A new method of preparation of aerogel-like materials using a freeze-drying process (original) (raw)
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A new route to aerogels: Monolithic silica cryogels
Journal of Non-Crystalline Solids, 2012
We have explored several different gel syntheses and drying procedures for producing silica cryogels with similar properties to those of silica aerogels, particularly in terms of monolithicity, density, porosity and surface area. These materials could be a suitable alternative to silica aerogels and ambigels. Some successful preparation methods are presented and properties of the corresponding cryogels are discussed, including comparison of these materials to supercritically dried products and an assessment of the effects of the experimental variables in the preparation process on the properties of the resultant cryogels. Two routes for the preparation of cryogels are highlighted, one of which is especially attractive as it has the advantage (compared to the known syntheses of APD aerogels) of not requiring any solvent exchange step.
SORPTION PROPERTIES OF AEROGEL IN LIQUID NITROGEN
Aerogel products are now available as insulation materials of the future. The Cryogenics Test Laboratory at the NASA Kennedy Space Center is developing aerogel-based thermal insulation systems for space launch applications. Aerogel beads (Cabot Nanogel ®) and aerogel blankets (Aspen Aerogels Spaceloft ®) have outstanding ambient pressure thermal performance that makes them useful for applications where sealing is not possible. Aerogel beads are open-celled silicone dioxide and have tiny pores that run throughout the body of the bead. It has also recently been discovered that aerogel beads can be used as a filtering device for aqueous compounds at room temperature. With their hydrophobic covering, the beads absorb any non-polar substance and they can be chemically altered to absorb hot gases. The combination of the absorption and cryogenic insulating properties of aerogel beads have never been studied together. For future cryogenic insulation applications, it is crucial to know how the beads react while immersed in cryogenic liquids, most notably liquid nitrogen. Aerogel beads in loose-fill situation and aerogel blankets with composite fiber structure have been tested for absorption properties. Depending on the type of aerogel used and the preparation, preliminary results show the material can absorb up to seven times its own weight of liquid nitrogen, corresponding to a volumetric ratio of 0.70 (unit volume nitrogen per unit volume aerogel). These tests allow for an estimate on how much insulation is needed in certain situations. The theory behind the different processes of sorption is necessary for a better understanding of the preparation of the beads before they are used in an insulation system.
An Analysis on Synthesizing Techniques of Aerogel and its Applications
2020
1,2Dept. of Mechanical Engineering, A.P. Shah Institute of technology, Maharashtra, India ----------------------------------------------------------------------***--------------------------------------------------------------------AbstractIn this paper the current techniques for synthesizing aerogel and its applications are reviewed in which the fillers are supercritical CO2. In this paper we have tried to provide the synthesizing techniques with a computational overview. Aerogel which is also called as frozen smoke was first created by Samuel Stephan kister in 1931. It is a material which is produced by extracting liquid components of a gel through the process of supercritical drying whose process and significance is mentioned in this paper. The application of aerogel along with its properties are mentioned below in a tabular form systematically arranged. This paper also reveals the use of aerogel as a sensors. Aerogel can be positively used as a sensors due to its high surface act...
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.
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.
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.
Journal of Non-Crystalline Solids, 2021
Alumina aerogels in the form of monoliths were obtained within a work. Gels were synthesized using a sol-gel process. An inorganic salt, aluminum chloride hexahydrate, was used as a precursor. Hydrolysis and condensation reactions were carried out in a medium of water and ethanol, and epichlorohydrin was the initiator of the gelation process. In order to identify an appropriate compositions of components gels preparation the phase equilibrium of the «epichlorohydrin-ethanol-water» system was experimentally studied. The effect of the precursor-water-ethanol ratio on the aerogels' final characteristics was investigated. To determine the characteristics of alumina aerogels method of nitrogen adsorption, helium pycnometry, transmission and scanning transmission electron microscopy with x-ray microanalysis were used. The resulting materials have high specific surface area from 464 to 859 m 2 /g and low density from 65.6 to 121.1 kg/m 3. The structure of alumina aerogels consists of flake-like particles which form the interconnected network. Such materials are promising for use as heat-insulating materials and catalysts.
The influence of various pH values on the aerogel physical properties by sol-gel technique
Iraqi Journal of Physics, 2019
Hydrophobic silica aerogels were successfully preparation by an ambient pressure drying method from sodium silicate (Na2SiO3) with different pH values (5, 6, 7, 8, 9 and 10). In this study, acidic HCl (1M), a basic NH4OH (1M) were selected as a catalyst to perform the surface modification in a TMCS (trimethylchlorosilane) solution. The surface chemical modification of the aerogels was assured by the Fourier transform infrared (FTIR) spectroscopic studies. Other physical properties, such as pore volume and pore size and specific surface area were determined by Brunauer-Emmett- Teller (BET) method. The effect of pH values on the bulk density of aerogel. The sol–gel parameter pH value in the sol, have marked effects on the physical properties of the silica aerogels produced by this technique. The density and the hydrophobic tendency of the aerogels were significantly influenced.
Morphology of Silica Aerogels Obtained from the Process Catalyzed by NH 4 F and NH 4 OH
Langmuir, 1997
Twelve samples of silica aerogels were synthesized using different concentrations of catalysts (NH4F, NH4OH), and their morphology was investigated by means of small-angle X-ray scattering (SAXS), N2 adsorption, and transmission electron microscopy. Both SAXS and the nitrogen adsorption method consistently show a different influence of the catalysts; fluoride anions promote formation of a tenuous branched-polymeric structure, with mass fractal dimension Dm = 2.3-2.5, while addition of hydroxyl ions results in a sparser structure (Dm = 2.1-2.2) self-similar over a narrower range. The morphology of aerogels obtained from the process catalyzed by fluoride hydroxide resembles that of aerogels produced by the two-step method. This may suggest similar mechanisms of gel formation in both processes, and, consequently, a dramatic acceleration of the hydrolysis reaction caused by NH 4F.