Argon Evaporation Research Articles

Formation of bismuth nanoparticles on nanoporous substrates

Substrates with a layer of anodized aluminum oxide are obtained by one-stage and two-stage anodization. The samples had different porosity in volume and on the surface. Bismuth nanoparticles were obtained by thermal evaporation in argon by condensation onto substrates with a layer of anodized aluminum oxide. The distribution of sizes, shapes, and numbers of nano- and microparticles was studied using images obtained with a scanning electron microscope. The largest number of nanoparticles (21%) on the sample with a surface layer of aluminum oxide without pores had a diameter of 70 nm. It was assumed that the presence of pores on the surface affected the migration of deposited atoms and particles of bismuth melt until stable condensation centers were formed. The presence of pores with a diameter of 20–100 nm led to a decrease in the diameter of the most common bismuth nanoparticles from 80 to 40 nm. Nanoparticles with a diameter of 90 nm were predominant (25%) in the sample with pores with a diameter of 60–220 nm. The largest number of spherical crystallites on all substrates had a diameter of 110 nm. It was established that a uniform distribution of particles was obtained on a sample, the surface of which was not subjected to chemical polishing.

Molecular dynamics investigation of argon evaporation on copper surfaces covered with graphene and carbon nanotubes

The recent advancements in nanoscience have led to a significant improvement in surface characteristics, which play a critical role in heat transfer. In this study, the heat transfer of modified copper-based surfaces covered with graphene sheets and carbon nanotubes (CNTs) is investigated using molecular dynamics method, and compared with the heat transfer of a simple copper surface. To modify the surfaces, various numbers of carbon nanotubes with different heights are used. Investigations are conducted on the dynamics of liquid argon clusters, the distribution of argon density, temperature changes, and the phase transition from liquid to the gas. In comparison to the pre-defined reference surface, heat transfer shows an improvement for the copper-based surface coated with graphene platelets. Similarly, simulation results indicate that the increase in both the quantity and height of carbon nanotubes leads to the enhancement of the heat transfer of the surface. Additionally, it is found that, when CNTs are inserted on a surface, a shorter time is required for liquid argon to reach the temperature of solid surface.

Structure and Properties of NbMoCrTiAl High-Entropy Alloy Coatings Formed by Plasma-Assisted Vacuum Arc Deposition

The paper analyzes the structure and properties of metal, cermet, and ceramic NbMoCrTiAl high-entropy alloy (HEA) coatings formed on solid substrates by plasma-assisted vacuum arc deposition (from multicomponent gas-metal plasma through Nb, Mo, Cr, and TiAl cathode evaporation in argon and/or a mixture of argon and nitrogen). The analysis shows that all coatings represent a nanocrystalline (3–5 nm) multilayer film. The metal coating has a bcc lattice (a = 0.3146 nm). The ceramic coating has an fcc lattice (an uncertain lattice parameter due to highly smeared diffraction peaks). The coating hardness increases in the order of metal, cermet, and then ceramic, reaching 43 GPa at Young’s modulus equal to 326 GPa. When heated in air, the metal and cermet coatings start to oxidize at 630–640 °C, and the ceramic coating at 770–780 °C.

MONITORING COMPOSITION AND STRUCTURE OF MOCVD ZrO2-BASED MULTICOMPONENT FILMS BY INNOVATIVE MIXED METAL-ORGANIC PRECURSORS

Three volatile mixed-metal precursors [ZrL4Pb(hfa)2] (1), [ZrL4PbL2] (2), and [ZrL4La(dpm)3] (3) (L = 2-methoxy-2,6,6-trimethyl-3,5-heptanedionate; dpm = 2,2,6,6-tetramethyl-3,5-heptanedionate; hfa = 1,1,1,5,5,5-hexafluore-2,4-pentanedionate) are used to prepare ZrO2-based multicomponent films by metalorganic chemical vapor deposition (MOCVD). The deposition experiments are carried out in a hot-wall reactor at 600-750 °C on silicon substrates under 20 Torr in the presence of oxygen. According to X-ray powder diffraction, the main crystal phases in the films prepared from precursors 1 and 2 are solid solutions based on tetragonal and cubic ZrO2. Lead does not form separate crystal phases but is dissolved in the oxide form within the ZrO2 matrix, as is indicated by X-ray photoelectron spectroscopy data. La2Zr2O7 films are prepared from 3 using two ways of precursor supply: evaporation in argon and by direct liquid injection (DLI). It is shown that the composition and structure of obtained films are determined by the precursor composition. The results obtained for thermal behavior of precursors in condensed and gas phases are discussed.

Thermal transport during thin-film argon evaporation over nanostructured platinum surface: A molecular dynamics study

Investigation of thermal transport characteristics of thin-film liquid evaporation over nanostructured surface has been conducted using molecular dynamics simulation with particular importance on the effects of the nanostructure configuration for different wall–fluid interaction strengths. The nanostructured surface considered herein comprises wall-through rectangular nanoposts placed over a flat wall. Both the substrate and the nanostructure are of platinum while argon is used as the evaporating liquid. Two different wall–fluid interaction strengths have been considered that essentially emulate both hydrophilic and hydrophobic wetting conditions for three different nanostructure configurations. The argon–platinum molecular system is first equilibrated at 90 K and then followed by a sudden increase in the wall temperature at 130 K that induces evaporation of argon laid over it. Comparative effectiveness of heat and mass transfer for different surface wetting conditions has been studied by calculating the wall heat flux and evaporative mass flux. The results obtained in this study show that heat transfer occurs more easily in cases of nanostructured surfaces than in case of flat surface. Difference in behavior of argon molecules during and after the evaporation process, that is, wall adsorption characteristics, has been found to depend on the surface wetting condition as well as on presence and configuration of nanostructure. A thermodynamic approach of energy balance shows reasonable agreement with the present molecular dynamics study.

Atomistic modelling of thin film argon evaporation over different solid surfaces at different wetting conditions

In the present study, non-equilibrium molecular dynamics (MD) simulations have been performed to reveal the effect of solid–liquid interfacial wettability on the evaporation characteristics of thin liquid argon film placed over the flat solid surface. The atomistic model considered herein comprises of a three-phase simulation domain having a solid wall over which liquid argon and argon vapour co-exist. Initially, the system is thermally equilibrated at 90 K for a while after which rapid increase in the solid wall temperature induces a phase change process, i.e. evaporation. Both hydrophilic and hydrophobic wetting conditions of the solid surface have been considered at an evaporation temperature of 130 K for three different surface materials such as platinum, silver, and aluminium. The simulation results show that both the surface wettability and surface material have a significant role in phase transition phenomena of thin liquid film, particularly the surface wettability for the present system configuration. The thermal transport phenomena between the wall and liquid thin film have been studied thoroughly and discussed in terms of wall heat flux, evaporative mass flux, upper bound of maximum possible heat flux etc. The results obtained in the present MD simulation study are compared with the macroscopic predictions based on classical thermodynamics. Interestingly, a very good agreement has been found indicating that macroscopic thermodynamics approach can predict the characteristic of phase change phenomena of nanoscale thin liquid film.

Removal of Lead from Fe–Pb Mixtures by Evaporation in Stagnant Argon at 1 448 K and the Determination of the Lead Vapor-Argon Interdiffusivity

Lead, considered as a tramp element in steel, was successfully removed from a 50 % Fe-50 % Pb mixture by evaporation in argon at 1 448 K. The progress of the removal shows that it takes 5 h to completely remove the lead from the mixture, the nonvolatile iron (solid) being left behind in the crucible (alumina). Results point toward a diffusion-controlled evaporation, and the system, which closely simulates Stefan's tube for studying evaporation of a liquid through a stagnant gas film, allowed the measurement of the interdiffusivity of lead and argon (D Pb-Ar ). Since this study dealt with a pseudosteady state diffusion, rather than the steady state one for which the diffusion flux equation is well established, the required pseudosteady state equation was first developed, and, subsequently, utilized with the help of the rate data to find the D Pb-Ar at 1 448 K. The measured value, which is 1.766(+0.378) cm 2 /s, compares very well with the estimated value of 1.774 cm 2 /s.

Gas-phase vibrational spectroscopy and ab initio calculations of Rb +(H 2O) n and Rb +(H 2O) nAr cluster ions

The competition between ion–water electrostatic interactions and water–water hydrogen bonding in cluster ions depends on several factors, including charge density of the ion and temperature of the system. Infrared photodissociation spectra of Rb +(H 2O) n =2–5 and Rb +(H 2O) n =1–5Ar are presented here and compared to previous experiments involving potassium and cesium. The temperature, or internal energy, of hydrated rubidium cluster ions is controlled by varying the evaporative path available for cluster formation. Warmer clusters (with effective temperatures of ∼250–500 K) are formed by the evaporation of water, while colder clusters (∼40–120 K) can be formed by argon evaporation. Colder cluster ions tend to favor conformers with more hydrogen bonds compared to those cluster ions at warmer temperatures. Previous work from this laboratory has shown significant and dramatic differences between the spectra of hydrated potassium and cesium ions. With a charge density intermediate between that of K + and Cs +, Rb + plays an important role in bridging the gap in our previous studies.

A grand canonical Monte Carlo study of capillary condensation in mesoporous media: effect of the pore morphology and topology.

We study by means of Grand Canonical Monte Carlo simulations the condensation and evaporation of argon at 77 K in nanoporous silica media of different morphology or topology. For each porous material, our results are compared with data obtained for regular cylindrical pores. We show that both the filling and emptying mechanisms are significantly affected by the presence of a constriction. The simulation data for a constricted pore closed at one end reproduces the asymmetrical shape of the hysteresis loop that is observed for many real disordered porous materials. The adsorption process is a quasicontinuous mechanism that corresponds to the filling of the different parts of the porous material, cavity, and constriction. In contrast, the desorption branch for this pore closed at one end is brutal because the evaporation of Ar atoms confined in the largest cavity is triggered by the evaporation of the fluid confined in the constriction (which isolates the cavity from the gas reservoir). This evaporation process conforms to the classical picture of "pore blocking effect" proposed by Everett many years ago. We also simulate Ar adsorption in a disordered porous medium, which mimics a Vycor mesoporous silica glass. The adsorption isotherm for this disordered porous material having both topological and morphological defects presents the same features as that for the constricted pore (quasicontinuous adsorption and steep desorption process). However, the larger degree of disorder of the Vycor surface enhances these main characteristics. Finally, we show that the effect of the disorder, topological and/or morphological, leads to a significant lowering of the capillary condensation pressure compared to that for regular cylindrical nanopores. Also, our results suggest that confined fluids isolated from the bulk reservoir evaporate at a pressure driven by the smallest size of the pore.

Evaporatively cooled M+ (H2O)Ar cluster ions: infrared spectroscopy and internal energy simulations.

Rotationally resolved IR spectra of M+ (H2O)Ar cluster ions for M=Na, K, and Cs in the O-H stretch region were measured in a triple-quadrupole mass spectrometer. Analysis of the spectra yields O-H stretch vibrational band origins and relative IR intensities of the symmetric and asymmetric modes. The effect of the alkali-metal ions on these modes results in frequency shifts and intensity changes from the gas phase values of water. The A-rotational constants are also obtained from the rotational structure and are discussed. Experimentally, the temperatures of these species were deduced from the relative populations of the K-rotational states. The internal energies and temperatures of the cluster ions for Na and K were simulated using RRKM calculations and the evaporative ensemble formalism. With binding energies and vibrational frequencies obtained from ab initio calculations, the average predicted temperatures are qualitatively consistent with the experimental values and demonstrate the additional cooling resulting from argon evaporation.

Femtosecond photodissociation dynamics of van der Waals cationic clusters: a tool for detecting metastable isomers of organic cations

A femtosecond pump (266 nm)–probe (800 nm) experiment as been performed on small clusters of tetrakis(dimethylamino)ethylene (TDMAE) with argon (TDMAE(Ar) ≪10). The pump excites TDMAE and initiates deformations of the molecule within the cluster. Then, the probe ionises TDMAE and disposes enough excess energy in the cluster to evaporate all argon atoms. However, ionisation during the first 150 fs after the pump gives preferentially access to a metastable configuration of the TDMAE + ion, unquenched by the argon environment. This takes up enough of the excess energy to prevent complete argon evaporation. Observing the non-total-evaporation of weakly bond clusters formed of an organic cation with argon appears as a useful tool to reveal the existence of such metastable configurations in organic cations.

Controlled Deposition, Soft Landing, and Glass Formation in Nanocluster-Surface Collisions

Molecular dynamics simulations have been used to investigate the dynamics and redistribution of energy during the impact of a nanocrystal with adsorbed liquid films. Although impact of a 32-molecule NaCl cluster on a solid surface at 3 kilometers per second leads to melting, disordering, fragmentation, and rebounding, the same size cluster colliding with a liquid neon film transfers its energy efficiently to the liquid for a controlled soft landing. Impact on a higher density film (argon) leads to rapid attenuation of the cluster velocity, accompanied by fast heating. Subsequent disordering, melting, and fast cooling by evaporation of argon quench the cluster to a glassy state. These results suggest a method for the controlled growth of nanophase materials.

Size-selective Raman spectroscopy of carbazole–(Ar)n clusters at sub-wave-number resolution

Size-selective Raman spectra of carbazole–(Ar)n clusters (n=0–14) obtained between about 1305 and 1325 cm−1 by mass-selective ionization-loss stimulated Raman spectroscopy are reported. The spectra are interpreted with respect to vibrational energy flow and argon evaporation, and are compared with vibronic spectroscopic results on the species.

Crystal structure and habit of silicon and germanium particles grown in argon gas

Silicon and germanium particles prepared by evaporation in argon at low pressure have been studied by electron microscopy. The particles are crystalline and show frequently distinct crystal habits. The typical habit of silicon particles is a {311} icositetrahedron truncated by eight {111} faces, and that of germanium particles is a {311} icositetrahedron. For germanium, multiply-twinned particles have also been found when they were prepared in a clean atmosphere. The crystallites with these habits have the ordinary diamond structure. Germanium particles with a new structure (tetragonal system, a=0.537 nm and c=0.904 nm) are also found besides those with the ordinary diamond structure. The particles with the new structure are not formed when evaporation is carried out in an atmosphere containing a small amount of air.

Selective absorption of solar energy in ultrafine chromium particles

Ultrafine chromium particles prepared by evaporation in argon + air are found to be highly absorbing over the solar spectrum and highly transparent in the infrared. Such spectral selectivity is the distinctive feature of a coating in an efficient photothermal converter for solar energy. Optical transmittance through coatings with mass density ≲0.5 g/m2 agrees well with calculations based on the Maxwell-Garnett theory, whereas coatings with larger mass density exhibit a transmittance too low to be directly reconciled with this theory.

Electron Microscope and Electron Diffraction Study of Fine Particles Prepared by Evaporation in Argon at Low Pressure. IV. Fine Particles of Tellurium

Tellurium particles produced by evaporation in argon at a pressure of several Torr are observed to occur with two distinct shapes. They are either spherical or egg-shaped or else they are rod-shaped. The rods are hexagonal in cross section, bounded by {10.0} planes and have their axes parallel to the [00.1] direction. Both kinds of particle are single crystals with the normal hexagonal structure of α-tellurium. They nucleate and grow in different zones of the tellurium smoke. The fact that particles grown in different zones have different shapes is explained in terms of the temperature dependence of the spatial anisotropy of the γ-plot where γ is the specific surface free energy.

Crystal habit and crystal structure of fine chromium particles: An electron microscope and electron diffraction study of fine metallic particles prepared by evaporation in argon at low pressures (III)

Fine Cr particles formed by evaporation in argon at low pressures were found to have well-defined crystal habits corresponding to the following solids: (i) the icositetrahedron bounded by 24 {211} planes; (ii) the rhombic dodecahedron bounded by 12 {110} planes; and (iii) the cube or the rectangular parallelepiped bounded by six {100} planes. The first two particle shapes were consistent with δ-Cr and the third with ordinary b.c.c. Cr. Re-examination of the crystal structure of δ-Cr by means of fine single crystals suggested that it was consistent with the A-15 type structure proposed by Forssell and Persson ( J. Phys. Soc. Japan, 29 (1970) 1532). A disordered structure derivable from the A-15 type, reported previously by the present authors, is therefore withdrawn.

Structure of chromium-rich Cr-Ni, Cr-Fe, Cr-Co, and Cr-Ni-Fe alloy particles made by evaporation in argon

Structural analyses were performed on alloy particles of chromium-rich Cr-Ni, Cr-Fe, Cr-Co, and Cr-Ni-Fe systems. Fine alloy particles (100 to 1000A in diameter) were prepared by evaporation of parent alloys in argon at 20 torr. In addition, alloy structures of bulk specimens of the Cr-Ni system were investigated using X-ray diffraction techniques to confirm the results obtained from the particulate alloys. In these binary systems, δ phase with W3O structure (A-15) and a phase withβ-uranium structure (D 8 ) were identified in addition to the α (bcc) and γ (fcc) terminal solid solutions. The compositional ranges for the σ phase in the Cr-Ni, Cr-Fe, and Cr-Co systems are from low chromium to 68, 63.4, and 62.1 wt pct Cr, respectively. The δ phase exists in the range from pure chromium to 68, 58, and 54 wt pct Cr in the respective Cr-Ni, Cr-Fe, and Cr-Co alloy systems. Similarly, in the Cr-Ni-Fe system, it was found that δ phase occurs in the chromium corner while σ phase exists in the region bridging the two binary σ phases of the Cr-Ni and Cr-Fe systems. Possible modification of phase diagrams of these systems is discussed in view of these results.

An Electron Microscope and Electron Diffraction Study of Fine Manganese Particles Prepared by Evaporation in Argon at Low Pressures

Fine manganese particles prepared by evaporation in argon, or in xenon, at low pressures were studied mainly by electron microscopy and electron diffraction. The particles showed clear-cut crystal habits and they were classified into three types; firstly, particles of the shape of a tristetrahedron bounded by twelve {211}-planes, the crystal structure of which was α-man-ganese; secondly, particles of the shape of a rhombic dodecahedron bounded by twelve {110}-planes, the crystal structure of which was β-manganese, thirdly particles of the shape of a needle-like rod, the crystal structure of which is still unknown. Extra spots {00 l }, (\(l{\backslash\llap{=}}4n\)), observed in the selected area diffraction pattern from a β-manganese particle are explained by dynamical double reflections.