Primary Particle Size Distribution Research Articles

Characterization of Fractal Structures by Spray Flame Synthesis Using X-ray Scattering.

In this work, we take on an in-depth characterization of the complex particle structures made by spray flame synthesis. Because of the resulting hierarchical aggregates, very few measurement techniques are available to analyze their primary particle and fractal properties. Therefore, we use small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) to investigate the influence of the precursor concentration on the fractal structures of zirconia nanoparticles. The combination of information gained from these measurement results leads to a detailed description of the particle system, including the polydispersity and size distribution of the primary particles. Based on our findings, unstable process conditions could be identified at low precursor concentrations resulting in the broadest size distribution of primary particles with rough surfaces. Higher precursor concentrations lead to reproducible primary particle sizes almost independent of the initial precursor concentration. Regarding the fractal properties, the typical shape of aggregates for aerosols is present for the investigated range of precursor concentrations. In conclusion, the consistent results for SAXS and TEM show a conclusive characterization of a complex particle system, allowing for the identification of the underlying particle formation mechanism.

Estimating particle-size distribution from limited soil texture data: Introducing two new methods

Most soil databases present the mass fractions of sand, silt, and clay particles without detailed primary particle-size distribution (PSD). In the present paper two simple methods are introduced to estimate the PSD using the fractions of sand, silt, clay, and very coarse sand. A total of 504 soil samples with different texture classes from different regions, i.e., southern Iran (245), south-eastern Brazil (116), and UNSODA database (143) were used. PSD of the Iranian and Brazilian soil samples were determined using a combination of wet-sieving and sedimentation (hydrometer for Iranian and pipette for Brazilian soils) approaches and that of the others extracted from UNSODA database. We developed two new methods including very coarse sand-dependent (VCS-D) and very coarse sand-independent (VCS–I) to predict PSD of the soils. PSD were also predicted using the revised Skaggs method (R-Skaggs) and compared with the measured ones. Both VCS-D and VCS-I approaches with very simple equations accurately estimated full range of PSD in a wide range of the fine to medium textured soils)clay, clay loam, loam, silty clay, silty clay loam, silt loam, and silt texture classes). Accuracy of predictions was slightly lower in coarse textured soils (sandy clay, sandy clay loam, sandy loam, loamy sand, and sand textures) than that of the other texture classes. For these coarse textured soils, it is strongly recommended that the fraction of particles between 0.05 and 2 mm diameters be estimated using the VCS-D or VCS-I methods and the fraction of particles between 0.002 and 0.05 mm diameters be estimated using the R-Skaggs method. The R-Skaggs method could only predict the PSD in the soils with nearly 25–60% sand content, accurately. Whereas, the new proposed methods could predict full range of PSD with high accuracy (NRMSE <10%) in soils with sand content less than 53%. Overall, the proposed methods could accurately predict full range of PSD curve by using only the content of primary soil particles in nearly 73% of soil classes in texture triangle. • New methods (VCS-D and VCS-I) proposed to estimate particle-size distribution (PSD). • The methods are very simple and accurate to estimate PSD from limited texture data. • The methods are very accurate to estimate PSD in 73% of soil texture triangle. • The methods excellently estimate PSD in fine and medium textured soils. • The methods were more accurate than R-Skaggs in fine and medium textured soils.

A robust trivariate approach for characterizing the primary particle size distribution of soot aggregates using time-resolved laser-induced incandescence

This study proposes a new trivariate retrieval method based on a moderate-fluence laser-induced incandescence (LII) model. By changing the undesirable mathematical properties present in our previous work, this upgraded new method is able to obtain robust estimates of the two primary particle size distribution parameters and one assembled parameter, and still maintains the advantage of not requiring prior knowledge of thermal accommodation coefficient and aggregate structure. The robustness of the upgraded method is verified on a large number of numerical results, as evidenced by significantly improved retrieval accuracy and substantially reduced estimation uncertainty. The generality of the upgrade method is demonstrated over a large range of three variables. Furthermore, the influence of several key factors (the number of shots used for averaging noisy LII signal, the intensity ratio of the measurement termination, and the sublimation sub-model error) on the retrieval results of the upgraded method is investigated numerically.

Layered Cathode Materials: Precursors, Synthesis, Microstructure, Electrochemical Properties, and Battery Performance.

The exploitation of clean energy promotes the exploration of next-generation lithium-ion batteries (LIBs) with high energy-density, long life, high safety, and low cost. Ni-rich layered cathode materials are one of the most promising candidates for next-generation LIBs. Numerous studies focusing on the synthesis and modifications of the layered cathode materials are published every year. Many physical features of precursors, such as density, morphology, size distribution, and microstructure of primary particles pass to the resulting cathode materials, thus significantly affecting their electrochemical properties and battery performance. This review focuses on the recent advances in the controlled synthesis of hydroxide precursors and the growth of particles. The essential parameters in controlled coprecipitation are discussed in detail. Some innovative technologies for precursor modifications and for the synthesis of novel precursors are highlighted. In addition, future perspectives of the development of hydroxide precursors are presented.

Characteristic of TiO2&Ag0 nanocomposites formed via transformation of metatitanic acid and titanium (IV) isopropoxide

Composite nanoparticles TiO2&Ag0 were synthesized by coprecipitationin a weakly alkaline medium in the presence of various precursors - metatitanic acid and titanium (IV) isopropoxide. Comparative study of TiO2&Ag0 included analysis of morphology (SEM), phase (XRD) and chemical compositions (EDS). The formation of anatase structures was confirmed at a relatively low concentration of silver (0–8 wt% for metatitanic acid and 0–4 wt% for Ti (IV) isopropoxide). An increase in the Ag fraction to 8 wt% led to the appearance of the Ti2O3 and β-TiO2 phases in the Ti(IV) isopropoxide system. The size distribution of primary particles is 8.3–10.6 nm (Ti(IV) isopropoxide system) and 12.3–14.0 nm (metatitanic acid system). The anatase lattice parameters decrease with Ag wt% increasing in Ti(IV) isopropoxide system and do not change for the metatitanic acid system. TG-DTA study showed the effects corresponding to the removal of structural water (210 °C), destruction of isopropoxide and combustion of organic matter (300–410 °C), crystallization of anatase (510 °C) and the anatase transformation into the rutile phase (720–760 °C), but in the presence silver, the anatase structure did not change to 1000 °C. Morphological study shows high crystallinity of both kinds of structures but less aggregative stability composites formed in the metatitanic acid solution. All type nanocomposites include four chemical elements – Ti, Ag, O, and Na as well as a small number of various admixtures - S or Cl and K depending on precursor species. At the same time, a relatively high concentration of argentum in the initial solution led to obtaining chemically pure composite nanoparticlesandcore&shell type structures.

Measurement of soot primary particle size in co-flow laminar diffusion flames of n-heptane at elevated pressures

To comprehensively understand soot formation in flames of liquid fuel, the soot primary particle size (PPS) in n-heptane\\air diffusion flame at pressures up to 5 bar was investigated in this work. A series of experiments were conducted in a high-pressure laminar flame burner system, and the spatial distributions of soot primary particle size in the flames at atmospheric and elevated pressures were evaluated by two-dimensional Time-Resolved Laser-Induced Incandescence (TiRe-LII). The results indicated a tendency that the conunt mean diameter (Dp,mean) of soot primary particle increased with enhancing laser fluence, which might be attributed to inaccuracy of the thermal accommodation coefficient at the increasing laser fluences. The spatial distribution of Dp,mean varied with pressures. Further, the evolution of Dp,mean along the centerline and the radial distribution of Dp,mean at the pressures of 1.0–3.0 bar were also presented. The zone of the maximum Dp,mean moved from the flame centerline to the flame edge as the pressure increases. The spatial distribution of Dp,mean at 3.5–5.0 bar showed the area of maximum Dp,mean located at the annular wing region in flames and being floated as the pressure increased. This study complemented the existing theoretical knowledge of spatial primary particle size distribution (PPSD) in n-heptane laminar diffusion flames.

Effect of Impeller and Gas Stirring on Agglomeration Behavior of Polydisperse Fine Particles in Liquid

Agglomeration, coalescence and flotation of non-metallic inclusions in steel melt are effective for obtaining “clean steel.” In this study, the agglomeration and breakup behaviors of particles with a primary particle size distribution (hereinafter, polydisperse particles) in a liquid under impeller and gas stirring were compared by numerical calculations and model experiments. The particle-size-grouping (PSG) method in the numerical agglomeration model of particles was combined with a breakup term of agglomeration due to bubble bursting at the free surface. Polydisperse and monodisperse polymethylmethacrylate (PMMA) particles were used in the agglomeration experiments. The agglomeration rate of the polydisperse particles under impeller stirring was increased by an increasing energy input rate, whereas the agglomeration rate under gas stirring decreased under this condition due to the larger contribution of the breakup of agglomerated particles during bubble bursting in gas stirring. At the same energy input rate, agglomeration of polydisperse particles was larger under impeller stirring than under gas stirring. The agglomeration rate of polydisperse particles was larger than that of monodisperse particles under both impeller and gas stirring at the same energy input rate. The computational temporal changes in the total number of particles were in good agreement with the experimental results. This means that the difference in the agglomeration behaviors observed in impeller and gas stirring can be explained by the turbulent coagulation and subsequent agglomerated particle breakup in gas stirring. The computational temporal change in the number of each group approximately agreed with the experimental change in both impeller and gas stirring.

Carbon nanostructure and reactivity of soot particles from non-intrusive methods based on UV-VIS spectroscopy and time-resolved laser-induced incandescence

The objective of this study is to derive morphological and nanostructural properties of soot as well as the reactivity against low-temperature oxidation by O2 from easily measurable optical properties. First, ex-situ experiments utilizing thermogravimetric analysis (TGA) and high-resolution transmission electron microscopy (HRTEM) serve to evaluate the kinetics of soot oxidation with O2 and relate reactivity to particle morphology and nanostructure. Second, ultraviolet–visible (UV-VIS) absorption spectra provide wavelength-dependent absorption cross sections and refractive-index functions E(m~,λ). From these, optical band gap energies, EOG, and coefficients ξ∗ for single parameter functions describing the wavelength-dependency of E(m~,λ) are obtained. Third, from time-resolved laser-induced incandescence (TR-LII) ratios of the refractive-index functions E(m~,λi)/E(m~,λj) at three excitation wavelengths and primary particle size distributions are acquired.The ex-situ experiments show that the size of the graphene layers predominantly determines soot reactivity against oxidation. Graphene layer size and, therefore, soot reactivity are reflected in the UV-VIS absorption spectra and E(m~,λ), EOG, and ξ∗, respectively. Similarly, scattering-corrected ratios E(m~,λi)/E(m~,λj) from TR-LII also reflect graphene layer size and, hence, soot reactivity. The established strong correlations between the optical properties, nanostructural characteristics and reactivity against oxidation make UV-VIS spectroscopy as well as TR-LII useful fast in-situ diagnostic methods for soot reactivity.

Influence of Low Ambient Temperatures on the Exhaust Gas and Deposit Composition of Gasoline Engines

Abstract To comply with future emission regulations for internal combustion engines, system-related cold start conditions in short-distance traffic constitute a particular challenge. Under these conditions, pollutant emissions are seriously increased due to internal engine effects and unfavorable operating conditions of the exhaust aftertreatment systems. As a secondary effect, the composition of the exhaust gases has a considerable influence on the deposition of aerosols via different deposition mechanisms and on fouling processes of exhaust gas-carrying components. Also, the performance of exhaust gas aftertreatment systems may be affected disadvantageously. In this study, the exhaust gas and deposit composition of a turbocharged three-cylinder gasoline engine are examined in situ upstream of the catalytic converter at ambient and engine starting temperatures of −22 °C to 23 °C using a Fourier-transform infrared spectrometer and a particle spectrometer. For the cold start investigation, a modern gasoline engine with series engine periphery is used. In particular, the investigation of the behavior of deposits in the exhaust system of gasoline engines during cold start under dynamic driving conditions represents an extraordinary challenge due to an average lower soot concentration in the exhaust gas compared to diesel engines and so far has not been examined in this form. A novel sampling method allows ex situ analysis of formed deposits during a single driving cycle. Both, particle number concentration and the deposition rate are higher in the testing procedure of real driving emissions (RDEs) than in the inner city part of the worldwide harmonized light vehicles test cycle (WLTC). In addition, reduced ambient temperatures increase the amount of deposits, which consist predominantly of soot and to a minor fraction of volatile compounds. Although the primary particle size distributions of the deposited soot particles do not change when boundary conditions change, the degree of graphitization within the particles increases with the increasing exhaust gas temperature.

A proof-of-concept study on characterization of primary particle size distribution and thermal accommodation coefficient of soot aggregate by using conduction-dominated laser-induced incandescence

Laser-induced incandescence (LII) is a powerful tool for in-situ measurement of soot aggregate properties, including the primary particle size distribution (PPSD). When integrating the shielding factor in the LII model to quantify the aggregate shielding effect, thermal accommodation coefficient (TAC) and shielding factor appear as a product. It is impossible to determine them independently without their accurate prior values. To overcome this problem, this study defines a new parameter fractal-dependent TAC (αT, f) which combines the overall impact of both TAC and shielding factor on the all detected aggregates. Given the ill-posed inverse problem is sensitive to the uncertainty from model parameters, inferring αT, f simultaneously with μdp and σdp (two characteristics parameters of PPSD) from conduction-dominated LII has the remarkable advantage of being independent of most LII model parameters. To evaluate the feasibility of the inversion from the respective of model parameter uncertainty, three relatively simple two-dimensional (2D) invers problems are investigated: (1) inferring μdp and σdp when prior αT, f is biased; (2) inferring μdp and αT, f when prior σdp is biased; (3) inferring σdp and αT, f when prior μdp is biased. The results and analysis presented in this study provide a theoretical basis for inferring μdp, σdp, and αT, f from conduction-dominated LII.

Simultaneous determination of primary particle size distribution and thermal accommodation coefficient of soot aggregates using low-fluence LII.

For the ill-posed inverse problem of LII-based nanoparticle size measurement, recovered primary particle size distribution (PPSD) is sensitive to the uncertainty of LII model parameters. In the absence of reliable prior knowledge, the thermal accommodation coefficient (TAC) and fractal-dependent shielding factor are often required to be inferred simultaneously with the PPSD. In the simplified LII model for low fluence regime, TAC and fractal-dependent shielding factor are combined to define a new fractal-dependent TAC. The present study theoretically verified the feasibility of inferring PPSD and fractal-dependent TAC from the normalized LII signals. Moreover, the inversion is independent of prior knowledge of most full LII model parameters, which is attributed to low laser fluence, normalized signal, and fractal-dependent TAC.

Impact of the competition between aggregation and surface growth on the morphology of soot particles formed in an ethylene laminar premixed flame

Soot particle surface growth may have a significant impact on aggregation kinetics, particle morphology, and both aggregate and monomer size distributions. Particularly, the morphological effects of surface growth are currently poorly understood. This is complex since surface growth also affects the kinetics of aggregation resulting in a time-dependent competition between both phenomena, producing very different morphological markers. The present study aims at improving our understanding of the morphological impact of surface growth by implementing this effect in a Monte Carlo discrete element code for the simulation of soot formation in an ethylene premixed flame. An asymptotic average primary particle overlapping ∼30% is observed for different flame conditions. The primary particle coordination number is highly sensitive to surface growth rate and particle volume fraction, and maximum values are on average within the 4 - 8 range. The particle local compacity, as quantified by the packing factor, is also considerably increased when aggregates experience surface growth, contrarily to the fractal structure of individual aggregates that seems more dependent on the aggregation regime. Finally, surface growth narrows both the primary particle and aggregate size distributions.

Effect of potassium chloride addition on soot formation during ethylene pyrolysis in a flow reactor

In this work, we investigated the effect of potassium chloride (KCl) addition on soot formation during ethylene pyrolysis in a flow reactor by using micro-orifice probe sampling in tandem with a series of systematic characterizations. The particle size distributions measured by a scanning mobility particle sizer showed that, with the addition of KCl, the nucleation of soot particles was not affected while the size growth of soot particles was inhibited. Then the morphology and mass of soot particles were examined by a transmission electron microscopy (TEM) and a centrifugal particle mass analyzer, respectively. The TEM images and mass-mobility relationship further showed that, soot particles were generally more compact and the primary particle size distributions were narrower when KCl was added. To understand what happened to the particle growth process, the charge characteristics were examined via tandem differential mobility analyzers. The number of particles with both double and triple positive charges was found to decrease with the increase of KCl concentration, indicating that the multi-charged particles were neutralized by KCl, which could lead to slower coagulation rates and thus smaller particles.

Nanostructural and morphological characteristics of single soot aggregates during low-temperature oxidation

In this paper the evolution of morphological and nanostructural properties of soot primary particles during low-temperature oxidation with oxygen is presented. Soot primary particles and aggregates from three soot samples with varying morphological and nanostructural initial properties were investigated by high-resolution transmission electron microscopy (HRTEM) at consecutive steps of oxidation. Soot samples were deposited on a TEM grid and then oxidized repetitively. After each oxidation step labelled identical aggregates were scanned and reanalyzed by HRTEM. Size distributions of primary particles and their nanostructure was tracked using an image analysis algorithm. The oxidation conditions (temperature 850 K, O2 concentration 5% by volume) correspond to those of the real application, e.g. for regeneration of particle filters.The tracking of different aggregates revealed that the morphological changes of the primary particles are determined by the predominant oxidation mode, which is governed by the nanostructure of the primary particles. Extended graphene layers favor internal oxidation, while bent, short and reactive layers favor both, external and internal oxidation. In surface oxidation mode, primary particles do not shrink homogeneously, but rather form a blackberry-like cluster of smallest nucleated particles.While a partly expansion of graphene layers is detected in the early stages of oxidation, the number density of graphene layers gradually decreases with increasing oxidation time. Additionally, graphene layers are oxidized by fragmentation until final degradation. Tracking of selected primary particles at different positions within the aggregate brought about that each primary particle is subjected to its own oxidation history. Therefore, each observed and analyzed primary particle possibly exhibits a different oxidation stage at the same oxidation time.

Why Soot is not Alike Soot: A Molecular/Nanostructural Approach to Low Temperature Soot Oxidation

Due to worldwide increasingly sharpened emission regulations, the development of Gasoline Direct Injection and Diesel Direct Injection engines not only aims at the reduction of the emission of nitrogen oxides but also at the reduction of particulate emissions. Regarding present regulations, both tasks can be achieved solely with the help of exhaust after treatment systems. For the reduction of the emission of particulates, Gasoline (GPF) and diesel Particulate Filters (DPF) offer a solution and their implementation is intensely promoted. Under optimal conditions particulates retained on particulate filters are continuously oxidized with the exhaust residual oxygen so that the particulate filter (PF) is regenerated possibly without any additional intervention into the engine operating parameters. The regeneration behavior of PF depends on the reaction rates of soot particles with oxidative reactants at exhaust gas temperatures. The reaction rates of soot particles from internal combustion engines (ICE) often are discussed in terms of order/disorder on the particle nanoscale, the concentration and kind of functional groups on the particle surfaces, and the content of (mostly polycyclic aromatic) hydrocarbons in the soot. In this work the reactivity of different kinds of soot (soot from flames, soot from ICE, carbon black) under oxidation conditions representative for PF regeneration is investigated. Soot reactivity is determined in dynamic Temperature Programmed Oxidation (TPO) experiments and the soot primary particle morphology and nanostructure is investigated by High-Resolution Transmission Electron Microscopy (HRTEM). An image analysis method based on known methods from the literature and improving some infirmities is used to evaluate morphology and nanostructural characteristics. From this, primary particle size distributions, length and separation distance distributions as well as tortuosities of fringes within the primary particle structures are obtained. Further, UV–visible spectroscopy and Raman scattering and other diagnostic techniques are used to study the properties connected to the reactivity of soot and to corroborate the experimental findings. It is found that nanostructural characteristics predominantly affect reactivity. Oxidation rates are derived from TPO and interpreted on a molecular basis from quantum chemistry calculations revealing a replication/activation oxidation mechanism.

Soot particles in piston-top pool fires and exhaust at 5 and 15 MPa injection pressure in a gasoline direct-injection engine

This study shows the structure of soot particles sampled directly from wall wetting-induced pool fires formed on the piston top in a spark-ignition direct-injection (SIDI) engine. Of particular interest is its variation with injection pressure considering the current trend of high-pressure DI system development to reduce engine-out particulate emissions. Thermophoretic particle sampling was performed for transmission electron microscope (TEM) imaging, which was post-processed for statistical analysis of key morphology and internal structure parameters. These include the size distribution of soot aggregates and primary particles as well as carbon-layer fringe-to-fringe gap and concentricity. With the fixed engine speed and load conditions, in-flame soot particles are compared to the exhaust particles sampled simultaneously at selected 5 and 15 MPa injection pressures corresponding to low-speed/low-load and high-speed/high-load in practical engine operation. From the TEM images and statistical analysis, it was found that more concentrated and taller pool fire developed for 5-MPa injection leads to smaller soot aggregates composed of smaller soot primary particles due to suppressed soot growth in fuel-rich flames. However, the soot particles formed in 15-MPa injection-induced pool fires are at a more reactive status evidenced by less defined core-shell boundaries and higher fringe separation. The higher soot reactivity results in enhanced soot oxidation, which explains smaller soot aggregates and primary particles found for the 15-MPa injection in the exhaust sample.

In vitro intestinal toxicity of commercially available spray disinfectant products advertised to contain colloidal silver

The use of colloidal silver-containing products as dietary supplements, immune boosters and surface disinfectants has increased in recent years which has elevated the potential for human exposure to silver nanoparticles and ions. Product mislabeling and long-term use of these products may put consumers at risk for adverse health outcomes including argyria. This study assessed several physical and chemical characteristics of five commercial products as well as their cytotoxicity using a rat intestinal epithelial cell (IEC-6) model. Concentrations of silver were determined for both the soluble and particulate fractions of the products. Primary particle size distribution and elemental composition were determined by transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS), respectively. Hydrodynamic diameters were measured using nanoparticle tracking analysis (NTA) and dynamic light scattering (DLS). The effect of gastrointestinal (GI) simulation on the colloidal silver products was determined using two systems. First, physical and chemical changes of the silver nanoparticles in these products was assessed after exposure to Synthetic Stomach Fluid (SSF) resulting in particle agglomeration, and the appearance of AgCl on the surfaces and between particles. IEC-6 cells were exposed for 24 h to dilutions of the products and assessed for cell viability. The products were also treated with a three-stage simulated GI system (stomach and intestinal fluids) prior to exposure of the IEC-6 cells to the isolated silver nanoparticles. Cell viability was affected by each of the consumer products. Based on the silver nitrate and commercial silver nanoparticle dose response, the cytotoxicity for each of the colloidal silver products was attributed to the particulate silver, soluble silver or non‑silver matrix constituents.

A novel method for synthesizing narrow particle size distribution of holmium–doped strontium hexaferrite by sol-gel auto-combustion

In this study, a new technique put in to the sol-gel auto-combustion method which is the preparation of solutions and treatment of products by using ultrasonic liquid sonicate. Different atomic concentrations of Holmium-doped strontium hexaferrite (SrFe12−xHoxO19) been synthesized by the conventional sol-gel auto-combustion method. This new technique estimates as a novel method which escalates the amount of Ho+3 present in strontium hexaferrite of produced powder by 500% compared to the conventional method when Ho+3 substitution was 15% of atomic weight. Characterization of produced powders, porosity, unit cell volume, and lattice parameters were tested and calculated from x-ray diffraction. Morphology, particle size distribution, and shape of nanoparticles were analyzed by field emission scanning electron microscope. Primary particle size and particle size distributions were studied by transmission electron microscope. The purity and elemental composition of products were tested by energy dispersive spectrometer. Bulk density, crystallinity, and purity increased with decreased crystal size and porosity when nanoparticles were treated by ultrasonic method. Morphology, purity, narrow particle size distribution, and nanoparticles with isomorphic geometry can be controlled with Ho+3 substitution by 15% of atomic weight when powder is produced by this novel method. This new technique provides an improved new method for narrowing particle size distribution, increasing density, and substitution limit of rare earth ions to hexaferrites.

Simulation of primary particle size distributions in a premixed ethylene stagnation flame

Numerical simulation of soot formation in a laminar premixed burner-stabilized ethylene stagnation flame was performed with a detailed population balance model (DPBM) capable of tracking full structural details of aggregates as well as their chemical composition. A thorough parametric sensitivity study was carried out to understand the influence of individual sooting processes on the computed primary particle size distributions (PPSDs). The rate of production of pyrene, coagulation efficiency and surface growth rate were found to have significant effects on the computed PPSDs. Besides, we found that the instantaneous sintering between small primary particles (PP) can affect the computed PPSDs drastically while sintering between large PPs within aggregates only had mild effects. For an ethylene premixed flame with stagnation plate height being 1.2 cm (Combust. Flame, 198:428-435, 2018), good agreement was obtained between both the computed and measured PPSD and fractal dimension, which supports the current mechanisms contributing to the evolution of PPs, i.e. nucleation, coagulation, surface growth and sintering. Moreover, time scale analysis for individual sooting processes was performed to determine the dominant particle processes at different periods of time, which helped explain the evolution of soot morphology.

A Simple Model for the High Temperature Oxidation Kinetics of Silicon Nanoparticle Aggregates

Silicon nanoparticles are an emerging and promising material in many fields including electronics, catalysis, and biomaterial engineering. Synthesis in the gas phase via aerosol routes allows tuning and engineering material features such as primary particle size distribution, agglomerate size distribution, crystallite size, and morphology. Proper control of these features as well as post-synthesis processing such as surface oxidation is very relevant for making the nanoparticles chemically stable. Therefore, a kinetic expression for determining the extent of oxidation in silicon nanoparticles is necessary. Significant work has been devoted to understanding the kinetics of monocrystalline silicon, and generally accepted models such as the one by Deal and Grove provide a very accurate description of bulk silicon oxidation. However, these models are not as accurate for the first hundreds of angstroms of the oxidation. While this might be acceptable for bulk wafers, it has critical results for silicon nanoparticles with diameters around such range. Furthermore, many applications involve silicon nanoparticle aggregates with a shape far away from perfect spheres. For this reason, in this work, we propose an expression for the parabolic oxidation kinetics of polycrystalline silicon nanoparticle aggregates at 1000 °C with surface area in the range of 3 to 19 m2/g. and crystallite sizes from 50 to 70 nm. The activation energy of the process is also reported to be linked to the crystallite size.