Aerogels

Abstract

The article contains sections titled: 1. Introduction 2. Synthesis 2.1. Network Formation and Structure 2.1.1. Inorganic Aerogels 2.1.1.1. SiO 2 Aerogels 2.1.1.2. Metal Oxide Aerogels 2.1.2. Inorganic/Organic Hybrid Aerogels 2.1.3. Organic Aerogels 2.2. Drying Methods 2.2.1. Supercritical Drying 2.2.1.1. Drying in Organic Solvents 2.2.1.2. Supercritical Drying with CO 2 2.2.2. Freeze Drying 2.2.3. Ambient Pressure Drying 2.3. Modification of Aerogels after Drying 3. Properties 3.1. Structural Properties 3.2. Physical and Chemical Properties 3.2.1. Optical Properties 3.2.2. Mechanical and Acoustic Properties 3.2.3. Thermal Conductivity 3.2.4. Hydrophobicity 4. Applications 4.1. Çerenkov Detectors 4.2. Thermal Insulation 4.3. Catalysis 4.4. Application as Storage Media 4.5. Electrical and Electronic Applications 4.6. Acoustic and Mechanical Applications 4.7. Optical Applications 4.8. Other Applications 5. Future Perspectives Aerogels are unique among solid materials. They have extremely low densities (up to 95 % of their volume is air), large open pores, and a high inner surface area. This combination of properties results in interesting physical properties, e.g., for silica aerogels extremely low thermal conductivities and low sound velocities are combined with optical transparency. Aerogels are typically prepared via wet chemical processes, such as sol – gel processing, and thus their pore system is initially filled with liquid. Only special drying techniques allow for exchange of the pore liquid against air while maintaining the filigrane solid framework. The most common drying technique is supercritical extraction, and sometimes chemical modification of the inner surface can be used to facilitate drying. The structure of the gel network, and thus the physical properties of aerogels, decisively depends on the choice of the precursors and the chemical reaction parameters for preparing the gels. Therefore, the later materials properties can be deliberately designed in the starting reaction solution.

Thermodynamic assessment of the stability of bulk and nanoparticulate cobalt and nickel during dry and steam reforming of methane

“…In turn, the thermodynamically stable phase of nanoparticles may differ from the one in larger particles. 36 For example, the fcc-Co structure ( Fig. 1 ) is the preferential phase for nanoparticles below 20 nm (ref.…”

Nanostructure of Aerogels and Their Applications in Thermal Energy Insulation

“…Aerogels are open-cell materials with unique properties such as low density, high specific surface area, high porosity, low dielectric constant as well as a superlow thermal conductivity, and so on. , A silica wet gel was dried via supercritical drying without eradication of the gel structure for the first time in 1932 by Kistler; − it is called “‘aerogel’”. Additionally, Kistler synthesized alumina, tungstic, ferric, stannic oxide, nickel tartrate, cellulose, nitrocellulose, gelatin, agar, and egg albumin aerogels. In the 1980s, a resorcinol-formaldehyde aerogel was produced by Pekala via polycondensation method, and a carbon aerogel was discovered by pyrolyzing products in the 1990s. , In the recent century, fascinating developments occurred in the aerogel world, such as carbon nanotube (CNT) aerogel, graphene aerogel, gradient aerogel, and so forth. − According to statistical data of Scopus (Figure ), the publications on thermal insulation aerogels have increased year by year.…”

Mechanical properties of polymer-modified silica aerogels dried under ambient pressure

“…Therefore, the difference in shrinkage for polymermodified silica aerogels is due to the different shrinkage during the drying process. When silica wet gels were dried under ambient pressure, they were subjected to very large capillary forces exerted by the meniscus of the pores liquid as liquid evaporated from the wet gels [1,2]. Fig.…”

“…Their unique physical properties make them attractive for use in various applications [1][2][3][4]. However, silica aerogel monoliths have limited use in specialized environments, such as in Cerenkov radiation detectors in certain nuclear reactors [5][6][7], collectors of hypervelocity particles in space, and thermal insulators in space vehicles [6][7][8][9].…”