The Pore Structure Control And Properties Analysis Of RF/SiO₂ And C/SiO₂ Aerogels

Aerogels possess on the characteristics of high surface areas, low densities and low thermal conductivity due to their unique three-dimensional network structures,and can be widely applied in the fields of thermal insulation, catalysis, hydrogen storage and optics, et al. In order to control nano-scale structures and tailor properties,it is an effective method to realize organic-inorganic multicomponent hybrid aerogels,which is a hotspot and emphasis in scientific research fields. In this paper,resorcinol-formaldehyde（RF）/SiO₂ and C/SiO₂ aerogels were prepared by changing the order of sol, the ratio of silicon sources and drying methods. Their microstructures and properties were analyzed by several analytical methods. The active mechanism in sol-gel process and influence factors under different synthesized methods was discussed.In the course of preparing RF/SiO₂ aerogels, 3-aminopropyltriethoxysilane（APTES） and tetraethyl orthosilicate（TEOS） were used as co-precursors of silicon,and resorcinol（R） and formaldehyde（F） were chosen as carbon sources. Three methods in the sol-gel process, i.e. sols of carbon source first（RF-S）, sols of silicon source fist（S-RF） and one-step mixed sols（SRF）, and three drying methods, i.e.supercritical drying, freeze drying and ambient drying, were used to synthesized RF/SiO₂ aerogels, respectively. Then, C/SiO₂ aerogels were obtained by high temperature carbonization.The results show that RF/SiO₂ and C/SiO₂ aerogels both are non-crystalline structures under different preparing methods. The skeletons of gels are formed by linking silicon and oxygen elements provided by APTES and TEOS. The skeleton is constituted by the state of three and four bonding for APTES acting as precursor and three bonding for TEOS, respectively. The time of gelation reduces with the addition of APTES. With the increase of APTES, the time of gelation reduces further. The strength increases and the diameters of particles decrease with the increase of amount of APTES. The specific surface area（SSA） and pore volume increase and average porous diameter decreases after drying.There is no influence for the skeleton linkage in gel system under different sol orders. For RF-S gelation, the degree of crosslinking in silicon sources is low and the skeleton is mainly constituted by RF system, the aerogels of RF-S method are provided with higher thermal stability. After freeze drying, the microstructure of RFS is compact. The microstructures of S-RF and SRF are similar, which possesses on typical chain networks. The pore diameter is uniform for the samples prepared by RF-S, but those are nonuniform for the ones obtained by SR-F and SRF.After ambient drying, the linear shrinkage and density are high for RF/SiO₂ aerogels whose nano-particle is uniform and structure is compact. However, after supercritical drying or freeze drying, RF/SiO₂ aerogels render uniform three dimension networks and their linear shrinkage is small and densities only are one tenth of the one after ambient drying. The SSA of aerogels after ambient drying is lower than the one of aerogels after supercritical drying or freeze drying. The porous structures are mesopores.C/SiO₂ aerogels were obtained from RF/SiO₂ aerogels after carbothermal reduction.Carbonation causes the collapse of the skeletons and leads to decrease of SSA and porous size. The-CH3 and-CH₂- groups decompose and the Si-O-Si groups are not changed. The thermal stable temperature of C/SiO₂ aerogels is about 460℃ and the thermal stability is not influenced by the orders of the sols. For S-C2 and SC2,however, due to their high pore volumes, the mass losses are high when they are heated between 400℃ and 600℃.