Nano-Porous Silica, Alumina Aerogels And Thermal Super-Insulation Composites

In terms of the thermal protection system of advanced missiles and aerospace vehicles as well as the insulation part of thermal battery, advanced insulation materials with high temperature stability, low density, high performance thermal insulation properties and good mechanical properties are urgently required. In this dissertation, the preparation process, structures and properties of nano-porous SiO2, Al2O3 aerogel and their thermal insulation composites were investigated. The applications of SiO2 aerogel composites such as molding capability, machinability and thermal insulation property were also researched. The results are shown as follows.Nano-porous SiO2 aerogels were prepared by sol-gel method and supercritical drying process. The effects of processing parameters such as precursor, catalyst, water and solvent on structures and properties of SiO2 aerogels were researched. It is found that the porous structure and properties of SiO2 aerogels could be controlled by two step acid (HCl)-base (NH3·H2O) catalyzed using tetraethoxysilane (TEOS) as the precursor. The molar ratio of TEOS:HCl:NH3·H2O:H2O:EtOH is optimized to be 1:1.8×10-3:3.6×10-3:4:2-20. The SiO2 aerogels are composed of Si-O-Si and the porosity is above 90%. The particle size, pore size, and specific surface area of the SiO2 aerogels are 5.4 nm,35 nm and 527.8m2/g, respectively. Due to the ultra fine skeleton particles and nano-porous structure, SiO2 aerogels have low solid thermal conductivity and low gas thermal conductivity. The preparation of SiO2 aerogels set up the foundation of preparing new SiO2 aerogel insulation composites.SiO2 aerogel insulation composites were prepared by mixing inorganic ceramic fibers with SiO2 sol. The influences of fiber species, fiber volume fraction and EtOH/TEOS molar ratio on mechanical and thermal properties of SiO2 aerogel composites were investigated. The results show that aerogel composites with low thermal conductivity can be obtained by using fibers with small diameter, few amounts of impurity and high infrared radiation absorption and scattering. It has been observed that with the increase of fiber volume fraction from 4% to 8%, the tensile and bending strength increases first and then decreases, and the composites with the highest strength is fabricated with 7% fiber volume fraction. Moreover, the thermal conductivity at high temperature decreases gradually with the increase of fiber volume fraction. With the increase of EtOH/TEOS molar ratio from 2 to 12, the mechanical properties increase while the thermal conductivities remain almost constant.The optimal conditions for producing SiO2 aerogel insulation composites are confirmed as follows, using mullite fibers as reinforcement, with 6.0%-7.0% fiber volume fraction and the molar ratio of EtOH/TEOS ranging from 4 to 12. The SiO2 aerogel composites show high temperature stability (<800℃), low density (-0.35g/cm3), high thermal insulating performance and good mechanical behavior. The thermal conductivities at 200℃and 800℃are 0.017W/m-K and 0.042W/m-K, respectively, which are decreased by 50% and 55% compared with mullite fiber insulations. The bending strengths at 200℃and 800℃are 1.31MPa and 1.80MPa, respectively. The void in the fiber matrix are filled with the high porosity, nano-porous SiO2 aerogels, essentially no fiber-fiber contacts remain, thus forming a well interface adhesion between the aerogels and inorganic ceramic fibers. As a result, SiO2 aerogel composites show well mechanical behavior and excellent heat insulation properties.Nano-porous Al2O3 aerogels were prepared using aluminum alkoxides as precursors. The effects of sol-gel parameters such as precursors, molar ratios of water, solvent and chelating reagent on the structure and properties of Al2O3 aerogels were studied. It is found that a clear Al2O3 sol is obtained using tri-sec-butoxide (ASB) as precursor when heated up to 60℃. With the increase of ethanol, the gelation time of the sols prolongs while the density and strength of the Al2O3 aerogels decrease. The Al2O3 sol usually precipitates with high molar ratio of H2O/ASB. The addition of chelating reagent ethyl acetoacetate (Etac), which controls the hydrolysis-condensation of ASB, improves the stability of Al2O3 sol. The molar ratio of ASB:EtOH:H2O:Etac is optimized to be 1:8-16:1.2:0-0.1. Al2O3 aerogels are composed of poly-crystallized boehmite, and the porosity and average pore size are 97.9% and 26 nm, respectively. The specific surface area at ambient temperature and 1200℃are 440m2/g and 73m2/g, respectively. It is found that Al2O3 aerogels are consisted of randomly connected leaf-like particles that could control the surface/bulk diffusion, thus the sintering process is retarded and the high temperature stability is improved.Al2O3 aerogel insulation composites with high temperature stability (<1200℃) were prepared by mixing mullite fibers with Al2O3 sol. Due to Al2O3 aerogels still keeping the nano porous textures after high temperature treatment, Al2O3 aerogel composites show excellent thermal insulating performance. The thermal conductivity of Al2O3 aerogel composites at 1000℃is 0.064W/m-K, which decreases by about 47.5% compared with mullite fiber insulation, thus Al2O3 aerogel composites exhibit well thermal stability and insulating performance at high temperature.The applications of the SiO2 aerogel composites were researched. The composites show well molding capability and machinability. It can be machined to complex insulation component by cutting, drilling and grinding. It sets up the foundation for the application of aerogel composites in the advanced missiles and aerospace vehicles as well as the insulation part of thermal battery. The SiO2 aerogel insulation parts of xx missile rudder cabin and thermal battery were successfully tested in practical circumstances. The results show that aerogel insulation parts achive the demand of the missile and the thermal battery. Compared with the conventional insulation, the life time of battery is prolonged about 40% by using SiO2 aerogel insulation.