Nucleation Frequency Research Articles

Кинетика зарождения жидкой фазы в растянутом ГЦК-кристалле: молекулярно-динамическое моделирование

AbstractThe kinetics of a spontaneous formation of liquid phase in a stretched (superheated) Lennard-Jones fcc crystal is studied. Molecular dynamics experiments are used to determine the main parameters of the nucleation process: nucleation frequency J , diffusion coefficient of nuclei D _*, nonequilibrium Zel’dovich factor Z , and critical nucleus size R _*. The calculations are performed at negative pressures from the endpoint of the melting line and at positive pressures that are higher by a factor of eight than the critical pressure. The simulation results are compared to the classical homogeneous nucleation theory. It is found that the theory qualitatively correctly reproduces the dynamics of developing the process. The theory and the simulation demonstrate good quantitative agreement for the transition rate of the liquid phase nucleus through the critical size, but there is large difference in the numbers of critical nuclei in the unit volume of the metastable phase. In the case of significant superheatings and negative pressures, the contribution of the energy of elastic stresses to the moving force of the phase transformation is small and it can be neglected in a first approximation. The mismatch between the theory and the simulation results can be eliminated taking that the surface free energy of a curved “crystal–liquid droplet” interface is smaller than that of a plane interface by 30–35%.

Epitaxial growth of LiCoO2 thin films with (001) orientation

The layered structure of LiCoO2 implies anisotropic ionic conduction; however, experimental data have never demonstrated this. The anisotropy can be observed clearly in epitaxial films with controlled orientations. Our previous study had reported that LiCoO2 grows epitaxially on Nb-doped SrTiO3 (100) and (110) substrates with complete (104) and (018) orientations, respectively. On the other hand, the growth on SrTiO3 (111) substrates with (001) orientation was accompanied by the inclusion of (012)-oriented domains, although the (012) orientation is higher in the energy state than the (001). The present study reveals that lower laser energy density (fluence) and lower substrate temperature decrease the amount of inclusions; that is, the occurrence of the (012) orientation in spite of its higher energy is governed by these factors. Higher fluence leading to higher deposition rates does not provide sufficient time for the cations to be rearranged into the (001) orientation, and the higher substrate temperature increases the nucleation frequency for the (012) orientation. A micrograph of the final (001)-oriented film reveals that the LiCoO2 film grows in an island growth mode.

Dimensionality of the crystal growth, exponents of the power laws and activation energy for nucleation and growth processes in glass-crystal transformations under non-isothermal regime. Application to the crystallization of the Sb0.13As0.35Se0.52 glassy semiconductor

A procedure has been described to establish the dimensionality of the crystal growth, to evaluate the exponents of the power laws corresponding to time-dependence both for the nucleation frequency and for the crystal growth rate and to calculate the activation energy for nucleation and growth processes. From the linear relation existing between the logarithms of the heating rate and of the temperature corresponding to the maximum crystallization rate, it is possible to deduce a linear relation between the logarithm of the quotient of the mentioned temperature at two different heating rates and the difference between the inverse of these temperatures. Known the kinetic exponent, the activation energy and the thermal process type, continuous nucleation or “site saturation”, it is determined the dimensionality from the slope of the last relation. From the dimensionality and the values of the kinetic exponent for the as-quenched and reheated material, the exponents of the power laws have been evaluated, from which, the activation energy both for nucleation and crystal growth has been calculated. This procedure has been applied to the crystallization kinetics of the Sb0.13As0.35Se0.52 glassy semiconductor. The results confirm the reliability of this procedure to analyse the non-isothermal transformation of the studied glassy alloy.

Boiling water jet outflow from a thin nozzle: spatial modeling

This study presents dual-temperature two-phase model for liquid-vapor mixture with account for evaporation and inter-phase heat transfer (taken in single-velocity single-pressure approximation). Simulation was performed using the shock-capturing method and moving Lagrangian grids. Analysis was performed for simulated and experimental values of nucleation frequency (for refining the initial number and radius of microbubbles) which affect the evaporation rate. Validity of 2D and 1D simulation was examined through comparison with experimental data. The peculiarities of the water-steam formation at the initial stage of outflow through a thin nozzle were studied for different initial equilibrium states of water for the conditions close to chosen experimental conditions.

Spatial extent of new particle formation events over the Mediterranean Basin from multiple ground-based and airborne measurements

Abstract. Over the last two decades, new particle formation (NPF), i.e., the formation of new particle clusters from gas-phase compounds followed by their growth to the 10–50 nm size range, has been extensively observed in the atmosphere at a given location, but their spatial extent has rarely been assessed. In this work, we use aerosol size distribution measurements performed simultaneously at Ersa (Corsica) and Finokalia (Crete) over a 1-year period to analyze the occurrence of NPF events in the Mediterranean area. The geographical location of these two sites, as well as the extended sampling period, allows us to assess the spatial and temporal variability in atmospheric nucleation at a regional scale. Finokalia and Ersa show similar seasonalities in the monthly average nucleation frequencies, growth rates, and nucleation rates, although the two stations are located more than 1000 km away from each other. Within this extended period, aerosol size distribution measurements were performed during an intensive campaign (3 July to 12 August 2013) from a ground-based station on the island of Mallorca, as well as onboard the ATR-42 research aircraft. This unique combination of stationary and mobile measurements provides us with detailed insights into the horizontal and vertical development of the NPF process on a daily scale. During the intensive campaign, nucleation events occurred simultaneously both at Ersa and Mallorca over delimited time slots of several days, but different features were observed at Finokalia. The results show that the spatial extent of the NPF events over the Mediterranean Sea might be as large as several hundreds of kilometers, mainly determined by synoptic conditions. Airborne measurements gave additional information regarding the origin of the clusters detected above the sea. The selected cases depicted contrasting situations, with clusters formed in the marine boundary layer or initially nucleated above the continent or in the free troposphere (FT) and further transported above the sea.

Influence of feedstock concentration on tetragonality and particle size of hydrothermally synthesized barium titanate powders

Abstract Barium titanate (BaTiO 3 ) powders were synthesized through hydrothermal process with Ba(OH) 2 ·8H 2 O and TiO 2 . By increasing the feedstock concentration (FC) from 0.25 to 1.50 M, BaTiO 3 powders maintain a stable average particle size (~180 nm and ~6.4441 m 2 /g) with an increasing tetragonality ( c/a : 1.0065–1.0075). Johnson-Mehl-Avrami and standard reaction rate equations were adopted to analyze the kinetic process of BaTiO 3 formation. The reaction is governed by first-order and phase-boundary-controlled mechanism for 0.25 M and 1.50 M, respectively. Lower extent of reaction is believed to lead to the better tetragonality for BaTiO 3 powders fabricated with higher FC. On the other hand, the relative stable particle size is correlated with the unvaried nucleation frequency and grain growth rate with various FC. This work can provide a guideline to manipulate the properties of BaTiO 3 powders used in electronic industry.

Direct estimation of the global distribution of vertical velocity within cirrus clouds

Cirrus clouds determine the radiative balance of the upper troposphere and the transport of water vapor across the tropopause. The representation of vertical wind velocity, W, in atmospheric models constitutes the largest source of uncertainty in the calculation of the cirrus formation rate. Using global atmospheric simulations with a spatial resolution of 7 km we obtain for the first time a direct estimate of the distribution of W at the scale relevant for cirrus formation, validated against long-term observations at two different ground sites. The standard deviation in W, σw, varies widely over the globe with the highest values resulting from orographic uplift and convection, and the lowest occurring in the Arctic. Globally about 90% of the simulated σw values are below 0.1 m s−1 and about one in 104 cloud formation events occur in environments with σw > 0.8 m s−1. Combining our estimate with reanalysis products and an advanced cloud formation scheme results in lower homogeneous ice nucleation frequency than previously reported, and a decreasing average ice crystal concentration with decreasing temperature. These features are in agreement with observations and suggest that the correct parameterization of σw is critical to simulate realistic cirrus properties.

On the Application of Image Processing Methods for Bubble Recognition to the Study of Subcooled Flow Boiling of Water in Rectangular Channels.

This work introduces the use of machine vision in the massive bubble recognition process, which supports the validation of boiling models involving bubble dynamics, as well as nucleation frequency, active site density and size of the bubbles. The two algorithms presented are meant to be run employing quite standard images of the bubbling process, recorded in general-purpose boiling facilities. The recognition routines are easily adaptable to other facilities if a minimum number of precautions are taken in the setup and in the treatment of the information. Both the side and front projections of subcooled flow-boiling phenomenon over a plain plate are covered. Once all of the intended bubbles have been located in space and time, the proper post-process of the recorded data become capable of tracking each of the recognized bubbles, sketching their trajectories and size evolution, locating the nucleation sites, computing their diameters, and so on. After validating the algorithm’s output against the human eye and data from other researchers, machine vision systems have been demonstrated to be a very valuable option to successfully perform the recognition process, even though the optical analysis of bubbles has not been set as the main goal of the experimental facility.

In-situ study of morphology and growth of primary α-Al(FeMnCr)Si intermetallics in an Al-Si alloy

Morphology and growth of primary α-Al(FeMnCr)Si intermetallics have been studied in-situ during solidification of a commercial secondary aluminum alloy employing X-radiographic imaging combined with deep-etching. The α-Al(FeMnCr)Si intermetallics were found to nucleate primarily on surface oxides, and the continued growth yielded both rhombic dodecahedrons and elongated rod-like morphologies. Both morphologies were observed as hopper and massive types, where the hopper intermetallics had the higher growth rates. The growth rate, which determines the type, appears to be linked to nucleation frequency; higher nucleation frequency promoted massive types and lower nucleation frequency promoted hopper intermetallics.

Tubulin acetylation protects long-lived microtubules against mechanical ageing

Introductory ParagraphLong-lived microtubules endow the eukaryotic cell with long-range transport abilities. While long-lived microtubules are acetylated on lysine 40 of α-tubulin (αK40), acetylation takes place after stabilization1 and does not protect against depolymerization2. Instead, αK40 acetylation has been proposed to mechanically stabilize microtubules3. Yet how modification of αK40, a residue exposed to the microtubule lumen and inaccessible from MAPs and motors1,4, could affect microtubule mechanics remains an open question. Here we develop FRET-based assays that report on the lateral interactions between protofilaments and find that αK40 acetylation directly weakens inter-protofilament interactions. Congruently, αK40 acetylation affects two processes largely governed by inter-protofilament interactions, reducing the nucleation frequency and accelerating the shrinkage rate. Most relevant to the biological function of acetylation, microfluidics manipulations demonstrate that αK40 acetylation enhances flexibility and confers resilience against repeated mechanical stresses. Thus, unlike deacetylated microtubules that accumulate damages when subjected to repeated stresses, long-lived microtubules are protected from mechanical aging through their acquisition of αK40 acetylation. Thus, unlike other tubulin post-translational modifications that act through MAPs, motors and severing enzymes, intraluminal acetylation directly tunes the compliance and resilience of microtubules.

Effects of Al grain size on metal-induced layer exchange growth of amorphous Ge thin film on glass substrate

Metal-induced layer exchange (MILE) has attracted increasing attention as a way to lower the crystallization temperature of amorphous semiconductor thin films on insulating substrates. This paper demonstrates that the quality of the catalytic Al layer strongly influences the growth properties in the MILE of amorphous Ge. The growth velocity of the MILE significantly decreases with increasing the deposition temperature of Al (TAl: RT–200°C), while the grain size of crystallized Ge becomes maximum (28μm) at TAl=100°C. This behavior is attributed to the Al grain size depending on TAl, which influences both the nucleation frequency and the lateral growth velocity of Ge in Al. These findings give new insight into MILE for fabricating high-quality semiconductor thin films at low temperatures on inexpensive substrates.

Nucleation of two-dimensional islands on Si (111) during high-temperature epitaxial growth

The process of two-dimensional island nucleation at the surface of ultra large Si (111) during hightemperature epitaxial growth is studied by in situ ultrahigh-vacuum reflection electron microscopy. The critical terrace size D crit, at which a two-dimensional island is nucleated in the center, is measured in the temperature range 900–1180°C at different silicon fluxes onto the surface. It is found that the parameter D crit 2 is a power function of the frequency of island nucleation, with the exponent χ = 0.9 ± 0.05 in the entire temperature range under study. It is established that the kinetics of nucleus formation is defined by the diffusion of adsorbed silicon atoms at temperatures of up to 1180°C and the minimum critical nucleus size corresponds to 12 silicon atoms.

On Wetting Angles and Nucleation Energies during the Electrochemical Nucleation of Cobalt onto Glassy Carbon from a Deep Eutectic Solvent

This work deals with the electrochemical nucleation of cobalt, onto a glassy carbon electrode, from a deep eutectic solvent formed by choline chloride and urea in a 1:2 molar ratio. The nucleation mechanism was studied through electrochemical techniques. From the analysis of the experimental potentiostatic current transients, the kinetic parameters such as the nucleation frequency, A, and the number density of active sites, N0, were determined. In this work, A was also expressed as a function of the wetting angle, θ, (A(θ)) where the local distribution of θ was calculated through a stochastic simulation of the misorientation angle distribution derived from a created random texture. The A(θ) obtained from such a stochastic approach was in good agreement with the experimental data thus leading to determination of the Gibbs free energy of nucleation ΔG(nc) and the critical nucleus size nc, aside from other kinetic parameters such as the transfer coefficient α, the exchange current density j0, and the surface tension σ. Furthermore, a new method of calculating the nucleation energy from a voltammetric trace recorded during cobalt nucleation onto glassy carbon was proposed and validated. This method provides a novel tool to assess the experimental magnitude and to measure the nucleation energy.

Experimental study of multi-scale heat transfer characteristics at pool boiling

This study presents the results of the experimental investigation of local and integral characteristics of heat transfer at liquid pool boiling. Saturated ethanol and water were used as the working fluids. Thin, resistively heated indium-tin oxide films deposited onto the sapphire substrates were used as the heaters. The synchronized measurements of the heater surface temperature field and dynamics of vapor bubbles were performed by high-speed infrared thermography with the frame rate of 1000 fps and resolution of up to 0.13 μm/px and high-speed video recording. In this paper new data on major local boiling characteristics, such as nucleation site density, dynamics of vapor bubbles, temporal characteristics and nucleation frequency at different heat fluxes and superheating and their comparison with correlations are presented.

Three-dimensional nucleation with diffusion controlled growth: A comparative study of electrochemical phase formation from aqueous and deep eutectic solvents

The nucleation of Ag onto vitreous carbon from aqueous 3M NaCl or 0.6M NaClO4 and deep eutectic solvent (DES) 1:2M mixture of choline chloride:urea solutions containing Ag+, has been studied analyzing the chronoamperometric response to single potential steps. From the coordinates of the maxima observed in the current responses, the nucleation frequencies A (s−1) and number densities of nucleation sites N0 (cm−2) were obtained from the standard model of nucleation with diffusion-controlled three-dimensional growth. Analysis of the overpotential dependence of nucleation frequencies using the classical electrochemical nucleation theory allowed to calculate the Gibbs free energy of nucleation ΔG˜nc and critical nucleus size nc as well as the exchange current density j0, transfer coefficient α and surface tension σ of silver nuclei. The kinetics of Ag+ reduction is two orders of magnitude slower in DES compared to both aqueous systems studied, and values of α≪0.5 where found in both aqueous and DES media, indicating either that the intermediate state for metal ion reduction is located close to the initial state, i.e., the solvated or complexed metal ion in solution, or that the metal ion is specifically adsorbed on the surface and the symmetry factor involved requires an alternative electron transfer formalism. The low ΔG˜nc and nc values observed indicate that the discharge of a single Ag ion on the surface already becomes a supercritical nucleus, involving a very low Gibbs energy barrier, characteristic of a non-activated process.

Computational fluid dynamics and population balance modelling of nucleate boiling of cryogenic liquids: Theoretical developments

The main focus in the analysis of pool or flow boiling in saturated or subcooled conditions is the basic understanding of the phase change process through the heat transfer and wall heat flux partitioning at the heated wall and the two-phase bubble behaviours in the bulk liquid as they migrate away from the heated wall. This paper reviews the work in this rapid developing area with special reference to modelling nucleate boiling of cryogenic liquids in the context of computational fluid dynamics and associated theoretical developments. The partitioning of the wall heat flux at the heated wall into three components – single-phase convection, transient conduction and evaporation – remains the most popular mechanistic approach in predicting the heat transfer process during boiling. Nevertheless, the respective wall heat flux components generally require the determination of the active nucleation site density, bubble departure diameter and nucleation frequency, which are crucial to the proper prediction of the heat transfer process. Numerous empirical correlations presented in this paper have been developed to ascertain these three important parameters with some degree of success. Albeit the simplicity of empirical correlations, they remain applicable to only a narrow range of flow conditions. In order to extend the wall heat flux partitioning approach to a wider range of flow conditions, the fractal model proposed for the active nucleation site density, force balance model for bubble departing from the cavity and bubble lifting off from the heated wall and evaluation of nucleation frequency based on fundamental theory depict the many enhancements that can improve the mechanistic model predictions. The macroscopic consideration of the two-phase boiling in the bulk liquid via the two-fluid model represents the most effective continuum approach in predicting the volume fraction and velocity distributions of each phase. Nevertheless, the interfacial mass, momentum and energy exchange terms that appear in the transport equations generally require the determination of the Sauter mean diameter or interfacial area concentration, which strongly governs the fluid flow and heat transfer in the bulk liquid. In order to accommodate the dynamically changing bubble sizes that are prevalent in the bulk liquid, the mechanistic approach based on the population balance model allows the appropriate prediction of local distributions of Sauter mean diameter or interfacial area concentration, which in turn can improve the predictions of the interfacial mass, momentum and energy exchanges that occur across the interface between the phases. Need for further developments are discussed.

Crystallographic Investigation of the Initial Solidification Grain Structure in Al-Si Alloy

As-solidified structure of an ingot is composed of the chill, columnar and equiaxed zones. The whole solidified structure is strongly affected by the chill crystals. Some initial solidification grains have been observed on the ingot surface and thought to be traces of the nucleation point. The aim of this study is, therefore, to develop the experiment technique to make one ‘grain’ and to crystallographically investigate the initial solidification grain using EBSD analysis. In order to start solidification at a very specified position, a small metallic protrusion was installed on an insulating plate. Al-6 wt%Si alloy was melted at 800 °C and was poured on the metallic protrusion. In this study, the amount of protrusion was varied to investigate the growth mechanism of the initial solidification grain. The longitudinal cross section of the specimen was observed by an optical microscope, a scanning electron microscope. The starting position of solidification was the area that was on the metallic protrusion. In this initial solidification grain, it was difficult to observe the dendritic structure. The shape of this grain was about hemispherical. The grain area seemed to increase with increasing the amount of protrusion. The results of EBSD analysis showed that almost all initial solidification grains were composed by several crystals. The reason of this is that the nucleation frequency may increase with the amount of protrusion. The dendrite grew radially from the initial solidification grain continuously. The crystallographic structure was also continuous on the boundary of the initial solidification grain.

Molecular dynamics study of the nucleation rate of nanopores in aluminum at a negative pressure

The rate of a homogeneous nucleation of nanovoids in expanded aluminum is researched in this work. Typical lifetime of the system in a metastable state at a negative pressure as well as coefficients of surface tension and nucleation frequency at the temperatures 300 and 500 K are obtained with the help of molecular dynamic simulation. The large value of the pre-exponential factor should be noted, which requires further detailed investigations.

Modification of the Ti40Cu36Zr10Pd14 BMG Crystallization Mechanism with Heating Rates 10-140 K/min

The article presents investigations of Ti40Cu36Zr10Pd14 bulk metallic glass crystallization process heated with the rates of 10, 60, 100 and 140 K/min. High heating rates experiments were performed in a new type of differential scanning calorimeter equipped with a fast responding thermal sensor. Phase composition and microstructure were studied with x-ray diffraction and transmission electron microscopy. The observed crystallization proceeded in two separate steps. Applied high rates of heating/cooling resulted in the crystallization of only one CuTi phase, replacing typical multi-phase crystallization. The microstructure after crystallization was polycrystalline with some amount of amorphous phase retained. Kinetic parameters were determined with the use of the Kissinger and Friedman iso-conversional analysis and Matusita–Sakka iso-kinetic model. The kinetic analysis supplies results concerning autocatalytically activated mechanism of primary crystallization with decreasing activation energy and small density of quenched-in nuclei, in good agreement with previous structural investigations. The mechanism of secondary crystallization required dense nuclei site, increasing activation energy and large nucleation frequency. The amorphous phase of Ti40Cu36Zr10Pd14 BMG revealed high thermal stability against crystallization. Application of high heating rates in DSC experiments might be useful for the determination of mechanism and kinetic parameters in investigations of metallic glasses crystallization, giving reasonable results.

Modification of the surface morphology of 4H-SiC by addition of Sn and Al in solution growth with SiCr solvents

For solution growth of 4H-SiC with Si0.6−x−yCr0.4AlxSny solvents, the changes in surface morphology and polytype induced by the addition of Sn and Al to the Si0.6Cr0.4 solvent were investigated. Growth with Si0.6Cr0.4 solvents resulted in a rough surface covered with large macrosteps that were several micrometers high, and the polytype of the grown layer transformed to 6H and 15R-SiC. The surface roughening and polytype instability were suppressed when more than 2at% Al was added to the SiCr0.4 solvent. We also found that the combined addition of both 2–4at% Sn and 0.5–1at% Al resulted in smooth surface morphology. We discussed the modification of the surface morphology of 4H-SiC caused by the additives in terms of the wetting properties of the solvents. Based on the results of experiments and thermodynamic calculations, the addition of both Sn and Al increased the liquid/solid interfacial energy. Because the two-dimensional nucleation energy increases with the interfacial energy, we conclude that smooth step flow growth of 4H-SiC was achieved by lowering the frequency of two-dimensional nucleation on the growth surface.