Spectroscopic study of the decomposition process of tetramethylsilane in the N2–H2 and N2–Ar low pressure plasma

The optical emission spectroscopy has been used to study the decomposition process of tetramethylsilane in a low pressure plasma. The Si(CH₃)₄–N₂–H₂ and Si(CH₃)₄–N₂–Ar reactive mixtures, which are used for the deposition of SiCN layers, have been studied here. High energy active species were identified in the plasma phase and the electron excitation, vibrational and rotational temperatures as well as electron number density were determined for various compositions of the reactive mixtures. The electron excitation temperature (Si I, Ar I, H) was found to be higher than the vibrational temperatures (CN, N₂, N₂⁺) and considerably higher than the N₂⁺ rotational temperature, as the results of nonequilibrium state of plasma generated. It was observed that introduction of tetramethylsilane as well as growth of hydrogen percentage led to lowering of the electron density and the rotational temperature. Optical actinometry was applied to study the Si(CH₃)₄–N₂–H₂ reactive mixture.     

Effective Diffusivities and Convective Coefficients for CaO‐CaSO4 and CaO‐CaCl2 Pellets

Diffusion and convective flow in the pores of pellets formed by compressing mixtures of calcined limestone and CaSO₄/CaCl₂ powders have been studied experimentally by using the single pellet moment technique. The experiments were conducted in a diffusion cell by flowing nitrogen gas (carrier) through both faces of the pellet. Limestone powder was calcined in an atmosphere of N₂ at 800 °C and mixed with CaSO₄/CaCl₂ for diffusion experiments. Effective diffusivity of helium has been estimated by exposing the upper face of the pellet to a pulse of and matching the response peak on the lower face of the pellet with theoretical expressions. The values of the effective diffusivities increased with temperature, but decreased with increasing CaSO₄/CaCl₂ content in the pellet. The convective flow contribution to the diffusion flux was found to increase with increasing pressure drop across the pellet.     

Multicomponent diffusion in porous media

A transport equation for multicomponent diffusion through porous media, which has been proposed in the literature, is tested experimentally by studying the pressure dependence of ternary diffusion (pH₂/oH₂/He resp.pH₂/oH₂/Ar) through a fritted glass disk with a mean pore radius of ¯r ≈ 1μ. The diffusion resistance has been measured in a pressure range between 2 and nearly 600 mm Hg using the experimental method of the “diffusion—reaction cell”. The linear dependence on pressure predicted theoretically has been observed for higher pressures only (p > 200 mm Hg). At lower pressures considerable deviations from the expected linear behavior can be found. In particular, at low pressures a minimum is observed in the resistance vs. pressure relationship.Taking into account the additional resistances due to the finite length of the transport pores one can partially explain these deviations. Finally the results are used to calculate the pressure dependence of the effectiveness factor for diffusion controlled o-p-H₂ conversion. At low pressures lower values for this factor are obtained than predicted by the equation mentioned above.A surprising result is that the pressure dependence of the effectiveness factor shows a minimum for the o-p-H₂-He mixtures.     

Multistage mechanism of thermal decomposition of hydrogen azide

A kinetic mechanism for combustion of hydrogen azide (HN₃) comprising 61 reactions and 14 flame species (H₂, H, N, NH, NH₂, NNH, NH₃, HN₃, N₃, N₂H₂, N₂H₃, N₂H₄, N₂, and Ar) was developed and tested. The CHEMKIN software was used to calculate the flame speed at a pressure of 50 torr in mixtures of HN₃ with various diluents (N₂ and Ar), as well as the self-ignition parameters of HN₃ (temperature and pressure) at a fixed ignition delay. The modeling results of the flame structure of HN₃/N₂ mixtures show that at a 25–100% concentration of HN₃ in the mixture, the maximum temperature in the flame front is 25–940 K higher than the adiabatic temperature of the combustible mixture. Analysis of the mechanism shows that burning velocity of a HN₃/N₂ mixture at a pressure of 50 torr is described by the Zel’dovich-Frank-Kamenetskii theory under the assumption that the burn rate controlling reaction is HN₃ + M = N₂ + NH + M (M = HN₃) provided that its rate constant is determined at a superadiabatic flame temperature. The developed mechanism can be used to describe the combustion and thermal decomposition of systems containing HN₃.     

Gas Phase Reactions in Silent Electric Discharges Part III: Investigation of Chemical Reactions Involving Electron-Molecule Collisions in Various Mixtures of O2 with Ar,N2 or CO

Electron-molecule collisions play a significant role in chemical transformations in a silent electric discharge. The reaction rate coefficients of electron-molecule collisions in various gaseous mixtures of Ar, N₂ or CO with O₂ have been investigated. These coefficients can be determined experimentally and calculated theoretically. The measured values are in good agreement with those calculated by theoretical methods. The reaction rate coefficients of electron-O₂ molecule collisions increase with decrease of the amount of O₂ in Ar/O₂ mixtures, but decrease with decrease of the amount of O₂ in N₂/O₂ or CO/O₂ mixtures.     

Effect of diluents on rapid phase transition of water induced by combustion

When exploding CH₄/O₂/N₂ mixtures with high oxygen contents in a nonadiabatic vessel, the pressure–time histories display oscillations of different frequencies and very high pressure peaks (hundreds of bars). We have attributed this anomalous behavior (combustion‐induced rapid phase transition, cRPT) to the occurrence of cycles of condensation/evaporation of water at the vessel walls, followed by superheating of the liquid film due to radiative heat transfer from the flame, which culminates in the water rapid phase transition. We now report a detailed analysis of the role played by the addition of a diluent (CO₂, N₂, He, and Ar) on the anomalous behavior. The limit values of the diluent concentration at which the cRPT phenomenon is suppressed have been found and correlated to the kinematic viscosity and the thermal diffusivity through the Prandtl number. The less effective diluent has been found to be Ar followed by He, N₂, and CO₂ in the order listed. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

Effects of plasma gas composition on bond strength of hydroxyapatite/titanium composite coatings prepared by rf-plasma spraying

The effects of plasma gas composition on the bond-strength of HA/Ti composite coatings were investigated. HA/Ti composite coatings were deposited on titanium substrates by a radio-frequency (rf) thermal plasma spraying method with input powers of 10–30 kW. The ratio of the HA and Ti powders supplied into the plasma was precisely controlled by two microfeeders so as to change the coating's composition from Ti-rich at the bottom to HA-rich at its upper layer. The bond (tensile) strength of the obtained HA/Ti composite coatings was 40–65 MPa when sprayed with plasma gas containing N₂ (i.e., Ar–N₂). On the other hand, HA/Ti composite coatings prepared with plasma gas containing O₂ (i.e., Ar–O₂) had significantly lower bond strength (under 30 MPa). XRD patterns of Ti coatings without HA showed that titanium nitride and titanium dioxide formed, respectively, on titanium deposits sprayed with Ar–N₂ and Ar–O₂ plasma. Scanning electron microscopic (SEM) observation showed an acicular texture on the Ti deposits prepared with Ar–N₂ plasma. SEM observations implied that, when sprayed with Ar–O₂ plasma, a thin TiO₂ layer formed at the interfaces between the Ti splats in the deposits.     

Micromechanical properties of BN and B–C–N coatings obtained by r.f. plasma-assisted CVD

We have obtained highly transparent and hard BN films in a capacitively coupled r.f. plasma-assisted CVD reactor from three different gas mixtures: B₂H₆–H₂–NH₃, B₂H₆–N₂ and B₂H₆–N₂–Ar. It was found that the films were smooth, dense, and had a textured hexagonal structure with the basal planes perpendicular to the film surface. The microhardness, friction coefficient and adhesion of these coatings were measured by nanoindentation and microscratching. BC_xN_y films were also prepared in the same plasma-assisted CVD reactor from B₂H₆–N₂–CH₄ gas mixtures. The carbon content in the films was varied by using different CH₄ flow rates. These films had a less ordered structure. The mechanical properties of these films had been compared to those of hexagonal BN films. Microhardness measurements showed that there is a correlation between film composition and hardness of the BCN films.     

High-current electron emission of thin diamond films deposited on molybdenum cathodes

The results of investigation of emission characteristics of cold cathodes employing diamond and related films are presented. The films were deposited in a new millimeter wave plasma-assisted CVD reactor using Ar–H₂–CH₄ and Ar–H₂–CH₄–N₂ gas mixtures. To study the emission properties of the high-current cathodes they were subjected to ~ 50 ns high-voltage pulses with amplitudes up to 100 kV. Experiments show that the emission properties strongly depend on methane and nitrogen concentration in gas mixture. The homogeneous emission with current density of 220 A/cm² has been obtained. The prepared cathodes were successfully tested in high-power rf pulse compressor employing electron beam triggering.     

Measurement of binary diffusion coefficient of gases and organic vapours by chromatography

The binary diffusion of organic vapours such as benzene, nitrobenzene, aniline, n-hexane, acetone, ethyl alcohol and toluene in H₂, Ar and N₂ at different temperatures and the diffusion coefficients of the gas pairs; Ar-O₂, Ar-N₂, Ar-SO₂, N₂-NH₃, N₂-O₂, N₂-CO₂ & N₂-SO₂ at 25°C have been studied using a gas chromatograph (GCHF-18). Good agreement was found between observed and theoretically calculated values of diffusion coefficients.     

Solubilite de l'eau dans le melange gazeux (N2 + 3 H2) comprime—teneurs a la saturation—methode de calcul

The solubility of water in the synthesis gas (N₂ + 3H₂) has been measured between ?20°C and +15°C, at pressures varying from 100 to 400 atm. These results are coherent with those formerly published concerning higher temperatures.An easy method for calculating solubility is explained. It gives an accurate account of measures relating to the systems containing water or ammonia and a nonpolar component. For instance, the author presents the results obtained for the mixtures (H₂O + N₂ + 3H₂) and the mixtures (NH₃ + N₂ + 3H₂).     

Estimating the self-diffusion and mutual diffusion coefficients of binary mixtures on the basis of a modified van der Waals model

The previously proposed model is used to determine the values of the self-diffusion coefficient of He, Ne, Ar, Kr, Xe, H₂, D₂, N₂, O2, CO₂, NH₃, and CH₄ in the liquid and dense gaseous states, which were compared with the experimental data obtained at a pressure of ≈200 MPa and a temperature of ≈500 K. The calculations are carried out with the use of the equation of state of these substances in the form of a modified van der Waals model. The self-diffusion model was generalized for the case of mutual diffusion in binary mixtures, which is based on the modified model of the van der Waals state equation for mixtures. The modeled coefficient of mutual diffusion for a great number of binary mixtures of the above-mentioned individual substances is determined, and the results are compared with the known data. Without special calibration for the experiment, the model correctly predicts the relationship of the self-diffusion and mutual diffusion coefficients (with their variation by several orders of magnitude in the case where the density changes from gaseous to liquid) with both pressure and temperature. For most substances considered in the paper, the maximum deviations of calculations from the experiment do not exceed 30–50%.     

Effects of diluents on NO x formation in coflow CH4/air diffusion flames

The effect of diluent addition on NO _x formation in a laminar CH₄/air coflow diffusion flame was investigated by numerical simulation with experimental verification. The hydrocarbon fuel stream was diluted with N₂, CO₂, and Ar. The volume fraction of diluents systematically changed from 0.0 to 0.5. The simulation data agree well with the experimental one. The computational results indicate that overall the three diluents reduce the formation of NO and the effects vary from weak to strong in the order: N₂, Ar and CO₂. Differences between the influences of the various diluents are discussed in terms of the thermal, the dilution and the direct chemical effects, respectively. Further, the addition of CO₂ reduces the formation of NO₂, while the addition of N₂ or Ar has little effect on it. However, the formation rate of N₂O increases by each of the added diluents.     

The Effect of Gaseous Diluents on Ozone Generation from Oxygen

Ozone generation in a negative corona discharge has been experimentally investigated using both a pure oxygen and in binary mixtures of oxygen with several gases. The concentration of ozone (O₃) in such mixtures is found to be dependent both on the input energy density η, dissipated in unit volume of gas mixture and on the type and the concentration of the additives. The experimentally measured dependencies of ozone concentration on the input energy density (O₃) = f(η) have been fitted using the Vasiliev–Kobozev–Eremin formula and the specific rate coefficients for ozone formation K_f and ozone decomposition K_d have been calculated. Using Ar, N₂ or CO₂ as admixtures, an increase in the specific rate coefficient for ozone generation was observed for increasing concentrations of added gaseous impurity into oxygen. In contrast, admixtures with SF₆ or CCl₂F₂ caused a substantial reduction of K_f values. The absolute values of ozone concentration at constant input energy density were observed to decrease with decreasing concentrations of oxygen in all mixtures.     

Comparison of the adsorption dynamics of air on zeolite 5A and carbon molecular sieve beds

Adsorption and desorption in zeolite 5A and CMS beds were compared by using a ternary mixture (N₂/ O₂/Ar; 78 : 21 : 1 vol%). Because the breakthrough curves for both beds show a tail by temperature variance, a non-isothermal mathematical model was applied to the simulation of adsorption dynamics. The LDF model with a constant rate parameter was enough to predict the experimental breakthrough and temperature curves of an equilibrium separation bed, while the modified LDF model with a concentration-dependent parameter should be applied to a kinetic separation bed. In the CMS bed initially saturated with He, Ar was the first breakthrough component with N₂ following after a short interval. Then, after a long interval, the breakthrough of O₂ occurred with a broad roll-up due to its fast diffusion rate and the relatively slow diffusion rate of N₂. In the CMS bed initially saturated with O₂, the breakthrough curves of O₂ and N₂ showed a very broad shape because of the slow diffusion of N₂ into CMS. In the zeolite 5A bed, the breakthrough time sequence was Ar, O₂, and N₂ at very close time intervals. After the sharp roll-ups of O2 and Ar, the variation of the breakthrough curves was negligible. The inflection of the temperature profile in the zeolite 5A bed was caused by the crossover of the O₂ and N₂MTZs, while in the CMS bed it was caused by the difference in the diffusion rates of O₂ and N₂.     

Validation of a kinetic scheme for numerical investigation of hydrogen–methanol–air flames

Normal burning velocities in methanol–air mixtures and in the same mixtures with added 4.5 and 7.2% hydrogen as a second fuel were measured over a wide range of equivalence ratio and for initial conditions of 0.16 MPa and 354 K. It has been shown that the mechanism previously proposed for the combustion of mixtures of CO, CH₂O and CH₃OH with air is applicable to multicomponent mixtures containing hydrogen and methanol.     

Multicomponent isothermal diffusion and forced flow of gases in capillaries

Isothermal transport is described of a multicomponent gas mixture in a capillary. The governing equation (eqn 24) derived in the paper accounts for sim and pressure. The equation applies to the transition region between the Knudsen diffusion and the region of continuum and provides also for the slip on equations for the H₂C₂H₂, H₂C₂H₂C₂H₄ and H₂N₂NH₃ mixtures indicates give rise to a pressure gradient representing the driving force for additional transport. Notions of a critical component and diffusion-stoichiometric     

Multicomponent gas diffusion in nonuniform tubes

In many technical processes gas, multicomponent diffusion takes place in confinements that are rarely uniform in direction of their long axis (e.g., catalysts pores). Here, we show that in conical tubes multicomponent diffusion is hindered. This effect increases with ratio of inlet to outlet cone radius Λ, indifferent of the orientation of the tube. Based on the Maxwell–Stefan equations, predictive analytical solution for ideal multicomponent diffusion in slightly tapered ducts is developed. In two‐bulb diffusion experiments on a uniform tube, the results of Duncan and Toor (1962) were reproduced. Comparison of model and experiment shows that the solution presented here provides a reliable quantitative prediction of the temporal change of H₂, N₂, and CO₂‐concentration for both tube geometries, uniform and slightly conical. In the demonstrated case (Λ = 3.16), mass diffusion is 68% delayed. Thus, for gaseous diffusion in “real,” typically tapered pores the transport limitation is more serious than considered so far. © 2014 American Institute of Chemical Engineers AIChE J, 61: 1404–1412, 2015     

High pressure measurements and molecular modeling of the water content of acid gas containing mixtures

Water content of three carbon dioxide containing natural gas mixtures in equilibrium with an aqueous phase was measured using a dynamic saturation method. Measurements were performed up to high temperatures (477.6 K = 400°F) and pressures (103.4 MPa = 15,000 psia). The perturbed chain form of the statistical associating fluid theory was applied to predict water content of pure carbon dioxide (CO₂), hydrogen sulfide (H₂S), nitrous oxide (N₂O), nitrogen (N₂), and argon (Ar) systems. The theory application was also extended to model water content of acid gas mixtures containing methane (CH₄). To model accurately the liquid‐liquid equilibrium at subcritical conditions, cross association between CO₂, H₂S, and water was included. The agreement between the model predictions and experimental data measured in this work was found to be good up to high temperatures and pressures. © 2015 American Institute of Chemical Engineers AIChE J, 61: 3038–3052, 2015     

Optimization of wear–corrosion properties of a–C:N films using filtered cathodic arc deposition

Applying a 4-factor (negative substrate bias voltage, arc current, Ar flow and N₂/Ar ratio) 3-level (L9) orthogonal array design using the Taguchi method to optimize the wear–corrosion properties of a–C:N in Filtered Cathodic Vacuum Arc (FCVA) deposition was investigated. The influence of N₂/Ar ratio, over the range of 1/6 to 2/3, is shown by the increase in the following: the wear–corrosion current density changes from 36 to 78 nA/cm², and the friction coefficient changes from 0.042 to 0.086. A higher N₂/Ar ratio induces the a–C:N film's structure to reduce the sp³ content (sp² rich) which implies the formation of loose networks due to N₂ incorporation. Based on the analysis, the N₂/Ar ratio is the most significant control factor as its percentage contribution is 69% for wear–corrosion current density and 32% for frictional coefficient, respectively. There is a tendency for a higher friction coefficient by increasing the following three factors; level of negative substrate bias voltage, Ar flow and N₂/Ar ratio. We observed an increase of N content and sp² bonding which correlated to the decrease of hardness and an also rougher a–C:N surface with increasing the factors level. Over the design range of 4 factors, the optimum wear–corrosion properties and friction coefficient obtained from the Taguchi analysis were obtained using a 20 V negative substrate bias, 30 A arc current, 30 sccm Ar flow and 1/6 N₂/Ar ratio respectively. Overall, the results show an optimum design when compared with the current design as it was able to reduce 84% of the wear–corrosion current density (35.7 down to 5.6 nA/cm²) and 58% of the frictional coefficient (0.060 down to 0.025), respectively.