Preferential Evaporation Research Articles

Effect of evaporation on surface morphology of epitaxial ZnO films during postdeposition annealing

We investigated the effect of evaporation on the surface morphology of c-oriented epitaxial ZnO (40 nm thick)/Al 2O 3(0 0 0 1) films during postdeposition annealing using real time synchrotron X-ray scattering and atomic force microscopy (AFM). We find that evaporation as well as grain coalescence play crucial roles on the surface morphology of the ZnO/Al 2O 3(0 0 0 1) films. Grain growth occurring in initial stage of annealing forms facets with higher surface energies than the (0 0 0 1) planes. By the preferential evaporation of the prism planes, the surface morphology of the ZnO film eventually evolves into 2D flat (0 0 0 1) surface at 800 °C, as confirmed by AFM. The real time measurement of the film thickness during annealing also supports the effect of the evaporation on the morphology. The evaporation rate is high in initial stage by the preferential evaporation from high energy facets but slows down after transition to the flat (0 0 0 1) surface.

Influence of Surfactant and Humidity on Sol-Gel Macroporous Organosilicate Coatings

In the preparation of macroporous hydrophobic organosilicate films using methyltriethoxysilane (MTES) as precursor, the effects of surfactant addition, surfactant properties and atmospheric humidity were explored. As films dried, preferential evaporation of the ethanol resulted in an increase of the relative water content. This led to development of phase separation between the hydrophobic gel and the aqueous liquid and ultimately the formation of macropores. In the presence of surfactant, surfactant adsorption at the aqueous phase/gel interface affected the extent of phase separation therefore the resulting pores. Span 20 surfactant (HLB = 8.6) has lower compatibility with the aqueous phase than Tween 20 (HLB = 16.7) and effectively increases the hydrophobicity of the gel phase leading to the formation of larger pores. An increase in Span 20 content from 2 wt.% to 5 wt.% also increased pore size. Film porosity also increased significantly with humidity inside the coating chamber. It would appear that the increased porosity is a result of increased phase separation caused by reduced water evaporation at the higher humidity. Highly macroporous (up to ∼80% porosity), reproducible and uniform films were obtained by incorporating Span 20 surfactant into the coating solutions and performing dip coating at ∼80% relative humidity.

Effects of Fuel Properties on the Evaporation of a Binary Miscible Fuel Droplet on a Heated Wall

An experimental study has been made of the evaporation of a fuel droplet of miscible binary mixture on a heated wall. Primary attention is toward the phenomena which are characteristic of the evaporation of a binary fuel droplet as distinguished from the phenomena for a pure fuel droplet. Fuels tested are the mixture of various couples of normal paraffine hydrocarbons. The results show that two minima and two maxima appear in the lifetime curve. The preferential evaporation of the highly volatile component of the fuel is responsible for the first maximum in the film evaporation region. The increase in the content of the highly volatile component results in the first minimum and maximum shifted toward higher wall temperature, as well as the second minimum and maximum toward lower wall temperature. Four different modes of the droplet evaporation are observed.

In-situ nanostructured film formation during physical vapor deposition

An approach to directly make controlled nanostructured films in situ with physical vapor thin film deposition is presented. Co films were formed into lines with 3.2±0.8 nm thickness and were regularly spaced 550±20 nm apart on a Si(100) surface. The line pattern results during pulsed laser deposition of Co simultaneous with two-beam laser interference irradiation of the Si surface. Preliminary results indicate that preferential evaporation is not the primary mechanism. This patterning is likely due to anisotropic diffusion modified growth of Co under thermal gradients. This one step patterning process, without need of any pre- or post-patterning of the substrate or film, is promising as an economical and simple nanostructured film fabrication approach.

Investigation of Decisive Mixture Effects in Nucleate Boiling of Binary Mixtures Using a Theoretical Model

Abstract In the present paper an attempt is made to clarify the influence of mixture effects upon heat transfer in nucleate boiling of binary mixtures. The studies are based on a theoretical model that is briefly summarized. Evaluating heat and mass transfer around a single vapor bubble emphasizes a strong influence of the so-called micro region where the liquid-vapor phase interface approaches the wall. Due to the preferential evaporation of one component of the mixture, strong concentration gradients occur in the micro region. These microscale composition effects cause diffusive mass transfer, Marangoni convection, and a variation of the liquid-vapor phase equilibrium as well as a variation of the thermophysical properties. From a macroscopic point of view the bubble site density and the departure diameter vary with the composition of the liquid. By means of parameter studies decisive mixture effects are identified and their relevance in the nucleate boiling process is stated. The heat transfer coefficient crucially depends on the bubble site density and departure diameter. For increasing bubble site density, the influence of microscopic concentration gradients increases. But only the variation of liquid-vapor phase equilibrium becomes important, while diffusive mass transfer and Marangoni convection can be neglected.

Evaporation characteristics of the 3-pentanone–isooctane binary system

The results of a numerical and experimental investigation into the evaporation characteristics of the 3-pentanone–isooctane binary system are presented. The behaviour of the system is shown to be nonideal, exhibiting positive deviations from Raoult's law. Thus, the evaporation characteristics of 3-pentanone–isooctane mixtures are shown to be controlled by the relative mole fractions of the two components contained within the liquid phase. The preferential evaporation of 3-pentanone from mixtures containing low concentrations of 3-pentanone is predicted numerically and is confirmed experimentally. The implications of this result with respect to the use of 3-pentanone as a fluorescence marker for isooctane fuel in engine research are discussed at length. The influence high 3-pentanone concentrations on the atomisation behaviour and the optical depth of 3-pentanone–isooctane mixtures is predicted and discussed.

A semi-theoretical model for predicting refrigerant/lubricant mixture pool boiling heat transfer

This paper outlines the framework of a semi-theoretical model for predicting the pool boiling heat transfer of refrigerant/lubricant mixtures on a roughened, horizontal, flat pool-boiling surface. The predictive model is based on the mechanisms involved in the formation of the lubricant excess layer that exists on the heat transfer surface. The lubricant accumulates on the surface in excess of the bulk concentration via preferential evaporation of the refrigerant from the bulk refrigerant/lubricant mixture. As a result, excess lubricant resides in a thin layer on the surface and influences the boiling performance, giving either an enhancement or degradation in heat transfer. A dimensionless excess layer parameter and a thermal boundary layer constant were derived and fitted to data in an attempt to generalize the model to other refrigerant/lubricant mixtures. The model inputs include transport and thermodynamic refrigerant properties and the lubricant composition, viscosity, and critical solution temperature with the refrigerant. The model predicts the boiling heat transfer coefficient of three different mixtures of R123 and lubricant to within ±10%. Comparisons of heat transfer predictions to measurements for 13 different refrigerant/lubricant mixtures were made, including two different refrigerants and three different lubricants.

The crystallization of amorphous Fe 2MnGe powder prepared by ball milling

We synthesized for the first time the intermetallic compound Fe 2MnGe. To avoid preferential evaporation of volatile components we exploited mechanical alloying. Amorphous Fe 2MnGe alloy powder was prepared by planetary ball milling elemental starting materials. The amorphous-to-crystalline transition was studied by means of differential scanning calorimetry (DSC) and X-ray diffraction (XRD). A cubic D0 3 phase is formed at low temperature and transforms to a high-temperature hexagonal D0 19 phase. The apparent activation energy was determined by means of the Kissinger method.

Experimental study of incongruent evaporation kinetics of enstatite in vacuum and in hydrogen gas

Variations in bulk Mg/Si ratios in the various groups of chondritic meteorites indicate that Mg/Si fractionation occurred in the primitive solar nebula. Enstatite (MgSiO3) evaporates incongruently forming forsterite (Mg2SiO4) as an evaporation residue; therefore, evaporation of enstatite produces Mg/Si variations in solid (Mg-rich) and gas (Si-rich) and must be considered as a probable process responsible for Mg/Si fractionation recorded in chondrites. To understand the evaporation kinetics of enstatite, incongruent evaporation experiments on enstatite single crystals have been carried out in vacuum and in hydrogen gas at temperatures of 1300 to 1500°C. A polycrystalline forsterite layer is formed on the surface of enstatite by preferential evaporation of the SiO2 component, both in vacuum and in hydrogen gas. The thickness of the forsterite layer in vacuum increases with time in the early stage of evaporation and later the thickness of the forsterite layer remains constant (several microns). This is due to the change in the rate limiting process from surface reaction plus nucleation and growth to diffusion in the surface forsterite layer. The activation energy of the diffusion-controlled evaporation rate constant of enstatite is 457 (±58) kJ/mol. A thinner forsterite layer is formed on the surface of enstatite in hydrogen gas than in vacuum. Evaporation of enstatite in hydrogen gas is also considered to be controlled by diffusion of ions through the forsterite layer. The thin forsterite layer formed in hydrogen gas is ascribed to the enhanced evaporation rate of forsterite in the presence of hydrogen gas.The results are applied to incongruent evaporation under the solar nebular conditions. The steady thickness of the forsterite of nebular pressure-temperature conditions is estimated to be submicron because of the enhanced evaporation rate of forsterite under hydrogen-rich nebular conditions if evaporated gases are taken away immediately and no back reaction occurs (an open system). Because enstatite grains in the solar nebula would be comparable to the estimated steady thickness of forsterite, evaporation of such enstatite grains under kinetic conditions could play an important role in producing variations in Mg/Si ratios between solid and gas in the solar nebula.

A deep large-area search for very low-mass members of the Hyades open cluster

ABSTRA C T We present the results of a comprehensive I- and Z-band photometric survey of 10.5 deg 2 of the Hyades to search for low-mass stellar and substellar members. The survey, which is * 95 per cent photometrically complete to IC < 20:3, has unearthed a total of 20 candidates, 14 of which are previously uncatalogued. Despite follow-up observations indicating that nine display spectral energy distributions consistent with cluster membership, a detailed astrometric study of all 20 reveals that only one, the previously known stellar member RHy297, displays a proper motion typical of a Hyad. We discuss our failure to detect further low-mass members in terms of the shape of the present-day mass function of the Hyades and find that dynamical evolution has probably led to their preferential evaporation from the cluster.

Self-assembled aerogel-like low dielectric constant films

A series of new spin-on nanoporous silica films with controlled porosity and dielectric constants were prepared by evaporation induced self-assembly (EISA) during spin-coating. Starting with a homogeneous solution of soluble silica and surfactant/swelling agent prepared in an alcohol/water solvent, preferential evaporation of alcohol during spin-coating results in the hierarchical self-assembly of surfactants plus swelling agent and soluble silica into a templated microemulsion system. Silicate polymerization followed by removal of surfactant and swelling agents via immediate calcination results in a highly porous silica film of the liquid crystalline assembly with controlled porosity ranging from 50% to 90% and dielectric constant ranging from 1.3 to 2.6. TEM and surface acoustic wave (SAW) techniques were used to characterize the microstructure of the films. The dielectric constant, Young's modulus, and hardness of the final films are also discussed.

In Situ X-Ray Photoelectron Spectroscopy Study of Etch Chemistry of Methane-Based Reactive Ion Beam Etching of InP Using N2

The effects of nitrogen gas on the methane-based reactive ion beam etching (RIBE) of InP were studied in detail. The surface was very rough and covered with In droplets and heavy polymer when it was etched by the mixture of CH4/H2 without nitrogen gas. In contrast, the surface smoothness was improved markedly when nitrogen gas was added, realizing a root mean square (RMS) roughness comparable to that of the initial InP surface before etching. From the detailed X-ray photoelectron spectroscopy (XPS) analysis, it was found that the formation of P–N bonds on the surface could suppress the preferential evaporation of P atoms in the PH3 form and balance the In- and P-reactions during the etching process. This resulted in the decrease of In droplets and a smoother etched InP surface. It was also found that the rate limiting process for etching was the In-related reaction. The study of Raman scattering showed that the crystal quality of the etched surface was improved when N2 gas was added to the CH4/H2 mixture.

Surface Patterns by Solvent Evaporation of Colloidal Zeolite Suspension

ABSTRACTSurface patterns of porous zeolite structures such as knotted-rope web and wrinkled honeycomb were obtained by dynamic self-assembly of zeolite nanoparticles during solvent evaporation of colloidal zeolite suspension. The study shows that extra ethanol in zeolite synthesis solution is crucial for pattern formation. The addition of ethanol helps produce zeolite nanoparticles with a specific range of particle size during the hydrothermal synthesis. It also provides uniform dynamic driving force for pattern formation during its preferential evaporation. In addition, surface patterns vary with suspension compositions. The patterned zeolite structures have a well-defined bimodal pore size distribution (i.e., 0.55 nm and 2.6 nm) with high BET surface area of 680∼750m2&amp;/g.

Formation Mechanism of Silica/Diblock Mesophases by Solvent Evaporation-Induced Self-Assembly

ABSTRACTIntermediate structures were trapped during the mesophase transition from lamellae to higher curvature structures in a sol-gel matrix. The target structures included normal hexagonally arranged cylinders and/or normal spheres in a cubic array distributed in a hydrophilic matrix. The present system is believed to be the first to trap these intermediates. Through solvent evaporation-induced self-assembly (EISA), mesostructured silica/diblock films with large characteristic length scales were prepared. The structure-directing agents were polystyrene-block-poly(ethylene oxide) (PS-b-PEO) diblock copolymers with high molecular weight, which are water insoluble and alcohol insoluble. We believe that no micellization took place in the present system; a disorder-to-order transition occurred due to the cooperative self- assembly of the diblock and silicates as the solvent preferentially evaporated from a film cast from a dilute homogeneous solution. During further preferential evaporation, the morphogenic effect of the increase of species concentration facilitates the mesophase development in the direction of a normal cubic to hexagonal to lamellar pathway. However, the morphogenic effects of both the decrease of the PS coil dimension and the siloxane condensation drive the mesophase development in opposite directions. The decrease of the PS coil dimension plays an important role in the present self-assembly process. Trapping of the intermediates and coexisting multiple mesophases are related to the facts that PS has high Tg and high hydrophobicity in particular, as well as to the fact that polymers have relatively low mobility in general.

Star cluster ecology -- IV. Dissection of an open star cluster: photometry

The evolution of star clusters is studied using N-body simulations in which the evolution of single stars and binaries are taken self-consistently into account. Initial conditions are chosen to represent relatively young Galactic open clusters, such as the Pleiades, Praesepe and the Hyades. The calculations include a realistic mass function, primordial binaries and the external potential of the parent Galaxy. Our model clusters are generally significantly flattened in the Galactic tidal field, and dissolve before deep core collapse occurs. The binary fraction decreases initially due to the destruction of soft binaries, but increases later because lower mass single stars escape more easily than the more massive binaries. At late times, the cluster core is quite rich in giants and white dwarfs. There is no evidence for preferential evaporation of old white dwarfs, on the contrary the formed white dwarfs are likely to remain in the cluster. Stars tend to escape from the cluster through the first and second Lagrange points, in the direction of and away from the Galactic center. Mass segregation manifests itself in our models well within an initial relaxation time. As expected, giants and white dwarfs are much more strongly affected by mass segregation than main-sequence stars. Open clusters are dynamically rather inactive. However, the combined effect of stellar mass loss and evaporation of stars from the cluster potential drives its dissolution on a much shorter timescale than if these effects are neglected. The often-used argument that a star cluster is barely older than its relaxation time and therefore cannot be dynamically evolved is clearly in error for the majority of star clusters.

Preferential evaporation of precipitants from polymer solutions in mixed solvents

Partial vapor pressures were measured for the volatiles of solutions of polysulfone or polyethersulfone in mixtures of N,N-dimethylformamide (DMF, solvent) and water or acetone (precipitants) by means of headspace gas chromatography. The results demonstrate that the enrichment of water in the gas phase increases exponentially with rising polymer concentration, in contrast to that of acetone which remains constant. The reinforced expulsion of water resulting from the presence of polymers is theoretically conceivable and should be useful in the field of separation techniques.

Kinetics of diffusion-controlled evaporation of Fe-Mg olivine: experimental study and implication for stability of Fe-rich olivine in the solar nebula

Evaporation is a process that caused chemical fractionation in the solar nebula. The evaporation rates of meteorite-forming minerals and silicate melts are key parameters for constraining the timescale of high temperature processes in the early solar nebula and for understanding the mechanisms of cosmochemical fractionation. The kinetics of evaporation of olivine (initial Fo = 92), the most common silicate in meteorites, was experimentally investigated at 1414°C for various duration under continuous evacuation. The surface of olivine becomes more magnesian and the interior becomes zoned as evaporation proceeds, indicating preferential evaporation of the fayalite (Fa) component. The weight loss, surface composition, and interior zoning profile were analyzed using a semi-infinite one-dimensional diffusion model with boundary migration, and the controlling model parameters were optimized by fitting the experimental results. The evaporation rate of olivine shows notable composition and orientation dependence. As olivine becomes more magnesian, the evaporation rates of the (100), (010), and (001) surfaces decrease, among which the rate at the (001) surface is greatest (1.0 μm/h at Fo = 91.8), which is about 2 orders of magnitude faster than that of pure synthetic forsterite. The estimated Fe-Mg fractionation factor shows anisotropy ranging from 0.017 to 0.035, which is significantly greater than the equilibrium value of the Fe-Mg distribution coefficient between olivine and gas, suggesting kinetic suppression of evaporation of the Fa component. The Fe-Mg interdiffusion coefficient at 1414°C, which also shows notable composition and orientation dependence, is estimated to be 8.7 × 10 −15 m 2/sec along the c-axis. The results were applied to constrain the timescale of heating for a spherical Fo50 olivine grain to retain the Fa component. Olivine dust ∼1 μm in diameter can retain 80% of the initial Fa component for one minute in vacuum, and for ∼30 min at ∼10 Pa P H2 when heated at 1414°C. Therefore, Fo50 olivine in chondrite matrices could have survived an incipient heating event depending on the ambient hydrogen pressure, if they were originally present in the solar nebula.

Nanoscale compositional analysis of Fe/Tb multilayers: A field ion microscopy and tomographic atom probe study

Sputtered Fe/Tb multilayered systems have been successfully deposited on needle-shaped substrates. These specimens have been observed by field ion microscopy and the layered sequence is evidenced together with the preferential field evaporation of terbium versus iron. The first direct atomic scale concentration data of Fe/Tb multilayers by means of the tomographic atom probe are given. The three-dimensional reconstructions of the layers were obtained and the compositional modulation was observed across the specimens.

減圧下脱炭による溶鋼からの蒸発脱銅・脱すずの促進

For accelerating the removal of copper and tin by evaporation under practical reduced pressure, 1.3×102 Pa, decarburization by weak oxidizing agents such as SiO2 and MgO has been investigated in laboratory scale experiments. The experimental results are as follows;(1) An effective removal of copper was obtained in the order of MgO>SiO2>O2 at a given amount of decarburization. The order is the same as that of oxygen potential of those oxidizers.(2) 50% of copper in iron melt was removed in 40 min in MgO crucible, which was reacted as an oxidizer. The rate of decopperization was 1.53 times higher than that without decarburization.(3) The reason why the acceleration of the removal of copper was achieved is assumed to be due to the increase in reaction interfacial area obtained by turbulence with decarburization. However, low oxygen content is preferable, because oxygen is the surface active element and is assumed to retard the evaporation reaction.(4) In high sulfur melt, 0.1 mass%, removal of copper was slightly lowered by suppresion of decarburization. However, removal of tin was accelerated. The phenomenon is considered to be caused by preferential evaporation as SnS.

INFLUENCE OF MICROSCALE CONCENTRATION GRADIENTS IN NUCLEATE BOILING HEAT TRANSFER OF BINARY MIXTURES

Nucleate boiling heat transfer coefficients of binary mixtures can be calculated using a micro region model. The model is described and assessed by comparing calculated heat transfer coefficients with experimental values. The model includes the governing physical phenomena, such as the meniscus curvature, the adhesion pressure, and the interfacial thermal resistance as well as the local variation of composition and liquid-vapor equilibrium and the Marangoni effect. Due to the preferential evaporation of one component of the mixture, strong concentration gradients occur close to the heated wall. The influence of these microscale concentration gradients is investigated. By means of various calculations the physical phenomena are quantified and their relevance in the nucleate boiling process is evaluated.