Preparation And Characterization Of Al₂O₃ And Al₂O₃-SiO₂ Aerogel And The Composites

Aerogel possesses many fantastic properties, such as low density, low thermal conductivity, high specific surface area, a high porosity and so on, and great potential due to its unique three-dimensional network and nano-porous structure. It is an ideal heat insulating material with great research value. On account of these properties, aerogel has been applied in various fields, including military industry, energy, chemical industry. In this dissertation, the preparation technology, structures and properties of Al₂O₃ and Al₂O₃-SiO₂ aerogel and the composites were investigated.Al₂O₃ and Al₂O₃-SiO₂ aerogel were prepared through sol-gel process and ethanol supercritical drying technology using aluminum chloride hexahydrate?AlCl3·6H2O? and tetraethoxysilane?TEOS? as precursors. Under the condition of the propylene oxide?PO? used as gel initiators, during the process, Al₂O₃ and Al₂O₃-SiO₂ aerogel with low bulk density and large specific surface area were achieved in our experiments. The thermal insulation composites which possess low thermal conductivity, low coefficient of thermal expansion and excellent mechanical properties were synthesized through vacuum impregnation technology, integral forming process and ethanol supercritical drying technology, using fibrous perform as supports. Structures and properties of Al₂O₃ and Al₂O₃-SiO₂ aerogel were investigated by means of BET, XRD, SEM, FTIR, TG-DTA. The heat-insulating property and mechanical properties of the thermal insulation composites were investigated through thermal conductivity tester, dilatometer and universal testing machine.The influences of content of propylene oxide, ethyl alcohol and deionized water on gel time and pore structure of Al₂O₃ aerogel were studied. The optimal molar ratio of Al₂O₃ aerogel was established, n?AlCl3·6H2O?: n?PO?: n?EtOH?: n?H2O? =1:10:12:35.The structure and properties of Al₂O₃ aerogel were studied. At room temperature, the specific surface area of Al₂O₃ aerogel was 465 m²·g^-1 and the average pore size was 19.24 nm. The three-dimensional network skeleton was composed of needle or rod particles. Al₂O₃ aerogel went through the transformation process of ?-AlOOH ? ?-Al₂O₃ ? ?-Al₂O₃ ? ?-Al₂O₃ when heat treatment temperature increased from room temperature to 1200?. In this process, the specific surface area of Al₂O₃ aerogel firstly increased and then decreased. At 1200?, the specific surface area was 56 m²·g^-1, the average pore size was 66.57 nm.The influences of Al/Si molar ratio on Al₂O₃-SiO₂ aerogel were investigated. As the content of Si increased, the specific surface area increased, and the gel gradually extended. With comparing the properities of samples with different Al/Si molar ratio, it was indicated the sample with Al/Si=3/1 possessed better heat-insulating property.The structure and properties of Al₂O₃-SiO₂ aerogel were studied. At room temperature, the specific surface area of Al₂O₃-SiO₂ aerogel was 574 m²·g^-1, and the average pore size was 26.34 nm. The three-dimensional network skeleton was composed of leaf shape particles. Al₂O₃-SiO₂ aerogel went through the transformation process of ?-AlOOH + amorphous SiO₂ ??-Al₂O₃+amorphous SiO₂ ? Mullite?3Al₂O₃·2SiO₂? when heat treatment temperature increased from room temperature to 1200?. At 1200?, the specific surface area was 162 m²·g^-1, the average pore size was 56.29 nm. It was indicated that Al₂O₃-SiO₂ aerogel possessed better high temperature stability and heat insulation performance.The preparation technology of fiber-aerogel composites was studied. In this dissertation, fibrous performs are fiber mat and thermal insulation rigid tiles. The Al₂O₃-SiO₂ aerogel-fiber mat composities possessed good flexibility, the thermal conductivity is 0.0167W·m-1·K-1 at room temperature, and 0.109W·m-1·K-1 at 1200?. The Al₂O₃-SiO₂ aerogel-thermal insulation tiles composities possessed better mechanism properities, compressive strength of 3%, 5%, 10% strain, tensile strength and bending strength was 1.50 MPa, 1.57 MPa, 1.63 MPa, 0.61 MPa, 6.4MPa respectively. It was indicated that Al₂O₃-SiO₂ aerogel-thermal insulation tiles composities possessed excellent processability.