Markus Sander

SURFACE CHEMISTRY AND PARTICLE SHAPE: PROCESSES FOR THE EVOLUTION OF AEROSOLS IN TITAN'S ATMOSPHERE

We use a stochastic approach in order to investigate the production and evolution of aerosols in Titan’s atmosphere. The simulation initiates from the benzene molecules observed in the thermosphere and follows their evolution to larger aromatic structures through reaction with gas-phase radical species. Aromatics are allowed to collide and provide the first primary particles, which further grow to aggregates through coagulation. We also consider for the first time the contribution of heterogenous processes at the surface of the particles, which are described by the deposition of the formed aromatic structures on the surface of the particles, and also through the chemical reaction with radical species. Our results demonstrate that the evolution of aerosols in terms of size, shape, and density is a result of competing processes between surface growth, coagulation, and sedimentation. Furthermore, our simulations clearly demonstrate the presence of a spherical growth region in the upper atmosphere followed by a transition to an aggregate growth region below. The transition altitude ranges between 500 and 600 km based on the parameters of the simulation.

A Detailed Model for the Sintering of Polydispersed Nanoparticle Agglomerates

In this study the coagulation, condensation, and sintering of nanoparticles is investigated using a stochastic particle model. Each stochastic particle consists of interacting polydisperse primary particles that are connected to each other. In the model sintering occurs between each individual pair of neighboring primary particles. This is important for particles in which the range of the size of the primary particles varies significantly. The sintering time is obtained from the viscous flow model. The model is solved using a stochastic particle algorithm. The particles are represented in a binary tree that contains the connectivity as well as the degree of sintering information. Particles are forme, coagulate, sinter, and experience condensation according to known rate laws. The particle binary tree, along with it the degree of sintering, is updated after each time step according to the rates of the different processes. The stochastic particle method uses the technique of fictitious jumps and linear process deferment. The theoretical results are fitted against experimental values for the formation of SiO 2 nanoparticles and computer generated TEM pictures are presented and compared to experiments.

First-principles thermochemistry for silicon species in the decomposition of tetraethoxysilane.

Tetraethoxysilane (TEOS) is used as a precursor in the industrial production of silica nanoparticles using thermal decomposition methods such as flame spray pyrolysis (FSP). Despite the industrial importance of this process, the current kinetic model of high-temperature decomposition of TEOS to produce intermediate silicon species and eventually form amorphous silica (R-SiO2) nanoparticles remains inadequate. This is partly due to the fact only a small proportion of the possible species is considered. This work presents the thermochemistry of practically all of the species that can exist in the early stages of the reaction mechanism. In order to ensure that all possible species are considered, the process is automated by considering all species that can be formed from the reactions that are deemed reasonable in the standard ethanol combustion model in the literature. Thermochemical data for 180 species (over 160 of which have not appeared in the literature before) are calculated using density functional theory with two different hybrid functionals, B3LYP and B97-1. The standard enthalpy of formation (DeltafH(298.15K) degrees) values for these species are calculated using isodesmic reactions. It is observed that internal rotation may be important because the barriers to rotation are reasonably low. Comparisons are then made between the rigid rotor harmonic oscillator approximation (RRHO) and the RRHO with some of the vibrational modes treated as hindered rotors. It is found that full treatment of the hindered rotors makes a significant difference to the thermochemistry and thus has an impact on equilibrium concentrations and kinetics in this system. For this reason, all of the species are treated using the hindered rotor approximation where appropriate. Finally, equilibrium calculations are performed to identify the intermediates that are likely to be most prevalent in the high-temperature industrial process. Particularly, Si(OH)4, SiH(OH)3, SiH2(OH)2, SiH3(OH), Si(OH)3(OCH3), Si(OH)2(OCH3)2, the silicon dimers (CH3)3-SiOSi(CH3)3 and SiH3OSiH3, and the smaller hydrocarbon species CH4, CO2, C2H4, and C2H6 are highlighted as the important species.