sensitization of large band gap semiconductor devices.

Hydrogen induced changes in the metallurgical interactions at PtSi–Si interfaces

This paper addresses the role of hydrogen in the kinetics of interfacial reactions between PtxSi1−x (x=0.67, 0.5) thin films, prepared by reactive sputtering of Pt in a silane plasma, and Si substrates and consequent changes in the electrical characteristics of PtSi/Si Schottky diodes. The effects of the deposition temperature and the alloy composition on the degree of the interfacial reactions have been examined with Rutherford backscattering and x‐ray diffraction techniques. The absolute hydrogen concentration in the films, as measured using ion beam forward scattering, decreases with increasing temperature of deposition and also with increasing Pt concentration. An important effect of hydrogen incorporation in the films with x<0.5 is a reduction in the kinetics of PtSi formation at the film–substrate interface. The diode parameters, (φb and η), for all the compositions of the films, improve with increasing deposition temperature until 400 °C; above 400 °C however, the barrier height decreases and the I–V characteristics deviate from ideality. The deterioration of the diode parameters has been found to be due to a significant degree of Pt diffusion into the bulk Si at the higher temperatures of deposition.

Dynamic interfacial tensions in acidic crude oil/caustic systems. Part II: Role of dynamic effects in alkaline flooding for enhanced oil recovery

AbstractA model for caustic propagation in a core during continuous injection is combined with the considerations of interfacial reactions proposed in Part I in order to examine the role of dynamic effects in continuous alkaline flooding. Axial dispersion of caustic and its reversible uptake by the reservoir rock are taken into account. From this model, the interfacial tension between an isolated oil globule and the surrounding flood water can be predicted as a function of time using the knowledge of the kinetics of interfacial reactions, the rock adsorption equilibria, and the longitudinal dispersion coefficient.

Microstructure of Metal-GaAs Interfaces

ABSTRACTThe interfacial reactions and microstructures of metal-GaAs contacts are, in general, much more complicated and difficult to control than the corresponding metal-Si contacts. They are very sensitive to reaction temperature, ambient, metal layer thickness, and the GaAs surface cleaning procedure. Many of the ohmic and gate contacts to GaAs currently in use or under development for GaAs FET devices are comprised of more than one metal species and in some cases a doping element as well. In dealing with these complexities, information about the microstructure at the contact interface is critically needed: for the evaluation of a specific contact metallurgy, for the definition of an optimum fabrication process, and, most important of all, for generating new ideas for better contact schemes. In this paper, our TEM and STEM studies of several ohmic and gate contacts that are of technological interest will be described. Attention will be drawn to the link between the interfacial microstructures and their electrical behavior, the kinetics of interfacial reactions, and thermal stability. The current constraints on obtaining ideal, reliable and controllable metal-GaAs contacts will also be discussed.

Thermal oxidation of silicon: Chemisorption and linear rate constant

This is the first of a series of three papers which, by introducing new concepts and new formulations that are consistent with experimental observations, attempt to treat a number of topics related to the kinetics and the mechanisms of the thermal oxidation of silicon. The present paper deals with the surface reaction aspect of oxidation. A new oxide growth law has been formulated to be consistent with the observed power-law pressure dependence of interface reaction kinetics. Then the subject of chemisorption, proposed here as an intermediate step of the thermal oxidation of silicon, is treated at length. It is shown, from a first-principles analysis, that the energy of chemisorption should vary approximately logarithmically with the amount of chemisorption, that this leads to an adsorption isotherm which is related to the oxygen pressure by a power law, and that the exponent of the power law should vary linearly with temperature within a reasonable span. The effects of substrate doping and of substrate damage are also analyzed.

Low temperature compound formation in [formula omitted] thin film couples

Transmission electron microscopy (TEM) and Auger electron spectroscopy have been used to study the morphology and kinetics of compound formation in vapor-deposited polycrystalline bimetallic Cu Sn thin film diffused at low transport temperatures. The TEM observations show that two concurrent mechanisms, namely the interstitial transport of copper in tin and the grain boundary diffusion of tin in copper, lead to the sequential formation of η′ ( Cu 6 Sn 5) and ε ( Cu 3 Sn) intermetallic compounds. Preliminary Auger depth profiling data indicate linear growth kinetics for the η′ compound at 86°C implying that interfacial reaction kinetics is the rate-controlling step. A detailed model for the interdiffusion and reaction process in Cu Sn thin films is presented.

Reservoir Simulation: State of the Art (includes associated papers 11927 and 12290 )

Distinguished Author Series articles are general, descriptiverepresentations that summarize the state of the art in an area of technology bydescribing recent developments for readers who are not specialists in thetopics discussed. Written by individuals recognized as experts in the area, these articles provide key references to more definitive work and presentspecific details only to illustrate the technology. Purpose: to informthe general readership of recent advances in various areas of petroleumengineering. Introduction The purpose of this paper is to describe the current level of development inreservoir simulation. This requires some discussion of what a simulation modelis and why it is needed or used. Following a brief history of simulation and ageneral description of a simulation model, two sections describe the reservoirsimulator through discussions of recovery mechanisms and model methodology. Thesecond of these sections discusses past and recent developments and summarizesthe technology currently used in simulation models. The two descriptivesections are followed by a discussion of why simulation is used (i.e., typicalreservoir performance questions addressed by computer simulation), a sectionwith examples pertinent to simulation today, and a summary. A Brief History In it broad sense. reservoir simulation has been practiced since thebeginning of petroleum engineering in the 1930's. Simulation is simply the useof calculations to predict reservoir performance (to forecast recovery orcompare economics of alternative recovery methods). Before 1960. thesecalculations consisted largely of analytical methods, zerodimensional materialbalances, and one-dimensional (ID) Buckley-Leverett calculations. The term"simulation" became common in the early 1960's. as predictive methods evolvedinto relatively sophisticated computer programs. These programs represented amajor advancement because they allowed solution of large sets offinite-difference equations describing two- and three-dimensional(2- and 3D), transient, multiphase flow in heterogeneous porous media. This advancement wasmade possible by the rapid evolution of large-scale, high-speed digitalcomputers and development of numerical mathematical methods for solving largesystems of finite-difference equations. During the 1960's. reservoir simulationefforts were devoted largely to two-phase gas/water and three-phase black-oilreservoir problems, Recovery methods simulated essentially were limited todepletion or pressure maintenance. It was possible to develop a singlesimulation model capable of addressing most reservoir problems encountered.This concept of a single, general model always has appealed to operatingcompanies because it significantly reduces the cost of training and usage, and, potentially, the cost of model development and maintenance. During the 1970's, the picture changed markedly. The sharp rise in oil prices and governmentaltrends toward deregulation and partial funding of field pilot projects led to aproliferation of enhanced-recovery processes. This led to simulation of newprocesses that extended beyond conventional depletion and pressure maintenanceto miscible flooding, chemical flooding, C02 injection, steam or hot waterstimulation/ flooding. and in-situ combustion. A relatively comfortableunderstanding of two-component(gas and oil) hydrocarbon behavior in simpleimmiscible flow was replaced by a struggle to unravel and characterize thephysics of oil displacement under the influence of temperature, chemicalagents, and complex multicomponent phase behavior. In addition to simplemultiphase flow in porous media, simulators had to reflect chemical adsorptionand degradation, emulsifying and interfacial tension (IFT) reduction effects, reaction kinetics, and other thermal effects and complex equilibrium phasebehavior. The proliferation of recovery methods in the 1970's caused adeparture from the single-model concept as individual models were developed torepresent each of these new recovery schemes. Thus, the emphasis today is onexamining and fine tuning the equations and related assumptions pertinent tothese techniques. Research during the 1970's resulted in many significantadvances in simulation model formulations and numerical solution methods. Theseadvances allowed simulation of more complex recovery processes and/or reducedcomputing costs through increased stability of the formulations and efficiencyof the numerical solution methods.

Kinetics of interfacial reaction in bimetallic CuSn thin films

Kinetics of formation of Cu 6Su 5 between Cu and Sn thin films at room temperature and of Cu 3Sn between Cu 6Sn 5 and Cu at temperatures from 115° to 150°'C were measured by Rutherford backscattering spectroscopy and glancing-incidence X-ray diffraction. The growth of Cu 6Sn 5; showed a linear rate and the reduction of Cu 6Sn 5 due to the growth of Cu 3Sn showed a parabolic rate with an activation energy of 0.99 eV. A flash of W film was deposited between Cu and Sn as diffusion markers during their room temperature reaction. Marker displacement measured by the backscattering technique showed that Cu is the dominant diffusing species in forming Cu 6,Sn 5. By annealing a Cu 3Sn/Cu sample at 640°C for 20 min and by cooling (not a rapid quenching) it to room temperature, we found that the high temperature phase Cu 4Sn can be made metastable at room temperature.

The influence of sulfur on interfacial reaction kinetics in the decarburization of liquid iron by carbon dioxide

The kinetics of decarburization of continuously carbon-saturated liquid iron by CO2 have been studied between 1280 and 1600‡C at sulfur concentrations between 0.01 and 1 wt pct. The results are consistent with a surface blockage mechanism by chemisorbed sulfur which shows an essentially ideal adsorption isotherm. The adsorption coefficient of sulfur, in (wt pct)-1, is given by the equation logK = 3600/T + 0.57 for carbon-saturated alloys. A small residual rate at apparent surface saturation is observed. This leaves about 1.4 pct of the active surface sites available for reaction, essentially independent of temperature. Studies with varying carbon concentration suggest that to a first approximation, and above about 3 wt pct C, the adsorption equilibrium for sulfur depends only on the thermodynamic activity of sulfur.

Observations on the interaction of dislocations with a migrating interphase boundary

Observations of defect generation during the β ( b. c. c.) ⇆ ζ ( h. c. p.) transformation in the AgAl system have been performed by hot stage electron microscopy. Upon cooling, during the β → ζ reaction both the generation and absorption of dislocations are quite active at the transformation front. At high under-cooling, dislocation generation into the ζ phase was observed to be related to a growth accident process. In the β phase, dislocation emission from a migrating β-ζ interface appeared to be associated with the relaxation of transformation strains and was connected with the presence of step-type interfacial defects. During further transformations, the dislocations emitted into the β phase were absorbed within the same boundary at which they had been generated. Transformation induced dislocation emission was found to contribute to the development of a non-uniform interphase boundary migration and a modified boundary morphology at high under-coolings. These observations indicate that the interaction of dislocations with a migrating interphase boundary can have an important influence on the interfacial reaction kinetics during a phase transformation.

Interfacial reaction kinetics in the decarburization of liquid iron by carbon dioxide

The kinetics of decarburization of liquid iron have been studied between 1160 and 1600°C under conditions where mass transport of reactants is not rate determining. Studies with continuously carbon-saturated iron and of iron with varying carbon concentration have been used to show that the slow step at high concentrations of carbon is independent of carbon concentration and is first order with respect to the pressure of CO2. For high purity iron, the forward rate constant, in mole cm2 s-1 atm-1, is given by the equation ln kf = -11,700/T-0.48. It is concluded that the data are consistent with the chemisorption process as the rate limiting step. A marked sensitivity of the rate to trace amounts of sulfur has been found and it is shown that this is consistent with ideal adsorption of sulfur and is in fair accord with the existing measurements of the depression of the surface tension of iron-carbon alloys by sulfur.

The kinetics of fast interfacial reactions

The kinetics of fast interfacial reactions