Monolithic Silica Aerogels Research Articles

Isocyanate-crosslinked silica aerogel monolith with low thermal conductivity and much enhanced mechanical properties: Fabrication and analysis of forming mechanisms

The applications of silica aerogels are restricted due to their intrinsic fragile property. Polymerization of di-isocyanates can be templated onto the mesoporous surface of the –NH2 group modified silica clusters, resulting in the conformal crosslinked coating on surface of silica clusters. Aminopropyltriethoxysilane (APTES), as the silica co-precursor and amine group modification agent, is involved containing tetramethyl orthosilicate (TMOS) silica precursor, while hexamethylene diisocyanate (HDI) is incorporated as the polymer crosslinking agent. The effects of different amounts of APTES on the physicochemical properties of the resulting crosslinked aerogels are investigated. The results show that the optimized APTES/TMOS volume ratio can be determined at 0.5:1. The resulting optimal crosslinked silica aerogel possesses large BET specific surface area of 150.9 m2/g, low thermal conductivity of 0.037 W/(m·K), and the Young's modulus is as large as 18 MPa under strain of 4.2%, much higher than that in the previously published works. The polymerization reaction mechanism forming the polyurethane chains has also been proposed. In addition, the interactions between silica clusters and polymer chains are studied by molecular mechanics and molecular dynamics. The interactions are mainly dependent on non-bonding energy, and the electrostatic energy has decisive impact on the combination of silica clusters and polymer chains. The density field of C, H, N, O, and Si elements overlaps with each other, indicating that the polymer crosslinked silica aerogel maintains typical three-dimensional porous structure. The N element enriches in the region between silica clusters, further verifying the formation –CONH–(CH2)6–CONH- polyurethane chains, which is actually responsible for the much enhanced mechanical property.

Development of a New Silica Aerogel-Polypropylene Foam Composite as a Highly Flexible Thermal Insulation Material

A new flexible thermal insulation sheet, a composite of silica aerogel with polypropylene (PP) foam, has been developed. Even though a large volume ratio of silica aerogel (97%) was included, the composite showed high flexibility. Thermal conductivity of the composite was 0.016 W/(m⋅K) at 298 K, which is as low as silica aerogel monolith. Silica aerogel flaking, which has been a problem in practical applications, is very low in this composite due to skin layers of the polymer-foam composite. A supercritical drying process for rolled sheets of the material for large-scale production is also described. Simulations of extraction of 2-propanol from a silica alcogel revealed that rolled composite sheets with small spaces between the sheet composite are able to dry in a reasonable extraction time. These findings led to effective on production of this material on a pilot industrial scale.

A review of recent progress on the silica aerogel monoliths: synthesis, reinforcement, and applications

The silica aerogel monoliths (SAMs) with three-dimensional (3D) porous structures showing unique physical and chemical properties promise a new era of technologies. However, the critical challenge remains in these materials due to their poor mechanical properties in large-scale fabrication. Several attempts have been adopted to enhance the mechanical properties of SAMs to overcome this challenge and explore their applications in various communities. In this review, we introduce the general synthetic methods of SAMs and highlight the use of mechanical reinforcement strategies to improve their properties. Furthermore, the applications of SAMs in killer applications have been introduced, including heat and acoustic insulation, catalysis, and electronic devices. Critically, perspectives on the challenges and opportunities of SAMs are highlighted as potential targets for commercialization.

Aesthetically Enhanced Silica Aerogel Via Incorporation of Laser Etching and Dyes.

A procedure for aesthetically enhancing silica aerogel monoliths by laser etching and incorporation of dyes is described in this manuscript. Using a rapid supercritical extraction method, large silica aerogel monolith (10 cm x 11 cm x 1.5 cm) can be fabricated in about 10 h. Dyes incorporated into the precursor mixture result in yellow-, pink- and orange-tinged aerogels. Text, patterns, and images can be etched onto the surface (or surfaces) of the aerogel monolith without damaging the bulk structure. The laser engraver can be used to cut shapes from the aerogel and form colorful mosaics.

Effect of sodium bicarbonate solution on methyltrimethoxysilane-derived silica aerogels dried at ambient pressure

Here we present an economical ambient pressure drying method of preparing monolithic silica aerogels from methyltrimethoxysilane precursor while using sodium bicarbonate solution as the exchanging solvent. We prepared silica aerogels with a density and a specific surface area of 0.053 g·cm−3 and 423 m2·g−1, respectively. The average pore diameter of silica aerogels is 23 nm as the pore specific volume is 1.11 cm3·g−1. Further, the contact angle between water droplet and the surface of silica aerogels in specific condition can be as high as 166°, which indicates a super-hydrophobic surface of aerogels.

Contactless Visible Luminescence Thermometry Based on β-Phase Zinc Silicate Confined in Silica Glass Matrix

Nanophosphors zinc silicate β-phase in glass matrix with adequate manganese concentration were synthesized by a sol–gel method in three steps. In the first one, a sol-gel process was used for the synthesis of ZnO:Mn nanopowder. In the second one, the elaborated nanopowder was incorporated in silica aerogel monolith using supercritical conditions of ethanol and in the third step, a simple solid-solid reaction under natural atmosphere for 2 h at 1500 °C was performed. The obtained samples are composed by grains with an average size of about 70 nm in a triclinic phase dispersed in silica host matrix. This protocol leads to a very stable 15 at.% Mn doped zinc silicate β-phase at high temperature. The analysis of the obtained nanophosphor material showed a strong broad yellow emission at 584 nm attributed to 4T1(4G) to 6A1(6S) transition. The intensity and the time decay of this yellow emission depend on the measurement temperature. In fact, the decreases rate of the PL intensity is of 12.7% and the rate of lifetime change is of about 2.7%. These results are considered as promising values compared to those obtained until now. These two potential behaviours make it possible to predict the use of this material in various technological applications, particularly in contactless luminescence thermometry.

Thermal Insulation Performance of Monolithic Silica Aerogel with Gas Permeation Effect at Pressure Gradient and Large Temperature Differences

Thermal Insulation Performance of Monolithic Silica Aerogel with Gas Permeation Effect at Pressure Gradient and Large Temperature Differences

Structural, out-gassing and nanomechanical properties of super-hydrophobic transparent silica aerogels developed by ambient pressure drying for space application

Transparent super-hydrophobic monolithic silica aerogels are prepared by the cost-effective ambient pressure drying. The drying of aerogel is performed at various temperatures ranging from 25 to 200°C. Oxidation states of different silica aerogels are investigated by XPS which shows the presence of both SiO and SiO2 phases. Thermal stability of hydrophobic aerogel is found as ~320°C investigated by TGA and DSC. The chemical bonds (i.e., –CH3) responsible for the hydrophobic (WCA > 170°) nature of synthesized silica aerogels are identified by FTIR. Further, out-gassing properties of the silica aerogels have been investigated and they are found in limit for the space application. The highest hardness and Young’s modulus are measured by the nanoindentation technique for the aerogel dried at 120°C which has higher density, while density decreases for the aerogel dried at 200°C and the corresponding nanomechanical properties are found to be lowest as expected.

Estimation of the Surface Free Energy of Hydrophobic Monolithic Silica Aerogels

AbstractAerogels are prepared by quantifying the sol–gel process of the organo‐metallic precursors. The objective of the present article is to synthesize ultralow density, low index of refraction, and monolithic silica aerogels. The quantification of hydrophobicity of the silica aerogels using water drop contact angle is not only crucial but also the estimation of the surface free energy is an important parameter to understand the interfacial behavior of the aerogel surface and liquid. To provide insight on this behavior, calculation of the surface free energy is done by using Neumann's equation of state. These synthesized aerogels emerged with ultralow density 24 kg m−3 and low thermal conductivity ∼0.091 W m−1 K−1.

Engineering mesoporous silica for superior optical and thermal properties

Abstract

Advanced Opacified Fiber-Reinforced Silica-Based Aerogel Composites for Superinsulation of Exhaust Tubing Systems in Semi-Stationary Motors.

Within this study, monolithic three-dimensional silica aerogel (SA) composite parts with super insulating properties are presented. A generic part based on fiber-reinforced (FR) silica aerogel for thermal insulation of the exhaust tubing system—to keep the exhaust gases as hot as possible to improve the efficiency of the catalyst system—was produced via a sol-gel-based molding process in combination with a supercritical drying using scCO2. A thermal conductivity of 16 mW m−1 K−1 was measured via a heat flow meter technique. In this manuscript, we present a full cycle of the material compound design, starting with fundamental material evaluation including aerogel optimization, opacifier influence, and casting process. The obtained generic part in shape of a half-shell for pipe insulation is characterized under real conditions.

Rapid synthesis and characterization of monolithic ambient pressure dried MTMS aerogels in pure water

In this work, monolithic silica aerogels were prepared in pure water within 4 h by using MTMS as the precursor under ambient pressure drying (APD). The effects of the volume ratio of H2O/MTMS, CTAB content and NH3·H2O concentration on the properties of MTMS-based aerogels (MSA) were investigated experimentally. The realization of the rapid synthesis of MSA under APD is attributed to getting rid of the aging process, solvent exchange and surface modification. It demonstrates that the microstructure of MSA is comprised of coralloid branches. Furthermore, the greater volume ratio of H2O /MTMS, larger CTAB content and smaller NH3·H2O concentration cause relatively slimmer coralloid branches, smaller pores and weakened connectivity of skeletons. It also finds that the MSA with a greater volume ratio of H2O/MTMS presents larger Young’s modulus and smaller compressive stress, which is because of the correspondingly different microstructures. The as-prepared MSA shows a low density of 0.064 g/cm3 and a contact angle as high as 143.4°, which provides their potentials for some engineering applications.

Preparation of Silica Aerogels Monoliths from Hydrophobic Silica Gels and Pluronic10R5 via Sol–Gel Process

Silica aerogel, the most common type of aerogels, comprised of 95% air in its structure which made the aerogel has a high surface area, high porosity, low density, and low thermal conductivity. Because of its structure and high porosity, one of its major weakness compared to other materials is being very brittle. This study aims at strengthening the connection points between silica nanoparticles using Pluronic10R5 (poly(propylene oxide)8–poly(ethylene oxide)22–poly(propylene oxide)8) where the Pluronic10R5 was used to reduce phase separation during the silica condensation reaction in the sol–gel process. Silica aerogel monoliths were prepared via a sol–gel process from hydrophobic silica gels and Pluronic10R5 with an ambient pressure drying (APD) process. Results from the compression test showed that the Pluronic10R5/silica aerogels have improved mechanical property by ten times that of unmodified silica aerogels. A thermogravimetric analysis (TGA) showed a mass loss at 300–400 °C that is attributed to the surface methyl group, while a mass loss at 200 °C refers to the loss of Pluronic10R5 which confirms the incorporation of Pluronic10R5 into the monolith. Moreover, infrared (IR) images revealed that the top surface temperature of Pluronic10R5/silica aerogels monolith is about 80 °C differs from the bottom heat source temperature of 160 °C.

Oil spills clean-up by super hydrophobic organo-modified silica aerogel monoliths treated by different solvents in ambient condition

Water-repellent silica aerogel with a high surface area and extremely low-density considered a very successful adsorbent for oil spills clean-up in the water surface treatment. The present work exhibits new route for the preparation of superhydrophobic organo-modified silica aerogel monoliths from tetraethoxysilane/vinyltriethoxysilane/Ethylene glycol (1:1:1 volume ratio) using sol-gel process and drying in ambient condition. The effects of different solvents (methanol, ethanol and propanol) in the solvent exchange step during the samples preparation procedure were studied. Characterization was performed by FT-IR, XRD, BET and SEM with EDX. The bulk density of the aerogel samples and contact angle of water droplet were measured. The as-prepared monolithic aerogel samples demonstrated a good adsorbing capacity (from 4.83–6.94 gram oil/gram aerogel) against two kind of commercial engines oils which have different specific weights.

Ambient pressure dried flexible silica aerogel for construction of monolithic shape-stabilized phase change materials

Flexible monolithic silica aerogel was prepared using an ambient pressure drying method without extra modifying and solvent-exchange process by controlling the crosslinking degree of the network and in-situ modification of methyl groups to the siloxane backbone. The introduction of the CH3-riched trimethylmethoxysilane (TMMS) molecules transforms silica clusters from originally rigid network structure with high crosslinking degree into flexible network structure with low crosslinking degree, not only improving the flexibility of the aerogel, but also endowing the hydrophobic property. The monolith feature, flexibility, hierarchical porous structure and hydrophobicity make the aerogel an ideal porous support for fabricating shape-stabilized phase change materials (PCMs). The obtained monolithic Paraffin (PA)/Aerogel composite shows high loading amount of PCMs, enhanced phase change enthalpy and good thermal stability. It is convenient to produce diverse shapes of monolithic composite PCMs to satisfy a range of specific requirements for energy storage applications.

Polysilsesquioxane-based silica aerogel monoliths with embedded CNTs

Abstract Monolithic mechanically reinforced CNTs-silica aerogel composites have been successfully synthesized by two-step sol-gel methodology, using methyltrimethoxysilane as the main silica precursor, combined with an ecofriendly ethanol/water mixture as solvent, and dried at ambient pressure. For an effective linkage of the carbon nanotubes with the silica network, the CNTs were submitted to a silanization treatment. With this approach, the chemical binding between the two phases takes place, with the silica particles following the elongated shape of CNTs, which leads to an improvement of the mechanical properties, with Young's modulus up to 201.5 kPa, when compared to the corresponding silica aerogel counterpart without CNTs (104.9 kPa). Moreover, the developed materials show high porosities (up to 95.8%), low densities (as low as 61.3 kg m −3 ), good thermal insulation performance of 31.2–44.1 mW m −1 K −1 and superhydrophobic character. The significant improvements regarding physical and mechanical properties make these composite aerogels attractive for several applications, ranging from thermal insulators to adsorbents.

Harnessing Heat Beyond 200 °C from Unconcentrated Sunlight with Nonevacuated Transparent Aerogels

Heat at intermediate temperatures (120-220 °C) is in significant demand in both industrial and domestic sectors for applications such as water and space heating, steam generation, sterilization, and other industrial processes. Harnessing heat from solar energy at these temperatures, however, requires costly optical and mechanical components to concentrate the dilute solar flux and suppress heat losses. Thus, achieving high temperatures under unconcentrated sunlight remains a technological challenge as well as an opportunity for utilizing solar thermal energy. In this work, we demonstrate a solar receiver capable of reaching over 265 °C under ambient conditions without optical concentration. The high temperatures are achieved by leveraging an artificial greenhouse effect within an optimized monolithic silica aerogel to reduce heat losses while maintaining high solar transparency. This study demonstrates a viable path to promote cost-effective solar thermal energy at intermediate temperatures.

Dual modification of silica aerogel monoliths

A facile and versatile method based on hydrolysis and subsequent condensation of silica alkoxides through sol–gel approach is proposed to obtain an internal-hydrophobic and surface-hydrophilic silica aerogel monoliths. Wet gel monoliths were modified in turn by the hydrophobic agent trimethychlorosilane (TMCS) and hydrophilic modifier 3-aminopropylsilanetriol and finished with supercritical CO2 drying. The silica aerogels were characterized by scanning electron microscopy (SEM), fourier transform infrared spectroscopy (FT-IR), water contact angle, and microcomputer-controlled electron universal tests. The characterization on microstructure of prepared internal-hydrophobic and surface-hydrophilic silica aerogel monoliths indicated that the monoliths have a typical network structure with a porous interior, which exhibits hydrophobic performance (Max. contact angle = 137°) and a hydrophilic surface (Min. contact angle = 60°). The dually modified silica aerogel monoliths were coated with a pure acrylic emulsion layer through which the compressive strength was enhanced greatly which proved that the proposed approach is valuable to silica aerogel modification.

Optical and visual experimental characterization of a glazing system with monolithic silica aerogel

Aerogel is a promising building envelope material that can be used to improve thermal and lighting performance. In monolithic form, aerogels allow visual interaction between the built and outdoor environments, unlike aerogels in granular form. However, the high production cost of large monoliths limits their use. In this work, monolithic silica aerogels were fabricated using tetramethyl orthosilicate as the precursor in a base-catalysed recipe, and a novel rapid supercritical extraction method. The method produces transparent aerogel monoliths in as little as six hours. A 30 × 30 cm glazing sample was constructed from nine 15-mm-thick aerogel monoliths sandwiched between two pieces of 4.7-mm-thick float glass. The glazing sample was characterized in terms of directional and hemispherical optical and visual properties. Measurements were made with an in-house-constructed spectrophotometer equipped with a 75-cm-diameter integrating sphere in the 380- to 2300- nm wavelength range with variable incidence angles. The light and direct solar transmittance are 0.69 and 0.62, while the light and direct solar reflectance are 0.25 and 0.19, respectively. The sample is also characterized by a high general colour rendering index (Ra = 96), allowing indoor visual comfort. Gonio-photometric and directional spectral transmittance measurements show negligible visual distortion and colour variation at different observation angles. The light transmittance of the prototype is lower than that of glazing units present on the market, but about 10% higher than an equivalent unit with granular aerogel. This performance is promising for applications where view through a window is required. However, the size of the samples currently fabricated with the innovative process is limited to 14 × 14 cm and they are characterized by some optical defects that must be removed if the process is to be used for commercial product.

Theoretical and experimental investigation of haze in transparent aerogels.

Haze in optically transparent aerogels severely degrades the visual experience, which has prevented their adoption in windows despite their outstanding thermal insulation property. Previous studies have primarily relied on experiments to characterize haze in aerogels, however, a theoretical framework to systematically investigate haze in porous media is lacking. In this work, we present a radiative transfer model that can predict haze in aerogels based on their physical properties. The model is validated using optical characterization of custom-fabricated, highly-transparent monolithic silica aerogels. The fundamental relationships between the aerogel structure and haze highlighted in this study could lead to a better understanding of light-matter interaction in a wide range of transparent porous materials and assist in the development of low-haze silica aerogels for high-performance glazing units to reduce building energy consumption.