| For the past few years, polycrystalline silicon industry in China has developed rapidly due to the increasing demand for solar photovoltaic cell. Incomplete statistics show that the country has a total of dozens of production enterprise and the capacity over 100000 t/a up to now. The current predominant productive technique employed by most of these polycrystalline silicon enterprises is so-called"modified Siemens process", and a mass of by-products, or silicon tetrachloride, give off during the production. Silicon tetrachloride possessing low boiling point and strong causticity easily causes environmental pollution. Thus, the scientific, reasonable and efficient utilization of the resulting silicon tetrachloride is becoming a problem to be urgently solved. The preparation of silica using silicon tetrachloride as feedstock is one of its effective transformation way. Owing to its non-toxic, tasteless and reinforcing properties, silica has been applied to such fields as chemical engineering, material and electrommunication, and so on. Especially, the silica with hydrophobicity and special morphology has found more and more potential applications in many new fields shch as drug controllable release, catalyst supproter, superpolymer reinforcing, sensor and advanced separation processes, etc. because of its particular surface structures and shapes.This paper mainly involves the following several aspects: firstly, in the mixed solvents of aliphatic alcohol and water, the preparation of SiO2 micro/nano particles using silicon tetrachloride as reaction material has been detailedly investigated, and the optimized preparation conditions have been obtained from the following parameters: pH value of solution, reaction temperature, stirring speed, the molar ratio of alcohol to water, dropping speed of silicon tetrachloride and its concentration in the solution; secondly, the silica with different morphologies such as bar-like, needle-like, short stick, hollow tubule were prepared by adding surface active agent and templates; finally, hydrophobic silica was prepared by way of surface modification of ihexamethyldisizane. The as-prepared products were characterized by x-ray diffraction (XRD), transmission electron microscope (TEM), scaned electron microscope (SEM) and Fourier transform infrared spectroscopy (FT-IR), and the BET nitrogen adsorption technique was employed for determination of specific surface area of the powdery silica. The formation conditions and mechanism of the nano-silica with multifold shapes are discussed in detail.The results show that: (1) when the pH value of the reaction system was controlled in the range of 3 to 4, temperature 10 to 30℃, stirring speed 50 to 200 r.min-1, silicon tetrachloride dropping speed 0.5 to 2.0 g.min-1, silicon tetrachloride concentration in mixed solvent 0.2 to 2.0 mol.l-1 and the molar ratio of alcohol to water 1:3 to 5:3, the spherical and amorphous nanoscale silica powder with the particle size of 100 to 300nm can be obtained. (2) When the molar ratio of aliphatic alcohol to water is 4:3, the needle-like nano-silica can be prepared via 60℃water-bath heating. (3) The CTAB concentration has a direct effect on the morphology of the products. Bar-like and hollow tubular nano-silica was respectively achieved when the CTAB concentration was kept at 1.0×10-2 mol.l-1 and 4.0×10-2 mol.l-1; the mole ratio of alcohol to water and reaction time also influence the appearances of the product. (4) The hollow tubular micron-sized silica with widening characteristic diffraction peak, micron particle size of 3 5μm and tubule length of 50 100μm could be achieved using the ammonium tartrate as template when the dropping speed of silicon tetrachloride and quiescence time were respectively kept at 0.1g.min-1 and 2h. (5) The nano-silica with good hydrophobic performance could be achieved when the HMDS concentration was controlled at 6.7×10-5mol.l-1, reaction temperature at 60℃and reaction time for 4h. |
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