Effective diffusion coefficient in the low temperature process of silica aerogel production (original) (raw)
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Diffusion of ethanolcarbon dioxide in silica gel
Journal of non-crystalline …, 1998
Extraction of a primary solvent with liquid or supercritical carbon dioxide is the most difficult to control stage in low-temperature silica aerogel production. Diffusion of primary ethanol through alcogel structure to surrounding CO was 2 investigated in carefully controlled experiments with cylindrical alcogel samples. Changes of the alcohol concentration in carbon dioxide leaving the autoclave were followed with on-line chromatograph analysis and experiments were repeated for temperatures around the critical point of carbon dioxide. On the basis of the concentration histories during the drying, the temperature dependence of diffusion coefficient in the ethanol-CO mixture inside the silica gel at constant pressure was 2 identified.
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.
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.
Pore network connectivity and effective diffusivity of silica aerogels
Chemical Engineering Science, 1995
The analysis method recently proposed by Seaton and coworkers was applied to determine the pore network connectivity of hyperporous solids-silica aerogels. The mean coordination number of the pore network of silica aerogels was found to lie between three and eight, depending on the pore volume of mesopores and micropores, and independent of bulk density of the aerogels. Calculations of the effective diffusivities of aerogel samples clearly show that silica aerogels exhibit remarkable mass transport properties, significantly higher than conventional silica or alumina porous supports.
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.
The Journal of Supercritical Fluids, 2010
The surfaces of monolithic silica aerogels were rendered hydrophobic using hexamethyldisilazane (HMDS) as surface modification agent and scCO 2 as solvent. The treatment led to hydrophobic silica aerogels which are as transparent as untreated aerogels. The effects of HMDS concentration in the fluid phase and the reaction time were investigated and the contact angles were found to be 130 • at different conditions. FTIR spectra indicated a reduction in hydrophilic surface silanol groups and the emergence of hydrophobic CH 3 groups. The bubble point pressures of the HMDS-CO 2 system were obtained at temperatures 298.2 K, 313.2 K, 327.7 K and 342 K at various concentrations. At a fixed temperature, the bubble point pressure decreased as the concentration of HMDS increased. At a fixed composition, bubble point pressure increased as the temperature increased. The bubble point pressures were modeled using the Peng-Robinson Stryjek-Vera Equation of State (PRSVEOS) and compared well with the experimental data.
Gels, 2021
This work aims to contribute to the theoretical and experimental research of supercritical processes for intensification and combination in one apparatus. Investigation is carried out to improve production technology of organic alginate aerogels. It is proposed within the investigation to carry out the solvent exchange stage, an important stage of organic aerogels production, under pressure in a carbon dioxide medium in the same apparatus used for supercritical drying. The phase behavior in the system “carbon dioxide–water–2-propanol”, which arises during such a solvent exchange stage, is studied theoretically. An experimental study of the process of step-by-step solvent exchange under pressure was carried out through multiphase and homogeneous regions of the phase diagram of such a system. As a result, new highly efficient technology for the production of organic aerogels was proposed, which can be implemented by combining the two main stages of the process.
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.
Impact of depressurizing rate on the porosity of aerogels
Microporous and Mesoporous Materials, 2012
The effects of supercritical drying of diluted resorcinol-formaldehyde (RF) gels in CO 2 and in acetone are compared. We show that, for both processes, depressurizing rate of the autoclave after drying has a significant influence on the resultant shrinkage, and hence on resultant bulk density, surface area and pore volumes. At depressurizing rates below 2 MPa/min, acetone leads to much lower shrinkage than does CO 2 , and is 1000 times cheaper. However, supercritical drying with CO 2 remains interesting because it is faster, easier to control, less sensitive to the experimental conditions and cleaner. We indeed show that acetone is degraded and leads to various compounds that might partly remain in the porosity of the dried RF gels. Supercritical CO 2 also clearly leads to the highest surface areas and micropore volumes, whereas supercritical acetone is in favour of higher mesopore volumes, especially at high depressurizing rates.