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• 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]

Silica aerogels are advanced materials with outstanding properties, including high specific surface area and porosity, low thermal conductivity, density, dielectric constant, and refractive index. However, the excellent properties of silica aerogels have shown limited utility in industrial applications, primarily as thermal insulation materials, where they hold only a small (1.1–2.6%) market share. The emergence of clean energy technologies stimulated renewed interest in silica and other aerogels. Here the recent advances in the production and applications of silica aerogels are summarized, and the current challenges and future development opportunities are discussed. The conventional and emerging syntheses and characterization methods of silica aerogels are introduced, followed by a discussion on thermophysical properties and potential applications. The fabrication processes of silica aerogels are analysed at different industry development stages. Recent advances in aerogels research are considered, highlighting the obstacles and possible solutions for the successful translation and industry uptake and focusing on the emerging applications in electric vehicles and building insulation. [ABSTRACT FROM AUTHOR]

[Display omitted] • Unique structure of particulate silica aerogel embedded in fibrous polyimide aerogel. • Nanocomposite inherits hydrophobicity and temperature stability from silica aerogel. • Composite maintains superior mechanical and low dielectric property from PI aerogel. • Low dielectric constant and loss are stable at various frequencies from 10-1 to 3 × 1010. Polymer aerogels are a promising, non-brittle alternative to silica aerogel, but are limited by their very poor high-temperature stability. Polyimide is widely known for its high heat-resistance, however, a high degree of volume shrinkage is common for polyimide aerogels after exposure to temperatures above 200 °C. Here, we present the aerogel-in-aerogel composites that comprise silica aerogel grains with a nanoparticulate microstructure embedded in a nanofibrous polyimide aerogel matrix. The mixing procedure and synthesis protocol were optimized to avoid excessive infiltration of the polyimide sol into the silica aerogel mesopores. The composites display a unique, heterogeneous structure with a high surface area, >600 m2 g−1, a low thermal conductivity down to 17.5 mW m−1 K−1, a very low dielectric constant (∼2.5 at 10-1-106 Hz, ∼1.2 at 8–12.5 GHz, ∼1.2 at 26.5–32 GHz) and dielectric loss (10-3 to 10-1 at all studied frequencies). The hydrophobic silica aerogel component contributes a high water resistance, with high water contact angles (>150°) and a low humidity uptake (<4 wt% at 88% relative humidity), and a much-reduced volume shrinkage at high temperature. The polyimide component imparts excellent mechanical properties to the composites, including improved compressive modulus, compressive and bending strengths. The composite aerogels offer great potential for applications that require high mechanical strength, a low dielectric constant and/or thermal conductivity and/or high-temperature stability. [ABSTRACT FROM AUTHOR]