Russo, Carmela (2013) Characterization and synthesis of carbon aggregates in high temperature environment. [Tesi di dottorato]

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Abstract

Carbon materials in all its forms, from the natural carbon solid materials, as coal and graphite, to the synthesized carbon materials, as carbon black, pitch fibers, fullerenes, carbon nanotubes, etc,. have been object of many studies regarding their characteristics and behaviour due to their importance in the energy and industrial sectors. Recently, most of the research efforts have been focused on the synthesis of new carbon materials and in particular on their physico-chemical properties which are very variable as a function of the production process. Arc discharge and laser ablation methods, chemical vapour deposition, pyrolysis and flame synthesis are the processes mostly used for production of special carbon materials as fullerenes, nanotubes, graphene, etc., whose structures are relatively simple and fairly well known. By contrast, traditional high-temperature pyrolytic and combustion processes in very fuel-rich conditions of gaseous, liquid, and solid fuels also produce a carbon material, namely soot, that is important when the enhancement of radiative transfer is required, but undesired for the pollutant effect. A special form of soot is carbon black that is an industrially-manufactured product used as filler in rubber products and as a component in ink for printers. Soot and related high mass aromatic species, as polycyclic aromatic hydrocarbons, constitute as a whole the carbon particulate matter, in form of particles and aggregates, that can be produced starting from organic precursors, generally hydrocarbons, in high temperature partially oxidative conditions (fuel-rich) typical of combustion processes. Soot exhibits a mixed ordered/disordered character having a nano-structure mainly based on two-dimensional graphene layers grouped, cross-linked and/or layered each other in a disordered (turbostratic) way. The degree of order/disorder depends on the process temperature and fuel source. This PhD thesis is focused on the study of the structural characteristics of carbon particulate matter, synthesized in the well-controlled combustion conditions of a premixed fuel-rich flame, where it is possible to study the parameters affecting their formation and, consequently, controlling their features. In this context the main objectives of this thesis are: i) to explore the diagnostic potentialities of fast and non-destructive spectroscopic tools for carbon characterization, ii) to get a deeper understanding of the formation mechanism of carbon materials in flames with the perspective of controlling and modulating the properties of carbon materials obtained. Structural characterization of carbon materials at a nanoscale implies the determination of three key parameters that determine their structure and properties: the fraction of sp2-bonded carbon sites, the hydrogen content and the ordering and clustering of sp2 sites. Various characterization methods have been developed and implemented to determine these structural parameters. Among them, the use of optical features have been in deeper investigated and focused on the upgrade and the exploitation of the diagnostic potentiality of spectroscopic tools. In particular UV-Visible, FT-IR and Raman spectroscopy, have been developed for the quantitative and qualitative characterization of flame-derived carbon soot materials and their related micro- and nano-structure. A reconstruction method of UV-Visible spectra has been developed as diagnostic tool discriminating the contribution and the spectral features of different molecular weight aromatic species to the spectral properties of carbonaceous species formed in flame. A detailed procedure for the quantitative FT-IR analysis of aromatic and aliphatic hydrogen has been set up by using different standard aromatic and aliphatic species and implemented on hydrogen-rich and hydrogen-poor carbon materials. Regarding Raman spectroscopy, there are no multiwavelength Raman data for flame formed soot. Part of this work was devoted to fill this gap by providing Raman spectra at different excitation energies and exploiting the resonant Raman process as a powerful mean for soot structural and electronic characterization. On the basis of the experimental results obtained, relationships between synthesis/combustion conditions and specific carbon properties, such as size distribution, chemical composition, optical properties and internal structural organization, have been found. This has led to give further insights on the carbonaceous species formation mechanism into a flame environment.

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