Research On Synthesis Of Titanium Dioixde Nanoparticles By Gas-phase Deflagration And Detonation Method

Gas-phase Deflagration and Detonation method is a new synthesis technology for preparing nanomaterials by detonating the combustible gases, gas-phase oxidants and precursors. Not only is it simple operation, easy control, high efficiency, low costs and energy efficiency, but also it is high output, high purity and easy for industrialization. In this paper, TiO₂ nanoparticles were produced by gas-phase deflagration and detonation method, and the synthesis mechanism had also been studied. The main work and results are as follows:1. The detonation pressure of hydrogen-air deflagration and hydrogen oxygen detonation were measured. The flame propagation processes of deflagration and detonation were photographed by high speed cameras. In the deflagration reaction of H₂ and air, the maximum flame velocity is about 250 m/s in the obserbing window, and the maximum detonation pressure is about 0.5 MPa. In the initial stage of the reaction H₂ and O₂, it is under transition from deflagration to detonation. Then the frame front produced distortion due to the turbulence, which leads to the increase of combustion velocity. Finally the reaction is up to detonation at the tail of pipe. The flame velocity is about 1300 m/s at the distance of 0.46 m from the initiation, and the velocity becomes higher. The maximum detonation pressure is about 2.0 MPa when the reaction finished. The ambient temperature has less influence on the maximum pressure and the pressure rising rate in the detonation reaction of H₂ and O₂. With the ambient temperature increase, the maximum pressure and the pressure rising rate rise in the deflagration reaction of H₂ and air.2. The numerical simulation processes of hydrogen-air reaction and hydrogen-oxygen reaction are studied. It can be proved that the hydrogen-air reaction is deflagration reaction, and the hydrogen-oxygen reaction is under detonation. In the reaction of H₂ and air, the pressure and temperature rise with time. Furthermore, compression wave appears ahead of flame front. Otherwise, gas density decreases behind of flame front. With the process of reaction, the flame propagates from the ignition to another end until the reaction finished. In the reaction of H₂ and O₂, the deflagration reaction gradually turns to the detonation reaction. Compared with the experimental results, the numerical simulation results are identical.3. With changing the ambient temperature and the content of precursor, we could selectively synthesize TiO₂ nanoparticles by controlling the particle size and shapes of TiO₂ particles. According to Clausius-Clapeyron equation, we deduced the relation curve between vapor pressure of titanium tetrachloride and gasification temperature. When the ambient temperature did not reach the gasification temperature of titanium tetrachloride, the titanium tetrachloride turned into aerosol, which would change to foggy droplets nearby the inner wall of pipe. In the above case, it would have great influence on the particle size and the shapes of TiO₂ particles.4. Using titanium tetrachloride as the source of TiO₂, the experiments for synthesizing TiO₂ nanoparticles were carried out by detonating the premixed gas of H₂ and O₂. The structures and properties of the as-prepared TiO₂ nanoparticles were also characterized. Compared with the reaction of H₂ and air, the reaction of H₂ and O₂ more easily reach detonation which react faster and order more heat in the same conditions. The shapes of TiO₂ nanoparticles which were synthesized by gas-phase detonation tend to sphere, and it could have good dispersibility. The results indicated gas-phase detonation synthesis method was a promising method for industrial production in the future.5. Based on the reaction thermodynamics theory, the nucleation and growth processes of TiO₂ nanoparticles were analyzed in the detonation. The influencing factors of chemical reaction, crystal nucleation, grain growth and intercrystalline adsorption-condensation were studied. Moreover, the kruis model was improved. And the growth model of crystal nuclear proliferation for gas deflagration and detonation synthesis was derived. The calculation results are close to the experimental results. The phase transition mechanism of grains synthesized by gas deflagration and detonation was analyzed and discussed. In the end, combining with experimental results and theoretical formulas, it is found that some TiO₂ particles are formed by pyrohydrolysis outside the combustion reaction.