Preparation and characterization of monolithic alumina aerogels

Alumina aerogels were prepared by a sol-gel method combined with the ethanol supercritical drying technique using aluminum tri-sec butoxide and nitric acid as the precursor and catalyzer respectively. This method affords high-surface-area alumina aerogel monoliths without the use of complexing agents. The structure and morphology of the aerogels were investigated by TEM, XRD, FTIR and BET techniques. The results confirmed that the as-prepared alumina aerogel possessed a network microstructure made up of pseudoboehmite fibers and a surface area of 690 m²/g. It was transformed to γ-Al₂O₃ after heat treatment at 800 °C without a significant loss in surface area. DMA analysis and hotdisk thermal analysis were performed to characterize the mechanical and thermal properties of the samples. The results indicated that the alumina aerogel was robust and exhibited excellent thermal insulating properties. The elastic modulus was up to 11.4 MPa after drying, which is the one of the highest modulus of alumina aerogels ever reported. The thermal conductivities at 30 °C and 400 °C were 0.028 W/mK and 0.065 W/mK respectively.     

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.     

Analysis of a rapid supercritical extraction aerogel fabrication process: Prediction of thermodynamic conditions during processing

Aerogels have unusual mechanical and thermal properties that render them useful in a range of applications from thermal insulators to chemical sensors. However, aerogel fabrication can be difficult, typically requiring the use of supercritical extraction of solvent from the sol-gel matrix. We employ a rapid supercritical extraction (RSCE) technique for aerogel fabrication that is faster and simpler than the standard methods. This technique relies on a hydraulic hot press to heat the chemical precursors and provide the required temperature and pressure increase needed to reach a supercritical state within a contained mold. Experimental results show that the pressure-temperature curve is characterized by a ‘take-off point’ and a ‘leak point’. The take-off point occurs at the start of a rapid increase in pressure and the leak point occurs where the pressure increase subsides. This paper presents an analytical model that predicts the pressure-temperature relationship of the aerogel precursors during the RSCE fabrication process and shows that the model can be extended to other RSCE systems. Using pure methanol, water and aerogel precursors in separate tests, the effects of initial liquid volume and press force on the pressure and temperature during processing were studied. We find that the take-off point can be estimated using a maximum specific volume model, which is a function of the initial percent volume fill of liquid used in the test. We are also able to predict the rapid pressure increase region observed during the process using a constant volume model. Leaking is determined to be a function of the mechanical forces acting on the system and occurs when the pressure in the contained mold nears the value of the pressure imparted onto the gasket by the hot press. The leak point pressure is found to be independent of the initial percent volume of liquid used in the test; however the leak point temperature decreases with increasing initial percent volume fill.     

Monolithic copper oxide aerogel via dispersed inorganic sol-gel method

To avoid the use of rare copper alkoxides, copper oxide aerogels were prepared using copper chloride, polyacrylic acid and propylene oxide via the dispersed inorganic sol-gel method, a supercritical fluid drying process and a 500 °C thermal treatment. The morphology and composition of the aerogel without thermal treatment (copper-based aerogel) and copper oxide aerogel were both characterized and analyzed. Based on studies of the gelation mechanism, it was demonstrated that polyacrylic acid guides the sol formation including providing a steric effect. Analysis of the thermal treatment process shows that the copper-based aerogel has four primary thermal reactions and the major composition of samples treated above 420 °C is monoclinic copper oxide.     

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.     

Synthesis of sodium silicate-based hydrophilic silica aerogel beads with superior properties: Effect of heat-treatment

This work demonstrates the synthesis of hydrophilic and hydrophobic high surface area silica aerogel beads with a large pore volume. Wet gel silica beads were modified and heat-treated under atmospheric pressure after modification of the surface by trimethychlorosilane (TMCS). The effects of heat treatment on the physical (hydrophobicity) and textural properties (specific surface area, pore volume, and pore size) of silica aerogel beads were investigated. The results indicated that hydrophobicity of the silica aerogel beads can be maintained up to 400 °C. The hydrophobicity of the silica aerogel beads decreased with increasing temperature in the range of 200-500 °C, and the beads became completely hydrophilic after heat treatment at 500 °C. The specific surface area, cumulative pore volume, and pore size of the silica aerogel beads increased with increasing temperature. Heating the TMCS modified bead gel at 400 °C for 1 h resulted in silica aerogel beads with high surface area (769 m²/g), and large cumulative pore volume (3.10 cm³/g). The effects of heat treatment on the physical and textural properties of silica aerogel beads were investigated by field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM), thermogravimetric and differential analysis (TG-DTA), Fourier-transform infrared spectroscopy (FT-IR), and Brunauer, Emmett and Teller (BET) and BJH nitrogen gas adsorption and desorption methods.     

Large deformation of chlorotrimethylsilane treated silica aerogels

Silica aerogels were prepared through an acid-base process and surface modified with chlorotrimethylsilane. This novel application of a common non-crosslinking surface modification to improve mechanical properties allows the treated aerogels to deform plastically to compressive strains greater than 80% without macroscopic damage. This improvement in mechanical properties remains after heating in air at 500 °C for 3 h, as do residual organic groups. Heating at 700 °C for 1 h removes all organics and the aerogel behaves similar to the unmodified control. The treated aerogels also exhibit a greater resistance to sintering. Nitrogen adsorption measurements show a reduction in the number of micropores with surface modification. It is concluded that the organic monolayer increases the ductility of the silica network by filling and strengthening surface micropores that serve as crack initiators, and that these organics remain effective at elevated temperatures.     

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.     

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.     

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.     

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.     

Cost-effective synthesis of silica aerogels from fly ash via ambient pressure drying

Silica aerogels were synthesized from the industrial fly ash by ambient pressure drying method. The process consists of two stages, preparation of sodium silicate solution from fly ash by hydrothermal reaction with sodium hydroxide, and synthesis of porous silica aerogels from the obtained sodium silicate solution. Silica wet gels were formed by vitriol-catalysis or resin-exchange-alkali-catalysis of the obtained sodium silicate solution. The trimethylchlorosilane(TMCS)/ethanol(EtOH)/hexane mixed solution was used for solvent exchange/surface modification of the wet gel so as to obtain porous silica aerogels via ambient pressure drying. The results indicated that the synthesized silica aerogels were lightweight and hydrophobic. The BET specific surface area, pore volume and average pore diameter were 362.2-907.9 m² g^− 1, 0.738-4.875 cm³ g^− 1, and 7.69-24.09 nm respectively. Particularly, the synthesized silica aerogels by resin-exchange-alkali-catalysis method showed uniform mesoporous structure, and had much higher specific surface area (907.9 m² g^− 1) and pore volume (4.875 cm³ g^− 1) than that of by vitriol-catalysis process.     

Synthesis and characterization of germanium sulfide aerogels

The synthesis of germanium sulfide gels by thiolysis reactions of a non-aqueous solution of Ge(OEt)₄, followed by supercritical fluid extraction to create aerogels, is described. Analysis of the as-prepared GeS_x aerogels by powder X-ray diffraction (PXRD) and surface area analysis reveals an amorphous structure exhibiting very high surface areas, 755 m²/g, that rival those of the best SiO₂ aerogels when compared on a per mole basis. Transmission electron microscopy shows that the aerogel material is composed of a continuous network of GeS_x colloidal particles assembled in a three-dimensional architecture. A detailed comparison of GeS_x aerogels and their xerogel (bench-top dried) counterparts in terms of the influence of the synthetic methodology on morphology and surface area is reported. In the presence of adventitious moisture, the amorphous GeS_x is oxidized to a crystalline phase identified by X-ray photoelectron spectroscopy, Raman spectroscopy and PXRD cell refinement to be hexagonal GeO₂.     

The growth of carbon nanostructures on cobalt-doped carbon aerogels

By carbonizing cobalt-doped aerogel precursors directly at various temperatures, or by carbon monoxide decomposition of cobalt-doped carbon aerogels, different carbon nano-features such as carbon nano-filaments and graphitic nano-ribbons were grown on cobalt-doped carbon aerogel samples. Transmission electron spectroscopy, X-ray photoelectron spectroscopy and X-ray diffraction characterization results showed that metallic cobalt nano-particles form when heating the cobalt-doped aerogel samples over 500 °C. At low heating temperature, many highly oriented carbon thin films can be found on metallic cobalt nano-particles. When heating the samples at 850 °C, some carbon nano-filaments are obtained. While heating the samples at 1050 °C, many graphitic nano-ribbons are grown and the framework of the interconnected carbon particles of the sample is changed. Graphitic nano-ribbons can also be grown by CO decomposition of the cobalt-doped carbon aerogels. We can therefore control and modify the nanostructures of cobalt-doped carbon aerogels by heating them at different temperatures or by using CO decomposition.     

Synthesis and characterization of monolithic Gd2O3 aerogels

In this thesis, we will elaborate on the synthesis and characterization of monolithic Gd₂O₃ aerogel. We conducted the experiment in the following procedure. Use gadolinium nitrate or gadolinium chloride, a kind of inorganic gadolinium salt as raw material, and polymerize it in ethanol with propylene oxide as gelation initiator in the way of sol-gel. After this step, we can obtain the wet gel. Then, dry the wet gel by supercritical CO₂, at last we will get aerogel. The product has strong transparency and also shows some thermal stability. XRD characterization shows that it is amorphous. Nitrogen adsorption/desorption analysis tells clearly its surface area (223 m²/g), average pore diameters (42 nm) and large pore volume (1.83 ml/g). It is also characterized by transmission electron microscopy and high-resolution transmission electron microscopy.     

Effect of uni-axial loading on the nanostructure of silica aerogels

Aerogels are unique materials offering a combination of remarkable properties that make them useful in a wide range of applications. However, aerogel materials can be difficult to work with because they are fragile. The intent of the work presented here was to study the relationship between axial loading and pore structure in aerogel material. Silica aerogel samples with a bulk density of 0.1 g/mL were compressed by uni-axial force loads from 1 to 5 kN which resulted in stress levels up to 23 MPa. The resulting change in the pore distribution was observed using nitrogen desorption analysis and scanning electron microscopy. Uncompressed aerogel samples exhibit peak pore volume at diameters of about 20 nm. As the aerogels are subjected to increased loading, the location of the peak volume moves to smaller diameters with a reduced volume of pores occurring above this diameter. The peak diameter, the average pore diameter and pore volume all decrease and scale with increasing maximum stress while the surface area of the aerogel samples remains unaffected at about 520 m²/g. When combined with data from the literature, the relation between maximum pore diameter and applied stress suggests a failure mechanism dominated by bending induced fracture.     

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.     

Nitridation of SiO2–B2O3 aerogels

SiO₂–B₂O₃ aerogels have been prepared by drying wet gels at a supercritical condition for ethanol in an autoclave. Aerogels have been nitrided for 6 h in flowing ammonia at the temperature of 1200 °C. It has been found that the amount of nitrogen incorporated in these aerogels always exceeds 20 wt%. This is a much higher value compared with the amount of nitrogen incorporated in a pure silica aerogel nitrided at the same conditions. The specific surface area of SiO₂–B₂O₃ aerogels has been between 312 and 359 m²/g. After nitridation some shrinkage of aerogels has been observed and the surface area decreases about 20%. In FTIR spectra of SiO₂–B₂O₃ aerogels a typical bands for SiO₂ are observed. After nitridation a shift and broadening of 1100 cm^?1 band to lower wavenumbers indicates that Si–N and B–N bonds are formed in nitrided aerogels.     

Synthesis and structural characteristics of carbon aerogels with a high content of Fe,Co, Ni,Cu, and Pd

High content metal carbon aerogels have been prepared by sol–gel polymerization of formaldehyde with potassium salt of 2,4-dihydroxybenzoic acid, followed by K⁺-exchange with Fe(II), Fe(III), Co(II), Ni(II), Cu(II) and Pd(II) ions from an aqueous or acetoneous solution and subsequent supercritical drying with CO₂. Carbonization at 1050 °C, under an inert atmosphere, transforms the metal ion doped organic aerogels into metal and/or metal oxide nanoparticles-doped carbon aerogels. The resulting materials were characterized by means of elemental analysis, nitrogen adsorption, transmission electron microscopy and X-ray diffraction. The structural properties and metal concentration of the doped carbon aerogel depend on the type and valence of the precursor metal salt. The presence of some graphitic nano-ribbons was evidenced in the case of Fe-, Co-, and Ni-doped carbon aerogels.     

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).