Proposed Nucleation Mechanism Research Articles

A new insight into the formation of β-crystals of isotactic polypropylene induced by zinc dicarboxylic acids

Some zinc dicarboxylic acids can induce numerous β-crystals formation in isotactic polypropylene (iPP), thus improving the crystallization behavior and mechanical properties of iPP. However, the corresponding nucleation mechanism of β-crystals remains inadequate, leading to the absence of a theoretical basis for the design and preparation of new and high-efficiency β-nucleating agents (β-NAs). In this work, we demonstrate a new mechanism that β-crystals can be formed when the interfacial interaction between β-NAs and iPP is comparable to that between β-PP and iPP. The interfacial interaction occurs when iPP epitaxially crystallizes on the NA surface, which will affect the folding and arrangement of iPP chains, thereby leading to formation of different crystalline forms of iPP. The interaction energy of iPP chain with α-PP and β-PP is used as a judgment criterion, which is resolved by molecular dynamics (MD) simulation. We surprisingly found that when the interaction energy of NA/iPP was close to that of β-PP/iPP, the NA can be acted as β-NA; β-crystals were inclined to form when the interaction energy of NA/iPP was close to that of β-PP/iPP. Furthermore, the simulation results are well verified by the crystallization behavior of iPP in the presence of aromatic zinc dicarboxylic acids of zinc terephthalate (ZnCC1) and zinc 4-carboxybenzoate (ZnCC2), lending strong support to the proposed nucleation mechanism and method. This work proposes a new insight into the formation of β-crystals of iPP based on the interaction energy of NA and iPP, and demonstrates a method to rapidly predict and evaluate the β-nucleation effect of NAs, thus may serve as a theoretical guidance for the design of NAs that can induce iPP with specific crystalline forms.

Effect of crystal structures on nucleation mechanism in hydrothermal synthesis of MZrO3 (M=Ba, Sr, Ca)

Hydrothermal method is widely used in the synthesis of perovskite-type oxides, whereas few studies are reported for the nucleation mechanism, so that the relationship between the crystal structures and reactive activities of reactants and products is still unknown. Herein, the reaction processes are analyzed on the basis of XRD, SEM and Raman characterizations, and the nucleation mechanism is investigated for the hydrothermal synthesis of MZrO3 (M = Ba, Sr, Ca). We propose that the negative charged cyclic tetramer complexes [Zr4(OH)8(OH)16]8- form in the hydrothermal reaction, which play major roles in the nucleation process. The tetramer complexes continually dehydrate and condensate to form substructural units composed of alkali-earth ions and 6-fold Zr tetramers; substructural units further dehydrate and distort to form perovskite structures. The reactive activation energy increases with the decreasing of M2+ (M = Ba, Sr, Ca) ionic radius because the incorporation of smaller A site ions in the perovskite structure is accompanied by greater rotation and distortion of the ZrO6 octahedra, leading to the decrease of reactive activity accordingly. In a word, the proposed nucleation mechanism in this paper is of great significance for the study of perovskite.

Effect of structural relaxation on crystal nucleation in a soda‐lime‐silica glass

AbstractThe influence of structural relaxation on crystal nucleation has been underexplored and remains elusive. This article discusses its possible effect on the nucleation process using a stoichiometric soda‐lime‐silica (2Na2O·CaO·3SiO2) glass as a model system. We show that the relaxation effect is powerful at low temperatures, close and below the glass transition, , and leads to a continuous increase in the nucleation rate. At any given temperature, the nucleation rate eventually reaches its ultimate steady‐state corresponding to the fully relaxed supercooled liquid (SCL). However, the time to reach the steady‐state is two to three orders of magnitude longer than the average relaxation time estimated by the Maxwell relation (shear viscosity / shear modulus). The proposed nucleation mechanism and model, which take relaxation into account, and related experimental results also explain the alleged “breakdown” of CNT at low temperatures reported for various glasses. It confirms a few recent papers that this apparent flaw is merely because most researchers did not prolong nucleation treatments enough to complete the relaxation process to achieve a steady state. Another remarkable result is that the actual maximum nucleation temperature, , is significantly lower than the previously reported values. Finally, a comparative analysis of the kinetic coefficient using viscosity versus growth velocity favors the last. These results for this soda‐lime‐silica glass extend and validate recent findings for lithium disilicate on the significant (but often neglected) effect of relaxation on crystal nucleation.

Molecular level nucleation mechanisms of hierarchical MFI and MOR zeolite structures via non-stochastic pathways.

Understanding the chemical mechanism of crystal nucleation at the molecular level is crucial for the design of architectural structures of valuable materials in the future. In this study, it has been revealed that amorphous silicate precursors, which play a role in the nucleation processes of zeolitic frameworks, can be regularly fragmented in mass spectroscopy due to the hydroxyl functional groups in their molecular structures. In this way, by using the mass spectra acquired from LDI-TOF MS, the systematic evolution stages of a common 1D precursor converting to the 3D unit cells of MFI and MOR zeolite structures observed in the same reaction medium were constructed through a nucleation mechanism at the molecular level for the first time. Here we show a novel nucleation pathway that does not occur via stochastic assembly of atoms or distinct building blocks by molecular recognition. Each of the proposed nucleation mechanisms of these different frameworks carrying structural similarities is from different combinations of sequential self-attaching intramolecular covalent couplings of identical origin precursors. The dynamic molecular structure capable of forming finite building units of target frameworks during the nucleation process of this precursor, which is the polymerized form of simple 6-membered siloxane chains, has been arranged around structure directing agents before a hydrothermal reaction.

Direct measurement of nanostructural change during in situ deformation of a bulk metallic glass

To date, there has not yet been a direct observation of the initiation and propagation of individual defects in metallic glasses during deformation at the nanoscale. Here, we show through a combination of in situ nanobeam electron diffraction and large-scale molecular dynamics simulations that we can directly observe changes to the local short to medium range atomic ordering during the formation of a shear band. We observe experimentally a spatially resolved reduction of order prior to shear banding due to increased strain. We compare this to molecular dynamics simulations, in which a similar reduction in local order is seen, and caused by shear transformation zone activation, providing direct experimental evidence for this proposed nucleation mechanism for shear bands in amorphous solids. Our observation serves as a link between the atomistic molecular dynamics simulation and the bulk mechanical properties, providing insight into how one could increase ductility in glassy materials.

Utilizing in Situ Ec-S/TEM to Study Electrochemical Nucleation of Copper from a Bromide Electrolyte

Electrochemical nucleation of metals have long been investigated using chronoamperometry1, cyclic voltammetry2, and electrochemical impedance spectroscopy3 . The current/potential time transients from these methods are used as basis for models for various proposed nucleation mechanisms such as instantaneous and progressive nucleation4. Physical insight has been primarily inferred from the current/potential time transient models and ex situ analysis of the deposit thereafter. However, surface restructuring is known to occur ex situ. In situ methods, such as in situ STM5 and in situ AFM6, minimize ambiguous understanding due to surface restructuring; however, the time resolution of these methods are lacking7. Thus direct correlation between the nucleation processes and electrochemical measurements have not been studied in great detail due to the present lack of in situ characterization methods that permit the simultaneous acquisition of electrochemical measurements with high spatial resolution imaging and high time resolution processing within the electrolyte. Recent advances in liquid cell in situ electrochemical scanning/transmitting electron microscopy (ec-STEM), mitigates ambiguity due to surface restructuring following nucleation7, 8. In situ ec-STEM can acquire and analyze time-resolved images of the nucleation and growth process at the nanometer-scaled spatial resolution while quantitative electrochemical measurements are concurrently performed. Therefore, this technique can also provide clarity between nucleation growth and classical electrochemical nucleation models such as the Scharifker and Hills4 model and the Sluyter-Rehbach9 model. Recent advances in an all-copper flow battery has created a renewed interest in copper electrodeposition from a high halide electrolyte10. The high halide electrolyte is unique compared to traditional damascene plating in that the copper (I) oxidation state is stabilized. Here, we apply the in situ ec-STEM approach to investigate the electrochemical nucleation of copper in order to understand the negative electrode reaction in these all-copper batteries. This presentation compares the electrochemical nucleation of copper of high bromide ion electrolyte on a carbon versus copper electrode and discusses the nuances of in situ liquid cell ec-S/TEM. 1. Heerman, L.; Tarallo, A. Journal of Electroanalytical Chemistry 1999, 470, (1), 70-76. 2. Williams, D. E.; Wright, G. A. Electrochimica Acta 1976, 21, (11), 1009-1019. 3. Cachet, C.; Saïdani, B.; Wiart, R. Electrochimica Acta 1989, 34, (8), 1249-1250. 4. Scharifker, B.; Hills, G. Electrochimica Acta 1983, 28, (7), 879-889. 5. Itaya, K.; Tomita, E. Surface Science 1988, 201, (3), L507-L512. 6. Rynders, R. M.; Alkire, R. C. Journal of The Electrochemical Society 1994, 141, (5), 1166-1173. 7. Williamson, M. J.; Tromp, R. M.; Vereecken, P. M.; Hull, R.; Ross, F. M. Nat Mater 2003, 2, (8), 532-536. 8. Unocic, R. R.; Sacci, R. L.; Brown, G. M.; Veith, G. M.; Dudney, N. J.; More, K. L.; Walden, F. S., II; Gardiner, D. S.; Damiano, J.; Nackashi, D. P. Microscopy and Microanalysis 2014, 20, (02), 452-461. 9. Sluyters-Rehbach, M.; Wijenberg, J. H. O. J.; Bosco, E.; Sluyters, J. H. Journal of Electroanalytical Chemistry and Interfacial Electrochemistry 1987, 236, (1–2), 1-20. 10. Lloyd, D.; Magdalena, E.; Sanz, L.; Murtomäki, L.; Kontturi, K. Journal of Power Sources 2015, 292, (0), 87-94.

Cluster-assisted nucleation of silicon phase in hypoeutectic Al–Si alloy with further inoculation

The paper discusses the responses of eutectic silicon and eutectic cells in Al–10Si alloy upon inoculation with an Al–10Si–2Fe master alloy. The further inoculation hardly destroys the modification effect of Sr but significantly refines the eutectic cells in Sr-modified samples, while in unmodified samples, it stimulates the occurrences of polyhedral silicon particles and divorced eutectic. Thermal analysis, scanning electron microscopy, (high-resolution) transmission electron microscopy and scanning and transmission electron microscopy have been used to elucidate the underlying mechanism. A cluster-assisted nucleation mechanism responsible for the enhanced nucleation of silicon phase upon inoculation is proposed. Icosahedral (AlFeSi) clusters are speculated to evolve from the added Al–10Si–2Fe master alloy in Al–10Si melt, around which aggregations of silicon atoms form. Through a series of structural evolutions, these clusters transform into precursors of a silicon crystal. The subsequent formation of silicon particles is achieved by the agglomerations and attachments of these precursors and individual silicon atoms. This hypothesis is further consolidated by the increased characteristic temperatures of eutectic and the anomalous appearance of a high density of nanoscale particles, as well as the abnormal disappearance of Sr-induced twins in further inoculated silicon particles. The increased characteristic temperatures are strong indications of the enhanced nucleation of the silicon phase. The high density of nanoscale particles with an indeterminate crystal structure are the survivors of these precursors. In an Sr-modified and further inoculated sample, the formation of Sr-induced twins is consequently inhibited due to the participation of these precursors during the growth of silicon particles. Furthermore, based on the proposed nucleation mechanism, the dependence of eutectic cell size on Sr level is elucidated in detail.

ChemInform Abstract: Nucleation of Organic Crystals — A Molecular Perspective

AbstractReview: 106 refs.

Discontinuous Bubble Nucleation Due to a Metastable Condensation Transition in Polymer–CO2 Mixtures

We combine a newly developed density-functional theory with the string method to calculate the minimum free energy path of bubble nucleation in compressible polymer-CO2 mixtures. Nucleation is initiated by saturating the polymer liquid with high pressure CO2 and subsequently reducing the pressure to ambient condition. Below a critical temperature, we find that there is a discontinuous drop in the nucleation barrier with increased initial CO2 pressure, as a result of an underlying metastable transition from a CO2-rich-vapor phase to a CO2-rich-liquid phase. This phenomenon is different from previously proposed nucleation mechanisms involving metastable transitions.

Nucleation of Organic Crystals—A Molecular Perspective

The outcome of synthetic procedures for crystalline organic materials strongly depends on the first steps along the molecular self-assembly pathway, a process we know as crystal nucleation. New experimental techniques and computational methodologies have spurred significant interest in understanding the detailed molecular mechanisms by which nuclei form and develop into macroscopic crystals. Although classical nucleation theory (CNT) has served well in describing the kinetics of the processes involved, new proposed nucleation mechanisms are additionally concerned with the evolution of structure and the competing nature of crystallization in polymorphic systems. In this Review, we explore the extent to which CNT and nucleation rate measurements can yield molecular-scale information on this process and summarize current knowledge relating to molecular self-assembly in nucleating systems.

Nucleation mechanism for the direct graphite-to-diamond phase transition

Graphite and diamond have comparable free energies, yet forming diamond from graphite in the absence of a catalyst requires pressures that are significantly higher than those at equilibrium coexistence. At lower temperatures, the formation of the metastable hexagonal polymorph of diamond is favoured instead of the more stable cubic diamond. These phenomena cannot be explained by the concerted mechanism suggested in previous theoretical studies. Using an ab initio quality neural-network potential, we carried out a large-scale study of the graphite-to-diamond transition assuming that it occurs through nucleation. The nucleation mechanism accounts for the observed phenomenology and reveals its microscopic origins. We demonstrate that the large lattice distortions that accompany the formation of diamond nuclei inhibit the phase transition at low pressure, and direct it towards the hexagonal diamond phase at higher pressure. The proposed nucleation mechanism should improve our understanding of structural transformations in a wide range of carbon-based materials.

Connection of Sulfuric Acid to Atmospheric Nucleation in Boreal Forest

Gas to particle conversion in the boundary layer occurs worldwide. Sulfuric acid is considered to be one of the key components in these new particle formation events. In this study we explore the connection between measured sulfuric acid and observed formation rate of both charged 2 nm as well as neutral clusters in a boreal forest environment A very short time delay of the order of ten minutes between these two parameters was detected. On average the event days were clearly associated with higher sulfuric acid concentrations and lower condensation sink (CS) values than the nonevent days. Although there was not a clear sharp boundary between the nucleation and no-nucleation days in sulfuric acid-CS plane, at our measurement site a typical threshold concentration of 3.10(5) molecules cm(-3) of sulfuric acid was needed to initiate the new particle formation. Two proposed nucleation mechanisms were tested. Our results are somewhat more in favor of activation type nucleation than of kinetic type nucleation, even though our data set is too limited to omit either of these two mechanisms. In line with earlier studies, the atmospheric nucleation seems to start from sizes very close to 2 nm.

On the nucleation and dissolution process of Z-phase Cr(V,Nb)N in martensitic 12%Cr steels

Precipitation of large Z-phase particles, Cr(V,Nb)N, replacing fine MX nitrides, (V,Nb)N, has recently been identified as a major cause of premature breakdown in long-term creep strength of a number of new 9–12%Cr martensitic steels. The Z-phase precipitates slowly during long-term exposure at around 650 °C accelerated by high Cr content in the steels. It appears that the nucleation process controls the precipitation rate of Z-phase. A 12%Cr steel, which had precipitated Z-phase during long-term operation at 660 °C/12,000 h, was further heat treated in order to investigate the dissolution and reappearance processes for the Z-phase. In both cases it appears that Z-phase and MX phase are in physical contact and have a preferred crystallographic orientation relationship. The proposed nucleation mechanism is a chromium diffusion controlled transformation of MX into Z-phase, which explains the rather low precipitation rate of Z-phase.

Kinetics of emulsion polymerization of styrene using the reactive surfactant HITENOL BC20

AbstractThe effect of the reactive surfactant HITENOL BC20 (polyoxyethylene alkylphenyl ether ammonium sulfate; 20 U of ethylene oxide (EO)) on the emulsion polymerization of styrene was studied via reaction calorimetry (Mettler RC1; 70°C). For polymerizations carried out above the cmc of BC20, the reaction kinetics and evolution of the number of particles (Np) resembled those reported using the conventional surfactant sodium lauryl sulfate, indicating that nucleation proceeded by a combination of micellar and homogenous nucleation (Interval I and Stage 2). The reaction rate (Rp) not only increased with increasing initiator (K2S2O8) and surfactant concentrations, as expected, but the increase in rate in Stage 2 was dependent on the initiator concentration and independent of the surfactant concentration. This is consistent with the proposed nucleation mechanisms. The molecular weight increased with increasing surfactant concentration and decreasing initiator concentration as would also be expected for a conventional surfactant. The dependencies of Rp and Np on the BC20 and initiator concentrations, however, were lower than the classical Smith‐Ewart values (Rp ∝ [E]0.47–0.52; Rp ∝ [I]0.21–0.25) although Rp was found to be directly proportional to Np. Chain transfer to the reactive surfactant is considered a likely source of divergence. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Improved quasi-unary nucleation model for binary H2SO4–H2O homogeneous nucleation

Aerosol nucleation events have been observed at a variety of locations worldwide, and may have significant climatic and health implications. Binary homogeneous nucleation (BHN) of H2SO4 and H2O is the foundation of recently proposed nucleation mechanisms involving additional species such as ammonia, ions, and organic compounds, and it may dominate atmospheric nucleation under certain conditions. We have shown in previous work that H2SO4-H2O BHN can be treated as a quasi-unary nucleation (QUN) process involving H2SO4 in equilibrium with H2O vapor, and we have developed a self-consistent kinetic model for H2SO4-H2O nucleation. Here, the QUN approach is improved, and an analytical expression yielding H2SO4-H2O QUN rates is derived. Two independent measurements related to monomer hydration are used to constrain the equilibrium constants for this process, which reduces a major source of uncertainty. It is also shown that the capillarity approximation may lead to a large error in the calculated Gibbs free energy change for the evaporation of H2SO4 molecules from small H2SO4-H2O clusters, which affects the accuracy of predicted BHN nucleation rates. The improved QUN model-taking into account the recently measured energetics of small clusters-is thermodynamically more robust. Moreover, predicted QUN nucleation rates are in better agreement with available experimental data than rates calculated using classical H2SO4-H2O BHN theory.

A review of particle formation events and growth in the atmosphere in the various environments and discussion of mechanistic implications

This review highlights recent observations from a large number of studies investigating formation and growth within different environments and discusses the importance of various mechanisms of particle formation and growth between the different environments. Whilst, several mechanisms for new particle formation which proposed the importance of each mechanism are still the centre of much debate. Proposed nucleation mechanisms include condensation of a binary mixture of sulphuric acid and water; ternary nucleation of sulphuric acid, water and a third molecule, most likely ammonia; ion-induced nucleation; secondary organic aerosol formation involving condensation of low- or non-volatile organic compounds and homogeneous nucleation of iodine oxides. Laboratory and modelling studies have shown these mechanisms can occur in the atmosphere although the contribution depends on the concentrations of precursor compounds present. In addition, atmospheric particle formation events are significantly affected by environmental factors, such as temperature, humidity and the surface area of pre-existing particles, which is also discussed here. One major problem hampering our current understanding is that these new particles are smaller than the lower size detection limit of most instruments and are only observed after some particle growth has occurred. Particles growth occurs through condensation of supersaturated vapours on the surface of the nucleated particles. This requires a lower degree of supersaturation than nucleation and thus condensation of the nucleating species reduces the rate of particle formation. Therefore, it is believed that particle growth often occurs through the condensation of other gases, including organic and inorganic compounds, than those responsible for nucleation. This decoupling of nucleation and growth means that the individual gases responsible for nucleation and growth can be unclear. Since observations of particle formation only occur following growth to observable sizes it is possible that a pool of undetectable particles exist at all times but are only observed following significant condensational growth.

Fast crystallization of poly(3‐hydroxybutyrate) and poly(3‐hydroxybutyrate‐co‐3‐hydroxyvalerate) with talc and boron nitride as nucleating agents

AbstractThe crystallization behavior of poly(3‐hydroxybutyrate) (PHB) and poly(3‐hydroxybutyrate‐co‐3‐hydroxyvalerate) (PHBV) induced by two kinds of nucleating agents, boron nitride (BN) and talc, was investigated by differential scanning calorimetry, polarized optical microscopy and X‐ray diffraction. Both BN and talc have good nucleating ability in the crystallization of PHB and PHBV. From these results, combined with molecular weight measurement by gel permeation chromatography, the mechanism of nucleation by BN and talc in the crystallization of PHB and PHBV has been proposed. BN acts as a nucleating agent itself and initiates nucleation in the crystallization of PHB and PHBV. Talc acts in a different way. It reacts as a chemical reagent with the molten chains of PHB/PHBV, while the reaction product acts as the true nucleating agent, which lowers the crystallization barriers of PHB and PHBV. 1H NMR spectroscopy provides evidence for the reaction between PHB and talc and supports the proposed nucleation mechanism. Copyright © 2005 Society of Chemical Industry

A model for the nucleation of diamond clusters on Si(111) substrates

A theoretical study of the nucleation, size, and structure of diamond phase carbon clusters on Si(111) substrates is presented. Molecular mechanics analysis has been utilized to predict energetically and entropically feasible pathways for nucleation of the carbon clusters. Several mechanistic pathways for nucleation of carbon clusters are examined with CH3 and/or C2H2 as the nucleation precursors. A possible model for the nucleation mechanism of diamond-phase carbon clusters on the β-SiC(111) surface, which forms epitaxially on Si(111) substrates, is presented. The critical size of the carbon clusters is computed based on the atomistic theory of nucleation and the proposed nucleation mechanisms.

A mechanism for reactive diffusion between Si single crystal and NbC powder compact

Based on our recent experimental observations, a growth mechanism for the reactive diffusion between Si single crystal and NbC powder compact is proposed. In Si–NbC diffusion couples annealed at 1300 °C, a two-phase NbSi2 + SiC reaction layer formed with NbSi2 as the matrix and SiC as discontinuous particles. The NbSi2 grain sizes and SiC particle sizes are both in the μm range. We propose that the SiC particles nucleated at the void surfaces in the NbC powder compact. This proposed nucleation mechanism offers a potential way of controlling the SiC particle size by changing the void size and void density of the NbC powder compact. It is also pointed out that this microstructure requires Si to be the dominant diffusing species. Si must diffuse through the reaction layer, while C only has to undergo local rearrangement, and Nb need not diffuse at all.

Bidimensional mixtures of cholesterol and poly-γ-methyl- l-glutamate in α conformation at the water—air interface: mechanism of collapse

The mechanism of the collapse process of bidimensional mixtures of poly-γ-methyl- l-giutamate in α conformation and cholesterol was studied. It was shown that the collapse mechanism is described by a process of nucleation and growth. The Prout—Tompkins and the second-order kinetic laws were in agreement with the experimental data. The kinetic analysis of the collapse process confirmed the bidimensional immiscibility found in static conditions from the analysis of the thermodynamic parameters. Further information on the mechanism of collapse was obtained from the values of Δ G +, Δ H +, and Δ S + calculated applying the transition-state theory. Scanning electron microscopy supported the proposed nucleation mechanism and the bidimensional immiscibility of the components. Biological implications were also considered.