What is aerogel？

It is a kind of light porous material with high porosity and low thermal conductivity. It is the lowest density solid currently known, 0.16 mg per cubic centimeter, 98% of which is air, even if the air inside is pumped A vacuum will result in a solid lighter than air.
However, the most out-of-loop nature of aerogel is its ultra-low thermal conductivity, which is lower than static air at room temperature, only 0.015 W/(m·K). These two characteristics are derived from the special three-dimensional network of aerogel The structure, and the invention of aerogel is derived from a "gambling" between two scientists 90 years ago. In 1931, Samuel Kistler of California Pacific University and his colleagues made a bet and proposed to prove that a solid gel with a continuous network structure of the same size had the same shape as a wet gel. An analogy is to take out the liquid in the jelly but then It does not affect the overall structure. Someone has to ask, can this also allow two scientists to bet on a lunch? I just threw the jelly into the oven and boiled the water. However, if you want to maintain the structure of the gel, simply let the wet gel dry! The gel will severely crack and shrink, eventually breaking into small pieces. Kistler speculates that the reason for the gel shrinkage and breakage is that when the liquid evaporates from the micropores, there is a large capillary action and surface tension at the solid-liquid interface, which pulls on the inner wall of the hole, causing the fragile gel microporous structure to collapse. Kistler tried many methods. Finally, through solvent replacement, the water in the gel was replaced with ethanol, and then high temperature and high pressure (243 ℃, 6.38 MPa) was applied in a closed container to make the ethanol reach a supercritical state and eliminate solid and liquid. The surface tension between the two successfully eliminated the liquid from the gel and retained the complete microporous structure of the gel for the first time. The final result was published on Nature under the title "Copolymerized Diffusion Aerogel and Jelly", so , Aerogel is finally born!

Why can aerogels be prepared by heating and pressurizing ethanol to a supercritical state? This is because the supercritical fluid is neither a liquid nor a gas, but at the same time it has the characteristics of a liquid and a gas. The liquid in this special state, such as ethanol, will drift around like smoke, and the gas-liquid interface will disappear. If there is a solid-liquid boundary or a gas-liquid boundary, the surface tension or capillary action that caused the gel to shrink and break will no longer exist, and the liquid remaining in the micropores can escape from the pores to the environment very smoothly and almost unimpeded. , To achieve the drying of the wet gel.