The Effect of Inelastic Collisions of Molecules of a Diatomic Gas with Rotational Degrees of Freedom

The problem on Barnett slip of gas along a plane surface is solved within the suggested kinetic model for a diatomic gas with rotational degrees of freedom of molecules, which takes into account transitions from rotational degrees of freedom to translational and vice versa. The Barnett slip coefficient is obtained in the form of a function dependent on the frequency of inelastic collisions of gas molecules and on the coefficient of accommodation of tangential momentum.     

The Temperature Dependence of the Coefficients of Thermal and Isothermal Slip of a Diatomic Gas with Rotational Degrees of Freedom

A model kinetic equation is suggested for diatomic gases with rotational degrees of freedom of molecules. The free parameters of the model are related to Eucken's partial factors. This model is used to solve the problem on the slip of a diatomic gas along a plane surface. The coefficients of thermal and isothermal slip are obtained in the form of functions dependent on the transport coefficients and the frequency of inelastic collisions of gas molecules and, as a consequence, on the temperature.     

Direct simultaneous measurement of rotational and translational accomodation coefficients

When molecules of a low density diatomic gas strike a solid surface both translational energy and the internal energy modes of rotation and vibration will contribute the energy exchange that occurs. Theoretical studies indicate that accomodation coefficient for rotational energy should be less than that for translational energy, and this is confirmed by experimental results. The experimental apparatus described in this paper uses the electron bream fluorescence detector to measure simultaneously both rotational and translational energy accommodation coefficients of room temperature nitrogen reflecting from a solid surface. A bakeable ultra high vacuum system was built to provide a clean vacuum environment for control of the solid surface properties. In addition to being the only known direct measurement of rotational accomodation coefficient the system offers an advantage over some previous methods of translational accommodation measurement in that there are few restrictions on solid surface temperature or composition.     

Kinetic Coefficients in the Boundary-Value Problem on the Slip of a Multiatomic Gas with Rotational Degrees of Freedom

The boundary-value problem on the slip of an inhomogeneous multiatomic gas along the spherical surface of small curvature is solved. For this purpose, a model kinetic equation that includes the rotational degrees of freedom of molecules is proposed. The solution is performed using the method of half-space moments. Gas-kinetic slip coefficients and jumps of macroscopic parameters of gas of the first and second order in the Knudsen number are obtained. These gas-kinetic coefficients are represented in the form of functions depending on the accommodation coefficient of tangential momentum, on the accommodation coefficients of the translational and rotational components of energy, and on the Prandtl number. For a number of multiatomic gases, calculations of the above-mentioned coefficients are performed.     

A Proposed Dynamic Pressure and Temperature Primary Standard

Diatomic gas molecules have a fundamental vibrational motion whose frequency is affected by pressure in a simple way. In addition, these molecules have well defined rotational energy levels whose populations provide a reliable measure of the thermodynamic temperature. Since information concerning the frequency of vibration and the relative populations can be determined by laser spectroscopy, the gas molecules themselves can serve as sensors of pressure and temperature. Through measurements under static conditions, the pressure and temperature dependence of the spectra of selected molecules is now understood. As the time required for the spectroscopic measurement can be reduced to nanoseconds, the diatomic gas molecule is an excellent candidate for a dynamic pressure/temperature primary standard. The temporal response in this case will be limited by the equilibration time for the molecules to respond to changes in local thermodynamic variables. Preliminary feasibility studies suggest that by using coherent anti-Stokes Raman spectroscopy we will be able to measure dynamic pressure up to 10⁸ Pa and dynamic temperature up to 1500 K with an uncertainty of 5%.     

Rotational and translational accommodation coefficients of nitrogen on nickel,silver and gold

An electron beam excited fluorescence technique was used to make simultaneous measurements of rotational and translational accommodation of nitrogen on nickel, silver, gold and stainless steel solid surfaces. In addition to the advantage of being able to make independent measurements of rotational accommodation, this apparatus allows a wide range of solid surface temperatures to be used and operates at lower gas density than most previous experiments.Rotational accommodation coefficients for nickel and gold varied from α_R = 0.12 at 400°K to α_R = 0.18 at 850°K while the value of α_R for silver increased more quickly from α_R = 0.03 at 430°K to α_R = 0.2 at 700°K.A linear variation of translational accommodation coefficient, α_T, with surface temperature was observed. An empirical relationship based on molecular weights was able to account for variation in dα_T/dT_s observed. The metal surfaces used were thought to be free of adsorbed gas and oxide layers.     

Prediction of translational accommodation coefficient

A simple empirical equation has been developed from experimental data for prediction of the translational accommodation coefficient. It contains such basic parameters of gas-surface interaction as the atomic weights of gas and solid surface atoms, their temperatures and the ratio of specific heats of the gas. Adequate agreement with experimental data available in the literature for monatomic, diatomic and polyatomic gases is obtained for a wide range of gas-surface parameters.     

The effect of the pattern of gas/surface interaction on the relation for Knudsen layer under conditions of strong subsonic condensation

The moments method is used within the model of diffuse reflection to investigate the effect of the temperature accommodation coefficient α_T on the flow behind the Knudsen layer under conditions of strong subsonic condensation. An explicit expression is obtained for dimensionless pressure as a function of temperature and Mach number M, as well as of the coefficients of evaporation, condensation, and temperature accommodation. It is demonstrated that the difference of α_T from unity may have a significant effect on the flow parameters and, in particular, on the value of Mach number that is the maximal value attainable under conditions of subsonic condensation. It is further demonstrated that this effect increases in the case of mirror reflection of molecules from the surface. The adiabatic exponent is investigated as well.     

Dynamics of ethane and propane in zeolite ZSM-5 studied by quasi-elastic neutron scattering

Quasi-elastic neutron scattering has been used to study the dynamics of ethane and propane in zeolite ZSM-5. The experiments were performed at different loadings for the two alkanes and at different temperatures for propane. The long-range translational motion of the molecules has been observed and has been interpreted with a jump diffusion model, the mean jump lengths being of the order of 10 Å, slightly decreasing with increasing loading. The self-diffusion coefficients of ethane are about 2 × 10⁻⁹ m² s⁻¹ at 300 K, those of propane at the same temperature being smaller by a factor of two. An activation energy of 5 kJ mol⁻¹ is obtained for the self-diffusion of propane. These results are in good agreement with the pulsed-field gradient n.m.r. measurements. In addition to the translational motion, a rotational motion of the two molecules is also observed; it is described by an uniaxial rotational diffusion model and rotational diffusion constants are derived.     

Measurements of the relative momentum accommodation coefficient for different gases with a viscosity vacuum gauge

The viscosity vacuum gauges are based on the gas momentum transfer phenomena between a moving part of the gauge and a stationary surface. Thus, they may be used for the study of the momentum accommodation coefficient for various combinations of gas species and surfaces. The aim of the present work is to determine the momentum accommodation coefficient by means of the viscosity vacuum gauge with vibrating metal ribbon. The relative accommodation coefficient was computed from the measurements for Xe, Ar, He and H₂ on the bronze ribbon of the gauge in the molecular conditions. The values of the relative coefficients were 0.90 for Xe, 0.95 for Ar, 1 for He (values were normalised to data obtained for He) and 0.94 for H₂.     

Calculation of the heat flux from a spherical particle in a molecular gas

The problem of calculating the heat flux from a uniformly heated spherical particle in a molecular gas is considered. The results of numerical calculations performed within the framework of the Hanson-Morse model using the linearized Wang Chang-Uhlenbeck equation are presented, which are obtained under the condition of purely diffuse reflection of gas molecules from the particle surface. The values of the temperature jump coefficient for particular gases have been determined.     

Molecular level investigation into selective permeability of silicon based membranes with potential use in gas separation processes

Abstract

The effect of the modification of the molecular structure on the permeability coefficients of typical rubbery and glassy silane and siloxane polymers at different temperatures was experimentally investigated. It was shown that carbon dioxide had higher permeability coefficients than those of nitrogen and oxygen due to the higher affinity of the various polymers toward the gas molecules. In order to provide a detailed understanding into the effect of the molecular structure on the gas diffusion behaviour in polymers, molecular modelling of carbon dioxide diffusion in silicon based membranes was used. The polymer molecules were shown to have lower self-diffusion coefficients than the gas ones related to the small size of the gas molecules as compared to the large size of the polymeric segments, thus allowing the gas molecules to jump from one unoccupied site to another through a series of connected pores or channels within the polymeric matrix. Increasing the temperature was shown to have a proportional effect on the mean square displacement, possibly due to the increase in the kinetic energy available to the systems. At high temperatures, the glassy siloxane molecules had similar values for the mean square displacement to those of the gas molecules since the polymer in this case is in close proximity to its glass transition temperature. The presence of the alternating oxygen atoms in the main backbone of the polymeric chains led to higher values for the selfdiffusion coefficients for the siloxane polymers as compared to those of the silane polymers as a result of the change in the bond angle about the oxygen atom (~ 144°) as compared to the tetrahedral angle (~ 110°) about the silicon atoms.     

Influence of the coefficient of vaporization on the parameters of the gas near the surface

The dependence of the coefficient of concentration jump on the coefficient of vaporization for a diluted binary mixture, when the concentrations of the components are significantly different, has been investigated. The analytical expression of the concentration jump for the case where the frequency of collisions of molecules is in proportion to their velocity has been obtained. __________ Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 80, No. 2, pp. 121–126, March–April, 2007.     

Nonstationary Heat and Mass Transfer in Molecular Gases

The analytical solution of the problem on a nonstationary heat source in a diatomic gas has been given. The characteristics of the temperature and sound waves excited by it have been obtained. The distribution of the pressure, temperature, and concentration of the gas molecules has been considered.     

Molecular dynamics simulation of pressure effect on phase transition behaviour and dynamics in the isotropic and discotic nematic phases

Molecular dynamics simulation was performed at constant temperature and pressure to investigate the effect of pressure on molecular dynamics for disc-shaped molecules. The generic Gay-Berne model, GB(0.345, 5.0, 1, 3), was used to study the phase transition behaviour, and translational and rotational dynamics, under two different reduced pressures P^?, 10.0 and 20.0. Obvious shifts were detected in the transition temperatures. Both systems have the same phase sequence with different pressures: isotropic, discotic nematic and columnar phases. Translational motion is characterised by the parallel and perpendicular components of diffusion coefficients, with respect to the director in the orientational ordered phase. With regard to rotational dynamics, the correlation time of the first-rank orientational time autocorrelation function, which corresponds to end-over-end rotational motion of a molecule, has been investigated. A clear jump in the temperature dependence of the correlation time has been found at the isotropic-nematic phase transition point. The retardation factor g_|| as a function of the reduced temperature T^?/T_NI^? shows an apparent pressure effect on the rotational dynamics.     

Pressure and Temperature Coefficients of the More Commonly Used Buffer Gases in Rubidium Vapor Frequency Standards

The temperature and pressure coefficients of three buffer gases, N2, Ar, Kr, commonly used in passive Rb57 frequency standards have been accurately determined. Techniques for filling cells reproducibly, are described. From the results obtained, calculations are made on the expected characteristics of gas mixtures. Graphs are given which can be used to select the proper buffer gas mixture and pressure to obtain specially required frequency shift and temperature coefficient in a given cell.     

Orientational effects in adsorption III: In-plane ordering in localized monolayers of diatomic molecules on crystalline solids

Orientational effects in monolayer films of diatomic molecules adsorbed on a single (100) face of a cubic crystal are discussed. The adsorbed molecules are assumed to take up an orientation parallel to the solid surface with their centres of mass located directly above the centres of the adsorption sites.It is shown that various orientationally ordered structures can appear and that their properties depend on the elongation of the adsorbed molecules, the size of the solid lattice, the density of the adsorbed film and the temperature. The heat capacities and the one-particle orientational distribution functions calculated for various systems are used to determine orientational properties of adsorbed films and to locate rotational transitions.     

Experimental study of the influences of degraded vacuum on multilayer insulation blankets

The paper presented experimental investigation on the heat transfer of MLI with different rarefied gases at different pressures. The investigations were carried out using an innovative static liquid nitrogen boil-off rate measurement system in the case of the small temperature perturbations of cold and warm boundaries. The heat fluxes for a number of inert and some polyatomic gases have been analyzed at different heat transfer conditions ranging from molecular to continuum regime, apparent thermal conductivities of the multilayer insulation were measured over a wide range of temperature (77 K–300 K) and pressure (10⁻³–10⁵ Pa) using the apparatus. The experimental results indicated that under degraded vacuum condition, the influences of rarefied gas on the MLI thermal performance very depend on the gas rarefaction degree which impacted by the MLI vacuum degree. Under the condition of molecular regime heat transfer, the MLI thermal performance was greatly influenced by gas energy accommodation coefficients (EAC), when under the continuum regime, the performances depend on the thermal conductivity of rarefied gas itself. Compared to the results of N₂, Ar, CO₂, Air and He as interstitial gases in the MLI, Ar was the better selection as space gas because of its low EAC and thermal conductivity characteristics on the different vacuum condition ranging from high pressure to vacuum. So different residual gases can be utilized according to the vacuum level and gas energy accommodation coefficient, in order to improve the insulation performance of low vacuum MLI.