Effect of ceria nanoparticles on soot inception and growth in toluene-oxygen-argon mixtures

Soot formation from the combustion of toluene (C₆H₅CH₃) and of two concentrations of nano-sized-ceria-laden toluene was monitored using a shock tube to observe the effect of the organometallic additive on the formation of soot from its point of inception. Two concentrations of ceria, of chemical composition CeO_1.63, were employed to examine the effect on soot production of toluene over the range of temperature 1588-2370 K using two levels of inert gas dilution in which reflected-shock pressure was maintained near 1.5 atm. The ceria nanoparticles were synthesized using a microemulsion technique which employs sodium dioctyl sulfosuccinate (AOT), a surfactant, to retard agglomeration. Introduction of the nanoparticles into the shock tube is achieved using a novel, two-stage injection procedure. Soot yield measurements reveal that the presence of ceria has no direct implications on peak soot concentration near 1950 K. A shift in the parabolic soot profile of toluene in the direction of increased temperature was observed for each concentration of ceria with a larger shift occurring for increased concentration of ceria, although the same effect was exhibited for the toluene-AOT mixtures in absence of ceria, supporting an inefficaciousness of ceria on soot suppression on kinetic timescales. It is evidenced in measured soot delay times that the presence of the surfactant in absence of ceria significantly slows the rate of soot growth for T < 2000 K, while the presence of ceria has a relatively negligible impact. Under conditions of higher fuel concentration, a remarkable decrease in soot accumulation on the shock tube walls was observed in experiments using the ceria-toluene mixtures over that yielded by pure toluene combustion. In the present paper, the authors report the first measurements of nanoparticle-influenced combustion of a hydrocarbon as performed in a shock tube.     

Experimental study of the E(m, λ)/E(m, 1064) ratio as a function of wavelength,fuel type,height above the burner and temperature

The optical properties of soot have been studied for many years with a particular attention focused on refractive index. In the present study, the two-excitation wavelength laser-induced incandescence technique has been applied to determine the ratio of the soot absorption function as a function of the wavelength. The advantage of this technique is to provide the determination of the E(m) ratio using a non-intrusive laser-based method without being disturbed by scattering. Measurements have been carried out in a methane premixed flat flame and in a diesel turbulent spray one. Four pairs of wavelength have been used to evaluate the spectral behavior of E(m) ratios from ultraviolet (UV) to near infrared (NIR). The two-excitation wavelength LII method implies heating soot the same way using two different laser excitations. Particular operating conditions must be selected to insure the equality of the LII signals, such an equality being necessary to derive the E(m) ratio. A laser excitation at 1064 nm has been chosen as a reference, and the obtained results have been compared with those issued from the use of UV and visible wavelengths of 266, 355, 532 and 660 nm. Results show a significant decrease of the E(m) ratio from UV to visible while it tends to become constant from 532 nm to NIR. The use of different experimental conditions allows to analyze the dependence of the E(m) ratios with the height above the burner, the fuel type and the soot temperature. No significant influence of these parameters has been pointed out on the relative E(m) values determined in the flame conditions investigated here.     

Effect of fuel/air ratio and aromaticity on the molecular weight distribution of soot in premixed n-heptane flames

Soot growth from inception to mass-loading is studied in a wide range of molecular weights (MW) from 10⁵ to 10¹⁰u by means of size exclusion chromatography (SEC) coupled with on-line UV-visible spectroscopy. The evolution of MW distributions of soot is also numerically predicted by using a detailed kinetic model coupled with a discrete-sectional approach for the modeling of the gas-to-particle process. Two premixed flames burning n-heptane in slightly sooting and heavily sooting conditions are studied. The effect of aromatic addition to the fuel is studied by adding n-propylbenzene (10% by volume) to n-heptane in the heavily sooting condition. A progressive reduction of the MW distribution from multimodal to unimodal is observed along the flames testifying the occurrence of particle growth and agglomeration. These processes occur earlier in the aromatic-doped n-heptane flame due to the overriding role of benzene on soot formation which results in bigger young soot particles. Modeled MW distributions are in reasonable agreement with experimental data although the model predicts a slower coagulation process particularly in the slightly sooting n-heptane flame. Given the good agreement between model predictions and experiments, the model is used to explore the role of fuel chemistry on MW distributions. Two flames of n-heptane and n-heptane/n-propylbenzene in heavily sooting conditions with the same temperature profile and inert dilution are modeled. The formation of larger soot particles is still evident in the n-heptane/n-propylbenzene flame with respect to the n-heptane flame in the same operating conditions of temperature and dilution. In addition the model predicts a larger formation of molecular particles in the flame containing n-propylbenzene and shows that soot inception occurs in correspondence of their maximum formation thus indicating the importance of molecular growth in soot inception.     

A surrogate fuel for kerosene

Experimental and numerical studies are carried out to develop a surrogate that can reproduce selected aspects of combustion of kerosene. Jet fuels, in particular Jet-A1, Jet-A, and JP-8 are kerosene type fuels. Surrogate fuels are defined as mixtures of few hydrocarbon compounds with combustion characteristics similar to those of commercial fuels. A mixture of n-decane 80% and 1,2,4-trimethylbenzene 20% by weight, called the Aachen surrogate, is selected for consideration as a possible surrogate of kerosene. Experiments are carried out employing the counterflow configuration. The fuels tested are kerosene and the Aachen surrogate. Critical conditions of extinction, autoignition, and volume fraction of soot measured in laminar non premixed flows burning the Aachen surrogate are found to be similar to those in flames burning kerosene.A chemical-kinetic mechanism is developed to describe the combustion of the Aachen surrogate. This mechanism is assembled using previously developed chemical-kinetic mechanisms for the components: n-decane and 1,2,4-trimethylbenzene. Improvements are made to the previously developed chemical-kinetic mechanism for n-decane. The combined mechanisms are validated using experimental data obtained from shock tubes, rapid compression machines, jet stirred reactor, burner stabilized premixed flames, and a freely propagating premixed flame. Numerical calculations are performed using the chemical-kinetic mechanism for the Aachen surrogate. The calculated values of the critical conditions of autoignition and soot volume fraction agree well with experimental data. The present study shows that the chemical-kinetic mechanism for the Aachen surrogate can be employed to predict non premixed combustion of kerosene.     

Experimental comparison of soot formation in turbulent flames of Diesel and surrogate Diesel fuels

Soot formation is compared in turbulent diffusion flames burning a commercial Diesel and two Diesel surrogates containing n-decane and α-methylnaphthalene. A burner equipped with a high-efficiency atomisation system has been specially designed and allows the stabilisation of liquid fuels flames with similar hydrodynamics conditions. The initial surrogate composition (70% n-decane, 30% α-methylnaphthalene) was previously used in the literature to simulate combustion in Diesel engines. In this work, a direct comparison of Diesel and surrogates soot tendencies is undertaken and relies on soot and fluorescent species mappings obtained respectively by Laser-Induced Incandescence (LII) at 1064 nm and Laser-Induced Fluorescence at 532 nm. LIF was assigned to soot precursors and mainly to high-number ring Polycyclic Aromatic Hydrocarbons (PAH). The initial surrogate was found to form 40% more soot than the tested Diesel. Consequently, a second surrogate containing a lower α-methylnaphthalene concentration (20%) has been formulated. That composition which presents a Threshold Soot Index (TSI) very close to Diesel one is also consistent with our Diesel composition that indicates a relatively low PAH content. The spatially resolved measurements of soot and fluorescent soot precursors are quite identical (in shape and intensity) in the Diesel and in the second surrogate flames. Furthermore the concordance of the LII temporal decays suggests that a similar growth of the primary soot particles has occurred for Diesel and surrogates. In addition, the comparison of the LII fluence curves indicates that physical/optical properties of soot contained in the different flames might be similar. The chemical composition present at the surface of soot particles collected in Diesel and surrogate flames has been obtained by laser-desorption ionisation time-of-flight mass spectrometry. An important difference is found between Diesel and surrogate samples indicating the influence of the fuel composition on soot content.     

Evaluation and optimisation of phenomenological multi-step soot model for spray combustion under diesel engine-like operating conditions

In this work, a two-dimensional computational fluid dynamics study is reported of an n-heptane combustion event and the associated soot formation process in a constant volume combustion chamber. The key interest here is to evaluate the sensitivity of the chemical kinetics and submodels of a semi-empirical soot model in predicting the associated events. Numerical computation is performed using an open-source code and a chemistry coordinate mapping approach is used to expedite the calculation. A library consisting of various phenomenological multi-step soot models is constructed and integrated with the spray combustion solver. Prior to the soot modelling, combustion simulations are carried out. Numerical results show that the ignition delay times and lift-off lengths exhibit good agreement with the experimental measurements across a wide range of operating conditions, apart from those in the cases with ambient temperature lower than 850 K. The variation of the soot precursor production with respect to the change of ambient oxygen levels qualitatively agrees with that of the conceptual models when the skeletal n-heptane mechanism is integrated with a reduced pyrene chemistry. Subsequently, a comprehensive sensitivity analysis is carried out to appraise the existing soot formation and oxidation submodels. It is revealed that the soot formation is captured when the surface growth rate is calculated using a square root function of the soot specific surface area and when a pressure-dependent model constant is considered. An optimised soot model is then proposed based on the knowledge gained through this exercise. With the implementation of optimised model, the simulated soot onset and transport phenomena before reaching quasi-steady state agree reasonably well with the experimental observation. Also, variation of spatial soot distribution and soot mass produced at oxygen molar fractions ranging from 10.0 to 21.0% for both low and high density conditions are reproduced.     

Detailed investigation of soot formation from jet fuel in a diffusion flame with comprehensive and hybrid chemical mechanisms

A numerical and experimental study is performed to investigate soot formation from jet fuel in a laminar coflow diffusion flame. The combustion chemistry of the fuel is simulated using (1) the MURI jet fuel surrogate (Dooley et al. 2012) with a modestly reduced Ranzi mechanism (Ranzi et al. 2012), and (2) the recently proposed HyChem model (Xu et al. 2018) combined with the KAUST PAH mechanism 2 (Wang et al. 2013). The two reaction mechanisms are coupled with a sectional soot model to simulate a coflow diffusion flame of methane doped with the MURI jet fuel surrogate. The combined laser extinction and two-angle elastic light scattering method is used to perform non-intrusive in situ measurements of soot volume fraction, primary particle diameter and number density. The good agreement including soot particle size and number density between the experimental data and the simulation results computed with the reduced Ranzi mechanism demonstrate the robustness of the soot model to changes in fuel composition, as the model parameters are unchanged with a previous numerical study of soot formation of n-propylbenzene/n-dodecane mixtures (Zhang and Thomson, 2018). The computation with the combined HyChem/KAUST mechanism predicts similar results as the computation with the detailed chemistry of the reduced Ranzi mechanism for fuel breakdown, thus the basic premise of the HyChem model that the fuel decomposition process can be greatly simplified with the lumped reaction steps is supported. The results also show that by adding a PAH growth scheme to the HyChem model, the approach can be used to predict soot formation from jet fuel combustion in a laminar coflow diffusion flame. Finally, the dependency of the soot prediction on PAH chemistry is discussed and it is suggested that more experimental data is needed to validate the PAH mechanism and improve the predictive accuracy of the model.     

Modification to Nagle/Strickland-Constable model with consideration of soot nanostructure effects

The oxidation rates of diesel soot from the combustion chamber of a running diesel engine were calculated based on the particle size distributions at different crank angles. The primary particle diameter and nanostructure of soot were obtained by means of high-resolution transmission electron microscopy (HRTEM). The soot characteristics were also investigated by oxidative thermogravimetry and Raman scattering spectrometry.

The results showed the soot nanostructures were dependent on engine operation conditions and combustion phases. The oxidation rates were found to differ by nearly fourfold from that calculated by the Nagle/Strickland-Constable (NSC) model for the soots studied here. The varied oxidation rates were interpreted in terms of differences in nanostructure between the soots. The experimental results were used to modify the NSC model and the fringe length of in-cylinder diesel soot was chosen to describe the influence of graphitisation on the oxidation of soot. The modified NSC model lessened the deviation between measurements and predictions.     

Theoretical analysis on the exergy destruction mechanisms and reduction under LTC relevant conditions

Low temperature combustion (LTC) is a potential thermodynamic pathway to maximize the thermal efficiency of internal combustion (IC) engines. However, high exergy loss is also observed within this combustion concept. The present study focuses on the homogeneous combustion process and examines the detailed exergy destruction mechanisms under representative LTC engine conditions. By varying both equivalence ratios (φ) and temperatures (T) at initial pressure of 50?bar, it is found that the decreased total exergy destruction fraction (f_ED) with increasing initial temperature mainly results from the decreased exergy destruction in the high temperature heat release stage, while using rich mixture can significantly reduce the f_ED in the ignition delay stage, which is dominated by the reactions involving large molecules (C7 species). Reaction pathway analysis reveals that the detailed exergy destruction sources are significantly affected by the reaction pathways. Furthermore, a qualitative exergy loss φ-T map was created to illustrate the exergy loss reduction potential. It is concluded that the combustion pathway that reforming the rich fuel/air mixtures before ignition followed by the low temperature combustion of lean reforming products offers the potential to simultaneously reduce exergy destruction and avoid soot and NOx formation. However, the potential advantages of this exergy reduction combustion concept still require further work.     

Low- and high-temperature study of n-heptane combustion chemistry

n-Heptane has been used extensively in various fundamental combustion experiments as a prototypical hydrocarbon fuel. While the formation of polycyclic aromatic hydrocarbon (PAH) in n-heptane combustion has been studied preferably in premixed flames, this study aims to investigate the combustion chemistry of n-heptane in less-studied diffusion flame and highly rich high-temperature homogeneous oxidation configurations by using a counterflow burner and a flow reactor, respectively. This work addresses the formation of higher-molecular species in the mass range up to about 160 u in both configurations. Samples are analyzed by time-of-flight (TOF) molecular beam mass spectrometry (MBMS) using electron-impact (EI) and single-photon ionization (PI). Highly resolved speciation data are reported. Laminar flow reactor experiments cover a wide temperature range. Especially the measurements at low temperatures provide speciation data of large oxygenates produced in the low-temperature oxidation of n-heptane, which are scarce in the literature. Important precursor molecules for PAH and soot formation, such as C₉H₈, C₁₀H₈, C₁₁H_10, and C₁₂H₈, are formed during the high-temperature combustion process in the counterflow flame, while oxygenated growth species are observed under low-temperature conditions, even at the fuel-rich equivalence ratio of ?=4.00.Numerical modeling for both conditions is performed by using a newly developed kinetic model of n-heptane, which includes the n-heptane and PAH formation chemistry with state-of-the-art kinetic knowledge. Good agreement between model predictions and experimental data of counterflow flame and flow reactor is observed for the major species and some intermediates of n-heptane oxidation. While the concentrations of benzene and toluene measured in the counterflow burner are well-reproduced, the numerical results for flow reactor data are not satisfactory. Differences are found between the formation pathways of fulvene, from whose isomerization benzene is produced in diffusion flame and flow reactor.     

Effect of H-atom concentration on soot formation in premixed ethylene/air flames

Soot formation is a major challenge in the development of clean and efficient combustion systems based on hydrocarbon fuels. Fundamental understanding of the reaction mechanism leading to soot formation can be obtained by investigating the role of key reactive species such as atomic hydrogen taking part in soot formation pathways. In this study, two-dimensional laser induced incandescence (LII) measurements using λ?=?1064?nm laser have been used to measure soot volume fraction (f_V) in a series of rich ethylene (C₂H₄)/air flames, stabilized over a McKenna burner fitted with a flame stabilizing metal disc. Moreover, a comparison of UV (λ?=?283?nm), visible (λ?=?532?nm) and IR (λ?=?1064?nm) laser excited LII measurements of soot is discussed. Recently developed, femtosecond two-photon laser-induced fluorescence (fs-TPLIF) technique has been applied for obtaining spatially resolved H-atom concentration ([H]) profiles under the same flame conditions. The structure of the flames has also been determined using hydroxyl radical (OH) planar laser induced fluorescence (PLIF) imaging. The results indicate an inverse dependence of f_V on [H] for a range of C₂H₄/air rich flames up to an equivalence ratio, Φ?=?3.0. Although an absolute relationship between [H] and f_V cannot be easily derived owing to the multiple steps involving H and other intermediate species in soot formation pathways, the present study demonstrates the feasibility to couple [H] and f_V obtained using advanced optical techniques for soot formation studies.     

The importance of thermal radiation transfer in laminar diffusion flames at normal and microgravity

The importance of radiation heat loss in laminar and turbulent diffusion flames at normal gravity has been relatively well recognized in recent years. There is currently lack of quantitative understanding on the importance of radiation heat loss in relatively small scale laminar diffusion flames at microgravity. The effects of radiation heat transfer and radiation absorption on the structure and soot formation characteristics of a coflow laminar ethylene/air diffusion flame at normal- and microgravity were numerically investigated. Numerical calculations were conducted using GRI-Mech 3.0 combustion chemistry without the NO_x mechanism and complex thermal and transport properties, an acetylene based soot formation model, and a statistical narrow-band correlated-k non-grey gas radiation model. Radiation heat transfer and radiation absorption in the microgravity flame were found to be much more important than their counterparts at normal gravity. It is important to calculate thermal radiation transfer accurately in diffusion flame modelling under microgravity conditions.     

Numerical study on effects of hydrothermal aging to composite regeneration in CeO2-based catalyzed diesel particulate filter

The effects of hydrothermal aging on the performance of CeO₂-based catalyzed diesel particulate filter (CDPF) was numerically investigated in this study based on a zero-dimensional model using the plug flow reactor in which a chemical reaction kinetic mechanism was established and validated by the simulated gas environment experiment. The effects of regeneration temperature, O₂ concentration in the ultimate emission conditions, the ratio of NO₂ in NOx (α) and the ratio of NOx to soot (β) on catalyst deactivation temperature and soot oxidation rate were investigated with fresh and hydrothermal aging CeO₂-based CDPF. The results show that hydrothermal aging of CeO₂-based catalysts raises the regeneration temperature from 613 to 783 K and shifts the soot catalytic combustion reaction path from complete to incomplete oxidation. Soot oxidation rate of fresh catalyst first increases rapidly at 516 K and then starts to slow down gradually at 633 K, but for hydrothermal aging catalysts, are 601 K and 789 K, respectively. With O₂ concentration increased from 1.5 mol/m³ to 5.5 mol/m³, the catalyst deactivation of fresh and hydrothermal aging catalyst increased from 609 K to 602 K, 791 K to 818 K, respectively. The increase in α and β leads to an increase in soot oxidation rate and a decrease in regeneration temperature. The deactivation temperature of catalyst is increased in higher α (1.0) and lower β (0.1), which the highest is 821 K. Synergistic mechanisms of NOx, regeneration temperature, and hydrothermal aging effects on soot catalytic combustion in CeO₂-based CDPF are revealed deeply with the help of zero-dimensional model.     

Impact of post-injection strategy on the physicochemical properties and reactivity of diesel in-cylinder soot

Post injection has significant benefit in the reduction of diesel soot emissions. Therefore, there is a need to understand the effect of post-injection strategy on soot physicochemical properties and reactivity because they play an important role in soot oxidation process that governs the final soot emissions. This work focuses on the impact of post injection on the physicochemical properties and reactivity of diesel in-cylinder soot using a main plus post injection (M*P) and a single injection (M) strategy. The soot was sampled by a developed total cylinder sampling system, and the dividing points of soot formation-dominant and oxidation-dominant phases were used for studying the impacts of post injection on the characteristics of in-cylinder soot. The physicochemical properties of the soot samples, including primary particle size, nanostructure, carbon chemical state and surface functional groups, were characterized. The soot reactivity was evaluated in terms of peak temperature, burnout temperature and apparent activation energy. In the oxidation-dominant phase, the M*P soot initially possesses smaller primary particle size, shorter fringe length, larger tortuosity, lower sp²/sp³ hybridization ratio of carbon atoms and higher content of aliphatic CH groups than the M soot. The beneficial influence of physicochemical properties on soot reactivity when using post injection is validated by the thermogravimetric data, which shows that the M*P soot is more reactive than the M soot at the onset of the oxidation-dominant phase. In the M*P case, the soot generated from the main-injection combustion has lower reactivity than the soot from the post-injection combustion after they experience the soot formation-dominant phase. The results indicate that the use of post injection leads to in-cylinder soot with physicochemical properties that favor reactivity. The enhancement of reactivity means that the soot will be more readily oxidized in the subsequent combustion process, and consequently contributes to a reduction in final soot emissions.     

Modeling of soot aggregate formation and size distribution in a laminar ethylene/air coflow diffusion flame with detailed PAH chemistry and an advanced sectional aerosol dynamics model

Soot aggregate formation and size distribution in a laminar ethylene/air coflow diffusion flame is modeled with a PAH-based soot model and an advanced sectional aerosol dynamics model. The mass range of solid soot phase is divided into 35 discrete sections and two variables are solved for in each section. The coagulation kernel of soot aggregates is calculated for the entire Knudsen number regime. Radiation from gaseous species and soot are calculated by a discrete-ordinate method with a statistical narrow-band correlated-k based band model. The discretized sectional soot equations are solved simultaneously to ensure convergence. Parallel computation with the domain decomposition method is used to save computational time. The flame temperature, soot volume fraction, primary particle size and number density are well reproduced. The number of primary particles per aggregate is overpredicted. This discrepancy is presumably associated with the unitary coagulation efficiency assumption in the current sectional model. Along the maximum soot volume fraction pathline, the number-based and mass-based aggregate size distribution functions are found to evolve from unimodal to bimodal and finally to unimodal again. The different shapes of these two aggregate size distribution functions indicate that the total number and mass of aggregates are dominated by aggregates of different sizes. The PAH-soot condensation efficiency γ is found to have a small effect on soot formation when γ is larger than 0.5. However, the soot level and primary particle number density are significantly overpredicted if the PAH-soot condensation process is neglected. Generally, larger γ predicts lower soot level and primary particle number density. Further study on soot aggregate coagulation efficiency should be pursued and more experimental data on soot aggregate structure and size distribution are needed for improving the current sectional soot model and for better understanding the complex soot aggregation phenomenon.     

Soot particle size distribution measurements in a turbulent ethylene swirl flame

There is a need to better understand particle size distributions (PSDs) from turbulent flames from a theoretical, practical and even regulatory perspective. Experiments were conducted on a sooting turbulent non-premixed swirled ethylene flame with secondary (dilution) air injection to investigate exhaust and in-burner PSDs measured with a Scanning Mobility Particle Sizer (SMPS) and soot volume fractions (f_v) using extinction measurements. The focus was to understand the effect of systematically changing the amount and location of dilution air injection on the PSDs and f_v inside the burner and at the exhaust. The PSDs were also compared with planar Laser Induced Incandescence (LII) calibrated against the average f_v. LII provides some supplemental information on the relative soot amounts and spatial distribution among the various flow conditions that helps interpret the results. For the flame with no air dilution, f_v drops gradually along the centreline of the burner towards the exhaust and the PSD shows a shift from larger particles to smaller. However, with dilution air f_v reduces sharply where the dilution jets meet the burner axis. Downstream of the dilution jets f_v reduces gradually and the PSDs remain unchanged until the exhaust. At the exhaust, the flame with no air dilution shows significantly more particles with an f_v one to two orders of magnitude greater compared to the Cases with dilution. This dataset provides insights into soot spatial and particle size distributions within turbulent flames of relevance to gas turbine combustion with differing dilution parameters and the effect dilution has on the particle size. Additionally, this work measures f_v using both ex situ and in situ techniques, and highlights the difficulties associated with comparing results across the two. The results are useful for validating advanced models for turbulent combustion.     

Fragmentation of pulverized coal in a laminar drop tube reactor: Experiments and model

Fragmentation during pulverized coal particles conversion shifts the particle size distribution of the fuel towards smaller particle sizes, affecting both conversion rates and heat release. After pyrolysis of a high volatiles Colombian coal in CO₂ atmosphere in a drop tube reactor at 1573?K, solid carbonaceous particles of different size, from 100?µm of the particle feed down to the nanometric size, have been observed. A fragmentation model has been used to predict the fate of Colombian coal particles under the experimental conditions of the drop tube experiment and predict the particle size distribution (PSD). Model and experimental results are in very good agreement and indicate that in the DTR experiment the coal underwent almost complete pyrolysis and that fragmentation generated a 36?wt% population of particles with size close to 30?µm. The close match between the PSDs obtained from experiments and from the fragmentation model is an important novelty. It demonstrates that fragmentation occurs not only under fluidized bed conditions but also under the conditions of pulverized coal combustion. Experimentalists are warned against the fact that the fine particulate sampled at the outlet of laminar flow reactors and boilers is not always composed of soot only. Char fragments can be misidentified as soot. The implementation of fragmentation submodels in pulverized fuel combustion and gasification codes is highly recommended.     

Structure of fullerene-containing soot at different formation stages of arc discharge vaporization of graphite

Electron microscopy is used in a study of nanoclusters of the carbon soot deposited on a probe in different areas of arc discharge during graphite vaporization under conditions favorable for fullerene synthesis. It is found that the spatial network of soot nanoclusters consists of alternating regions of higher density or associates of carbon particles. Two types of nanoclusters have been identified with the correlation radii of the associates equal to 0.6–0.8 and 1.6–2.2 nm, respectively. Type I nanoclusters are dominant in the soot microparticles, and their structure shows practically no variations with increasing separation r of the soot collector from the discharge axis over the range of distances studied, r=1–9 cm. The effective radius R ₀ of the “elementary” particles making up the associates in the soot nanoclusters of Type I calculated with the use of scaling relationships is 0.15–0.17 nm and is close to the gas-kinetic radius of carbon atoms. Type II nanoclusters have been identified in soot collected at r>3 cm. Values of R ₀ calculated in this case are 0.6–0.9 nm and decrease with increasing r, which indicates the presence of fullerene molecules in these nanocluster associates.     

Judd-Ofelt analysis of luminescence emission from Zn2SiO4:Eu nanoparticles obtained by a polymer-assisted sol-gel method

In this research we prepared Zn₂SiO₄:Eu³⁺ phosphor nanopowders using a combination of sol-gel and combustion synthesis with the aim to examine the influence of synthesis conditions on the optical properties of the phosphor. As combustion fuels we used polyethylene glycol (PEG) with different average molecular weights, and the combustion was performed in two ways—in a microwave oven and a conventional furnace. Optical properties were examined by photoluminescence spectroscopy and spectra of all samples showed intense red emission, typical for f-f electronic transitions of the Eu³⁺ ions. Emission decays exhibited classical one exponential behavior at longer times and nonlinear nature at short times, with average lifetimes varied from 0.49 to 0.71 ms between samples. Judd-Ofelt theory was applied to experimental data for the quantitative determination of optical parameters such as Ω_2,4 Judd-Ofelt parameters, radiative and nonradiative transition rates and emission quantum efficiency. Calculated parameters vary moderately between samples prepared with different PEGs and combusted in different manner.