Preparation Of Al-Doped Silica Sols,Aerogels And Their Composite Materials

Silica aerogels possess many excellent properties,such as high specific surface area,high porosity,high visible light transmittance,low refractive index,low dielectric constant,low sound velocity and low thermal conductivity.They have been applied in many fields including heat insulation,adsorption and separation,sound insulation and sound absorption,catalyst carrier,drug carrier,energy storage and detection,especially in the field of thermal insulation.However,silica aerogels exhibit poor thermal stability at high temperature above 600?due to the collapse of the structure above 600?,which will significantly affect insulation performance at high temperature.In general,the doping can improve thermal stability of silica aerogels at high temperature,and decrease the thermal conductivity at high temperature,which becomes one of the most important development directions of silica aerogels.Based on the previous research of silica sols,silica aerogels and their composites,this works focuses on the preparation of aluminium?Al?doped silica aerogels and their composites.With methyl orthosilicate?TMOS?and tetraethyl orthosilicate?TEOS?as precursors,two Al-doped silica sols were synthesized by acid catalyzed sol-gel method,and the colloidal properties of Al-doped silica sols were analyzed.The corresponding Al-doped silica aerogels and their glass fiber composite materials were prepared by supercritical drying technology,and Al-Ti-doped silica aerogels and their composites were also achieved by adding titania sols into the silica sols.The pore structure characteristics and thermal stability of aerogels and their composites were studied,which builds an important foundation for the application of silica aerogels in high temperature insulation.The main research results are as follows:?1?Synthesis of Al doped silica sol by acid catalyzed sol gel method.With TMOS and TEOS as precursors,aluminum chloride(AlCl_3·6H₂O)as aluminum source,hydrochloric acid as catalyst and water/ethanol?EtOH?as solvents,two Al-doped silica sols were prepared by acid catalyzed sol-gel method respectively.The Al-doped silica sol in TMOS system with a solid content of 12%,the average size of sol particles of about 2 nm,a viscosity of 3.60 MPa·s,a pH value of 1.83 and zeta potential of-0.182MV,can be obtained,and the Al-doped silica sol is stable at room temperature for about12 days.The Al-doped silica sol in TEOS system with a solid content of 6%,an average size of sol particles of about 2 nm can be achieved,and the resultant silica sol is stable at room temperature for about 20 days.?2?Preparation of Al-doped and Al-Ti-doped silica aerogels by supercritical drying technology.Al-doped silica aerogels were prepared by acid catalyzed sol-gel method in TEOS system combined with supercritical drying technology,and Al-Ti-doped silica aerogels were further prepared by adding titania sol into Al-doped silica sol as a light screening agent.The high temperature stability of two silica aerogels before and after doping was analyzed.The structure of the silica aerogels without doping collapses at 800?,and the specific surface area and pore volume decrease obviously.The Al-doped silica aerogel has excellent stability at 800 ^oC because of its thinner skeleton structure.The specific surface area and the pore volume of Al-doped silica aerogel are 538 m²·g^-1 and 1.62 cc·g^-1.The Al-Ti-doped silica aerogel has better stability over 200?.The specific surface area and the pore volume of Al-doped silica aerogel are 455 m²·g^-1 and 1.30 cc·g^-1.?3?Preparation of doped silica aerogel/fiber composites by supercritical drying.With glass fiber as reinforcing materials,Al-doped and Al-Ti-doped silica aerogel/fiber composite materials were prepared by physical impregnation combined with supercritical drying,and the high temperature insulation properties of the resultant silica aerogel/fiber composite materials were analyzed.The silica aerogels uniformly distribute in the glass fiber,and the aerogels cover the fiber surface and fill the gap between fibers.The thermal conductivity of Al-doped silica aerogel/fiber composites at room temperature and 600?are 0.031 and 0.107 W·m^-1·K^-1,respectively.The thermal conductivity of Al-Ti-doped silica aerogel/fiber composites at room temperature and 600?is 0.029 and 0.110 W·m^-1·K^-1,respectively.