Argon Molecules Research Articles

Atmospheric pressure argon plasma jets I – Measurements of spatial distribution for visible emission spectrum and vacuum ultraviolet

AbstractThis paper describes the observed position dependent emission spectrum and vacuum ultraviolet (VUV) to clarify the optical structure of argon atmospheric pressure plasma jets (APJs). Visible emission spectra of argon, nitrogen, and OH molecules were measured as a function of position from the nozzle of plasma jet generator to gas flow direction. From the results, we realized that a certain electric field is required for producing high energy excited atoms of argon in the jet from the high voltage electrode in the device to the ground positioned at downstream side. Moreover, VUV was detected along the plasma jet through the downstream direction. Thus, we could picture that the light pattern of plasma jet of which intensity weaken depend to the distance from the nozzle is sustained by the above two factors with the present steady state experiments.

Radiation-induced transformations of isolated dimethyl disulphide molecules in solid argon

The radiation-induced transformations of isolated dimethyl disulphide molecules in a solid argon matrix occurring under the action of X-rays at 4.5 K were studied by FTIR spectroscopy. It was shown that decomposition was quite efficient (the corresponding radiation-chemical yield of Me2S2 degradation was estimated as 4.5 molecules per 100 eV). The radiolysis proceeds via two primary channels yielding CH3SH plus H2CS and CH4 plus H2CSS, respectively. These two channels of dimethyl disulphide decay during radiolysis lead to various secondary products appearing during prolonged radiolysis. Spectroscopic characteristics of the observed products and possible implications of the results are discussed.

CO Oxidation to CO2 on Uranium Dioxide Molecules through Intermediate O2U(η1-CO).

Investigations on the reactions of uranium oxide molecules with CO offer new inspiration for the design of promising high-efficiency catalysts for CO activation using actinide materials. Herein, we contribute a combined matrix-isolation infrared spectroscopic and theoretical study of CO oxidation to CO2 on uranium dioxide (UO2) molecules in solid argon. The reaction intermediate O2U(η1-CO) is generated spontaneously at the bands of 1893.0, 870.6, and 801.3 cm-1 during codeposition and annealing. Upon the following irradiation, CO2 is substantially produced by the consumption of O2U(η1-CO), indicating the catalytic conversion of CO to CO2 through the intermediate O2U(η1-CO). In C18O isotopic substitution experiments, the yields of 16OC18O convincingly confirm that one of the oxygen atoms in CO2 derives from UO2. The reaction pathways are discussed based on the theoretical and experimental results.

The Relationships in the Law of Conservation of Mechanical Energy and the Ideal Gas Equation in Argon Molecules based on Moldy and Gnuplot

Science and technology have been progressing by leaps and bounds over the past few decades all over the world, resulting in the rapid development of advanced computer programs. Under such circumstances, we are able to prove plenty of theories in a new way with the help of different kinds of modern computer programs. For example, we are capable of investigating the relationship between kinetic energy and potential energy, as well as the connection between temperature and pressure, by applying computer programs to draw graphs. In this paper, moldy and gnuplot which are progressive computer programs are used to paint the graphs about the two relationships in the conservation law of mechanical energy and ideal gas equation respectively in argon gas with the help of the information obtained from the Internet. These two computer programs not only remove the burden of calculation, but also make the relationship clearer. In the end, this paper proves that the sum of kinetic energy and potential energy in argon molecules is invariable in a closed system under different temperatures. Pressure is also tested to have a positive linear association with temperature although the density changes when the argon gas is an ideal gas.

An EPR study on the radiolysis of isolated ethanol molecules in solid argon and xenon: matrix control of radiation-induced generation of radicals in cryogenic media.

This paper addresses the basic question of the impact of a chemically inert environment on the radiation-induced transformations of isolated organic molecules in icy media at cryogenic temperatures with possible implications for astrochemical issues. The radicals produced by X-ray irradiation of isolated ethanol molecules (C2H5OH and CH3CD2OH) in solid argon and xenon matrices at 7 K were characterized by electron paramagnetic resonance (EPR) spectroscopy. It was shown that methyl (CH3˙) and formyl (HCO˙) radicals resulting from the C-C bond cleavage were mainly produced in the case of solid argon, which was attributed to the significant role of "hot" ionic fragmentation and inefficient energy dissipation to medium. In contrast, irradiation in xenon results in the predominant formation of α-hydroxyethyl radicals (CH3˙CHOH or CH3˙CDOH(D) in the cases of C2H5OH and CH3CD2OH, respectively). Remarkably, the experiments with selectively deuterated ethanol provide strong indirect evidence for the primary formation of ethoxy (CH3CD2O˙) radicals due to O-H bond cleavage, which convert to the α-hydroxyethyl radicals due to isomerization occurring at 7 K. The α-hydroxyethyl radicals adopt a specific rigid conformation with a non-rotating methyl group at low temperatures, which is an unusual effect for neutral CH3˙CHX species, and exhibit free rotation in solid xenon only at ca. 65 K.

Acoustic cavitation characteristics of bubble in compressible liquid

The newly proposed Gilmore-NASG model is used to study the acoustic cavitation characteristics of bubble in compressible liquid under the boundary condition of considering the compressible effect of the liquid, and comparison is made between the results calculated by the Gilmore-NASG model and original KM-VdW model without considering the mass exchange, chemical reaction and heat exchange between the gas in the bubble and the surrounding liquid. The results suggest that, compared with the KM-VdW model, the Gilmore-NASG model which employs a new equation of state to describe the gas, liquid and variations of liquid density and sound velocity due to compressibility, can give a larger compression ratio of cavitation bubble, a deeper collapse depth, higher temperature and pressure peaks. This is mainly because that the co-volume of argon molecule in the NASG equation of state is smaller than that in the VdW equation of state and the effect of the co-volume of water molecule is considered in the NASG equation of state, that is, the Gilmore-NASG model gives more comprehensive consideration to the liquid compressibility. When the bubble collapses violently, the Gilmore-NASG model takes into account the changes of sound velocity caused by the compressibility of the liquid at the bubble wall, effectively avoid the possibility of abnormal increase in the Mach number of the liquid at the bubble wall. With the increase in the driving sound pressure amplitude, the difference between the results given by the two models more and more significantly and the temperature and pressure peaks in the bubble given by the Gilmore-NASG model increase more significantly. With the rise of driving frequency, the difference between the results given by the two models gradually decreases and tends to be consistent under the high-frequency excitation. This indicates that the temperature and pressure in the bubble may arrive at higher values considering the compressibility of the medium caused by the co-volume changes of gas and surrounding liquid at different temperatures and pressures. In the meantime, the Gilmore-NASG model can accurately predict the changes in density, pressure and temperature of the liquid at the bubble wall as well as sound velocity, so this model has advantages in the study of bubble cavitation characteristics under high pressure and the effect of surrounding liquid on bubble cavitation characteristics. There will be important applications for the research on specific issues such as high-intensity focused ultrasound, shock wave lithotripsy treatment and sonochemistry.

Molecular dynamics simulations of thin-film evaporation: The influence of interfacial thermal resistance on a graphene-coated heated silicon substrate

Two-phase liquid cooling on nanoengineered surfaces has shown great promise for tackling overheating in high performance microelectronics. At the nanoscale, phase change heat transfer can be affected substantially by the thermal resistance at the solid–liquid interface (also known as the Kapitza resistance). For example, surface modification by nanocoatings has been shown to efficiently enhance boiling heat transfer. However, there have been few studies on the effect of the Kapitza resistance on thin-film evaporative heat transfer. Here, the transport behavior of a liquid argon nanofilm evaporating on silicon (100) surfaces coated with different layers of graphene is analyzed using non-equilibrium molecular dynamics (MD) simulations. The results show that increasing layers of graphene coatings lead to a 136% increase in the Kapitza resistance and a 62% reduction in the evaporation rate. The large increase in Kapitza resistance is attributed to the strong repellence between graphene and argon molecules, manifested by a higher contact angle. These findings demonstrate the dominant role of interfacial thermal transport in the evaporation behavior of thin liquid films, and provide guidelines for selecting new materials when designing nanoengineered surfaces to realize improved evaporative cooling performance. • MD simulations of evaporation of liquid Ar on Si coated with graphene. • Influence of surface wettability on Kapitza resistance and evaporation. • Strong correlation between evaporative heat transfer and intermolecular interaction at solid–liquid interface.

Computational Simulations of Nanoconfined Argon Film through Adsorption–Desorption in a Uniform Slit Pore

We performed hybrid grand canonical Monte Carlo/molecular dynamics (GCMC/MD) simulations to investigate the adsorption-desorption isotherms of argon molecules confined between commensurate and incommensurate contacts in nanoscale thickness. The recently proposed mid-density scheme was applied to the obtained hysteresis loops to produce a realistic equilibrium phase of nanoconfined fluids. The appropriate chemical potentials can be determined if the equilibrium structures predicted by GCMC/MD simulations are consistent with those observed in previously developed liquid-vapor molecular dynamics (LVMD) simulations. With the chemical potential as input, the equilibrium structures obtained by GCMC/MD simulations can be used as reasonable initial configurations for future metadynamics free energy calculations.

Modification of Zeolites Y and ZSM-5 adsorption of nanoparticles of transition metals from back-micellar solutions for separation of gas mixtures

On the basis of granular synthetic zeolites NaY, HY, and ZSM-5, adsorbents containing nanoparticles of silver, cobalt, molybdenum, and tungsten were obtained. The samples have a lower surface polarity in comparison with the initial zeolites, which is reflected in the selectivity of a number of samples with respect to argon. This is due to the fact that the argon molecule interacts with zeolites only through nonspecific forces. Modification was performed by interacting with reverse-micellar solutions of nanoparticles. The actual sizes of metal particles and their distribution over the surface of the modified samples of zeolites have been determined by the method of transmission electron microscopy. The samples’ equilibrium adsorption capacities for oxygen and argon (25°С and atmospheric pressure) and the separation coefficient of the argon–oxygen mixture as the ratio of Henry’s coefficients have been determined. It has been demonstrated that samples of the NaY zeolite modified with silver nanoparticles have the separation coefficient value of the argon–oxygen gas mixture equal to 1.6.

Molecular dynamics study on the conversion mechanism between momentum and energy components in gas-surface interaction

With the rapid development of micro-nano technology and aerospace technology, researches related to rarefied gas flows have received more and more attention. For micro-/nanoscale systems and spacecraft in a rarefied environment, the reduction in the frequency of intermolecular collisions in the flow field makes the interaction between gas molecules and the solid surface develop into a major factor affecting the flow state. However, the mechanism of gas-surface interaction in rarefied flow has not been fully revealed due to its microscopic nature and physical complexity, and the existing simulation methods cannot accurately reflect the effect of this process on the flow state. In this paper, molecular beam method is adopted to simulate the scattering process of argon molecules on platinum surface, and the impacts of incident velocity, angle and wall roughness on the momentum and energy conversion mechanism are explored. By simulating the molecular scattering process under the two incident angles of <inline-formula><tex-math id="M3">\begin{document}$ 5^{\circ} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20201192_M3.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20201192_M3.png"/></alternatives></inline-formula> and <inline-formula><tex-math id="M4">\begin{document}$ 75^{\circ}$\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20201192_M4.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20201192_M4.png"/></alternatives></inline-formula>, the following conclusions are obtained. When colliding with the wall at an angle close to vertical, both components of the momentum of the gas molecules are lost. The normal energy transfers to the tangential direction, and when the molecular velocity is not less than 2.0, the transfer rate is not significantly affected by the incident energy of the molecule and the surface roughness. The total energy loss of gas molecules after scattering becomes significant with the increase of incident velocity, and it is not sensitive to changes of surface roughness. When the gas molecules are incident at <inline-formula><tex-math id="M5">\begin{document}$ 75^{\circ} $\end{document}</tex-math><alternatives><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20201192_M5.jpg"/><graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="2-20201192_M5.png"/></alternatives></inline-formula>, the roughness of the surface has a significant impact on the conversion mechanism of molecular momentum and energy. After colliding with a smooth wall, the momentum and energy values of the gas molecules remain basically unchanged, only the direction of the momentum is reversed. The motion state of molecules is close to the mirror reflection, and the conversion between momentum and energy components is not obvious. The introduction of roughness enhances the degree of accommodation between gas molecules and metal surface, and promotes the transfer of molecular tangential momentum and kinetic energy to the normal direction. When incident at a large polar angle, as opposed to the small-angle cases, the total energy loss of molecules is not sensitive to changes of incident velocity, it goes up significantly with the surface roughness increasing. The research in this article not only explores the gas-surface interaction mechanism, but also provides a useful reference for the high-fidelity simulation of rare gas flow and the development of appropriate gas-surface interaction models.

Efficient approaches to solutions of partition function for condensed matters

The key problem of statistical physics standing over one hundred years is how to exactly calculate the partition function (or free energy), which severely hinders the theory to be applied to predict the thermodynamic properties of condensed matters. Very recently, we developed a direct integral approach (DIA) to the solutions and achieved ultrahigh computational efficiency and precision. In the present work, the background and the limitations of DIA were examined in details, and another method with the same efficiency was established to overcome the shortage of DIA for condensed system with lower density. The two methods were demonstrated with empirical potentials for solid and liquid cooper, solid argon and C60 molecules by comparing the derived internal energy or pressure with the results of vast molecular dynamics simulations, showing that the precision is about ten times higher than previous methods in a temperature range up to melting point. The ultrahigh efficiency enables the two methods to be performed with ab initio calculations and the experimental equation of state of solid copper up to ∼600 GPa was well reproduced, for the first time, from the partition function via density functional theory implemented.

Nonequilibrium molecular dynamics study on energy accommodation coefficient on condensing liquid surface—Molecular boundary conditions for heat and mass transfer

Nonequilibrium molecular dynamics (NEMD) studies have been conducted to determine molecular boundary conditions at vapor–liquid interfaces for the kinetic theory of condensation and evaporation. In previous studies, a microscopic formulation of the condensation coefficient was defined as the condensation probability of vapor molecules based on equilibrium molecular dynamics simulations and transition state theory. The condensation coefficient was presented as a function of the translation energy of incoming molecules and surface temperature. Based on this, the velocity distributions of evaporating and reflecting molecules were theoretically expressed under equilibrium conditions. In a practical nonequilibrium situation, the energy transfer by the reflecting molecules is important along with the condensation/evaporation probability. However, it is unclear whether the results obtained under equilibrium conditions can be applied under nonequilibrium conditions. This study, therefore, defines the energy accommodation coefficient of reflecting molecules by comparing the energy transfer due to reflection with that under equilibrium conditions. NEMD simulations are conducted using two surfaces facing each other, an evaporating surface and a condensing surface, for argon molecules under different nonequilibrium conditions. The results show that the velocity distribution of reflecting molecules deviates from those under equilibrium conditions, and the energy accommodation coefficient decreases as nonequilibrium conditions increase. Additionally, an inverted temperature profile is observed. Reflecting molecules play an important role in the sensible heat transfer on the condensing surface, and they are not accommodated on the condensing surface. Thus, they raise the temperature in the vicinity of the condensing surface under nonequilibrium conditions.

Calculation of Chemical Potential of a Molecule on the Basis of Radial Distribution Functions

The work is devoted to the application of the Hill method for calculating the chemical potential of a molecule in the one-component homogeneous molecular system within computer simulations. This method is based on the double integration of molecular radial distribution functions, which depend on the additional parameter that controls the strength of interactions between the molecules. The results of calculating the chemical potential of the argon molecule by the Hill method are compared with our data obtained by two other methods: the Widom test-particle method and the extended ensemble method within the Wang–Landau algorithm.

Effect of Low-Pressure Plasma Treatment Parameters on Wrinkle Features.

Wrinkles attract significant attention due to their ability to enhance the mechanical and optical characteristics of various optoelectronic devices. We report the effect of the plasma gas type, power, flow rate, and treatment time on the wrinkle features. When an optical adhesive was treated using a low-pressure plasma of oxygen, argon, and nitrogen, the oxygen and argon plasma generated wrinkles with the lowest and highest wavelengths, respectively. The increase in the power of the nitrogen and oxygen plasma increased the wavelengths and heights of the wrinkles; however, the increase in the power of the argon plasma increased the wavelengths and decreased the heights of the wrinkles. Argon molecules are heavier and smaller than nitrogen and oxygen molecules that have similar weights and sizes; moreover, the argon plasma comprises positive ions while the oxygen and nitrogen plasma comprise negative ions. This resulted in differences in the wrinkle features. It was concluded that a combination of different plasma gases could achieve exclusive control over either the wavelength or the height and allow a thorough analysis of the correlation between the wrinkle features and the characteristics of the electronic devices.

Molecular dynamics simulation on the phase change of argon molecules between two plates in a wide range of temperature variation

Understanding the complex physical mechanisms of phase change phenomena such as solidification, evaporation and condensation at the nanoscale depending on the temperature variation in a wide range is very important. This study carried out molecular dynamics simulation to investigate the motion of the argon molecules and their phase changes between the bottom cold and top hot walls whose temperatures cover a wide range of variation from the freezing temperature to the supercritical temperature. The motion of argon molecules and their corresponding final phases of solid, liquid, liquid-gas and supercritical fluid formed in the domain depend on the temperature values at the bottom cold and top hot walls, and were classified into seven different processes. The characteristics of final phases of solid, liquid, liquid-gas and supercritical fluid were analyzed by using the radial distribution function, the surface-averaged temperature distribution along the vertical direction and the volume-averaged temperature in the domain for different phases of argon molecules obtained at the final stage of the non-equilibrium simulation. Furthermore, the time history of the average density of argon molecules at the upper domain has also been reported.

Dependence of Surface Tension of a Droplet Formed on the Molecular Condensation Nucleus or Ion on the Droplet Radius

Abstract—The dependence of the thermodynamic surface tension of a small droplet formed on a molecular-size condensation nucleus on the droplet size, nucleus size, molecular field parameters, and nuclear charge in the case of an ion is investigated. Calculations have been performed for molecules of supersaturated argon vapor at different values of the chemical potential of the molecules in the framework of the gradient density functional theory (DFT) and the Carnahan–Starling model to take into account the contribution of hard spheres. The interaction of argon molecules with an uncharged condensation nucleus has been described by the Lennard–Jones potential. In the case of an ion, the long-range Coulomb potential of electric forces is additionally taken into account. The dielectric constant is defined as a function of the local density of the number of argon molecules. As a variable describing the size of the droplet, the radius of the equimolecular surface of the droplet is chosen. The obtained dependences of the surface tension of the droplets have been compared with the dependence of the surface tension on the size of droplet without a condensation nucleus. When the effect of the solvation layer is discarded, the dependence of the surface tension on the radius of the equimolecular surface of a small droplet with a condensation nucleus exhibits similar behavior as in the absence of the nucleus with almost the same negative Tolman correction. The effect of the rigidity constant, however, is clearly influenced by the existence of a condensation nucleus. It is shown that when the first solvation layer is divided around the condensation nucleus, the dependence of surface tension on the radius of the equimolecular surface of a small droplet with a condensation nucleus exhibits similar behavior with almost the same negative Tolman correction as in the absence of a nucleus, but with a different correction, namely, with effective rigidity constant for the surface layer.

Determination of momentum accommodation coefficients and velocity distribution function for Noble gas-polymeric surface interactions using molecular dynamics simulation

Due to the momentum and energy exchange between the gas and solid surface molecules, by means of the accommodation coefficients in the Cercignani-Lampis-Lord (CLL) model, the velocity components used in the Direct Simulation Monte Carlo (DSMC) method can be studied more accurately. The coefficients can also be used in calculation of slip velocity, temperature jumping, drag force and shear stress. In the light of rising needs for polymers in industrial applications, the scattering behavior of noble gas molecules in their collision with a surface made of diglycidyl ether bisphenol A (DGEBA) epoxy resin, cured by tetrahydrophthalic anhydride (THPA) agent and reinforced by multi-layer graphite, was simulated and investigated by Molecular Dynamics (MD). The momentum and energy accommodation coefficients for gas–surface interactions were also calculated by MD simulation. The results indicated that temperature increase, change in the gas species, reduction in surface roughness, and increase in the Knudsen number (Kn) and the wall velocity could cause the accommodation coefficients to be less than one. Based on the scattering kernel of CLL boundary condition, a suitable distribution function for calculating the velocity components of noble gas molecules (e.g., argon molecules) after interaction with polymeric surface was proposed.

Study of microwave discharge at high power density conditions in diamond chemical vapor deposition reactor by optical emission spectroscopy

The paper presents results of measurements of spatial distributions of emission intensity of various lines of argon, atomic hydrogen, C2 and CH molecules in a wide pressure range from 40 to 500 Torr in diamond chemical vapor deposition (CVD) reactor. Microwave discharge plasma was maintained in reactor in continuous mode both in hydrogen and in a mixture of hydrogen-methane with a small addition of argon for diagnostic purposes. Intensity distribution of argon emission was characterized by a pronounced maximum near the substrate in reactor, which indicates that microwave power density (MWPD) absorbed in the plasma is also concentrated mainly near the substrate. Maximum emission intensity of hydrogen atoms is located at some distance from the substrate, and maxima of spatial distributions of emission intensity of carbon-containing species C2 and CH are shifted farther away from the substrate. This leads to the fact that the shape and size of discharge, which are observed visually or with help of a camera, significantly differ from the distribution of MWPD. There is a difference in the shape of spatial distribution of argon lines emission (750.4 nm and 811.5 nm), that can be explained by a change in the mechanism for quenching the excited levels of argon in different regions of discharge. This imposes some restrictions on the use of argon for measuring characteristics of microwave discharge.

INFLUENCE OF ARGON PRESSURE ON MICROSTRUCTURE AND OPTICAL PROPERTIES OF Zn0.9Se:Co0.1 THIN FILMS PREPARED BY PULSED LASER DEPOSITION

Zn[Formula: see text]Se:Co[Formula: see text] thin films were deposited on sapphire (Al2O3) substrates in argon atmosphere at various gas pressures by pulsed laser deposition. Influence of argon pressure on the thickness, surface morphology, crystal structure and optical properties of the thin films were investigated by various diagnosis tools. It was found that these physical properties were correlated to the argon pressure and appeared like a mutation between the pressure of 2[Formula: see text]Pa and 4[Formula: see text]Pa. As the deposition pressure changed from 2[Formula: see text]Pa to 4[Formula: see text]Pa, the collision probability between ablated species and argon molecule was increased, which resulted in the transformation of the deposition type from sputter effect to adsorption effect and the mutation of film thickness and structure. These in turn influence the transmission range and band gap of the films. All of the results suggest that the ambient pressure is a very important factor to the deposition of Zn[Formula: see text]Se:Co[Formula: see text] films by PLD.

Molecular dynamics simulation study on the mechanisms of liquid-phase permeation in nanopores

Liquid-phase permeation was simulated in nano-scale pores via non-equilibrium molecular dynamics (NEMD). Two virtual cristobalite membranes were prepared with pore diameters of 1.7 and 2.4 nm. NEMD simulation system was employed as an ideal experimental system to calculate the affinity between liquid argon molecules and membrane materials during permeation. When argon-membrane interactions decreased, permeation flux increased. With a smaller interaction the permeation flux exceeded the value posited by the Hagen-Poiseuille theorem, while a lower-than-expected level of permeation flux was observed when the interactions with the pore surface became greater. We focused on the viscosity change of liquid in a nano-scale pore due to attractive or repulsive interactions with the pore surface, and a mathematical model for describing the liquid permeation flux in a nanopore was proposed by solving the Navier-Stokes equation by considering the viscosity distribution of a liquid confined in a pore. The local viscosity of a liquid confined in a pore was calculated from the total potential distribution in the pore based on the Andrade equation. The predicted level of permeation flux, the velocity profiles of different pore sizes, and the interactions of the pore models all showed good agreement with the NEMD simulation results.