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Silica aerogels display exceptional properties and great application potential, with a mature market in thermal insulation. Both supercritical drying (SCD) and ambient pressure drying (APD) routes are implemented industrially. Herein, how aging and silica content affect the mechanical properties, and how these in turn determine the shrinkage, spring back, and density during APD are systematically investigated. The APD densities display a U‐shaped dependence of density w.r.t. silica concentration. At low silica concentrations, the gels cannot withstand the capillary forces during APD and dense xerogels are obtained. At intermediate to high concentrations, APD shrinkage is strongly reduced and density increases with silica concentration. A series of cylinders are prepared by SCD and investigated by uniaxial compression and their strain recovery is determined systematically. The mechanical responses are plastic, viscoelastic, and brittle in nature for low, intermediate, and high silica concentrations, respectively. The strain recovery of the SCD cylinders correlates to the degree of spring back during APD. The viscoelastic response of SCD aerogels having 6 wt% corresponds to the silica concentration where a minimum in APD aerogel density is observed. The importance of gel mechanics for silica aerogel spring back during APD, in addition to surface modification and hydrophobization is highlighted. [ABSTRACT FROM AUTHOR]

Carbon nanostructures-silica aerogel composites were synthesized and characterized to assess the effect of carbon nanotubes (CNTs) or graphene oxide (GO) on the silica aerogel properties. The sol-gel chemistry was based on methyltrimethoxysilane (MTMS) and 3-aminopropyltrimethoxysilane (APTMS) as silica precursor system, with varying amounts of APTMS (0–20% mol of Si). APTMS significantly impacted the materials' physical properties. The chemistry and microstructure were investigated by FTIR, NMR, TEM, SEM, SAXS and BET. The addition of CNTs induced the growth of the silica matrix around them; thus, an elongated shape was observed in the silica structural units. APTMS and CNTs have a synergistic effect on the mechanical properties, increasing the Young's modulus up to 14 MPa. Small amounts of carbon materials (∼1 wt%) in the MTMS-matrix improved its thermal insulation property, particularly for temperatures above 50 °C. In terms of electrochemical properties, the carbon nanostructures lead to higher specific capacitances and a reduction in resistance. The characterizations here performed allowed a better understanding of the interactions between the silica and carbon phases. The possibility to obtain materials with tailored properties demonstrates their application potential in several areas, such as thermal insulation and energy storage. [Display omitted] • CNTs and GO were incorporated into MTMS-based silica aerogel networks. • CNTs changes the silica network morphology to more elongated structural units. • Small amounts of CNTs enhance the thermal insulation of the silica aerogels. • APTMS and CNTs have a synergistic effect leading to higher Young's modulus. • Addition of CNTs or GO improves the electrochemical performance of silica aerogels. [ABSTRACT FROM AUTHOR]

Highlights • Aerogels can improve the mechanical properties of epoxy-based nanocomposites due to 3-dimensional nanoporous structure and high surface area. • The effect of silica aerogel was investigated on the rheological, mechanical and thermal properties of the epoxy nanocomposite. • Two main toughening mechanisms are identified: (a) crack pinning and deflection, and (b) plastic deformation (debonding). Abstract Silica aerogel is a promising candidate for improving the mechanical properties of epoxy-based nanocomposites due to the unique properties such as the 3-dimensional nanoporous structure and high surface area. In this study rheological, mechanical and thermal properties of the silica aerogel-epoxy nanocomposites were investigated. The rheological results demonstrated an increase in viscosity of the nanocomposite suspension compared to the neat resin. The results of dynamic mechanical along with the thermal analysis showed that the addition of 6 wt% of silica aerogel causes the storage modulus and glass transition point (Tg) to be increased by 11% and 5 °C, respectively. Also, significant improvements in the elastic modulus (35%), tensile strength (62%) and toughness (126%) were achieved with the optimal volume fraction of silica aerogel (6 wt%). Two main fracture mechanisms were involved in the fracture behavior of the silica aerogel-epoxy nanocomposite: (a) crack pining and deflection, and (b) plastic deformation (debonding). [ABSTRACT FROM AUTHOR]

Silica aerogels’ unique physical and chemical properties make them fascinating materials for a wide variety of applications. In addition to hydrophobization by silylation, aging is very important in the synthesis of silica aerogel by ambient pressure drying. Here we systematically study the effect of aging on the physico-chemical properties of silica aerogel with emphasis on ambient dried materials. Silica gels were aged for different times and at different temperatures in their gelation liquid (without solvent exchange), hydrophobized in hexamethyldisiloxane and subsequently dried either at ambient pressure or from supercritical CO 2 . Dynamic oscillatory rheological measurements demonstrate that aging reinforces the alcogels, particularly at high strain. The specific surface area decreases with increasing aging time and temperature as a consequence of Ostwald ripening processes during aging. With increasing aging time and temperature, the linear shrinkage and bulk density decrease and the pore size and pore volume increase for the ambient dried gels, but remain nearly constant for supercritically dried gels. Small-Angle X-ray scattering does not detect significant structural changes at length scales smaller than about hundred nanometers, but hints at systematic variations at larger length scales. The findings of this study highlight the importance of aging to increase the ability of the gel particle network to withstand irreversible pore collapse during ambient pressure drying. [ABSTRACT FROM AUTHOR]

Aerogels are mesoporous materials with a rapidly growing thermal insulation market interest, partly because ambient pressure drying (APD) reduces cost. Silica gels must be hydrophobized to prevent condensation reactions and irreversible shrinkage during APD. Here, we study how hydrophobization with mono-, di-, and trifunctional organoalkoxysilanes (M, D, T) and their mixtures, affects spring-back, density, surface area, pore size and hydrophobicity. All aerogels were synthesized from a polyethoxydisiloxane precursor prepared from a tetraalkoxysilane (tetraethoxysilane, TEOS). Aged gels, hydrophobized with trimethylethoxysilane (M100), display the highest spring-back and lowest density (0.100 g/cm3) upon APD drying. Hydrophobization with pure dimethyldiethoxysilane (D100) or methyltriethoxysilane (T100) leads to excessive shrinkage and respective densities of 0.440 and 1.450 g/cm3 due to the self-polymerization from the di- and trifunctional silanes: in the D100 material, the formation of polydimethyldisiloxane (PDMS) is evident in the 29Si NMR spectra, whereas 3D silica structures (beads) are apparent for the T100 sample. Aerogels hydrophobized with binary and ternary mixtures of M, D and T display systematic variations in density: nearly constant (<0.12 g/cm3) values are obtained after replacing up to 20% of M, but the density increases rapidly upon further substitution. In contrast to the strong effects on density, the humidity uptake remains below 2 wt% for most samples and conditions. The possibility to use mixtures of hydrophobization agents, rather than pure compounds, may decrease the production costs of silica aerogels, and can be readily carried out for up to 20%, but would require further process optimization for higher degrees of substitution. Image 1 • Up to 20% trimethylsilyl can be substituted without compromising aerogel density. • Higher degrees of replacement not possible without further optimization. • Self-polymerization of dimethyldiethoxysilane into polydimethylsiloxane. • Self-polymerization of methyltriethoxysilane into 100–200 nm large beads. [ABSTRACT FROM AUTHOR]

Silica aerogel is the ultimate thermal insulator thanks to its record-breaking low thermal conductivity (λ), open porosity and hydrophobicity. Silica aerogel's thermal conductivity is lowest at intermediate densities (ρ ≈ 0.11 g/cm3) and, because of the strong, power-law dependence of the E modulus on density, this rather low density so far led to low E moduli. Even with polymer reinforcement, increasing stiffness is only possible at higher density, thus higher conductivity. This paper explores the synthesis of silica aerogel granules using ambient pressure drying to provide enhanced mechanical stiffness whilst maintaining thermal conductivities well below 20 mW m−1 K−1. The aging and drying conditions affect the interplay between mechanical and thermal properties, and are varied to optimize the physical properties. The dense (ρ≤0.29 g/cm3), but superinsulating (λ≈15 mW m−1 K−1) silica aerogels presented in this paper challenge the community's understanding of heat transport in aerogels, and do not rely on polymer reinforcement. The underlying microscopic structural parameters affecting the mechanical and thermal transport properties are investigated by modelling and simulation of the aerogel back-bone. Short aging times reduce the cross-section of, and heat transport through, inter-particle necks, leading to an overall decrease in thermal conductivity through the solid skeleton (λ s). In addition, short-aged gels undergo a partial pore collapse during ambient pressure drying of the pore fluid due to less aged, hence weaker network structures. The resulting denser structure contains additional point contacts that increase stiffness, by up to an order of magnitude. However, heat transport through these newly formed point-contacts is limited and the gas phase conduction (λ s) is further suppressed due to the even smaller pore sizes. Strong and superinsulating particles are ideal fillers for aerogel composites, concrete and renders. The optimized APD aerogels, available as granules, are finally compiled in a composite thermal insulation board with an effective thermal conductivity down to 20 mW m−1 K−1 with improved strength: a 2-fold increase for E, compared to a board produced from classical silica aerogel granulate. The possibility to improve mechanical properties of pure silica aerogels can help aerogels to break into new high-strength, superinsulating structural applications needed to reduce carbon emissions of the built environment. [ABSTRACT FROM AUTHOR]

• A first critical review on the acoustic properties of the silica aerogels • Silica aerogels display resonance absorption behavior • Lack of systematic studies and fundamental insight in acoustic properties of aerogels • Evaluation of acoustics imperative to support aerogel thermal insulation application Silica aerogels are popular in terms of production volume and real-world applications. Although the current market growth rate is driven exclusively by thermal insulation, aerogels may also be attractive for acoustic applications with the potential in aiding sound absorption/insulation. This paper is a summary of the acoustics related studies of silica aerogel-based products. It introduces silica aerogels, some acoustic characterization methods, and reviews systematically the available data on sound absorption/insulation of silica aerogels, polymer-silica aerogel composites, nonwoven-silica aerogel blankets, and aerogel renders/glazing. The work identifies areas where further research is required, including experimental and theoretical work on the physics of sound absorption in mesoporous materials, and more systematic and standardized evaluations of the acoustic properties of aerogel and aerogel-composites. Aside from this call to action, the opportunities and barriers for the commercialization of silica aerogel products for acoustic applications are presented. [ABSTRACT FROM AUTHOR]

Silica aerogels are highly porous (~95%), predominantly mesoporous (20–50 nm) materials with very high surface area (~800 m2/g), but applications that valorize the chemical properties linked to the high surface area lag behind those that valorize the pore structure (thermal insulation). Here, we present widely applicable protocols to synthesize and characterize organo-functionalized silica aerogels with high surface area (356–1177 m2/g) and high mass loadings of surface functional groups (1.0–4.9 mmol/g, 0.9–3.9 molecules/nm2). Wet silica gels are soaked into dilute solutions containing pre-hydrolyzed organofunctional trialkoxysilanes, followed by supercritical CO 2 drying. The retention of the various functional groups in the dry aerogels is confirmed by FTIR and their concentrations quantified by solid-state NMR spectroscopy. The mild reaction conditions are compatible with all tested functional groups (silanol, ethoxy, trimethylsilyl, amino, vinyl, methyl methacrylate, epoxy, phosphonate), without the need to optimize for each specific functional silane, making the synthesis of organofunctionalized aerogels a routine task. Unlabelled Image • A flexible protocol to produce organo-functionalized silica aerogels is presented. • Grafting and retention of functional groups are validated by FTIR and solid-state NMR. • Content of functional groups are determined by quantitative NMR spectroscopy. • Potential applications are systemically reviewed for the investigated functionalities. [ABSTRACT FROM AUTHOR]

• Simple aerogel synthesis from low-cost silica precursor without ion exchange. • Superinsulating silica aerogel powder produced in less than three hours. • Phase transfer agent improves hydrophobization and application relevant properties. • Silica aerogel powder displays high surface area and mesopore volume. A fast and simple, one-step synthesis for the preparation of silica aerogel powders is reported. A waterglass based sol was prepared without prior ion-exchange and stirred with an immiscible organic phase (heptane) that contains the hydrophobization agent (hexamethyldisilazane). The addition of nitric acid triggers gelation and subsequently the hydrophobization of this hydrogel, which causes the gel particles to transfer from the aqueous into the organic phase, whilst the sodium and nitrate ions are retained in the aqueous phase. The resulting organogel slurry is then converted into a silica aerogel powder by means of ambient pressure drying. The addition of phase transfer agents, ethanol and isopropanol in particular, strongly influences the final silica aerogel properties, including tap density, hydrophobicity, particle size distribution, surface chemistry (solid-state NMR), and microstructure (TEM, SEM, nitrogen sorption). The addition of intermediate amounts of phase transfer agent strongly improves the aerogel properties, but these positive effects disappear when larger quantities are added. Under optimal conditions, silica aerogel powders with high mesoporosity, surface areas above 800 m2/g, tap densities as low as 0.083 g/cm3, and thermal conductivities below 20 mW m−1 K−1 can be produced in a one-step process within just over two hours. The direct use of waterglass as an inexpensive silica precursor provides a great opportunity to reduce the production cost of silica aerogel powders for thermal insulation applications. [ABSTRACT FROM AUTHOR]