Mechanical Properties Of Silica Aerogels Research Articles

Improvement of the Mechanical Properties of Silica Aerogels for Thermal Insulation Applications through a Combination of Aramid Nanofibres and Microfibres.

Reinforcement of silica aerogels, remarkable lightweight mesoporous materials with outstanding insulation performance, is still a challenging research topic. Among the strategies used to overcome their brittleness, one of the most effective is the manufacturing of aerogel composites with embedded fibres. In this work, the incorporation of nanofibres together with microfibres in a tetraethoxysilane-vinyltrimethoxysilane matrix is investigated for the first time for the development of novel aerogel nanocomposites. The nanofibres, synthesized from different aramid fibres, including Kevlar® pulp, Technora®, Teijinconex® and Twaron® fibres, were used in different combinations with microaramids and the resulting nanocomposites were thoroughly investigated for their physicochemical and thermomechanical features. The properties depended on the type and amount of the nano/microfibre used. While the microfibres exhibited low interaction with the silica matrix, the higher surface of the nanofibres ensured increased contact with the gel matrix. A low bulk density of 161 kg m-3 and thermal conductivity of 38.3 mW m-1 K-1 (Hot Disk®) was achieved when combining the nanofibres obtained from Kevlar® pulp with the Technora® or Teijinconex® long fibres. The nanofibres showed higher dispersion and random orientation and in combination with microfibres led to the improvement by a factor of three regarding the mechanical properties of the aerogel nanocomposites reinforced only with microfibres. The scale-up process of the samples and simulated tests of thermal cycling and vacuum outgassing successfully conducted indicate good compliance with space applications.

High-temperature heat transfer simulation and optimization of quartz fiber-aerogel composite multilayer 3D fabric

Quartz fabrics, which have excellent mechanical properties and high temperature resistance abilities, had been widely applicated in aerospace, military, and other thermal insulation fields. However, single-layer quartz fabric has poor thermal insulation performance, which is unfavorable to the high temperature application of this material. In order to improve the thermal insulation effect of quartz fiber fabric, silica aerogel materials with outstanding thermal insulation performance were introduced to the fabrication of quartz fabrics. Thermal insulation properties of quartz fiber and mechanical properties of silica aerogel were improved by a 3D braiding technique. Firstly, a 3D-fabric structure of quartz fiber-aerogel multilayer composite was designed. Silica aerogel and quartz yarn were woven into 3D composite fabric by textile technique. Through 3D braiding technique, silica aerogels were protected by quartz yarns while enhancing their mechanical properties. The thermal insulation performance and high temperature erosion resistance of the fiber-aerogel fabric are improved. Secondly, to explore the influence of the internal structure of the 3D fiber-aerogel composite fabric, a full-scale 3D simulation model for the heat transfer performance of fiber-aerogel multilayer composite was established. The thermal insulation performance of the composite fabric at 700 °C was predicted. Finally, the temperature of different positions inside the fabric in different environments (300 °C, 400 °C, 500 °C, 600 °C, 700 °C) was tested. It has been proved that introducing aerogel materials to the quartz fabric could reduce the surface temperature by about 60%.

Robust Silica-Agarose Composite Aerogels with Interpenetrating Network Structure by In Situ Sol-Gel Process.

Aerogels are three-dimensional nanoporous materials with outstanding properties, especially great thermal insulation. Nevertheless, their extremely high brittleness restricts their practical application. Recently, although the mechanical properties of silica aerogels have been improved by regulating the precursor or introducing a polymer reinforcer, these preparation processes are usually tedious and time-consuming. The purpose of this study was to simplify the preparation process of these composite aerogels. A silicic acid solution treated with cation exchange resin was mixed with agarose (AG) to gel in situ, and then composite aerogels (CAs) with an interpenetrating network (IPN) structure were obtained by aging and supercritical CO2 fluid (SCF) drying. Compared to previous works, the presented CAs preparation process is briefer and more environmentally friendly. Moreover, the CAs exhibit a high specific surface area (420.5 m2/g), low thermal conductivity (28.9 mW m−1 K−1), excellent thermal insulation properties, and thermal stability. These results show that these CAs can be better used in thermal insulation.

Mechanical properties of silica aerogels modelled by Movable Cellular Automata simulations

In this paper the mechanical properties of silica aerogels are studied by simulations with the usage of the Movable Cellular Automata (MCA) method. In the MCA approach a sample is considered as a set of interacting cellular automata. The procedure to prepare porous samples is based on random spheres method. The resulting structures have densities in the range from 111 to 2200 kg/m3 and porosity up to 95%. The structures of the samples are studied by obtaining the fractal dimension using the FD3 program. The Young's modulus is examined in the simulation of a compression test. The elastic modulus relate to density by a power law characterized by exponent 3.83 ± 0.18 for samples with densities from 111 to 568 kg/m3 and 3.17 ± 0.11 for samples with densities from 350 to 2200 kg/m3. A comparison to previously published data from experiments and comparable computer simulations is included. The results prove that the Movable Cellular Automata method is suitable to investigate the mechanical properties of silica aerogels.

Modeling and Simulation of the Aggregation and the Structural and Mechanical Properties of Silica Aerogels.

Mechanical properties of aerogels are controlled by the connectivity of their network. In this paper, in order to study these properties, computational models of silica aerogels with different morphological entities have been generated by means of the diffusion-limited cluster-cluster aggregation (DLCA) algorithm. New insights into the influence of the model parameters on the generated aerogel structures and on the finite deformation under mechanical loads are provided. First, the structural and fractal properties of the modeled aerogels are investigated. The dependence of morphological properties such as the particle radius and density on these properties is studied. The results are correlated with experimental small-angle X-ray scattering (SAXS) data of a silica aerogel. The DLCA models of silica aerogels are analyzed for their mechanical properties with finite element simulations. There, the aerogel particles are modeled as nodes and the interparticle bonds as beam elements to account for bond stretching, bending, and torsion. The scaling relation between the elastic moduli E and relative density ρ, E ∝ ρm, is investigated and the exponent m = 3.61 is determined. Backbone paths evidently appear in the 3-d network structure under deformation, while the majority of the bonds in the network do not bear loads. The sensitivity of particle neck-sizes on the mechanical properties is also studied. All the results are shown to be qualitatively as well as quantitatively in agreement with the experimental data or with the available literature.

Techniques for characterizing the mechanical properties of aerogels

In this paper, we present the different characterization techniques used to measure the mechanical properties of silica aerogels. The mechanical behaviour of aerogels is generally described in terms of elastic and fragile materials (such as glasses or ceramics) but also in terms of plastic media in compression testing. Because of these very different mechanical behaviors, several types of characterization techniques are proposed in the literature. We first describe the dynamic characterization techniques such as ultrasounds, Brillouin scattering, dynamic mechanical analysis (DMA) to measure the elastic properties: Young's modulus (E) , shear modulus (G), poisson ratio (υ) but also attenuation and internal friction. Thanks to static techniques such as three-point bending, uniaxial compression, compression we also access to the elastic modulus (E) and to the rupture strength (σ). The experimental results show that the value of the elastic and fracture moduli measured is several orders of magnitude lower than that of a material without porosity. With regard to the brittleness characteristics, Weibull's analysis is used to show the statistical nature of the fracture resistance. We also present the SENB (single edge notched beam technique) technique to characterize toughness (K1C) and the stress corrosion mechanisms, which are studied in ambient conditions and temperature by the double-cleavage drilled compression experiment (DCDC). In the last part of the paper, we show how, during the isostatic compression test, aerogels behave like plastic materials.The data allow calculating the bulk modulus (K), the amplitude of the plastic deformation and the yield strength (σel), which is the boundary between the elastic and plastic domains. These different techniques allow understanding which parameters influence the overall mechanical behavior of aerogels, such as pore volume, but also pore size, internal connectivity and silanol bounds content. It is shown that pore size plays a very important role; pores can be considered as flaws in the terms of fracture mechanics.

Durability of silica aerogel cementitious composites – freeze–thaw resistance, water resistance and drying shrinkage

Owing to its exceptional physical properties, silica aerogel is regarded as an attractive candidate material for building insulation applications; however, the widespread adoption of silica aerogel is limited by its inherently brittle nature and related sub-micrometre dust release. In this paper, the poor mechanical properties of silica aerogel were improved by combining the silica phase and cementitious materials to reach engineering strong composites and maintain relatively low thermal conductivity. The mechanical properties, durability (freeze–thaw resistance, water resistance and drying shrinkage) and thermal conductivity of the silica aerogel–cement composites were comprehensively studied. The composites show improved mechanical properties as compared to pure silica aerogel, but this decreases with the increase of silica aerogel content. With respect to the durability, surprisingly, the silica aerogel can efficiently improve the frost and water resistance of the composite, which might be benefited by the strong hydrophobicity of the silica aerogel. The optimal performance of the composite is observed at 60 vol.% aerogel loading. The thermal conductivity, compressive strength, flexural strength and bonding force are 0·065 W/(m/K), 3·81 MPa, 2·23 MPa and 0·27 MPa, respectively, and the drying shrinkage of the composite is only 0·12% after 90 d.

A novel constitutive model for the mechanical properties of silica aerogels

Most studies on the mechanical properties of silica aerogels neglected the influence of structural randomness and dead-ends of nano-porous materials, whereas both of them have a considerable effect on the mechanical properties of silica aerogels. In this study, we propose a cubic frame model with random orientation as the representative volume element of a silica aerogel to investigate the dead-ends' effect on its mechanical properties. To characterize the structural randomness in open-cell porous materials, a new constitutive model is established by a statistical average of the mechanical response of the random structure. This constitutive model reveals the inherent isotropy of open-cell porous materials. It also indicates that the widely used Gibson-Ashby model may overestimate the Young's modulus and shear modulus for open-cell porous materials. Then the mass ratio of dead-ends and effective skeleton is derived using the available experimental results. It shows that the mass ratio of dead-ends increases with the decreasing density of silica aerogels because of the additional space resulting from the slimmer effective skeleton, which results in the abnormal exponent of the Young's modulus-density power-law relation. We suggest the mass ratio of the effective skeleton to be an intuitive and practical index to reflect the connectivity in silica aerogels. Besides, the dead-ends can significantly enhance the compressive strength of silica aerogels, especially for low-density silica aerogels. We hope this work can provide a new idea to consider the structural randomness in porous materials and give guidance on the mechanical reinforcement of aerogels.

A Hybrid Electric and Thermal Solar Receiver

Summary Solar energy offers a promising renewable energy source; however, it is expensive to store electricity from photovoltaics (PV), the most widely deployed solar electricity technology. Solar thermal energy technologies can be paired with inexpensive thermal storage, but are more expensive overall. We have developed a solar receiver that combines PV and solar thermal systems to efficiently convert solar radiation to electricity (to be used immediately) and thermal energy (to be stored and converted to electricity on demand). This paper describes the Hybrid Electric And Thermal Solar (HEATS) receiver and models its performance. An idealized model predicts high solar-to-electricity efficiency (35.2%) with high dispatchability (44.2% of electricity from thermal energy) at an operating temperature of 775 K. Modeling using measured performance values for HEATS subcomponents predicts 26.8% efficiency and 81% dispatchability with silicon PV and 28.5% efficiency and 76% dispatchability with gallium arsenide PV, both operating at 700 K.

Simulation and Analysis of Mechanical Properties of Silica Aerogels: From Rationalization to Prediction.

Silica aerogels are highly porous 3D nanostructures and have exhibited excellent physio-chemical properties. Although silica aerogels have broad potential in many fields, the poor mechanical properties greatly limit further applications. In this study, we have applied the finite volume method (FVM) method to calculate the mechanical properties of silica aerogels with different geometric properties such as particle size, pore size, ligament diameter, etc. The FVM simulation results show that a power law correlation existing between relative density and mechanical properties (elastic modulus and yield stress) of silica aerogels, which are consistent with experimental and literature studies. In addition, depending on the relative densities, different strategies are proposed in order to synthesize silica aerogels with better mechanical performance by adjusting the distribution of pore size and ligament diameter of aerogels. Finally, the results suggest that it is possible to synthesize silica aerogels with ultra-low density as well as high strength and stiffness as long as the textural features are well controlled. It is believed that the FVM simulation methodology could be a valuable tool to study mechanical performance of silica aerogel based materials in the future.

Elasticity and strength of silica aerogels: A molecular dynamics study on large volumes

The nanoporous structure and the mechanical properties of silica aerogels are studied by molecular dynamics simulations on large samples. For the first time, atomistic simulations are able to reproduce a pore size distribution, centered on 10 nm, comparable to experimental values. Using an unprecedented combination for silica aerogels of large volumes, large strains, and relatively small strain-rates, direct evidence of their peculiar mechanical behavior at the nanoscale is provided, from elasticity to fracture. The surface stress that silica ligaments experience produces a significant tension-compression asymmetry and an unusual discontinuity in the Poisson effect. The ductility of highly porous silica aerogels arises from an interplay between surface stress and a significant amount of reorganization in ligaments. Taking advantage of the large volumes accessible here, the very heterogeneous nature of low-density silica aerogels is reproduced, with an impact both on their elasticity and on their strength. In particular, a clear dependence between tensile strength and sample volume is uncovered, which opens perspectives for the elaboration of multi-scale models.

Mechanics of Nanostructured Porous Silica Aerogel Resulting from Molecular Dynamics Simulations.

Silica aerogels are nanostructured, highly porous solids which have, compared to other soft materials, special mechanical properties, such as extremely low densities. In the present work, the mechanical properties of silica aerogels have been studied with molecular dynamics (MD) simulations. The aerogel model of 192 000 atoms was created with different densities by direct expansion of β-cristobalite and subjected to series of thermal treatments. Because of the high number of atoms and improved modeling procedure, the proposed model was more stable and showed significant improvement in the smoothness of the resulting stress-strain curves in comparison to previous models. Resulting Poisson's ratio values for silica aerogels lie between 0.18 and 0.21. The elasticity moduli display a power law dependence on the density, with the exponent estimated to be 3.25 ± 0.1. These results are in excellent agreement with reported experimental as well as computational values. Two different deformation scenarios have been discussed. Under tension, the low-density aerogels were more ductile while the denser ones behaved rather brittle. In the compression simulations of low-density aerogels, deformation occurred without significant increase in stress. However, for high densities, atoms offer a higher resistance to the deformation, resulting in a more stiff response and an early densification. The relationship between different mechanical parameters has been found in the cyclic loading simulations of silica aerogels with different densities. The residual strain grows linearly with the applied strain (≥0.16) and can be approximated by a phenomenological relation ϵp = 1.09ϵmax - 0.12. The dissipation energy also varies with the compressive strain according to a power law with an exponent of 2.31 ± 0.07. Moreover, the tangent modulus under cyclic loading varies exponentially with the compressive strain. The results of the study pave the way toward multiscale modeling of silica as well as reinforced silica aerogels.

Flexible silica aerogel based on methyltrimethoxysilane with improved mechanical property

Methyltrimethoxysilane (MTMS) based silica aerogels with excellent mechanical properties have been synthesized through sol-gel process under ambient pressure drying. The effect of water/ethanol mixed solvent system on the mechanical properties of silica aerogels was studied. The surfactant was used to promote the mixing of the MTMS-derived species and water in solvent. Low ethanol in the solvent system helps to form nano-scale and uniformly distributed pore and particle structure. And the increasing EtOH caused large clusters to microns in size. Surface modification makes sure that more methyl groups attached on the particle surface which repels each other when pressed. Low EtOH in the solvent system and surface modification results in higher reversible shrinkage and Young's Modulus.

Determination of dynamic mechanical properties of silica aerogel by resonance and non-resonance method

Silica aerogel has great potential in aeronautics, building construction, and offshore oil transportation. The dynamic mechanical properties of silica aerogel were measured by the resonance and non-resonance method. The results of the two methods were compared. While the Young’s modulus coincided well, the loss factor had big difference. To explain this phenomenon, the sensitivity of the two methods was analyzed. It shows that the loss factor determination of low-loss materials by the non-resonance method is quite sensitive to the experiment uncertainties. This probably results in the loss factor difference between the two methods.

Preparation and characterization of silica aerogels from diatomite via ambient pressure drying

The silica aerogels were successfully fabricated under ambient pressure from diatomite. The influence of different dilution ratios of diatomite filtrate on physical properties of aerogels were studied. The microstructure, surface functional groups, thermal stability, morphology and mechanical properties of silica aerogels based on diatomite were investigated by BET adsorption, FT-IR, DTA-TG, FESEM, TEM, and nanoindentation methods. The results indicate that the filtrate diluted with distilled water in a proportion of 1: 2 could give silica aerogels in the largest size with highest transparency. The obtained aerogels with density of 0.122–0.203 g/m3 and specific surface area of 655.5–790.7 m2/g are crack free amorphous solids and exhibited a sponge-like structure. Moreover, the peak pore size resided at 9 nm. The initial aerogels were hydrophobic, when being heat-treated around 400°C, the aerogels were transformed into hydrophilic ones. The obtained aerogel has good mechanical properties.

Mechanical properties of silica aerogels prepared from a mixture of TEOS and organo-alkoxysilanes of type R1SiX3

Silica aerogels were prepared from a mixture of tetraethylorthosilicate and organoalkoxysilanes. The effects of organo-alkoxysilanes on the mechanical properties of the silica aerogels were studied. The flexibility of silica aerogels was significantly improved by incorporation of organo-alkoxysilanes. When MTES and TEOS were combined as precursors of silica areogels, with the increased amount of MTES, the apparent elastic modulus and apparent compressive strength monotonously rose. At the same organoalkoxysilanes to TEOS ratio, the size of alkyl groups of the organo-alkoxysilanes had little effect on the mechanical properties. In series of MTES and TEOS, the lowest elastic modulus of silica skeleton and the highest compressive strength of silica skeleton were observed at MTES to TEOS ratio of around 50:50. At a certain organo-alkoxysilanes to TEOS ratio, the elastic modulus of silica skeleton increased and the compressive strength of silica skeleton decreased with the size increase of the alkyl groups.

Effects of processing conditions on silica aerogel during aging: Role of solvent, time and temperature

Effective parameters that enhanced mechanical properties of silica aerogel during aging were investigated. Silica aerogel was made by tetraethyl orthosilicate (TEOS), water, methanol and ammonium fluoride (NH4F) as catalyst using a one-step method. Aging on the samples were performed under three different solvents including n-hexane, methanol and water for different time periods and also different temperatures. Subsequently, solvent exchange with n-hexane, modification under trimethyl chlorosilane (TMCS) solution and the ambient pressure drying (APD) were applied on all samples. The Fourier transform infrared spectroscopy (FTIR), Brunauer, Emmett and Teller (BET) method and compression tests were carried out on the produced samples. Both the compression strength and compression modulus of the gel increased with increasing of the time and temperature of aging. Those samples aged under water showed higher mechanical properties. On the contrary, the specific compression strength and modulus of samples aged in water were declined drastically.

Investigation of the Bulk Modulus of Silica Aerogel Using Molecular Dynamics Simulations of a Coarse-Grained Model

Structural and mechanical properties of silica aerogels are studied using a flexible coarse-grained model and a variety of simulation techniques. The model, introduced in a previous study (J. Phys. Chem. C 2007, 111, 15792-15802), consists of spherical "primary" gel particles that interact through weak nonbonded forces and through microscopically motivated interparticle bonds that may break and form during the simulations. Aerogel models are prepared using a three-stage protocol consisting of separate simulations of gelation, aging, and a final relaxation during which no further bond formation is permitted. Models of varying particle size, density, and size dispersity are considered. These are characterized in terms of fractal dimensions and pore size distributions, and generally good agreement with experimental data is obtained for these metrics. The bulk moduli of these materials are studied in detail. Two different techniques for obtaining the bulk modulus are considered, fluctuation analysis and direct compression/expansion simulations. We find that the fluctuation result can be subject to systematic error due to coupling with the simulation barostat but, if performed carefully, yields results equivalent with those of compression/expansion experiments. The dependence of the bulk modulus on density follows a power law with an exponent between 3.00 and 3.15, in agreement with reported experimental results. The best correlate for the bulk modulus appears to be the volumetric bond density, on which there is also a power law dependence. Polydisperse models exhibit lower bulk moduli than comparable monodisperse models, which is due to lower bond densities in the polydisperse materials.

Hydrophobic Flexible Silica Aerogels Felts Fabricated by Ambient Pressure Drying

Hydrophobic flexible silica aerogels felts were fabricated successfully by two-steps sol-gel process via ambient pressure drying. First of all, the sol with various pH values was obtained from tetraethylorthosilicate (TEOS) as silicon source, hydrochloric acid/aqueous ammonia ethanol solution as catalysts. Then glass fiber felts was incorporated into the sol to increase the mechanical properties of silica aerogel. After the completion of solvent exchange and surface modification by using trimethylchlorosilane (TMCS)/n-hexane solution, the gel felts were dried under ambient pressure. The samples show excellent hydrophobic properties.

Effect of the thermal treatment on microstructure and physical properties of low-density and high transparency silica aerogels via acetonitrile supercritical drying

In this paper, we reported the experimental results about the effect of the thermal treatment on microstructure and physical properties of low-density and high transparent silica aerogels. From our results, with tetramethyl orthosilicate as precursor and via acetonitrile supercritical drying process, silica aerogel monolith was obtained possessing the properties as low-density (0.018 g/cm3), high surface area (923 m2/g), high optical transparency (87.9 %, 800 nm). It should be noted that high transparency of silica aerogel can be maintained up to 600 °C (91.5 %, 800 nm). The mechanical properties of silica aerogel decreased with increasing heat treated temperature to 600 °C, and silica aerogels still maintained crack-free monoliths completely and possessed high homogeneous density even after 600 °C thermal treatment. Furthermore, thermal conductivity of the monoliths at desired temperatures was analyzed by the transient plane heat source method. When the temperature flowed from 25 to 600 °C, thermal conductivity coefficients of silica aerogels changed from 0.021 to 0.065 W (m K)−1, revealed an excellent heat insulation effect in high-temperature area. Currently, the specific process developed for low-density aerogels affected by thermal treatment has not been reported in previous literature.