Mechanical strength of silica aerogels

Pure silica aerogels are obtained by hypercritical evacuation of the solvent. The strength is measured by the three-point flexural test on monolithic parallelepipedic samples and by a diametral compression test on cylindrical samples. The stress-strain curve shows a perfect elastic behaviour and the “conchoidal” fracture morphology indicates that the material is as brittle as a conventional glass. The mechanical properties are followed as a function of the bulk density. Aerogels with the highest porosity (P > 95%) reveal a maximum flexural strength lower than 10⁻² MPa. A model is proposed to account for the obtained mechanical properties.     

Superhydrophobic silica aerogels based on methyltrimethoxysilane precursor

The experimental results on the synthesis and physicochemical properties of superhydrophobic silica aerogels, with the highest ever obtained contact angle (173°), using methyltrimethoxysilane (MTMS) precursor, methanol (MeOH) solvent and ammonium hydroxide (NH₄OH) catalyst, are reported. The molar ratios of NH₄OH/MTMS (N), H₂O/MTMS (H) and MeOH/MTMS (M) were varied from 4.25 × 10⁻² to 3.5 × 10⁻¹, 2 to 10 and 1.75 to 17, respectively. It has been found that the gelation time decreases with increase in N and H values and it increases with increase in M values. The bulk density of the aerogels was found to decrease with increase in N, H and M values. It has been observed that the volume shrinkage increases with decrease in N and H values and increases with M values. In the case of catalyst concentration variation, the contact angle (θ) increases slightly from 159° to 167° with increase in N values. On the other hand, in the case of H₂O and MeOH variations, the θ first increases from 162° and 160° up to the values of 173° and 167° and then decreases to 160° and 159° with increase in H and M values, respectively. All the MTMS aerogels are opaque to the visible light. The aerogels retain their hydrophobicity up to a temperature of 480 ° C. The thermal conductivity of the aerogels was found to be around 0.095 W/m K except for the aerogels with higher bulk density (>0.3 g/cm³, at a lower H value of 2) whose thermal conductivity was around 0.109 W/m K. The aerogels have been characterized by Fourier transform infrared spectra (FTIR), thermogravimetric and differential thermal analyses (TGA-DTA) and scanning electron microscopy (SEM) techniques. The results have been discussed by taking into account the hydrolysis and condensation reactions and SEM observations.     

Enhancing mechanical properties of silica aerogels

Low-density monolithic silica and hexylene-bridged polysilsesquioxane aerogels were chemical vapor deposition (CVD) treated with hexamethyldisilazane or hexachlorodisilane silylating agents producing TMS (trimethylsilane) or Si layers on the aerogel. Reinforcing the weak aerogels by controlled deposition process improved their compressive strength and preserved their properties characteristic of low-density aerogels. When the silica and hexylene-bridged polysilsesquioxane aerogels were CVD treated with hexamethyldisilazane, the compressive modulus more than doubled in some cases. However, when treating hexylene-bridged aerogels with hexachlorodisilane the compressive modulus increased six fold. Not only did CVD treatment of the aerogels improve the compressive modulus, but the low densities, high surface areas, high porosities, and the transparency of the aerogels were not significantly affected.     

A SAXS study of silica aerogels

Aerogels produced by hypercritical drying of gels from hydrolysis of TMOS in various pH conditions and subjected to a densification process were studied by SAXS using the LURE synchrotron facility. The evaluation of scattering data combined with BET measurements leads to a model of aerogels consisting of a light density matrix in which meso- and macro-pores are embedded. No fractal features were observed for the gels, the Porod's limit having an exponent n = 4. This could mean that either these aerogels are not fractals or that the SAXS method suffers from an inherent ambiguity for fractal dimension D = 3.     

High resolution patterning of silica aerogels

Three-dimensional metallic structures are fabricated with high spatial resolution in silica aerogels. In our method, silica hydrogels are prepared with a standard base-catalyzed route, and exchanged with an aqueous solution typically containing Ag⁺ ions (1 M) and 2-propanol (0.2 M). The metal ions are reduced photolytically with a table-top ultraviolet lamp, or radiolytically, with a focused X-ray beam. We fabricated dots and lines as small as 30 × 70 μm, protruding for several mm into the bulk of the materials. The hydrogels are eventually supercritically dried to yield aerogels, without any measurable change in the shape and spatial resolution of the lithographed structures. Transmission electron microscopy shows that illuminated regions are composed by Ag clusters with a size of several μm, separated by thin layers of silica.     

Globally anisotropic high porosity silica aerogels

We discuss two methods by which high porosity silica aerogels can be engineered to exhibit global anisotropy. First, anisotropy can be introduced with axial strain (i.e. axial compression). In addition, intrinsic anisotropy can result during growth and drying stages and, suitably controlled, it can be correlated with preferential radial shrinkage in cylindrical samples. We have performed small angle X-ray scattering (SAXS) to characterize these two types of anisotropy. We show that global anisotropy originating from either strain or shrinkage leads to optical birefringence and that optical cross-polarization studies are a useful characterization of the uniformity of the imposed global anisotropy.     

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.     

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.     

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.     

Nanostructural damage associated with isostatic compression of silica aerogels

Isostatic compression of silica aerogels is known to allow densification of these highly porous materials. However, at the onset of compression, hydrophobic and consequently slightly reacting aerogels, exhibit a decrease in bulk modulus. This unusual behavior is associated with damage occurring at low pressures which recovers with further density increase. Damage development and healing are analyzed measuring elastic modulus and, for the first time, internal friction as a function of compression. It is proposed that the origin of damage and healing could be associated with the rupture of tenuous links between clusters of dense silica particles at low density levels, and with the creation of new links between the resulting arms and reacting species that are revealed at cluster interface under higher pressure.     

A new route to aerogels: Monolithic silica cryogels

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.     

Mechanical properties of polymer-modified silica aerogels dried under ambient pressure

Silica gels prepared by copolymerizing tetraethylorthosilicate with 3-aminopropyltriethoxy-silane were modified using polymer derived from toluene diisocyanate and dried under ambient pressure. The successful preparation of silica aerogels depended on the effective control of shrinkage during drying. The resulting material, polymer-modified silica aerogel, was then characterized by thermogravimetric analysis and uniaxial compression tests. Results indicated that the apparent elastic modulus and compressive strength of the polymer-modified silica aerogels decreased with increasing amounts of incorporated polymer because of decreasing shrinkage and density, while the strains at the surface cracking point and the final failure point increased significantly during compression tests. The strength and modulus of the silica skeleton could be calculated from the apparent strength and modulus of the silica aerogels respectively. It was interestingly shown that the elastic modulus of the silica skeleton of the silica aerogels increased because of the incorporated polymers, while the polymers had no effects on the compressive strength of the silica skeleton. In addition, the relationships between the apparent elastic modulus or the apparent compressive strength of the polymer-modified silica aerogels and their shrinkage were quantitatively expressed.     

Porous texture characteristics of a homologous series of base-catalyzed silica aerogels

Eighteen samples of base-catalyzed silica aerogels obtained using various tetraethylorthosilicate (TEOS) and water contents in the reaction system were characterized using N₂ adsorption. From the observed pore size distribution and adsorption isotherms the volumetric and surface fractal dimensions were determined and also the range of power-law behaviour of the porous network. The porous texture of both the bulk aerogel and surface of clusters appeared to be dependent on the concentration of reactants. The water content, however, emerges as a factor controlling surface fractal structure, its principal effect being exerted on the pore volume in the mesopore size range.     

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.     

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.     

A 3-D numerical heat transfer model for silica aerogels based on the porous secondary nanoparticle aggregate structure

A 3-D finite volume numerical model based on the porous secondary nanoparticle random aggregate structure was developed to predict the total thermal conductivity of silica aerogels. An improved 3-D diffusion-limited cluster–cluster aggregation (DLCA) method was used to generate an approximately real silica aerogel structure. The model includes the effects of the random and irregular nanoparticle aggregate structure for silica aerogels, solid–gas coupling, combined conduction and radiation, nanoparticle and pore sizes, secondary nanoparticle porosity and contact length between adjacent nanoparticles. The results show that the contact length and porosity of the secondary aerogel nanoparticle significantly affect the aerogel microstructure for a give density and, thus, greatly affect the total thermal conductivity of silica aerogels. The present model is fully validated by experimental results and is much better than the model based on a periodic cubic array of full density primary nanoparticles, especially for higher densities. The minimum total thermal conductivity for various silica aerogel microstructures can be well predicted by the present model for various temperatures, pressures and densities.     

Improvement in optical transmission of the ambient pressure dried hydrophobic nanostructured silica aerogels with mixed silylating agents

The experimental results on the preparation of transparent and hydrophobic silica aerogels based on ion exchanged sodium silicate (Na₂SiO₃) using mixed silylating agents of trimethylchlorosilane (TMCS), hexamethyldisiloxane (HMDSO) and hexamethyldisilazane (HMDZ) are reported. Hydrogels were prepared with ion exchanged Na₂Si0₃ of 1.10 specific gravity with 1 N ammonium hydroxide (NH₄OH). The resulted gels were exchanged with methanol solvent followed by silylation using HMDZ + HMDSO or TMCS + HMDZ or TMCS + HMDSO mixture in methanol and hexane and dried the gels at room temperature for 24 h, at 50 and 200 °C for 1 h each. It has been observed that the percentage of the silylating agent in the mixture, time interval of addition of one agent followed by another and volume of silylating mixture have an effect on density, % of optical transmission, % of porosity, porevolume, thermalconductivity and refractive index of the aerogels. Hydrophobicity of the aerogels was studied by contact angle measurements. The TMCS + HMDSO aerogels have been found more hydrophobic (contact angle > 150°) than the other aerogels. Further, aerogels have been characterized by pore size distribution, Fourier Transform Infra Red Spectroscopy (FTIR), Transmission Electron Microscopy (TEM) and thermogravimetric analysis and differential thermal analysis (TGA–DTA) techniques. It has been found that the weight increase is the highest (325%) for HMDSO + HMDZ aerogels and lowest (1.2%) for HMDSO + TMCS aerogels by keeping the aerogels in the atmosphere over a period of 18 months. Low density (0.042 g/cc) transparent (85%), low thermal conductive (0.047 W/m K), low refractive index (1.0088) and hydrophobic (152°) silica aerogels were obtained with equivolume TMCS + HMDSO mixture of 10 ml, time interval of addition of 12 h between the two reagents prior addition of TMCS followed by HMDSO for 24 h of silylation period. It was found from the TGA–DTA of the aerogels that hydrophobicity of the aerogels remained up to the temperature of 425 °C.     

An analytical model for combined radiative and conductive heat transfer in fiber-loaded silica aerogels

An improved analytical model for the total thermal conductivity of fiber-loaded silica aerogels was developed based on the complex refractive index, size, orientation, volume fraction and morphology of the fibers and silica aerogel. A cubic array of spherical porous secondary nanoparticles and a modified parallel-series model were proposed to model the combined solid and gaseous thermal conductivities. An anomalous diffraction theory (ADT) was used to predict the fiber extinction coefficient. Five common fiber types in the composites were studied including amorphous SiO₂ glass, silicon glass, common float glass, soda lime silica glass and borosilicate glass. The results show that the total extinction coefficient of the silica aerogel system is largest by loading with the common float glass fiber and lowest by loading with the soda lime silica glass among the five fiber types. The model provides theoretic guidelines for material designs with optimum parameters, such as the type, inclination angle, volume fraction and diameter of the fibers as well as the aerogel nanoparticle and pore sizes. The optimum fiber for improved thermal insulation should have a large spectral complex refractive index throughout the infrared region.     

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.