Nucleation Period Research Articles

Complex Aerosol Experiment in Western Siberia (April – October 2013)

The primary project objective was to accomplish the Complex Aerosol Experiment, during which the aerosol properties should be measured in the near-ground layer and free atmosphere. Three measurement cycles were performed during the project implementation: in spring period (April), when the maximum of aerosol generation is observed; in summer (July), when atmospheric boundary layer height and mixing layer height are maximal; and in late summer – early autumn (October), when the secondary particle nucleation period is recorded. Numerical calculations were compared with measurements of fluxes of downward solar radiation. It was shown that the relative differences between model and experimental values of fluxes of direct and total radiation, on the average, do not exceed 1% and 3% respectively.

Complex experiment on studying the microphysical, chemical, and optical properties of aerosol particles and estimating the contribution of atmospheric aerosol-to-earth radiation budget

Abstract. The primary objective of this complex aerosol experiment was the measurement of microphysical, chemical, and optical properties of aerosol particles in the surface air layer and free atmosphere. The measurement data were used to retrieve the whole set of aerosol optical parameters, necessary for radiation calculations. Three measurement cycles were performed within the experiment during 2013: in spring, when the aerosol generation is maximal; in summer (July), when atmospheric boundary layer altitude and, hence, mixing layer altitude are maximal; and in late summer/early autumn, during the period of nucleation of secondary particles. Thus, independently obtained data on the optical, meteorological, and microphysical parameters of the atmosphere allow intercalibration and inter-complement of the data and thereby provide for qualitatively new information which explains the physical nature of the processes that form the vertical structure of the aerosol field.

Silanization-Based Zeolite Crystallization: Participation Degree and Pathway.

A clear and deep understanding of zeolite crystallization with the addition of organosilane is desirable for the reasonable design and preparation of hierarchical zeolites. Herein, the effects of different organosilanes on zeolite crystallization were systematically studied. It was found that organosilane plays the role of an inhibitor in the silanization-based zeolite preparation, and this inhibition effect was determined by its participation degree. An organosilane with a high participation degree can result in the prolongation of nucleation and growth periods of zeolite as well as the variation of product properties. More importantly, a dynamic participation pathway of organosilane is proposed, that is, the growth of zeolite is accompanied by the continuous removal of organosilane, leading to an increase of product crystallinity as well as the decrease of mesoporosity. This study gives a new insight into the role that organosilane plays in zeolite crystallization, which will help to direct the rational selection of organosilane and design of crystallization condition for the optimal synthesis of hierarchical zeolites.

Atomic Layer Deposition of Cobalt Silicide Thin Films Studied by in Situ Infrared Spectroscopy

Atomic layer deposition of cobalt silicide (CoSi2) thin films on H-terminated Si(111) surfaces, using the cobalt-based precursor tertiarybutylallylcobalttricarbonyl (tBu-AllylCo(CO)3) and trisilane, is investigated by in situ Fourier transform infrared spectroscopy (FTIR) and ex situ X-ray photoelectron spectroscopy (XPS) to uncover the film growth mechanisms. The strong reactivity of tBu-AllylCo(CO)3 with H-terminated silicon surfaces and inertness with silicon oxide surfaces, as previously determined by IR spectroscopy [Chem. Mater. 2012, 24, 1025], opens the door for selective deposition. Deposition of CoSi2 is observed after a brief nucleation period (∼3 cycles), during which the stabilization of the cobalt precursor takes place, as evidenced by a shift of the stretch frequency of the carbonyl groups bonded to the Co center from 2010 to 1980 cm–1. This shift is evidence for completion of the catalytic reaction and leads to a surface termination and configuration that is favorable for subsequent ligand...

Study of the synthesis and crystallization kinetics of magnesium hydroxide

The crystallization kinetics of magnesium hydroxide in low concentration solutions has been studied using an electrical conductivity method. Using various experimental relationships, several kinetic factors could be determined during the nucleation and growth process. From measurements of ion concentrations at various temperatures, the induction period of nucleation could be determined. As concentration and temperature increase, the crystallization rate increases affecting the nucleation and crystal growth of Mg(OH)2. The crystal-solution interfacial surface tension was also calculated and shown to be affected by the magnesium anion source. The results show that a uniform particle size distribution of nuclei is formed during the initial homogeneous nucleation process and the nucleation and crystal growth rates can be represented by a first order reaction equation. Through the study of crystallization kinetics, a control strategy is proposed to synthesize a uniform particle size distribution of magnesium hydroxide.

Effect of Elastic Strain Fluctuation on Atomic Layer Growth of Epitaxial Silicide in Si Nanowires by Point Contact Reactions.

Effects of strain impact a range of applications involving mobility change in field-effect-transistors. We report the effect of strain fluctuation on epitaxial growth of NiSi2 in a Si nanowire via point contact and atomic layer reactions, and we discuss the thermodynamic, kinetic, and mechanical implications. The generation and relaxation of strain shown by in situ TEM is periodic and in synchronization with the atomic layer reaction. The Si lattice at the epitaxial interface is under tensile strain, which enables a high solubility of supersaturated interstitial Ni atoms for homogeneous nucleation of an epitaxial atomic layer of the disilicide phase. The tensile strain is reduced locally during the incubation period of nucleation by the dissolution of supersaturated Ni atoms in the Si lattice but the strained-Si state returns once the atomic layer epitaxial growth of NiSi2 occurs by consuming the supersaturated Ni.

Kinetics of nucleation with decreasing rate of growth

Extension of analytical description of the stage of nucleation to the case of the slow growth rates of the embryos growth has been constructed. The metastable phase consumption by the already formed embryos affects the nucleation rate which leads to the non-linear evolution. The power exponentials which are smaller than that for the diffusion growth are chosen as the model laws of the embryos growth. All main characteristics of the nucleation period including the form of the embryos sizes spectrum are found. Analytical description of nucleation in the closed systems as well as in the open systems with the metastable phase influx is presented. It is shown that the relative errors of this description are small.

Ultra-large scale synthesis of high electrochemical performance SnO2 quantum dots within 5 min at room temperature following a growth self-termination mechanism

SnO2 quantum dots are prepared at an ultra-large scale by a productive synthetic procedure without using any organic ligand. The grinding of solid mixture of SnCl2⋅2H2O and ammonium persulphate with morpholine in a mortar at room temperature produces 1.2nm SnO2 quantum dots within 5min. The formation of SnO2 is initiated by the reaction between tin ions and hydroxyl groups generated from hydrolysis of morpholine in the released hydrate water from SnCl2⋅2H2O. It is considered that as water is rapidly consumed by the hydrolysis reaction of morpholine, the growth process of particles is self-terminated immediately after their transitory period of nucleation and growth. As a result of simple procedure and high toleration to scaling up of preparation, at least 50g of SnO2 quantum dots can be produced in one batch in our laboratory. The as prepared quantum dots present high electrochemical performance due to the effective faradaic reaction and the alternative trapping of electrons and holes.

Gravitational Level Effects on Coupling Phenomena Between Morphological Variations of Electrodeposited Film of ZnO and Mass Transfer Rates

Coupling Phenomena between morphological or microstructural evolution rate of electrodeposited film and mass transfer rates must be well understood in order to tailor the unique physical properties of nanostructured films. It is well known microstructure of electrodeposited ZnO film is drastically influenced by the adsorption phenomena of surfactant molecule. However, the mass transfer rate of surfactant has been seldom discussed, especially when natural convection accompanying oxide layer synthesis reaction is induced in an electrolytic cell. The surfactant concentration is significantly influenced at the liquid/soli. interface. Now it is discussed how gravitational level influences the nucleation and growth phenomena of electrodeposited ZnO. ZnO electrodeposition onto FTO/ITO substrate is firstly performed in 0.1MZn(NO3)2-LiNO3 aqueous solution (pH=5.1) at 70°C without addition of surfactant molecule. The substrates were ultrasonically cleaned sequentially in acetone, ethanol and deionized water for 15 min., respectively. The counter electrode was a sheer of pure zinc (Nilaco Corp.). Effective surface of workin and counter electrodes were 10x10mm. They were embedded in each side of channel walls. Ag/AgCl was used as a reference electrode. The cell was arranged in two different configurations: (1) a horizontally installed cathode facing downward over a horizontal anode (C/A) and (2) an anode over a cathode (A/C). No natural convection is principally expected under the former configuration. Current transients were measured at various electrode potential. No apparent differences were observed between -0.85 and -0.8V. Below -0.9V, they were categorized into four stages. Stage 1: Double layer charging and start of NO3 -reduction Stage 2: Induction time for nucleation Stage 3: ZnO nucleation and growth Stage 4: Transport limited The initial three stages can be described by a model including progressive nucleation and kinetically controlled 3D growth. Surface OH- concentration during the nucleation period was estimated by using a simple model. We assume ZnO nucleation can be caused by the production of precursor like Zn(OH)4 2-. Surface morphology and XRD spectrum are changed by the gravitational field direction. The effect may be attributed to natural convection, eventually, concentration of reactant near the substrate cathode surface during electrodeposition.

Preparation of Fe3O4/MnOOH core–shell nanoparticles by a high-frequency impinging stream reactor

Well-defined Fe3O4/MnOOH nanoparticles with 61.1emu·g−1 in magnetization intensity and 90.53m2·g−1 in surface area have been synthesized by a new-style of high-frequency impinging stream (HFIS) reactor. In this reactor, two streams first collided together to form nano Fe3O4 suspension, which subsequently flew through an S-shaped main channel to generate high-frequency reversing high-gravity fields. At the same time, 24 thin liquid sheets impinged into the main channel at the frequencies higher than 100Hz to create nano Fe3O4/MnOOH colloids. The obtained powders were characterized by transmission electron microscopy/energy dispersive spectrometer (TEM/EDS), X-ray diffraction (XRD), Brunner–Emmet–Teller (BET) and vibrating sample magnetometer (VSM). Experimental results indicated that low coating ratio prolonged the induction period of heterogeneous nucleation. The high-frequency impingements of 24 thin liquid sheets greatly accelerated the macro-mixing and the initial dispersion. The high-frequency reversing high-gravity fields promoted the meso- and micro-mixing. As a result, nano Fe3O4 cores were fleetly and uniformly covered by MnOOH precursor. As a continuously operated and static high-gravity reactor, the high-frequency impinging stream (HFIS) reactor is being developed to the large-scaled and low-cost production of various nanocomposites.

Kinetics of transformation on ZIF-67 crystals

Herein we report the structural evolution of ZIF-67 as a function of time at room temperature. We have identified the different stages of ZIF-67 formation (nucleation, crystallization, growth, and steady state periods) and elucidated its kinetics of transformation. It was found that the nucleation and growth of ZIF-67 crystals followed the classic Avrami׳s kinetics. Fundamental studies on the kinetics of formation of porous crystals demonstrated here should facilitate the preparation of MOF phases with controlled crystal size and extent of crystallinity.

Effect of Seed Size, Suspension Recycling and Substrate Pre-Treatment on the CVD Growth of Diamond Coatings

CVD growth of uniform conformal polycrystalline diamond (PCD) coatings over complex three dimensional structures is very important material processing technique. It has been found that the nucleation and subsequent growth period is very critical for successful development of CVD diamond based technologies. There are many methods of enhancing diamond nucleation on foreign substrates-ultrasonic treatment with diamond seed suspension being the best among them. A combination of ultrasonic seeding (US) technique with prior treatment (PT) of the substrate under CVD diamond growth conditions for brief period of time, has found to be very effective in enhancing the diamond nucleation during CVD growth—together they are known as NNP. But successive usage of the same seeding suspension up to ten cycles deteriorates the seeding efficiency. 6th seeding cycle onwards the silicon substrates are barely get covered by diamond crystallites. Five different diamond micron grits were used for seeding the silicon substrates and it is observed that US with the sub-micron particles (0.25 μm) is very effective in efficient nucleation of PCD on Si substrates. PT of the substrate somewhat negates the effect of successive use of the same seeding slurry but it is best to avoid recycling of the same seeding suspension using micron size diamond grits.

Optimization of two-stage cooling profile in unseeded batch crystallization

A two-stage cooling protocol for batch cooling crystallization was optimized to maximize mean crystal size while minimizing the variance in size. A population balance model represents the crystallization process, incorporating crystal growth and nucleation, for use in a multi-objective optimization. At all points along the Pareto front, the optimal strategy has the nucleation event occurring on the temperature plateau between the two cooling stages. This shortens the nucleation period–supersaturation is rapidly depleted, ultimately generating fewer crystals of larger size. In comparison to a simple linear cooling profile, the two-step strategy produces crystals of larger size with smaller variance.

From heterogeneous to homogeneous nucleation for precipitation in austenite of microalloyed steels

This paper studies the influence of strain on precipitate nucleation in austenite for three microalloyed steels with different microalloying element (Nb, V) contents. Precipitation start–time–temperature diagrams were determined by means of hot torsion tests and nucleation periods were measured at strains of 0.20 and 0.35. The increase in the dislocation density caused by the strain was calculated for both strains, and the driving forces for precipitation were also calculated. The results show that the influence of the strain on the nucleation time (t0.05) is dependent not only on the strain magnitude but also on the driving force for precipitation. When the driving force is high, or low in absolute terms, the influence of the strain, i.e. the increase in the dislocation density, gives rise to a notable reduction in the t0.05 value due to heterogeneous nucleation on the dislocation nodes. In contrast, when the driving force is low, or high in absolute terms, the influence of the strain on t0.05 decreases considerably and the nucleation of precipitates becomes preponderantly homogeneous. Therefore, the driving force value is responsible for the transition from heterogeneous nucleation to homogenous nucleation.

Study of Nucleation of CVD Diamond by DC Arcjet Method

Arcjet plasma enhanced CVD was used to grow diamond. Nucleation of diamond was studied at the early stage of growth cause. The micro-structural probes (transmission electron microscope (TEM), high resolution electronic microscope (HREM), selected area diffraction (SAD) and electron energy loss spectra (EELS) were used to characterize the nuclei. It was found that nuclei formed following the amorphous carbon formation. The critical nucleus size was deduced to be less than 20 nm. The growth of nucleus would form the network-like structure. The incubation period of nucleation was deduced as 6–8min under the condition of high concentration of CH4 in H2.

Alternate layer-by-layer assembly of graphene oxide nanosheets and fibrinogen nanofibers on a silicon substrate for a biomimetic three-dimensional hydroxyapatite scaffold.

Layer-by-layer (LBL) assembly is a simple and effective method for the fabrication of a three-dimensional (3D) scaffold for nanotechnological and biomedical applications. Herein, a novel 3D scaffold based on an alternate LBL assembly of graphene oxide (GO) nanosheets and fibrinogen nanofibers (Fg NFs) on a silicon substrate was fabricated and utilized to create a 3D hydroxyapatite (HA) scaffold by biomimetic mineralization in 1.5× simulated body fluid for different nucleation periods. The obtained 3D (GO-NF)n-HA scaffold was characterized using atomic force microscopy, scanning electron microscopy, transmission electron microscopy, X-ray diffraction, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. The results demonstrate that the Fg NFs could promote the nucleation and growth of HA crystals along the axis. The 3D (GO-NF)10-HA scaffold composed of 10 layers of GO alternating with 10 layers of NFs was successfully created by LBL assembly and subsequent biomimetic mineralization. The effects of the created 3D (GO-NF)10-HA scaffolds on the adhesion, morphology, and proliferation of L-929 cells were investigated. The in vitro cell culture indicates that the 3D (GO-NF)10-HA scaffold has a higher proliferation ability and better cytocompatibility than the other control samples.

Direct visualization of subcritical nuclei in molecular film growth

The growth and structure of 4,4'-biphenyldicarboxylic-acid (BDA) on Cu(001) at temperatures between 300 K and 400 K was studied by LEEM and μ-LEED. BDA is a linear molecule consisting of two phenyl rings with a carboxylic-acid group at opposite ends. During growth on Cu(001) the adsorbed BDA molecules form first a disordered 2D gas phase. Once this phase reaches a sufficiently large density, a crystalline phase nucleates, in which the molecules form a hydrogen-bonded 2D supramolecular network. By a careful analysis of the LEEM bright-field image intensity we can measure the density of the 2D gas phase, which is up to 40% of that in the crystalline phase. From the equilibrium densities at different temperatures we can construct the 2D phase diagram and extract the cohesive energy (0.35 eV). During the distinct nucleation period we can observe a fascinating phenomenon: sub-critical nuclei form, grow up to 4000 nm2in size and decay with lifetimes of several seconds. These sizes are considerably larger than what is usually seen in epitaxial growth and we explain this observation with the relatively weak intermolecular interactions.

Nucleation and Growth of Silver Nanoparticles by AB and ABC-Type Atomic Layer Deposition

In this work, we report synthesis strategies to produce Ag nanoparticles by AB-type and ABC-type atomic layer deposition (ALD) using trimethylphosphine- (hexafluoroacetylacetonato) silver(I) ((hfac)Ag(PMe3)) and formalin (AB-type) and (hfac)Ag(PMe3), trimethylaluminum, and H2O (ABC-type). In situ quartz crystal microbalance measurements reveal a Ag growth rate of 1−2 ng/cm 2 /cycle by ABC-type ALD at 110 °C and 2−10 ng/cm 2 /cycle for AB-type ALD at 170−200 °C. AB-type Ag ALD has a nucleation period before continuous linear growth that is shorter at 200 °C. Transmission electron microscopy reveals that AB-type Ag ALD particles have an average size of ∼1.8 nm after 10 cycles. ABC-type Ag ALD particles have an average size of ∼2.2 nm after 20 cycles. With increasing ALD cycles, ABC-type Ag ALD increases the metal loading while maintaining the particle size but AB-type Ag ALD results in the formation of bigger particles in addition to small particles. The ability to synthesize supported metal nanoparticles with well-defined particle sizes and narrow size distributions makes ALD an attractive synthesis method compared to conventional wet chemistry techniques.

The formation and growth of ultrafine particles in two contrasting environments: a case study

Abstract. Formation of ultrafine particles and their subsequent growth have been examined during new particle formation (NPF) events in two contrasting environments under varying ambient conditions, one for a tropical semi-urban coastal station, Trivandrum, and the other for a high-altitude free-tropospheric Himalayan location, Hanle. At Trivandrum, NPF bursts took place in the late evening/night hours, whereas at Hanle the burst was a daytime event. During the nucleation period, the total number concentration reached levels as high as ~ 15 900 cm−3 at Trivandrum, whereas at Hanle, the total number concentration was ~ 2700 cm−3, indicating the abundant availability of precursors at Trivandrum and the pristine nature of Hanle. A sharp decrease was associated with NPF for the geometric mean diameter of the size distribution, and a large increase in the concentration of the particles in the nucleation regime (Dp < 25 nm). Once formed, these (secondary) aerosols grew from nucleation (diameter Dp < 25 nm) to Aitken (25 ≤ Dp ≤ 100 nm) regime and beyond, to the accumulation size regimes (100 ≤ Dp ≤ 1000 nm), with varying growth rates (GR) for the different size regimes at both the locations. A more rapid growth ~ 50 nm h−1 was observed at Trivandrum, in contrast to Hanle where the growth rate ranged from 0.1 to 20 nm h−1 for the transformation from the nucleation to accumulation – a size regime that can potentially act as cloud condensation nuclei (CCN). The faster coagulation led to lifetimes of < 1 h for nucleation mode particles.

Ab Initio Emulsion and Miniemulsion Polymerization of Styrene Mediated by a Cyclohexenyl-Functionalized Amphiphilic RAFT Agent

We designed and used a cyclohexenyl-functionalized amphiphilic macromolecular reversible addition–fragmentation chain transfer (macro-RAFT) agent as surfactant and polymerization mediator for (mini)emulsion polymerization of styrene (St), aiming at synthesizing nanoparticles with vinyl-enriched surface and well-defined polymer chains. Unexpectedly, the pendent cyclohexenyl units in the macro-RAFT agent were found to undergo significant copolymerization with St exclusively during the nucleation period in the ab initio emulsion polymerization, leading to the consumption of cyclohexenyl groups, positive deviation of molecular weights, and broad molecular weight distributions. By contrast, in miniemulsion polymerization, the radical polymerization exhibited superb selectivity toward St monomers relative to the pendent cyclohexene vinyl units just as in the solution polymerization. The obtained polymer chains retained designed molecular weights, low polydispersity indexes, and quantitative cyclohexenyl functio...