Heterogeneous Nucleation Kinetics Research Articles

THERMOPHYSICAL PROPERTIES OF HYPOEUTECTIC, EUTECTIC AND HYPEREUTECTIC Al-Si AUTOMOTIVE ALLOYS UNDER AGEING TREATMENT

Influence of different levels of silicon under a variety of thermal treatment is investigated on the thermophysical behaviour of Al-Si-Cu-Mg automotive alloys. Conventional cast alloys are subjected to T6 heat treatment for age hardening. Hardness values, thermal conductivity along with microstructural observation under different heat-treated conditions are well thought-out to comprehend the ageing performance and the precipitation behaviour of the alloys. From these investigational results it is indicated that two successive hardening peaks take place in the agedalloys. Ageing sequence consists of different phases like a supersaturated solid solution, GP zones, intermediate βʹʹ, intermetallic βʹ, equilibrium β and the Q phase but GP zones and the metastable phases are most likely responsible for these ageing peaks. Thermal conductivity decreases for these precipitates formation during ageing and increases for relieving the internal stress, dissolution of metastable phase and coarsening the precipitation of the alloys. Addingup of Si to the alloy showed earlier ageing peaks with higher intensities intended for its increasing properties of heterogeneous nucleation and diffusion kinetics. After ageing around at 200°C for four hours the alloys offer the height hardness. Microstructural study reveals that eutectic silicon makes the grain boundary coarsen and beyond the eutectic composition the star-shaped blocky primary Si is formed. Subsequent to ageing at 350oC for one hour the alloys reach completely recrystallized state.

Kinetic study on heterogeneous nucleation and incubation period during chemical vapor deposition.

The kinetics of heterogeneous nucleation during chemical vapor deposition (CVD) is still unclear despite its importance. Nucleation delay is often observed in many CVD processes, which is known as the incubation period (τi). In this study, the effects of concentration (C) and sticking probability (η) of film-forming species on τi were formulated based on our kinetic model. To discuss the kinetics, τi -1 with the rate dimension was used and formulated using C and η. Because η onto heterogeneous surfaces (ηhetero) is difficult to evaluate, the study was initiated with η onto homogeneous surfaces (ηhomo), followed by a discussion on its reasonability. The formulation was validated using the experimental dataset for SiC-CVD from CH3SiCl3/H2 onto BN underlayers because CVD involves multiple film-forming species with different ηhomo ranging from 10-6 to 10-2 and thus is a suitable system for studying the effect of ηhomo. High-aspect-ratio (1000:1) parallel-plate microchannels consisting of τi-involving BN and a τi-free Si surface were utilized to separate these film-forming species along the microchannel depth. τi was exceptionally long, up to several hours, depending on the CVD conditions. τi -1 was found to be proportional to Cn, where n is the reaction order. n was quantified as ≈1.6, suggesting the initial nucleation was triggered by the impingement of two adspecies in the second order and lowered possibly by the discrepancy between C in the gas-phase and that actually producing adspecies on the surface. τi -1 was also found to be proportional to ηhomo. The exceptionally long τi was likely originated from the significantly lower ηhetero than ηhomo and the higher activation energy for ηhetero than that for ηhomo.

Atomic insights into heterogeneous nucleation and growth kinetics of Al on TiB2 particles in undercooled Al-5Ti-1B melt

Titanium diboride (TiB2) is an effective grain refiner of Al alloys in the industry that facilitates casting processes by forming uniformly refined microstructures. Although our understanding of the underlying refinement mechanisms has advanced, the atomic kinetics of heterogeneous nucleation of Al on TiB2 remains unknown. Here, we report atomic-scale observations of the heterogeneous nucleation and growth kinetics of Al on self-formed TiB2 particles by in situ heating of undercooled Al-5Ti-1B films. We demonstrate that an ordered Al monolayer forms on the Ti-terminated {0001}TiB2 surface; then, the surrounding Al atoms are initiated to form an island-shaped Al nucleus with face-centered cubic {111} stacking without the assistance of a Ti-rich buffer layer. The interfacial lattice mismatch between {111}Al and {0001}TiB2 causes remarkable out-of-plane strain that decreases gradually with Al nucleus layers increasing to 6 atomic layers. The elastic strain energy originating from this interfacial strain increases the free energy of the Al/TiB2 heterostructure, hence impeding the rapid growth of the Al nucleus. We found that TiB2 particles stabilize the Al nuclei rather than activating their free growth into grains when the experimental undercooling ΔT is lower than the onset undercooling ΔTfgin Greer's free growth model. Our findings provide an atomic-scale physical image of the heterogeneous nucleation and growth mechanisms of Al with inoculator participation and elucidate the strain-dependent growth kinetics of Al nuclei.

Bulk Perovskite Crystal Properties Determined by Heterogeneous Nucleation and Growth

In metal halide perovskites, charge transport in the bulk of the films is influenced by trapping and release and nonradiative recombination at ionic and crystal defects. Thus, mitigating the formation of defects during the synthesis process of perovskites from precursors is required for better device performance. An in-depth understanding of the nucleation and growth mechanisms of perovskite layers is crucial for the successful solution processing of organic-inorganic perovskite thin films for optoelectronic applications. In particular, heterogeneous nucleation, which occurs at the interface, must be understood in detail, as it has an effect on the bulk properties of perovskites. This review presents a detailed discussion on the controlled nucleation and growth kinetics of interfacial perovskite crystal growth. Heterogeneous nucleation kinetics can be controlled by modifying the perovskite solution and the interfacial properties of perovskites adjacent to the underlaying layer and to the air interface. As factors influencing the nucleation kinetics, the effects of surface energy, interfacial engineering, polymer additives, solution concentration, antisolvents, and temperature are discussed. The importance of the nucleation and crystal growth of single-crystal, nanocrystal, and quasi-two-dimensional perovskites is also discussed with respect to the crystallographic orientation.

On Thermodynamics and Kinetics of Interface‐Induced Crystallization in Polymers

AbstractCrystallization initiated at interfaces is a ubiquitous phenomenon, covering both simple liquids and polymer melts. Understanding the physical process underlying interface‐induced crystallization in polymers is of fundamental interest and is relevant for many applications, especially for films of functional semicrystalline polymers. Interface‐induced crystallization of liquids can proceed either by heterogeneous nucleation or by the less explored equilibrium phenomenon of prefreezing. This study reports recent progress in the theoretical and experimental investigation of the effect of substrate‐material interactions on the thermodynamic properties of prefreezing and on the kinetics of heterogeneous nucleation in model polymers on different flat substrates. Through a combination of various microscopy and scattering techniques, supported by theoretical analysis, a detailed picture of the two different phenomena of interface‐induced crystallization in polymers is achieved.

The roles of nucleation and growth kinetics in determining Li metal morphology for Li metal batteries: columnar versus spherical growth

The formation mechanism of columnar and spherical Li metal is demonstrated in terms of competition among the kinetics of heterogeneous Li nucleation on the substrate, Li growth, and homogeneous Li nucleation on Li metal.

Effect of oscillatory flow conditions on crystalliser fouling investigated through non-invasive imaging

Conditions that lead to undesired fouling events in oscillatory flow crystallisers are investigated. The moving fluid oscillatory baffled crystalliser is used to mimic the operating conditions of continuous oscillatory baffled crystallisers, while reducing materials and energy consumption. A non-invasive imaging method is deployed to determine fouling induction times as a function of oscillatory flow conditions and supersaturation in crystallisation of glycine and L-glutamic acid from aqueous solutions. Heterogeneous nucleation kinetics are extracted from the distribution of fouling induction times, showing that higher fouling nucleation rates are observed when the frequency and amplitude of the oscillatory flow are increased. Higher oscillatory Reynolds numbers result in increased fluid shear in the crystalliser, promoting heterogeneous nucleation at the glass-solution interface and leading to subsequent fouling. It is therefore essential to consider the dependence of fouling kinetics on operating conditions to enable rational design of continuous crystallisers that ensure smooth and robust operation.

Numerical investigation of barite scaling kinetics in fractures

Barite stands out as one of the most ubiquitous scaling agents in deep geothermal systems, responsible for irreversible efficiency loss. Due to complex parameter interplay, it is imperative to utilise numerical simulations to investigate temporal and spatial precipitation effects. A one-dimensional reactive transport model is set up with heterogeneous nucleation and crystal growth kinetics. In line with geothermal systems in the North German Basin, the following parameters are considered in a sensitivity analysis: temperature (25 to 150 °C), pore pressure (10 to 50 MPa), fracture aperture (10−4 to 10−2 m), flow velocity (10−3 to 100 m s−1), molar volume (50.3 to 55.6 cm3 mol−1), contact angle for heterogeneous nucleation (0° to 180°), interfacial tension (0.07 to 0.134 J m−2), salinity (0.1 to 1.5 mol kgw−1 NaCl), pH (5 to 7), and supersaturation ratio (1 to 30). Nucleation and consequently crystal growth can only begin if the threshold supersaturation is exceeded, therefore contact angle and interfacial tension are the most sensitive in terms of precipitation kinetics. If nucleation has occurred, crystal growth becomes the dominant process, which is mainly controlled by fracture aperture. Results show that fracture sealing takes place within months (median 33 days) and the affected range can be on the order of tens of metres (median 10 m). The presented models suggest that barite scaling must be recognised as a serious threat if the supersaturation threshold is exceeded, in which case, large fracture apertures could help to minimise kinetic rates. The models further are of use for adjusting the fluid injection temperature.

Development and validation of a semi-analytical framework for droplet freezing with heterogeneous nucleation and non-linear interface kinetics

We present a novel, semi-analytical framework which accurately predicts every stage of droplet freezing. Supercooling degree of the freezing droplet is computed by coupling the 1-D transient heat conduction equation in spherical coordinates with a modified heterogeneous nucleation model. The coupling connects atomic-scale thermodynamic properties with bulk thermophysical properties and yields interesting insights into the freezing process. The study also develops a novel dendritic growth model for crystal propagation that takes into account the effects of non-linear interface kinetics and surface curvature. This non-equilibrium phase-change formulation during the recalescence stage is coupled with quasi-steady phase change approximation using perturbation series for equilibrium freezing stage to yield an accurate closed form solution of freezing dynamics. The temperature curves, nucleation rates, dendritic growth velocities and mass fraction evolution from the model are validated or verified with the available experimental or numerical data in the literature. The analytical framework developed is a reduced-order freezing model that is fast to compute and yields accurate results. The framework presented can be useful as a subgrid model for high-fidelity crystal growth and spray freezing simulations.

Modeling Crystallization Processes of a Nanomodified Binary Alloy

A mathematical model of nonequilibrium crystallization of a binary aluminum alloy (Al-Si) with modifying refractory nanoparticles, which are the centers of nucleation of the crystalline phase, is proposed. The model describes thermodynamic processes, as well as heterogeneous nucleation and crystallization of the α-component and the β-component of a melt. The crystalline phase nucleates on the surface of nanoseeds during the undercooling of a melt. The liquidus temperature in the melt depends on the concentration of the dissolved alloying component, which is determined from the equation of the nonequilibrium lever. The α-component of the alloy crystallizes upon cooling the metal to the eutectic temperature, and the β-component of the alloy eutectically crystallizes upon further cooling. The growth rate of the crystalline phase is proportional to the undercooling. The solid volume formed around the nucleus characterizes the size of the grain structure in the solidified alloy. The numerical simulation of the solidification of the melt in a cylindrical mold is carried out. The heat transfer parameters of the melt–mold system with the environment are determined as a result of the experiments. The kinetics of heterogeneous nucleation and crystallization of the cooling melt are considered. It is found that the nucleation conditions, crystallization rate, undercooling, and solidification time significantly differ within the molding. The results lead to the conclusion that volume-sequential crystallization of the metal takes place as the melt cools. The region with the finest structure of the solidified metal is located near the mold’s wall. The size evaluation of the grain structure in the molding is in good agreement with the experimental results. The reliability of the proposed model is confirmed by comparing the results of the numerical calculations with the data of the physical experiment on the measuring temperature during solidification of the melt and studying the properties of the molding.

Molecular insight into the interfacial chemical functionalities regulating heterogeneous calcium-arsenate nucleation

Heterogeneous nucleation induced by natural organic matter (NOM) can lower the energy barrier for calcium arsenate (Ca-As) precipitation, which aids in immobilizing arsenate (AsⅤ). However, it remains unclear how certain chemical functionalities of NOM affect Ca-As nucleation at the molecular scale. By analyzing changes in the local supersaturation and/or interfacial energy, the present work investigates the Ca-As heterogeneous nucleation kinetics and mechanisms on functional-group–modified model surfaces.Mica surfaces modified by functional groups of amine (NH2), hydroxyl (OH), or carboxyl (COOH) through self-assembled monolayers were used to investigate how chemical functionalities affect the Ca-As heterogeneous nucleation, in which the distributions of formation kinetics and size (as measured by the change in particle height) of nucleated Ca-As particles were measured by using in situ atomic force microscopy. In a parallel analysis, a quartz-crystal microbalance with dissipation was used to detect the buildup of Ca2+ and/or HAsO42− ions at the solid–fluid interface. PeakForce quantitative nanomechanical mapping and dynamic force spectroscopy using functional-group–modified tips made it possible to calculate the binding energies holding functional groups to Ca-As particles. Nucleated Ca-As particles were characterized by using Raman spectroscopy and high-resolution transmission electron microscopy.The results indicate that the height of amorphous Ca-As particles formed on a modified mica surface may be ranked in descending order as NH2 > OH > bare mica > COOH, as determined by the buildup of Ca2+ and HAsO42− ions at the solid–fluid interface and the decrease of interfacial energy due to the functional groups. These nanoscale observations and molecular-scale determinations improve our understanding of the roles played by chemical functionalities on NOM in immobilizing dissolved As through heterogeneous nucleation in soil and water.

Numerical simulation of solidification of an iron-based binary alloy modified with nanoscale particles

A mathematical model of solidification of an iron-based binary alloy (Fe-C) modified with nanosize high-refractory particles is proposed. The processes of alloy heterogeneous nucleation on the surface of nanosize particles and those of alloy crystallization in a cylindrical mold are described. For liquidus temperature, a linear approximation of the concentration of dissolved carbon is adopted; the law of change of this temperature obeys the non-equilibrium lever rule. The volume of the solid phase formed around a nucleus during the crystallization determines the characteristic size of the grain structure in the solidified alloy. Numerical simulation of melt solidification in a cylindrical crucible was carried out, and the kinetics of heterogeneous nucleation and growth of the solid phase was analyzed. It was found that the conditions for nucleation and crystallization differ substantially within the casting. It is shown that, as the melt undergoes cooling, volume-sequential crystallization gets established.

Numerical simulation of nucleation and solid phase growth in a nanomodified binary alloy

A mathematical model of solidification of a binary alloy with eutectic-type phase diagram modified by refractory nanoscale particles as nucleation centers of the crystalline phase is proposed. Thermodynamic processes in the melt and solid phase, heterogeneous nucleation and growth of particles of the main (α) phase of the crystallizing material are described. The crystal phase origin occurs on the surface of nanoscale particles. The volume of the solid phase grown on separate crystallization centers determines the character of the grain structure in the solidified alloy. On the example of a binary alloy based on iron (Fe-C) numerical simulation of melt solidification in a cylindrical casting is carried out. The features of the kinetics of heterogeneous nucleation and crystallization in the melt were considered. It is determined that the undercooling magnitudes, the conditions of nucleation, the rate of crystallization and solidification time vary significantly inside the casting. It is established that as the melt cools, volume-sequential crystallization takes place. The area with the finest structure of the solidified metal is located near the wall of the mold.

Spatially Mapping Heterogeneous Nucleation Kinetics of Silver Nanocrystals with Liquid Cell Scanning Transmission Electron Microscopy

Spatially Mapping Heterogeneous Nucleation Kinetics of Silver Nanocrystals with Liquid Cell Scanning Transmission Electron Microscopy

Nanoscale Mapping of Nonuniform Heterogeneous Nucleation Kinetics Mediated by Surface Chemistry.

Nucleation underlies the formation of many liquid-phase synthetic and natural materials with applications in materials chemistry, geochemistry, biophysics, and structural biology. Most liquid-phase nucleation processes are heterogeneous, occurring at specific nucleation sites at a solid-liquid interface; however, the chemical and topographical identity of these nucleation sites and how nucleation kinetics vary from site-to-site remain mysterious. Here we utilize in situ liquid cell electron microscopy to unveil counterintuitive nanoscale nonuniformities in heterogeneous nucleation kinetics on a macroscopically uniform solid-liquid interface. Time-resolved in situ electron microscopy imaging of silver nanoparticle nucleation at a water-silicon nitride interface showed apparently randomly located nucleation events at the interface. However, nanometric maps of local nucleation kinetics uncovered nanoscale interfacial domains with either slow or rapid nucleation. Interestingly, the interfacial domains vanished at high supersaturation ratio, giving way to rapid spatially uniform nucleation kinetics. Atomic force microscopy and nanoparticle labeling experiments revealed a topographically flat, chemically heterogeneous interface with nanoscale interfacial domains of functional groups similar in size to those observed in the nanometric nucleation maps. These results, along with a semiquantitative nucleation model, indicate that a chemically nonuniform interface presenting different free energy barriers to heterogeneous nucleation underlies our observations of nonuniform nucleation kinetics. Overall, our results introduce a new imaging modality, nanometric nucleation mapping, and provide important new insights into the impact of surface chemistry on microscopic spatial variations in heterogeneous nucleation kinetics that have not been previously observed.

Analysis of MnS Inclusions Formation in Resulphurised Steel via Modeling and Experiments.

Controlling the formation of MnS inclusions during solidification influences the mechanical properties and machinability of the resulfurized steel. A coupled segregation–nucleation–growth model was developed by the finite-difference method involving solute redistribution, heterogeneous nucleation and growth kinetics. Laboratory solidification experiments were performed under various cooling rates in resulphurised 49MnVS steel. In this work, the influence of cooling rate on solute redistribution and growth size of MnS inclusions were simulated using the current coupled model, and the calculated results can provide a valuable reference for MnS formation. Increasing of the cooling rate led to early precipitation and refinement of formed MnS inclusions. Based on the simulation results and experimental data, mathematical relationships between the growing size of MnS with the cooling rate in the low ductility temperature region and in the whole solidification were obtained.

Surface Wetting Controls Calcium Carbonate Crystallization Kinetics

Because of the widespread presence of foreign substrates in natural settings, mineral precipitation usually occurs via heterogeneous nucleation. This process is controlled by the interplay between the fluid supersaturation and interfacial energies present between the fluid, nucleus, and substrate. Among a number of physicochemical parameters, the surface wetting properties have been shown to be a key parameter controlling heterogeneous nucleation. The present study aims at elucidating the pathway and kinetics of CaCO3 heterogeneous nucleation on a set of phlogopite micas with and without fluorine/hydroxyl substitutions, yielding substrates with contrasting hydrophilicity. Our results show that, irrespective of surface wetting properties, amorphous calcium carbonate (ACC) is formed during the early stages. The surface wetting properties have a strong effect on the crystallization kinetics: ACC precipitates persist longer on the hydrophilic (hydroxylated) surface than on the less hydrophilic (fluorinated) one. We show that this stabilization could have a thermodynamic origin because of the lower interfacial free energy between the hydrated amorphous precursor and the hydrophilic substrate. These results are highly relevant for biomineralization studies, where differences in wetting properties of organic moieties present in calcifying organisms could be used to accelerate or decelerate the crystallization of the initially formed amorphous precursor phase.

Homogeneous and Heterogeneous Crystal Nucleation in Glass of the Li2O–SiO2 System

The results of studying the kinetics of homogeneous (spontaneous) and heterogeneous (catalyzed) crystal nucleation in silicate glass is presented for glasses in the Li2O–SiO2 system. Nucleation is catalyzed via the photosensitive mechanism by adding poorly soluble impurities, shifting the glass composition towards the higher content of one of the components (autocatalysis), and by passing steam through the glass melt. The fundamental characteristics of crystal nucleation are obtained. The composition ranges with the maximum and minimum steady-state crystal nucleation rates are identified.

Heterogeneous nucleation and grain growth of inoculated aluminium alloys: An integrated study by in-situ X-radiography and numerical modelling

A comprehensive study on the heterogeneous nucleation and grain growth of Al-10 wt.%Cu alloys inoculated with Al-5Ti-1B was carried out. To further reveal the solute segregation stifling mechanism, in-situ near-isothermal melt solidification experiments with constant cooling rates and greatly suppressed melt convection were realized by in-situ microfocus X-radiography study. The kinetics of heterogeneous nucleation and grain growth under the isolated influence of cooling rate and addition level of inoculant particles has been quantitatively studied. Moreover, novel image processing and analysis approaches have been proposed, to determine the maximum nucleation undercooling and solid volume fraction at nucleation ceasing. To better understand the heterogeneous nucleation and grain growth behaviors under the in-situ experiment conditions, a new grain size prediction model in which both pure globular growth kinetics and dendritic growth kinetics including spherical/globular to dendritic transition (GDT) has been developed. The quantitative agreements between the simulation results and experimental results in terms of grain size, maximum nucleation undercooling and solid fraction at nucleation ceasing, have confirmed the validity of the solute segregation stifling mechanism for castings without recalescence. Furthermore, it is demonstrated that globular growth kinetics is an acceptable approximation for grain size prediction purposes of well grain-refined aluminum alloys. However, for poorly inoculated aluminum alloys with well-developed coarse dendritic grains, an application of dendritic growth kinetics significantly improves the grain size prediction power of the model.

Anomalous Temperature Dependence of Isotactic Polypropylene α-on-β Cross-Nucleation Kinetics

A particular kind of heterogeneous nucleation, i.e., cross-nucleation, is sometimes observed in polymorphic substances, when a new crystal structure nucleates on the surface of a crystal of a different modification. Here we show a unique and apparently incongruous nucleation behavior in polymorphic isotactic polypropylene (i-PP). The rate of cross-nucleation of the monoclinic α-phase on the trigonal β-phase crystals increases with increasing temperature, in the vicinity of the α-crystals melting point. This behavior is contrary to that of the heterogeneous nucleation kinetics of the same crystal on various solid substrates, and also to the previously reported cases of cross-nucleation rate of other polymorphic systems, both exhibiting the expected decrease with temperature in the same range of undercoolings. i-PP α-on-β cross-nucleation apparently eludes the nucleation theory. The results are explained as a manifestation of a kinetic competition between α-on-β cross-nucleation and growth of β-crystalline ...