Morphology And Structure Of Al₂O₃ Nanomaterials Synthesized Via Flame Aerosol Methods

The potential of nanoscale particles in high-performance applications has been identified, scientific and commercial interest has increased immensely. However, nanomaterials can perform their multifunctional tasks only if they are customized in terms of chemical composition, size, and morphology to suit the application at hand. The flame aerosol synthesis is a well-known chemical manufacturing technique for an extensive variety of nanoscale particles. Since the synthesis process have many advantages, such as high efficiency of reaction fast, simple post treatment, easy and suitable for the industrial manufacture. The study of the nucleation, growth, phase transformation, structural control, formation mechanism for the preparation of nanomaterials by the flame aerosol synthesis has important significance in theory and practice. Al2O3 nano-particles and hollow nanospheres were successfully prepared in the multi-jet flame reactor and flame spray pyrolysis reactor. The morphology, structure, crytallinty and pore size distribution were systematically characterized by Transmission Electron Microscopy, Scanning Electron Microscopy, X-ray diffraction, N2 dsorption and Laser Diffraction Particle Size Analyzer. In addition, the formation mechanism of the complex nannostrcutures via the flame aerosol process is discussed. In this thesis, the main research work carried out includes:1. Nano-Al2O3 was prepared via H2/Air flame synthesis in the multi-jet reactor with anhydrous AlCl3 as precursor. The results showed that the particles size raised from 7.6 nm to 34.4 nm with the flame temperature increased. At the same time, a simple y-phase gradually transformed toδ,δ*-phase, as the reaction temperature and the residence time in the flame increased. Nano-AlO3 particles have strong hydrophilic, and the dispersion has strong stability after stewing 8 days. The film-gas on the top of Multi-jet flame reactor affected the nano-AlO3 particles size and distribution by changing the flow field and cooling rate of high temperature flow. The morphology and particle size of the prepared nanoparticles lay on the competition among the surface growth rate (Rg), the sintering and agglomeration rate (Rs) at different flame temperature (T) and residence time (t). Higher reaction temperature and shorter residence time is conducive to the formation of a chain-like structure of small particles, the increase of particle size and sintered aggregates would be obtained for higher reaction temperature and longer residence time.2. Al2O3 nano-microspheres were prepared by flame spray pyrolysis with Al(NO3)3·9H2O in ethenol as precursor solution. The results showed that Air as a dispersion/oxidation gas, Al2O3 hollow spheres were obtained; using O2 as the dispersion/oxidation gas, Al2O3 solid particles were obtained. With the increasing of the gas flow rate, the size of prepared Al2O3 nano-microspheres decreased, SSA and total pores volume increased both for air and O2. With Air flow rates increased, Al2O3 hollow spheres changed into Al2O3 solid particles. The morphology and structure of the prepared Al2O3 lay on the combined effect of the initial droplet size (Dd), flame temperature (T) and residence time (t). The droplets were formed by well mixed precursor solution atomization. The air to liquid volume ration is the key factor to determine the spray droplet size, with the ratio increase, the spray droplet diameter decreased; increasing the dispersion gas flow rates, the spray droplet diameters decreased with constant liquid flow rate. Using O2 as dispersion/oxidation gas, the flame was more intense, brighter and higher temperature, the flame length was shorter than that of the same air flow rate; with increasing of air or O2 flow rates, the flame lengths became shorter. At the same gas flow rate, O2 flame temperature is higher than the air. At the same gas flow rate, surface precipitation and nucleation occurred on larger droplets, leading to the formation of hollow structure at lower air flame temperature; O2 flame temperature higher than Air flame by 1000 K, accelerating the combustion, then nucleate on the surface catalyze by precipitation throughout the droplet, solid Al2O3 particles formed. Higher air or O2 flow rate, the smaller initial droplets lower the concentration gradient within the droplet, the occurrence of volume precipitation leading to solid particles formed.3. Al2O3 hollow nanospheres with well-defined structure and disorder pores were successfully prepared by flame spray pyrolysis via the non-ionic surfactant-assisted pyrolysis. The hollow nanospheres was composed of 60-80 nm in size and a shell thickness about 10-20 nm. The same condition, Al2O3 prepared from Al(NO3)3·9H2O and AlCl3 as aluminum source had difference structures. The former were a mixture of Al2O3 hollow vesicles and small particles; the later were a mixture of small particles and large particles. The different conversion process of the precursors is the reason. AlCl3 as a volatile aluminum source undergoes the vapor-to-particle conversion process, Al(NO3)3·9H2O as a non-volatile one undergoes the droplet-to-particle conversion process. The same condition, Al2O3 prepared by different surfactants was also difference structures. Al2O3 prepared by adding cationic surfactant cetyltrimethylammonium bromide (CTAB) was hollow nanospheres with thick shell and irregular sintered shape. CTAB added into an ethanol solution with Al(NO3)3·9H2O makes the surface tension and viscosity increased, resulting in the increased diameter of spray droplets. Since CTAB has a good solubility in ethanol and NO3- weakens the repulsion among CTAB ions, CTAB diffuses into the droplet center continuously while ethanol evaporation, which transfers heat to the center from the surface, the thick hollow shell formed with the nucleation extension. So the experiment choosed Al(NO3)3·9H2O as precursor and PEG-2000 as surfactant. The effect of the addition of non-ionic surfactant PEG-2000 significantly affected the characteristics of the spray droplet by reducing the liquid-phase equilibrium surface tension and the kinematic viscosity, which made the smaller droplets formed. The cross-linking system between Al3+ and PEG-2000 molecular is beneficial to the stability of spray, keeping the spherical droplets in high temperature fluid. As the result, the Al2O3 hollow nanospheres prepared with the addition of PEG-2000 have a better spherical shape, with disorder pores and larger SSA than that of without the addition of surfactants.