![]() ![]() Where S 0 is the solubility of a flat surface of the solid phase, V m is the molar volume of the solid, \(\gamma_\) exist. The difference in solubility ( S) for surfaces with different curvatures ( r) drives the transportation of material, which can be represented by the Kelvin Equation (Eq. ( 1)). During the process, the material will be transported to the neck region where the colloidal particles gives a firmer network. During the aging process, two different mechanisms, including Neck growth and Ostwald ripening, will operate simultaneously to influence the structure and features of a gel at different rates. The obtained gel in the first step will be further aged in its mother liquor. Generally, the gels are classified based on the dispersion medium, as examples, water for hydrogel or aquagel, alcohol for alcogel and air for aerogel. For silica gel preparation, for example, the precursor tetramethoxysilane (TMOS) is dissolved in ethanol/water (4:1) mixture and then 0.1 M HCL is added to catalyze the gelation. In the process, the required precursor is dispersed in a solvent first and then a catalyst is introduced to facilitate the gel formation. The gel phase is commonly obtained by the sol–gel method. Their application progress in coatings are well summarized with focusing more on the flexible substrates (textile and leather) on account of the rapid development of flexible wearable devices. In this work, we attempt to review the general preparation methods and properties of aerogels. Other aerogel coating approaches mainly include the dipping, spraying and spinning coatings as well as the bar and flow coatings. Through the sol–gel strategy, several researchers have synthesized silica aerogels as functional agents to coat on fibers and fabrics with multifunctionality including super-thermal insulation, super-hydrophobicity, UV radiation protection, colorfastness of materials as well as antimicrobial preservation. In the process, the sol is coated on a substrate and the dispersing medium constituting the sol is evaporated to form the gel layer. The sol–gel method produces a thin film coating of nanoparticle clusters or nanoporous aerogels/xerogels on a substrate in a single coating step. The surface coatings of aerogels on different substrates can be carried out by a sol–gel coating technique. The coating of aerogels on different substrates is among the solutions to rectify the aforementioned problem. However, the fiber-reinforced aerogel composite has a major problem that is dust-releasing from the fiber assemblies. Synthesizing fiber-reinforced aerogel composites is one of the most effective methods to reduce the fragility of the aerogels. However, aerogels also have some disadvantages, such as fragility, rigidity, dust formation, bulkiness, et al., which limit their extensive applications. Īerogels can also be used in heavy metal removal, gas absorption, radioactive waste confinement, drug delivery, sensors, nuclear particle detection (Cherenkov), optics (light guides), Knudsen pump, electronic devices (solar cells, capacitors), catalysis and surface coatings (self-cleaning coatings, chemical resistant coating, etc.). Compared with conventional insulation, this silica aerogel had an obviously lower thermal conductivity due to their small pore size that could limit the gas phase conduction. For instance, silica aerogel is a super thermal insulator in possession of a thermal conductivity lower to 12 mW/(mK). Low thermal conductivity is the typical feature of aerogel investigated most widely, which has been prominently used in high-performance thermal insulation. As is well known, silica, alumina and carbon, are the main sources for fabricating the aerogels, especially when the synthetic polymers, biopolymers, and other organic precursors are emerging sources. After a special drying, the solvent will be removed from the gel structure, followed by filling the network with air, as such the dried product is called aerogel. An aerogel is prepared using a sol–gel method which generally involves the conversion of a liquid system (sol) into a gel phase through chemical reactions. Also, the aerogels can be defined as the gels in which the liquid has been replaced by air, while the solid network shrinks very moderately. Based on IUPAC, the aerogels are defined as the gels that comprise a microporous solid in which the dispersed phase is gas. Aerogels are generally known as solid air, solid smoke, frozen smoke, or blue smoke. They are predominantly mesoporous solids. Aerogels are the special materials with porosity, extremely lightweight, low-density and thermal insulation. ![]()
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