Effects of TEOs aerogel particles size of TEOS aerogel on its mesoporous structure and thermal behavior via supercritical drying and high temperature (original) (raw)
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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.
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
Materials Chemistry and Physics, 1999
The experimental results on the physical properties such as monolithicity, optical transmission, thermal conductivity and porosity of silica aerogels processed by three different precursors: (a) tetraethoxysilane (TEOS) (b) polyethoxydisiloxane (PEDS) (c) tetramethoxysilane (TMOS) are reported. The aerogels have been prepared by sol±gel polymerization of the parent solvent diluted precursor in the presence of a catalyst, followed by supercritical CO 2 solvent exchange and drying. It has been found that the monolithicity of the aerogels is strongly dependent on the type of catalyst used for each precursor. For TEOS and PEDS precursors, acid catalysts and for TMOS precursor base catalysts resulted in monolithic aerogels. It has been found that the optical transmission at 900 nm for 1 cm thick sample of the TMOS and PEDS precursor aerogels are the highest (>92%) and far lower ($70%) for the TEOS precursor aerogels. The thermal conductivities of the PEDS and TMOS aerogels have been found to be lower (0.015 and 0.020 W m À1 K À1 , respectively) compared to the TEOS (0.060 W m À1 K À1 ) aerogels. The pore sizes obtained from the N 2 adsorption measurements varied from 30 to 180 nm, 60 to 190 nm and 80 to 200 nm in the TEOS, TMOS and PEDS precursor aerogels, respectively. The scanning electron microscopy studies of the aerogels indicated that the PEDS and TMOS aerogels show narrow and uniform pores while particles of the SiO 2 network are very small. On the other hand, TEOS aerogels show non-uniform pores such that the number of smaller size pores are less compared to the pores of larger size while SiO 2 particles of the network are larger compared to both the PEDS and TMOS aerogels. Hence, the surface area of the aerogels prepared using TEOS precursor has been found to be the lowest ($800 m 2 g À1 ) compared to the PEDS ($1100 m 2 g À1 ) and TMOS ($1000 m 2 g À1 ) aerogels. # 1999 Elsevier Science S.A. All rights reserved.
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).
A COMPREHENSIVE REVIEW ON SILICA AEROGEL
Silica Aerogel is a nano-structured material that is also known as “frozen smoke” or “blue smoke,” which is a lighter, low density, low dielectric constant, highly porous, and super-dried material, and it has many substantial properties and most exceptional property is its high thermal insulation. However, this research focuses on Silica Aerogel (SA) synthesis, characterization, properties, and its comparison with experimental results. Several precursors can be used for producing aerogels and its precursor is responsible for the properties of aerogel. Different silicon alkoxide precursors are either tetramethoxysilane (TMOS) or tetraethoxysilane (TEOS). TEOS is often preferred because it is cheap and less hazardous. The process for aerogel synthesis is known as “The Sol-Gel” method. After making the solution, it is converted into a waxy gel and after the gelation, the gel is transformed to a solid network by a drying process. Several drying procedures are used to transform the gel into a solid network like the widely used supercritical drying (SCD), or ambient pressure drying (APD). Modern research on silica aerogel is still in progress to improve its thermal, mechanical, and optical efficiency by introducing hybrid aerogels. For the determination of various properties of silica aerogel several characterization techniques are used such as for its external and internal structure, pore size, and morphology SEM, TEM, and AFM are used. Other techniques like XRD, Raman spectroscopy, FTIR, NMR, and thermal gravimetric analysis, etc., are also used to measure different properties. This research theoretically includes the predicted properties of silica aerogels and its comparison with experimental results. Mostly all the properties are discussed in chapter IV and the last section is all about the applications of silica aerogels in various fields.
Silica Aerogel: Synthesis and Applications
Journal of Nanomaterials, 2010
Silica aerogels have drawn a lot of interest both in science and technology because of their low bulk density (up to 95% of their volume is air), hydrophobicity, low thermal conductivity, high surface area, and optical transparency. Aerogels are synthesized from molecular precursors by sol-gel processing. Special drying techniques must be applied to replace the pore liquid with air while maintaining the solid network. Supercritical drying is most common; however, recently developed methods allow removal of the liquid at atmospheric pressure after chemical modification of the inner surface of the gels, leaving only a porous silica network filled with air. Therefore, by considering the surprising properties of aerogels, the present review addresses synthesis of silica aerogels by the sol-gel method, as well as drying techniques and applications in current industrial development and scientific research. tion and its development. In the 1930s, Samuel Stephens Kistler first produced silica aerogels by formulating the idea of replacing the liquid phase by a gas with only a slight shrinkage of the gel. He prepared aerogels from many other materials, including alumina, tungsten oxide, ferric oxide, tin oxide, nickel tartarate, cellulose, cellulose nitrate, gelatin, agar, egg albumen, and rubber, which are out of scope of the discussion. Kistler's method involves tedious and timeconsuming procedures, and as such there was no follow-up interest in the field of aerogels until 1968 when rediscovery of aerogels took place by a team of researchers headed by Professor S. J. Teichner at the University Claude, Bernard, Lyon, France. They substantially simplified the procedure by carrying out the sol-gel transition in a solvent, which was then removed at supercritical conditions. The first Cerenkov radiation detector based on silica aerogels was developed in 1974 by Cantin et al. Since then, aerogels have been used or considered for use in laser experiments, sensors, thermal insulation, waste management, for molten metals, for optics and light guides, electronic devices, capacitors, imaging devices, catalysts, pesticides, and cosmic dust collection. More recently, several groups around the 2 Journal of Nanomaterials world began working in the field of silica aerogels for the various applications mentioned above. Strictly speaking, to understand silica aerogels, it is necessary to first understand sol-gel chemistry and related physicochemical aspects. In the following, we shall discuss sol-gel chemistry, synthetic strategy of silica aerogels, and some recent developments in applications of aerogels.
Effect of mixed Catalysts system on TEOS-based silica aerogels dried at ambient pressure
Applied Surface Science, 2008
In the present paper, the experimental results on the effect of mixed Catalysts system on the physical properties of the TEOS-based silica aerogels, are reported and discussed. The aerogels were produced by the single-step as well as two-step sol-gel process followed by atmospheric pressure drying. In the single-step process, only the NH 4F was used as a catalyst, whereas in the two-step process, NH 4F as well as a mixed catalysts, i.e., NH 4F and NH 4OH, were used after 12 h of acid (oxalic acid) addition. Effect of various exchanging solvents, viz., xylene, toluene, heptane or hexane and silylating agents, viz., MTMS, TMES, TMCS, HMDSO or HMDZ on the physical properties of the as prepared aerogels were studied. The volume of the NH 4OH, the molar ratios of MeOH/TEOS and HMDZ/TEOS were varied from 0.2 to 1 ml, and 5.5 to 27.5, 0.34 to 0.9, respectively, by keeping the volume of NH 4F and the concentrations of NH 4F and NH 4OH constant at 0.5 ml, 0.1 and 1 M, respectively. Remarkable results were obtained by using mixed catalyst system, hexane as exchanging solvent and surface chemical modification with 5% HMDZ in hexane. The aerogels were characterized by bulk density, optical transmission, thermal stability and contact angle measurements. The surface chemical modification was confirmed by Fourier Transform Infrared (FTIR) spectroscopy. The microstructural studies of the aerogels were done by Transmission Electron Microscopy (TEM), which revealed highly ramified self-similar polymeric structure in large length scale. The thermal stability of the aerogels were tested using TG-DT analyses. It was found that low bulk density (0.065 g/cm 3), superhydrophobic (153°), high thermal stability (380 °C) and high optical transmission (95%) of the as produced aerogels obtained at the molar ratio of TEOS:MeOH:oxalic acid:NH 4F:NH 4OH:HMDZ for 1:16.5:0.81:0.62:0.63:0.41, respectively.
Journal of Sol-Gel Science and Technology, 2006
In the present paper the experimental results of the effect of sol-gel processing temperature on the physical properties of the TEOS based silica aerogels are reported and discussed. The aerogels were produced by the two step sol-gel process at various temperatures in the range of 26-70 • C followed by supercritical drying using methanol solvent extraction. A remarkable reduction in the gelation time was observed from three and a half days at room temperature to a mere 18 hours at 50 • C. The best quality aerogels in terms of low density and high optical transmission were obtained for 6 hours hydrolysis time. The aerogels were characterized by the measurements of bulk density, volume shrinkage, porosity, refractive index and optical transmission. Monolithic aerogels with ultra low density (∼0.018 g/cm 3), extremely high porosity (∼99%) and optimum optical transmission at 700 nm (∼75%) were obtained for the molar ratio of TEOS:MeOH:acidic water:basic water at 1:99:10.42:14.58 respectively.
Preparation and thermal properties of chemically prepared nanoporous silica aerogels
Journal of Sol-Gel Science and Technology, 2014
This work focuses on the dependence preparation conditions-structure-physical properties of hydrophobic silica aerogels, all of them prepared under subcritical drying conditions (70°C and 0.4 atm.), thus aiming at potential application as case insulation filling in heat pumps. The so prepared, millimeter scaled nanoporous hydrophobic silica aerogel granules were analyzed with standard electron microscope and atomic force microscopy, IR spectroscopy, UV/Vis spectroscopy, differential scanning calorimetry and thermal conductivity measurements. The physical properties of the aerogels were compared with commercial aerogel granules. A method for contact angle measurement of micro-droplets situated on the silica granules was proposed to quantify the level of their hydrophobicity.