Properties Of Argon Research Articles

How well does the Tang-Toennies potential represent the thermodynamic properties of argon?

The potential function suggested by Tang and Toennies is parametrised to represent the thermodynamic properties of argon. The fit of its parameters is carried out on the basis of different data obtained by molecular dynamics simulation and integration of Mayer's f function. Apart from usual optimisation procedures, an efficient strategy based on reduced units is employed. The Tang-Toennies model with five adjustable parameters outperforms the Lennard-Jones model with only two parameters by far for the second virial coefficient. For other properties, the agreement with reference data is only slightly better. The critical point is still overestimated significantly. Since for pair potentials even a substantial increase of the number of parameters does not yield satisfying results, the only remedy is presumably the consideration of three-body interactions.

Fate of subducted argon in the deep mantle

The physical properties of argon (Ar) are investigated to 382 GPa and 3000 K using diamond anvil cell experiments and first-principles molecular dynamics. The estimated density of Ar is smaller that of the Preliminary reference Earth model (PREM) mantle, which indicates that the density crossover does not occur at the bottom of the lower mantle. A large volume dependence of the thermal pressure of Ar is revealed at pressures higher than 200 GPa, and a significant temperature dependence of the calculated effective Grüneisen parameters is confirmed at high pressures. A melting temperature of Ar is estimated from the calculation data and a significant pressure dependence is confirmed. If the pressure-temperature path of the subducted slab is lower than the critical condition, ~750 K and ~7.5 GPa, solid Ar can be carried down into the deep mantle. Melting of solid Ar in the upwelling mantle plume occurs at the bottom of the transition zone. Thus, solid Ar plays an important role in Ar recycling in the Earth’s interior.

Efficient Generator of Low-Temperature Argon Plasma with an Expanding Channel of the Output

To study the thermophysical, electrophysical and optical properties of argon, as well as the implementation of various plasma-chemical reactions, a direct-current generator of a high-enthalpy argon plasma jet with a self-adjusting arc length and an expanding channel of the output electrode has been developed. A comparative analysis of the electrophysical characteristics (current–voltage characteristics—CVC, efficiency) in the expanding and cylindrical channels of constant cross section was carried out. Electrical, calorimetric and spectral studies have shown that the created generator of low-temperature plasma provides the formation of a slightly divergent plasma jet of argon with a diameter of 5–8 × 10–3 m and enthalpy of 5–10 MJ/kg and a mass-average temperature at the outlet of the gas-discharge channel of 5–12 × 103 K with an electron concentration in the axial plasma of 1017 cm–3, the total electric power of the arc discharge 2–10 kW and the plasma-forming gas consumption rate of 1.5–3 × 10–3 kg/s. Depending on the initial conditions at a distance of 0–3 × 10–2 m from the nozzle section of the low-temperature plasma generator, the plasma flow velocity varies from 990 to 300 m/s.

The equation of state of symmetric extended Lennard-Jones fluids

This paper developed the conceptual model of symmetric extended Lennard-Jones fluid based on an extended Lennard-Jones potential function. Therein, important relationships between averaged intermolecular potential field (AIPF) and thermodynamic properties were presented, in which the equation of state was included. These formulascan not only predict the p-V-T properties of argon, nitrogen, carbon monoxide and methane that are representatives of spherical, polar and non-polar molecular fluids in acceptable accuracy, but also physically interpret the p-V-T properties from the AIPF. Obtained results have theoretical significance and can serve as reference or base in the analysis and computation of the p-V-T properties of simple fluids.

On Failures of van der Waals’ Equation at the Gas–Liquid Critical Point

This comment is in response to a recent “new comment” by Umirzakov on the article “Gibbs density surface of fluid argon: revised critical parameters.” It was incorrectly asserted that van der Waals equation “proves” the existence of a scaling singularity with a divergent isochoric heat capacity (Cv). Van der Waals’ equation, however, is inconsistent with the universal scaling singularity concept; it erroneously predicts, for instance, that Cv is a constant for all fluid states. Van der Waals hypothetical singular critical point is based upon a common misconception that van der Waals equation represents physical reality of fluids. A comparison with experimental properties of argon shows that state functions of van der Waals’ equation fail to describe the thermodynamic properties of low-temperature gases, liquids and of gas–liquid coexistence. The conclusion that there is no “critical point” singularity on Gibbs density surface remains scientifically sound.

Recommended gas transport properties of argon at low density using ab initio potential

The most important transport properties of argon have been calculated using classical kinetic theory expressions in conjunction with high-quality ab initio potential energy values computed by Patkowski and Szalewicz. Dilute gas transport properties have been calculated for the viscosity, thermal conductivity, self-diffusion coefficient and thermal diffusion factor from 83 to 10,000 K. Comparisons between experimental transport property data and values presently calculated indicate that the present theoretical predictions may be employed as recommended values for this set of transport properties over a wide temperature range.

Adsorption of argon on graphite and graphite with preadsorbed xenon monolayer: Simulation using the Monte Carlo technique

The Monte Carlo technique in the “Gibbs” ensemble is used to calculate Henry constants of adsorption of argon on graphite and graphite with preadsorbed xenon monolayer with an ideal commensurate \(\left( {\sqrt 3 \times \sqrt 3 } \right)R30^ \circ\) structure. The calculation algorithm is described; it is shown that the obtained results agree well with the literature data. Simulation was carried out in the approximation of additivity of atom-atom potentials in the Lennard-Jones form (6,12) with parameters found from the properties of argon, xenon, and graphite without introducing any corrections. The results of calculations for both adsorption systems point both to translation mobility and vibrations of adsorbed argon atoms normally to the adsorbent surface. The contribution of the vibrational degree of freedom to the thermodynamic characteristics of adsorption of argon can be approximately accounted for within the harmonic oscillator model.

Adsorption properties of argon on Ti doped SBA-15.

Thermodynamic properties of argon on Ti doped Santa barbara amorphous No. 15 (SBA-15) were investigated in the temperature range of 77-89 K to understand the interaction of gas molecules with porous materials. When the total amount of adsorbed molecules is plotted as a function of the equilibrium vapor pressure of the adsorbed Ar, the results exhibit two distinct isotherm steps. The first step appears at the beginning of the isotherm while the second step locates at 0.7 of the normalized pressure. The existence of the second isotherm step which spanned in the normalized pressure from 0.7 to 0.9 is confirmed when the isotherm data were plotted in terms of the 2-dimensional compressibility values. The total amount of adsorbed molecules forming the second isotherm step is 2.5 times greater than the one for the first step. These adsorption behaviors are typical patterns noted from porous materials and far different from the ones observed from non-pore materials. Our observations demonstrate that most of adsorbed molecules reside in the pores and the height of the second isotherm step is strongly associated with filling pores with gas molecules.

A New Model for the Thermal Conductivity of Molten Salts

Anewmodelbasedonroughhard-spheretheoryisproposedforthethermal conductivity of molten salts. The model incorporates a smooth hard-sphere contribu- tion using the properties of argon, as well as characteristic parameters based on the melting point of the molten salt. It is demonstrated that it is possible to correlate the thermal conductivity of monovalent and multivalent molten salts within experimen- tal error using this approach. Furthermore, in salts with a common anion, the single adjustable parameter in the model exhibits regular behavior with the molecular weight ofthesalt.Itisalsoshownthatthethermalconductivityofseveralmolten-saltmixtures can be predicted without any mixture parameters.

Effect of ion irradiation on the properties of carbon nanotube buckypapers

Optical, electrical and structural properties of argon (Ar) ion-irradiated buckypapers of multi-walled carbon nanotube (MWCNT) at various doses prepared by a vacuum filtration method were investigated. It was found that the direct current (DC) conductivity and absorption spectra in the visible range were decreased with an increasing Ar ion irradiation dose. A subsequent heating of nanotube buckypapers at 800 K in a vacuum at each irradiation dose improved the conductivity of buckypapers, whereas optical absorption was unchanged. Moreover, the graphite structure of MWCNTs was transformed to amorphous structure with an increasing Ar ion irradiation dose. The decrease of optical absorption and electrical conductivity of MWCNT buckypaper at room temperature can be ascribed to the increase of defects in the irradiated MWCNTs.

A new approach for long range corrections in molecular dynamics simulation with application to calculation of argon properties

In the application of long range corrections to calculation of potential energy and pressure utilizing molecular dynamics simulation, it is common to use a longer distance of cut-off to obtain more accurate results. The radial distribution function is assumed to be equal to unity in conventional terms for the long range corrections. The main objective of this work is to introduce a methodology for obtaining a correlation for radial distribution function from preliminary simulation runs and then use it in other simulation runs with a smaller ratio of cut-off distance to collision diameter. This, in turn, results in much lower CPU time requirements. The radial distribution function is considered as a convergent oscillating function. A number of simulations were performed in NVT ensemble with the ratio of cut-off distance to collision diameter set to five. Also, two sets of simulations with the ratio of two were performed. The first set used the conventional term of long range corrections and the second used the term presented in this work. With the ratio of two, calculating the pressure of argon in liquid and supercritical fluid states resulted in much lower CPU time and less error. In predicting pressure, applying the new correlation for radial distribution function reduced the CPU time by a factor of twelve. Assuming cut-off distance to be twice the collision diameter also resulted in a decrease of nearly 4% in relative error. In addition, the effect of this modification on argon properties: such as heat capacity, thermal pressure coefficient and isothermal compressibility are investigated using molecular dynamics simulation data in fluctuation formulas.

MFM and gas adsorption isotherm analysis of proton beam irradiated multi-walled carbon nanotubes

To enhance the gas adsorption properties and modify the physical properties of carbon nanotubes, multi-walled carbon nanotubes (MWCNTs) were irradiated by high-energy proton beams, and the physical properties including morphology and local surface structure were investigated by using a transmission electron microscope (TEM), magnetic force microscope (MFM) and a gas adsorption isotherm apparatus which can deeply probe the fine structure of surface. Interestingly, clearer MFM images were obtained from the proton irradiated samples which supports that carbon exhibits magnetism under proton bombardments, although the intrinsic magnetic property is not understood. The layering properties of argon on MWCNTs were measured from 59 to 69 K and the interaction of argon on the surface was analyzed. The calculated values of isosteric heat of adsorption demonstrated that higher interaction of gas molecules with surface is found from the proton irradiated MWCNTs. This result strongly supports that the local surface modification, partial defects, for example, were created due to the external high energy impacts. Our results are worthy to note that gas adsorption technique can provide the fine atomic resolution which beyond the one of TEM and MFM.

A model of the liquid-crystal phase transition and the quasicrystal model of liquid

We compare properties of the crystal structure and of the quasicrystal model of liquid. We show that if the Lennard-Jones potential is used to model the properties of argon, then the temperature of the phase transition between the densely packed face-centered and the body-centered cubic structure is very close to the liquid-crystal transition temperature. Based on this, we propose a model of the phase transition consisting in a change of just the short-range order, which then leads to a change of the long-range order.

The local structure of various hydrogen bonded liquids: Voronoi polyhedra analysis of water, methanol, and HF

The local structure of three hydrogen bonded liquids comprising clusters of markedly different topology: water, methanol, and HF are investigated by analyzing the properties of the Voronoi polyhedra (VP) of the molecules in configurations obtained from Monte Carlo computer simulations. For comparisons, the properties of argon as a nonassociating reference liquid are also calculated. It is found that in liquid HF the fluctuation of the local density is much larger than in the other three liquids, and hence, besides the dense regions, this liquid contains numerous large voids as well. The size of the largest of these voids is found to be commensurate with a cluster of 11 HF molecules, whereas the number of voids as large as one molecule is in the order of the number of molecules. It is found that the number of the hydrogen-bonded neighbors of the HF molecules located at the boundary of such voids does not differ considerably from that of the other molecules, and thus they do not have any specific topological position (e.g., chain terminal) in the hydrogen bonded chains of the molecules. The shape of the VP is found to reflect the arrangement of the nearest neighbors of the molecules. Thus, the VP are considerably less spherical and their topological properties are distributed in a wider range in the associated liquids than in argon. Similarly, in the hydrogen bonded liquids considerably more geometric neighbors are found than the number of molecules belonging to the first coordination shell. On the other hand, in liquid argon, which is a closely packed system, the two values are found to be close to each other. The inhomogeneities of the local density of liquid HF are also reflected in the shape of the VP. The molecules located at the boundary of the large voids are found to have more spherical VP with more and larger faces than the other ones.

A new pair potential for liquid argon

Recently, the well-known point molecular attractive pair term, r-6 only asymptotically correct, has been modified (Lu J. X. and Marlow W. Ç. (1995) Phys. Rev. Lett. 74. 1724) by taking into account the effect of a molecular size through a parameter a characteristic of the atom. Our interest is to test this specific feature on the properties of argon in the liquid state. A new pair potential has been generated by adding to the short-range repulsion, modelled by an r-12 term. the attractive term proposed by Lu and Marlow. Parameters have been fitted on the excess internal energy for argon over a wide range of densities and temperatures. In order to achieve this test, the hybridized-mean spherical approximation (HMSA) integral equation has been used. The HMSA has proven to be very accurate to reproduce the structure and the thermodynamics self-consistently. Structural and thermodynamic results with the new pair potential are in very good agreement with the experiments as well as simulation data.

Theoretical radiative emission results for argon/copper thermal plasmas

Theoretically determined radiative properties for argon and argon/copper plasmas at one atmosphere, in the temperature range from 3000 to 25000 K, are presented. The spectrum from 0.03 to 25 μm is covered. The spectral emission coefficient, integrated emission coefficient, and net emission coefficient are the properties considered. To determine the limits of the optically thin regime, the intensity for a line-of-sight in an isothermal, homogeneous plasma versus pathlength is compared to the intensity calculated with the optically thin assumption. To obtain information on the dominating emission mechanisms, the fractional contributions of the bound bound (line), free-bound, and free-free emission are shown. To obtain information on the dominating species, plots of the fractional contributions of each of the atom and ion types is presented. Because it is of interest to know how much of the total radiation is in the vacuum ultraviolet wavelength region, fractional plots of this are presented too.

Computer simulation of enclosed inductively coupled plasma discharges. Part 2. Molecular gases

An enclosed inductively coupled plasma (EICP) discharge is simulated with argon and neon, and the thermodynamic and fluid flow fields are calculated for a sealed discharge. The influence of discharge container geometry and generator power and frequency are examined. Similarities between the model discharges reflect the thermodynamic and transport properties of argon and neon. Neon model discharges are found to be more stable than their argon counterparts. They exhibit higher maximum temperatures and lower wall temperatures than argon at similar operating frequencies. The velocities in the neon discharge simulation are higher than in an equivalent argon discharge.

An effective triplet potential for argon

Abstract Miyano Y., 1994. An effective triplet potential for argon. Fluid Phase Equilibria , 95: 31-41. An effective triplet potential is proposed. This triplet potential expresses the effects of Axilrod-Teller three-body interactions on a pair of molecules, and is given as a simple function of density and the distance between the pair of molecules. The computer time for Monte Carlo simulations of fluids with both a pair potential and the effective triplet potential is almost equivalent to that of fluids with only the pair potential. By using an accurate pair potential together with this effective triplet potential, Monte Carlo simulations are performed to estimate the thermodynamic properties of argon, and the results are compared with the experimental properties, the saturated vapor and liquid densities, the heats of varporization and the second virial coefficients. The comparisons show that the simulation results for fluids with only the pair potential give poor agreement with the experimental data, especially for the heats of vaporization where the simulation results give very large deviations. However, the simulation results for fluids with both pair potential and triplet potential give very good agreements with the experimental data of argon.

Predicting transport properties without adjustable parameters: A test application of the Hulburt-Hirschfelder potential to argon

Abstract Accurate estimates of the transport properties of gaseous systems under conditions where experimental transport data are sparse of unavailable are important in a number of applications. The Hulburt-Hirschfelder (HH) potential for monatomic gas interactions, which is determined entirely by spectroscopic constants of diatomic molecules, provides a basis for calculating transport properties without adjustable parameters. In this paper we report test calculations of the viscosity, thermal conductivity and self-diffusion coefficients for argon. Comparison with the comprehensive correlation of thermophysical properties for argon by Kestin and co-workers shows very reasonable agreement for the transport properties at moderate and high temperatures. Deviations at lower temperatures may be attributed to inaccuracies in the long-range part of the HH potential, whereas the core and well of the potential appear to be adequately represented. These results strongly support the use of the HH potential for estimating the transport properties of monatomic gases at high t

Magnetic and thermal properties of adsorbed binary systems ofO2-Ar andO2-N2

We report a high-resolution study of the magnetic susceptibility and heat capacity of submonolayer films of oxygen-argon and oxygen-nitrogen mixtures on Grafoil from 3 to 70 K. These systems behave quite differently, in consequence of the different registry properties of argon and nitrogen on graphite.