Gas-phase Nucleation Research Articles

Fractal approach of nucleation in liquid and gas phases

g→+ In the kinetic equations derived by taking into consideration nucleation and nuclei growth, the fractal character of the new phase nuclei was introduced. In this note a higher generality of the gas phase or liquid phase nucleation fractal treatment is suggested. This treatment enables to describe, in principle, besides the generation of the solid product from gas phase reactants, crystallization from liquids and solutions and precipitation [6]. For the quantitative treatment the bulk disappearance of the potential nuclei which turn into real ones should be considered. Besides one has to take into account that potential nuclei can disappear due to the surface reaction as they could be incorporated by the growing nuclei [6]. One has to mention that nucleation can be normal or it can occur according to a branched chain mechanism.

High-Growth-Rate Chemical Vapor Deposition of Silicon: an Experimental and Modeling Approach

Abstract The chemical vapor deposition (CVD) process of silicon from silane has been analyzed by a combined experimental and modeling approach with the aim to optimize growth rate, yield and deposition quality of polycystalline silicon while minimizing gas phase nucleation. The effects of precursor concentration, flow rate and buffer gas have been investigated. Modeling of the gas phase and of the deposition process has been used in a parameter screening to select appropriate deposition conditions in a stagnation-point cold-wall CVD reactor. Deposition experiments have been performed for several silane concentrations with helium, nitrogen and hydrogen as the buffer gases. Polycrystalline films of good quality were obtained with growth rates up to 8 μm/min. The observed trends in the experiments are in good qualitative agreement with model predictions. To achieve high film growth rates and minimal gas phase nucleation, hydrogen is suggested as the diluent gas, potential reasons for this being its involvement in gas phase and surface reactions.

Size measurement of plutonium particles from internal sputtering into air

During the past century, the results of spontaneous translocation of radioactivity in air, biological media and groundwater have been reported. Here, we report the first measurements of the size characteristics in air of the particles participating in this translocation phenomenon. For the plutonium material powering radioisotope thermal generators, we find two narrow, well-separated fractions, one corresponding to particles below a nanometer and one at or below 10 nm. These results are interpreted as a gas-phase nucleation phenomenon arising from internal sputtering. They suggest fruitful directions for further research with immediate implications for accounting for the effects of radiological terrorism, for identifying new signatures for nuclear materials of possible use in antiterrorism and other covert nuclear materials operations, for radioactive and mixed materials storage handling, for reactor safety and source term modeling and for other materials processes.

Tungsten oxide nanoparticles synthesised by laser assisted homogeneous gas-phase nucleation

Tungsten oxide nanoparticles were generated by excimer (ArF) laser assisted chemical vapor deposition from WF 6/H 2/O 2/Ar gas mixtures. The deposited particles were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy. The deposition rate as a function of the partial pressures of the reactants and of the laser fluence was measured by X-ray fluorescence spectroscopy. The mean diameter of the deposited tungsten oxide particles varied with the experimental parameters and was typically 23 nm. Particles with a higher degree of crystallinity were observed at a laser fluence exceeding 130 mJ/cm 2, and X-ray amorphous particles were obtained below 110 mJ/cm 2. The amorphous tungsten oxide had a stoichiometry ranging from WO 2.7 to WO 3. Deposits were formed only when hydrogen was present in the gas mixture.

Synthesis of TiO2 Nanoparticles Using Chemical Vapor Condensation

AbstractNano-sized TiO2 particles are of interest for many applications, including use as photocatalysts and in heat transfer fluids (nanofluids). In the present study, TiO2 nanoparticles with controllable phase and particle size have been obtained through homogeneous gas-phase nucleation using chemical vapor condensation (CVC). The phase and particle size of TiO2 nanoparticles under various processing conditions have been characterized using x-ray diffraction and transmission electron microscopy. Chamber temperature and pressure were found to be two key parameters affecting particle phase and size. Pure anatase phase was observed for synthesis temperatures as low as 600 °C with chamber pressure varying from 20-50 Torr. When the furnace temperature was increased to 1000 °C at a pressure of 50 Torr, a mixture of anatase and rutile phases was observed, with the predominant phase being anatase. The average particle size under all the experimental conditions was observed to be less than 20 nm.

Reactions of amines with CVD diamond nanopowders

Diamond powders have been synthesised by inducing gas phase nucleation within a multicathode direct current plasma. The surfaces of the spherical particles, with diameters in the range of 150–600 nm, were thermally oxidised at 550 °C under pure oxygen at one atmosphere pressure. Subsequently, the powders were successfully functionalised via reaction with a molecule, 4-(trifluoromethyl)benzylamine containing the reactive amine, –NH 2, functional group. The F(1s) and N(1s) signals in the XPS spectra of the sample following reaction were evident after prolonged washing with methanol solvent indicating covalent binding to the surface of the particles.

A microvisual study of solution-gas-drive mechanisms in viscous oils

The literature suggests that oil-phase viscosity has a profound effect on oil production during solution gas drive. Some heavy-oil reservoirs show higher than expected production rates, produced gas–oil ratio (GOR) close to solution GOR, and relatively high recovery. The reasons for this behavior are not clear, but it has been suggested that gas-phase nucleation, growth of gas bubbles, and eventual coalescence into a continuous gas phase must be examined in detail to interpret better recovery. To this end, solution-gas-drive (depletion) experiments were conducted in micromodels, oil-lens drainage was observed and modeled in cornered capillary tubes, and bubble growth modeled. Experiments and calculations were conducted with viscous and low-viscosity liquids to probe explicitly the effect of solution viscosity. A micromodel is a two-dimensional representation of pore space. The micromodels employed are etched from silicon and exhibit pore body and throat sizes equivalent to a representative sandstone. Likewise, they capture many aspects of pore-wall roughness. Micromodels are housed in a pressure vessel to allow experimentation at elevated pressure. Pore-level events are viewed through an optical microscope. Spatial resolution of events within the micromodel is on the order of 1 μm. Nucleation occurs repeatedly and regularly at surface roughness. Nucleated bubbles grow to fill pores before they are mobilized. Interestingly, solution viscosity appears to slow considerably the coalescence of gas. For coalescence of two gas bubbles into a single bubble, the liquid lens separating the bubbles must drain. Experimental results in micromodels and triangular capillary tubes indicate that when oil-phase viscosity is high, the rate of coalescence of gas bubbles is slow. Capillary tube experiments are modeled exactly with no adjustable parameters. It is shown that the period of time for drainage is at least linearly proportional to the liquid viscosity. The implication for recovery of viscous oil is that the produced gas–oil ratio and gas-phase relative permeability remain relatively low because gas bubbles remain dispersed. Correspondingly, oil-phase relative permeability remains high contributing to relatively efficient oil recovery despite high oil-phase viscosity.

Atmospheric ion‐induced nucleation of sulfuric acid and water

Field studies show that gas phase nucleation is an important source of new particles in the Earth's atmosphere. However, the mechanism of new particle formation is not known. The predictions of current atmospheric nucleation models are highly uncertain because the models are based on estimates for the thermodynamics of cluster growth. We have measured the thermodynamics for the growth and evaporation of small cluster ions containing H2SO4 and H2O, and incorporated these data into a kinetic aerosol model to yield quantitative predictions of the rate of ion‐induced nucleation for atmospheric conditions. The model predicts that the binary negative ion H2SO4/H2O mechanism is an efficient source of new particles in the middle and upper troposphere. The ion‐induced HSO4−/H2SO4/H2O mechanism does explain nucleation events observed in the remote middle troposphere, but does not generally predict the nucleation events observed in the boundary layer.

Study of CNT synthesis mechanism by in-situ spectroscopy

Optical emission spectra (OES) of the plasma during synthesis of carbon nanotubes (CNTs) were in-situ examined for the purpose of understanding the formation mechanism of CNTs. It is observed that simultaneous presence of abundant amount of neutral Fe-atoms and the neutral (or singly-ionized) carbon-atoms in the plasma, but without the induction of carbon clusters, is necessary for growing CNTs. The optimum chamber pressure required is 400 Torr in 100 W microwave powers, which corresponds to 5000 K plasma temperature. The CNTs are presumed to form via gas phase nucleation process and thus obtained CNTs are multi-wall, approximately 30–70 nm in diameter.

Aerosol dynamics modeling of silicon nanoparticle formation during silane pyrolysis: a comparison of three solution methods

A numerical model has been developed to predict gas-phase nucleation, growth, and coagulation of silicon nanoparticles formed during thermal decomposition of silane. A detailed chemical kinetic model of particle nucleation was coupled to an aerosol dynamics model that includes particle growth by surface reactions, coagulation with instantaneous coalescence, and convective transport. Solution of the aerosol general dynamic equation was handled by three approaches: (1) the efficient and reasonably accurate method of moments; (2) the quadrature method of moments, which requires no prior assumption of the shape of the particle size distribution; and (3) a computationally more expensive sectional method (SM). The SM includes a conceptually simple and computationally efficient algorithm for treating coagulation that is found to be superior to widely used methods from the literature. All three approaches gave similar results for the evolution of the first few moments of the particle size distribution. The SM was able to capture the bimodal distribution that appears briefly at short residence times due to simultaneous nucleation and coagulation. At longer residence time, only coagulation remains important and both the sectional and moment methods give very similar results as they approach the self-preserving size distribution.

Experimental Thermodynamics of Cluster Ions Composed of H2SO4 and H2O. 2. Measurements and ab Initio Structures of Negative Ions

Gas-phase nucleation in the atmosphere may be initiated by the condensation of sulfuric acid and water around ions. The thermodynamics of stepwise cluster ion growth must be known in order to model ion-induced nucleation of the H2SO4/H2O system. In the companion paper, we measured temperature-dependent equilibrium constants for the reactions of H2O with the H+(H2SO4)s(H2O)w cluster ions using an ion flow reactor. In this work, H2O bond enthalpies and entropies for the HSO4-(H2SO4)s(H2O)w, s ≤ 6 and w ≤ 10, cluster ions were measured. The thermodynamics of H2SO4 ligand bonding in these clusters were also determined using data from previous experiments. The small positive and negative cluster ions exhibit very different chemical characteristics, but both families show trends analogous to bulk solution thermodynamics as clusters grow in size. Small negative cluster ions have a lower affinity for H2O than positive ion species, but H2SO4 bonding is considerably stronger in the negative clusters. Ion-induced nucleation of the negative H2SO4/H2O cluster ions is therefore thermodynamically favored. Addition of H2O to certain HSO4-(H2SO4)s(H2O)w cluster ions results in unusually large reaction entropies. Ab initio calculations were performed to investigate ligand bonding in the HSO4-(H2SO4)s(H2O)w cluster ions. Stable cluster geometries comprised of multiple ions were identified for s ≥ 2. Large experimental entropy changes upon H2O addition may be due to multiply ionic species formed by proton transfer within the product cluster.

Experimental Thermodynamics of Cluster Ions Composed of H2SO4 and H2O. 1. Positive Ions

Predictions from gas phase nucleation models are highly sensitive to the thermodynamics of the initial stages of particle growth, where free energies calculated using classical nucleation theory are expected to be the least accurate. There is strong evidence that H2SO4 and H2O are directly involved in atmospheric nucleation. These species are also principal components of atmospheric ions, suggesting that they may play important roles in ion-induced nucleation mechanisms. In part 1 of this work, we measured equilibrium constants for the reactions of H2O with the cluster ions, H+(H2SO4)s(H2O)w, s ≤ 4 and w ≤ 15, over a range of temperatures using an ion flow reactor. H2O bond enthalpies and entropies were derived from van't Hoff analyses, and results for the H+(H2O)w system are in agreement with literature values. Thermodynamics of H2SO4 binding in the H+(H2SO4)s(H2O)w cluster ions were also estimated based on comparisons with the HSO4-(H2SO4)s system, whose bond enthalpies were measured previously. As clusters grow in size, some thermodynamic trends begin to reflect those for bulk H2SO4/H2O solutions. A stable population of the H+(H2O)w cluster ions is present in the atmosphere, but for typical concentrations, incorporation of the first H2SO4 molecule to form H+(H2SO4)(H2O)w is thermodynamically unfavorable at 270 K. As a result, significant growth and subsequent nucleation of the H+(H2SO4)s(H2O)w system is not anticipated in the middle or lower troposphere.

Investigation of a Novel Condensation Aerosol Generator: Solute and Solvent Effects

Part I of this article presents results of an experimental study on gas-phase nucleation for three model solutes and their solvent, propylene glycol (PG), with variables being solute concentration and the nature of the solute substance. A single manifestation of an aerosol generator, which forms condensation aerosols by cooling of hot vapor issuing from an electrically heated, pumped capillary, is described and used for all experiments. The effects of solute concentration and solute type were studied for deoxycorticosterone (DOC), benzil (BZ), and phenyl salicylate (PhS). Suppression of homogeneous nucleation and occurrence of heterogeneous condensation of the solvent was observed at different solute concentrations for BZ, PhS, and DOC. The nature and concentration of the solute dissolved in the solvent was shown to determine the final particle size distribution of the condensed aerosol. In the case of the least volatile solute, DOC, solute aerosol and total aerosol size distributions were identical at low solute concentrations. A transitional concentration region then existed in which a bimodal solute aerosol was formed, followed at high concentrations by increasing separation of the solvent-dominated aerosol size distribution and that of the solute. In Part II of this article, the effect of DOC dissolution in different solvents was studied at fixed solute concentration. The effects of six glycol solvents--i.e., PG, ethylene glycol (EG), dipropylene glycol (DPG), diethylene glycol (DEG), triethylene glycol (TEG), and tetraethylene glycol (TetEG)--and three nonglycol solvents--i.e., dimethyl sulfoxide (DMSO), formamide (FORM), and oleyl alcohol (OA)--were studied, as these affected the resultant aerosol sizes. Suppression of total aerosol mass median aerodynamic diameter (MMAD) was observed on dissolution of 0.5% w/w DOC in each solvent, although suppression occurred to different extents. It was shown that the boiling point or volatility of the solvent used to dissolve the less volatile DOC had an effect on the final particle size distribution of the condensed aerosol and whether the aerodynamic size distributions for the solute and the total aerosol were the same or different.

The role of total pressure in gas-phase nucleation: A diffusion effect

A numerical modeling study is presented concerning the role of total pressure in gas-phase nucleation. We consider a generic self-clustering mechanism for particle production in which the cluster transport properties are assumed to scale with monomer properties. A steady-state isothermal plug flow reactor model is considered in which axial diffusion of gas species is allowed. Species conservation equations with appropriate boundary conditions are solved in the one-dimensional reactor space. A steady-state nucleation current is shown to be achievable when the monomer concentration is fixed, regardless of the total pressure. In the case when the monomer concentration is varied, we demonstrate that diffusion can play a critical role at low pressures, where the nucleation rate is found to be highly pressure-sensitive. In this regime steady-state nucleation is not possible. As pressure increases, the nucleation rate becomes gradually pressure-insensitive and approaches the high-pressure limit. A dimensionless analysis is performed for a series of irreversible clustering reactions, and the effects of dimensionless system parameters on the nucleation rate are presented as a function of pressure. The results indicate that to minimize particle formation in chemical vapor deposition reactors while maintaining high deposition rates one should operate in the pressure-dependent regime, with low total reactor pressures and high partial pressures of reactants.

Modeling gas-phase nucleation in inductively coupled silane-oxygen plasmas

A detailed chemical kinetics mechanism was developed to model silicon oxide clustering during high density plasma chemical vapor deposition of SiO2 films from silane-oxygen–argon mixtures. An inductively coupled plasma reactor was modeled in a one-dimensional multicomponent two-temperature framework. Spatial distributions of species concentrations were calculated. The effects of discharge parameters and the main processes contributing to cluster formation were examined. A sensitivity analysis was conducted to determine the dominant reactions that affect the model results.

Deposition process of laminar pyrocarbon from propane

The deposition mechanisms of smooth laminar (SL) and rough laminar (RL) pyrocarbons from propane are found to be very different, as evidenced by (i) gas phase analysis, (ii) nucleation experiments and (iii) analysis of the deposit composition and nanostructure. SL pyrocarbon results mainly from aliphatic species; its nucleation needs surface reactive sites where chemisorption and hydrogen abstraction occur; its growth leads to extended but highly distorted graphene layers which can enclose internal nanoporosity. RL pyrocarbon results mainly from polycyclic aromatic hydrocarbons (PAH); its nucleation, which does not depend on the substrate, occurs widely by physisorption of PAH; the corresponding graphene layers are not very extended but parallel to each other and parallel to the anisotropy plane.

Size Distribution Change of Titania Nano-Particle Agglomerates Generated by Gas Phase Reaction, Agglomeration, and Sintering

In the manufacturing of nanometer-sized material particlulates by aerosol gas-to-particle conversion processes, it is important to analyze how the gas-phase chemical reaction, nucleation, agglomeration, and sintering rates control the size distribution and morphology of particles. In this study, titania particles were produced experimentally by the thermal decomposition of titanium tetraisopropoxide (TTIP) and oxidation of titanium tetrachloride (TiCl 4 ) using a laminar flow aerosol reactor. The effect of reaction temperature on the size and morphology of the generated particles was investigated under various conditions. The size distributions of agglomerates were measured using a DMA/CNC system. The size distributions of primary particles were measured using TEM pictures of the agglomerates sampled by a thermophoretic aerosol sampler. In order to model the growth of both agglomerates and primary particles simultaneously, a two-dimensional discrete-sectional representation of the size distribution was em...

Particle generation and thin film surface morphology in the tetraethylorthosilicate/oxygen plasma enhanced chemical vapor deposition process

Particle generation by gas-phase nucleation in the plasma enhanced chemical vapor deposition process and its effects on thin film surface morphology were studied experimentally for a conventional radio frequency plasma reactor using tetraethylorthosilicate (TEOS) vapor and oxygen gases. The particles suspended in the plasma space and deposited on the film were observed simultaneously by in situ laser light scattering methods and an ex situ scanning electron microscopic method. The generated particles were trapped in the plasma/sheath boundary under all four experimental conditions, in which TEOS concentrations were 0.5 and 5.0 vol % and reactor pressures were 66.7 and 533.3 Pa (0.5 and 4.0 Torr). The size and amount of particles and the film morphology were found to depend on the TEOS concentration and the reactor pressure. Under the conditions in which highly concentrated particles were generated in plasma, dome-like structures of 50–400 nm in diameter were observed on the thin film surface. The average size of the dome-like structure was comparable to that of the particles generated in plasma.

Deposition mechanism of gold by thermal evaporation: approach by charged cluster model

The charged cluster model states that chemical vapor deposition (CVD) begins with gas-phase nucleation of charged clusters followed by cluster deposition on a substrate surface to form a thin film. Gold deposition by thermal evaporation was studied in a two-chambered CVD system in order to determine if this mechanism applies to the present case. The presence of nanometer-sized gold clusters was confirmed by transmission electron microscopy (TEM). The charge on the primary clusters was found to be positive. Smaller clusters were found to be amorphous and combined with clusters already deposited on a substrate surface to form larger amorphous clusters on the surface. Larger clusters were also observed by TEM to form after 120 and 300 s. These clusters exhibited a lattice structure and may have grown by attachment of small clusters in the gas phase.

Modeling of gas phase nucleation during silicon carbide chemical vapor deposition

Abstract An advanced model taking into account silicon cluster formation in the gas phase is suggested. 2D simulation is carried out at the growth parameters typical for chemical vapor deposition (CVD) of SiC in a vertical reactor. It is found that two main parameters have a significant effect on the nucleation and transport of the clusters in the gas phase: the input flow rate of silane and the thermophoretic force. Using a special criterion, the growth conditions favorable for parasitic graphite and silicon phases formation are determined. The obtained results are in good agreement with experimental data.