Successive Nucleation Research Articles

Reductively PEGylated carbon nanomaterials and their use to nucleate 3D protein crystals: a comparison of dimensionality.

A range of carbon nanomaterials, with varying dimensionality, were dispersed by a non-damaging and versatile chemical reduction route, and subsequently grafted by reaction with methoxy polyethylene glycol (mPEG) monobromides. The use of carbon nanomaterials with different geometries provides both a systematic comparison of surface modification chemistry and the opportunity to study factors affecting specific applications. Multi-walled carbon nanotubes, single-walled carbon nanotubes, graphite nanoplatelets, exfoliated few layer graphite and carbon black were functionalized with mPEG-Br, yielding grafting ratios relative to the nanocarbon framework between ca. 7 and 135 wt%; the products were characterised by Raman spectroscopy, TGA-MS, and electron microscopy. The functionalized materials were tested as nucleants by subjecting them to rigorous protein crystallization studies. Sparsely functionalized flat sheet geometries proved exceptionally effective at inducing crystallization of six proteins. This new class of nucleant, based on PEG grafted graphene-related materials, can be widely applied to promote the growth of 3D crystals suitable for X-ray crystallography. The association of the protein ferritin with functionalized exfoliated few layer graphite was directly visualized by transmission electron microscopy, illustrating the formation of ordered clusters of protein molecules critical to successful nucleation.

連続的核生成が炭素鋼δ－γ massive的変態に及ぼす寄与

連続的核生成が炭素鋼δ－γ massive的変態に及ぼす寄与

In-situ observation of hydrothermal growth of ZnO nanowires on patterned Zn substrate and their photocatalytic performance

ZnO nanowire thin film was synthesized in situ on patterned Zn substrate via a simple and seed-free hydrothermal method. The as-synthesized products were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersion X-ray analysis(EDS), UV–visible diffuse reflectance spectroscopy (UV–vis-DRS), and photoluminescence (PL) spectroscopy. The time-, temperature- and ammonia concentration-dependent morphology evolution were observed. The results showed that the Zn MPs, in the presence of the reaction solution, can be gradually dissolved, thus providing the Zn source for successive nucleation and growth of ZnO nanowires. The increase of the light absorption as well as a red shift of the band edge was related to the increase of the quantity and the defects of ZnO nanowires. A growth mechanism was proposed to understand the formation of ZnO nanowires on the plated Zn substrate. Under UV irradiation, the as-synthesized ZnO nanowire film exhibited good photocatalytic activity and high photochemistry stability in a wide pH range of 5–10. Furthermore, the ZnO nanowire film can be easily separated, demonstrating great potential for practical application in the degradation of the organic pollutant while exhibiting high reusability.

Temperature dependence of strain-induced crystallization in natural rubber: On the presence of different crystallite populations

The effect of the temperature on strain induced crystallization (SIC) of natural rubber (NR) is studied by in situ wide angle X-rays scattering (WAXS) experiments performed along different thermo-mechanical paths (cyclic deformation at different temperatures or crystallization and melting in the deformed state). The crystallinity index (CI), the average crystallite size (L200) and the average volume of the crystallites (V) increase (decrease) similarly if the sample is cooled (heated) in the deformed state or stretched (unstretched) at fixed temperature. These experiments are analyzed through a thermodynamic description assuming that SIC is controlled by successive nucleation of crystallite populations. Such description predicts that for a stretching ratio λ above (below) 4 and a temperature above (below) room temperature, crystallization first occurs in chains with the highest (lowest) network chain density, leading to the smallest (largest) crystallites.

Experimental screening method for flash-crystallization

Flash-crystallization is a process to produce small, fine crystals (L50,3=30–90μm) from solutions of substances which are well soluble. Such substances tend to crystallize readily at low supersaturations to rather coarse size ranges above 200μm (Mersmann and Kind, 1988). For such substances, the feasibility of the flash-crystallization process has been well examined on a laboratory scale and on a pilot plant scale. It is ready to be transferred to production scale. However, not all combinations of solute and solvent can be flash-crystallized. Nucleation and growth kinetics must allow for the fast formation of crystals. Here, we introduce an experimental screening method for a fast but reliable determination of the general suitability of any so far untested solute–solvent combination for the flash-crystallization process. It is found that the particle size of the product and residual supersaturation after flash-crystallization are key indicators for a successful nucleation in the droplets. Offline investigation of atomized droplets shows that one droplet carries approximately one nucleus and crystal. Additionally, this allows us to estimate the median particle size of so far untested solute–solvent combinations. They are in good agreement with measured particle sizes.

Grain Refinement Mechanism of Al-5Ti-1B Master Alloy by <i>Ab Initio</i> Calculations

To exactly understand the grain refining mechanism of α-Al by the Al-5Ti-1B master alloy, the structural properties of α-Al/solid-TiB2 (S/S) and liquid-Al/solid-TiB2 (L/S) interfaces were studied using the first-principles method. Different ordered structures were formed on the interfaces with different terminations of TiB2 (0001) surface, which determines the nucleant potency of TiB2. The heterogeneous nucleation of α-Al on the B-terminated surface is frustrated by an AlB2-like structure formed at the interface. In contrast, a five-layer quasi-solid region with stacking sequence of fcc-Al (111) planes forms on the Ti-terminated TiB2 (0001) surface, which is the basis of successful heterogeneous nucleation of α-Al. Moreover, when redundant Ti solute being added into the liquid Al region of Ti-terminated liquid-Al/TiB2 interface, the quasi-solid Al region further extends until entire solidification. The reason for using the Al-5Ti-1B master alloy rather than TiB2 powders as the commercial refiner in Al industry lies in two aspects: the excessive Ti atoms in the master alloy could guarantee sufficient Ti chemical potential to form Ti-terminated surface of TiB2, and the redundant Ti solute in inoculated melts could facilitate the growth of quasi-solid Al region at the solid/liquid interface.

On the entropic nucleation barrier in a martensitic transformation

The nucleation of martensite in alloys is hindered by a free energy nucleation barrier, hence comprising contributions of the potential energy and the entropy. The leading effect is commonly attributed to the potential energy barrier due to strain fields. In this contribution, we investigate the nature of the entropic barrier by means of molecular dynamics (MD) simulations. We study a transformation process of an undercooled single crystal and examine two nucleation events observed under adiabatic conditions using vibrational mode analysis of the atomic trajectories. Our analysis shows that martensitic nucleations are indicated by transit from a state of uncorrelated into a state of correlated atomic motions. This correlation process is built up locally by a small group of atoms even before the product lattice can be recognized morphologically and it produces vibrational ‘soft’ modes along transformation paths. Phase space analyses unveil that the correlation process is characterized by narrow domains – ‘nucleation channels’ – the atomic trajectories have to pass, connecting the phase space domains of the parent and the product lattice. For a successful nucleation event, the nucleus atoms have to pass this channel collectively, which stochastically represents a rare event. Thermal fluctuations prevent finding the channel at elevated temperature and give rise for entropic stabilization of the parent phase. This ‘entropic nucleation barrier’ is reduced in the undercooled state but still effective, thus preventing the parent phase from collapsing into the product. The entropic barrier may be interpreted as the probability of a group of atoms to simultaneously pass the nucleation channel. Such group then represents a nucleus.

Modeling the nucleation statistics in vapor–liquid–solid nanowires

The statistics of nucleation events in nanowires growing via the vapor–liquid–solid mechanism in the mononuclear regime is studied theoretically. A semi-analytical model is developed which is capable of describing the distributions of time intervals between the successive nucleation events and some other useful characteristics of nucleation statistics. Our model accounts for desorption from the droplet, which was not included in the previous studies of nucleation statistics. It is shown that the relative dispersion of nucleation distributions increases with the nanowire radius and at a higher desorption rate from the droplet, leading to the corresponding broadening of the length distribution. Using the model is also shown to fit well experimental data available on nucleation statistics in the Au-catalyzed Si and III–V nanowires.

The effect of domain patterns on 180° domain switching in BaTiO3 crystals during antiparallel electric field loading

In this Letter, the effect of domain pattern on 180° domain switching behavior in BaTiO3 crystals was investigated by using polarized light microscopy during antiparallel electric field loading. Results show that for nearly perfectly in-plane poled specimen, 180° domain switching is accomplished by successive antiparallel domain nucleation and forward domain wall motion; whereas for specimen with in-plane a-a domain pattern, 180° domain switching is achieved by two-step 90° ferroelastic domain switching. This discrepancy is explained by combined effects of the depolarization field and the mechanical constraint from the adjacent domains during 90° ferroelastic domain switching.

Direct electrochemical synthesis and characterization of polypyrrole nano- and micro-snails

Novel nano- and microstructures of polypyrrole were synthesized by an electrochemical template-free method on Nickel (Ni) electrode. Scanning Electron Microscope (SEM) study revealed nano- and micro-snails of polypyrrole after the one-step potentiostatic electropolymerization of pyrrole in aqueous alkaline solution. The influence of the anodic polarization time on the morphological, mechanistic and electrochemical characteristics of these novel polymeric films was investigated. An appropriate mathematical model was established and showed that two simultaneous 2D growth instantaneous nucleation processes and a successive 3D growth progressive nucleation mechanism are involved in the polypyrrole micro-snails formation under a diffusion control. Polypyrrole layers generated from these processes appeared to be in different overoxidation states from X-ray Photoelectron analysis and from the electroactivity study of the modified Ni electrode.

Stochastic modeling of twin nucleation in polycrystals: An application in hexagonal close-packed metals

Twinning in hexagonal close-packed (hcp) metals is a multi-scale process that depends on the microstructural and mechanical response details at the polycrystalline aggregate, grain, micro, and atomic scales. Twinning can generally be regarded as a two-step process, a nucleation event followed by propagation and growth. This articles presents a stochastic model for the nucleation of deformation twins in hcp polycrystals. Twin nucleation is modeled through its dependence on lower length scale material details, such as the defect configurations at potential nucleation sites within grain boundaries, and mechanical details such as highly localized stress concentrations at the microscale in a probabilistic manner. These two aspects, the material and mechanical, must align for a successful nucleation event. The nucleation process is cast as a survival model parameterized by the local stress at the grain boundary. The model gives an explicit form for the probability distribution for the critical stress values required for twin nucleation. The model is implemented into a viscoplastic self-consistent (VPSC) crystal plasticity framework in order to test its predictive capability against previously reported statistical characterization in deformed zirconium at multiple temperatures. For implementation in VPSC, the stress concentrations are sampled from a distribution calibrated to full-field crystal plasticity simulations and a three-dimensional model of grain neighbors and distribution of grain boundary areas are implemented.

Introducing Grain Boundary Influenced Stochastic Effects into Constitutive Models

Twinning is an important deformation mechanism in hexagonal close-packed (hcp) metals such as Mg, Zr, Ti, and Be. Twinning in hcp materials is a multiscale process that depends on microstructural and mechanical response details at the mesoscale, microscale, and atomic scales. Twinning can generally be understood as a two-step process, a nucleation step followed by propagation. The nucleation of twins is governed by both material details such as the defect configurations at potential nucleation sites within grain boundaries, as well as the highly local mechanical field near grain boundaries. These two factors, the material and mechanical, must align for a successful nucleation event. In this article, we present a stochastic constitutive law for nucleation of twins and describe its implementation into a homogenized crystal plasticity simulation. Twin nucleation relies on the dissociation of grain boundary defects under stress into the required twinning partials. This dissociation is considered to follow a Poisson process where the parameters of the Poisson distribution are related to the properties of the grain boundaries. The rate of the process is a direct function of the local stress concentration at the grain boundary. These stress concentrations are randomly sampled from a distribution calibrated to full-field crystal plasticity simulations.

Tantalum-oxide catalysed chemical vapour deposition of single- and multi-walled carbon nanotubes

Tantalum-oxide thin films are shown to catalyse single- and multi-walled carbon nanotube growth by chemical vapour deposition. A low film thickness, the nature of the support material (best results with SiO2) and an atmospheric process gas pressure are of key importance for successful nanotube nucleation. Strong material interactions, such as silicide formation, inhibit nanotube growth. In situ X-ray photoelectron spectroscopy indicates that no catalyst reduction to Ta-metal or Ta-carbide occurs during our nanotube growth conditions and that the catalytically active phase is the Ta-oxide phase. Such a reduction-free oxide catalyst can be technologically advantageous.

Influence of composition on monomodal versus multimodal γ′ precipitation in Ni–Al–Cr alloys

This study investigates the influence of alloy composition on γ′ precipitation in Ni–8Al–8Cr and Ni–10Al–10Cr at.% during continuous cooling from a supersolvus temperature. When subjected to the same cooling rate, Ni–8Al–8Cr develops a monomodal population, whereas Ni–10Al–10Cr develops a multimodal (primarily bimodal) population of γ′ precipitates. The bimodal γ′ precipitate size distribution in Ni–10Al–10Cr alloy can be attributed to two successive nucleation bursts during continuous cooling while the monomodal γ′ size distribution in Ni–8Al–8Cr results from a single nucleation burst followed by a longer time—wider temperature window for nucleation resulting in a larger number density of precipitates. Three-dimensional atom probe investigations reveal that while local equilibrium is achieved across the γ/γ′ interface, far-field γ compositions still retain an excess of Al and a depletion of Cr with respect to equilibrium, thus giving direct experimental evidence of the driving force for subsequent nucleation bursts during continuous cooling in case of Ni–10Al–10Cr. Contrastingly, in case of the Ni–8Al–8Cr alloy, while the γ phase retains an excess of Al and a depletion of Cr with respect to equilibrium after continuous cooling to room temperature, the additional driving force is consumed during subsequent isothermal annealing by growth of the large number of γ′ precipitates without any new nucleation.

On Multiple Nucleation Bursts During Solution Crystallization in Pure and Impure Solvent

It is widely accepted that during batch cooling crystallization processes, uncontrolled cooling can produce multiple nucleation events which have detrimental effects on the crystal size distribution (CSD). Controlling the CSD therefore requires mastering nucleation during the whole batch development: this is the starting point of many studies devoted to temperature control during industrial crystallization operations. When nucleation phenomena take place throughout the batch process, the time allocated to every crystal to grow is widespread, which enlarges the width of the final CSD. Successive nucleation phenomena are referred below to as multiple nucleation bursts (MNBs). Oscillatory supersaturation dynamics (OSD) are sometimes assumed to accompany the trend of MNBs, and it is suggested in the following that MNBs should not be misread as oscillatory supersaturation profiles. Experimental results obtained during the crystallization of ammonium oxalate in the presence of impurities (NiSO4) are presented to support and illustrate OSDs crystallizations which, even though they are often presented as a basic pattern of uncontrolled batch crystallization systems, were never observed before. A population balance equations (PBEs) simulation model based on Kubota–Mullin’s pinning mechanism is developed to represent the observed oscillations of supersaturation. The latter are shown to be rather likely when inhibiting impurities are present in solution, but it is suggested that oscillations of supersaturation are unlikely in pure solvents. These latter two results shed some new light on the need to control temperature during batch cooling crystallization processes.

Steric Crowding and the Kinetics of DNA Hybridization within a DNA Nanostructure System

The ability to generate precisely designed molecular networks and modulate the surrounding environment is vital for fundamental studies of chemical reactions. DNA nanotechnology simultaneously affords versatility and modularity for the construction of tailored molecular environments. We systematically studied the effects of steric crowding on the hybridization of a 20 nucleotide (nt) single-stranded DNA (ssDNA) target to a complementary probe strand extended from a rectangular six-helix tile, where the number and character of the surrounding strands influence the molecular environment of the hybridization site. The hybridization events were monitored through an increase in the quantum yield of a single reporter fluorophore (5-carboxyfluorescein) upon hybridization of the 20-nt ssDNA, an effect previously undocumented in similar systems. We observed that as the hybridization site moved from outer to inner positions along the DNA tile, the hybridization rate constant decreased. A similar rate decrease was observed when noncomplementary single- and double-stranded DNA flanked the hybridization site. However, base-pairing interactions between the hybridization site of the probe and the surrounding DNA resulted in a reduction in the reaction kinetics. The decreases in the hybridization rate constants can be explained by the reduced probability of successful nucleation of the invading ssDNA target to the complementary probe.

Heterogeneous nucleation and depletion effect in nanowire growth

Recent experiments by Ross and co-workers proved the possibility of a mononuclear regime with heterogeneous nucleation as well as jerky growth in the vapor–liquid–solid (VLS) process for silicon nanowires. In this work, a theoretical model is presented which incorporates the effects of (i) a mononuclear regime with layer by layer growth, (i) heterogeneous nucleation of each new layer at the edge of a Au–Si droplet, (iii) drop of supersaturation after each successful nucleation and respective fast layer growth, (iv) time-dependent nucleation barrier during each new waiting period and (v) correlation between subsequent waiting periods (non-Markovian sequence of waiting periods).

Type I Antifreeze Proteins Enhance Ice Nucleation above Certain Concentrations

In this study, we examined the effects that antifreeze proteins have on the supercooling and ice-nucleating abilities of aqueous solutions. Very little information on such nucleation currently exists. Using an automated lag time apparatus and a new analysis, we show several dilution series of Type I antifreeze proteins. Our results indicate that, above a concentration of ∼8 mg/ml, ice nucleation is enhanced rather than hindered. We discuss this unexpected result and present a new hypothesis outlining three components of polar fish blood that we believe affect its solution properties in certain situations.

Evaluation on nonisothermal crystallization kinetics of polypropylene/kaolin composites by employing Dobreva and Kissinger methods

The influences of kaolin content, processing temperature, and shear stress on crystallization of all samples were investigated by differential scanning calorimetry (DSC). The crystallization activation energy calculated using Kissinger’s method displayed a decreasing trend with increasing kaolin content, processing temperature, and shear stress. A study of nucleation activity, which could indicate the influence of filler on polymer matrix, revealed that kaolin filler had a slight nucleation effect on polypropylene (PP) matrix. A thorough observation on nucleation effect also revealed that the incorporation of kaolin in tandem with increasing temperature and shear stress have contributed to successive heterogeneous nucleation in the system.

Miscibility enhancement and formation of interpenetrating spherulites in ternary blends of crystalline polymers

AbstractMiscible blends of three crystalline polymers, namely poly(butylene succinate) (PBS), poly(ethylene succinate) (PES), and poly(oxyethylene) (POE), exhibited interpenetrating spherulites, where a spherulite of one component grows inside the spherulites of other components. PBS and PES were immiscible above the melting points, Tm, of these substances, while ternary blends with POE showed miscibility, which depended on the molecular weight of POE. PBS and PES exhibited the same spherulitic growth process as in a miscible binary blend when they were crystallized from a homogeneous ternary melt. Spherulites of PBS, which is the highest‐Tm component, filled the whole volume first when a miscible ternary blend was quenched below Tm of POE, the lowest‐Tm component. Then, the blends showed either two types of crystallization processes. One was successive nucleation and growth of PES and POE spherulites, that is, PES nucleated and developed spherulites inside the PBS spherulites and then POE spherulites grew inside the interlocked spherulites of PBS and PES. The other was simultaneous growth and the formation of interpenetrating spherulites of PES and POE inside the PBS spherulites. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 706–711, 2010