Native Silica Aerogels Research Articles

Mechanical properties of silica aerogels controlled by synthesis parameters

Development of silica aerogels for real applications requires control of their mechanical properties. Based on the sol-gel procedure and using the alkoxide precursor of silica (TMOS), native silica aerogels were synthesized. This work explores the influence of three synthesis parameters, namely, TMOS concentration, catalyst concentration, and solvents (MeOH, EtOH, IPA), on the gel point, shrinkage and mechanical properties of aerogels. Structural studies were conducted to determine how the analysed synthesis parameters influenced the spatial network of aerogels and their elasticity. The analysis of the relationship between Young's modulus (E) and bulk density (ρ), E∝ρα, showed that the catalyst concentration appeared the most interesting parameter because, while the ρ did not change with an increase in the catalyst concentration, there was a significant increase in the E. We associate this observation with an increase in specific surface area, reflecting an increase in bond diameter between secondary particles formed in the aerogel.

Improved Polydopamine Deposition in Amine-Functionalized Silica Aerogels for Enhanced UV Absorption.

Silica aerogels are interesting porous materials with extremely low density and high surface area, making them advantageous for a number of aerospace and catalysis applications. Here, we report the preparation of polydopamine (PDA)-functionalized silica aerogels using an in situ coating method, wherein the dopamine monomer was allowed to diffuse through the underlying structure of the gels in the absence of any external base and polymerize on the surface of the gel. The use of a siloxane precursor with an amine functionality decorates the silica backbone, allowing for a superior PDA coating, as evident in the darker color of PDA-coated amine-functionalized silica gels than PDA-coated silica-only gels and the X-ray photoelectron spectroscopy results. Furthermore, by varying the coating time, a series of aerogels with increasing optical absorption are prepared. Analyses using Brunauer-Emmett-Teller, scanning electron microscopy, and pycnometry show that the in situ PDA coating does not affect the inherent properties of the silica aerogels as opposed to PDA coatings deposited using an external base. Aerogels coated for 12 h and 24 h offer a surface area of 614 ± 35 and 658 ± 15 m2/g along with a porosity of 92.6 ± 0.9 and 92.4 ± 0.7%, respectively, properties similar to the native silica aerogels. PDA-coated aerogels have the potential to serve as UV ray mitigating materials due to the tortuosity of the underlying structure and the unique chemical properties of the PDA coating.

Molecular dynamics simulations of silica aerogel nanocomposites reinforced by glass fibers, graphene sheets and carbon nanotubes: A comparison study on mechanical properties

Silica aerogel is a brittle as well as a low-tensile strength highly porous solid. Therefore, the primary goal of the present work is to incorporate glass fibers, graphene sheets and carbon nanotubes into the silica aerogel matrix using molecular dynamics (MD) simulations to improve not only the mechanical properties, e.g., tensile strength, elastic modulus, and toughness, but also the deformation characteristics due to the pre-existing cracks. This work investigates and compares the mechanical properties obtained from tension and compression tests of native silica aerogels and their nanocomposites. The results show that the axial and lateral direction mechanical properties of carbon-based nanocomposites are significantly higher than the native and glass fibers reinforced silica aerogels. For example, in the axial loading, the elastic modulus of the proposed glass fibers, graphene sheets, and carbon nanotubes reinforced nanocomposites were ∼3.5, ∼9.5 and ∼11.5 times higher than the native silica aerogels, respectively. Moreover, the influence of crack depth has been studied in the tensile fracture simulations, and we found that in nanocomposites, the tensile load bearing candidates are mainly reinforced materials. The outcome of this work is a vital step in the process of designing advanced nanocomposites.

Composites of silica aerogels with organics: a review of synthesis and mechanical properties

Aerogels are considered as outstanding future materials owing to their wide surface area and three-dimensional network of silica particles, low density, low-thermal conductivity, high porosity, and low dielectric property. Their outstanding characteristics represent excellent potential applications in thermal insulation systems, aeronautical domains, environmental clean-up and protection, as heat storage devices, transparent windows, thickening agents in paints, etc. Among these applications, thermal insulating materials can play a vital role in living systems and for saving energy in various domestic and industrial processes. However, native silica aerogels are fragile and sensitive to relatively low pressures, which limit their application. More robust aerogels with higher strength and stiffness can be obtained by compounding silica networks with organoalkoxysilanes, polymers or using porous scaffolds as supports. This paper presents a review on the approaches for mechanical reinforcing methods for silica aerogels and recent achievements toward improving the strength of native silica aerogels. In addition, various characteristics derived from composite aerogels are analyzed synthetically.

Nanoindentation of Graphene-Reinforced Silica Aerogel: A Molecular Dynamics Study.

In the present work, we performed nanoindentation tests using molecular dynamics (MD) simulations on graphene, native silica aerogels, and single- and multi-layered graphene-reinforced silica aerogel nanocomposites. This work mainly focused on the two aspects of nanoindentation simulations: first, the resultant indentation force–depth curves, and second, the associated mechanical deformation behavior. We found that in the single-layer graphene-reinforced silica aerogel nanocomposite, the indentation resistance was four-fold that of native silica aerogels. Moreover, the combined system proved to be higher in stiffness compared to the individual material. Furthermore, the indentation resistance was increased significantly as we proceeded from single- to two-layered graphene-reinforced silica aerogel nanocomposites. The results of the study provide a detailed understanding of the mechanical behavior during the indentation tests of nanocomposites, which helps to design advanced nanoscale multi-layered materials.

Facile fabrication of superhydrophobic, mechanically strong multifunctional silica-based aerogels at benign temperature

We first reported the simultaneous in-situ amine-modification and hydrophobization of a native silica aerogel at the benign temperature of 0°C for further isocyanate cross-linking, compared with the traditional method at the harsh temperature of −77°C. The hydrolysis of tetraethylorthosilicate (TEOS) was slower than that of tetramethoxysilane (TMOS) and each methyltrimethoxysilane (MTMS) monomer had one non-hydrolyzable methyl group. The hydrophobicity of the native silica aerogels could be adjusted by tunning the MTMS/TEOS molar ratio. The 3-aminopropyltriethoxysilane (APTES) was used to provide a flexible linking group in the silica structure, while the amine group in APTES was further used as a reactive site for reacting with the isocyanate. As a result, a lightweight, mechanically strong and porous silica-based composite material was obtained by encapsulating the skeletal framework of amine-modified silica aerogels with isocyanate.

Fabrication of native silica, cross-linked, and hybrid aerogel monoliths with customized geometries

In this work, a previously developed (White et al 2015 J. Mater. Chem. A 3 762) rapid synthesis approach is used to fabricate native and cross-linked aerogel monoliths with customized geometries. This technique does not require solvent exchange, therefore fabrication times do not depend on part size. To prove this, parts with a smallest dimension of approximately 3.6 cm were fabricated within the same time scale as that of small cylinders with a diameter of 7 mm. In addition, monoliths with customized geometries exhibiting physical detail on the order of 1 mm were produced to demonstrate the versatility of this technique. Furthermore, hybrid materials consisting of native silica aerogel integrated with selected regions of polymer cross-linking were produced. The cross-linked regions allow for adhesion to other surfaces or labeling while the majority of the material retains the physical characteristics of a native silica aerogel. The physical and thermal properties of all aerogel components were examined. All aerogel materials produced in this work exhibited characteristics that were within the range of aerogel materials produced using more conventional methods. For native silica materials, this includes densities in the range of 0.03–0.116 g cm−3, surface areas between 342–799 m2 g−1, mode pore sizes in the range of 30–39 nm, and thermal conductivities in the range of 0.020–0.026 W m−1 K−1. For cross-linked aerogel materials, densities ranged between 0.154–0.340 g cm−3, surface areas were between 291–388 m2 g−1, mode pore sizes were in the range of 29–41 nm, and thermal conductivities were in range of 0.038–0.066 W m−1 K−1.

A New Approach for Synthesizing the Hybrid Silica Aerogels

Uniform Polyurethane/silica hybrid aerogels were achieved at ambient pressure via an approach in which an aqueous polyurethane dispersion was used and the hybridization step carried out before gelation step. The FT-IR spectra confirmed the incorporation of polymer chains into the silica network and hybrid network formation.As the H2O/TEOS molar ratio increased after hybridization in this method, the effect of H2O/TEOS molar ratio changes and PU content on the density of hybrid aerogels was investigated separately. The density of native silica aerogels decreases commonly with increasing the water content while increasing the aqueous PU dispersion content increases the density of hybrid aerogels. The density difference between the hybrid aerogel sample with a certain aqueous PU dispersion and the similar native silica aerogel with the same H2O/TEOS molar ratio demonstrated the intensive polyurethane chain effect on the aerogel density. The results showed that the simultaneous increase of H2O/TEOS molar ratio and PU content could be led to the uniform structure of hybrid aerogels. However, the effect of polyurethane on the aerogel density was more sensitive than H2O/TEOS molar ratio effect especially in high polyurethane content (≥10wt.%) that increased the density of hybrid aerogels.

Effect of polymer molecular weight and deposition temperature on the properties of silica aerogel/hydroxy-terminated poly(dimethylsiloxane) nanocomposites prepared by reactive supercritical deposition

Monolithic nanocomposites of silica aerogels with hydroxy-terminated poly(dimethylsiloxane) (PDMS(OH)) were prepared by reactive supercritical deposition technique. The depositions were performed by using PDMS(OH) having two different molecular weights (Mn=2750g/mol and 18,000g/mol) and at three different temperatures (313.2K, 323.2K, 333.2K) and the effects of deposition temperature and polymer molecular weight on the properties of nanocomposites were investigated. The polymer uptake of the nanocomposites was found to increase with increasing deposition temperature indicating faster reaction rates at higher temperatures. PDMS(OH) molecules with lower molecular weight were homogenously distributed throughout the cylindrical composites. On the other hand, the samples that were deposited with high molecular weight PDMS(OH) were not homogenous with a higher polymer concentration near the surface than at the center. The pore volumes and BET surface areas of the nanocomposites decreased upon deposition of the polymer. The reductions in pore volumes were higher by a factor of two than the volume of the deposited polymer indicative of blocking of pores. Moreover, the compressive modulus of the nanocomposite was found to be more than three times greater than the compressive modulus of the native silica aerogel.

Synthesis, pore structure and properties of polyurethane/silica hybrid aerogels dried at ambient pressure

Polyurethane/silica hybrid aerogels were synthesized at ambient pressure drying conditions with the specific surface area, pore volume and pore diameter in the range of 530–850m2/g, 0.71–2.83cm3/g, and 5–13nm, respectively. TGA results showed that almost all polyurethane chains preserved as a part of gel network. The pore structure and properties of these hybrid aerogels showed an interesting dependence on the polyurethane content. The improved structure and properties compared to native silica aerogel was achieved with introducing 3v/v% aqueous polyurethane dispersion into silica sol. High polyurethane content led to low porosity (61%) and small surface area (535m2/g).

An overview on silica aerogels synthesis and different mechanical reinforcing strategies

Silica aerogels are lightweight and highly porous materials, with a three-dimensional network of silica particles, which are obtained by extracting the liquid phase of silica gels under supercritical conditions. Due to their outstanding characteristics, such as extremely low thermal conductivity, low density, high porosity and high specific surface area, they have found excellent potential application for thermal insulation systems in aeronautical/aerospace and earthly domains, for environment clean up and protection, heat storage devices, transparent windows systems, thickening agents in paints, etc. However, native silica aerogels are fragile and sensitive at relatively low stresses, which limit their application. More durable aerogels, with higher strength and stiffness, can be obtained by proper selection of the silane precursors, and constructing the silica inorganic networks by compounding them with different organic polymers or different fiber networks. Recent studies showed that adding flexible organic polymers to the hydroxyl groups on the silica gel surface would be an effective mechanical reinforcing method of silica aerogels. More versatile polymer reinforcement approach can be readily achieved if proper functional groups are introduced on the surface of silica aerogels and then co-polymerized with appropriate organic monomers. The mechanical reinforced silica aerogels, with their very open texture, can be an outstanding thermal insulator material for different industrial and aerospace applications.This paper presents a review of the literature on the methods for mechanical reinforcing of silica aerogels and discusses the recent achievements in improving the strength and elastic response of native silica aerogels along with cost effectiveness of each methodology.

Regioselective cross-linking of silica aerogels with magnesium silicate ceramics

Nanoporous monoliths with a typical aerogel core and a mechanically robust exterior ceramic layer were synthesized from silica aerogels cross-linked with polyacrylonitrile (X-PAN aerogels). Following supercritical drying, X-PAN aerogels were wrapped in Mg and heated to 850 °C in inert atmosphere to generate Mg-PAN monoliths. Mg penetrated 2–3 mm inside the monolith, reacted with the silica framework and formed a mechanically strong crust with a hardness of 28 MPa, a reduced Young modulus of 374 MPa and a surface area of 170 m2 g−1. The core of the monoliths maintained chemical and physical characteristics close to those of native silica aerogels with a hardness of 0.81 MPa, a modulus of 7.42 MPa and a surface area of 370 m2 g−1. The cross-linking crust was composed of forsterite (Mg2SiO4), enstatite (MgSiO3), cristobalite and magnesium oxide (MgO). SEM and TEM showed that the Mg silicates bridged between silica aggregates, thus realizing the first known example of ceramic cross-linking of aerogels. Control experiments showed that carbon from PAN was responsible for the crust formation. A thin, mechanically weak crust formed in all other samples and was composed of MgO and Mg2SiO4. EELS showed the presence of disordered, graphite-type carbon in Mg-PAN aerogels and traces of amorphous carbon in all other control samples. The role of PAN was confirmed by using masking techniques, whereas acrylonitrile was photopolymerized in selected regions of aerogel monoliths. The crust formed only in these exposed regions. The role of carbon in crust formation is discussed, as well as potential applications of our technique for the fabrication of anisotropic ceramics.

Tailoring Mechanical Properties of Aerogels for Aerospace Applications

Silica aerogels are highly porous solid materials consisting of three-dimensional networks of silica particles and are typically obtained by removing the liquid in silica gels under supercritical conditions. Several unique attributes such as extremely low thermal conductivity and low density make silica aerogels excellent candidates in the quest for thermal insulation materials used in space missions. However, native silica aerogels are fragile at relatively low stresses. More durable aerogels with higher strength and stiffness are obtained by proper selection of silane precursors and by reinforcement with polymers. This paper first presents a brief review of the literature on methods of silica aerogel reinforcement and then discusses our recent activities in improving not only the strength but also the elastic response of polymer-reinforced silica aerogels. Several alkyl-linked bis-silanes were used in promoting flexibility of the silica networks in conjunction with polymer reinforcement by epoxy.

Formation of Polycyanoacrylate−Silica Nanocomposites by Chemical Vapor Deposition of Cyanoacrylates on Aerogels

Here, we report the first CVD reinforcement of silica aerogels using cyanoacrylates to create a silica-polycyanoacrylate nanocomposite. Deposition is fast and homogeneous through monolithic aerogels. The nanocomposite results in a 32× improvement over native silica aerogels while not diminishing the aerogels attractive physical properties.

The compressive behavior of isocyanate-crosslinked silica aerogel at high strain rates

Aerogels are low-density, highly nano-porous materials. Their engineering applications are limited due to their brittleness and hydrophilicity. Recently, a strong lightweight crosslinked silica aerogel has been developed by encapsulating the skeletal framework of amine-modified silica aerogels with polyureas derived by isocyanate. The mesoporous structure of the underlying silica framework is preserved through conformal polymer coating, and the thermal conductivity remains low. Characterization has been conducted on the thermal, physical properties and the mechanical properties under quasi-static loading conditions. In this paper, we present results on the dynamic compressive behavior of the crosslinked silica aerogel (CSA) using a split Hopkinson pressure bar (SHPB). A new tubing pulse shaper was employed to help reach the dynamic stress equilibrium and constant strain rate. The stress-strain relationship was determined at high strain rates within 114–4386 s−1. The effects of strain rate, density, specimen thickness and water absorption on the dynamic behavior of the CSA were investigated through a series of dynamic experiments. The Young’s moduli (or 0.2% offset compressive yield strengths) at a strain rate ∼350 s−1 were determined as 10.96/2.08, 159.5/6.75, 192.2/7.68, 304.6/11.46, 407.0/20.91 and 640.5/30.47 MPa for CSA with densities 0.205, 0.454, 0.492, 0.551, 0.628 and 0.731 g cm−3, respectively. The deformation and failure behaviors of a native silica aerogel with density (0.472 g cm−3), approximately the same as a typical CSA sample were observed with a high speed digital camera. Digital image correlation technique was used to determine the surface strains through a series of images acquired using high speed photography. The relative uniform axial deformation indicated that localized compaction did not occur at a compressive strain level of ∼17%, suggesting most likely failure mechanism at high strain rate to be different from that under quasi-static loading condition. The Poisson’s ratio was determined to be 0.162 in nonlinear regime under high strain rates. CSA samples failed generally by splitting, but were much more ductile than native silica aerogels.

Isocyanate-crosslinked silica aerogel monoliths: preparation and characterization

Polymerization of di- and tri-isocyanates can be templated onto the mesoporous surface of a preformed network of sol–gel-derived silica nanoparticles, resulting in a conformal ‘crosslinked’ coating that renders the interparticle neck zone wider. Upon drying, these crosslinked networks yield aerogels which are up to ∼3× more dense than native aerogels based on the underlying silica framework, but also up to 10× less hygroscopic and they may take more than 300× the force to break. These results have been obtained with one-step based-catalyzed sol–gel silica networks, as well as with gels derived through a two-step process involving an acid-catalyzed sol and a based-catalyzed gel. Furthermore, it has been also found that crosslinking increases the dielectric constant only by ∼35% relative to values reported in the literature for native silica aerogels of about the same porosity. Chemical investigations into the polymerization reaction have shown that the process of crosslinking involves reaction of the isocyanate with: (a) OH groups at the surface of silica to form carbamate; and (b) adsorbed water, to form an amine and carbon dioxide. This amine then reacts with additional isocyanates resulting in polymer chain extension and bridging of particles with urethane-terminated polyurea.