The New Quality Characterization Technique Of Aerogels And The Study Of SiO₂-P₂O₅ Aerogels

Silica aerogels are porous materials with three-dimensional network structure which are formed by the accumulation of nanoparticles.The porosity can be up to 99%,and the pore size range from 1nm to 100nm.Silica aerogels have excellent properties such as high specific surface area,high optical transmittance and low thermal conductivity.Therefore,silica aerogels have wide applications in energy-saving materials.Butthe cost of SiO₂ aerogel is high which lead to limits its practical application,due to complex and low-yield drying process.In this thesis,the relationship between relative transmittance at 500nm of silica wet-gel in different TEOS:H₂O:EtOH molar ratios were studied by Fourier infrared spectrum?FTIR?,UV-Visible spectrophotometer?UV-Vis?,Nitrogen adsorption and desorption isotherms,Transmission Electron Microscopy?TEM?and Solid-state ²⁹Si nuclear magnetic resonance?NMR?.The degree of silicate hydrolysis and polymerization can be characterized by the intensity and shift of Si-O-Cpeak,Si-O-H peak and Si-O-Si peak in FTIR spectra.The relative transmittance of wet-gel was the highest when the molar ratio of TEOS:H₂O:EtOH was 1:3:6.It is shown relative transmittances of the silica wet-gels at 500nm had good linear correlation with the specific surface areas of corresponding aerogels.In addition,the Qⁿ[?SiO?_n-Si-?OH?_4-n]environment of silica aerogel are characterized by NMR.We find the value of Q²[?SiO?₂-Si-?OH?₂]/Q⁴[?SiO?₄-Si]can be used as the criterion for the degree of polymerization.It is noted that relative optical transmittance of the silica wet-gels at 500nm had good linear correlation with the degree of silicate polymerization in the corresponding aerogel.This research presents a simple,cheap and safe technique to identify properties of silica wet-gels.It will benefit to remove defective silica wet-gelsbefore the supercritical drying,improve the yield and reduce the costs of supercritical drying.Phosphorus was doped into silicon aerogels ofthe optimum TEOS:H₂O:EtOH molar ratioto improve the infrared radiation propety of SiO₂ aerogels.The effects of different P:Si molar ratio on the structure of SiO₂-P₂O₅ aerogels were studied,and the structure and morphology were analyzed by BET,SEM and NMR.The result shows thatSiO₂-P₂O₅aerogels are nanoporous materials which can control nanoporous structure by altering experimental conditions.The average diameter of silica particles is about 10nm.The pore diameter is about10nm.The specific surface area is up to 805.4m²/g.The radiant emissivity of SiO₂-P₂O₅aerogel is up to 0.95.The effects of different content of mullite fiberon the structure of SiO₂-P₂O₅ aerogels were studied.It has been found that there was no crack in the aerogel composite material with 10%fiber.Its maximum compressive strength can be up to 1.9MPa,and the thermal conductivity is 0.09W/?m·K?.