Nucleation Frequency Research Articles

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.

Homogeneous nucleation in liquid nitrogen at negative pressures

The kinetics of spontaneous cavitation in liquid nitrogen at positive and negative pressures has been studied in a tension wave formed by a compression pulse reflected from the liquid–vapor interface on a thin platinum wire heated by a current pulse. The limiting tensile stresses (Δp = ps–p, where ps is the saturation pressure), the corresponding bubble nucleation frequencies J (1020–1022 s–1 m–3), and temperature induced nucleation frequency growth rate GT = dlnJ/dT have been experimentally determined. At T = 90 K, the limiting tensile stress was Δp = 8.3 MPa, which was 4.9 MPa lower than the value corresponding to the boundary of thermodynamic stability of the liquid phase (spinodal). The measurement results were compared to classical (homogeneous) nucleation theory (CNT) with and without neglect of the dependence of the surface tension of critical bubbles on their dimensions. In the latter case, the properties of new phase nuclei were described in terms of the Van der Waals theory of capillarity. The experimental data agree well with the CNT theory when it takes into account the “size effect.”

An alternative theoretical procedure to analyze the non-isothermal glass-crystal transformation kinetics: crystallization of the Sb0.13As0.35Se0.52 glassy semiconductor

A procedure has been developed for analyzing the evolution with temperature of the actual transformed volume fraction and for calculating the kinetic parameters at non-isothermal reactions in materials involving formation and growth of nuclei. By considering the assumptions of extended volume and spatially random nucleation, as well as, assuming that both the nucleation frequency and the crystal growth rate depend on time as a power law, an equation of the extended transformed volume fraction has been obtained as a function of the temperature. From the quoted equation and according to the Jhonson–Mehl–Avrami model, it has been deduced an expression for the actual transformed volume fraction. The theoretical procedure described has been applied to the non-isothermal crystallization kinetics of the Sb0.13As0.35Se0.52 glassy alloy, with and without previous reheating, using DSC technique. In accordance with the study carried out, it is possible to state that the reheating did not cause the appearance of nuclei, but that the as-quenched material already contains a sufficient number of them. Likewise, it has been analyzed the variations of the kinetic exponent and the activation energy with the temperature. This analysis suggests that the transformation cannot be described by a mechanism of single-step, which demonstrates a mechanism of complex multi-step. Moreover, the experimental curve of the transformed fraction shows a satisfactory agreement with the theoretical curve corresponding to the considered procedure, confirming the reliability of the same to analyze the transformation kinetics of the studied alloy. The phase at which the alloy crystallizes after the thermal process has been identified by X-ray diffraction. The obtained diffractogram suggests the presence of micro crystallites of Sb2Se3, remaining an additional amorphous matrix.

The interaction between Eu and P in high purity Al-7Si alloys

The interaction between Eu and P in high purity Al-7Si alloys was investigated. Interrupted solidification experiments were undertaken to study macrostructure evaluation and thermal analyses were performed to obtain characteristic temperatures of the eutectic reaction. Without the presence of Eu, increasing P concentrations results in a significant increase in the number of eutectic grains due to the formation of AlP particles, which can act as nucleation sites for eutectic Si. With the presence of Eu, a decrease in nucleation frequency and thereby an increase in eutectic grain size was observed in alloys containing 5 and 30ppm P due to the interaction between Eu and P, which can be further supported by the results of μ-XRD patterns and SADP of TEM. It was found that most P-rich particles were covered by Eu-rich intermetallics, which are indexed to be Al2Si2Eu phase, indicating that the formation of the pre-eutectic Al2Si2Eu phase may lead to a depletion of pre-existing AlP particles or residual P in the melt. This investigation highlights the effect of the interaction between Eu and P on the nucleation of eutectic Si and eutectic grains.

Application of high-speed IR thermography to study boiling of liquids

A method for studying local and integral characteristics of heat transfer at pool boiling on a thinfilm heater using a high-speed thermal-imaging camera is described. Experiments have been carried out at pool boiling of saturated ethanol with a heat flux as high as 40 W/cm2. A 1-μm-thick indium−tin oxide (ITO) film sputtered on a 0.4-mm-thick sapphire substrate by the vapor deposition method was used as a heater. Based on simultaneous measurements of the temperature field distribution at the back side of the ITO film using an IR thermal-imaging camera in combination with visual recording of vapor bubble growth and departure directly on the heater surface, it is possible to judge the temperature field evolution under individual vapor bubbles, the nucleation site density and the nucleation frequency, as well as the time of bubble growth and departure during boiling.

The isokinetic behavior in diffusion controlled growth processes

The isokinetic behaviour of crystallization processes controlled by diffusion is analysed under both isothermal and continuous cooling/heating conditions. The kinetic function is computed employing an approximate expression for the so called exponential integral. Diffusion controlled 3D growth of homogeneously nucleating crystalline grains is an isokinetic reaction if the nucleation and diffusion activation energies are identical. In the non isokinetic range, the kinetic functions in both continuous heating and isothermal transformations are the same except for a factor which depends on the nucleation and diffusion activation energies ratio. This development is applied to the crystallization of AgGeSe glasses. The primary crystallization kinetic of glasses with compositions (Ge25Se75)100−y Agy (with y = 10, 15, 20 and 25 at. %) was studied in a previous work using differential scanning calorimetry and X-ray diffraction. The analysis is grounded on the Kolmogorov-Johnson-Mehl-Avrami model generalized to account for the compositional changes of the parent phase, responsible for the decreasing of both the nucleation frequency and the growth rate of the primary grains. The kinetic study of the crystallization process from continuous heating calorimetric data has been performed applying the master curve method. The primary crystallization product is the ternary phase γ-Ag8GeSe6. The values of apparent activation energy Ea, obtained are in the range: 2.0 eV/at < Ea < 2.6 eV/at. A model of diffusion controlled 3D growth with decreasing homogeneous nucleation and soft impingement has been developed that reproduces the rate of transformation obtained experimentally for the studied alloys.

Wall-temperature distributions of nucleate pool boiling surfaces vs. boiling curves: A new approach

Boiling is one of the most effective heat transfer mechanisms and has a great potential in applications where superior thermal energy management capability is required. During nucleate boiling the fluid vaporises on a surface heated above the fluid’s saturation temperature and the latent heat of vaporisation allows dissipating large heat flux. The boiling performance is usually represented by a boiling curve that plots the spatio-temporal averaged heat flux versus the wall superheat. Here, we show a new approach to evaluating the process on the basis of wall-temperature distributions, calculated from spatio-temporal thermographs of the boiling surface. Instead of a single data point on the boiling curve, the presented distributions provide spectra of information, such as the maximum, minimum and mean wall superheat and the standard deviation of the wall temperature. These parameters are all associated with the nucleation frequencies and the nucleation site densities. The experimental results from various state-of-the-art surfaces suggest that the distributions reveal the stability of the boiling process, which opens a new path towards a better understanding and the development of heat transfer technology.

The influence of Cu, Mg and Ni on the solidification and microstructure of Al-Si alloys

The influence of alloying elements (Cu, Mg, and Ni) on eutectic nucleation, eutectic grain morphology and the final microstructure of an Al-10Si commercial purity alloy in unmodified and Sr-modified conditions was investigated. It was found that the nucleation and eutectic grain growth morphologies of both the unmodified and Sr-modified Al-Si eutectic were significantly influenced by the addition of ternary alloying elements to a degree dependent on when the intermetallic phase formed during the solidification of the alloy with respect to the Al-Si eutectic. In cases where an intermetallic phase nucleated prior to the onset of the Al-Si eutectic reaction, the eutectic nucleation frequency was affected by changes to the available nuclei population. In cases where the intermetallic nucleated after the Al-Si eutectic, segregation of the ternary solutes in front of the Al-Si eutectic interface changed the nucleation and macroscopic growth dynamics. The changes in nucleation and growth dynamics of the Al-Si eutectic due to the presence of solute altered the morphology of the eutectic silicon considerably. This study has revealed a number of insights into the mechanisms of nucleation and growth of the Al-Si eutectic.

Measurement of liquid–vapor phase distribution on nano- and microstructured boiling surfaces

Many studies have reported that the boiling heat transfer coefficient (HTC) and critical heat flux (CHF) are significantly enhanced by nano- and microstructures on a boiling surface, which may effectively change the nucleation characteristics of bubbles and the micro-hydrodynamics of liquid layers underneath growing bubbles. However, there has been no direct observation of how these surface structures alter such physical phenomena at the bubble scale. Here, we report the first direct observations of the distribution and dynamics of the liquid and vapor phases and triple contact line on nano- and microstructured surfaces during boiling of saturated water using an infrared-based detection technique, DEtection of Phase by Infrared Thermometry (DEPIcT). Fundamental boiling parameters at the level of individual bubbles, such as nucleation site density and frequency, dry area fraction, triple contact line density, and bubble base diameter, were measured from the observed results. Finally, the changes in the HTC and CHF values on the structured surfaces were interpreted using quantified boiling parameter data.

マグネシウムハイドライドの水素放出における潜伏期に及ぼす比表面積の影響

Although Mg based hydrogen storage materials absorb hydrogen immediately under appropriate thermodynamic conditions, an obvious incubation period exists for hydrogen desorption. This fact implies that the mechanisms of hydrogen absorption and desorption are different from each other. During hydrogen desorption process, nucleation and growth of Mg are occurring in MgH2. It is considered that free surface is the only nucleation site of Mg in hydrogen desorption process. The nucleation frequency per unit specific surface area should be constant under a fixed temperature and H2 partial pressure. Therefore, it is estimated that the incubation period is proportional to the inverse of specific surface area. The relationship between incubation period and specific surface area of MgH2 was investigated in order to understand the mechanism of incubation period on magnesium-hydrogen system. MgH2 with several kinds of specific surface area were manufactured by changing milling time of a planetary ball mill. A primary Gibbs free energy for dehydrogenation was conducted as −1.94 kJ mol−1 by controlling H2 pressure for measuring incubation period. The results revealed that the incubation period was proportional to the inverse of specific surface area.

Modeling dynamic recrystallization of L-605 cobalt superalloy

The evolution of the microstructure of L-605 cobalt superalloy during high-temperature deformation is investigated using mean-field models combined with a reverse-engineering method. This method provides the unknown material parameters such as mobility and nucleation frequency, and it helps to quantify the occurrence of recrystallization. The effect of alloying elements on the mobility is quantified by comparison with the pure metal. Despite the large concentration of elements, the solute-drag effect can still be properly described by the Cahn model. A comparison with experimental results sheds new light on the occurrence of joint mechanisms that are usually neglected, such as self-heating and meta-dynamic recrystallization. The inclusion of these phenomena is not significantly more complicated when compared to the basic model, but it significantly improves the predictability for large strain rate.

Profilin-Dependent Nucleation and Assembly of Actin Filaments Controls Cell Elongation in Arabidopsis.

Actin filaments in plant cells are incredibly dynamic; they undergo incessant remodeling and assembly or disassembly within seconds. These dynamic events are choreographed by a plethora of actin-binding proteins, but the exact mechanisms are poorly understood. Here, we dissect the contribution of Arabidopsis (Arabidopsis thaliana) PROFILIN1 (PRF1), a conserved actin monomer-binding protein, to actin organization and single filament dynamics during axial cell expansion of living epidermal cells. We found that reduced PRF1 levels enhanced cell and organ growth. Surprisingly, we observed that the overall frequency of nucleation events in prf1 mutants was dramatically decreased and that a subpopulation of actin filaments that assemble at high rates was reduced. To test whether profilin cooperates with plant formin proteins to execute actin nucleation and rapid filament elongation in cells, we used a pharmacological approach. Here, we used Small Molecule Inhibitor of Formin FH2 (SMIFH2), after validating its mode of action on a plant formin in vitro, and observed a reduced nucleation frequency of actin filaments in live cells. Treatment of wild-type epidermal cells with SMIFH2 mimicked the phenotype of prf1 mutants, and the nucleation frequency in prf1-2 mutant was completely insensitive to these treatments. Our data provide compelling evidence that PRF1 coordinates the stochastic dynamic properties of actin filaments by modulating formin-mediated actin nucleation and assembly during plant cell expansion.

Nanoindentation of gold and gold alloys by molecular dynamics simulation

The nanoindentation hardnesses and stacking fault energies (SFE) for pure and alloyed Au are determined from classical molecular dynamics simulations. Rather than a traditional force–displacement dependence that is examined in many previous nanoindentation works, we analyze the hardness vs. force in this study, which shows features that allow us to distinguish defect nucleation processes from hardening processes. During nanoindentation, homogeneously nucleated defects interact to form V-shape lock structures, and finally form four-sided dislocations that are continuously released into the bulk, in a manner similar to the heterogeneous Frank-Read dislocation generation mechanism. Hardness in the alloy system is predicted to be critically controlled by the ease and frequency of nucleation of new defects. Consistent with previous simulation results, the difference of the unstable and stable SFE, rather than the stable SFE along, is found to be closely related to this nucleation process, and thus to hardness.

The influence of ternary alloying elements on the Al–Si eutectic microstructure and the Si morphology

The influence of the ternary alloying elements Cu, Mg and Fe on the Al–Si eutectic microstructure is investigated using a commercial purity Al–10wt%Si alloy in unmodified and Sr-modified conditions. A change in the Al–Si eutectic microstructure was associated with a change in the nucleation density of the eutectic grains caused by the addition of ternary alloying elements. When the ternary alloying element addition resulted in an increase in the eutectic nucleation frequency, a fibrous to flake-like transition was observed within the eutectic grain. When the ternary alloying element addition decreased the eutectic nucleation frequency significantly, a change in the eutectic morphology from flake-like to a mixture of flake-like and fibrous morphologies was observed. The mechanism of Al–Si eutectic modification is discussed. The growth velocity of the eutectic grain – liquid interface and the constitutional driving force available for growth are proposed as important parameters that influence the degree of eutectic modification in Al–Si alloys.

Enhanced pool-boiling heat transfer on laser-made hydrophobic/superhydrophilic polydimethylsiloxane-silica patterned surfaces

This study presents the application of hydrophobic polydimethylsiloxane-silica coating used for the development of biphilic surfaces that are designed to enhance the heat transfer during boiling. Surface analyses showed that this coating exhibits a high hydrophobicity due to its hierarchical structure and the use of hydrophobic polymer. An appropriate thermal treatment leads to the oxidation of the methyl groups and a formation of silicon oxide and silicon carbide that result in a wettability transition from hydrophobic to superhydrophilic. On this basis, we manufactured hydrophobic/superhydrophilic patterns on stainless-steel foils using a pulsed Nd:YAG laser. The uniform, superhydrophilic surface exhibited a 350% larger critical heat flux (CHF) than bare stainless-steel foil. High-speed IR thermography revealed that the increased wettability reduced the bubble contact diameter, allowed a higher density of active nucleation sites, and delayed the dry-out. The biphilic surfaces with differently sized hydrophobic spots exhibited the highest heat transfer coefficients, with an up to 200% higher CHF compared to the bare stainless steel. Smaller hydrophobic spots reduced the bubble diameter and increased the nucleation frequency. However, surfaces with larger hydrophobic regions promoted boiling incipience and exhibited higher heat transfer coefficients at low heat fluxes. These results suggest that the optimal biphilic pattern could only be determined for a particular operating point. Our data provide a new insight into the complex phenomena of nucleate pool boiling on chemically and mechanically heterogeneous surfaces.

Fluctuation emergence of bubbles under a rapid drop of pressure in a liquid

Explosive cavitation at the front of a negative-pressure pulse has been studied. Conditions for the emergence of bubbles by the mechanism of homogeneous fluctuation nucleation were identified. Those conditions feature a high rate of the phase transformation, with the vapor formation process being concentrated in time at the instant of attainment of a certain pressure. Under such conditions, the liquid cavitation strength is maximal, and its value can be predicted by the homogeneous nucleation theory. For implementing the regime with high nucleation frequency, a method based on passing a negative-pressure pulse across a region with locally heated liquid was employed. The cavitation kinetics was examined by monitoring the perturbation of the heat flow from a miniature heater. The experimental data were generalized using the theory of explosive vapor formation in shock boiling mode. A method for calculating the cavitation in the regime of the fluctuation emergence of bubbles was approbated.

The influence of ternary Cu additions on the nucleation of eutectic grains in a hypoeutectic Al-10 wt.%Si alloy

The Influence of the ternary alloying element Cu on eutectic nucleation in an Al-10 wt.%Si alloy in unmodified and Sr-modified conditions was studied. Cu additions had a relatively minor effect on the unmodified eutectic nucleation frequency. In Sr-modified Al–Si alloys where the nucleation frequency of the eutectic grains is very low compared to the unmodified alloys, the addition of Cu significantly increased the nucleation frequency. Further increases in the Cu concentration resulted in a continuous increase in the number of eutectic grains and an associated decrease in their size. It is proposed that constitutional supercooling plays an important role in promoting the nucleation of eutectic grains ahead of the solidifying interface especially in the case of Sr-modified Al–Si alloys.

Microstructure and magnetic properties of (Nd,Dy)FeB and (Nd,Pr)FeB strips

In this paper, we investigate the effect of Dy and Pr substitution for Nd on the microstructure and the magnetic properties of Nd-Fe-B alloys fabricated by using the strip-casting process. The addition of Dy and Pr changes the melting temperature of the strips, which results in differences in undercooling during solidification. The alteration of undercooling induces variations in the nucleation frequency and the cooling rate in the strips and changes the grain size and alignment. These microstructural changes have a large effect on the coercivity of the strips. The addition of Dy improves the coercivity of the strips due to the presence of more refined and well-oriented grains, which is attributed to the high thermal gradient. The addition of Pr to strips with a low thermal gradient reduces the coercivity due to the large grain size.

Adipic Acid Primary Nucleation Kinetics from Probability Distributions in Droplet-Based Systems under Stagnant and Flow Conditions

In this work, we present a microfluidic approach that allows performing nucleation studies under different fluid dynamic conditions. We determine primary nucleation rates and nucleation kinetic parameters for adipic acid solutions by using liquid/liquid segmented flow in capillary tubes in which the crystallizing medium is partitioned into small droplets. We do so by measuring the probability of crystal presence within individual droplets under stagnant (motionless droplets) and flow (moving droplets) conditions as a function of time, droplet volume, and supersaturation. Comparing the results of the experiments with the predictions of the classical nucleation theory model and of the mononuclear nucleation mechanism model, we conclude that adipic acid nucleates mainly via a heterogeneous mechanism under both fluid dynamic conditions. Furthermore, we show that the flow conditions enhance the primary nucleation rate by increasing the kinetic parameters of the process without affecting the thermodynamic parameters. In this regard, a possible mechanism is discussed on the basis of the enhancement of the attachment frequency of nucleation caused by the internal recirculation that occurs within moving droplets.

Quantitative evaluation on the heterogeneous nucleation of amino acid by a thermodynamic analysis

Quantitative evaluation of heterogeneous nucleation is important in order to control crystal form or particle morphology. In the present work, heterogeneous nucleation was quantitatively estimated from induction time, which was defined as the period required for nucleation. The induction time of heterogeneous nucleation was thermodynamically measured with a differential scanning calorimeter. As a result, the free energy change of glycine nucleation in the supersaturated solution was calculated from the heterogeneous nucleation rate. The free energy change decreased with an increase in the occurrence frequency of heterogeneous nucleation. This method suggested that there was a possibility of making the distinction between homogeneous nucleation and heterogeneous nucleation from quiet supersaturated solution. Characteristic parameters of critical nucleus of homogeneous nucleation gave a good agreement with the experimental results. We could quantitatively predict the characteristic properties of heterogeneous nucleation introducing substance-specific parameters.