Flame Synthesis Silica Nanoparticles

Flame synthesis nanoparticle (FSN) is the main method to manufacture more than 90% of nano-scale commodities in industry. FSN has many advantages. Nanoparticles synthesized by FSN possess the characters of small particle size, low size distribution, and high chemical activity and so on. Besides, not involving wet chemistry process, these products are of high purity and easy to separate. However, there are still some drawbacks in FSN, such as difficulty to precisely control particle size, and uncleanness in particle formation mechanism.To solve the problems in SFN, this dissertation using Hexamethyl-disiloxane as precursor, argon as carrier gas, methane as fuel, and oxygen as oxidant synthesizes silica nanoparticles in diffusion flame reactors. Effecting factors of FSN, particle formation mechanism in flames and method for preparing hydrophobic silica nanoparticles are studied. Main research contents and results have been summarized as follows:(1) Effects of process parameters such as flame configuration, precursor flow rate, methane flow rate, and oxygen flow rate on size and shape of silica nanoparticles are systematically studied. It is found that, as there are no particles deposited on burner mouths, II type flame configuration is suitable for producing nanoparticles for long time; precursor flow rate has an important influent on particle size, and particle size increases linearly as the precursor flow rate grows; particle size increases as methane flow rate grows; and particle size firstly increases and then decreases as the oxygen flow rate grows. In conclusion, factors deciding particle size are precursor flow rate, flame temperature, and resident time.(2) On-line particle characterization system based on ELPI, TEM grid in-situ thermophoretic sampling and infrared thermometer technique are used to study the diffusion flame temperature profile, and number density and shape of particles at different flame height. It is found that particles experience chemical reaction, nuclear, surface reaction, particle gasification, coalescence, and coagulation to arrive the ultimate form.(3) Based on the traditional diffusion flame reactors, a new argon (branch) flow is introduced to further decrease particle resident time in flames, and silica nanoparticles with 7nm-size are successfully prepared, which is smaller than particles prepared in traditional reactors (13nm).(4) Further increasing argon (branch) flow rate can produce hydrophobic silica nanoparticles, which has a contact angle of 131°. FTIR, TG, XPS, and NMR are used to confirm the structure of the hydrophobic silica nanoparticles. It is found that Methyl groups staying on the surface of silica nanoparticles though surface reaction make silica be hydrophobic.In this dissertation, our knowledge of FSN in affecting factors, formation mechanism, particle controlling methods, and hydrophobic silica preparation is improved, which builds a solid foundation for China's industrialization of silica nanoparticles synthesized by FSN.