Structures of thermal double donors in silicon

Accurate total-energy calculations are used to study the structures and formation energies of oxygen chains as models for thermal double donors (TDD's) in Si. We find that the first three TDD's (TDD0-TDD2) consist of one four-member ring, with one or two adjacent interstitial O atoms. These metastable TDD's form bistable negative-U systems with the corresponding stable, electrically inactive staggered structures. The TDD3-TDD7 structures are found to consist of four-member rings with adjacent interstitial O atoms at both ends. The TDD's with a central "di-Y-lid" core are found to become energetically competitive with the four-member ring TDD's only for clusters larger than ten O atoms.     

Solvable Aggregation-Migration-Annihilation Processes of a Multispecies System

An aggregation-migration-annihilation model is proposed for a two-species-group system. In the system, aggregation reactions occur between any two aggregates of the same species and migration reactions between two different species in the same group and joint annihilation reactions between two species from different groups. The kinetics of the system is then investigated in the framework of the mean-field theory. It is found that the scaling solutions of the aggregate size distributions depend crucially on the ratios of the equivalent aggregation rates of species groups to the annihilation rates. Each species always scales according to a conventional or modified scaling form; moreover, the governing scaling exponents are nonuniversal and dependent on the reaction details for most cases.     

Solvable Aggregation-Migration-Annihilation Processes of a Multispecies System

An aggregation-migration-annihilation model is proposed for a two-species-group system. In the system,aggregation reactions occur between any two aggregates of the same species and migration reactions between two different species in the same group and joint annihilation reactions between two species from different groups. The kinetics of the system is then investigated in the framework of the mean-field theory. It is found that the scaling solutions of the aggregate size distributions depend crucially on the ratios of the equivalent aggregation rates of species groups to the annihilation rates. Each species always scales according to a conventional or modified scaling form; moreover, the governing scaling exponents are nonuniversal and dependent on the reaction details for most cases.     

A DFT-based microkinetic theory for Fe2O3 reduction by CO in chemical looping

Redox kinetics of oxygen carrier in chemical looping is an important component for material preparation, reactor design and process demonstration. How to bridge the gap between the microscale density functional theory (DFT) and the macroscale redox kinetics and develop a first-principle-based theoretical model is still a challenge in the field of chemical looping. This study addresses this challenge and proposes a DFT-based microkinetic rate equation theory to calculate the heterogeneous kinetics of Fe₂O₃ reduction by CO in chemical looping. Firstly, the DFT calculation is adopted to search the reaction pathways and to obtain the energy barriers of elementary reactions. Secondly, the DFT results are introduced into the transition state theory (TST) to calculate the reaction rate constants and build the rate equations of elementary surface reactions. Finally, by considering the bulk diffusion, a rate equation is developed to bridge the gap between the elementary surface reactions and the grain conversion. In the theory, the reaction mechanism obtained from DFT and kinetic rate constants obtained from TST are directly implemented into the rate equation to predict the reduction kinetics of oxygen carriers without fitting experimental data. The accuracy of the developed theory is validated by experimental data of two Fe₂O₃ oxygen carriers obtained from the thermogravimetric analyzer (TGA). The microkinetic rate equation theory is based on the first principles calculation and can predict directly the redox kinetics of oxygen carriers without depending on the experimental kinetic data, therefore, it provides a powerful theoretical tool to screen the oxygen carrier materials and optimize the microstructure of oxygen carriers.     

Modeling the process of chemical regeneration of air in airtight habitable facilities

Physical experiments and mathematical modeling are used to study the kinetics of the reactions of carbon dioxide and water with potassium superoxide accompanied by oxygen release at various values of the temperature and humidity of the breathing gas mixture. The kinetics of the chemisorption is demonstrated to be limited by the rate of air regeneration in an airtight habitable facility. Experimental and analytical approaches are applied to determine the kinetic coefficients of the chemical reactions using the experimental data and a mathematical model of chemisorption kinetics. To perform the above chemical reactions, an original-design chemisorption reactor was developed, which contains plates with potassium superoxide nanocrystalline fixed on the fibers and pore surface of a fibrous polymer matrix. A mathematical model of chemical air regeneration is developed to calculate the guaranteed values of the parameters of the reactor and the protective effect time of the chemisorbent during which, at a given load, the reactor provides the appropriate concentrations of oxygen and carbon dioxide in the breathing gas mixture in an airtight habitable.     

Understanding oxygen reactions in aprotic Li-O_2 batteries

Although significant progress has been made in many aspects of the emerging aprotic Li-O_2 battery system, an indepth understanding of the oxygen reactions is still underway. The oxygen reactions occurring in the positive electrode distinguish Li-O_2 batteries from the conventional Li-ion cells and play a crucial role in the Li-O_2cell's performance(capacity, rate capability, and cycle life). Recent advances in fundamental studies of oxygen reactions in aprotic Li-O_2 batteries are reviewed, including the reaction route, kinetics, morphological evolution of Li_2O_2, and charge transport within Li_2O_2. Prospects are also provided for future fundamental investigations of Li-O_2 chemistry.     

Peculiarities of diffusion-controlled recombination kinetics at long time and/or for great initial reagent concentrations

The earlier developed generalized theory of bimolecular reactions between reagents (defects) in solids and liquids, based on a more complete than the standard one decoupling of the hierarchy of equations for many-point densities, is used for detailed calculations of theA+BAB reaction kinetics at long time (great reaction depth). The effects of great initial concentrations and relative diffusion coefficients upon the reaction kinetics are also studied. The earlier predicted lowering of the reaction rate due to a dynamical aggregation of similar (non-interacting) reagents is confirmed. The greater the initial reagent concentration and if one kind of reagents (A's orB's) are immobile, the more pronounced is the aggregation effect.One of the authors (E. K.) is greatly indebted to the Institute of Physics of Charles University for warm hospitality during his visit in autumn 1983 and Dr. L. Skála for the help in computer calculations at the early stage of this work.     

On the mechanism of explosive etching of PMDA-ODA film in nonequilibrium oxygen plasmas

The explosive acceleration of PMDA-ODA film etching in the oxygen plasma generated by a high-frequency inductive discharge at a low pressure (P=0.2 Pa) is described. The peak in the etch rate is shown to depend on the energy of bombarding ions and film thickness. The explosive kinetics of PMDA-ODA film etching is controlled by the cyclodehydration reaction transforming it into a polyimide film. The imidization is accelerated by the exothermic effect of oxidation reactions that take place on the film surface. As a result, the two reactions form a positive feedback loop, which promotes the ion-induced chain process of PMDA-ODA film etching and imidization.     

化学反应与饱和模型对化学氧碘激光性能的影响

 将速率方程（RE）模型与化学动力学模型相结合，讨论了增益饱和模型与化学反应系统对COIL性能的影响。流动为预混的一维模型，考虑了10种成分和21个化学反应，分析计算了未分解碘分子，激发态氧产率，水含量以及温度等因素对COIL性能的影响。计算结果表明，碘流量过多，混合和反应过程中消耗大量能量；碘流量过低，导致粒子数反转和增益过低，对于能量的提取不利。     

Positive ion emission during the interaction of dissociated gases with alkali-halide crystals

Radical-recombination emission (RRE) from the surfaces of a number of alkali-halide crystals has been studied in an atmosphere of nitrogen, hydrogen, and oxygen. It is found that RRE of positive ions in an atmosphere of atomic hydrogen is much greater than from atmospheres of either nitrogen or oxygen. The kinetics of positive ion emission have also been investigated. It is found that the emission intensity is proportional to the surface coverage by chemisorbed atoms. The experimental data are consistent with the assumption that RRE is due to sputtering of alkali-metal ions (which together with a halide form the matrix of the crystal lattice) during surface atomic recombination reactions.     

Theoretical kinetics study of thymine tautomerism and interaction of Na+ with its tautomers

The current work is a theoretical study of the tautomerism of thymine in the gas phase. Eighteen structures were found in the isomerisation reaction of thymine, some of which are reported for the first time. Thirty hydrogen transfer reactions were carried out. In 24 of the reactions, the hydrogen abstractions N―H→O, N―H→C and C―H→O were considered. The potential energy surface for all trajectories was determined for 18 tautomers and 40 transition states. The RRKM-TST model was used to calculate the rate constants of the reactions to examine their kinetics. Nonlinear least-squares fitting was used to calculate the rate constants expressions. The interaction of sodium ion and tautomers in the gas phase was also investigated. Three types of interaction of metal cations with thymine were found. In the first, metal cations interact with a lone pair of nitrogen or oxygen tautomers. The second type is the interaction of metal cations with two nitrogen and oxygen of tautomers. The last type is the interaction of metal cations and the electron density of the π-system of thymine in which the metal ion is perpendicular to the ring of tautomers. The stability ranking of the thymine tautomers and their complexes was also determined.     

Role of adsorption complexes in catalytic acceleration of heterogeneous reactions

The universal equation that imposes restrictions on the kinetics of heterogeneous chemical reactions has been derived. A method of studying the participation of chemisorbed or physisorbed gas molecules in heterogeneous chemical reactions has been proposed. It has been shown that the heterogeneous reaction rate is independent of the degree of occupation of the catalyst surface by the reactants as long as the product is formed by chemisorbed complexes and physisorbed molecules. It has been found that the formation of CO₂ in the heterogeneous chemical reactions CO+ O → CO₂ and 2CO + O₂ → 2CO₂ is participated by physisorbed particles and chemisorbed complexes of oxygen atoms and CO molecules.     

A transient study of the co-adsorption and reaction of CO and oxygen on Ir(110)

The heterogeneously catalyzed reaction of CO and O₂ to form CO₂ over Ir(110) has been studied through measurements of the transient kinetics of the various elementary reactions that may limit the steady state rate. Rate expressions for these elementary reactions — the desorption of CO, the oxidation of CO via the Langmuir-Hinshelwood mechanism, the adsorption of CO and the adsorption of oxygen — were developed using thermal desorption mass spectrometry. Several phenomena were observed: (1) the activation energies for CO desorption and CO oxidation depend markedly upon the composition of the adlayer; (2) diffusion in the adlayer may limit the rates of CO desorption and CO oxidation; (3) the formation of a surface oxide modifies these four rate processes; and (4) chemisorbed CO blocks sites for oxygen adsorption, but chemisorbed oxygen does not block sites for CO adsorption.     

Bistable kinetics of simple reactions on solid surfaces: lateral interactions, chemical waves, and the equistability criterion

In heterogeneous reactions, the rate constants of desorption, diffusion and elementary reaction steps are usually strongly dependent on reactant coverages due to adsorbate-adsorbate lateral interactions. We analyze the effect of this factor on the bistable regime of the reaction kinetics. As an example, we consider CO oxidation on Pt(111). The equistability lines in the bistable region for this reaction are calculated by analyzing propagation of chemical waves and taking into account the coverage dependence of the CO diffusion coefficient. The results of simulations are compared with the available experimental data. We show that it is possible to obtain the relationship between various kinetic parameters, for example, between CO and oxygen sticking probabilities, by studying special features of the bistable kinetics.     

Aggregation beyond the gel point: A new class of exactly solvable models

The existing models describing the kinetics of aggregation in the presence of an infinite cluster, orgel, are reviewed, and a new class of post-gel models is proposed. In this new class of models, clusters are assumed to be acyclic and the rate constant for reactions involving the gel can be varied. The model is called thegeneralized acyclic model (GAM) since it generalizes Stockmayer's model and Ziff's third model. It is shown that the GAM can be solved exactly in terms of standard (hypergeometric) functions. The solutions are analyzed in detail, both asymptotically in various limits and numerically.Dedicated to Matthieu Ernst, on the occasion of his 60th birthday.     

Aggregation kinetics of CeO<Subscript>2</Subscript> nanoparticles in KCl and CaCl<Subscript>2</Subscript> solutions: measurements and modeling

To characterize the environmental transport and health risks of CeO₂ nanoparticles (NPs), it is important to understand their aggregation behavior. This study investigates the aggregation kinetics of CeO₂ NPs in KCl and CaCl₂ solutions using time-resolved dynamic light scattering (TR-DLS). The initial hydrodynamic radius of CeO₂ NPs measured by DLS was approximately 95 nm. Attachment efficiencies were derived both from aggregation data and predictions based on the Derjaguin–Landau–Verwey–Overbeek (DLVO) theory. The deviations of the DLVO predictions were corrected by employing the extended DLVO (EDLVO) theory. The critical coagulation concentration (CCC) of CeO₂ NPs at pH = 5.6 is approximately 34 mM for KCl and 9.5 mM for CaCl₂. Furthermore, based on the EDLVO theory and the von Smoluchowski’s population balance equation, a model accounting for diffusion-limited aggregation (DLA) kinetics was established. For the reaction-limited aggregation (RLA) kinetics, a model that takes fractal geometry into account was established. The models fitted the experimental data well and proved to be useful for predicting the aggregation kinetics of CeO₂ NPs.     

Dynamic observations of interface motion during the oxidation of silicon

We describe real time observations of the behaviour of the silicon-oxide interface during oxidation in situ in an ultrahigh vacuum transmission electron microscope. We have formed clean, flat Si(111) surfaces by heating under UHV and allowed oxidation or oxygen etching to proceed in the microscope. We have examined the kinetics of both the oxidation and etching reactions using an imaging technique based on the use of forbidden reflections in silicon. We find that oxidation to form SiO₂ occurs by the reaction of discrete monolayers with no flow of surface steps. This is in dramatic contrast to oxygen etching, during which the volatile oxide SiO evaporates preferentially from step edges.     

Electron-active silicon oxidation

Visible and ultraviolet light was employed in a series of refining experiments to reveal the kinetics of photon-enhanced oxidation of silicon. The experimental evidence then gave rise to an electron-active silicon oxidation process involving electron emission from Si into SiO₂ moderating the dissociation of molecular oxygen near the interface. Photonicallystimulated oxidation enhancement then straightforwardly derives from an enhanced level of electron emission. This simple model helps to clarify normal dry thermal oxidation growth kinetics, as well as thin oxide rapid growth behavior and the charged-versus-neutral oxidant controversy. Finally, attention is called to the not generally recognized but likely role that secondary electron emission from surfaces may play in moderating the chemistry of various electron, ion and photon beam-induced surface reactions.     

Influence of C60 Aggregation on Pseudorotation in the Excited Triplet State Probed by Multifrequency Time-Resolved EPR

The influence of C₆₀ aggregation on time-resolved (TR) electron paramagnetic resonance (EPR) of C₆₀ in the excited triplet state was investigated by multifrequency EPR techniques. Temperature-independent X-band (9.7 GHz) TR-EPR spectra were observed in a fresh toluene solution, while temperature-dependent ones were reported in literatures. The experimental spectra in this study indicated that the pseudorotation of pristine C₆₀ in frozen toluene solution is not frozen out even at lower temperatures. Careful investigations of TR-EPR and its decay kinetics demonstrated that the pseudorotation can be affected by C₆₀ aggregation. A comparison between X- and W-band (94.9 GHz) results indicated that the aggregation can be accelerated by a capillary effect. Three decay constants were extracted from the analysis of the decay kinetics. The fastest component was ascribed to the pseudorotation, which was independent of temperature in the range of 10–40 K. The temperature dependences of the decay kinetics showed that the pseudorotation is not affected by C₆₀ aggregation at higher temperatures.