Pascazio, Laura (2017) Nucleation and characterization of combustion-generated nanoparticles: a molecular dynamics approach. [Tesi di dottorato]


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Item Type: Tesi di dottorato
Lingua: English
Title: Nucleation and characterization of combustion-generated nanoparticles: a molecular dynamics approach.
Date: 11 December 2017
Number of Pages: 120
Institution: Università degli Studi di Napoli Federico II
Department: dep08
Dottorato: phd038
Ciclo di dottorato: 30
Coordinatore del Corso di dottorato:
Mensitieri, GiuseppeUNSPECIFIED
Date: 11 December 2017
Number of Pages: 120
Uncontrolled Keywords: Nucleation; Soot; Nanoparticles; Molecular Dynamics
Settori scientifico-disciplinari del MIUR: Area 09 - Ingegneria industriale e dell'informazione > ING-IND/25 - Impianti chimici
Date Deposited: 06 Jan 2018 03:10
Last Modified: 19 Mar 2019 12:10


Combustion-generated carbonaceous particles are generally considered as unwanted byproducts of energy production and significant atmospheric pollutant. Several studies have been conducted, during last decades, on the chemical-physical mechanisms responsible for the formation of carbon compounds in flames, with the aim of improving the combustion efficiency and reducing the emission of pollutants from combustion devices. Moreover, since such species have many features similar to carbon compounds used in new technologies (fullerene, graphite, carbon blacks and so on), a deeply knowledge on the key steps that control the formation and growth of such species can be a starting point to develop new synthesis routes for specific and tailored carbonaceous structures designed for innovative technologies. Nucleation of combustion-generated particles remains the least understood process in particle formation in combustion and it is a challenge for both experimental and modelling sides. The peculiar combustion environment, which is characterized by high temperatures and a large number of radicals, make the study of this process very complex. Despite of the efforts spent on the topic, a conclusive description of the process is not reached yet. Molecular Dynamics (MD) approach allows to describe punctually the physical interaction and evolution of the particles and of their principal constituents, i.e., the polycyclic aromatic hydrocarbons (PAHs). Monitoring the internal coordinate of atoms in the system over the time, MD allows to predict qualitatively the behaviour of a system at conditions that cannot be accessed experimentally. The objective of this study is to explore nucleation process and the properties of particles obtained by using MD. Firstly, the role of different PAHs in the physical nucleation process is investigated using MD. Two different types of molecules present in a flame environment are studied: pericondensed aromatic hydrocarbons (PCAHs) and aromatic aliphatic-linked hydrocarbons (AALHs). MD allows to treat the nucleation in a physical way. However, the investigation of AALH evolution permits to treat indirectly also the chemical pathway because every ALLH molecule can be viewed as the result of a chemical reaction between PCAH molecules. The effect of different molecular masses and morphologies of precursor molecules as well as temperatures on the nucleation mechanism is analysed. Moreover, for the first time, statistical analysis of the morphology of the formed clusters are reported, introducing the internal distance distribution and a structural parameter. Successively, a study of the effect of the intermolecular potential function on MD simulation of PAHs stacking and cluster morphology is made. The accuracy and reliability of a MD simulation mainly depends on the quality of the force field employed to model the intra- and intermolecular interactions. Many intermolecular potential functions and parametrizations for PAH molecules have been proposed in literature. Four intermolecular potentials, which differ for their function forms and parametrizations used in other MD PAH evolution studies, are tested in order to analyse the difference not only in cluster formation but also in the formed cluster morphologies, an aspect never considered in this kind of studies. The use of a systematic approach to analyse the internal structure and shape of the formed clusters allowed to have a better comparison between the potentials and their capability to reproduce realistic configurations. Collision efficiencies of pyrene and coronene are also determined from MD results, in order to relate the MD findings with soot formation kinetic models. Finally, mechanical properties of cross-linked PAHs are tested. The mechanical properties of a material are strictly related to its internal structure. Many HR-TEM studies on soot have been carried on during the years, gaining new insights on its properties, but if its constituents are chemical or only physical bonded is still unclear. MD simulations of uniaxial tensile tests on PAHs boxes with different degree of cross-linking are made. Hardness of each sample is calculated and compared with nanoindentation experiments of soot particles. The results clearly show that the soot structure must present cross-links between its constituent PAH molecules to have a comparable value of the hardness found experimentally.

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