Successive Nucleation Research Articles

Petrological evidence for the existence and disruption of a 500 km-sized differentiated planetesimal of enstatite-chondritic parentage

Two samples of a unique achondritic lithology of the Almahata Sitta meteorite (MS-MU-019 and MS-MU-036) contain three coexisting pyroxene species: orthoenstatite, clinoenstatite and augite. The silicate assemblage appears to be the restite after extraction of melts of broadly basaltic and metal-sulfide composition from an enstatite chondrite protolith. Transmission electron microscopy (TEM) provides evidence that clinoenstatite within the lithology formed from earlier protoenstatite. The absence of pigeonite despite the successful nucleation of augite and the persistence of orthoenstatite during cooling suggests that the sub-solidus formation of the three coexisting pyroxenes occurred at a pressure of about 0.1 GPa. Rapid cooling at >1 K/h below 1260°C is documented by the cessation of augite equilibration, preservation of the 3-pyroxene assemblage and a low volume fraction of nanoscale orthoenstatite lamellae formed during the transformation of protoenstatite to clinoenstatite. The pressure implies a diameter of roughly 500 km of the differentiated parent body, putting petrological constraints on the size of planetesimals that may have contributed to the accretion of the terrestrial planets including Earth. The high cooling rate indicates a catastrophic disruption of this large planetesimal early in its history. The lithology studied here underlines that planetesimals which existed in the inner Solar System were more diverse than previously thought, and included potentially large differentiated bodies with very FeO-poor, enstatite-dominated mantles. Remains of these bodies are poorly represented in meteorite collections, which points to efficient accretion in the inner Solar System or to removal and little re-distribution of material into the present-day asteroid belt.

Controlling ice formation on gradient wettability surface for high-performance bioinspired materials.

Ice-templating holds promise to become a powerful technique to construct high-performance bioinspired materials. Both ice nucleation and growth during the freezing process are crucial for the final architecture of the ice-templated material. However, effective ways to control these two very important factors are still lacking. Here, we demonstrate that successive ice nucleation and preferential growth can be realized by introducing a wettability gradient on a cold finger. A bulk porous material with a long-range lamellar pattern was obtained using a linear gradient, yielding a high-performance, bulk nacre-mimetic composite with excellent strength and toughness after infiltration. In addition, cross-aligned and circular lamellar structures can be obtained by freeze-casting on surfaces modified with bilayer linear gradient and radial gradient, respectively, which are impossible to realize with conventional freeze-casting techniques. Our study highlights the potential of harnessing the rich designability of surface wettability patterns to build high-performance bulk materials with bioinspired complex architectures.

Oxygen-enriched combustion mechanism of lignite

In this work, thermogravimetric experiments were carried out in a thermogravi-metric analyzer under O2/N2 atmosphere with an oxygen content ranging from 21 vol.% to 70 vol.%. Malek method combined with iso-conversional method and non-isothermal method was employed to determine the burning dynamical function of lignite in high temperature burning region with different oxygen concentrations. The results indicated that the lignite has different burning dynamical function in different oxygen conditions. The combustion mechanism function of lignite belonged to 3-D model when the oxygen concentration is below 30%. The combustion mechanism of lignite belongs to a random successive nucleation growth model when the oxygen concentration is between 40% and 50%. Kinetic burning model of lignite in high burning temperature region with different oxygen concentrations was established. The kinetic parameters were obtained from the kinetic burning model of lignite using Kissinger-Akah-Sunose method.

On the ductile damage of nanotwinned copper crystal with prolate void defect at the twin boundary

Void growth and coalescence is one of the primary causes that are responsible for the ductile damage of metals. Extensive literature works have been conducted for exploring the dislocation-based damage mechanisms due to cylindrical or spherical voids embedded in single or polycrystals. In this paper, molecular dynamics simulations were performed for elucidating the ductile damage mechanisms of nanotwinned copper crystal containing prolate void defects sitting along the twin boundary. The effects of length scale, strain rate, temperature and void fraction on the strength, stiffness and dislocation evolution of the proposed nanotwinned model are studied in detail. Starting at the critical yield stress, the successive nucleation, propagation and reaction of Shockley partial dislocations are responsible for the continuous void growth process and result in a dramatic stress drop immediately following the ultimate strength. An obvious dependence on length scale is found for both the yield and the peak stress. The continuum Lubarda model that was developed for characterizing the critical stress of spherically voided nanocrystals is found to appreciably underestimate the strength of nanotwinned copper embedded with prolate voids. Simulation results also reveal that twin boundaries function as a dislocation barrier that is able to prevent dislocations from penetrating through. It is by this mechanism that twin boundaries affect the strength of nanotwinned crystals.

Effect of misalignments of individual grains’ easy axis on magnetization-reversal process in granular NdFeB magnets: A finite-element micromagnetic simulation study

We studied, using finite-element micromagnetic simulations, magnetization-reversal processes of exchange-decoupled granular NdFeB magnets for a cubic model system that consists of 27 polyhedral grains of different crystallographic orientations along with non-magnetic grain boundaries. In the simulations, we varied the degree of easy-axis alignment of individual grains with different uniaxial orientations. For perfect uniaxial alignment, i.e., α = 1, the normalized remanence magnetization and the normalized coercivity along with the maximum energy product (BH)max were estimated to be 1, 0.64, and 64.71 MGOe, respectively, and were decreased to 0.59, 0.42, and 20.56 MGOe, representing reductions of 41, 34, and 68%, respectively, for a randomly oriented alignment of the grains’ easy axis, α = 0.6. From a comparison of the magnetization-reversal processes for different easy-axis alignments, i.e., α = 1, 0.98, 0.94, 0.87, 0.82, 0.79, and 0.60, it was found that all of the magnetization reversals consisted of three different sub-processes: the nucleation of a reversed domain within a grain and the expansion of nucleated domains to complete the reversal in that grain via domain-wall motion; magnetization-reversal propagation to the next grain via successive nucleation in other neighboring grains; further rotation of the magnetizations of all of the grains for cases of larger misalignment of grains’ easy axis. According to these reversal sub-processes, as the degree of easy-axis alignment, α, decreased, the nucleation of reversed domains occurred at smaller field strengths and the apparent grain-by-grain reversal propagation became slower. The lower the value of α, the lower the energy required for the nucleation of reversed domains and the more predominant the pinning of the apparent grain-by-grain reversal propagation. These reversal processes led to hard-magnet parameters such as normalized remanence magnetization and coercivity and the maximum energy product (BH)max. The values of the parameters for the perfect uniaxial alignment of grains became smaller with increasing misalignment of the grain’s easy axis. The results provide both a better understanding of micro-scale magnetization-reversal processes and guidelines for optimal design of hard-magnet microstructures for higher (BH)max values.

Effect of ultrasonic treatment on the alloying and grain refinement efficiency of a Mg – Zr master alloy added to magnesium at hypo- and hyper-peritectic compositions

Magnesium – Zirconium master alloys are the dominant grain refiner for a range of commercial magnesium alloys that exhibit good elevated temperature properties. However, the grain refining efficiency achieved by the master alloy is relatively low because Zr particles quickly settle to form sludge. In this study ultrasonic treatment (UST) is applied to investigate whether the grain refinement efficiency can be improved by increasing the Zr solute content, producing a more uniform dispersion of Zr particles and minimizing sludge formation. It was determined that, for a range of Zr contents from 0.2 to 1.0 wt% Zr, UST of the melt above the liquidus temperature decreased the grain size due to an increase in the amount of solute Zr and the number of Zr particles. For Zr contents less than 0.5 wt% the grain size decreased further when UST was applied from above to below the liquidus temperature to include the onset of nucleation of α – Mg grains. Additionally, the application of UST substantially decreased the amount of sludge which increased the number density of Zr particles and improved their likelihood of successful nucleation. The Interdependence nucleation model was successfully applied to predict the grain size of each casting condition where the parameter z relates to the temperature gradient which in turn is affected by the acoustically-induced convection. A lower value of z when UST is applied facilitates nucleation and the survival of grains. The factors affecting grain refinement under each condition are evaluated in terms of solute and particle Zr contents, cavitation and acoustic streaming and their effect on nucleation and as-cast grain size.

A Unified Three-Dimensional Numerical Model for Boiling Curve in a Temperature Controlled Mode1

In the present study, level set method is used to simulate the entire boiling curve in a temperature-controlled mode spanning all the three regimes viz., nucleate, transition, and film boiling with a unified numerical model supplemented with correlations specifying nucleation site density and waiting time between successive nucleations. In order to improve the performance of the code, parallel computing has also been implemented. Vapor evolution process along with temporal- and spatial-averaged wall heat flux and wall void fraction are computed for a uniform wall superheat case. Wall void fraction is found to increase with increase in wall superheat nonlinearly as different regimes of boiling were traversed. Energy partitioning from wall into liquid, interface, and microlayer has also been examined where it is found that as the wall void fraction increases, the percent energy going into liquid decreases while the microlayer contribution peaks around critical heat flux (CHF). Numerical simulations are carried out in 3D with water as test liquid and contact angle of 38 deg.

Ultrasonic Processing of Aluminum⁻Magnesium Alloys.

This study evaluated the effect of UltraSonic Treatment (UST) on a range of Al–Mg alloys. Previous research was carried out on single magnesium compositions. However, the amount and type of the alloy addition are known to affect the grain size even under UST, and the aim of this study was to determine whether or not alloy composition plays a similar role in the case of Al–Mg alloys. By testing binary Al–Mg alloys cast under regular casting conditions and under the presence of an ultrasonic field, it was found that while the addition of Mg solute is important, the amount of solute has little effect when UST is applied. It was observed that the grain size was barely affected by extra solute additions in this condition. This is due to the application of UST during solidification, which resulted in a dramatic reduction in the size of the nucleation free zone thus promoting many more successful nucleation events. Acoustic streaming is proposed as the main cause of this reduction in grain size.

Tethered multifluorophore motion reveals equilibrium transition kinetics of single DNA double helices

We describe a tethered multifluorophore motion assay based on DNA origami for revealing bimolecular reaction kinetics on the single-molecule level. Molecular binding partners may be placed at user-defined positions and in user-defined stoichiometry; and binding states are read out by tracking the motion of quickly diffusing fluorescent reporter units. Multiple dyes per reporter unit enable singe-particle observation for more than 1 hour. We applied the system to study in equilibrium reversible hybridization and dissociation of complementary DNA single strands as a function of tether length, cation concentration, and sequence. We observed up to hundreds of hybridization and dissociation events per single reactant pair and could produce cumulative statistics with tens of thousands of binding and unbinding events. Because the binding partners per particle do not exchange, we could also detect subtle heterogeneity from molecule to molecule, which enabled separating data reflecting the actual target strand pair binding kinetics from falsifying influences stemming from chemically truncated oligonucleotides. Our data reflected that mainly DNA strand hybridization, but not strand dissociation, is affected by cation concentration, in agreement with previous results from different assays. We studied 8-bp-long DNA duplexes with virtually identical thermodynamic stability, but different sequences, and observed strongly differing hybridization kinetics. Complementary full-atom molecular-dynamics simulations indicated two opposing sequence-dependent phenomena: helical templating in purine-rich single strands and secondary structures. These two effects can increase or decrease, respectively, the fraction of strand collisions leading to successful nucleation events for duplex formation.

Inward growth by nucleation: Multiscale self-assembly of ordered membranes.

Striking morphological similarities found between superstructures of a wide variety of seemingly unrelated crystalline membrane systems hint at the existence of a common formation mechanism. Resembling systems such as multiwalled carbon nanotubes, bacterial protein shells, or peptide nanotubes, the self-assembly of SDS/β-cyclodextrin complexes leads to monodisperse multilamellar microtubes. We uncover the mechanism of this hierarchical self-assembly process by time-resolved small- and ultrasmall-angle x-ray scattering. In particular, we show that symmetric crystalline bilayers bend into hollow cylinders as a consequence of membrane line tension and an anisotropic elastic modulus. Starting from single-walled microtubes, successive nucleation of new cylinders inside preexisting ones drives an inward growth. As both the driving forces that underlie the self-assembly behavior and the resulting morphologies are common to systems of ordered membranes, we believe that this formation mechanism has a similarly general applicability.

Horizontal Dendritic Stacking of Methanofullerene Single Crystals upon Diffusion‐Limited Aggregation

AbstractWithin the binary amorphous homogeneous mixtures of conjugated polyalkylfluorene and methanofullerene, the annealing‐induced nucleation and growth of methanofullerene single crystals under the influences of molecular supply and mixing tendency have been studied in this research. As the molecular supply is satisfactory at selected annealing temperatures, the crystallization of methanofullerene is identified to advance mainly via the concurrent growth of board‐like crystals. When the diffusion is relatively limited, the local self‐aggregation of methanofullerene retards the further growth of preexistent crystals. Consequently, the successive nucleation of new single crystals on preexistent crystals emerges as the dominant mechanism of crystallization. With the advance of horizontal stacking of single crystals, the fractal‐like stacking patterns are able to evolve with long‐range continuity and facilitate the charge transportation. Nevertheless, upon the increase of annealing temperature and side‐chain length, the mixing tendency is enhanced, declining the tendency of self‐aggregation and thus successive horizontal stacking of single crystals.

Duplex Heterogeneous Nucleation Behavior of Precipitates in C-Mn Steel Containing Sn

The two successive heterogeneous nucleation behaviors of FeSn2-MnS-Al2O3 complex precipitates in ultrahigh Sn-bearing steel were investigated. First, Al2O3 was the nucleation site of the MnS at the end of solidification. Then, FeSn2 nucleated heterogeneously on the MnS particles that nucleated on the Al2O3 particles. The formation sequence of the precipitated phase caused the duplex heterogeneous nucleation to occur consecutively at most twice.

Nucleation of samarium-iron-garnet Sm3Fe5O12/α-Fe junctions in cm-scale foam-like carbon films continuously filled with α-Fe

The fabrication of rare-earth garnet-ferrites in the form of films is generally limited by numerous problems associated with diffusion, impurities, agglomeration or pH control. Here we show the successful nucleation of Sm3Fe5O12/α-Fe junctions in cm-scale foam-like carbon films continuously filled with α-Fe through an advanced CVD approach. The key feature of this method is the α-Fe filling which can be used as host for the garnet nucleation through interaction with Sm2O3 nanoparticles. The morphology, structure and magnetic properties of the films are characterized by scanning electron microscopy, scanning transmission electron microscopy, energy dispersive X-rays, X-ray diffraction, Raman Spectroscopy and T-dependent vibrating sample magnetometry.

Laminar flow continuous settling crystalliser. Part 1. Initial exploration

Conventional lactose crystallisations give large spans that are not suitable for direct use in inhaler grade lactose. The factors causing large spans are successive nucleation events and growth rate dispersion. This paper (Part 1 of 2) explores a novel lactose crystallisation technique, laminar flow continuous settling crystallisation, proposed as a method for directly producing a narrow particle size distribution with a span of less than 1. A theoretical model was developed that modelled the growth and settling of individual crystals with growth rate dispersion within a column full of lactose solution flowing upwards in laminar flow. The model predicted a d50 of 73.2 ± 0.9 μm and a span of 0.47 ± 0.01. In an experimental crystalliser crystals obtained had a d50 between 50 and 90 μm but the span was greater than 1. The difference has been attributed to agglomeration and flow variations from true laminar flow, which is reported in Part 2.

Phase Field Simulations of Autocatalytic Formation of Alpha Lamellar Colonies in Ti-6Al-4V

We present phase field simulations incorporating contributions due to chemical free energy, anisotropic interfacial energy, and elastic energy due to transformation strain, to demonstrate the nucleation and growth of multiple variants of alpha from undercooled beta in Ti-6Al-4V under isothermal conditions. A new composite nucleation seeding approach is used within the phase field simulations to demonstrate that the presence of a pre-existing strain field can cause the nucleation of specific crystallographic variants of alpha based on minimization of local elastic strain energy. Under conditions where specific combinations of elastic strains exist, for example in the vicinity of one or more pre-existing alpha variants, the nucleation of a new alpha variant is followed by the successive nucleation of the same variant in the form of a lamellar colony by an autocatalytic mechanism. At a given thermodynamic undercooling, the colony structure was favored at a nucleation rate that was low enough to allow sufficient growth of previously nucleated variants before another nucleus formed in their vicinity. Basket weave morphology was formed at higher nucleation rates where multiple nuclei variants grew almost simultaneously under evolving strain fields of several adjacent nuclei.

Nanograin nucleation at the growth front in melt crystallization of syndiotactic polystyrene

Melt-crystallization of syndiotactic polystyrene (sPS) is studied using simultaneous small/wide angle X-ray scattering (SAXS/WAXS). Emergence, intensification, and saturation of WAXS reflections within 120 s at isothermal crystallization temperature Tc = 250 °C after quenching from 300 °C illustrate crystallinity development through successive nucleation of instantaneously stabilized crystallites of lateral (hk0) coherence length Λ ≈ 75 nm. The corresponding SAXS profiles exhibit increased density heterogeneity due to formation of nanograins; fitting of time-resolved SAXS data with a model of arrayed disks reveals slightly thickened (lc = 7.4–8.4 nm) disks of radius R ≈ 9 nm and long period L ≈ 19 nm with polydisperse stacking number slowly enhanced from an average of 1.2 to 2.4. The moderate ratio of Λ/2R ≈ 4 indicates imperfect coalescence of nanograins in the lamellar assembly process. More importantly, with successively increased Tc from 250 to 260 °C the development of density heterogeneity of SAXS invariant Qinv is found to precede that of the WAXS-determined crystallinity Xc increasingly more, suggesting a precursor mesophase with non-crystalline nanograins. Avrami analysis for the developments of Xc and Qinv further reveals the same Avrami exponent n ≈ 3, consistent with heterogeneous nucleation of crystallites; the corresponding Avrami rate constant κ1/n(Tc) extracted from Xc is correlated to the crystal growth rates G(Tc) described by the Hoffman-Lauritzen theory with a common surface-nucleation rate constant Kg. A lateral surface energy σ ≈ 17 mJ m−2 can be deduced on the basis of Kg using previously determined fold surface energy σe ≈ 27 mJ m−2. Correspondingly, κ1/n(Tc) extracted from Qinv is described by a mesophase model proposed by Strobl with a zero-growth temperature of the nanograin mesophase ca. 30 K below the β-crystal melting point of sPS. Together, these results suggest that sPS melt-crystallization proceeds with nucleation of non-crystalline nanograins (SAXS-before-WAXS regime) at the crystal growth front, followed by crystallization of the nanograins with similar kinetics to sustain the growth front. Branching/twisting can occur at the lamellar growth front due to frustrated nanograin alignment, favoring the microscopically observed sheaf-like crystal morphology.

Modeling of data retention statistics of phase-change memory with confined- and mushroom-type cells

Data retention statistics of phase-change memory with two representative cell schemes, confined and mushroom cells, were investigated using phase-field method that can correctly model successive nucleation events and their growth, simultaneously. Several directions of cell structure engineering are suggested. An interesting point is that reducing only one lateral dimension below a characteristic length can improve the data retention. Most importantly, it was found that the cumulative distribution of the retention time is Weibull for the mushroom cells while that is lognormal for the confined cells.

The effect of the melt thermal gradient on the size of the constitutionally supercooled zone

Recent verification of the analytical Interdependence model by a numerical solidification model (µMatIC) confirmed the critical role of constitutional supercooling (CS) in achieving sufficient undercooling to trigger successful nucleation events. The location of the maximum amount of CS (ΔTCSmax) is some distance from the interface of the previously growing grain and this distance contributes to the final as-cast grain size. The effect of the thermal gradient, G, on the size of the CS zone (CSZ) was neglected in that work. However, G is expected to affect the size of the CSZ (i.e. the length of the CSZ, x’CSZ, and the location of ΔTCSmax, x’CSmax). This investigation assesses the effect of G on x’csz and x'CSmax. A range of G values is introduced into both the analytical and the numerical models to obtain a correlation between the value of G and the dimensions of the CSZ. The result of a test case from the analytical model shows that x’CSmax initially decreases rapidly and then decreases gradually approaching zero at very high values of G. Independent of the analytical model, the results from the numerical model replicate the trend obtained from the analytical model.

Dynamic Remodeling of the Magnetosome Membrane Is Triggered by the Initiation of Biomineralization.

ABSTRACTMagnetotactic bacteria produce chains of membrane-bound organelles that direct the biomineralization of magnetic nanoparticles. These magnetosome compartments are a model for studying the biogenesis and subcellular organization of bacterial organelles. Previous studies have suggested that discrete gene products build and assemble magnetosomes in a stepwise fashion. Here, using an inducible system, we show that the stages of magnetosome formation are highly dynamic and interconnected. During de novo formation, magnetosomes first organize into discontinuous chain fragments that are subsequently connected by the bacterial actin-like protein MamK. We also find that magnetosome membranes are not uniform in size and can grow in a biomineralization-dependent manner. In the absence of biomineralization, magnetosome membranes stall at a diameter of ~50 nm. Those that have initiated biomineralization then expand to significantly larger sizes and accommodate mature magnetic particles. We speculate that such a biomineralization-dependent checkpoint for membrane growth establishes the appropriate conditions within the magnetosome to ensure successful nucleation and growth of magnetic particles.

Use of SSA to detect structural changes in metallocenic ethylene/α-olefin copolymers and their free radical post-reactor modifications

The successive self nucleation and annealing (SSA) methodology has been shown to be successful to characterize the lamellae structure of many polyolefins. The differences in molecular structure of metallocenic α-olefin copolymers with varied comonomer type and content were characterized by this technique with a conventional DSC. This thermo-structural characterization process was also applied to analyze the changes induced on a 1-octadecene - ethylene copolymer modified with different post-reactor methods: several doses of gamma-radiation, peroxide modification and oxidative degradation. These modifications induced crosslinking and branching on the original copolymer affecting its molecular weight and the crystalline structure. The SSA technique revealed detailed changes in the lamellae formation of the materials obtained as a consequence of the different methods of modification employed in this work. Mainly, crosslinking and branching sites restrained the mobility and length of CH2 sequences to form lamellae, while degradation led to preferential chain scission at branched carbons making easier the formation of higher melting temperature lamellae.