Preparation And Properties Of Silica Aerogels And Their Composites

Silica aerogels are light-weight and porous nanostructured materials with unique framework and insulation properties, which are potential candidates for various applications such as lightweight thermal insulation, adsorption and catalyst. However the high production cost and poor mechanical properties limit the application of silica aerogels. To achieve larger scale commercial application of silica aerogels, low-cost simple preparation method and develop new products should be studied.In this paper, silica aerogels were made from water glass or tetraethoxysilane using one-step and two-step sol-gel process. The effects sol-gel process parameters and modification process parameters on the properties of resulting aerogels have been investigated. Using of low thermal conductivity and high temperature resistance of silica aerogels, silica aerogels papers and silica aerogels/polyimide composite films were developed. The influences of different amounts of silica aerogels on the structures and properties of composite materials have been studied. In this paper, the main conclusions include:In the preparation of silica aerogels by the acid and alkali two step catalytic process, the effects of the pH value, the ammonia concentration, the type and ratio of drying control chemical additive(DCCA), the volume ratio of trimethylchlorosilane and hexane, the modified method were explored. The results indicate that when pH value is 5, the ammonia concentration at 1mol℃L-1, the molar ratio of formamide and silicon at 0.25, the volume ratio of trimethylchlorosilane and hexane at 1:10, one-step solvent exchange-surface modification optimum silica aerogels can be obtained. The result shows that: silica aerogels was a kind of amorphous material with density 0.078g℃cm-3, porosity 96.5%, specific surface area 814m2℃g-1, pore size 9.27nm, pore volume 3.07cm3℃g-1. The optimum method of aerogels prepared with one-step process by water glass is keeping pH at 5, the volume ratio of water glass and water at 1:4, using one-step solvent exchange-surface modification. The result shows that density is 0.069g·cm-3, porosity is 96.9%, specific surface area is 524m2·g-1 pore size is 17.62nm, pore volume is 3.21cm3·g-1.When V(NH3·H2O)/V(TEOS) is 0.25, V(H2C2O4)/V(TEOS) is 1.5, V(CH3CH2OH)/V(TEOS) is 2, hydrolysis time is 24h, silica aerogels prepared by tetraethoxysilane had the best performance. The result shows that silica aerogels was a kind of amorphous material with density 0.065g·cm-3, porosity 97.0%, specific surface area 835m2·g-1, pore size 10.23nm, pore volume 3.36cm3·g-1. After compared the properties of two different silicon sources and balanced the costs and properties, two-step process with water glass as the silicon source can accelerate the industrial application of aerogels.With alumina silicate fiber slurry as matrix, high flexibility and high temperature resistant silica aerogels papers were prepared by adding silica aerogels powder in papermaking process. The thermal conductivity of the composites decreases gradually with the increase of the aerogels. When the additive amount of aerogels is 15% the thermal conductivity is 0.02W·m-1vK-1, the mass losing of composites at the 1000℃ is 12.5%.High temperature resistant composite polyimide films were prepared by adding different content of silica aerogels powder. At 540℃ the mass losing of the polyimide film with 15% aerogels is 5%. Compared with pure polyimide, the temperature increased by 137 degrees. At 550℃ the mass losing of the polyimide film with 15% aerogels is 10%. Compared with pure polyimide, the temperature increased by 70 degrees. The thermal conductivity of pure polyimide is 0.43Wm·1·K-1, the thermal conductivity of the polyimide film with 15% aerogels is 0.25W·m-1·K-1, 0.18W·m-1·K-1 lower than that of pure polyimide. The two kinds of composites have good application prospect in the field of high temperature resistant and thermal insulation.