Preparation Of Silica Aerogels And Relating Hybrid Aerogels System From Diatomite

Silica aerogels are novel materials with fascinating proerties, they have quite attractive application prospect in fields of thermal insulation, electrics, optics, adsorption, etc. For Decades, extensive research have been done on preparation technology and basic applications for silica aerogels and got lots of achievements. However, drawbacks as expensive raw materials, long preparation periods, high risk drying technology, brittleness, high thermal conductivity in high temperature highly restrict its development and practical application. Though many work have been made at home and abroad, we saw little progress. Diatomite was employed as a cheap silica source in this study, amorphous silica was extracted by using alkali dissolving method, Box-Behnken experiment design of response surface methodology was adopted to optimize sodium silicate preparation process. The optimum technical parameters are:alkali-silica ratio3:10, NaOH concentration10%, reaction temperature90℃, weighted average of waterglass modolus and silica extraction ratio achieved the maxium79.91%.Silica aerogels were synthesized further under ambient pressure drying, the aerogels obtained were characterized for phase structure, pore structure, morphology structure, surface chemical property and thermal stability, experimental results show that silica aerogels synthesized from diatomite have a similar appearance of that from expensive silica source. The pore structure of aerogels based on diatomite are typical mesoporous with a uniformly pore size distribution and the peak pore diameter resided at10nm. The hydrophobicity of aerogels could maintain up to400℃In order to improve mechanical and infrared optical properties of aerogels, multi-walled carbon nanotubes and carbon nanofibers doped silica aerogel composites were synthesized by incorporated them into silica sol. The hybrid aerogels were studied by XRD, BET, FESEM, TEM, FT-IR, DTA/TG, Nanoindentor, Compression test, Laser thermal conductance method. The results show that the incorporating of MWCNTs and CNFs have little influence on morphology of aerogels, the dosage of MWCNTs and CNFs have a significant effect on properties of aerogels, when doped with5vol.%MWCNTs, the aerogels achieved the lowest density and linear shrinkage, while for CNFs, the best content is0.5wt.%. Both of the MWCNTs and CNFs could increase the extinction coeffiency of aerogel composites, which inllustrate a better infrared shading performance. The mechanical test s show that the incorporating of CNFs improved the robustness of aerogels, with the content of CNFs increasing, the absolute stress value increases and elastic modulus decreases.Equilibrium molecular dynamics method was adopted to simulate the thermal conductivity of gas in silica aerogel nanopores. The results show a linear relationship between the thermal conductivity of nitrogen and pore diameter, and the thermal insulation properties improved with the pore size reduces. Heat-transfer mechanism of silica aerogels were proposed based on EMD simulation results, the exists of skeleton structure bounded the freedom movement of molecular to a large extent, which causes a drop of gasous heat transfer, and exhibit a better adiabatic property.