Synthesis And Fundamental Applications Of The Silica-based Hybrid Aerogels

Aerogel is a unique porous material with low density and high absorption property, which makes this material a thermal insulator, catalysis support, drug delivery and dielectric materials etc., however, in order to satisfy some special applications, combining SiO2 aerogel with other materials is a trend. First part of this paper focuses on the synthesis and relating properties of two pure aerogels, by using Ti(OC4H9)4 as the precursor, the TiO2 cryogels was prepared and dried at freeze-drying condition, then, SiO2 aerogels were synthesized by using water-glass and investigating the effect of four different drying methods on the morphology and properties of the SiO2 porous materials. Based on the preparation of these 2 gels, combining the TiO2 and SiO2 gels to synthesize TiO2/SiO2 binary aeorgels, and analysize its photocatalytic degradation and thermal insulation applications. The surface group, thermal stability, morphology and microstructure of silica aerogels cryogels and hybrid aerogels were investigated by FT-IR, DTA-TG, XRD, SEM, TEM and BET adsorption method. UV/VIS spectrometry was used to test the photocatalytic degradation activity of the hybrid aerogels and the zeta potential of the sol was tested by mixed with different fibers, respectively. Finally, the thermal comductivity of TiO2/SiO2/fiber composite was tested by transient hot-wire method.First, the TiO2 aerogel was prepared by using Ti(OC4H9)4 as precursor and sol-gel method, the water in the wet gel was exchanged with t-butanol to protect the whole gel structure. The properties and applications of the TiO2 cryogels were analysized. The results show that the apparent density decrease and the porosity increase with the decrease of the freeze temperature. When the temperature was -10℃, the morphology of the cryogel is powder, with the temperature increased to -15℃, the porosity of the gel can reach 80.4%, some bulk sample was observed. When the sintering temperature is 200℃, the XRD result show an amorphous structure, an anatase structure appeared when the temperature reach 550℃, and a rutile structure can be observed at 800℃.The effects of drying techniques on the microstructure, morphology and pore structure of porous silica gels were studied in the Chapter 3. The gels were prepared by using sol-gel process and different drying routes:freeze-drying (FD), low pressure drying (LPD), high temperature drying (HTD) and chemical modification & ambient drying (CMD) techniques. Observation under pore distribution and structural properties showed that CMD technique led to homogenous mesoporous silica material with specific surface area of 745 m2g-1, and the average pore size around 20nm, while LPD and HTD resulted in loosely packed particles with non-isotropic aggregation pattern. The specific surface areas of LPD and HTD samples are 419 and 513 m2 g-1 respectively, and the pore size distribution of the samples are observed distributing widely in range of 10-100nm. Freeze drying method is a new but prospective way to prepare mesoporous silica. The specific area of FD sample is around 500 m2g-1.Mesoporous binary TiO2/SiO2 aerogel, as a novel photocatalyst, has been synthesized at an ambient pressure with two kinds of silica precursors of tetraethoxysilane(TEOS) and water-glass and a cheap titania precursor-titanium tetrachloride (TiCl4). Using an about-to-gel SiO2 sol as nanoglue, TiO2 was immobilized into a three-dimensional silica networks, this hybrid aerogel has an optimized nanostructure and superior photocatalytic degradation performance for organic waste, especially by using the water-glass-based aerogel. The porosity of the gel can reach low to 80%, and the apparent density is around 0.5 g/cm3. However, the improval properties can be observed comparing to pure TiO2 gels. The XRD results show the main crystalline phase of the binary gels are still amorphous at 550℃, but some anatase peaks can be seen during the main curves.800℃show obvious and clear anatase peaks, but the whole structure of the gel collapse at the high temperature, so 550℃has been chosen as the parameter for the continuing experiments. The decolorizing efficiency of Methyl Orange (MO) has been reached to above 84.9% after exposed to UV light for 210 min. The recycle experiments show the degradation effect can reach 70% at 210min after 4th use.At last, the aerogel composite was synthesized by dope 4 different fibers:mullite fiber, ceramic fiber, glass fiber and PET fiber. The dispersion of the fibers was investigate by using zeta-potential test. And the microstructure, monolithic and thermal conductivity of the composite were analyzed. The results show that the zeta-potential declined with the fiber doping, the tread shows the glass fiber and mullite fiber doped gels have the best stability. The linear shrinkage of the composite decreased from 15-30% to 3-4% after doping fibers, so the monolitic structre can be got by this method. The thermal conductivity of the samples was measured by transient hot-wire test show the compoite has higher value compare to pure SiO2 aerogel samples. By comparing study the properties of the gels, our paper want to induce a further interest in finding a proper porous silica gels for low price use in environmental remediation and thermal insulation.