The ultrahighly nanoporous aerogel is recognized as a state of matter rather than as a functional material, because of its qualitative differences in bulk properties, transitional density and enthalpy between liquid and gas, and diverse chemical compositions. In this review, the characteristics, classification, history and preparation of the aerogel were introduced. More attention was paid to the sol-gel method for preparing different kinds of aerogels, given its important role on bridging the synthetic parameters with the properties. At last, preparation of a novel single-component aerogel, design of a composite aerogel and industrial application of the aerogel were regarded as the research tendency of the aerogel state in the near future.

Titanium oxide coatings have been deposited by the sol-gel dip-coating method. Crystallization of titanium oxide coatings was then achieved through thermal annealing at temperatures above 400 °C. The structural properties and surface morphology of the crystallized coatings were studied by micro-Raman spectroscopy and atomic force microscopy, respectively. Characterization technique, based on least-square fitting to the measured reflectance and transmittance spectra, is used to determine the refractive indices of the crystallized TiO₂ coatings. The stability of the synthesized sol was also investigated by dynamic light scattering particle size analyzer. The influence of the thermal annealing on the optical properties was then discussed. The increase in refractive index with high temperature thermal annealing process was observed, obtaining refractive index values from 1.98 to 2.57 at He-Ne laser wavelength of 633 nm. The Raman spectroscopy and atomic force microscopy studies indicate that the index variation is due to the changes in crystalline phase, density, and morphology during thermal annealing.

The electrical, optical, and (bio)chemical properties of porous Si have attracted considerable interest for battery, solar cell, display, sensor, biomedical, and other applications. Free-standing monolithic Si aerogels have been generated for the first time via low-temperature magnesiothermic reduction of SiO₂ aerogels. Under appropriate conditions, the reaction of Mg gas with SiO₂ produced a co-continuous mixture of predominantly MgO and Si. Selective MgO dissolution then yielded nanocrystalline, porous, photoluminescent Si aerogels with a high degree of porosity (97.3 vol%) and a high specific surface area (615 m² g⁻¹).

Silica aerogel powders (SAp) are the scrap materials derived from the industrial production and use of silica-aerogel products and become the challenges to be disposed. In this paper, SAp play a role as the shrinkage inhibiter to fabricate the SAp “jammed” polyimide gels. Herein, the “jammed” effect means that the polyimide backbones have been “stretched up” by the micron-scale SAp. Namely, the strong chains-packing effect, which is always taking placing during the gelation process and results in a high shrinkage of the polyimide backbones, has been effectively prevented by a simple physical mixture of SAp. This “jammed” effect greatly reduces the shrinkage as low as 7.5%. Meanwhile, SAp are treated as the pore formers to make a conversion to polyimide aerogel with ultra-low density of about 0.02 g cm⁻³. In addition, incorporation of SAp also dramatically enhances the hydrophobicity (water contact angle up to 152°). Overall, SAp “jammed” polyimide aerogels have low thermal conductivity (0.028–0.031 W m⁻¹ K⁻¹), high mechanical strength, and high thermal stability. This combination of properties makes them more applicable as thermal insulators in harsh environments. Certainly, recycling of the scraped SAp by simple physical mixture with organic PI is also an advocated way for energy conservation.

TiO₂-SiO₂ binary coatings were deposited by a sol-gel dip-coating method using tetrabutyl titanate and tetraethyl orthosilicate as precursors. The structure and chemical composition of the coatings annealed at different temperatures were analyzed by Raman spectroscopy and Fourier Transform Infrared (FTIR) spectroscopy. The refractive indices of the coatings were calculated from the measured transmittance and reflectance spectra. An increase in refractive index with the high temperature thermal annealing process was observed. The Raman and FTIR results indicate that the refractive index variation is due to changes in the removal of the organic component, phase separation and the crystal structure of the binary coatings.