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.     

SiO2气凝胶分子动力学模拟研究进展

二氧化硅(SiO2)气凝胶是一种拥有三维骨架网络结构的纳米多孔材料,具有高孔隙率、低密度和低热导率等许多独特的性能.但是由于二氧化硅气凝胶本身的脆性及高温稳定性差等原因,限制了其大规模应用.二氧化硅气凝胶的热力学性能与其内部的三维骨架和孔结构紧密相关,掌握二氧化硅气凝胶内部微结构演化规律与宏观性能的关联,是改善其热力学...     

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.     

Simulation of the microstructural evolution of a polymer crosslinked templated silica aerogel under high-strain-rate compression

Surfactant-templated mesoporous silica aerogels (or nanofoams) with their entire skeletal framework nanoencapsulated conformally by a thin polyurea layer are emerging as materials with high specific strength and high energy absorption. In this paper a modified split Hopkinson pressure bar was used to investigate their mechanical behavior under dynamic compression at high strain rates. The evolution of the mesoporous structure under such dynamic impact conditions was simulated using the Material Point Method (MPM). The material point model was generated from X-ray micro-computed tomography whereas each voxel was converted to a material point corresponding to the local skeletal density of the material. Simulation results agree well with the experimental data, indicating that the MPM can effectively model the compression of complex mesoporous structures. Simulations indicate a nearly uniform deformation at all three stages of compression: the elastic region, compaction and the final densification due to the low ratio of pore size to wall thickness and random distribution of the pores. Simulations have also indentified the function of the conformal polymer coating as a reinforcing factor, showing that different porosities, obtained by varying the skeletal wall thickness, affect the local stress distribution. Eventually, simulations confirm that the stress-strain behavior of aerogels under compression follows a power-law relationship with the initial bulk density, consistent with experimental results.     

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.     

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.     

Transport properties of copper-doped carbon aerogels

The magnetic susceptibility, conductivity, magnetoresistance (MR) and Hall effect of copper-doped carbon aerogels are measured and compared with corresponding results from the original carbon aerogels. The experimental results indicate that the temperature-dependent magnetic susceptibility of the copper-doped and of the original carbon aerogels is well fit by a Curie function at low temperatures. The copper-doped carbon aerogels show a higher susceptibility and spin concentration than the original carbon aerogel. After doping by copper, the materials exhibit a more linear current-voltage curve than the original carbon aerogel under the same measurement conditions. The electrical resistance of the copper-doped carbon aerogels is strikingly lower than that of the original carbon aerogels, and decreases with increasing copper content in the samples. The temperature-dependent resistivity ρ(T) of all of the copper-doped and original carbon aerogels can be fitted by an exp(T^−1/2) dependence for T<100 K. The copper-doped and pristine carbon aerogels follow a quadratic MR behavior Δρ/ρ=AB² in the magnetic field range B investigated (up to 5 T), except at very low temperatures (T<4 K).     

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.     

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.     

Comparison between flexural and uniaxial compression tests to measure the elastic modulus of silica aerogel

The Young’s moduli of a set of silica aerogels have been measured by two techniques: 3-point bending and uniaxial compression. The data found by the two methods differ strongly. The uniaxial compression test gives generally underestimated values of Young’s modulus, because of geometrical effects. The appropriate gauge lengths were estimated based on the discussion of Euler buckling and nonuniform stress distribution. The measured compressive moduli were analyzed to correct for machine compliance and possible misalignment under compression of the aerogels. Similarly, moduli obtained by 3-point bending depend on the length/thickness ratio of the sample, reaching equilibrium only for ratios above about 10. The corrected compressive moduli were comparable to those measured by 3-point bending on samples of sufficient length.     

SiO2气凝胶纳米多孔结构的自组装过程模拟

基于分子动力学理论,模拟并计算了纳米多孔SiO2气凝胶的原子尺度模型和力学性能.SiO2气凝胶网络结构的自组装形成过程表明,当密度为0.078 g/cm3时,形成的结构以纳米团簇为主,难以形成连通的骨架结构;当密度为0.172 g/cm3及以上时,硅氧元素分布已扩展形成了连通的无定形骨架结构.通过对不同密度体系模型单轴施加应变并计算相应的应力值,得到应力-应变关系曲线,并依据弹性范围求得弹性模量.模拟结果表明,弹性模量与密度成一次线性关系,当气凝胶密度在0.078～0.443 g/cm3时,弹性模量为0.1265～0.7889 MPa.     

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.     

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.     

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.     

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.     

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.     

High pressure elastic and plastic deformations of silica: In situ diamond anvil cell Raman experiments

Normal silica glass is usually referred to as low density amorphous silica as it can be converted to high density amorphous silica by a hydrostatic pressure (polyamorphic transition). In this work in situ Raman experiments are performed in a diamond anvil cell up to 18 GPa. The pressure effects on the structure of silica after successive compression decompression experiments are analyzed. The mode Grüneisen parameters corresponding to the elastic compression of high density amorphous silica are obtained and compared with those of normal silica. A reorganization of the high density amorphous silica below 3 GPa is evidenced.     

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.     

The preparation of carbon aerogels based upon the gelation of resorcinol-furfural in isopropanol with organic base catalyst

Carbon aerogels with high BET surface area were developed by sol-gel polycondensation of resorcinol and furfural in isopropanol using hexamethylenetetramine (HMTA) as a catalyst, and then directly drying the organic gels under isopropanol supercritical conditions, followed by carbonization under a nitrogen atmosphere. The preparation conditions of carbon aerogels were explored by changing the mole ratio of resorcinol to basic catalyst HMTA (R/C), the ratio of resorcinol to isopropanol (R/I), and the mole ratio of resorcinol to furfural (R/F). The effect of these preparation conditions on the porous structure of the carbon aerogels obtained was studied by nitrogen adsorption isotherms. According to the characterizations of TEM, SEM and nitrogen adsorption, the carbon aerogels obtained have a three-dimensional network that consists of carbon nano-particles with size from 20 to 30 nm, which define numerous micropores, mesopores and macropores. HMTA reacts not only as a catalyst but also as a reagent in the gelation polymerization. XRD characterization indicates that carbon aerogels have disordered nanocrystalline structures similar to activated carbon.     

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.