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Molecular Dynamics Simulations Of Surface Processes Oxygen Recombination On Silica Surfaces At High Temperature
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Book Synopsis Molecular Dynamics Simulations of Surface Processes: Oxygen Recombination on Silica Surfaces at High Temperature by :
Download or read book Molecular Dynamics Simulations of Surface Processes: Oxygen Recombination on Silica Surfaces at High Temperature written by and published by . This book was released on 2007 with total page 23 pages. Available in PDF, EPUB and Kindle. Book excerpt: The interaction between a molecule and a solid surface can lead to a great variety of elementary processes such as elastic, inelastic and reactive. Of particular importance is the dissociative chemisorption of a diatomic molecule where a molecule chemisorbed at the surface in a specific roto-vibrational state (v, j) dissociates with the two atoms adsorbed or scattered into the gas-phase. Of great importance is also the atom recombination on surfaces. Here two atoms recombine thus forming a diatomic molecule that can be either chemisorbed or reflected in the gas-phase in a given internal energy state. Reactions (1)-(2) are very often the rate determining step of complex heterogeneous systems of interest in different branches of industrial and technological applications, as for example in the ammonia synthesis, hydrocarbon production, chemical vapour deposition, etching and thin solid film deposition via plasma, nuclear rector technologies. Both processes are of central importance in aerothermodynamics and the chemistry of interstellar media. Thus, the recombination of atomic O and N on silica and UHTC materials plays a central role for the thermal protection system of the space shuttles entering into the terrestrial atmosphere, whereas the recombination of hydrogen atoms on ice grains covered by carbon is, very likely, the main source of molecular hydrogen observed in the interstellar media. The interaction of chemical species with surfaces can lead to other non-reactive chemico-physical processes such as the inelastic processes and adsorption. The adsorption processes occur when the particle is trapped in a chemisorption site and its available energy is not enough to escape from the chemisorption potential well. The inelastic processes can be of two types: direct and indirect.
Book Synopsis Fundamental Aspects of Plasma Chemical Physics by : Mario Capitelli
Download or read book Fundamental Aspects of Plasma Chemical Physics written by Mario Capitelli and published by Springer Science & Business Media. This book was released on 2015-11-26 with total page 330 pages. Available in PDF, EPUB and Kindle. Book excerpt: Describing non-equilibrium "cold" plasmas through a chemical physics approach, this book uses the state-to-state plasma kinetics, which considers each internal state as a new species with its own cross sections. Extended atomic and molecular master equations are coupled with Boltzmann and Monte Carlo methods to solve the electron energy distribution function. Selected examples in different applied fields, such as microelectronics, fusion, and aerospace, are presented and discussed including the self-consistent kinetics in RF parallel plate reactors, the optimization of negative ion sources and the expansion of high enthalpy flows through nozzles of different geometries. The book will cover the main aspects of the state-to-state kinetic approach for the description of nonequilibrium cold plasmas, illustrating the more recent achievements in the development of kinetic models including the self-consistent coupling of master equations and Boltzmann equation for electron dynamics. To give a complete portrayal, the book will assess fundamental concepts and theoretical formulations, based on a unified methodological approach, and explore the insight in related scientific problems still opened for the research community.
Book Synopsis Molecular Dynamics Simulations of Si Binding and Diffusion on the Native and Thermal Silicon Oxide Surfaces by : Saketh Bharadwaja
Download or read book Molecular Dynamics Simulations of Si Binding and Diffusion on the Native and Thermal Silicon Oxide Surfaces written by Saketh Bharadwaja and published by . This book was released on 2012 with total page 103 pages. Available in PDF, EPUB and Kindle. Book excerpt: Amorphous silicon (a-Si) thin-film solar cells grown via plasma-enhanced chemical vapor deposition (PECVD) are of significant technological interest. As a result, there is significant interest in understanding the physical processes which control the a-Si thin-film structure and morphology. In particular, since the early stages of a-Si growth on the silicon oxide substrate play a key role in determining the subsequent evolution, it is important to obtain a better understanding of this stage of a-Si growth. The key objectives of the work presented in this thesis are to obtain a better understanding of the structure and morphology of the silicon-oxide substrate used in a-Si growth via PECVD as well as of the key processes of Si diffusion on the substrate which control the nucleation of a-Si islands. In particular, motivated by experimental and simulation results, we have carried out molecular dynamics simulations of the formation of a thermal silicon oxide substrate (corresponding to oxide formation at high-temperature) as well as of the room-temperature oxidation of "native" silicon oxide thin-films. In addition, for the case of a native silicon oxide surface, we have studied the binding energies, binding sites, and diffusion barriers for Si diffusion in order to gain insight into the critical length-scales for a-Si island formation. In the case of thermal silicon oxide formed at high temperature, our molecular dynamics simulations were carried out using an effective Munetoh potential which takes into account the "average" charge transfer as well as bond angles and energies. In this case, due to the relatively high temperature the surface was found to be extremely rough and highly disordered, while the thin-film structure was found to be amorphous. In contrast, in our simulations of the formation of native silicon oxide thin-films at room temperature, a more sophisticated ReaxFF potential was used which properly takes into account the effects of O2 molecular dissociation and rebinding at the surface, as well as the long-range Coulomb interaction and local charge-transfer. We have also studied the binding and diffusion of Si atoms for this case in order to try to explain recent experiments and simulations in which it was shown that 3D a-Si islands with a typical island diameter of approximately 30 A are formed in the early stages of growth. For the case of native silicon-oxide our results for the oxygen penetration profile and surface roughness were found to be in good qualitative agreement with experiments. Our results also indicate that while the typical binding energies for Si adatoms on the SiO2 surface are significantly lower than for Si/Si(100), due to the disordered structure of the surface the barriers for diffusion are typically significantly higher. As a result, at the deposition temperature of 200oC used in low-temperature PECVD, these sites may act like "trapping sites" for deposited Si atoms. We note that these results are consistent with recent experiments on the relaxation of SiO2 microstructures at high temperatures. However, they also imply that the characteristic length-scale for 3D islands in the early stages of a-Si growth via PECVD cannot be explained by a combination of homogenous diffusion and a critical island-size, as is typically found in epitaxial growth.
Book Synopsis Development of a ReaxFF Reactive Force Field for Silicon/oxygen/hydrogen/fluorine Interactions and Applications to Hydroxylation and Friction by : Jejoon Yeon
Download or read book Development of a ReaxFF Reactive Force Field for Silicon/oxygen/hydrogen/fluorine Interactions and Applications to Hydroxylation and Friction written by Jejoon Yeon and published by . This book was released on 2016 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Molecular dynamics (MD) simulations with the ReaxFF reactive force field were carried out to find the atomistic mechanisms for tribo-chemical reactions occurring at the sliding interface of fully-hydroxylated amorphous silica and oxidized silicon as a function of interfacial water amount. The ReaxFF-MD simulations showed a significant amount of mass transfer across the interface occurs during the sliding. In the absence of water molecules, the interfacial mixing was initiated by dehydroxylation followed by the Si-O-Si bond formation bridging two solid surfaces. In the presence of sub-monolayer thick water, the dissociation of water molecules can provide additional reaction pathways to form the Si-O-Si bridge bonds and mass transfers across the interface. However, when the amount of interfacial water molecules was large enough to form full monolayer, the degree of mass transfer was substantially reduced since the silicon atoms at the sliding interface were terminated with hydroxyl groups rather than forming interfacial Si-O-Si bridge bonds. The ReaxFF-MD simulations clearly showed the role of water molecules in atomic scale mechano-chemical processes during the sliding and provided physical insights into tribochemical wear processes of silicon oxide surfaces observed experimentally. In addition to this, we performed reactive force field molecular dynamics simulation to observe the hydrolysis reactions between water molecules and locally strained SiO2 geometries. We improved the Si/O/H force field from Fogarty et al.1, to more accurately describe the hydroxylation reaction barrier for strained and non-strained Si-O structures, which are about 20 kcal/mol and 30 kcal/mol, respectively. After optimization, energy barrier for the hydroxylation shows a good agreement with DFT data. The observation of silanol formation at the high-strain region of a silica nano-rod also supports the concept that the adsorption of water molecule: hydroxyl formation favors the geometry with higher strain energy. In addition, we found three distinct hydroxylation paths -- H3O+ formation reaction from the adsorbed water, proton donation from H3O+, and the direct dissociation of the adsorbed water molecule. Because water molecules and their hydrogen bond network behave differently with respect to temperature ranges, silanol formation is also affected by temperature. The formation of surface hydroxyl in an amorphous silica double slit displays a similar tendency: SiOH formation prefers high-strain sites. Silanol formation related with H3O+ formation and dissociation is observed in hydroxylation of amorphous SiO2, similar with the results from silica nano wire simulation. These results are particularly relevant to the tribological characteristics of surfaces, enabling the prediction of the attachment site of the lubrication film on silica surfaces with a locally strained geometry.
Book Synopsis A Model for Oxygen Atom Recombination on a Silicon Dioxide Surface by : William Arthur Seward
Download or read book A Model for Oxygen Atom Recombination on a Silicon Dioxide Surface written by William Arthur Seward and published by . This book was released on 1986 with total page 102 pages. Available in PDF, EPUB and Kindle. Book excerpt: A steady state model for oxygen atom recombination for temperatures from 300 K to 2000 K on a silicon dioxide surface was developed based on the Langmuir-Rideal heterogeneous recombination mechanism. The bonding of atomic oxygen to the surface and the thermal desorption of atomic oxygen from the surface were also included in the model. The hypothesis was made that the gas-phase oxygen atoms combine directly with the oxygen atoms that constitute the silicon dioxide surface, with other gas-phase oxygen atoms replacing the lost atoms that were bonded on the silicon dioxide surface matrix. The model agrees with the experimental data that is available in the literature, and provides an insight into the processes that control the recombination as a function of temperature. A set of two-dimensional, steady-state, laminar boundary-layer calculations was made using explicit numerical methods to demonstrate the use of this model and to explore the rational limits which may be placed on the role of catalysis for oxygen recombination on a silicon dioxide surface like that of the Space Shuttle. The predicted catalycity of the silcion dioxide surface did reduce the overall heating below that which would be predicted by a fully catalytic surface.
Book Synopsis Journal of Thermophysics and Heat Transfer by :
Download or read book Journal of Thermophysics and Heat Transfer written by and published by . This book was released on 1997 with total page 642 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Book Synopsis Structure and Dynamics of Molecules at Water/silica and Water/carbon Dioxide Interfaces by : Hui Zhang
Download or read book Structure and Dynamics of Molecules at Water/silica and Water/carbon Dioxide Interfaces written by Hui Zhang and published by . This book was released on 2010 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Abstract: The interface between silica and water is one of the most technologically relevant surfaces. An especially important aspect of this system is its inherent negative charges at most pH values, and the resulting electrokinetic phenomena that take place in the fluid region. We have constructed a realistic model for the charged silica/water interface where many of these standard models can be tested. The model allows for undissociated and dissociated silanol groups. We have also conducted ab initio MD simulations of a smaller system consisting of a hydrated silica slab. The comparison of the radial distribution functions from the ab initio MD simulations and those obtained from the empirical model are favorable. The hydrophobic and hydrophilic nature of silanol-poor and silanol-rich regions of the amorphous silica surface observed in our empirical model is reproduced in the ab initio MD simulations of the smaller slab. In the initial stages of our ab initio MD simulations, we observe various chemical processes that represent different hydroxylation mechanisms of the surface. To explain why dynamical properties of an aqueous electrolyte near a charged surface seem to be governed by a surface charge less than the actual one, the canonical Stern model supposes an interfacial layer of ions and immobile fluid. However, large ion mobilities within the Stern layer are needed to reconcile the Stern model with surface conduction measurements. Modeling the aqueous electrolyte/amorphous silica interface at typical charge densities, a prototypical double layer system, the flow velocity does not vanish until right at the surface. The Stern model is a good effective model away from the surface, but cannot be taken literally near the surface. Indeed, simulations show no ion mobility where water is immobile, nor is such mobility necessary since the surface conductivity in the simulations is comparable to experimental values. Our studies suggest a richer, microscopic picture that allows for much greater mobility near the surface without a sharp boundary between mobile fluid and immobile ion layer, but still accounts for observed phenomena. The effect of salt concentration, surface charge density (which would be controlled experimentally by varying the pH) and local water viscosity on electrokinetic phenomena is explored. The structural properties of the interface between water and carbon dioxide are very important in many areas of chemistry and physics, such as supercritical extraction, electrochemistry and ion transport across membranes. In my study, the structural properties of the interface of water and CO2 are investigated by means of molecular dynamics (MD) simulations. The capillary wave theory is used to find the interface positions and the shape of the interface is determined by this theory. The density profiles of CO2 and water are extracted based on capillary wave theory. The density profiles are very helpful to calculate the surface excess and check whether there is a wetting transition when the pressure is increased. Molecular orientations of water and CO2 are calculated to give detailed information of the interface structure. Both water and CO2 molecules near the interface prefer to laying parallel with respect to the surface. The preferred orientational distribution of molecules near the surface gives rise to the surface potential which is calculated to better understand the electrodynamics of the interface.
Book Synopsis Structural Configuration of Amorphous Silica Surface with Different Terminating Groups Using Classical and Ab Initio Molecular Dynamics by : Md Azharul Islam
Download or read book Structural Configuration of Amorphous Silica Surface with Different Terminating Groups Using Classical and Ab Initio Molecular Dynamics written by Md Azharul Islam and published by . This book was released on 2023 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: The surface interaction properties of amorphous silica are crucial various sectors of technology, yet many aspects of these interactions are still not fully understood because amorphous silica's nature is non-periodic. In this study, computational simulations are used to understand the bonding mechanism and atomic structure of the amorphous silica surfaces with different passivating groups. Amorphous silica surfaces are generated by melt-quench dynamics using classical molecular dynamics (MD). Then, subdomains of these surfaces containing an undercoordinated surface atom are selected for ab initio density functional theory calculations. Relaxed surface geometries including hydroxyl, methyl, and fluoromethyl passivating groups are determined from density functional theory (DFT)-based structural relaxation calculations, along with Born-Oppenheimer MD at 300 K to determine the bond dissociation energy, bond length, and angle. This study provides a deeper understanding of the interactions between silica surfaces and this information can be used to improve the production and processing of silica-based materials.
Book Synopsis Reactions and Interfacial Behaviors of the Water-Amorphous Silica System from Classical and Ab Initio Molecular Dynamics Simulations by : Jessica M. Rimsza
Download or read book Reactions and Interfacial Behaviors of the Water-Amorphous Silica System from Classical and Ab Initio Molecular Dynamics Simulations written by Jessica M. Rimsza and published by . This book was released on 2016 with total page 214 pages. Available in PDF, EPUB and Kindle. Book excerpt: Due to the wide application of silica based systems ranging from microelectronics to nuclear waste disposal, detailed knowledge of water-silica interactions plays an important role in understanding fundamental processes, such as glass corrosion and the long term reliability of devices. In this dissertation, atomistic computer simulation methods have been used to explore and identify the mechanisms of water-silica reactions and the detailed processes that control the properties of the water-silica interfaces due to their ability to provide atomic level details of the structure and reaction pathways. The main challenges of the amorphous nature of the silica based systems and nano-porosity of the structures were overcome by a combination of simulation methodologies based on classical molecular dynamics (MD) simulations with Reactive Force Field (ReaxFF) and density functional theory (DFT) based ab initio MD simulations. Through the development of nanoporous amorphous silica structure models, the interactions between water and the complex unhydroxylated internal surfaces identified the unusual stability of strained siloxane bonds in high energy ring structure defects, as well as the hydroxylation reaction kinetics, which suggests the difficulty in using DFT methods to simulate Si-O bond breakage with reasonable efficiency. Another important problem addressed is the development of silica gel structures and their interfaces, which is considered to control the long term residual dissolution rate in borosilicate glasses. Through application of the ReaxFF classical MD potential, silica gel structures which mimic the development of interfacial layers during silica dissolution were created A structural model, consisting of dense silica, silica gel, and bulk water, and the related interfaces was generated, to represent the dissolution gel structure. High temperature evolution of the silica-gel-water (SGW) structure was performed through classical MD simulation of the system, and growth of the gel into the water region occurred, as well as the formation of intermediate range structural features of dense silica. Additionally, hydroxylated silica monomers (SiO4H4) and longer polymerized silica chains were formed in the water region, indicating that glass dissolution is occurring, even at short time frames. The creation of the SGW model provides a framework for a method of identifying how interfacial structures which develop at glass-water interfaces can be incorporated into atomistic models for additional analysis of the dissolution of silicates in water.
Book Synopsis International Aerospace Abstracts by :
Download or read book International Aerospace Abstracts written by and published by . This book was released on 1999 with total page 974 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Book Synopsis Recombination of oxygen atoms on surface at high temperature by : Miran Mozetič
Download or read book Recombination of oxygen atoms on surface at high temperature written by Miran Mozetič and published by . This book was released on 2005 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt:
Book Synopsis Scientific and Technical Aerospace Reports by :
Download or read book Scientific and Technical Aerospace Reports written by and published by . This book was released on 1991 with total page 1102 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Book Synopsis Scientific and Technical Aerospace Reports by :
Download or read book Scientific and Technical Aerospace Reports written by and published by . This book was released on 1988 with total page 1142 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Chemical Abstracts written by and published by . This book was released on 1991 with total page 2682 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Book Synopsis 7th AIAA/ASME Joint Thermophysics and Heat Transfer Conference by :
Download or read book 7th AIAA/ASME Joint Thermophysics and Heat Transfer Conference written by and published by . This book was released on 1998 with total page 562 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Ceramic Abstracts written by and published by . This book was released on 1998 with total page 250 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Book Synopsis Dissertation Abstracts International by :
Download or read book Dissertation Abstracts International written by and published by . This book was released on 2001 with total page 810 pages. Available in PDF, EPUB and Kindle. Book excerpt:
