Silica aerogels have drawn a lot of interest both in science and technology because of their low bulk density (up to 95% of their volume is air), hydrophobicity, low thermal conductivity, high surface area, and optical transparency. Aerogels are synthesized from molecular precursors by sol-gel processing. Special drying techniques must be applied to replace the pore liquid with air while maintaining the solid network. Supercritical drying is most common; however, recently developed methods allow removal of the liquid at atmospheric pressure after chemical modification of the inner surface of the gels, leaving only a porous silica network filled with air. Therefore, by considering the surprising properties of aerogels, the present review addresses synthesis of silica aerogels by the sol-gel method, as well as drying techniques and applications in current industrial development and scientific research.

Being able to dynamically shape light at the nanoscale is one of the ultimate goals in nanooptics 1 . Resonant light-matter interaction can be achieved using conventional plasmonics based on metal nanostructures, but their tunability is highly limited due to fixed permittivity 2 . Materials with switchable states and methods for dynamic control of lightmatter interaction at the nanoscale are therefore desired. Here we show that nanodisks of a conductive polymer can support localised surface plasmon resonances in the near-infrared and function as dynamic nanooptical antennas, with their resonance behaviour tuneable by chemical redox reactions. These plasmons originate from the mobile polaronic charge carriers of a poly[3,4-ethylenedioxythiophene:sulfate (PEDOT:Sulf) polymer network. We demonstrate complete and reversible switching of the optical response of the nanoantennas by chemical tuning of their redox state, which modulates the material permittivity between plasmonic and dielectric regimes via non-volatile changes in the mobile charge carrier density. Further research may study different conductive polymers and nanostructures and explore their use in various applications, such as dynamic metaoptics and reflective displays.We prepared thin conductive polymer films of poly [3,4-ethylenedioxythiophene:sulfate] (PEDOT:Sulf, see Fig. 1a), which can provide high electrical conductivity and metallic character 3,4 . Using vapour phase polymerization and sulfuric acid treatment (see Methods), we obtained films with electrical conductivity exceeding 5000 S/cm (see Supplementary Table.1). Their complex and anisotropic permittivity was determined by ultrawide spectral range ellipsometry, employing an anisotropic Drude-Lorentz model as described previously (see Supplementary Table . 2) 5 . Fig. 1b shows the resulting in-plane permittivity of a thin PEDOT:Sulf film with thickness of 32 nm (Supplementary Fig. 1 presents the raw data). The shaded area highlights a spectral region (0.8 to 3.6 μm) in which the film has negative real permittivity and lower magnitude imaginary permittivity, which we define as plasmonic regime. This optically metallic and plasmonic character is related to the high conductivity within the thin film due to high concentration (2.6 ×10 21 cm -3 , determined by ellipsometry, see Supplementary Table.1and Supplementary Information for details) of mobile positive polaronic charge carriers. We also note that the mobility is highly anisotropic 5,6 and the out-of-plane real permittivity (Supplementary Fig. 2a) is primarily positive throughout the measured range, making the conductive polymer thin film a natural hyperbolic material 7 (Supplementary Fig. 3).

Titanium diboride (TiB2) was hot‐pressed at a temperature of 1800°C, and silicon nitride (Si3N4) was added as a sintering aid. The amount of Si3N4 that was added had a significant influence on the sinterability and mechanical properties of the TiB2. When a small amount (2.5 wt%) of Si3N4 was added, the Si3N4 reacted with titania (TiO2) that was present on the surface of the TiB2 powder to form titanium nitride (TiN), boron nitride (BN), and amorphous silica (SiO2). The elimination of TiO2 suppressed the grain growth effectively, which led to an improvement in the densification of TiB2. The formation of SiO2 also was deemed beneficial for densification. The mechanical properties‐especially, the flexural strength‐were enhanced remarkably through these improvements in the sinterability and microstructure. On the other hand, when a large amount (greaterthan equal to5 wt%) of Si3N4 was added, the mechanical properties were not improved much, presumably because of the extensive formation of a glassy Si‐Ti‐O‐N phase at the grain boundaries.

We embedded IR-resonant microparticles as visible scatterers and thermal emitters in a transparent nanocellulose composite, to form a metamaterial that provides radiative cooling while simultaneously functioning as an optical diffuser.

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