Mass spectrometric gas phase diagnostics in particle forming flames

Functional materials in the form of nanoparticles and coatings offer a high potential for applications in energy conversion and in biomedicine due to their size-dependent properties. Alongside the chemical composition, the control of the morphology can open up new dimensions for the use of nanoparticles. The research unit FOR2284 of the German Research Foundation is focused on the gas phase synthesis of special materials consisting of iron oxide and silicon dioxide. The objective of the research unit is to develop design rules for technical synthesis processes based on a fundamental understanding of the gas phase processes. The initial step of the gas phase synthesis is the decomposition of the precursor by the reactions with flame species. Tetramethylsilane (TMS) is a frequently used precursor for the silicon dioxide synthesis. The reactivity of the precursor in the flame is investigated. It comprises the decomposition kinetics and flame interaction of different precursors. For the experimental analysis time-of-flight molecular beam mass spectrometry is used. A large number of intermediate species (silicon-containing monomers, hydrocarbon species and silicon containing clusters) are analyzed in tetramethylsilane (TMS)-doped H2/O2/Ar flames. Based on these findings, a chemical reaction mechanism for different flame equivalence ratios (φ =0.6 - 1.2) and different precursor concentrations (TMS = 400 - 800 ppm) is developed and evaluated. The decomposition kinetics of TMS are analyzed experimentally and simulatively by a systematic study. The results indicate that in the gas phase synthesis of silica particles, two distinct particle formation pathways are active, the path SiO → (SiO)n → Particle and the path Si(OH)4 → Si4O10H4 → Particle.

Iron pentacarbonyl is used as the precursor for the iron oxide synthesis. Some species in synthesis flames are not detectable with the time-of-flight molecular beam mass spectrometer system. For this reason, a novel and very sensitive sampling technique for the measurement of naturally occurring ions is used for the investigation of flames. Initially, this technique is evaluated in a methane flame. In an extensive study on methane ion chemistry, a quantification approach using equilibrium calculations is tested. After the evaluation of the technique, the flame structure of iron pentacarbonyl (Fe(CO)5)-doped H2/O2/Ar flames for the production of iron oxide is analyzed. The results indicate that iron oxides (Fe2O3, Fe4O5 and Fe5O5) and iron hydroxides (Fe(OH)2 and Fe(OH)3) are formed in synthesis flames. Two oxidation states of iron (Fe(II) and Fe(III)) occur spatially separated in the flame. A database comprising reaction kinetics data of species with Fe/C/O/H- and Si/C/O/H-elements is provided and used for the evaluation and creation of chemical reaction kinetics models. This work represents the first results for the optimization of the gas phase synthesis of special nanomaterials.

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