Insights into incipient soot formation by atomic force microscopy

Abstract Combustion-generated soot particles can have significant impact on climate, environment and human health. Thus, understanding the processes governing the formation of soot particles in combustion is a topic of ongoing research. In this study, high-resolution atomic force microscopy (AFM) was used for direct imaging of the building blocks forming the particles in the early stages of soot formation. Incipient soot particles were collected right after the particle nucleation zone of a slightly sooting ethylene/air laminar premixed flame at atmospheric pressure and analyzed by AFM after a rapid sublimation procedure. Our data shed light on one of the most complex and still debated aspect on soot formation, i.e., the nucleation process. The molecular constituents of the initial particles have been individually analyzed in detail in their chemical/structural characteristics. Our data demonstrate the large complexity/variety of the aromatic compounds which are the building blocks of the initial soot particles. Nevertheless, some fundamental and specific characteristics have been clearly ascertained. These include a significant presence of penta-rings as opposed to the purely benzenoid aromatic compounds and the noticeable presence of aliphatic side-chains. In addition, there were indications for the presence of persistent π radicals. Incipient soot was also investigated by Raman spectroscopy, the results of which agreed in terms of chemical and structural composition of the particles with those obtained by AFM

Soot inception: A DFT study of σ and π dimerization of resonantly stabilized aromatic radicals

Recent advances in the soot studies have shown experimental evidences of π-radicals and cross-linked structures among the molecular constituents of just-nucleated soot particles. π-radicals could have an important role in particle nucleation by increasing the binding energy between polycyclic aromatic hydrocarbons with respect to pure van der Waals interactions. In this work we use density functional theory by Grimme D3 dispersion correction (DFT-D3) with hybrid functional and localized Gaussian basis set (B3LYP/6-31G**) to analyze and classify the clustering behaviors of two aromatic radicals visualized experimentally by atomic force microscopy (Commodo et al. Combust. Flame 205: 154–164, 2019). These aromatic radicals have different topological structures and delocalization of the unpaired electron. The binding energy and energy bandgap characteristics of the clusters are calculated. The theoretical results show a different clustering behavior for the two aromatic radicals. The one with a partial localization of the unpaired electron tends to form a σ-dimer; conversely, the radical with a greater delocalization of the unpaired electron leads to π-stacking formation with a slight overbinding of few kcal mol−1 with respect to pure van der Waals interactions and a marked lowering of the energy bandgap. The formation of π-stacking induced by delocalized π-radicals could in part explain some spectroscopic evidences observed during soot nucleation. © 2020 Elsevier Lt

Production of He-4 and (4) in Pb-Pb collisions at root(NN)-N-S=2.76 TeV at the LHC

Results on the production of He-4 and (4) nuclei in Pb-Pb collisions at root(NN)-N-S = 2.76 TeV in the rapidity range vertical bar y vertical bar <1, using the ALICE detector, are presented in this paper. The rapidity densities corresponding to 0-10% central events are found to be dN/dy4(He) = (0.8 +/- 0.4 (stat) +/- 0.3 (syst)) x 10(-6) and dN/dy4 = (1.1 +/- 0.4 (stat) +/- 0.2 (syst)) x 10(-6), respectively. This is in agreement with the statistical thermal model expectation assuming the same chemical freeze-out temperature (T-chem = 156 MeV) as for light hadrons. The measured ratio of (4)/He-4 is 1.4 +/- 0.8 (stat) +/- 0.5 (syst). (C) 2018 Published by Elsevier B.V.Peer reviewe

Exploring Soot Particle Concentration and Emissivity by Transient Thermocouples Measurements in Laminar Partially Premixed Coflow Flames

Soot formation in combustion represents a complex phenomenon that strongly depends on several factors such as pressure, temperature, fuel chemical composition, and the extent of premixing. The effect of partial premixing on soot formation is of relevance also for real combustion devices and still needs to be fully understood. An improved version of the thermophoretic particle densitometry (TPD) method has been used in this work with the aim to obtain both quantitative and qualitative information of soot particles generated in a set of laminar partially-premixed coflow flames characterized by different equivalence ratios. To this aim, the transient thermocouple temperature response has been analyzed to infer particle concentration and emissivity. A variety of thermal emissivity values have been measured for flame-formed carbonaceous particles, ranging from 0.4 to 0.5 for the early nucleated soot particles up to the value of 0.95, representing the typical value commonly attributed to mature soot particles, indicating that the correct determination of the thermal emissivity is necessary to accurately evaluate the particle volume fraction. This is particularly true at the early stage of the soot formation, when particle concentration measurement is indeed particularly challenging as in the central region of the diffusion flames. With increasing premixing, an initial increase of particles is detected both in the maximum radial soot volume fraction region and in the central region of the flame, while the further addition of primary air determines the particle volume fraction drop. Finally, a modeling analysis based on a sectional approach has been performed to corroborate the experimental findings

Manufacture and Characterization of Nanoparticles for Energetic Applications

It is established nowadays in combustion science and technology that every flame can pro-duce particles, even when it appears to be particle-free. Starting from this assumption, flames are considered no longer only as a reactive flow with internal energy transfer, but also as a reactor for synthesizing gaseous and solid species. Traditional combustion aerosol technology is focused on studying nanosized particulate matter formation during combustion of hydrocarbons, considered as unwanted pollutants, in order to understand the onset of their formation and minimize their emis-sions. On the other hand, in flame aerosol synthesis nanoparticles are desired products, and the knowledge of their formation is the starting point to set up cheap flame reactors. The main objective of this Ph.D. thesis is the development and control of specific aerosol flame synthesis (AFS) systems for the production and the subsequent characterization of engineered nanomaterials. The work was focused on metal oxides based nanomaterials, carbonaceous nano-materials and carbon-metal oxides nanocomposite. The flame reactors used for the synthesis of carbon nanomaterials were constituted by un-doped flat laminar ethylene/air premixed flame, operated in fuel-rich condition, in which vapor car-bon precursors are given by the unburned fuel. Different carbonaceous particles were produced by changing flame equivalent ratio and particle residence time (i.e, sampling position). Properties of synthesized nanoparticles, such as 2D/3D character, optical features, chemical structure, electrical behavior and interaction forces, were characterized with respect to the flame reactor characteristics. Regarding the production of metal oxides nanoparticles and carbon-metal oxides nanocom-posites, an Aerosol Flame Synthesis (AFS) system was developed and successfully operated in VAFS mode to produce pure, monodisperse nanoparticles of magnesium oxide and titanium diox-ide, by feeding magnesium and titanium precursors to a fuel-lean hydrocarbon flame reactor. Finally, the AFS system was operated in fuel-rich flame reactor conditions, in order to syn-thesize pure TiO2 and carbon-TiO2 nanoparticles with similar dimension and compositions. A char-acterization of nanoparticle health effects, for personal care products applications, in terms of Reac-tive Oxygen Species (ROS) production was performed, showing that flame-synthesized titania pro-duces a lower amount of ROS with respect to commercial TiO2. The presence of carbon induces a further decrease of ROS production, leading to a reduction of nanopowder skin toxicity

Characterization of flame-generated 2-D carbon nano-disks

Two-dimensional carbon nanostructures have been produced via flame synthesis. Flame conditions and sampling methods have been optimized for the collection of the carbon nanostructures and for their characterization. Atomic Force Microscopy, Differential Mobility Analysis and Raman spectroscopy have been used to characterize the morphology and the chemical composition of these carbon nanostructures. Results show that a network of aromatic compounds connected by non-aromatic bonding arrange to form bi-dimensional structures that assume an atomically thin disk-like shape when deposited on a substrate. The in plane lateral dimension is of the order of tens of nanometers, and the height of about 3 Å. In spite of a significant amount of disorder, Raman spectroscopy evidences that lattice distortion is not very large in these flame nano-disks

Health issues concerning carbon-TiO2 nanomaterials produced by flame synthesis

Titanium dioxide TiO2 is one of the most important and most used materials in the modern era. Because of its properties it is suitable for a large number of applications in several different fields. In the bulk phase, it is primarily used as a white pigment. As nanomaterial, it is a component in dye-sensitized and organic solar cells as well as in food and personal care products. In the last years personal care applications are increasing; for example nano-powdered TiO2 is extensively employed as physical filter in commercial sunscreens, since it is capable to adsorb and scatter both UVA and UVB radiations and to eliminate the natural opacity typical of microsized sunscreens. However, together with its physical and chemical properties, it is important to assess health implications that the use of TiO2 powder in the nanometer scale might have in personal care applications. A possible way to reduce health implications, especially when related to the formation of freeradicals, is given by coating and doping of TiO2 nanoparticles with organic and inorganic additives. Different efficiencies in preventing the release of free radicals can be achieved, depending on the nature of coating and doping. In this work, the formation of pure TiO2 nanoparticles and carbon-titania nanocomposite by flame synthesis is presented. The procedure is based on the injection of Ti precursor solutions inside hydrocarbon flames with different equivalent ratios. This allows us to synthesize nano-sized particles (1-10 nm), with specific crystallinity phase and carbon content. Chemical and physical properties, composition and dimension of flame-synthesized nanoparticles are characterized by Raman spectroscopy, UV-visible light absorption and Atomic Force Microscopy. Analysis of Reactive Oxygen Species in human keratinocytes cellular lines is used to select the operating conditions of the synthesis process leading to the production of TiO2-carbon nanopowder with reduced adverse health effect

Flame-formed carbon nanoparticles: Morphology, interaction forces, and hamaker constant from AFM

Interaction forces acting between combustion-generated carbon particles were studied by using atomic force microscopy (AFM). To this aim, carbon nanoparticles were produced in fuel-rich ethylene/air laminar premixed flames with different equivalent ratios Φ, and analyzed at a fixed residence time in the flame. Particles were collected on mica substrates by means of a thermophoretic sampling system and then analyzed by AFM. A characterization of particle dimension and morphology were performed operating AFM in semicontact mode, showing that the shape of the particles collected on a sampling plate is never spherical. Increasing the flame-equivalent ratio, particle shape moves from an almost atomically thick object to thicker compounds, indicating the transformation from particles made of small, defective graphene-like sheets to particles containing stacked aromatic layers. Attractive and adhesive forces between a titanium nitride probe and sampled particles were calculated from force-distance curves acquired in AFM force spectroscopy mode. Assuming that van der Walls forces are the main contribution to attractive forces, the measurement of attractive forces allowed the evaluation of the Hamaker constant for the carbon particles as a function of the flame-equivalent ratio. The comparison of the measured Hamaker constants with the values for benzene and HOPG, suggests a continuous increase of the aromatic domains and the three-dimensional order within the particles when the flame-equivalent ratio increases