Performance Of Silica Aerogels Research Articles

Effects of Synthesis Conditions on the Performance of SiO2 Aerogel Prepared by CO2 Supercritical Drying

The article presents an investigation about effects of the synthesis conditions on the performance of silica aerogels. Tetraethyl orthosilicates (TEOS) were selected as the silicon source, and two acid-base catalysts were adopted to catalyze the reaction. CO2 supercritical drying was used to drive out the solvent of alcohols in the wet gels. Semi-transparent monolithic and crack-free SiO2 aerogels were successfully obtained. The solvents were prepared with a ratio of ethyl orthosilicate: absolute ethanol: H2O:HCl:NH3·H2O=1:3:5:3×10-3:4.8×10-3.The single effects of solution pH, water volume, and aging temperature synthesized were investigated. The aerogels were characterized with SEM, specific surface area analysis, thermal constant analysis, thermogravimetric analysis, Raman spectroscopy analysis, N2 adsorption-desorption analysis, and hydrophobic angle test. The as-synthesized silica aerogels have a porosity of 95.6%, a specific surface area of 756.9m2/g, an average pore diameter of 16.22nm, a density of 96.8kg/m2, and a thermal conductivity of 0.02W/(m· K). Hydrophobic silica aerogels were also obtained.

Microstructure and Adsorption Properties of MTMS / TEOS Co-precursor Silica Aerogels Dried at Ambient Pressure

The existing preparation processes of silica aerogels suffer several problems, such as high cost, long preparation period and unsatisfactory performance. Herein a novel methyltrimethoxysilane (MTMS)/tetraethoxysilane (TEOS) co-precursor silica aerogel was prepared by using the ambient pressure drying method with MTMS/TEOS as co-precursor and water as solvent. The effects of MTMS and TEOS molar ratios on the performance of silica aerogels were investigated by using SEM and TG-DSC etc. The results indicated that with the increase of TEOS content in co-precursor, the gelation time of silica aerogel increased from 11 min to 38 min, and the density increased from 0.08018 g·cm−3 to 0.3499 g·cm−3, and the porosity decreased from 96.36% to 84.10%. In addition, the particle and pore sizes of the silica aerogel decreased gradually, and the whole structure became denser. The maximum surface area and porosity were found to be 913 m2·g−1 and 96.36% at a molar ratio of MTMS to TEOS of 1:9. In this case, the adsorption rates of this silica aerogel towards organic matters were the fastest and the adsorption rates for epoxy resin, silicone oil, methanol, ethanol and hexane were 12.00 g/g, 10.86 g/g, 10.23g/g, 9.83 g/g and 6.48 g/g, respectively. Moreover, the thermal stability of silicon aerogel also increased from 280.34 °C to 331.31 °C with the increase of TEOS content.

Mechanical and Adiabatic Properties of Silica Aerogel Doped with TiO<sub>2</sub> Nanowire

A novel silica aerogel insulation composite was prepared by using TiO2 nanowires as modifier via sol-gel processing and supercritical drying methods in order to address issues of the poor mechanical and the infrared shading performance of silica aerogel. The effects of TiO2 nanowire content on the mechanical and adiabatic performance were investigated by X-Ray diffraction (XRD), nitrogen adsorption (BET), field emission scanning electron microscopy (FESEM), static compression test and thermal analysis. The results show that TiO2 nanowire reinforced aerogels had great thermal insulation properties, while high surface area, low density and mechanical strength were retained.

Effect of catalyst used in the sol–gel process on the microstructure and adsorption/desorption performance of silica aerogels

Silica aerogels prepared by the sol–gel process are often used as solid desiccants in enthalpy wheels for dehumidifying ventilation air in air-conditioning systems. These hygroscopic materials have good moisture adsorption and desorption characteristics due to their porous structure. The current study is focused on the evaluation of the mass diffusivity of silica aerogels, which determines the rate at which a dehumidification process can be performed. The mass diffusivity of silica aerogels is affected by their porous structure which depends on the synthesis technique used to prepare the silica aerogels. The sol–gel process is used to prepared silica aerogels using various basic (ammonium hydroxide, sodium hydroxide, potassium hydroxide) and acidic (hydrofluoric acid, steric acid, hydrogen peroxide) catalysts with the same precipitator (tetramethyl orthosilicate – TMOS) and solvent (methanol). Scanning electron microscopy is used to analyze the microstructure of supercritically dried aerogels. The dynamic vapor sorption method is used to determine the effective mass diffusivity for the different silica aerogels. It is found that the mass diffusivity is related to the microstructure of silica aerogels, which depends on the catalysts used in the sol–gel process. In addition, a parametric study is conducted to determine the effect of relative humidity and temperature on the adsorption and desorption mass diffusivity.