Successive Nucleation Research Articles

Corrosion of nickel foam electrodes during hydrothermal reactions: The influence of a simple protective carbon black coating

Nickel foam (NF) substrates are widely used to support electrocatalysts, and this is frequently achieved using hydrothermal reactions, where the NF is immersed in the hydrothermal reactor together with the electrocatalyst precursors. However, other reactions including the corrosion of the NF and changes to the pH occur simultaneously, and these can affect the quality of the final electrocatalyst. Herein, a simple approach is devised to minimise these unwanted reactions. Carbon black (CB) was non-covalently functionalised at room temperature using tannic acid to give very stable and good dispersions of fCB in deionised water. Using a simple sonication step, the NF was coated with a uniform layer of the dispersed fCB. This layer served to minimise the corrosion of the underlying NF during the hydrothermal reactions with very good protection observed up to a temperature of 160 °C in deionised water at a pH of 2.0. The corrosion currents of the NF and fCB@NF were estimated at 8.7 µA and 3.9 µA, respectively, at room temperature in this acidic solution. Using a model reaction, the successful nucleation and growth of MnCo2O4 cubes was observed at fCB@NF, but not at the corroding NF.

Determination of Photothermal and EMI Shielding Efficiency of Graphene-Silver Nanoparticle Composites Prepared under Low-Dose Gamma Irradiation.

Silver nanoparticles (Ag NPs) have been produced by low-dose (1-20 kGy) gamma irradiation of silver nitrate in the presence of graphene-based material (graphene oxide or electrochemically exfoliated graphene). The large surface area of those graphene-based materials combined with the presence of oxygen-containing functional groups on the surface provided successful nucleation and growth of Ag nanoparticles, which resulted in a uniformly covered graphene surface. The obtained Ag nanoparticles were spherical with a predominant size distribution of 10-50 nm for graphene oxide and 10-100 nm for electrochemically exfoliated graphene. The photothermal efficiency measurement showed a temperature increase upon exposure to a 532 nm laser for all samples and the highest photothermal efficiency was measured for the graphene oxide/Ag NP sample prepared at 5 kGy. Electromagnetic interference (EMI) shielding efficiency measurements showed poor shielding for the composites prepared with graphene oxide. On the other hand, all composites prepared with electrochemically exfoliated graphene showed EMI shielding to some extent, and the best performance was measured for the samples prepared at 5 and 20 kGy doses.

Atomistic Insights into Nucleation of Ferroelastic Domains in PbTiO3.

Understanding the nucleation mechanism of domains is essential for domain engineering of perovskite ferroelectric materials. We proposed and examined atomistic details for nucleating ferroelastic (FS) domains by integrating topological analysis and first-principles calculations. FS domains are crystallographically treated as deformation twins. The conventional shear-shuffle nucleation mechanism under simple shear deformation is ruled out because the 1-layer elementary twinning disconnection (TD) cannot nucleate and glide in a perfect matrix. Thus, the pure-shuffle nucleation mechanism under pure shear deformation is proposed due to kinetically favored atomic shuffling. The coherency stress associated with the coherent nucleus is relaxed via forming misfit dislocations, accompanied by formation and sharpening of diffused (110)m∥(110)d domain walls (DWs). The sharp DWs enable growth of the FS nucleus through successive nucleation and gliding of TDs. These findings enrich the knowledge of domain behavior in perovskite ferroelectric materials.

Insights into Aerosol Emission Control in the Postcombustion CO2 Capture Process: From Cluster Formation to Aerosol Growth.

Aerosols produced in the amine carbon capture process can lead to secondary environmental pollution. This study employs molecular dynamics (MD) simulations to investigate cluster formation, amine behavior, and aerosol growth of amines, essential for reducing amine aerosol emissions. Results showed that the cluster evolution process can be divided into cluster formation and growth in terms of molecular content, and the nucleation rate for the present systems was estimated in the order of 1028 cm-3 s-1. CO2 absorption was observed alongside successful nucleation, with CO2 predominantly localizing in the cluster's outer layer postabsorption. Monoethanolamine (MEA) exhibited robust electrostatic interactions with other components via hydrogen bonding, leading to its migration toward regions where CO2 and H2O coexisted within the cluster. While MEA presence markedly spurred cluster formation, its concentration had a marginal effect on the final cluster size. Elevating water content can augment the aerosol growth rate. However, altering the gas saturation is possible only within narrow confines by introducing vapor. Contrarily, gas cooling introduced dual, opposing effects on aerosol growth. These findings, including diffusion coefficients and growth rates, enhance theoretical frameworks for predicting aerosol formation in absorbers, aiding in mitigating environmental impacts of amine-based carbon capture.

Transient physics in the compression and mixing dynamics of two nanochannel-confined polymer chains.

We use molecular dynamics (MD) simulation and nanofluidic experiments to probe the non-equilibrium transient physics of two nanochannel-confined polymers driven against a permeable barrier in a flow field. For chains with a persistence length P smaller than the channel diameter D, both simulation and experiment with dsDNA reveal nonuniform mixing of the two chains, with one chain dominating locally in what we term "aggregates." Aggregates undergo stochastic dynamics, persisting for a limited time, then disappearing and reforming. Whereas aggregate-prone mixing occurs immediately at sufficiently high flow speeds, chains stay segregated at intermediate flow for some time, often attempting to mix multiple times, before suddenly successfully mixing. Observation of successful mixing nucleation events in nanofluidic experiments reveal that they arise through a peculiar "back-propagation" mechanism whereby the upstream chain, closest to the barrier, penetrates and passes through the downstream chain (farthest from the barrier) moving against the flow direction. Simulations suggest that the observed back-propagation nucleation mechanism is favored at intermediate flow speeds and arises from a special configuration where the upstream chain exhibits one or more folds facing the downstream chain, while the downstream chain has an unfolded chain end facing upstream.

Gold versus platinum for chemical modification of carbon quantum dots from carboxymethyl cellulose: Tunable biomedical performance

Urgent requirements for medication from chronic inflammation and cancer are considerably interested, while, the recent reports were considered with investigating simple methods for synthesis. Metal-modified carbon quantum dots (“M-CQDs”) were successfully ingrained from carboxymethyl cellulose under the assistance of infra-red irradiation. The current approach demonstrates a study for the effect of structural tuning for biomedical performance of CQDs via modifying of CQDs with either gold (Au-CQDs) or platinum (Pt-CQDs). Successive nucleation of Au-CQDs and Pt-CQDs was confirmed via different instrumental analyses like, TEM micrographs, Zeta potential, XRD, FTIR, 1HNMR&13CNMR spectra. The data reveal that, modification of CQDs (8.7 nm) with gold was reflected in insignificant effect on the mean size of CQDs (8.9 nm), whereas, doping of platinum resulted in slight enlargement of the size (12.4 nm). However, Pt-CQDs were exhibited with the highest anti-inflammatory (cell viability percent 78 %) and antimicrobial action. On the other hand, Au-CQDs were shown with the highest anticancer affinity (reduction of cell viability 83 %) compared to the others. The current study approved the superiority of CQDs modified with either gold or platinum to be successfully applicable as potential therapeutic reagents for the treatment of either cancer or inflammation diseases.

Modeling the effect of prior austenite grain size on bainite formation kinetics

While experiments show that refining the prior austenite grain size can either accelerate or decelerate bainite formation in steels, kinetic models based on the successive nucleation of bainitic ferrite subunits can only predict an acceleration. In this work we develop a physically-based model for bainite kinetics assuming a displacive growth mechanism which is able to reproduce both faster and slower bainite formation kinetics induced by austenite grain refinement. A theoretical analysis of the model and comparison against published experimental data show that slower kinetics for smaller grains is favored as the difference between the activation energy for grain boundary and autocatalytic nucleation of bainite increases, and as the austenite grain refinement results in finer bainite sub-units. We also theoretically analyze the density of initially present potential nucleation sites for bainite and show that the values of density used in other published bainite nucleation models are mostly underestimated. After using physically consistent values for the density of potential nucleation sites, we were able to calculate the apparent lengthening rate of bainite sheaves which were in line with experimentally measured lengthening rates.

Yield criterion for intergranular void coalescence under combined tension and shear

Intergranular ductile fracture is a failure mode that may arise in many metallic alloys used in industrial applications. It manifests as the successive nucleation, growth, and coalescence of cavities at grain boundaries. Thus, simulation of intergranular ductile fracture in polycrystals requires modeling those three different stages at the scale of grain boundaries, i.e. at the interface between two different crystals. In this study, a yield criterion for the coalescence of cavities at the interface between two isotropic materials obeying Mises plasticity is first developed by limit analysis in order to provide some insights into that phenomenon. This criterion is checked against numerical limit analysis under combined tension and shear and is found to agree with unit-cell simulations quantitatively. The model is then extended to crystals so as to account for the complex coupling between loading state, crystallographic orientations, and void microstructure in intergranular coalescence. This second criterion is also assessed through comparisons to numerical limit analysis for an FCC crystal lattice. The agreement is very good in the case of coalescence by internal necking and the trends displayed by coalescence under combined tension and shear are captured correctly. Some implications of the model on the competition between transgranular and intergranular ductile fracture are discussed. Finally, by combining this model with an existing criterion for void growth at grain boundaries, a multi-surface yield function relevant to intergranular ductile fracture is obtained and compared to unit-cell simulations.

Multistep, site-selective noncovalent synthesis of two-dimensional block supramolecular polymers.

Although the principles of noncovalent bonding are well understood and form the basis for the syntheses of many intricate supramolecular structures, supramolecular noncovalent synthesis cannot yet achieve the levels of precision and complexity that are attainable in organic and/or macromolecular covalent synthesis. Here we show the stepwise synthesis of block supramolecular polymers from metal-porphyrin derivatives (in which the metal centre is Zn, Cu or Ni) functionalized with fluorinated alkyl chains. These monomers first undergo a one-dimensional supramolecular polymerization and cyclization process to form a toroidal structure. Subsequently, successive secondary nucleation, elongation and cyclization steps result in two-dimensional assemblies with concentric toroidal morphologies. The site selectivity endowed by the fluorinated chains, reminiscent of regioselectivity in covalent synthesis, enables the precise control of the compositions and sequences of the supramolecular structures, as demonstrated by the synthesis of several triblock supramolecular terpolymers.

Alloying normal and inverse spinel (Zn–Co ferrite) nanostructures via direct precursor pyrolysis for enhanced supercapacitance and water splitting

The nanocrystals comprising of multiple redox-active cations are generally proficient electrocatalysts for renewable and sustainable energy applications. Ferrites have rich redox chemistry and crystallographically, they can be classified into normal spinel structures or inverse spinel structures, depending on the type of occupancy at tetrahedral and octahedral sites. Herein, taking advantage of the simplicity and environmental benignity of the solventless method, a series of mixed inverse and normal spinel nanocatalysts (Co1-xZnxFe2O4 (0 = x ≤ 1)) have been successfully synthesized and tested for supercapacitance and overall water splitting. The p-XRD and EDX analyses confirmed the successful nucleation of CoFe2O4 and ZnFe2O4 to form monophasic Co1-xZnxFe2O4 solid solutions over the entire composition range. The solid solutions with composition Co0·4Zn0·6Fe2O4 demonstrated higher discharge time, indicating higher specific capacitance of the material than other electrode compositions. The Co0·8Zn0·2Fe2O4 showed relatively low overpotentials of 317 mV to afford the current density of 10 mA/cm2 for oxygen evolution reaction (OER), while Co0·6Zn0·4Fe2O4 exhibited an overpotential of 169 mV for hydrogen evolution reaction (HER), outperforming most of the electrocatalysts reported in the literature. More impressively, such solid solutions demonstrated negligible deviation between the first and the 1 k cycles, suggesting high durability for the electrolysis of water.

A Microfluidic Approach to Investigate the Contact Force Needed for Successful Contact-Mediated Nucleation

Emulsions with crystalline dispersed phase fractions are becoming increasingly important in the pharmaceutical, chemical, and life science industries. They can be produced by using two-stage melt emulsification processes. The completeness of the crystallization step is of particular importance as it influences the properties, quality, and shelf life of the products. Subcooled, liquid droplets in agitated vessels may contact an already crystallized particle, leading to so-called contact-mediated nucleation (CMN). Energetically, CMN is a more favorable mechanism than spontaneous nucleation. The CMN happens regularly because melt emulsions are stirred during production and storage. It is assumed that three main factors influence the efficiency of CNM, those being collision frequency, contact time, and contact force. Not all contacts lead to successful nucleation of the liquid droplet, therefore, we used microfluidic experiments with inline measurements of the differential pressure to investigate the minimum contact force needed for successful nucleation. Numerical simulations were performed to support the experimental data obtained. We were able to show that the minimum contact force needed for CMN increases with increasing surfactant concentration in the aqueous phase.

Pursuing colloidal diamonds.

The endeavor to selectively fabricate a cubic diamond is challenging due to the formation of competing phases such as its hexagonal polymorph or others possessing similar free energy. The necessity to achieve this is of paramount importance since the cubic diamond is the only polymorph exhibiting a complete photonic bandgap, making it a promising candidate in view of photonic applications. Herein, we demonstrate that due to the presence of an external field and delicate manipulation of its strength we can attain selectivity in the formation of a cubic diamond in a one-component system comprised of designer tetrahedral patchy particles. The driving force of such a phenomenon is the structure of the first adlayer which is commensurate with the (110) face of the cubic diamond. Moreover, after a successful nucleation event, once the external field is turned off, the structure remains stable, paving an avenue for further post-synthetic treatment.

Influence of process parameters on sealing strength of polymer films in an innovative continuous heat sealing technology

An innovative process for packaging films sealing has been recently promoted: Heat is provided on the upper side of the polymer films. In this paper, sealing conditions are studied for two PET‐PE films. Melting temperature of the sealant layer material was determined by differential scanning calorimetry (DSC), through both conventional and successive self nucleation and annealing procedures. Interface temperature was measured during sealing, and a temperature model was fitted to compute the effective sealing time for each process parameter. Unconventional T‐peel tests were established with disk‐shaped seal samples. Strengths of the seals formed for different process parameters were measured. Results have shown that the melting temperature measured by DSC (108° C and 99° C for PE70 and PE120, respectively) is the seal initiation temperature of the materials. Results of T‐peel tests can be divided in two regions: A growth region where seal strength values increase, up to a threshold at which the seal strength is almost constant (20 N/10 mm). In the growth region, we propose a time–temperature relationship between seal strength, tool temperature, and effective sealing time in the form of an Arrhenius model . The value of the time exponent in the model is in agreement with what the chain diffusion theory predicted .

Visible light assisted mineralization of malachite green dye by green synthesized xanthan gum/agar@ZnO bionanocomposite

The current research presents the ecofriendly and cost-effective green synthesis of zinc oxide nanoparticles (ZnO NPs) through bio-reduction process by a blend of xanthan gum (XG)– agar (AA) biopolymer. The bionanocomposite (BNC) material was explored as a potential photocatalyst and characterized by various instrumental techniques and thus found to have a distorted hexagonal geometry observed through X-ray diffraction (XRD) data. The optical studies suggested the successful nucleation of ZnO NPs in the biopolymer matrix with reduction in optical band gap energy from 3.54 eV to 2.55 eV. The synthesized BNC material showed a strong affinity towards the removal of malachite green (MG) through photocatalytic degradation under visible light irradiation. Response surface methodology-central composite design (RSM–CCD) module was utilized as computational modeling for the optimization of photocatalytic reaction variables and the computed results were found to be as irradiation time (46 min), pH (8.0), MG concentration (70 mg L–1) and catalyst dose (7.0 g L–1). The rate of MG degradation was explained in terms of the pseudo first order kinetics (R2 = 0.999) i.e., formation of adsorption-desorption equilibrium first and then in presence of reactive oxidant species (ROS) ·O2–radicals degradation occurs. The material shows high stability towards photocatalytic experiments and can be reused for five consecutive cycles with high degradation efficiency.

Carbon-induced strengthening of bcc iron at the atomic scale

In steels, the interaction between screw dislocations and carbon solutes has a great influence on the yield strength. Fe-C potentials used in molecular dynamics (MD) simulations yield a poor description of screw dislocation properties---their core structure and Peierls barrier---compared to ab initio calculations. Here we combine two EAM potentials from the literature, which greatly improves dislocation property accuracy in FeC alloys. Using this hybrid potential, MD simulations of dislocation glide in random solid solutions confirm a powerful solute strengthening, caused by complex interaction processes. We analyze these processes in a model geometry, where a row of carbon atoms is inserted in the dislocation core with varying separations. We use a combination of MD simulations, minimum-energy path calculations, and a statistical model based on the harmonic transition state theory to explain the strengthening induced by carbon. We unveil that carbon disrupts the glide process, as unpinning requires the successive nucleation of two kink pairs. When solute separation is below about 100 Burgers vectors, the activation enthalpy of both kink pairs are markedly increased compared to pure iron, resulting in a strong dependence of the unpinning stress on solute spacing. Our simulations also suggest an effect of carbon spacing on the kink-pair activation entropy. This work provides elementary processes and parameters that will be useful for larger-scale models and, in particular, kinetic Monte Carlo simulations.

Contact-mediated nucleation in melt emulsions investigated by rheo-nuclear magnetic resonance.

Increasing the efficiency of disperse phase crystallization is of great interest for melt emulsion production as the fraction of solidified droplets determines product quality and stability. Nucleation events must appear within every single one of the μm-sized droplets for solidification. Therefore, primary crystallization requires high subcooling and is, thus, time and energy consuming. Contact-mediated nucleation is a mechanism for intensifying the crystallization process. It is defined as the successful nucleation of a subcooled liquid droplet induced by contact with an already crystallized droplet. We investigated contact-mediated nucleation under shear flow conditions up to shear rates of 457 s-1 for a quantitative assessment of this mechanism. Rheo-nuclear magnetic resonance was successfully used for the time-resolved determination of the solids fraction of the dispersed phase of melt emulsions upon contact-mediated nucleation events. The measurements were carried out in a dedicated Taylor-Couette cell. The efficiency of contact-mediated nucleation decreased with increasing shear rate, whereas the effective second order kinetic constant increased approximately linearly at small shear rates and showed a linear decrease for shear rates higher than about 200 s-1 . These findings are in accordance with coalescence theory. Thus, the nucleation rate is optimal at specific flow conditions. There are limitations for successful inoculation at a low shear rate because of rare contact events and at a high shear rate due to too short contact time.

Biomineralization processes in modern calcareous tufa: Possible roles of viruses, vesicles and extracellular polymeric substances (Corvino Valley – Southern Italy)

AbstractA modern lithifying biofilm, associated with fluvial calcareous tufa in southern Italy, is composed of a complex community of micro‐organisms including autotrophic and heterotrophic bacteria, together with likely viruses (bacteriophages) and bacterial vesicles. Produced by most of the bacterial forms, extracellular polymeric substances are widespread and are the main sites of mineral precipitation, rather than the microbial cells themselves, but viruses and vesicles also appear to be mineralized. Neoformed biominerals replace these organic substrates starting with an amorphous phase rich in Ca, Si and several other cations followed by two different types of proto‐crystal: a granular−irregular Ca‐rich type and a lamellar/fibrous Si‐Al‐Mg‐rich type. Their growth leads to the formation of Ca‐carbonate and Mg‐silicate crystals that preserve their initial different habits. The coeval precipitation of silicate and carbonate at the initial stage of bio‐mediated mineral formation is a process common within freshwater and saline to hypersaline‐water calcareous microbialites. This is most probably related to the initial formation of poorly‐crystallized Mg‐Si phases that precipitate within the extracellular polymeric substances, related to the rise of pH in the zone of active oxygenic photosynthesis, and the mineralization of viruses, associated with the successive nucleation of Ca‐carbonate in close proximity to bacterial degradation of organic matter.

Technical textiles modified with immobilized carbon dots synthesized with infrared assistance

Carbon quantum dots “CQDs” were investigated as photo-luminescent nanomaterials as it advantageous with nontoxicity to be alternative for metallic-nanomaterials in different purposes. Therefore, the presented report demonstrates an innovative strategy for industrialization of antimicrobial/fluorescent cotton textiles via exploitation of “CQDs”. Unique/novel infrared-assisted technique was currently investigated for clustering “CQDs” form carboxymethyl cellulose. The successive nucleation of “CQDs” (8.0 nm) was affirmed via infra-red, Raman spectroscopy, NMR, TEM and Zeta-potential analysis. The clustered “CQDs” showed antimicrobial and fluorescent characters. The minimal inhibition concentration for “CQDs” (100 mg/mL) against E. coli and C. albicans showed pathogenic reduction of 96% and 82%, respectively. Fluorescent emission spectra for “CQDs” showed two intense peaks at 415–445 nm. “CQDs” were loaded upon pristine and cationized cotton to prepare CQDs@cotton and CQDs@cationized cotton. While, their physical/mechanical properties (air and water vapor permeabilities, tensile strength and elongation %) and thermal stability (TGA & DTG analysis) were studied. The CQDs@cationized cotton exhibited excellent antimicrobial activity with good durability as after ten repretitive washings, inhibition zone diameter against E. coli, was diminished from 21.0 mm to 14.0 mm. The fluorescent emmision intensity was diminished from 741 to 287 after 10 washing cycles. The produced cotton fabrics could be safely used in the medical and military textiles.

Topological aspects responsible for recrystallization evolution in an IF-steel sheet – Investigation with cellular-automaton simulations

A cellular automaton algorithm for curvature-driven coarsening is applied to a cold-rolled interstitial-free steel’s microstructure - obtained through electron backscatter diffraction (EBSD). Recrystallization nucleation occurs naturally during the simulation, due to the highly heterogeneous and hence competitive growth among pre-existing (sub) grains. The spatial inhomogeneity of the subgrain growth that takes place derives from the large local variations of subgrain sizes and misorientations that comprise the prior deformed state. The results show that capillary-driven selective growth takes place to the extent that the prior elongated and deformed grains are replaced by equiaxed grains with no interior small-angle boundaries. Additionally, during the simulation certain texture components intensify and others vanish, which indicates that preferential growth occurs in a fashion that relates to the crystal orientations’ topology. The study of the early stages of recrystallization (i.e. nucleation) shows that the pre-existing subgrains that eventually recrystallize, exhibit certain topological characteristics at the prior deformed state. Successful nucleation occurs mostly for pre-existing matrix subgrains abutting shear bands or narrow deformation bands and particularly at regions where the latter intersect.

Comprehensive Phase Field Study on Directionally-Solidified MoSi<sub>2</sub>/Mo<sub>5</sub>Si<sub>3</sub> Eutectic Alloy

MoSi2/Mo5Si3 eutectic composites have been considered as one of the promising candidates for ultra-high temperature structural applications owing to their high melting point, good oxidation resistance, and low mass density. Their mechanical properties can be improved by controlling the eutectic structure (i.e. script lamellar structure) in directional solidification. It is important to elucidate the dominant factors underlining the unique pattern formation. We conducted a comprehensive phase field study to examine the influence of various factors on the MoSi2/Mo5Si3 eutectic microstructure with complicated morphology. First, the inclined lamellae have been attributed to the minimization of elastic strain energy due to the lattice mismatch between MoSi2 and Mo5Si3, which are partially relaxed by forming semi-coherent phase boundaries. Second, the maze-like pattern on the horizontal cross-section appeared when a two-fold anisotropy of interfacial energy is superimposed on the MoSi2/Mo5Si3 boundary. Third, the random and intersected lamellae have been obtained by assuming the instability of the solid-liquid interface and introducing successive nucleation of Mo5Si3 phase. These findings provide guidance for manipulating the eutectic structure and act as footsteps for further theoretical investigation.