Heterogeneous Nucleation Mechanism Research Articles

Facile Characterization of Isothermal Crystallization and Microphase Separation Kinetics of Polyamide 6 (PA6)‐Based Thermoplastic Elastomers

AbstractThe facile characterization of isothermal microphase separation kinetics in polyamide 6 (PA6)‐based thermoplastic elastomers (TPAE‐6) has long posed a challenge for the development of suitable melt spinning processes. In this study, this challenge is addressed through differential scanning calorimetry (DSC) measurements. It is assumed that the enthalpy changes of TPAE‐6 during the isothermal process are a linear superposition of enthalpy changes associated with microphase separation and crystallization of PA6 in hard phases, resembling that of TPAE‐6 without soft segments (TPAE‐6‐0). The study reveals that, as the concentration of soft segments in TPAE‐6 increases, the accelerated dynamics of phase separation become stronger than the dilution of soft segments to PA6 segments during isothermal process, resulting in an increase in the microphase separation rate of TPAE‐6. Furthermore, despite microphase separation, the overall crystallization rate of TPAE‐6 decreases with rising isothermal temperature and varies with increasing soft segment content at different temperatures. Additionally, the crystallization mode of TPAE‐6 follows two‐dimensional, three‐dimensional, or a combination of both crystal growth mechanisms, accompanied by a heterogeneous nucleation mechanism.

A new heterogeneous nucleation mechanism of AM30 alloy refined by carbon inoculation

The AM30 alloy experiences remarkable grain refinement through carbon inoculation, resulting in a significant reduction of grain size from 623±21μm to 82 ± 2 μm. To delve into the influence of Mn on carbon inoculation in the presence of Ca, advanced techniques such as EDS, FIB, and TEM were utilized. This study reports the initial discovery of a unique duplex-phase particle characterized by an amorphous carbon layer coating on the Al8Mn5 phase. Notably, the amorphous layer exhibits a thickness of approximately 100 nm. These distinctive duplex-phase particles are hypothesized to serve as effective nuclei for Mg grains. Supported by epitaxial growth theory and experimental data, these particles exhibit remarkable nucleation efficiency.

First-principles investigation on adsorption poisoning and heterogeneous nucleation mechanisms in Ca modified primary Mg2Si phase

We employed first-principles calculations to investigate the adsorption poisoning and heteroepitaxial nucleation mechanisms in the Ca modification of the primary Mg2Si phase. The extensive computations considering various factors such as lattice mismatch, interface energy, adsorption energy, charge density, work of adhesion, and interface mechanical properties, as well as comparisons of interface stacking modes reveal that Ca modification of the primary Mg2Si phase can occur through both adsorption poisoning and heteroepitaxial nucleation mechanisms simultaneously. The adsorption poisoning mechanism implies that Ca preferentially adsorbs on the Mg-terminated surfaces of Mg2Si(100) and Mg2Si(111). Meanwhile, the heteroepitaxial nucleation mechanism indicates the nucleation interface of Mg2Si(100)//CaSi2(001), where the Mg2Si(100) surface is terminated by Si, and the CaSi2(001) surface is terminated by Ca, with stacking modes of Si-Top and Si-Center. In addition, the inclusion of Ca in the primary Mg2Si phase through heteroepitaxial nucleation not only leads to refinement of the primary Mg2Si phase but also displays excellent interface mechanical properties, which suggests that Ca can be considered as an optimal modifier for the primary Mg2Si phase. This study provides a fundamental insight into the role of Ca in modifying the primary Mg2Si phase and has potential implications for the development of Si-containing magnesium alloys.

Mechanisms of Heterogeneous Nanoscopic Ice Nucleation

Mechanisms of Heterogeneous Nanoscopic Ice Nucleation

Inhomogeneity of microstructure evolution in stir zone of 2195Al thick-plate FSW joints

An appropriate friction stir welding (FSW) tool has been developed to perform high-quality joining of 12 mm thick Al-Li alloy plates. Threads and triple facets were machined on the stirring pin to improve the material flow. The differences in microstructure characteristics, recrystallization mechanisms, and precipitate evolution along the stir zone (SZ) thickness were systematically clarified. The results indicated that microstructures in SZ were refined by dynamic recrystallization (DRX) and showed an apparent top-to-bottom gradient distribution in grain size. The geometric topology of the stirring pin results in periodic ultrafine-grained bands throughout SZ. At the top and middle regions of SZ, high-density vacancies and dislocations were formed due to elevated temperature and strain rate, facilitating the precipitation of T1 (Al2CuLi) phases through a dislocation-induced heterogeneous nucleation mechanism. In comparison, icosahedral quasi-crystalline T2 (Al6Cu(Li, Mg)3) phases, which can be attributed to the segregation of atoms and high cooling rate, were detected along grain boundaries at SZ-bottom. Furthermore, the ultimate strength values of tensile specimens sectioned from the top, middle, and bottom parts of SZ are 452, 457, and 406 MPa, respectively. Studying the inhomogeneous microstructure and properties of welded joints provides a theoretical basis for manufacturing high-load components.

Heterogeneous Nucleation Mechanism of Potassium Iodide on Graphene Surface in Water

In this work, molecular dynamic (MD) simulations are applied to investigate the heterogeneous nucleation mechanism of KI on a graphene surface in water. As graphene is immersed in water, it mainly affects the structure of interfacial water (the topmost water layer at the interface between the substance and water). To maximize the hydrogen bonding of water, the dissolved solutes tend to accumulate to form the aggregate at the graphene surface, which undoubtedly affects the nucleation pathways of solutes in water. In comparison with homogeneous nucleation, a lower barrier may be expected during the heterogeneous nucleation of KI on a graphene surface in water. Therefore, as the graphene is immersed in water, this facilitates solute nucleation. From this work, it may be derived that heterogeneous nucleation may be closely related to the geometric characteristics of foreign surfaces, especially their geometric shape.

Heterogeneous nucleation of CO2 hydrate: A thermodynamics analysis considering effects of wall characteristics and solution activity

The massive emission of greenhouse gas CO2 has significant effects on global climate and environment, causing worldwide concern. It has been proposed to store CO2 in clathrate hydrates as a possible strategy for reducing atmospheric CO2 emission. Nevertheless, the CO2 hydrate formation rate is slow and its associated heterogeneous nucleation mechanisms are not well understood, which limit commercial applications of this technology. This study developed a thermodynamic model based on the change in availability function approach, considering effects of surface characteristics (including surface topography, roughness, curvature radius, and wettability), and CO2 dissolution on the solution's activity. The critical nucleation radius and energy barrier of CO2 hydrate nucleation are determined. Effects of pressure, temperature, wall characteristics and solution activity on the nucleation radius of CO2 hydrate are discussed. The results of this paper contribute to advancing carbon capture and storage technology based on hydrate methods.

Heterogeneous nucleation mechanisms in Mg(0001)/Al3BC(0001) interfaces: Insights for advanced Mg-based composites

In pursuit of establishing a theoretical framework for the Mg(0001)/Al3BC(0001) interface structure and revealing the heterogeneous nucleation mechanism of Al3BC particles within α-Mg-based composite materials, thus the adhesion work, interfacial energy and electronic structure of pristine and doped Mg(0001)/Al3BC(0001) interfaces are fully explored. The results shows that the B-terminated HCP2 stacking configuration at the Mg(0001)/Al3BC(0001) interface exhibiting the highest working adhesion and the lowest interfacial energy. These observations underscore the prominent covalent bond character between Mg-p and B-p orbitals, indicative of robust interfacial bonding strength and stability. Consequently, our research verified the potential of Al3BC particles as phase for heterogeneous nucleation of α-Mg grains. Moreover, The introduction of Al, Ca, and Ti at the interface not only significantly enhances working adhesion but also concurrently reduces the interfacial energy of the B-terminated HCP2 stacking Mg(0001)/Al3BC(0001) interface. This intriguing phenomenon actively promotes the nucleation of the Mg matrix. However, it is imperative to mention that the introduce of Sn and Zn at the interface results in an increase in interfacial energy, which proves unfavorable for the nucleation of Al3BC on the Mg matrix. Regarding the binding energy at the doped Mg(0001)/Al3BC(0001) interface, we found a regularity among the elements, ranked as follows: Ti > Al > Ca > Zn > Sn. These findings significantly enrich our comprehension of the distinctive characteristics of Mg(0001)/Al3BC(0001) interfaces and the underlying nucleation processes. These insights hold substantial promise for advancing the development of Mg-based composites.

Divergence among Local Structure, Dynamics, and Nucleation Outcome in Heterogeneous Nucleation of Close-Packed Crystals.

Heterogeneous crystal nucleation is the dominant mechanism of crystallization in most systems, yet its underlying physics remains an enigma. While emergent interfacial crystalline order precedes heterogeneous nucleation, its importance in the nucleation mechanism is unclear. Here, we use path sampling simulations of two model systems to demonstrate that crystalline order in its traditional sense is not predictive of the outcome of the heterogeneous nucleation of close-packed crystals. Consequently, structure-based collective variables (CVs) that reliably describe homogeneous nucleation can be poor descriptors of heterogeneous nucleation. This divergence between structure and nucleation outcome is accompanied by an intriguing dynamical anomaly, wherein low-coordinated crystalline particles outpace their liquid-like counterparts. We use committor analysis, high-throughput screening, and machine learning to devise CV optimization strategies and present suitable structural heuristics within the metastable fluid for CV prescreening. Employing such optimized CVs is pivotal for properly characterizing the mechanism of heterogeneous nucleation in metallic and colloidal systems.

A capillary-induced negative pressure is able to initiate heterogeneous cavitation.

A capillarity-induced negative pressure is of general importance for understanding the phase behaviors of liquids in small pores and cracks. A unique example is the embolism in the xylem of plants and the cavitation at the limiting negative pressure generated by evaporation of water from nanocapillaries in the cell walls of leaves. In this work, by combining the effect of a capillary and cavitation together, we demonstrate with molecular dynamics (MD) simulations that capillarity is able to induce spontaneous cavitation in the presence of hydrophobic heterogeneities. Our simulation results reveal separately how the capillary generates a negative pressure and how the generated negative pressure affects the onset of cavitation. We then interpret the cavitation mechanism and determine the occurrence of cavitation as a function of the hydrophobicity of the nucleating substrates where the cavitation initiates and as a function of the hydrophilicity of the capillary tube from which the negative pressure generates. Our results reveal that the capillary-induced cavitation can be described well with a heterogeneous nucleation mechanism, within the framework of classical nucleation theory.

Size-Independent Nucleation and Growth Model of Potassium Sulfate from Supersaturated Solution Produced by Stirred Crystallization.

This paper explores the kinetics of the crystallization of potassium sulfate in a stirred bed crystallizer through experimental investigation. Employing classical nucleation theory, the homogeneous and heterogeneous nucleation mechanisms of potassium sulfate were investigated. The induction time and critical nucleation parameters, including the surface tension (γ), critical nucleation radius (r*), critical nucleation free energy (ΔG*), and critical nucleation molecule number (i*), were meticulously determined under varying temperatures and supersaturation ratios. The experimental findings revealed that as the temperature and supersaturation ratio increased, the induction time, critical nucleation free energy, critical nucleation radius, and critical molecule number decreased whereas the nucleation rate increased. The crystalline shape remains relatively unaltered with respect to temperature and supersaturation ratio, yet the particle size (D10, D50, D90) increases as the supersaturation and temperature increase. The variations in the measured nucleation parameters align well with the predictions of classical nucleation theory. Furthermore, the kinetic equations of crystal nucleation and the growth rate in a stirred crystallization system were fitted using population balance equations. The results demonstrate that the growth rate increases with increasing supersaturation and stirring rates. Additionally, the effects of the parameters in the nucleation rate equation suggested that the suspension density exerted the greatest influence, followed by the supersaturation ratio and stirring rate. This extensive research provides invaluable theoretical guidance for optimizing the crystallization process and designing industrial crystallizers.

Sonoelectrochemical system mechanisms, design, and machine learning for predicting degradation kinetic constants of pharmaceutical pollutants

This study explores the mechanism, design, and application of machine learning in the sonoelectrochemical (US-EC) systems and select ibuprofen (IBP) as the target pollutant. Mechanism investigation shows that OH and S O4·- are the primary chemical oxidation species through scavenger experiments. Based on the heterogeneous nucleation mechanism, the electrodes in the ultrasound (US) system play the role of electrode-sonocataliytic and can promote the degradation kinetic constant of pharmaceutical pollutants. The effects of design parameters, including US frequency, voltage, electrolyte, electrode area, gap, and position on IBP degradation were investigated. The results demonstrated that under optimized parameters: US frequency of 35 kHz, voltage of 5 V, 0.1 M Na2SO4 electrolyte, 1.5 cm gap, and placement at P8, the US-EC system achieved a kinetic constant of 0.016 min−1. Chemiluminescence was used to visualize the spatial distribution of the oxidant, and provided theoretical support for mechanism and design parameter optimization. The eXtreme Gradient Boosting model was used to predict the kinetic constant of pharmaceutical contaminants including IBP, indicating excellent model performance with results of R2 and RMSE reaching 0.98 and 0.0005, respectively. SHapley Additive exPlanations was employed to assess the impact of design parameters on pharmaceutical pollutants degradation. The results showed that US frequency, US power, and the distance 'r' from the US transmitter to the anode have the most significant impact on the prediction performance of the model. Two sets of new experiments were verified using this model, and the prediction accuracy reached 76% and 82% respectively, demonstrating that machine learning can effectively predict the kinetic constants of pharmaceutical contaminants under complex factors affecting the US-EC system, assisting researchers in swiftly evaluating the system's pollutant treatment performance and simplifying the experimental workload.

The improved mechanical properties of Mg–Zn-Nd alloy with Ca addition via grain refinement and nano precipitates

Micro-alloying of Ca in Mg–1Zn-0.3Nd (wt.%) alloy is found to significantly improve tensile strength and process high ductility simultaneously. It closely correlates with grain refinement and the increased coherent nano Mg2Ca and (Mg, Zn)3Nd precipitates. Ca addition enhances alloying elements co-segregation at grain boundary and promotes grain refinement. Furthermore, nano Mg2Ca particles induced by Ca addition can significantly promote the precipitation of (Mg, Zn)3Nd phase via the heterogeneous nucleation mechanism owing to their excellent coherency relationship and ultimately lead to the pronounced strengthening effect. In addition, both fine grains and the coherent characteristics between Mg2Ca, (Mg, Zn)3Nd, and Mg matrix facilitate strain coordination during deformation and yield relatively high ductility.

Partially melted powder in laser based directed energy deposition: Formation mechanism and its influence on microstructure

The powder–melt pool interaction behavior is crucial in laser-based directed energy deposition (LDED). Partially melted particles, which are formed as a result of this interaction, significantly influence on the microstructure and mechanical performance of multi-material and metal-matrix composites fabricated via LDED. However, the presence of partially melted particles is a contentious issue that has been overlooked in single-material LDED studies. Furthermore, the investigation of partially melted particles is hindered by the difficulties in direct observation. To overcome this obstacle, this study was conducted using a single-bead Ti–6Al–4V printing experiment with a relatively high oxygen content to distinguish partially melted particles directly. The formation mechanism of the partially melted particles was revealed through experimental studies combined with numerical analysis using a self-established model. Additionally, the influence of partially melted particles on the grain structure of LDED–fabricated parts was investigated in a low–oxygen environment. The partially melted particles tend to survive close to the surface of the deposited layer. As the penetration depth increased, the particle size decreased and the aspect ratio increased. The formation of partially melted particles collectively depends on the laser power, scanning velocity, powder size and powder feed speed, differing from the common conclusion that an insufficient input energy results in poor powder melting behavior. Furthermore, a Ti–6Al–4V sample with high–fraction equiaxed grains was fabricated using optimized processing conditions. The partially melted particles significantly affected the solidification behavior. In addition to the heterogeneous nucleation mechanism caused by the partially melted particles, a novel seed crystal mechanism was proposed to support the abnormal formation of equiaxed grains. This study highlights the importance of partially melted particles in LDED, and provides useful insights into in-situ microstructural control in LDED.

Energy assessment and thermodynamic evolution of a novel semi-clathrate hydrate cold storage system with internally circulating gas bubble disturbance

Low energy storage density, intermittent phase changes, and heat transfer barriers have posed significant challenges in the implementation of hydrate energy storage systems. Based on the heterogeneous nucleation mechanism for tetrabutylammonium bromide (TBAB) hydrate phase change energy storage, a novel cold storage system with internally circulating gas disturbance was constructed for energy evaluation and thermodynamic evolution. The hydrate cold storage efficiency, response time, dynamic driving force, unique force pattern of the coils and guest molecule diffusion were analyzed for the first time. The bubbles generated by gas internal circulation provided numerous nucleation sites for hydrate formation, efficiently promoting the energy storage process. The disturbances caused by the bubbles also rapidly transmitted the energy generated by the phase change. A hydrate storage density of 57.4 kWh/m3 for this system was the maximum among known hydrate storage systems. Notably, a change in solution concentration (from 10 wt% to 40 wt%) resulted in a 2–3-fold increase in the cold charge capacity. Rapid dilution of local solutions and rapid release of cold were the ways to promote the cold discharge. Less damage to the stress structure of the coil was due to the loose accumulation of hydrate particles under gas disturbance. The microbubbles generated by the gas disturbance enabled heterogeneous nucleation and heat-mass transfer in the hydrate cold storage. The gas disturbance-based hydrate energy storage process holds significant guiding value for various applications such as refrigerated transport, building cooling systems, and grid peak shaving.

The effect of ZnO:Si3N4 nanocomposite interlayer on electrical properties and interface-states between metal-semiconductor in Al/p-Si (MS) structures

In the study, the fabrication processes of the Al/p-Si (MS), Al/ZnO/p-Si (MIS1), and Al/(ZnO:Si3N4)/p-Si (MIS2) structures were described and the effect of the ZnO:Si3N4 interface layer in MS was investigated in detail. The ZnO and (ZnO:Si3N4) thin films were coated on p-Si substrates and annealed at 500 °C for 1 h. Si3N4 and ZnO phases were identified via X-ray diffraction (XRD) and Raman spectra. FESEM images demonstrated that ZnO nanorods grew on Si3N4 particles with a heterogeneous nucleation mechanism. The I–V characterization was performed in the dark (±4 V). The basic diode parameters (barrier height (BH), series/shunt (Rs/Rsh) resistances, rectifying rate (RR=If/Ir), and ideality factor (n)) were calculated from various methods. The RR, and Rsh values increased, but n, Rs and BH decreased by insulating layer in the MIS2 The energy-dependent profiles of interface-states (Nss) were also extracted from the I–V plots according to voltage-dependent n and BH. The MIS2 provided the best passivation effect with decreasing interface traps.

A review of nanoparticles and their composites as pour point depressant for waxy crude oil

Adding pour point depressant is an important method to solve the poor low temperature rheology of waxy crude oil. Although the traditional polymer pour point depressant has good improvement effect, it has some problems, such as poor resistance to repeated heating and shear. Therefore, nanoparticles are widely used to improve the rheology of crude oil. Although nanoparticles have a certain influence on the improvement of low-temperature rheology, and nanocomposite particles have better improvement effect than single polymer pour point depressant, no one has combed and summarized the influence mechanism of nanoparticles and their composites. This paper introduces three kinds of nanoparticles and their composite materials, briefly describes the performance of the influence of various nanoparticles and their composite particles on the rheology of waxy crude oil, and analyzes the mechanism of their influence on the rheology of waxy crude oil. The analysis of this paper shows that the influence and mechanism of individual nanoparticles on the rheology of crude oil are different due to their different properties. All nanocomposite particles meet the heterogeneous nucleation mechanism, and the polar group of nanocomposite particles is an important factor affecting the effect of nanocomposite particles. Carbon based nanoparticles, especially graphene oxide, promise important research prospect.

A theoretical model on unfrozen water content in soils and verification

Phase transition of pore water is a fundamental reason for frost heave in cold regions. Therefore, studying the variation in unfrozen water content is of great significance for understanding the freezing process of pore water and revealing the freezing mechanism in soils. In this study, a prediction model for unfrozen water content was established based on a probabilistic ice-forming model. To determine the heterogeneous nucleation rate, which is the key variation in the probabilistic model, the relationship between the water film thickness and temperature variation in the soil was characterized based on the improved premelting theory. In this process, the equivalent particle size and density of impurities on the surface of the particles were determined by introducing the particle size distribution curve and average accumulation mode of the particles. Based on the heterogeneous nucleation mechanism of ice crystals and the variation in the water film, a relationship between the heterogeneous nucleation rate and temperature was established. Subsequently, a method for calculating the equivalent contact angle related to pore water in soils was presented. The results showed that the heterogeneous nucleation rate and equivalent contact angle are positively and negatively correlated with the equivalent particle size at the same temperature, respectively, and the angle decreases with increasing solution concentration. Furthermore, the experimental data of 10 different soils showed that the variation in water content obtained by our model was in good agreement with the experimental values, particularly during the stage of rapid water phase change. The study developed an effective correlation among heterogeneous nucleation rate and equivalent contact angle of pore water and physical properties of soil, by combining crystallization theory and premelting theory, which provided a new way to explain water freezing and predict unfrozen water content in soils.

Effect of TiB2 Nanoparticle Content on the Microstructure and Mechanical Properties of TiB2/Mg-4Al-1.5Si Composites

Coarse primary and eutectic Mg2Si phases were generally precipitated in Mg-Al-Si alloys during solidification at a low cooling rate, which tends to deteriorate the strength and ductility of magnesium alloys due to stress concentration. Different volume fractions of TiB2 nanoparticles (1%, 3%, and 5%) were added to an Mg-4Al-1.5Si alloy to refine the coarse Mg2Si phases based on a heterogeneous nucleation mechanism. The nanoparticles were incorporated and dispersed in the molten Mg alloys and by using semi-solid stirring followed by ultrasonic treatment (SSUT), and TiB2/Mg-4Al-1.5Si composites were obtained. The effect of TiB2 content on the microstructure and mechanical properties of the composites was studied. The results showed that the average size of primary Mg2Si phases and α-Mg grains decreased as the TiB2 content raised, the dendritic primary Mg2Si phases were refined into polygonal shapes with smaller sizes, and the refined primary Mg2Si phases were uniformly distributed in the alloys after adding 1 vol.% or 3 vol.% TiB2 nanoparticles. As the TiB2 content increased, the morphology of the eutectic Mg2Si phases was modified from coarse Chinese characters to short rod or fine dot shapes. Vickers hardness and yield strength of the composites reached a maximum (153 HV and 90.9 MPa, respectively) when TiB2 content was 5 vol.%, while the most superior ultimate tensile strength (142.4 MPa) and elongation (9.2%) were obtained when TiB2 content was 3 vol.%, which were improved by 173.2%, 31.5%, 69.8%, and 187.5%, respectively compared with the Mg-4Al-1.5Si alloys.

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.