The effect of aging on acid-catalyzed aerogels (original) (raw)
Journal of Sol-Gel Science and Technology, 2009
Experimental results on the physico-chemical properties of ambiently dried sodium silicate based aerogels catalyzed with various acids are reported. The aerogels were prepared by hydrolysis and polycondensation of sodium silicate followed by subsequent washings, surface chemical modification and ambient pressure drying using 10 various acid catalysts consisting of strong and weak acids. The strength and concentration of acids have the major effect on the gelation of sol and hence the physico-chemical properties of the silica aerogels. Strong acids such as HCl, HNO 3 and H 2 SO 4 resulted in shrunk (70-95%) aerogels whereas weak acids such as citric and tartaric acids resulted in less shrunk (34-50%) aerogels. The physical properties of silica aerogels were studied by measuring bulk density, volume shrinkage (%), porosity (%), pore volume, thermal conductivity, contact angle with water, Transmission Electron Microscopy (TEM), Atomic Absorption Spectroscopy (AAS), Fourier Transform Infrared Spectroscopy (FTIR), Thermo Gravimetric-Differential Thermal (TG-DT) analyses and N 2 adsorption-desorption BET surface analyzer. The best quality silica aerogels in terms of low density (0.086 g/cm 3), low volume shrinkage (34%), high porosity (95%), low thermal conductivity (0.09 W/m K) and hydrophobic (148°) were obtained for molar ratio of Na 2 SiO 3 :H 2 O:citric acid:TMCS at 1:146.67:0.72:9.46 with 20 min gelation time. The resulting aerogels exhibited the thermal stability up to around 420°C.
Is Supercritical So Critical? The Choice of Temperature to Synthesize SiO2 Aerogels
Russian Journal of Inorganic Chemistry, 2020
The structure of SiO 2-based materials obtained by hydrolysis of tetraethoxysilane and subsequent drying of SiO 2 lyogels at temperatures from 85 to 265°C has been analyzed in detail. It has been found that drying conditions have no marked effect on the specific surface area of SiO 2 , which constitutes ~1000 m 2 /g in all cases. Average pore size and specific pore volume monotonically increase with drying temperature, the contribution of large mesopores (>20 nm) into total material porosity rises at high drying temperature. Drying temperature has a considerable effect on the degree and character of individual SiO 2 particles aggregation.
Nanoengineered Silica-Polymer Composite Aerogels with No Need for Supercritical Fluid Drying
Journal of Sol-Gel Science and Technology, 2005
Owing to their low density, dielectric constant, thermal conductivity, high porosity and chemical inertness, monolithic aerogels could be useful in a variety of electronic, optical and chemical applications [1]. However, practical implementation has been slow, because aerogels are fragile, environmentally sensitive (hydrophilic) and most importantly, the final stage of their preparation involves supercritical fluid (SCF) extraction [1c]. It is reported herewith that for a nominal 3-fold increase in density, typical polymer crosslinked silica aerogels are not only stronger (>300×) and less hydrophilic (<10×) than the underlying silica backbone, but they can also withstand the capillary forces exerted upon their nanostructured framework by the residing meniscus of selected solvents, and thus they can be dried under ambient pressure without need for supercritical fluid (SCF) extraction. The best solvent identified for that purpose is pentane, and the resulting aerogels are both microscopically and macroscopically identical to their SCF-CO 2 dried counterparts. Being able to dry monolithic crosslinked aerogels without SCF extraction is expected to facilitate their commercial application.
Materials Chemistry and Physics, 2011
Silica aerogels were synthesized through an ambient pressure drying technique, and the porosities of the aerogels were tailored over a wide range, by varying the experimental parameters such as hydrolysis temperature, gelation pH, aging pH and aging solvent. Pore size, pore volume and surface area could tailor between 6.2-18 nm, 0.99-2.04 cc g −1 and 452-635 m 2 g −1 respectively by changing the hydrolysis-condensation conditions. By varying the aging pH from 3 to 11, pore size and pore volume was tailored in between 5.8-13.4 nm and 0.88-1.45 cc g −1 respectively. The investigation will be highly beneficial for the synthesis of tailor made silica aerogels at ambient conditions.
Korean Journal of Chemical Engineering, 2010
The experimental results of aging time and temperature on the textural properties of water-glass (sodium silicate)-based silica aerogels are reported and discussed. Aging of the hydrogel for different times and temperatures led to an ability to increase the stiffness and strength of the networks. These improvements enabled the gel to withstand ambient pressure drying (APD) and, consequently, preserve the highly porous silica network without collapse. The pore size and volume increased with increasing aging temperature and time, while the specific surface area decreased. Monolithic aerogels with extremely low bulk density (~0.069 g/cm 3 ), high specific surface area (820 m 2 g −1 ), large cumulative pore volume (3.8 cm 3 g −1 ), and high porosity (~96%) were obtained by aging at 60 o C for 18 hours. Therefore, easy synthesis of monolithic silica aerogels at ambient pressure is achievable using a relatively inexpensive silica precursor (sodium silicate).
In this work, silica aerogels crack free monoliths of proper properties were simply and successfully prepared through homemade autoclave. We have systematically studied the relationship between the densification temperature of the synthesis environment of silica aerogels on their resulting morphological, optical and electrical properties. SEM and BET measurements were employed as structural probes to ascertain the structural differences. There is a systematic correlation between the annealing temperature and aerogel surface area, porosity, as well as pore size. The implemented autoclave was able to produce aerogel monolith of surface area reach to 998.25 g/m2 and low electric conductivity arrive to 1.17*10P-4 P (s/m), associated with density of 0.047 g/cm3. The microstructure observed is categorized into three types, namely, open cellular foam (the substance that is formed by trapping pockets of gas in solid), fractal (showing a hierarchical repetition of structural features) and isotropic morphology (at the scale of the visible spectrum). the aerogel properties were remarkably varied. While the influence of annealing temperature the reaction setting has gradually influence on the final aerogel properties, however, it is obviously requested for achieving desirable optically and nano-featured products.
Journal of Alloys and Compounds, 2009
The experimental results on physical properties of water glass (sodium silicate) based silica aerogels prepared by single step sol-gel process, dried at atmospheric pressure are reported. The hydrolysis and condensation reactions of the sodium silicate precursor proceeded with tartaric acid as a catalyst. The hydrogel was vapour passed in order to remove sodium salt from the gel network. Solvent exchange was carried out using methanol and hexane as a solvents. Finally, surface chemical modification of the gel was done using trimethylchlorosilane (TMCS) followed by ambient pressure drying of the gel up to the temperature 200 • C. To get good quality aerogels various sol-gel parameters such as water vapour passing period varied from 0.5 to 2 h, gel aging from 1 to 4 h, Na 2 SiO 3 /H 2 O molar ratio from 3 × 10 −3 to 1.5 × 10 −2 , tartaric acid/Na 2 SiO 3 molar ratio from 0.3 to 1.9 and TMCS/Na 2 SiO 3 molar ratio from 4.8 to 12. The aerogels were characterized by percentage of volume shrinkage, bulk density, porosity and hydrophobicity. The hydrophobicity of the aerogel was confirmed by Fourier Transform Infrared (FTIR) Spectroscopy and contact angle measurements. Microstructural studies have been carried out by Scanning Electron Microscopy (SEM) and nitrogen adsorption BET analysis. From the TGA-DTA studies of the aerogels, it was found that the aerogels were thermally stable up to 470 • C. Low density (∼0.066 g/cm 3 ), high hydrophobicity (∼145 • ), high porosity (∼97 %), high pore volume, surface area of 510 m 2 /g aerogels have been obtained for Na 2 SiO 3 :H 2 O:tartaric acid (C 4 H 6 O 6 ):TMCS molar ratio at 1:166.6:2.5:12 respectively with half an hour water vapour passing.