| Aerogels that have properties of low thermal conductivity (as low as 0.02 W·m-1·K-1), low density (as low as 0.003 g/cm3), high specific surface area (400～1500 m2/g) and three-dimensional network structure have been widely used as thermal insulations in aerospace, defense and chemical industry. However, because of the poor mechanical properties and temperature resistance of SiO2 and Al2O3 aerogels, the application of aerogels are greatly restricted. In this work, Al2O3-SiO2 aerogels, a new kind of ultrahigh temperature insulation, were preared by sol-gel polymerization of tetraethoxysilane (TEOS) and crystal aluminum chloride (AlCl3·6H2O) followed by supercritical drying. The effects of process conditions on preparation and the properties of aerogels were systematically investigated. In addition, Al2O3-SiO2-TiO2 aerogels and Al2O3-SiO2-ZrO2 aerogels were also synthesized from tetraethoxysilane (TEOS), crystal aluminum chloride (AICl3·6H2O), titanic chloride (TiCl4), zirconium oxychloride (ZrOCl2·8H2O) in order to further improve the insulation ability. Furthermore, CNTs and mullite fibre felts reinforced Al2O3-SiO2 aerogels were prepared to improve the mechanical strength of aerogels. The major conclusions were as follows:(1) Effects of process conditions including the ratio of Al and Si and the content of propylene oxide (PO), EtOH and HCl on the properties of Al2O3-SiO2 aerogels were systematically investigated. The conditions for the preparation of Al2O3-SiO2 aerogels were optimized as:n(Al):n(Si)=3:1, n(EtOH):n(H2O):n(TEOS)=16:16:1 and n(PO):n(TEOS)= 11:1. The obtained Al2O3-SiO2 aerogels have a low density of 0.105 g/cm3, a short gelation time of 32 min and a slight volume shrinkage of 8.1% during supercritical drying. Moreover, they have low thermal conductivity 0.015 W·m-1·K-1 at room temperature. In addition, the specific surface area of Al2O3-SiO2-TiO2 aerogels and Al2O3-SiO2-ZrO2 aerogels was higher than that of Al2O3-SiO2 aerogel after these aerogels were calcined in the air at 800℃. It’s also suggested that the addition of TiO2 and ZrO2 could hinders the sintering of SiO2.(2) CNTs reinforced Al2O3-SiO2 aerogels were prepared by using CNTs dispersions to avoid the aggregation problem of CNTs. Results indicated that the addition of CNTs could significantly decrease the gelation time, volume shrinkage and the density of resulant Al2O3-SiO2 aerogels. It is noted that, at a low CNTs content of 1.5%, the compressive strength of aerogels were remarkably improved from 0.76 MPa to 8.59 MPa, being 10.5 times higher than that of CNTs-free aerogels.(3) Large-size aerogel composites, including Al2O3-SiO2, Al2O3-SiO2-TiO2 and Al2O3-SiO2-ZrO2 were prepared by using mullite fiber flets as structural reinforced framework. These aerogel composites have good mechanical strength, low density, low thermal expansion rate, and excellent thermal insulation properties. Espically for Al2O3-SiO2 aerogel, it has only a 0.5% thermal expansion ratio and low thermal conductivity of 0.098 W·m-1·K-1 at 1050℃. With the temperature increasing from 300℃ to 1050℃, the thermal conductivities of ASMF increased from 0.032 W·m-1·K11 to 0.098 W·m-1·K-1, the thermal conductivities of ASTMF increased from 0.027 W·m-1·K-1 to 0.085 W·m-1·K-1 and the thermal conductivities of ASZMF increased from 0.024 W·m-1·K-1 to 0.076 W·m-1·K-1. However, the increasing rate of of thermal conductivity of ASZMF was lowest. The compressive strength and the flexural strength of ASTMF was 0.98 MPa and 114.9 MPa. The compressive strength and the flexural strength of ASZMF was 0.98 MPa and 114.9 MPa, which was largest. |
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