Modeling of the elementary gas-phase reaction during chemical vapor deposition of silicon carbide from CH3SiCl3/H2

We established a gas-phase, elementary reaction model for chemical vapor deposition of silicon carbide from methyltrichlorosilane (MTS) and H₂, based on the model developed at Iowa State University (ISU). The ISU model did not reproduce our experimental results, decomposition behavior of MTS in the gas phase in an environment with H₂. Therefore, we made several modifications to the ISU model. Of the reactions included in existing models, 236 were lacking in the ISU model, and thus were added to the model. In addition, we modified the rate constants of the unimolecular reactions and the recombination reactions, which were treated as a high-pressure limit in the ISU model, into pressure-dependent rate expressions based on the previous reports (to yield the ISU+ model), for example, H₂(+M) → H + H(+M), but decomposition behavior remained poorly reproducible. To incorporate the pressure dependencies of unimolecular decomposition rate constants, and to increase the accuracies of these constants, we recalculated the rate constants of five unimolecular decomposition reactions of MTS using the Rice-Ramsperger-Kassel-Marcus method at the CBS-QB3 level. These chemistries were added to the ISU+ model to yield the UT2014 model. The UT2014 model reproduced overall MTS decomposition. From the results of our model, we confirmed that MTS mainly decomposes into CH₃ and SiCl₃ at the temperature around 1000°C as reported in the several studies.     

Mechanism and kinetics of the silane decomposition in the presence of acetylene and in the presence of olefins

Kinetic data and product studies are reported for the silane pyrolysis in the presence of olefins and acetylene. The kinetics of silane loss in the presence of acetylene was found to be identical to the initial gas phase silane decomposition step (SiH₄ + M → SiH₂ + H₂ + M) when corrected for pressure fall-off effects. This result and the absence of methane or ethane from the pyrolysis of SiH₄ in the presence of 1-butene or 1-pentene demonstrate that silyl radicals and H atoms are not involved in silane-olefin or silane-acetylene reactions. Qualitative aspects and kinetic data from the SiH₄ pyrolysis in the presence of propylene are in accord with propylsilane formation via propylsilylene formed by silylene addition to propylene.     

Silane-Based Coatings on Polypropylene,Deposited by Atmospheric Pressure Glow Discharge Plasmas

The aim of this paper is to show the possibility to synthesize silicon-based deposits on a polypropylene substrate, using a glow dielectric barrier discharge at atmospheric pressure, and to correlate the gas phase behavior with the properties of the thin film deposits. The discharge is generated in a mixture of nitrous oxide and silane, diluted in nitrogen. The influence of the [N₂O]/[SiH₄] ratio on the layer characteristics is mainly studied. Deposits are analyzed by XPS, SSIMS, AFM and wetting angle measurements. The discharges are also characterized by their optical emission spectra. Measurements are made as a function of the distance from the gas inlet, and they allow one to correlate these spectra with the film thickness and its chemical composition. Finally, chemical kinetics of the reactive gas decomposition reactions are proposed.     

Ozone‐Induced Band Bending on Metal‐Oxide Surfaces Studied under Environmental Conditions

Ozone adsorption and decomposition on metal oxides is of wide interest in technology and in atmospheric chemistry. Here, ozone‐adsorption‐induced band bending is observed on Ti‐ and Fe‐oxide model surfaces under dry and humid conditions. Photoelectron spectroscopic studies indicate the effect of charge transfer to O₃, which limits the surface coverage of the precursor to decomposition reactions. This is also consistent with the negative pressure dependence observed in previous studies. These results contribute to our fundamental understanding of ozone adsorption and decomposition mechanisms on metal oxides of environmental and technological relevance.     

A theory of limiting flux in a stirred batch cell

Theoretical and experimental investigations of ultrafiltration (UF) of albumin solutions were conducted with a batch stirred cell. The theoretical model predicted that the limiting flux u∞ is equal to (K'_m/ø), where K'_m is the effective mass transfer coefficient and ø is the rejection coefficient. This interesting finding simply says that the limiting flux is independent of the pressure applied as was observed by many workers. Also, the limiting flux u∞ was theoretically found to increase with the stirring speed and decreases with the bulk protein concentration which is consistent with previous experimental findings. Experimental studies confirmed these predictions. Albumin ultrafiltration rate appears to be limited by the increased adsorption rate of the protein on the membrane surface and onto the pores of the membrane.     

The dynamics of the SiF4 - sensitized processes initiated by a pulsed CO2 laser

A model enabling the examination of the dynamics of the SiF₄ - sensitized processes initiated by a pulsed CO₂ laser is outlined, taking into account three effects : energy absorption, energeticity of chemical reactions and expansion of hot gas created upon interaction of molecules with the infrared photon field. This approach was used to elucidate some aspects regarding the decomposition of PH₃ and GeH₄.     

Thermal decomposition of mechanically activated arsenopyrite

The changes in specific surface area and structure disorder of mechanically activated arsenopyrite were investigated. The rate of nonoxidative decomposition of mechanically activated arsenopyrite was increased almost 10-times when compared with nonoxidative decomposition of a non-activated sample. An empirical linear relationship was found (r=0.996) between the rate constant of decomposition and the ratio of specific surface to transmission of the absorption band of arsenopyrite at $\bar \nu = 370 cm^{ - 1} $ . This relationship enables us to arrange the reaction 4FeAsS→4FeS+As₄ among structure-sensitive reactions.     

Kinetics of the reactions of isopropoxy radicals with NO,NO2, and O2

A fluorescence excitation spectrum of (CH₃)₂CHO (isopropoxy radical) is reported following photolysis of isopropyl nitrite at 355 nm. Rate constants for the reaction of isopropoxy with NO, NO₂, and O₂ have been measured as a function of pressure (1–50 Torr) and temperature (25–110°C) by monitoring isopropoxy radical concentrations using laser-induced fluorescence. We have obtained the following Arrhenius expressions for the reaction of isopropoxy with NO and O₂ respectively: (1.22±0.28)×10^?11 exp[(+0.62±0.14 kcal)/RT]cm²/s and (1.51±0.70)×10^?14 exp[(?0.39±0.28)kcal/RT]cm³/s where the uncertainties represent 2σ. The results with NO₂ are more complex, but indicate that reaction with NO₂ proceeds more rapidly than with NO contrary to previous reports. The pressure dependence of the thermal decomposition of the isopropoxy radical was studied at 104 and 133°C over a 300 Torr range using nitrogen as a buffer gas. The reaction is in the fall-off region over the entire range. Upper limits for the reaction of isopropoxy with acetaldehyde, isobutane, ethylene, and trimethyl ethylene are reported.We have performed the first LIF study of the isopropoxy radical. Arrhenius parameters were measured for the reaction of i-PrO with O₂, NO, NO₂, using direct radical measurement techniques. All reactions are in their high-pressure limits at a few Torr of pressure. The rate constant for the reactions of i-PrO with NO and NO₂ reactions exhibit a small negative activation energy. Studies of the i-PrO + NO₂ reaction produce data which indicate that O(³P) reacts rapidly with i-PrO. Unimolecular decomposition studies of i-PrO indicate that the reaction is in the fall-off region between 1 and 300 Torr of N₂ and the high-pressure limit is above 1 atmosphere of N₂.     

Effect of H2 Flow Rate on High-Rate Etching of Si by Narrow-Gap Microwave Hydrogen Plasma

For the purpose of realizing a low-cost production process of silane (SiH₄) gas, we have proposed the high-rate etching of metallurgical-grade Si by narrow-gap microwave hydrogen plasma. In this paper, effect of hydrogen gas flow rate (0–10 L/min) on the etch rate has been investigated and correlated with the relative variation of hydrogen-atom density estimated by actinometry. By decreasing hydrogen gas flow rate, the etch rate gradually increases up to the maximum value of 11 μm/min at 2 L/min. This increase is well correlated with the increase of hydrogen-atom density due to the longer residence time of hydrogen molecules in the plasma. On the other hand, when the gas flow rate is lower than 2 L/min, the etch rate abruptly decreases with decreasing gas flow rate in spite of the increase of hydrogen-atom density. From the surface observations and Raman measurements, it is found that the decrease in etch rate in the lower flow rate range is attributed to the formation of microcrystalline Si particles due to the decomposition of generated-SiH₄ molecules in the plasma.     

Kinetics and mechanism of the reaction of Cl atoms with CH2 CO (Ketene)

The kinetics and mechanism of the gas-phase reaction of Cl atoms with CH₂CO have been studied with a FTIR spectrometer/smog chamber apparatus. Using relative rate methods the rate of reaction of Cl atoms with ketene was found to be independent of total pressure over the range 1–700 torr of air diluent with a rate constant of (2.7 ± 0.5) × 10⁻¹⁰ cm³ molecule⁻¹ s⁻¹ at 295 K. The reaction proceeds via an addition mechanism to give a chloroacetyl radical (CH₂ClCO) which has a high degree of internal excitation and undergoes rapid unimolecular decomposition to give a CH₂Cl radical and CO. Chloroacetyl radicals were also produced by the reaction of Cl atoms with CH₂ClCHO; no decomposition was observed in this case. The rates of addition reactions are usually pressure dependent with the rate increasing with pressure reflecting increased collisional stabilization of the adduct. The absence of such behavior in the reaction of Cl atoms with CH₂CO combined with the fact that the reaction rate is close to the gas kinetic limit is attributed to preferential decomposition of excited CH₂ClCO radicals to CH₂Cl radicals and CO as products as opposed to decomposition to reform the reactants. As part of this work ab initio quantum mechanical calculations (MP2/6-31G(d,p)) were used to derive Δ_fH₂₉₈(CH₂ClCO) = −(5.4 ± 4.0) kcal mol⁻¹. © 1996 John Wiley & Sons, Inc.     

Sequential Pd2(dba)3 participated C–C bond cleavage of O-bromophenyl cyclobutanones/Michael addition en route to benzospirones

A tandem Pd₂(dba)₃ participated C–C bond cleavage of O-bromophenyl cyclobutanone derivatives/Michael addition reaction sequence was realized. We disclosed the first intramolecular C–Br bond triggered ring opening reaction of arylcyclobutanones, distinct from related reports in which the reactions were initiated by arylboron, silane or unsaturated chemical motifs, among others. The in situ generated palladium species underwent ring expansion process leading to methyleneindanones, which further reacted with dba to provide benzospirones in one step.     

Superequilibrium increase in the chemical reaction rate in the front and other effects of gas detonation numerically simulated in the channel by instant heating of the flat end

Gas detonation was calculated by the Monte Carlo method at the molecular level on the basis of non-stationary statistical simulation. The detonation was initiated by instant heating of the flat end of the channel. The efficiency of the method and the used block decomposition of the model space is shown. It turned out that an increase in the reaction threshold from 90kТ ₁ to 400kТ ₁ (k is the Boltzmann constant, and Т ₁ is the initial temperature of the gas) resulted in the disappearance of the region of constant parameters behind the front of the detonation wave. The translational non-equilibrium formed in the detonation front strongly increases the rate of the reaction considered at the front edge. The further increase in the reaction threshold leads to the situation where no detonation occurs.     

Thermal decomposition of rare-earth trioxalatocobaltates

The thermal decomposition of rare-earth trioxalatocobaltates LnCo(C₂O₄)₃ · x H₂O, where Ln  La, Pr, Nd, has been studied in flowing atmospheres of air/oxygen, argon/ nitrogen, carbon dioxide and a vacuum. The compounds decompose through three major steps, viz. dehydration, decomposition of the oxalate to an intermediate carbonate, which further decomposes to yield rare-earth cobaltite as the final product. The formation of the final product is influenced by the surrounding gas atmosphere. Studies on the thermal decomposition of photodecomposed lanthanum trioxalatocobaltate and a mechanical mixture of lanthanum oxalate and cobalt oxalate in 1 : 2 molar ratio reveal that the decomposition behaviour of the two samples is different. The drawbacks of the decomposition scheme proposed earlier have been pointed out, and logical schemes based on results obtained by TG, DTA, DTG, supplemented by various physico-chemical techniques such as gas and chemical analyses, IR and mass spectroscopy, surface area and magnetic susceptibility measurements and X-ray powder diffraction methods, have been proposed for the decomposition in air of rare-earth trioxalatocobaltates as well as for the photoreduced lanthanum salt and a mechanical mixture of lanthanum and cobalt oxalates.     

Kinetics of surface pyrolysis of a silane–germane gas mixture under conditions of epitaxial film deposition of Si<Subscript>1–<Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> solid solutions

Specific features of the pyrolysis of hydride molecules on the surface of a Si_1–x Ge _x film under conditions of epitaxial film deposition of a mixture of silicon and germanium hydrides have been studied. Temperature dependences of the kinetic coefficients responsible for the rate of hydrogen desorption from the Si_1–x Ge _x film surface and the rate of dissociation of gas molecule radicals adsorbed by the surface of the growing film have been obtained for the first time, using the developed kinetic models of surface pyrolysis and the results of engineering experiments in the temperature range 450–800°C. A correlation between the dissociation frequencies of silane and germane molecules, as well as a correlation of the dissociation frequencies with other kinetic parameters of the system were revealed.     

Thermochemistry of hydride and phenyl groups on the surface of amorphous silicon dioxide

The thermodynamic properties of hydride and phenyl groups on the surface of amorphous silicon dioxide are investigated in the presented work. The characteristics of the surface silane centers (SiH) are determined from the data obtained by infrared spectroscopy and caloric measurements. The conversions of hydrogen and benzene with the surface are described by thermodynamic calculations at reactions take place in the gaseous phase.To model the reaction between hydrogen and the surface the thermodynamic data for (OH)_4−nSi_n (n=0-4) in the gaseous phase are used. The surface groups and the model molecules are comparable because the thermodynamic characteristics depend only from the local environment.The thermodynamic properties of (OH)₃SiC₆H₅ in the gaseous phase are determined to describe the reaction between benzene and the surface. The predications of these calculations are confirmed by the spectroscopic results. The properties of the surface phenyl groups (SiC₆H₅) are concluded from these data.     

Cobalt-Catalyzed Regiodivergent Double Hydrosilylation of Arylacetylenes

Double hydrosilylation of alkynes represents a straightforward method to synthesize bis(silane)s, yet it is challenging if α-substituted vinylsilanes act as the intermediates. Here, a cobalt-catalyzed regiodivergent double hydrosilylation of arylacetylenes is reported for the first time involving this challenge, accessing both vicinal and geminal bis(silane)s with exclusive regioselectivity. Various novel bis(silane)s containing Si−H bonds can be easily obtained. The gram-scale reactions could be performed smoothly. Preliminarily mechanistic studies demonstrated that the reactions were initiated by cobalt-catalyzed α-hydrosilylation of alkynes, followed by cobalt-catalyzed β-hydrosilylation of the α-vinylsilanes to deliver vicinal bis(silane)s, or hydride-catalyzed α-hydrosilylation to give geminal ones. Notably, these bis(silane)s can be used for the synthesis of high-refractive-index polymers (n_d up to 1.83), demonstrating great potential utility in optical materials.     

Mechanism of the silane decomposition. I. Silane loss kinetics and rate inhibition by hydrogen. II. Modeling of the silane decomposition (all stages of reaction)

Part I: Kinetic data for the static system silane pyrolysis (from 640–703 K, 60–400 torr) are presented. For conversion from 3–30%, first-order kinetics are obtained, with silane loss rates equal to half the hydrogen formation rates. At conversions greater than 40%, rate inhibition attributable to the back reaction of hydrogen with silylene occurs. Overall reaction rates are not surface sensitive, but disilane and trisilane yield maxima under some conditions are. A nonchain mechanism capable of describing quantitatively all stages of the silane pyrolysis is proposed. Post 1.0% initiation is both homogeneous (gas phase) and heterogeneous (on the walls), and reaction intermediates are silylenes and disilenes. Free radicals are not involved at any stage of the reaction. Rate data at high conversions and with added hydrogen provide kinetics for the addition of silylene to hydrogen [reaction (?1)1] relative to its addition to silane [reaction (2)]: k_?1,/k₂ = 10^?0.65 × e^{?3200 cal/RT}. With E₂ = 1300 cal, this gives a high pressure activation energy for silylene insertion into hydrogen of E_?1 = 8200 cal. Part II: An analysis is made of each rate constant of the silane mechanism and the modeling results are compared with experimental results. Agreement is excellent. It is concluded that the dominant sink reaction for silylene intermediates is 1,2—H₂ elimination from disilane (followed by Si₂H₄ polymerization and wall deposition). The model is in accord with slow isomerization between disilene and silylsilylene and near exclusive 1,2—H₂ elimination from Si₂H₆. It is also concluded that disilene is about 10 kcal/mol more stable than silylsilylene and that the activation energy for isomerization of silylsilylene to disilene is greater than 26 kcal/mol.     

Properties of surface compounds in methanol conversion on copper-containing catalysts based on CeO2 according to in situ IR-spectroscopic data

Formate and carbonate complexes and bridging and linear methoxy groups were detected on the surfaces of CeO₂ and 5.0% Cu/CeO₂ under the reaction conditions of methanol conversion using IR spectroscopy. The reaction products were H₂, methyl formate, CO, CO₂, and H₂O. The bridging and linear methoxy groups were the sources of formation of bi- and monodentate formate complexes, respectively. Methyl formate was formed as a result of the interaction of the linear methoxy group and the formate complex. The study demonstrated that the recombination of hydrogen atoms on copper clusters and the decomposition of methyl formate were the main reactions of hydrogen formation. Formate and carbonate complexes were the source of CO₂ formation in the gas phase, and the decomposition of methyl formate was the source of CO. It was found that the addition of water vapor to the reaction flow considerably decreased the rate of CO formation at a constant yield of hydrogen. The effects of water vapor and oxygen on the course of surface reactions and the formation of products are discussed. To explain the mechanism of methanol conversion, a scheme of surface reactions is proposed.     

Mechanisms and kinetics of the thermal decompositions of trichlorosilane,dichlorosilane, and monochlorosilane

Decomposition studies of trichlorosilane, dichlorosilane, and monochlorosilane at 921 K, 872 K, and 806 K, respectively, are reported. The studies were made at fixed reactant pressures over a range of total pressures in a wall conditioned, quartz reactor connected to a quadrupole mass-spectrometer. Products were monitored sequentially and continuously in time. The dichlorosilane decomposition was also studied by the comparative-rate single-pulse shock-tube method at temperatures around 1250 K. Two mechanisms of decomposition are considered: a silylene based mechanism initiated by molecular elimination reactions (Scheme I), and a free radical based mechanism initiated by bond fission reactions (Scheme V). Modeling tests of these mechanisms show that only the former is consistent with the experimental data. The decompositions are shown to be essentially nonchain processes initiated by the following pressure dependent reactions: HSiCl₃(SINGLEBOND)4→ SiCl₂+HCl, H₂SiCl₂(SINGLEBOND)1→ SiCl₂+H₂ and H₃SiCl(SINGLEBOND)5→ HSiCl+H₂. High pressure Arrhenius parameters recommended for these reactions are A_4,∞=A_1,∞=A_5,∞=10^14.5±0.5 s⁻¹, E_4,∞=71.9±2.1 kcal/mol, E_1,∞=69.2±2.0 kcal/mol, and E_5,∞=60.6±1.8 kcal/mol. © 1998 John Wiley & Sons, Inc. Int J Chem Kinet: 30: 69–88, 1998.     

Kinetics and mechanism of the silane decomposition

The homogeneous gas-phase decomposition kinetics of silane has been investigated using the single-pulse shock tube comparative rate technique (T = 1035–1184?K, P_total ≈? 4000 Torr). The initial reaction of the decomposition SiH₄ \documentclass{article}\pagestyle{empty}\begin{document}$ {\rm SiH}_{\rm 4} \mathop \to \limits^1 {\rm SiH}_{\rm 2} + {\rm H}_{\rm 2} $\end{document} SiH₂ + H₂ is a unimolecular process in its pressure fall-off regime with experimental Arrhenius parameters of logk₁ (sec^?1) = 13.33 ± 0.28–52,700 ± 1400/2.303RT. The decomposition has also been studied at lower temperatures by conventional methods. The results confirm the total pressure effect, indicate a small but not negligible extent of induced reaction, and show that the decomposition is first order in silane at constant total pressures. RRKM-pressure fall-off calculations for four different transition-state models are reported, and good agreement with all the data is obtained with a model whose high-pressure parameters are logA₁ (sec^?1) = 15.5, E_1(∞) = 56.9 kcal, and ΔE^0±₀(1) = 55.9 kcal. The mechanism of the decomposition is discussed, and it is concluded that hydrogen atoms are not involved. It is further suggested that silylene in the pure silane pyrolysis ultimately reacts with itself to give hydrogen: 2SiH₂ → (Si₂H₄)* → (SiH₃SiH)* → Si₂H₂ + H₂. The mechanism of H ? D exchange absorbed in the pyrolysis of SiD₄-hydrocarbon systems is also discussed.