Nanoparticle in high temperature environment: experimental techniques and aspects of synthesis properties
Sirignano, Mariano (2011) Nanoparticle in high temperature environment: experimental techniques and aspects of synthesis properties. [Tesi di dottorato] (Inedito)
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The production of materials with nanoscale features is of great interest for the enhancement of the properties linked with the reduced size. Actually, the cost for the production of the commercially available nanoparticles represents the largest part of the added value of these products. This consideration has pushed the scientific community to find an economic and feasible process able to produce on large scale nanoparticles with determined characteristics. Carbon rose as suitable raw material since it is widely available. Moreover carbon is present on the earth in different form, from coal to light and heavy hydrocarbons; all these compounds have a high chemical potential that can be used during transformation process to get final products. Finally carbon based nanoparticles showed a great moiety of structural arrangements that, in principle, makes them suitable for all purposes. In order to achieve this goal a deep knowledge of the nanoparticle formation process is mandatory to optimally set up reactor parameters. One of the most used and appealing reactor for particle production is represented by flames. Flame synthesis science has been used since the antiquity to produce engineered material. Flames can be alternatively seen as high temperature reactors and thus a set of controlling parameters can be individuated. Temperature history, residence time, mixing effect and fuel or additive structure can be changed in order to obtain the desired products. Moreover the process is autothermic, largely reducing the operative costs. Most of the knowledge about carbonaceous particle in flame is due to the decennials studies conducted in order to strongly reduce the emissions, considered noxious for health and climate. However, to produce smart nanoparticles the knowledge of particle formation has to be shifted toward a deeper level .In fact, the attention has to be paid not only to the total amount produced but also to particle features such as mean size, size distribution, chemical composition, morphological aspect, and internal structure. In this work, lab scale reactors have been used to test the effect of single parameters on the final features of the particles mentioned above. To characterize the particles produced in hydrocarbon flame advanced experimental techniques have been set up. In particular, optical measurements have been conducted by using a conventional laser induced emission (LIE) techniques. Successively a new aspect of LIE has been developed. Time-resolved laser induced fluorescence has been set up in order to gain information on chemical composition and structural properties of the particles produced in hydrocarbon flames. Spectral evolution together with fluorescence lifetime has been correlated to the aromatization degree of the carbonaceous particles and their level of organization at atomic scale. Large part of the work has been devoted to the development of a new numerical tool to predict and explore new fields of nanoparticle flame synthesis. The model has been developed starting from consideration around the nature of the nanoparticles and their evolution based on experimental evidences both produced during this thesis work and from literature data. On the other hand, the model has resulted fundamental to support the results obtained with experimental techniques and thus to draw conclusions. Moreover it reliability has been used to fast explore several combustion conditions, to give indications for experimental investigations and to set up parameters of different reactor configurations. The model has moved from the precious knowledge on the kinetic scheme of gas phase compounds. In order to rigorously treat the particle evolution a sectional method has been used. This method is based on the use of lumped species which allow to numerically treat particle reactions as in the gas phase. In the version developed in this work, the model can be defined as an Advanced Multi Sectional method. It starts from a previous discrete sectional approach for particle modeling based on lumped species, which allowed to describe total amount and size distribution. The final version of the model accounts for multiple discretization, which considers for each lumped species the number of carbon atoms, i.e. the size distribution, the H/C ratio, i.e. the composition on particles and the aggregation state, i.e. the morphological organization. This model represents in the current version the most advanced numerical description of particles evolution. Experimental techniques and modeling simulation has been applied to different reactor configurations. Premixed flames have been investigated to evidence the role of equivalence ratio and fuel structure on particle features during formation and evolution. Opposed flow diffusion flames have been investigated to understand the role of the mixing effect on the particle inception mechanisms and thus final particle characteristics. Coflow flames have been investigated to extend the consideration based on the results of the opposed flow diffusion flame and to evidence the role of oxidation and oxidation induced fragmentation as a controlling step for the determination of the final features of the particles. Finally a non-conventional reactor has been set up, both to study and produce particles in medium temperature regime. A plug flow reactor with controlled temperature has been used. The reactors allow to have a long residence time, in the order of seconds, with temperature up to 700K. Particles produced with a conventional premixed flame have been fed to this system. The size of the particle was previously determined with a Differential Mobility Analyzer. The effect of temperature on particle coagulation efficiency has been evaluated. Looking at particle synthesis, in this reactor controlling the temperature environment it is possible to produce particles with determined size having the same composition of those produced in combustion.
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