Synthesis Of Zirconia Aerogels By Ambient Pressure Drying

Zirconia aerogels have potential applications in cleaning of air and water, and thermal insulation of buildings, owing to their low thermal conductivity, high specific surface area and high porosity. Supercritical drying was conventionally used to prepare zirconia aerogels, which are unsafe, with high cost and not conducive to large-scale production. Therefore, the ambient pressure drying with a low cost and simple process has become a new research field.In this paper, zirconia aerogels were prepared by sol-gel method and ambient pressure drying. The type of additives, the molar ratios of zirconium to additives and air flow rate in drying on the properties of zirconia aerogels were investigated. The gel properties were characterized in situ by small angel X-ray scattering(SAXS) in the formation and drying of the gel, and density, TEM, FT-IR, BET and TG-DSC off line. The main conclusions were as follows:(1) Results showed that the factors that affected the preparation conditions of zirconia aerogels were in a descending order of polyacrylic acid type >zirconium/polyacrylic acid molar ratio > zirconium/formamide molar ratio > zirconium/ethyl acetoacetate molar ratio >zirconium/propylene oxide molar ratio > zirconium salt concentration >zirconium/water molar ratio, as revealed by the orthogonal experiments. The optimum polyacrylic acid type,zirconium/polyacrylic acid molar ratio and zirconium/formamide molar ratio were polyacrylic acid PAA45W, 24: 1 and 1:1, respectively, under the same molar ratio of zirconium to ethyl acetoacetate of 1: 1, the molar ratio of zirconium to propylene oxide of 1: 9, the zirconium salt concentration of 7wt%, and the molar ratio of zirconium to water of 1: 5.(2) The effect of air flow rate in drying on the properties of zirconia aerogels was studied. The air flow rate was set at 40, 25, 10 and 5 mL / min. The results showed that with the decrease of the air flow rate, the mass and density of the gel decreased, the particle sizes of the zirconia aerogels decreased and the weight loss rate of the sample decreased, the specific surface area of the aerogel sample after calcination at 900 ℃ increased, and the pore volume and average pore size decreased. When the gas flow rate was 5 mL/min, the density of zirconia aerogel was 0.1830 g / cm3, the particle sizes of the zirconia aerogel were mainly distributed between 3-10nm, the weight loss rate was 33.8% under heating in air, the specific surface area was 84.6 m2/g, and the average pore size was 2.7 nm.(3) The properties of the gel in formation and drying were characterized by SAXS in situ. The results showed that the size of the colloidal particles and the fractal dimension increased gradually with time in the sol-gel process, indicating that the colloidal particles grew in the sol-gel process, and the growth of colloidal particles was in a random mode. During the gel drying process, the pore size of the gel decreased and the partical fraction increased gradually with the drying time. The mass and pore fractal were present in the system. The mass fractal was present in the early stage of drying and the fractal dimension decreased gradually.And pore fiactal was present in the later stage and the fiactal dimension increased gradually.