Aerogel is the lightest solid because it is a microporous, dense foam with a gas component that replaces the liquid component. It has a high thermal resistance, a low density, a low optical index of refraction, a low dielectric constant, a high porosity, a high specific surface area, and thermal, acoustic, and impact damping properties. It is capable of withstanding loads up to 4000 times its own weight. After use, aerogel sponge can be recycled, indicating its environmental friendliness. Aerogel is widely used in the oil and gas, aerospace, medical, and electronics industries. The aerogel market is hampered by high production costs and poor mechanical strength.
The three main methods for determining the porosity of aerogels are gas adsorption, mercury porosimetry, and scattering method. Nitrogen at its boiling point is adsorbed into the aerogel sample during gas adsorption. The amount of gas adsorbed is determined by the size of the pores in the sample as well as the partial pressure of the gas relative to its saturation pressure. The volume of gas adsorbed is calculated using the Brunauer, Emmit, and Teller formula (BET), which yields the sample's specific surface area. The pore size distribution of the sample is given by the Kelvin equation at high partial pressure during adsorption/desorption.
The mercury is forced into the aerogels porous system to determine the size of the pores in mercury porosimetry, but this method is inefficient because the solid frame of the aerogel will collapse due to the high compressive force. The angle-dependent deflection of radiation within the aerogel sample is used in the scattering method. Solid particles or pores can be used as the sample. The radiation penetrates the material, determining the fractal geometry of the aerogel pore network. X-rays and neutrons are the best radiation wavelengths to use.
Aerogel is also an open porous network: the difference between an open porous network and a closed porous network is that in an open porous network, gases can enter and leave the substance freely, whereas a closed porous network traps the gases within the material, forcing them to stay within the pores. Because of their high porosity and surface area, silica aerogels can be used in a wide range of environmental filtration applications.
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