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What is Aerogel
Release time:
2021-09-17 16:48
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What is Aerogel?
It is a lightweight porous material with high porosity and low thermal conductivity. It is the lowest density solid known at present, 0.16 mg per cubic centimeter, of which 98% of the volume is air. Even if the air inside is pumped into a vacuum, a solid lighter than air will be obtained.
However, the most out-of-circle property of aerogel is ultra-low thermal conductivity, which is lower than still air at room temperature, only 0.015W/(m K). These two characteristics are derived from the special three-dimensional network structure of aerogel, while the invention of aerogel comes from a "bet" between two scientists 90 years ago ". In 1931, Kistler Samuel of the University of the Pacific in California, USA, made a bet with colleagues, proposing to prove that a solid gel with a continuous network structure of the same size, the shape of the same as the wet gel, for example, is to remove the liquid from the jelly without affecting the overall structure. Some people ask, will this also allow two scientists to gamble on lunch? I just throw the jelly in the oven and boil the water. However, in order to maintain the structure of the gel, it is not enough to simply let the wet gel dry! The gel will crack severely and shrink, eventually breaking into small pieces. Kistler speculate that the reason for the gel shrinkage is that when the liquid evaporates from the micropores, there is a large capillary action and surface tension at the solid-liquid interface, pulling the inner wall of the hole, making the fragile gel micropore structure collapse. Kistler tried many methods. Finally, after replacing the water in the gel with ethanol through solvent replacement, high temperature and high pressure (243 ℃,6.38 MPa) were applied in a closed container to make ethanol reach supercritical state and eliminate the surface tension between solid and liquid. For the first time, the liquid was successfully excluded from the gel and the complete gel microporous structure was retained. The final result was published on Nature, the title is "copolymer diffusion aerogel and jelly", so aerogel was finally born!
Why can aerogels be prepared by heating and pressurizing ethanol to a supercritical state? This is because supercritical fluid is neither liquid nor gas, but it has the characteristics of liquid and gas at the same time. Liquids in this special state, such as ethanol, will drift around like smoke, and the gas-liquid interface will disappear. At this time, there will be no solid-liquid boundary or gas-liquid boundary, so the surface tension or capillary action that leads to gel shrinkage and fragmentation will no longer exist, the liquid remaining in the micropores can escape from the pores to the environment very smoothly and almost unimpeded, achieving the drying of the wet gel.
Aerogel, Aerogel manufacturing, Aerogel properties, Aerogel characteristics
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