Strengthening of silica gels and aerogels by washing and aging processes (original) (raw)
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Strengthening and aging of wet silica gels for up-scaling of aerogel preparation
Journal of Sol-Gel Science and Technology, 2007
In order to enhance the mechanical properties of wet gels for aerogel production, aging studies by using three different routes was performed. The wet gels were prepared from a polyethoxydisiloxane precursor by using HF as a catalyst. The three different aging routes studied were i) aging in sealed mould, ii) aging in solvent and iii) aging in simulated pore liquid, i.e. a solvent with small amounts of water and HF resembling the mother liquor. All aging processes gave stronger and stiffer wet gels however, a maximum in strength and stiffness was observed after a certain aging time. The simulated pore liquids allowed short aging time in the range of 8 h to achieve the maximum mechanical strength, however the maximum in strength was lower than for the other two aging routes. From the wet gels, monolithic and transparent aerogels were obtained by supercritical drying at small-, mid-and large-scale. The aging strengthening process was successfully transferred to larger scales giv-ing both lower density and higher transparency compared to small-scale.
Journal of Porous Materials, 2008
Research on the preparation and characterization of silica aerogels has focused mainly on transparency and monolithicity. In this paper, we address the effect of supercritical drying conditions in ethanol on the shrinkage and porous texture of aerogels. The variables studied included the initial amount of ethanol added to the reactor, initial pressure of N2, heating rate and stabilization time above supercritical conditions. The starting material was an alcogel obtained by the sol–gel process in acidic media. All aerogels were amorphous. In general, skeletal density increased when the initial amount of ethanol added into the body of the autoclave was decreased and the volume fraction of porosity was above 91%. According to infrared spectra, skeletal SiO2 network was independent of supercritical drying conditions. N2 adsorption isotherms identify the macroporous character of aerogels, which was confirmed by SEM and TEM. Specific surface area significantly increased when the initial volume of ethanol added to the reactor was increased and the stabilization time above supercritical conditions decreased, whereas surface area decreased when autoclave pre-pressurization was increased.
Ultralow density silica aerogels by alcohol supercritical drying
Journal of Non-Crystalline Solids, 1998
Synthesis of ultralow density aerogels requires full transformation of diluted monomers into solid. Shrinkage during ageing and supercritical drying must be controlled as low as possible. Reported investigations deal with achievement in the optimisation of the two-step process parameters performed using classical acid and basic steps. Acid step involves the formation of partially hydrolysed and partially condensed silica mixture in ethanol. Different compositions of tetraethoxysi-Ž . lane, ethanol and hydrochloric acid TEOS:ETOH:HCl have been prepared to provide suitable gelling time and to enhance mechanical properties of final gels. Supercritical drying was performed using modified solutions mixtures which prevent silica dissolution. Under these conditions gel texture is preserved as demonstrated by the low shrinkage. This route permits positioning of uncracked transparent aerogels with densities lower than 3 kgrm 3 . Specific surface area values are in the range of usual aerogels. Therefore, because of their unusual large pores, specific surface area decreases with density. q 1998 Elsevier Science B.V. All rights reserved.
Modified aging process for silica aerogel
Journal of Materials Processing Technology, 2009
Supercritical drying a b s t r a c t To increase the porosity and monolithic performance of silica aerogel, two methods were used to age the silica gels derived from tetraethoxysilane (TEOS) by two-step sol-gel process: aging in 100 • C-autoclave with TEOS/ethanol mixed solution and in pure ethanol at room temperature. The structural characteristics and physical properties of the resultant two kinds of aerogels after supercritical CO 2 drying were investigated and compared with the help of infrared spectra (IR), N 2 absorption, differential scanning calorimetry (DSC)/thermogravimetry (TG) and high-resolution 29 Si nuclear magnetic resonance (NMR) techniques. Aging in 100 • C-autoclave yields silica aerogel with high pore size and pore volume, twice of that aged in ethanol at room temperature. High aging temperature and pressure can promote the dissolution and reprecipitation process of silica and the esterification process of silanols, which will enhance the backbone strength of silica gel, and hence produce silica aerogel with low bulk density, good monolithic performance and hydrophobic features. While the latter aging method produces silica aerogel with high bulk density and cracking appearance.
Science and Technology of Advanced Materials, 2008
An in-depth investigation into the synthesis of hydrophobic silica aerogels prepared by the surface derivatization of wet gels followed by subsequent drying at ambient pressure is reported. The following sol-gel parameters were examined for their effect on the physical properties of the derived aerogels: number of gel washings with water, percentage of hexane or methanol in silylating mixture, molar ratio of tartaric acid: Na 2 SiO 3 , gel aging period, weight% of silica, trimethylchlorosilane (TMCS) percentage, and silylation period. These parameters were varied from 1 to 4, 0 to 100%, 0.27 to 1.2, 0 to 4 h, 1.5 to 8 wt.%, 20 to 40% and 6 to 24 h, respectively. The properties of hydrophobic silica aerogels synthesized by this new route were investigated in terms of bulk density, percentage volume shrinkage, percentage porosity, thermal conductivity and contact angle with water, and by Fourier transform infrared spectroscopy (FTIR). The as-prepared hydrophobic silica aerogels exhibited high temperature stability (up to approximately 435 • C) as measured by thermogravimetric/differential thermal analysis (TGA-DTA). The optimal sol-gel parameters were found to be a molar ratio of Na 2 SiO 3 : H 2 O : tartaric acid : TMCS of 1 : 146.67 : 0.86 : 9.46, an aging period of 3 h, four washings with water in 24 h and the use of a 50% hexane-or methanol-based silylating mixture. Aerogels prepared with these optimal parameters were found to exhibit 50% optical transparency in the visible range, 84 kg m −3 density, 0.090 W mK −1 thermal conductivity, 95% porosity and a contact angle of 146 • with water.
The effect of process variables on the properties of nanoporous silica aerogels
Silica aerogel, a nanoporous material, was produced by using rice husk ash via sol–gel method. The aim of the study is to examine effects of the acid type (acetic, hydrochloric, nitric, oxalic and sulfuric acid), dryer type (air, freeze, oven and vacuum) and the addition of tetraethyl orthosilicate on the structural and physical properties of aerogels produced from rice husk ash. In addition, this is the first study investigating the effect of vacuum oven drying on the structure of rice husk based silica aerogel. Specific surface area and pore size of obtained silica aerogels have been analyzed by the N2 adsorption and desorption measurements at 77 K via Brunauer–Emmett–Teller (BET) and Barrett–Joiner–Halenda (BJH) methods, respectively. Surface functional groups were determined with fourier transform infrared spectroscopy (FTIR). Surface morphology was examined with scanning electron microscopy (SEM). Moreover, density was calculated by tapping method. The results showed that all of the variables had remarkable effects on the final properties of the silica aerogel. The BET specific surface area of the silica aerogels increased with the addition of tetraethyl orthosilicate, while the tapping density decreased. The BET specific surface area and pore size of silica aerogels varied between 140.7–322.5 m2 g−1, and 5.38–12.05 nm, respectively. Silica aerogel which was obtained by using oxalic acid, tetraethyl orthosilicate addition and air dryer had the highest BET specific surface area (322.5 m2 g−1).
Effects of aging and drying conditions on the structural and textural properties of silica gels
… and mesoporous materials, 2007
Sol-gel derived materials are widely used as porous matrices for preparing optical chemical sensors or biosensors. The porosity of the support matrix is a key variable that affects the sensitivity and response time of the sensors. The aim of this research is to study the effect of different aging and drying conditions on the structure and porosity of silica gels to obtain materials of tailored porosity. The gels were aged in ethanol or NH 3(aq) (0.5 M and 2.0 M), and dried under atmospheric (xerogels) or supercritical conditions for ethanol (aerogels). Aging and drying conditions strongly affected the porous texture of silica gels. The surface area and micropore volume was higher in xerogels than in aerogels. Xerogel aged in ethanol was mainly microporous, while xerogels aged in ammonia were mesoporous due to cross-linking reactions in alkaline media. The maximum from the BJH distribution appeared at 4.0 nm for the xerogel aged in 0.5 M NH 3(aq) and at 5.4 nm for the xerogel aged in 2 M NH 3(aq) . Aerogels were macroporous materials, and macropore volume constituted over 92% of the total pore volume, independently of the aging media. Esterification reactions inside the reactor promoted crosslinking, which resulted in a higher skeletal density and an increase in the absorbance of Si-O related IR bands. Cross-linking strengthens the gel network and reduces the amount of shrinkage under atmospheric conditions. SEM and TEM micrographs confirmed the textural properties of xero-and aerogels deduced by gas adsorption, Hg porosimetry and density measurements.
Structural investigation in monolithic silica aerogels and thermal properties
Journal of Non-Crystalline Solids, 1998
Ž . The influence, of the number of water molecules n used to synthesize polyethoxydisiloxane PEDS-Px silica Ž . precursors, on the internal structure of aerogels made with those precursors in ethylacetoacetate etac under HF conditions and dried under CO supercritical conditions was studied. Gas adsorption, water thermoporometry and mercury porosimetry 2 were used to characterize specific area and pore size distribution. The results were used to interpret the evolution of the apparent thermal conductivity of those aerogels with n ) . Optical transmission was also measured to estimate optical quality of the aerogels prepared. q
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).