cykao@csie.ntu.edu.tw.

The approach of utilizing combustion synthesis to make fine particles of SiO2, A1203 and TiO 2 is a quite modern technology. Through the chemical reaction in post-flame region, fine SiO 2 particles can be formed with high purity on plate surface. Therefore, the combustion synthesis of SiO2 powders is an important area for further research and development, especially for the application of SiO 2 in the semiconductor industry. This investigation proposes an experimental approach (i.e., a gas-phase combustion synthesis) using two different kinds of organic compounds, Hexamethyldisilazane (HMDSA) and Hexamethyldisioxane (HMDSO), as the silicon precursors. A premixed gas bumer is chosen with C3H 8 as fuel, air as oxidant and part of the air was used as the carrying gas to entrain HMDSA/HMDSO vapor into the combustible mixture. Observations show that the C3Hs/air flame changed color from a pale-blue flame to light yellow and then orange when different amounts of precursors were introduced. Through the chemical reaction in the post-flame region, fine SiO 2 particles were formed in the gas phase and then quenched and collected on an aluminum flat plate. The objective of this paper is to study the effects of HMDSO and HMDSA concentrations and flame temperatures on the synthesis of SiO 2 particles.

An experimental study of gas-phase combustion synthesis of silicon dioxide (SiO,) particles was conducted in C,H,/air and H,/air premixed flames. Two types of organosilicon compounds. HMDSO and HMDSA. were used as the silicon precursors of synthesis processes. The objective of this investigation was to study the effects of HMDSO and HMDSA concentration, flame temperature, water vapor. and oxygen mole fraction on the synthesis of Si0 2 particles. The entrained HMDSO and HMDSA concentrations were varied from 0.2 percent to 1.2 percent. The flame temperatures and oxygen mole fractions in the post-flame region ranged from 1350 to 1700 K and 0 to 8.5 percent, respectively. As a small amount of HMDSO or HMDSA was added into C 3H s / air premixed mixtures, an orange-color secondary flame was observed. With the increase in HMDSO (or HMDSA) concentration. a bright yellow flame brush appeared following the tip of inner luminous cone of the flame. The flame brush could represent a particle-laden region. It was found that the flames with the same mole fraction of HMDSO and HMDSA exhibited comparable emission luminosity and similar flame structure. The radial temperature profiles indicated a decrease of flame temperature with the increasing HMDSO and HMDSA mole fractions. The axial temperature profiles showed that a decrease of temperature occurred in the flame brush region. Based upon the composition analyses of synthesized particles by the energy dispersive spectrometry (EDS), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD), the synthesized particle represents a mixed oxide of SiO and SiO,. Results showed that high-purity SiO, particles were synthesized in both CJH,/air and H,/air flames with a low HMDSO or HMDSA conccntration of about 0.2 percent. A, the HMDSO concentration increased, the test condition with a higher flame temperature yielded particles containing a slightly higher percentage of SiO,. However, when HMDSA was used at high concentrations, the mole fraction of oxygen in the post-flame zone played a more important role in the formation of Si0 2 than did the flame temperature. Based upon the composition of synthesized particles in a fuelrich H 2/air flame, it was believed that the water vapor served as an important oxidizer in the synthesis mechanism of SiO l _

This study is using HMDSA (C6H19NSi2) or HMDSO (C6HIROSi2) vapor into C3Hs/air premixed flames to form SiO2 thin film on the surface of an aluminum plate. With the addition of HMDSO or HMDSA to premixed flames, an orange secondary flame or a flame brush appeared and was contributed to the formation of SiO2 particles. Based upon the EDS, XPS and FTIR analysis, it is believed that the synthesized products consist of mainly SiO2 and a small amount of SiO. The pure SiO2 crystal structure, was proved by XRD analysis, which may form from the SiO2 amorphous structure after high temperature (1300°C) thermal treatment. The nano-size SiO2 particles, which ranged from 2.5-25 nm, are proved by analysis of the BET and TEM. A 2-D CFD-RC code with 12 reduced chemical reaction mechanism, based upon the SIMPLER procedure, was successfully employed to predict the flame temperature and both of the SiO2 and SiO concentration profiles. Compared with the experimental results, the calculated temperature profiles in the post-flame region are in good agreement with the measured data and observation phenomena.

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