Intensive molecular gas evaporating from the surface of a spherical particle into a vacuum

The kinetic problem of intensive diatomic gas evaporation from the surface of a spherical particle in a vacuum is solved. The analytical expressions for the parameters of a vapor gas at the hydrodynamic evaporation boundary are obtained, and their dependences on the Knudsen number in the range 0相似文献   

The effect of the evaporation coefficient on the thermophoresis of a volatile single-component drop in a binary gas mixture

A theory of the uniform thermophoretic motion of a liquid volatile spherical drop in a binary gas mixture is developed based on hydrodynamic analysis. One of the components undergoes the phase transition on the surface. The solution of the problem makes it possible to estimate the effect of the evaporation rate on the rate and direction of thermophoresis, as well as on the distributions of the velocity, temperature, and concentration of the volatile component. The thermal diffusion of the gas mixture, together with Stefan and capillary phenomena, is taken into account. The velocity of thermophoretic transport is expressed through the evaporation coefficient of the drop by the formula that generalizes the known results of the conventional theories for the cases of weak and moderately intense diffusive evaporation of a liquid drop.     

Field evaporation of silicon surfaces

Quantitative measurements on field evaporation of Si(111) surfaces in hydrogen imaging gas have been carried out by field ion microscopy. The field evaporation rate is found to increase exponentially with increase of the reciprocal of tip temperature in the range 80–103 K. The evaporation field strength increases with increase of tip temperature in the investigated range, 80–300 K. Within the applied pressure range, 5× 10^?6 to 2 × 10^?4 Torr of hydrogen gas, the evaporation rate linearly increases with the gas pressure. Similar effects of temperature and gas pressure on field evaporation of Si(111) surfaces have been observed also in silane imaging gas. A model, based on a field-induced formation of surface hydrides as a rate-determining step, is proposed, which accounts for all the experimental results of the field evaporation process.     

Theoretical study of the Kirchhoff integral from a two-dimensional randomly rough surface with shadowing effect: application to the backscattering coefficient for a perfectly-conducting surface

Abstract

In this paper, the backscattering coefficient of a two-dimensional randomly rough perfectly-conducting surface is investigated using the Kirchhoff approach with a shadowing function. The rough surface height/slope correlations assumed to be Gaussian are accounted for in this analysis. The scattering coefficient is then formulated in terms of a characteristic function for the integrations over the surface heights, in terms of expected values for the integrations over the surface slopes. Numerical comparisons of Kirchhoff's approach (KA) with the stationary-phase (SP) approximation are made with respect to the choice of the one-dimensional surface height autocorrelation function and the shadowing effect. For an isotropic surface the results show that SP underestimated the incoherent backscattering coefficient compared with KA. Moreover, when the correlation between the slopes and the heights is neglected, the shadowing effect may be ignored.     

Molecular-dynamics simulation of evaporation of a liquid

The molecular-dynamics method is used to investigate high-temperature evaporation of a simple liquid. The interaction of the atoms is described by a Lenard-Jones 6–12 potential. The simulation shows that fluctuations of the binding energy in the surface layer play an important role in evaporation, thanks to which a significant contribution to the evaporated flux comes from atoms whose kinetic energy is of the same order of magnitude as the mean thermal energy. Such a mechanism of evaporation differs substantially from the traditional one [Ya. I. Frenkel’, Kinetic Theory of Liquids (Clarendon Press, Oxford, 1946)] based on the assumption that only those particles evaporate that have energies of the order of the binding energy, i.e., much larger than the mean thermal energy. The structure of the transitional layer between the bulk gas and liquid phases is investigated. Potential energy fluctuations and pairwise correlation functions in the bulk phases and transitional layer are calculated. The velocity distribution function of the atoms for evaporation into vacuum is found. Zh. éksp. Teor. Fiz. 111, 1328–1346 (April 1997)     

T-odd asymmetries for evaporation neutrons in nuclear fission

T-odd asymmetries in the angular distributions of evaporation neutrons emitted by thermalized fission fragments in the fission of axially symmetric deformed nuclei by cold polarized neutrons are investigated within the quantum theory of fission. The asymmetries in question are due to the anisotropy of angular distributions of evaporation neutrons in the center-of-mass systems of the fission fragments, and this anisotropy arises from the orientation of large-value fission fragment spins in the direction perpendicular to the direction K ₀ of the symmetry axis of the fissioning nucleus at the time of its scission, caused by zero wriggling vibrations of the fissioning nucleus. The angle of rotation of the vector k ₀ with respect to the asymptotic direction k ₀ of the fissioning nucleus symmetry axis is calculated with allowance for the interference of fission amplitudes of neutron resonances excited in a fissioning nucleus as it captures an incident neutron. It is shown that the T-odd asymmetry coefficient for evaporation neutrons is close in structure and value to the analogous coefficient for evaporation γ-rays.     

Field evaporation of silicon (111) surfaces in the presence of hydrogen

The field evaporation rate of Si(111) face in H₂ imaging gas is measured by counting the removal rates on individual net planes in a field ion microscope. The rate is found to decrease with increasing temperature and to be proportional to H₂ gas pressure. The evaporation voltage increases with temperature (T<300). A model explaining this temperature dependence is based on the rate-determining formation of surface hydrides due to field-induced chemisorption of hydrogen.     

Effect of oxygen segregation on the surface structure of single-crystalline niobium films on sapphire

Single-crystalline Nb films are grown on (1120) oriented sapphire substrates by electron-beam evaporation in ultra-high vacuum. The films are studied in-situ by RHEED and Auger analysis. At a substrate temperature T _S=750° C the RHEED pattern shows a smooth growth of bcc-Nb in the [110] direction. In addition to the fundamental streaks, we observe superlattice streaks of fractional order in several azimuthal directions. The reciprocal lattice of the surface is determined. The basic vectors of the superlattice in real space are given by b ₁=2a ₁, b ₂=–a ₁+3a ₂ where a ₁ and a ₂ are the basic vectors of the Nb (110) surface. Auger analysis shows that the surface of these films is contaminated with oxygen. Therefore, the superstructure is attributed to a modified surface structure due to segregated oxygen, possibly having diffused from the sapphire to the film surface. The superstructure dissappears during further evaporation of Nb at T _S<450° C with a concomitant decrease of the oxygen signal. Nb films on sapphire with a clean, oxygen-free surface can only be prepared at lower temperatures in an island-growth mode.     

On the transverse distribution of a resonance field excited by a Gaussian electromagnetic beam on the critical surface of a radially nonuniform plasma sphere

The transverse distribution of the resonance field excited by a Gaussian electromagnetic field on the critical surface of a radially nonuniform plasma sphere is studied. Analytical expressions are obtained for this distribution. It is shown that when a laser beam is focused in front of or behind a spherical target, identical values of the integrated resonance coefficient can correspond to substantially different (in width) distributions of the resonance field over the spherical critical surface. Zh. Tekh. Fiz. 67, 125–128 (October 1997)     

A reformulation of the one-dimensional surface field integral equations

Abstract

By starting from the matrix forms of the two coupled, inhomogeneous integral equations for the values of the magnetic field and its normal derivative on a one-dimensional, rough metal surface, or for the values of the electric field and its normal derivative on such a surface, we have obtained an equivalent pair of equations for these quantities in which the inhomogeneous terms are just the Kirchhoff approximations to them. The new pair of equations for the surface values of the magnetic field and its normal derivative is solved iteratively to generate a multiple-scattering expansion for the scattering amplitude when p-polarized light is scattered from a large RMS height, large RMS slope, one-dimensional, random silver surface, with the plane of incidence perpendicular to the generators of the surface. It is shown that the Kirchhoff approximation to the contribution to the mean differential reflection coefficient from the incoherent component of the scattered light displays no evidence of enhanced backscattering. However, the pure double-scattering contribution already displays this effect, stamping it as a multiple-scattering phenomenon.     

Relationship between the diffusiophoresis rate, evaporation coefficient, and size of a large volatile drop

A new theory of diffusiophoresis of large volatile spherical aerosol drops that is an extension of investigations [1–8] is developed. The influence of the radius of the drop, the surface tension coefficient varying over the surface of the drop, the evaporation coefficient α of the liquid, and the flows inside the drop on the diffusiophoresis rate are taken into account. Expressions obtained allow for direct determination of the velocity of large individual aerosol drops in a binary gas mixture nonuniform in component concentration. It is shown that both the magnitude and the direction of the diffusiophoresis velocity depend on α and the size of the drop. It is assumed that the size of the drop varies but remains considerably greater than the mean free path of gas molecules.     

Sputtering of tungsten, tungsten oxide, and tungsten-carbon mixed layers by deuterium ions in the threshold energy region

An original experimental method is developed for determining the sputtering coefficients of electrically conducting materials during bombardment by light gas ions at threshold energies. This information is very valuable in both purely scientific and practical terms. The basis of the method is a special field-ion-microscopic analysis regime. The procedure for measuring the sputtering coefficients includes cleaning the surface by field desorption and evaporation, with the subsequent work on an atomically clean and atomically smooth surface. The method permits identification of single vacancies on the irradiated surface, i.e., it is possible to count individual sputtered atoms. The method is tested on commercially pure tungsten, tungsten oxide, and a W-C mixed layer on tungsten under deuterium ion bombardment. The energy dependences of the sputtering coefficients of these materials for sputtering by deuterium ions at energies of 10–500 eV are obtained and analyzed. An important relationship between the energy threshold for sputtering and the conditions for oxidation of tungsten is found. The energy threshold for sputtering of an oxidized tungsten surface is 65 eV. The energy threshold for sputtering of the W-C mixed layer is almost equal to the corresponding value for pure tungsten. Zh. Tekh. Fiz. 69, 137–142 (September 1999)     

Radiation-enhanced diffusion and fission gas release from recrystallised grains in high burn-up \hbox{UO}_{2} nuclear fuel

The effective diffusion coefficient that gives a steady-state xenon concentration of 0.2-0.3wt% in the recrystallised grains of high burn-up UO 2 fuel is calculated to lie in the range 10 m 24 to 10 m 22 m 2 s m 1 . These values are one to three orders of magnitude lower than the value currently accepted for the radiation-enhanced diffusion coefficient. The time required to reach the steady-state concentration depends on the local fission rate, the grain size distribution and the precise magnitude of the radiation-enhanced diffusion coefficient, and can take from 2 to 10 years. Additional calculations reveal that substantially less than 10% of the fission gas inventory is released from the original UO 2 grains in the outer region of the fuel prior to recrystallisation. In contrast, with a diffusion coefficient of 10 m 22 m 2 s m 1 more than 80% of the fission gas is released from the recrystallised grains of the high burn-up structure in one year.     

The influence of surface coordination on field evaporation processes in tungsten

The sequence of evaporation of atoms on (111), (332), (433), and (411) surfaces of tungsten has been observed in the field ion microscope for the evaporation of successive layers. Atoms with many different types of surface coordination were observed to evaporate. The data were processed to give the relative probabilities of evaporation (ρ) for each type of atom using a statistical procedure adapted to give probability limits for the ρ values. The results showed that for (111) surfaces the ρ values were the same with helium or neon as image gas and that in general differences in ρ values between differently coordinated atoms at 78 K were observed to be greater than at 20 K. At both these temperatures however the atoms with high surface coordination generally had lower ρ values. With (111), (332) and (433) surfaces the atomic arrangement allowed prediction of surface diffusion paths and with atoms in (111) surfaces an easy diffusion path was essential before evaporation could occur. Amongst atoms which had a diffusion path available, ρ values were relatively lower when the path had a saddle point protruding from the surface and which allowed the atom to be subjected to a higher field. On (411) surfaces diffusion paths are less predictable and the correlation with ρ was less.     

Deuterium and Oxygen-18 in Surface Waters of GDR draining to the Baltic Sea

A general view is presented of deuterium and ¹⁸O measurements of water samples collected at running and standing surface waters in German Democratic Republic. The values confirm earlier observations that the surface waters are influenced by evaporation with respect to the isotopic composition of groundwater. Nevertheless, stronger evaporation effects are restricted to larger lakes. The amount of surface water discharge from GDR to the Baltic Sea and the δD and δ¹⁸O values are discussed. The river Oder provides about 90% of the whole surface run-off. The other watercourses to the coast are unimportand. The mean heavy isotope content of surface run-off was calculated to be ?8.3‰ for δ¹⁸O and ?61‰ for δD (vs. SMOW), respectively.     

Semi-empirical model for intense evaporation

A semi-empirical model based on the linear kinetic theory was developed for intense evaporation. The extrapolated drops for pressure and temperature at the condensed phase surface were calculated through summing of linear and squared terms. The analytical dependencies were obtained for gas parameters in gas-dynamic zone as functions of Mach number, condensation coefficient, and the number of degrees of freedom for molecules of ideal gas. The calculations from semi-empirical model are in agreement with results from known analytical and numerical studies.     

Application of the surface ionization for the detection of secondary particles in the secondary-ion mass spectrometry (SIMS)

A method for postionization in the course of ion sputtering that is based on surface ionization of the sputtered particles is developed. The estimations show that the method allows a significant increase in the sensitivity of the secondary-ion mass spectrometry for several elements. Nonadditive increase in the sputtering coefficient of indium is experimentally studied using the surface-ionization method of postionization when the number of atoms in projectile clusters Bi _m ⁺ (m = 1–7) increases at energies 2–10 keV. Such a scheme for the detection of neutral particles can be used in alternative methods for the surface analysis, in particular, laser evaporation of surface and electron-stimulated desorption.     

Auger electron spectroscopy of the kinetics of evaporation of palladium beaded films from sapphire substrate

) surface of sapphire were investigated by Auger electron spectroscopy in the temperature range of 1053–1083 K. It was found that the decrease of the effective thickness during heat treatments under 10^-8 mbar was caused by evaporation from the surface of the substrate. Using the model developed by Kaganovskii and Beke we have established that the process is controlled by surface diffusion. It was shown that the evaporation rate of palladium adatoms from an alumina surface is given by I_o=1.45×10³⁶exp{-(393±46 kJ/mol)/RT}ms, and, on the basis of an estimation of the surface diffusion length, the effective surface diffusion coefficient D_s was evaluated: Received: 30 March 1998/Accepted: 28 May 1998     

Surface glass transition in bimodal polystyrene mixtures

Using the cluster-embedding method of V. Zaporojchenko et al. (Macromolecules 34, 1125 (2000)), we measured the glass transition temperature T _g at the polystyrene/vacuum interface of bimodal mixtures of monodisperse polystyrenes of 3.5k and 1000k. Embedding of ≈ 1 nm Au clusters was monitored in situ by X-ray photoelectron spectroscopy (XPS). The clusters were formed by evaporation of Au onto the polymer surface. Only one glass transition was observed in the mixtures. The surface glass transition temperatures are correlated to but are below the bulk values of the mixtures and obey the Gordon-Taylor equation. The results suggest that the earlier reported molecular-weight dependence of the surface glass transition is not due to segregation of short chains to the surface.     

3D problem of heat and mass transfer at the ignition of a combustible liquid by a heated metal particle

A nonlinear nonstationary 3D problem of heat and mass transfer at gas phase ignition of a combustible liquid spread on the surface of a solid body by a metal particle heated to a high temperature is solved. This is done within the framework of a model taking into account the heat conduction and evaporation of the liquid, the diffusion and convection of the combustible vapors in the oxidizer medium, the crystallization of the ignition source, the kinetics of the processes of evaporation and ignition of liquids, the dependence of the thermophysical characteristics of the interacting substances on the temperature, and the moisture content of the oxidizer—air. The dependences of the ignition delay time of the liquid on the temperature and sizes of the heating source are established. Limiting values of the temperature and particle sizes at which the ignition conditions take place are determined. The influence of the air humidity on the inertia of the process being investigated is analyzed. A comparison of numerical values of typical parameters of the process under investigation for 2D and 3D models is performed.