The effect of aging on acid-catalyzed aerogels (original) (raw)
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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.
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.
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.
Strengthening of silica gels and aerogels by washing and aging processes
Journal of Non-Crystalline Solids, 2001
Gels were prepared from a polyethoxydisiloxane precursor by using HF as a catalyst. During washing in water solution a signi®cant increase in the permeability of the gels was observed, showing that dissolution-reprecipitation occurs. After washing, the gels were further soaked in a solution of polyethoxydisiloxane precursor to strengthen and stien the gel. As expected, a signi®cant enhancement of the mechanical properties of the wet gels was observed. It is also interesting to note, however, that the permeability does not decrease below the value for the as-prepared gels. Hence, a promising process has been developed where both the stiness and the strength have been increased as well as the permeability. The increase in permeability is of importance to facilitate the supercritical drying process. Reasonably successful scaling up of the supercritical drying of these gels to laboratory scale has been achieved, and monolithic and transparent gels are obtained. Optical properties have been measured on laboratory scale aerogels. The corresponding results have been correlated with structural characteristics measured by small-angle X-ray scattering (SAXS). Ó
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.
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.