ABSTRACT
Si-C-H-O-containing radicals are important intermediates during the combustion and pyrolysis of precursors applied for the gas-phase synthesis of silica nanoparticles. Despite the industrial importance of silica nanoparticles, a comprehensive thermodynamics database of organosilane species is still missing. This work presents thermochemical data of 91 Si-C-H-O radical species. Quantum-chemical calculations and isodesmic reaction schemes are used to determine the standard enthalpy of formation, entropy, and heat capacities covering the 298-2000 K temperature range. In addition, 90 group-additivity values (GAVs) are calculated, which cover all relevant group increments. A combinatorial approach is used to ensure that all possible group increments are considered. The theoretically calculated species are used as a training set to derive 90 GAVs of Si-C-H-O radical species for the first time. In addition, uncertainty contributions of GAVs were estimated. These uncertainty estimates also comprise GAVs that were previously derived to compute thermochemical data of stable Si-C-H species and radicals as well as stable Si-C-H-O compounds. Therefore, uncertainty contributions of GAVs for a whole set of 243 group increments used to predict thermochemical data of Si-organic species are reported.
ABSTRACT
Structure formation models describe the change of the particle structure, e.g., by sintering or coating, as a function of the residence time and temperature. For the validation of these models, precise experimental data are required. The precise determination of the required data is difficult due to simultaneously acting mechanisms leading to particle structure formation as well as their dependency on various particle properties and process conditions in the reactor. In this work, a model flow reactor (MFR) is designed and optimized, supported by a validated computational fluid dynamic simulation, to determine the structure formation of nanoparticles under well-defined conditions. Online instrumentation is used to measure the particle mass and different equivalent diameter to detect changes of the particle shape and to calculate the particle structure, defined by the primary particle size, the number of primary particles per agglomerate, coating thickness, effective density, and fractal dimension, by means of structural models. High precision is achieved by examining size-selected particles in a low number concentration and a laminar flow field. Coagulation can be neglected due to the low particle number concentration. Structure formation is restricted to a defined region by direct particle trajectories from the water-cooled aerosol inlet to the water-cooled outlet. A preheated sheath gas is used to concentrate the aerosol on the centerline. The simulated particle trajectories exhibit a well-defined and narrow temperature residence time distribution. Residence times of at least 1 s in the temperature range from 500 K to 1400 K are achieved. The operation of the MFR is demonstrated by the sintering of size-selected FexOy agglomerates with measurements of the particle size and mass distribution as a function of the temperature. An increase of the effective density, resulting from the decreasing particle size at constant particle mass, is observed.
ABSTRACT
Nanocrystalline titania was synthesized via liquid-fed spray-flame synthesis in a hermetically closed system at various pressures. Titanium tetraisopropoxide dissolved in isopropanol was used as precursor. The size, crystal structure, degree of agglomeration, morphology and the band gap of the as-prepared particles were investigated ex situ by nitrogen adsorption, transmission electron microscopy, X-ray diffraction, and UV-VIS absorption spectroscopy. In comparison to synthesis at atmospheric pressure it was found that decreasing pressure has a significant influence on the particle size distribution leading to smaller particles with reduced geometric standard deviation while particle morphology and crystal structure are not affected. Computational fluid dynamics simulations support the experimental findings also indicating a significant decrease in particle size at reduced pressure. Although it is well known that decreasing pressure leads to smaller particle sizes, it is (to our knowledge) the first time that this relation was investigated for spray-flame synthesis.
