Synthesis of Al 2O 3 and SnO 2 particles by oxidation of metalorganic precursors in premixed H 2/O 2/Ar low pressure flames

Abstract

Low pressure premixed H 2/O 2/Ar flames were doped with the metalorganic precursors Al(CH 3) 3 (trimethylaluminium) and Sn(CH 3) 4 (tetramethyltin), respectively. The dopant concentration in the feed gas mixture was varied between 96 ppm and 3066 ppm. Nanosized Al 2O 3-particles and SnO 2-particles were formed during the oxidation process. They were extracted at different heights from the flame zone by two thermophoretic sampling devices and a molecular beamprobe, which is part of an aerosol mass spectrometer, AMS. This instrument allows the in situ analysis of the combustion aerosol according to the chemical composition of the gas phase as well as the mass of charged particles. The thermophoretically sampled particles were analysedfor their chemical composition, specific surface area, crystal structure, particle size, and morphology by the use of FT-IR absorption spectroscopy, BET gas adsorption method, X-ray I electron diffraction, and brightfield TEM analysis. The gas phase of the combustion aerosol was analysedfor gaseous reaction products of the precursors. Neither in the Al(CH 3) 3 doped flames nor in the Sn(CH 3) 4 experiments are gaseous metals or metal compounds found. The oxidation of Al(CH 3) 3 was monitored by measuring the formation of the by-product CO 2. At x ≈35 mm the slope of the CO 2 concentration profile indicates complete oxidation of the precursor. The formation and growth of amorphous Al 2O 3 particles of spherical shape was found in the size range 4.7 nm ≤d p ≤8.4 nm. Their size depend on the precursor concentration and the sampling position, indicated by the flow co-ordinate x. In the case of SnO 2 nanoparticle formation, two different crystal structures were found, which were influenced by the flow coordinate and the temperature drop during the sampling procedure. The results obtained from X-ray and electron diffraction analysis indicate a crystal evolution in the flame starting from a metastable phase, which is up to now unidentified to the commonly known α-structure. In samples extracted at x = 25 mm, which represent the early phase of the flame, primary SnO 2 particles of spherical shape and monodisperse size distribution were found ranging between 2.7 nm ≤d p ≤8.3 nm depending on the burning conditions. At x = 60 mm compact agglomerates of primary particles were formed. A theoretical model was developed including the full H 2 O 2 reaction kinetics as well as the transport properties of a burner stabilised flame. A sectional model was applied to calculate the particle growth starting at the molecular scale of homogeneous oxide formation in the gas phase followed by Brownian coagulation of nanometerparticles. Results were compared to experimental data for both substances.