Query: argon in graphite

title: Pore structure evolution of IG 110 graphite during argon ion irradiation at 600 degC
abstract: Irradiation induced pore structure evolution of graphite is especially important for its applications in molten salt reactors. If irradiation expands the pores, more salt will intrude into graphite components, which is undesired. In this study, multi energy argon ions were used instead of neutrons to introduce displacement damage in IG 110 graphite at 600 degC, in order to investigate pore structure evolution of graphite during irradiation. About 71% of the pores shrank after irradiation to 21 dpa, which was proved to be a result of c axis swelling of graphite crystallites. In contrast, only 12% of the pores expanded. The total area of gas escape pores gradually decreased to 89% at 21 dpa. At the turnaround dose and beyond, extensively distributed submicron pores were observed, indicating an increase in newborn porosity. Combining the evolution of original pores and newborn pores, a model depicting the overall porosity's evolution during irradiation is given in this study. Because the original porosity decreases continuously with dose and newborn pores are small in sizes, this study strongly suggests that irradiation will not facilitate salt intrusion into graphite.

title: On the evolution of the heat spike in the isosteric heat versus loading for argon adsorption on graphite A new molecular model for graphite reconciliation between experiment and computer simulation
abstract: We have carried out an extensive computer simulation of argon adsorption on graphite at temperatures in the range 40 K 100 K, using a new molecular model for graphite, and compared our simulation results with new high resolution experimental data. The new model accounts for: (1) The energetic corrugation of the graphene surface. (2) The smaller collision diameter of the carbon atoms (0.28 nm) in the outermost graphene layer compared to 0.34 nm in the lower layers. (3) The increase in the interaction energy well depth between argon and the carbon atoms of the outermost layer. (4) The closer spacing between the first and second layers compared to that between the inner layers. The simulated adsorption isotherms and isosteric heats give an improved description of the experimental data, especially in capturing the low temperature transition from a 2D liquid to a solid like adsorbate and subsequently to an incommensurate solid, and of the variation in these properties with respect to temperature. Analysis of the isosteric heat versus loading reveals details of the rearrangement of the adsorbed molecules during the layer transition.

title: Interaction of the Helium, Hydrogen, Air, Argon, and Nitrogen Bubbles with Graphite Surface in Water.
abstract: The interaction of the confined gas with solid surface immersed in water is a common theme of many important fields such as self cleaning surface, gas storage, and sensing. For that reason, we investigated the gas graphite interaction in the water medium. The graphite surface was prepared by mechanical exfoliation of highly oriented pyrolytic graphite (HOPG) The surface chemistry and morphology were studied by X ray photoelectron spectroscopy, profilometry, and atomic force microscopy. The surface energy of HOPG was estimated by contact angle measurements using the Owens Wendt method. The interaction of gases (Ar, He, H2, N2, and air) with graphite was studied by a captive bubble method, in which the gas bubble was in contact with the exfoliated graphite surface in water media. The experimental data were corroborated by molecular dynamics simulations and density functional theory calculations. The surface energy of HOPG equaled to 52.8 mJ/m2 and more of 95% of the surface energy was attributed to dispersion interactions. The results on gas surface interaction indicated that HOPG surface had gasphilic behavior for helium and hydrogen, while gasphobic behavior for argon and nitrogen. The results showed that the variation of the gas contact angle was related to the balance between the gas surface and gas gas interaction potentials. For helium and hydrogen the gas surface interaction was particularly high compared to gas gas interaction and this promoted the favorable interaction with graphite surface.

title: Heat Transfer During Evaporation of Cesium From Graphite Surface in an Argon Environment
abstract: The article focuses on discussion of problem of graphite radioactive waste formation and accumulation. It is shown that irradiated nuclear graphite being inalienable part of uranium graphite reactor may contain fission and activation products. Much attention is given to the process of formation of radioactive cesium on the graphite element surface. It is described a process of plasma decontamination of irradiated graphite in inert argon atmosphere. Quasi one mathematical model is offered, it describes heat transfer process in graphite cesium argon system. Article shows results of calculation of temperature field inside the unit cell. Authors determined the factors which influence on temperature change.

title: Stopping of energetic argon cluster ions in graphite: Role of cluster momentum and charge
abstract: We show that the implantation depth for argon clusters in graphite scales linearly with cluster momentum. A plot of implantation depth versus the momentum scaled with the projected surface area of the cluster falls on the same universal plot as that shown for semiconductor and metallic clusters, thus providing a universal scaling law for cluster implantation. Molecular dynamics simulations provide some insight to the mechanisms behind the empirical observation.

title: Polyyne formation by graphite laser ablation in argon and propane mixed gases
abstract: Laser ablation of graphite in argon/propane mixed gases was performed to elucidate the process of carbon cluster growth and hydrogen termination to form polyynes. The carbon rich materials ejected by ablation were carried by the gas flow and captured by a cooled organic solvent, and then, the soluble products were analyzed by UV absorption. The yield of polyyne increased drastically with the increase of the propane ratios. The propane ratio dependent yields for polyynes of various lengths are consistent with a polyyne formation model based on H abstraction reactions of carbon clusters.

title: Molecular dynamics modeling of thermal resistance at argon graphite and argon silver interfaces
abstract: Heat conduction between two parallel solid walls separated by liquid argon is investigated using three dimensional molecular dynamics (MD) simulations. Liquid argon molecules confined in silver and graphite nano channels are examined separately. Heat flux and temperature distribution within the nano channels are calculated by maintaining a fixed temperature difference between the two solid surfaces. Temperature profiles are linear sufficiently away from the walls, and heat transfer in liquid argon obeys the Fourier law. Temperature jump due to the interface thermal resistance (i.e. Kapitza length) is characterized as a function of the wall temperature. MD results enabled development of a phenomenological model for the Kapitza length, which is utilized as the coefficient of a Navier type temperature jump boundary condition using continuum heat conduction equation. Analytical solution of this model results in successful predictions of temperature distribution in liquid argon confined in silver and graphite nano channels as thin as 7 nm and 3.57 nm, respectively.

title: Simulation study of two dimensional phase transitions of argon on graphite surface and in slit micropores
abstract: Molecular simulation has been increasingly used in the analysis and modeling of gas adsorption on open surfaces and in porous materials because greater insight could be gained from such a study. In case of homogeneous surfaces or pore walls the adsorption behavior is often complicated by the order disorder transition. It is shown in our previous publications (Ustinov and Do, Langmuir 28:9543 9553, 2012a; Ustinov and Do, Adsorption 19:291 304, 2013) that once an ordered molecular layer has been formed on the surface, the lattice constant depends on the simulation box size, which requires adjusting the box dimensions parallel to the surface for each value of loading. It was shown that this can be accomplished with the Gibbs Duhem equation, which results in decreasing lattice constant with an increase of the amount adsorbed. The same feature is expected to be valid for gas adsorption in narrow pores, but this has not been analyzed in the literature. This study aims at an extension of our approach to adsorption in slit graphitic pores using kinetic Monte Carlo method (Ustinov and Do, J Colloid Interface Sci 366:216 223, 2012b) The emphasis rests on the thermodynamic analysis of the two dimensional (2D) ordering transition and state of the ordered phase; if the ordered phase exists in narrow slit pores, simulation with constant volume box always leads to erroneous results, for example, seemingly incompressible adsorbed phase. We proposed a new approach that allows for modeling thermodynamically consistent adsorption isotherms, which can be used as a basis for further refinement of the pore size distribution analysis of nanoporous materials.

title: On the identification of the sharp spike in the heat curve for argon, nitrogen, and methane adsorption on graphite: Reconciliation between computer simulation and experiments
abstract: Experimental data for the adsorption of argon, nitrogen, and methane on a planar graphite surface under subcritical conditions exhibit a very sharp peak in the isosteric heat curve at loadings close to the monolayer concentration. The magnitude of this peak is much greater than the value expected from the extrapolation of the isosteric heat in the submonolayer region to the monolayer loading. This sharp and large peak has been interpreted as the transition of the first layer from a hypercritical fluid phase to a solid phase. Here, we argue that volumetric or calorimetric experiments are in fact carried out in a canonical system, where a dose of known mass of adsorptive is introduced into the system, rather than in an open system exposed to an infinite supply reservoir as assumed in GCMC simulations. We have carried out canonical simulations with the new Mu CMC scheme proposed recently by Fan et al. (, 115 (35) 10509 10517) and have reproduced the sharp peak in the heat curve. We show that this sharp spike is observed in a canonical ensemble (and in some cases grand canonical ensemble) whether the surface is unstructured or structured and no matter whether nitrogen is modeled as a 1CLJ or 2CLJ+3q or methane is treated as a 1CLJ or 5CLJ+5q model, suggesting that the heat spike is an intrinsic characteristic of the gas solid pair.

title: Thermodynamic analysis of ordered and disordered monolayer of argon adsorption on graphite.
abstract: We presented a detailed thermodynamic analysis of argon adsorption on a graphitized carbon black with a kinetic Monte Carlo scheme. In this study, we particularly paid attention to the formation of a hexagonal two dimensional molecular layer on a graphite surface and discuss conditions of its stability and thermodynamic properties of the adsorbed phase as a function of loading. It is found that the simulation results are substantially affected by the dimensions of the simulation box when the monolayer forms a hexagonal ordered structure. This is due to the fact that the lattice constant is constrained by the dimensions of the surface. To circumvent this, we presented a thermodynamic technique, which allows for the variation of the box size as a function of loading, to determine the "intrinsic" lattice constant (rather than apparent average value because of the fixed dimensions of the simulation box) and the thermodynamic functions for the adsorbed phase: the Helmholtz free energy, the chemical potential, and the surface tension. The tangential and normal pressures as a function of the distance from the surface are also discussed.