Synthesis of silica aerogel blanket by ambient drying method using water glass based precursor and glass wool modified by alumina sol

Silica aerogel blankets have been synthesized by ambient drying technique using cheap water glass as the silica source and glass wool modified by alumina sol. One step solvent exchange and surface modification were simultaneously conducted by immersing the wet hydrogel blanket in EtOH/TMCS/hexane solution. The synthesized silica aerogel blanket was light with the density of 0.143-0.104 g/cm³ and 89.4-95% porosity. The microstructure of silica aerogel blanket exhibits a porous structure consisting of glass fibers of diameter ∼2.5 μm interconnected with solid silica clusters (5-20 μm).     

Microstructural and physical properties of the ambient pressure dried hydrophobic silica aerogels with various solvent mixtures

The experimental results on the microstructural and physical properties of the ambient pressure dried hydrophobic silica aerogels with various solvent mixtures have been reported. The aerogels were prepared with sodium silicate precursor, ammonium hydroxide catalyst, trimethylchlorosilane (TMCS) silylating agent, solvent mixture of methanol-isopropanol (MeOH/IPA) and various aprotic solvent mixtures namely, hexane and benzene (HB), hexane and toluene (HT), hexane and xylene (HX), heptane and benzene (HpB), heptane and toluene (HpT), heptane and xylene (HpX). The physical properties of the aerogels such as % of volume shrinkage, density, % of optical transmission, surface area, % of porosity, pore volume, thermal conductivity and heat capacities of the aerogels were studied. The hydrophobicity of the aerogels was studied by contact angle measurements. The HX and HpX aerogels have been found to be more hydrophobic (contact angle, θ > 155°) than the other aerogels. It has been observed that the % of weight increase is highest (1%) for the HT aerogels and lowest (0.25%) for HpX aerogels by keeping them at 70% humidity for 350 h. Further, the aerogels have been characterized by pore size distribution (PSD), Fourier transform infra red spectroscopy (FTIR) and thermogravimetric and differential thermal (TG-DGA) analysis and transmission electron microscopy (TEM) techniques. The results have been discussed by taking into account the surface tension, vapor pressure, molecular weight and chain length of the solvents. Low density (0.051 g/cc), hydrophobic (165°), transparent (85%), low thermal conductive (0.059 W/m K), low heat capacity (180 kJ/m³ K) and highly porous (97.38%) silica aerogels were obtained with HpX solvent mixture.     

Synthesis of ultrahigh-pore-volume carbon aerogels through a “reinforced-concrete” modified sol-gel process

Ultrahigh-pore-volume carbon aerogels were synthesized by adding rigid silica nanoparticles to resorcinol-formaldehyde sols, followed by supercritical drying, pyrolysis and HF leaching. The presence of silica nanoparticles in polymer gels dramatically inhibits volume shrinkage and framework collapse during the supercritical drying and pyrolysis processes, resulting in the obtained carbon aerogels exhibiting very low bulk density and high pore volume. By changing the mass ratio of silica nanoparticles/resorcinol-formaldehyde resin, pore volumes of carbon aerogels can be tuned in the range of 2.8-6.0 cm³/g.     

Ultralow density silica aerogels prepared with PEDS

This paper deals with the synthesis of ultralow density silica aerogels using polyethoxydisiloxanes (PEDS) as the precursor via sol-gel process followed by supercritical drying using ethanol solvent extraction. Ultralow density silica aerogels with 5 mg/cc of density were made for the molar ratio by this method. A remarkable reduction in the gelation time was observed by the effect of the catalyst NH₄OH at room temperature. The microstructure and morphology of the ultralow density silica aerogels were characterized by the specific surface area, S_BET, SEM, TEM and the pore size distribution techniques. The results show that the diameter of the silica particles is about 13 nm and the pore size of the silica aerogels is about several nm. The specific surface area of the silica aerogel is 339 m²/g and the specific surface area, pore volume and average pore diameter decrease with increasing density of the silica aerogel.     

Transparent SiO2 aerogels prepared by ambient pressure drying with ternary azeotropes as components of pore fluid

Transparent SiO₂ aerogels were prepared by two-step sol–gel processing followed by ambient pressure drying at temperatures from 70 to 250 °C. The wet gels were synthesized via acid–base catalysis using tetraethyl orthosilicate as a silica precursor and isopropanol as a solvent. Isopropanol was exchanged with n-butanol, and the gel surface was modified using a trimethylchlorosilane solution in n-butanol. Next, the solvent was exchanged in several steps with saturated hydrocarbon in order to obtain pore fluids containing azeotropic mixtures of water, n-butanol and a corresponding hydrocarbon (hexane, heptane, octane, nonane). Ambient pressure drying was performed in two steps, at the boiling points of the ternary azeotropes and hydrocarbons, respectively. In this way, transparent, crack-free aerogels of different shapes, with a specific surface area of 1000 m²/g, average pore diameter of ～40–55 Å and density in the range 0.4–0.57 g/cm³ were obtained.     

Rapid synthesis of amine cross-linked epoxy and methyl co-modified silica aerogels by ambient pressure drying

The silica aerogels were synthesized by sol–gel method via ambient pressure drying. Tetraethyl orthosilicate (TEOS) was used as a main silica source, methyltriethoxysilane (MTES) as a co-precursor silica source and (3-Glycidoxypropyl)trimethoxysilane (GPTMS) as a silane coupling agent. The silica aerogels obtained were further undergoing cross-linking epoxy from GPTMS with amine from diethylenetriamine (DETA) which played a dual role of base catalyst and reagent. The cumulative volumes for open pores of the cross-linked aerogels were evaluated to be 1.4 cm³/g. The Young's modulus and maximum compression strength were 25.4 MPa and 6.17 MPa, respectively. The addition of MTES accelerated the solvent exchange of alcohol within the pores with n-hexane and reduced the shrinkage of aerogels network during the ambient pressure drying. The formation of organic network enhanced the strength of the cross-linked aerogels to prevent the crack generation and the subsequent failure of the monolith during the ambient drying, therefore, protected the nanoporous structure of aerogels.     

Interactions between silica sand and sodium silicate solution during consolidation process

This work is based on the knowledge of the consolidation of silica sand with an alkaline solution in order to determine the mechanisms that occur during the drying of sand and various alkaline solution mixtures. The investigations concern effects of sand distribution size, dilution of sodium silicate solutions and drying temperature of the mixtures on consolidation behaviour. The thermal analysis performed on fresh mixtures show a release of free-water from diluted silicate solution during the consolidation. SEM observations and compressive strength tests results indicate that interactions between sodium silicate binder and silica sand depend on drying temperature. Consequently two consolidation mechanisms are proposed: at low drying temperature (70 °C), sodium silicate acts as a thin layer of glue covering sand grains and bind them to each other, while at high temperature (150 °C), dissolution-precipitation reaction occurred in the mixture consolidating more strongly the granular system. The increase of Si/Na molar ratio in a sodium silicate solution containing silica sand is in accordance with the proposed mechanism.     

Structure and electrochemical properties of carbon aerogels synthesized at ambient temperatures as supercapacitors

In this paper, the carbon aerogels derived from organic sol-gel process were prepared by means of ambient drying technique. Morphology and physical properties of the carbon aerogels were characterized by scanning electron microscopy (SEM) and N₂ sorption isotherm. It was found that the carbon aerogels were porous materials with pearly network structure, the particle size of carbon aerogels increased with the increase of R/C ratio (molar ratio of resorcinol to catalyst). The BET surface areas of obtained carbon aerogels were in the range 600-1000 m²/g. Electrochemical performances of the carbon aerogels electrodes were studied by cyclic voltammetry, galvanostatic charge/discharge measurements and electrochemical impedance measurement. The results indicated that carbon aerogels electrodes had good electrochemical performance, high reversibility and high specific capacitance. Moreover, the utilization efficiency of carbon aerogels was promoted due to excellent electronic conductivity and extensive mesoporous network of carbon aerogels. The specific capacitance of the electrode is 183.6 F/g with R/C ratio 1500. In addition, the carbon aerogel electrode for the application of supercapacitor had low resistance, small leakage current and good electrical conductivity.     

Consolidation mechanism of materials obtained from sodium silicate solution and silica-based aggregates

A study based on the use of sodium silicate gels as binder for cold consolidation of silica-based aggregates has been investigated. The gels used as precursor of binder were synthesized by adding hydrochloric acid to a concentrated sodium silicate solution. Consolidated materials were obtained by mixing the previous solution before gelation with granular materials (fine silica powder and sand). The study of the gel-silica-sand ternary system shows that the existence domain of materials depends on the sand size distribution. The microstructure of gel-silica-sand ternary samples reveals the presence of the three components with a partial attack of grain surface. This was confirmed by FTIR experiments during the monitoring of the synthesis. Actually, the ν_asSiOSi broad band resulting from the average of the contribution of the set of Q⁴, Q³ and Q² units with a sharp peak located around 1078 cm^− 1, firstly shifts to lower wavenumber until 21 days and then to higher wavenumber characteristic of dissolution/precipitation reactions. On the other hand, the consolidation of the material is strong when the amount of fine silica in the material is high leading to efficient mechanical properties. Therefore, consolidation could be explained by the dissolution of small particles of silica and their precipitation into the grain boundary of sand.     

Preparation and characterization of carbon aerogel microspheres by an inverse emulsion polymerization

Carbon aerogel (CA) microspheres have been successfully synthesized by an inverse emulsion polymerization and characterized by scanning electron microscopy (SEM), N₂ sorption isotherm and X-ray diffraction (XRD). The results show that the size and pore characteristics of carbon microsphere obviously depend on stirring speed and concentration of surfactant in the emulsion polymerization process. The resultant CA microspheres are amorphous carbon structure with the size ranging from about 2 to 50 μm by changing the stirring speed. CA microspheres with S_BET of 414-603 m² g^− 1 and V_meso of 0.028-0.432 cm³ g^− 1 are synthesized using different SPAN80 concentrations. The results of cyclic voltammetry indicate that the CA microspheres prepared at a stirring speed of 480 rpm and at V_s/V_h = 0.01 have ideal supercapacitive behavior in 6 M KOH electrolyte, the maximum specific capacitance of the electrode reaches 180 F g^− 1.     

Immobilization and catalytic activity of lipase on mesoporous silica prepared from biocompatible gelatin organic template

Mesoporous silica (MPS) with high surface area (518.8 m²/g), tunable pore size (3.6-13.7 nm) was synthesized using inexpensive sodium silicate as the source of silica and the biodegradable gelatin as the structure directing agent. The amorphous mesostructures of the materials were characterized by N₂ adsorption-desorption isotherms, scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The synthesized MPS was then utilized as supports for immobilization of Porcine pancreatic lipase (PPL) by physical adsorption. The measurements of Fourier transform infrared spectroscopy (FT-IR), N₂ adsorption-desorption isotherms and energy-dispersive spectroscopy (EDS) demonstrated that PPL had been adsorbed into the channels of the MPS. The immobilized PPL was used for catalyzing hydrolysis reaction of triacetin, and its correlative catalytic activity was studied. Compared with free PPL, the immobilized PPL possessed superior adaptability in alkaline range (pH = 7.0-9.0), good thermal stability and excellent reusability in phosphate buffer solution in hydrolysis reactions of triacetin. The results also indicated that the immobilized PPL maintained 45% of its initial activity even after being reused six times.     

Polymer nano-encapsulation of templated mesoporous silica monoliths with improved mechanical properties

Macroporous (1–5 μm) monolithic silica aerogels consisting of both random but also ordered mesoporous walls have been synthesized via an acid-catalyzed sol–gel process from tetramethoxysilane (TMOS) using a triblock co-polymer (Pluronic P123) as a structure-directing agent and 1,3,5-trimethylbenzene (TMB) as a micelle-swelling reagent. Pluronic P123 was removed by Soxhlet extraction, and materials in monolithic form were obtained by extracting the pore filling solvent with liquid CO₂, which eventually was taken out supercritically. Although these monoliths are more robust than base-catalyzed silica aerogels of similar density, nevertheless, the mechanical properties can be improved dramatically by letting an aliphatic di-isocyanate (Desmodur N3200) react with the silanols on the macro- and mesoporous surfaces. As it turns out, the polymer fills the mesopores and coats conformally the macropores of templated samples, so that BET surface areas decrease dramatically, from 550–620 m² g^?1 to <5 m² g^?1. By comparison, polymer nano-encapsulation of non-templated acid-catalyzed aerogels preserves a large fraction of their mesoporous surface area, and BET values decrease from 714 m² g^?1 to 109 m² g^?1. Finally, since polymer nano-encapsulation preserves the macroscopic physical dimensions of the monoliths before drying, comparative analysis of the physical dimensions against XRD data of native versus polymer nano-encapsulated samples provides evidence that upon drying macropores (micron size regime) shrink less than mesopores (nanometer size regime).     

Preparation of cresol–formaldehyde carbon aerogels via drying aquagel at ambient pressure

Carbon aerogels were prepared from mixed cresol (Cm) and formaldehyde (F) via the sol–gel process followed by drying at ambient pressure and carbonization. The inexpensive feedstock of mixed cresol and the simple drying process could be as an alternative economical route to the classical resorcinol–formaldehyde synthesis process. In our process, organic precursor gels were synthesized via polycondensation of cresol with formaldehyde in an aqueous alkaline (NaOH) solution. After gelation the solvent was removed via drying at ambient pressure to obtain organic aerogels that exhibit a drying shrinkage below 5% (linear). Upon carbonization of the organic aerogels at 1173 K, monolithic carbon aerogels (denoted as CmF carbon aerogels) can be produced. Nitrogen adsorption results showed that the CmF carbon aerogels have abundant mesopores and micropores with a dominant pore diameter of 25–40 nm. An increase of the BET surface area and a modification of the pore size distribution of CmF can be realized by a CO₂ activation. The images of scanning electron microscopy (SEM) indicated that the microstructure of carbon aerogels can be effectively controlled and tailored by varying the synthetic conditions during the initial sol–gel process.     

Synthesis of cubic mesoporous silica and carbon using fly ash

A supernatant solution of silicate species extracted from coal fly ash in a power plant by alkali fusion was used under acidic conditions to prepare a mesoporous silica, SBA-16. SBA-16 was used as a template for the synthesis of a mesoporous carbon using sucrose as a carbon source. These mesoporous silica and carbon materials were characterized by XRD, N₂ adsorption-desorption, SEM, and TEM. Textural properties of the silica and carbon samples prepared using fly ash were found to be comparable to those prepared by pure chemicals, successfully demonstrating the feasibility of recycling fly ash for the synthesis of high quality porous materials.     

Silica-doped alumina cryogels with high thermal stability

Alumina cryogels with a dopant of silica in the content range of 0–10 wt% were synthesized from aqueous boehmite sol through the sol-gel technique and subsequent freeze drying. The higher thermal stability was achieved by the addition of 10 wt% silica; a γ-Al₂O₃ phase still remained after heating at 1200 °C for 5 h, and the surface area and pore volume were 47 m² g⁻¹ and 105 mm³ g⁻¹, respectively. The marked stability was ascribed to the synergetic effect of the very low bulk density (0.05 g cm⁻³) and the dopant. The thermal stability was lower for the cryogels than for the corresponding aerogels; however, it was also suggested that cryogel was highly durable in water in contrast to aerogel.     

Structural characteristics of silica sonogels prepared with different proportions of TEOS and TMOS

Sonohydrolysis of mixtures of tetraethoxysilane (TEOS) and tetramethoxysilane (TMOS) with different TMOS/(TMOS + TEOS) molar ratio R was carried out to obtain ∼2.0 × 10⁻³ mol SiO₂/cm³ and ∼86%-volume liquid phase wet gels. Aerogels were obtained by supercritical CO₂ extraction in autoclave. The samples were analyzed by small-angle X-ray scattering (SAXS) and nitrogen adsorption. The structure of the wet gels can be described as a mass fractal structure with fractal dimension D ∼ 2.2 and characteristic length ξ increasing from ∼4.6 nm for pure TEOS to ∼6.4 nm for pure TMOS. A fraction of the porosity is eliminated with the supercritical process. The fundamental role of the TMOS/(TMOS + TEOS) molar ratio on the structure of the aerogels is to increase the porosity and the pore mean size as R changes from pure TEOS to pure TMOS. The supercritical process increases the mass fractal dimension and shortens the fractality domain in the mesopore region. A secondary structure appearing in the micropore region of the aerogels can be described as a mass/surface fractal structure with correlated mass fractal dimension D_m ∼ 2.6 and surface fractal dimension D_s ∼ 2.3.     

Textural characterization of high temperature silica aerogels

Silica alcogels were synthetized by the sol–gel polymerization of tetraethylorthosilicate in acid media. Conventional and supercritical drying was performed in order to obtain xerogels and aerogels. Different process parameters of the supercritical drying were altered in order to control the texture of the resulting gel. The texture and the structural evolution of xero- and aerogels were studied by thermogravimetric-differential thermal analysis, Fourier-transform infrared spectroscopy, transmission electron microscopy and N₂ physisorption at 77 K. ²⁹Si magic angle spinning nuclear magnetic resonance experiments on silica samples were used to resolve various silicon local environments. Hydrophilic microporous xerogels and hydrophobic micro- or mesoporous silica aerogels were obtained, whose microscopic structure is very similar. However, the samples obtained by different drying procedures exhibit a different structural evolution with temperature.     

Mass fractal characteristics of sonogels prepared from sonohydrolysis of tetraethoxysilane with additions of dimethylformamide

Wet silica gels with ∼1.4 × 10⁻³ mol SiO₂/cm³ and ∼90 vol.% liquid phase were prepared from the sonohydrolysis of tetraethoxysilane (TEOS) with different additions of dimethylformamide (DMF). Aerogels were obtained by CO₂ supercritical extraction. The samples were studied mainly by small-angle X-ray scattering (SAXS) and nitrogen adsorption. Wet gels exhibit a mass fractal structure with fractal dimension D increasing from 2.23 to 2.35 and characteristic length ξ decreasing from ∼9.4 nm to ∼5.1 nm, as the DMF/TEOS molar ratio is increased from 0 to 4. The supercritical process apparently eliminates some porosity, shortening the fractality domain in the mesopore region and developing an apparent surface/mass fractal (with correlated mass fractal dimension D_m ∼ 2.6 and surface fractal dimension D_s ∼ 2.3) in the micropore region. The fundamental role of the DMF addition on the structure of the aerogels is to diminish the porosity and the pore mean size, without, however, modify substantially the specific surface area and the average size of the silica particle of the solid network.     

Formation and characterization of tin oxide aerogel derived from sol-gel process based on Tetra(n-butoxy)tin(IV)

Transparent and translucent SnO₂ aerogels with high specific surface area (>300 m²/g) have been prepared by sol-gel process using tetra(n-butoxy)tin(IV) as a starting compound, and supercritical drying technique for solvent extraction. Light scattering measurements reveal that the polymeric cluster size distribution in sol system is gradually broadened during sol-gel transition. SEM images show that the aerogels are made up of the cottonlike oxide agglomerates with a large number of pores. TEM images show that these aerogels seem to be self-similar at different magnifications. Their pore size distribution is pretty wide ranging from mesopore to macropore especially for that of translucent aerogel.     

Sol-gel synthesis of porous and dense silica microspheres

An acidic silica sol (35 ± 2 wt% equivalent SiO₂) having a gelling time of 9-10 min has been used as an aqueous phase for obtaining a w/o emulsion in CCl₄ as oil phase in presence of a surfactant, Tween 80. The silica sol was allowed to form gel at room temperature via polycondensation among the -Si-OH groups forming the porous silica gel microspheres. The surface area of the microspheres heated at 500°, 700° and 900 °C was found to be 227 m²/g, 167 m²/g and 81 m²/g indicating the gradual densification. The decreased surface area and unchanged -Si-O-Si- asymmetric stretching vibration at 1084 cm⁻¹ up to 700 °C probably indicate the formation of extensive cross-linked gel structure in the microsphere. The appearance of the -Si-O-Si- asymmetric stretching vibration at 1104 cm⁻¹ and the absence of porosity while heating at 1000 °C indicate the formation of dense silica glass microspheres.