TY  - THES
T1  - Characterization of soot particles from diesel engines and tin dioxide particles milled in stirred media mills
A1  - Ma?kovi?,Mirza
Y1  - 2012/07/13
N2  - Transmission electron microscopy, electron energy loss spectroscopy and Raman spectroscopy are used to investigate soot particles taken form the combustion chamber and exhaust of low emission diesel engines. The present work represents an enormous progress in soot particle characterization and the understanding of the complex soot formation mechanism. Nascent particles are found in the early pre-combustion phase and are assumed to be nuclei for the further soot formation. First soot particles are observed at the beginning of the main-combustion phase and the evolution of their internal and electronic structure is shown as a function of crank angle / time after the combustion of the diesel fuel begins, as well as a function of injection pressure. A sample taken at the late phase of the main-combustion shows that the chain-like agglomerates, which consist of several primary soot particles, are already formed in the combustion chamber of a diesel engine. Such chain-like agglomerates were previously only found in the exhaust of diesel engines.
Investigations of the soot samples taken from the exhaust of diesel engines show the dependence of the morphology and nanostructure on the operating conditions (for instance injection pressure, exhaust gas recirculation) given in a diesel engine.
The present work points out that the transmission electron microscopy techniques are powerful methods to investigate soot particles taken from the exhaust and combustion chamber of diesel engines. Coupling the engine parameters with the results from electron microscopy enabled to understand more in detail the mechanisms which lead to the formation of diesel engine soot. Furthermore, the combination of results achieved from exhaust diesel soot and combustion chamber soot might be very useful for the elimination of diesel soot particles and the lowering of diesel soot emissions.

One possibility for the fabrication of nanoparticles is wet grinding in stirred media mills. Motivated by the fact that the fracture mechanisms at the nanoscale are not yet fully understood, in the present work the evolution of the microstructure within tin dioxide particles, grinded in a stirred media mill, is investigated. HRTEM images show particles with sizes below 10 nm, while mean crystallite sizes of ?  9 nm were measured from XRD. TEM analysis were conducted to gain detailed insight into the microstructural effects which govern the grinding process. Using TEM the formation of stacking faults, shear bands and mechanical twins on nanoscale are revealed.
MD simulations are performed, where the uniaxial compression of particles with a diameter of 30 nm was simulated. The simulated particles shared microstructural details with the real samples, most importantly the shear bands which lead to significant plastic deformation. In situ nanoindentation tests describe the mechanical behaviour of tin dioxide particles to be plastic, but in some cases also brittle in nature. Deformation induced events from load-displacement curves are directly coupled to images recorded during the in situ nanoindentation tests. The internal microstructure produced during multiple particle stressing events in the mill and also observed in the simulations is directly linked to the fracture mechanism and the experimentally observed grinding limit.
The studies in this work indicate that TEM and MD simulations are suitable methods for the observation of structural changes that occur in a comminution process. Quantitative comparison between intraparticulate structures produced in a real mill and those observed in molecular dynamics simulations was not possible. One reason is that the stressing events in a stirred media mill are very complex and not yet known in sufficient detail in terms of number, intensity and direction. Another reason is the limited computer capacities in MD simulations, so several simplifications in the simulation model must be made. While in the milling experiment the stressing events represent an unknown combination of compression, shear, impact and friction, the loading case in MD simulations and in situ nanoindentation testing is simplified to single compression modes. In order to understand in detail the microstructural changes occurring during a nanomilling experiment, fluid mechanics, motion of the milling beads, and multiple stressing events of all kinds must be taken into account, which represents a complex process and thus a motivation for further investigations.
KW  - Durchstrahlungselektronenmikroskopie
KW  - Dieselmotor
KW  - Nanopartikel
CY  - Erlangen
PB  - Universitätsbibliothek der Universität Erlangen-Nürnberg
AD  - Universitätsstraße. 4, 91054 Erlangen
L2  - http://www.opus.ub.uni-erlangen.de/opus/volltexte/2012/3353
ER  -