Modeling insights into amorphous carbon etching by SO2/O2 low-pressure plasma

Heterogeneous reaction characteristics and its effects on homogeneous combustion of methane/air mixture in microchannels II. Chemical analysis

The general approaches to the elaboration of two-dimensional models of plasma activated chemical vapor deposition (PACVD) reactors are discussed. The models have been used to study the gas-phase and surface processes of diamond growth in continuous dc, pulsed dc and microwave discharge CVD reactors. The gas-phase and surface reaction mechanisms, the molecular diffusion and thermodiffiision, difhisional thermoeffects have been taken into account in transport equations that are numerically solved. The expressions for diamond growth and etching rates have been derived from surface kinetics. Dependencies of the growth rate and species distribution from variations of dc discharge reactor parameters (total current, gas pressure, carbon content) are obtained. A possibility of diamond growth rate enhancement in a pulsed dc discharge CVD reactor is examined.

The feedstocks of natural gas exceed the resources of crude oil by far. Therefore, there is a strong interest in the utilisation of natural gas with its main component methane for the production of more useful chemicals. This leads to a strong demand for compact and low-capital-cost reactors for the conversion of natural gas to methanol and liquid hydrocarbons and the production of H2. The combustion of natural gas is also considered for power generation in catalytic devices, because the flameless catalytic process results in lower emissions of NOx. Noble metals such as Pt, Pd, and Rh are very active catalysts for the conversion of CH4. However, the processes occurring during the partial and complete oxidation of CH4 on these catalysts are not completely understood. A detailed understanding of the reactor behaviour at all possible conditions is crucial for the technical realisation of these catalytic processes, because explosive mixtures are handled. In particular it is necessary to control the operation of the reactors at transient conditions such as light-off and shut-down. The objective of this work is to study total and partial oxidation of methane over Pt, Pd, and Rh catalysts for a wide range of operating conditions. Therefore, an experimental setup, which is easily applicable for modelling and numerical simulation, has to be developed. The experiment and analysis has to be arranged in a way that allows studies of transient processes such as light-off. Based on the experimentally derived conversion and selectivity surface-reaction schemes, available in literature, have to be evaluated. Available computational tools have to be used for numerically predicted reactor performance and compare these results with the experimental data. Crucial conditions, at which the reaction models need improvement, have to be found, and if necessary and possible improved. A flow reactor with associated analysis is designed to meet these requirements. In order to facilitate the simulation of the system catalysts exhibiting a simple geometry such as monolithic structures are used. The reactor consists of a 40 cm long quartz tube with varying inner diameters. Honeycomb catalysts coated with Rh, Pd, or Pt are placed inside. The gas temperature at the exit of the catalytic monolith and outside the quartz tube is. The reaction is ignited by heating up the reactor by a furnace, in which the flow system is integrated. After ignition, in the case of autothermal operation, the furnace can be switched off. The gases are premixed at room temperature and flow into the quartz tube at atmospheric pressure. The product composition is quantitatively analysed by a quadrupole mass spectrometer (QMS) in order to study transient phenomena. For the evaluation of reaction mechanisms, detailed numerical simulations of the physical and chemical processes using the computer program package DETCHEM and the commercially available CFD code FLUENT are applied. The numerical codes take into account detailed mechanisms of surface and gas-phase reactions as well as mass and heat transport processes in the channels and heat transport in the solid monolithic structure. The channels are simulated under steady-state conditions in a 3D elliptic approach using FLUENT and in two dimensions by a parabolic approach using DETCHEMCHANNEL. Transient phenomena are simulated by DETCHEMMONOLITH with 2D parabolic flow field simulations of a representative number of single channels including transient heat balances of the monolithic structure. First, the ignition of catalytic combustion of CH4 on a Pt loaded monolith is investigated. The conversions of CH4 and O2 as well as the product selectivities are experimentally determined and the results numerically simulated. The influence of H2 addition on the combustion is investigated. Reaction-rate oscillations of the catalytic combustion of methane on a Pd based catalyst are found experimentally at lean conditions in Pd coated monoliths and simulated with a surface-reaction mechanism. The second set of experimental focuses on CPO of CH4 over Rh coated monoliths. The experimentally derived conversion and selectivities are used to improve a detailed reaction mechanism. Next, the influence of the support material on the ignition process is investigated. In the experiment, either -alumina or cordierite as support material is applied and coated with Rh. The models are able to describe the light-off behaviour and are applied for catalyst design: The ignition behaviour of a set of virtual catalytic monoliths exhibiting varying physical properties is modelled. In addition, catalysts coated with Rh-nanopowder produced by laser ablation is experimentally examined and compared with wet coated catalysts. Finally, the CPO of methane is carried in a Pt, presenting an even more well-defined configuration for modelling. At CH4/O2 ratios above 1.9 complex patterns of oscillation in conversion and selectivity occur.

The chemical vapour deposition (CVD) technique could be used to fabricate a silicon carbide (SiC) epitaxial layer. Methyltrichlorosilane (CH3SiCl3, MTS) is widely used as a precursor for CVD of SiC with a wide range of allowable deposition temperatures. Typically, an appropriate model for the CVD process involves kinetic mechanisms of both gas-phase reactions and surface reactions. Here, we proposed the surface kinetic mechanisms of epitaxial SiC growth for MTS-H2 gaseous system where the MTS employed as the single precursor diluted in H2. The deposition face is assumed to be the Si face with a surface site terminated by an open site or H atom. The kinetic mechanisms for surface reactions proposed in this work for MTS-H2 gaseous system of epitaxial growth of SiC by CVD technique from mechanisms proposed for H-Si-C-Cl system are discussed in detail. Predicted components of surface species and growth rates at different mechanisms are discussed in detail.

The objective of this paper is the validation of a surface reaction mechanism for silicon etching in low-pressure chlorine plasmas. We employ a quasi-one-dimensional fluid model to model the experimental conditions of Khater and Overzet [Plasma Sources Sci. Technol. 13, 466 (2004)]. This model couples self-consistently the plasma fluid equations with the surface reaction mechanism derived from the available literature. Based on the comparison between the experiments and modeling results, the best set of etch yield parameters is proposed for the conditions typical for industrial plasma processing. The influence of these etch yield parameters on the gas-phase plasma is also discussed.

Investigation of wall chemical effect using PLIF measurement of OH radical generated by pulsed electric discharge

Intrinsic low-dimensional maniforlds of heterogeneous combustion processes

The gas phase reaction mechanism in the aluminum metalorganic chemical vapor deposition from dimethylethylamine alane (DMEAA) was investigated by Fourier transform infrared spectroscopy. DMEAA in the gas phase was dissociated into dimethylethylamine and alane even at room temperature and the dissociation rate was accelerated at higher gas temperatures. Alane, produced from the dissociation of DMEAA, disappeared rapidly and the intermediates, produced during the degradation of alane, may form Al particles. The dissociation reaction was found to be a first order reaction, and the activation energy was about 9.56 kcal/mol. In H2 carrier gas, the dissociation of DMEAA was slower than that in Ar gas. The deposition rate was decreased when the substrate temperature was above 150 °C because DMEAA decomposed rapidly in the gas phase. From the experimental observations, a gas phase reaction mechanism was proposed and the degradation mechanism of liquid DMEAA was suggested.

Understanding how hydrogen plasma etches various potential products during the diamond growth process can contribute to improving the knowledge of diamond growth. However, due to the absence of an in situ characterization technique during the etching process, the complex chemical reactions involved in the process obscure the atomic‐scale etching mechanisms. In this paper, the etching mechanisms of diamond (001), graphite (0001), and amorphous carbon substrates irradiated by hydrogen plasmas are investigated and compared using molecular dynamics simulations based on ReaxFF. When the incident energy of H atoms is 1 eV, the rate of carbon loss from graphite and amorphous carbon are far higher than that from diamond. As the incident energy of H atoms increases, the etching rate of diamond shows a slow increase, while the etching rates of amorphous carbon and graphite exhibit more significant increases. It can be concluded that the etching rate of diamond is significantly lower than that of graphite and amorphous carbon under H plasma. In the Chemical Vapor Deposition (CVD) process of diamond growth, the generated graphite and amorphous carbon are rapidly etched, leaving only diamond. This offers a plausible explanation for the growth mechanism of diamond through CVD.

Modeling of dry etching processes requires a detailed understanding of the relevant reaction mechanisms. This study aims to elucidate the gas-phase mechanism of reactions in the chemical dry etching process of SiO2 layers which is initiated by mixing NF3 gas with the discharged flow of an NH3/N2 mixture in an etching chamber. A kinetic model describing the gas-phase reactions has been constructed based on the predictions of reaction channels and rate constants by quantum chemical and statistical reaction-rate calculations. The primary reaction pathway includes the reaction of NF3 with H atoms, NF3 + H → NF2 + HF, and subsequent reactions involving NF2 and other radicals. The reaction pathways were analyzed by kinetic simulation, and a simplified kinetic model composed of 12 reactions was developed. The surface process was also investigated based on preliminary quantum chemical calculations for ammonium fluoride clusters, which are considered to contribute to etching. The results indicate the presence of negatively charged fluorine atoms in the clusters, which are suggested to serve as etchants to remove SiO2 from the surface.

Surface reaction mechanism in Si and GaAs crystal growth

Gas-phase and aqueous-surface reaction mechanism of Criegee radicals with serine and nucleation of products: A theoretical study

Aerosol particles (also referred to as particulate matter or particulates) play a major role in the overall air pollution problem. They are responsible for reduction of visibility and acidification of waters, and in certain size ranges are deposited in the lungs where they can cause a variety of health effects. The air pollution aerosol particles can consist of solid and/or liquid substances. Some of these, such as windblown dust, soot, and fly ash, originate in sources outside the atmosphere and are known as “primary” particles. Others are formed directly in the atmosphere by reactions among the primary particulate and gaseous species and are known as “secondary” pollutants. The nature of both these primary and secondary particulate species, as well as the extent to which atmospheric chemistry is governed by (photochemical) gas-phase or heterogeneous gas-particle reaction mechanisms, is presently a very active area of research.

Atmospheric chemistry of CF3C[tbnd]CH: Kinetics, products, mechanism of gas-phase reaction with OH radicals, and atmospheric implications-an effort for novel ‘class’ of refrigerant

In order to improve the Schottky diode characteristics of metal-In0.52Al0.48As, a new dry photochemical etching and sulfur passivation process has been developed. Excimer and deep UV lamps were used for photochemical etching and surface passivation, respectively. Auger electron spectroscopy measurement shows that this passivation process is as good as wet (NH4)2Sx and P2S5/(NH4)2S passivation in terms of sulfur coverage. Schottky diodes were fabricated on n-In0.52Al0.48As by electron beam evaporation of a multilayer metal contact of the form Mo/Ti/Pt/Au (2/30/300/200 nm). The leakage current has been reduced by three orders of magnitude after applying this passivation process. X-ray photoelectron spectroscopy measurements illustrate that these free S* radicals are capable of replacing As–O and In–O bonds to As–S and In–S bonds. The conclusion is that the As–O and In–O bonds are the primary causes of leakage current. These bonds are dissolved after this photochemical sulfur surface passivation.