2023 Author: Bryan Walter | [email protected]. Last modified: 2023-05-21 22:24
Scientists from the United States, China and Saudi Arabia have created an airgel that shrinks when heated and deformed. In addition, it completely recovers after being compressed twenty times and practically does not lose its mechanical properties with sudden changes in temperature of a thousand degrees or more, the authors of the article say in Science.
Aerogels are porous gels consisting of a solid phase and pores that occupy almost the entire volume, often more than 99 percent. This makes such materials convenient for use in many areas. Due to the almost complete absence of a solid phase, aerogels have an extremely low density. In addition, this makes them an almost ideal heat insulator, which has a very low thermal conductivity, and it decreases when the airgel is placed in a vacuum, which is especially convenient when used in space technology.
But aerogels also have disadvantages associated with specific uses. For example, when aerogels are used for thermal insulation under conditions of highly varying temperatures, they are subject to deformation due to thermal expansion. Because of this, especially with repeated and rapid heating, defects accumulate in the material, which can lead to loss of strength and the appearance of cracks.
Scientists led by Xiangfeng Duan of the University of California, Los Angeles have created an airgel that has a negative coefficient of thermal expansion - it shrinks when heated. The airgel consists of hexagonal boron nitride, but it is created on the basis of a graphene airgel. First, scientists create, using hydrothermal synthesis and non-contact lyophilization, an airgel consisting of graphene plates shaped like a hyperbolic surface. Then, a boron nitride precursor is deposited on the graphene surface using chemical vapor deposition, which is then converted into boron nitride itself. All operations, except for the last one, are carried out in a vacuum, and at the final stage the airgel is placed in air and heated to 600 degrees Celsius, as a result of which the graphene is burned out, while boron nitride remains, forming a porous structure of double walls.
Scheme of making boron nitride airgel
After receiving the airgel samples, the authors tested their properties. First of all, it has two unusual properties - negative coefficients of thermal expansion and Poisson. Due to this, under mechanical pressure, the material is compressed not only in the direction of deformation, but also in the plane perpendicular to it. In addition, the negative coefficient of thermal expansion leads to a decrease in the volume of the material when it is heated.
Manifestation of negative Poisson's ratio
In addition, the authors carried out quantitative estimates of the properties of the airgel. The researchers found that the airgel is able to completely restore its original volume after quasi-static compression, in which it shrinks 20 times. In addition, its Young's modulus practically does not decrease tenfold after one hundred compression cycles.
The researchers also measured the thermal properties of the material and found that it tolerates sudden changes in temperature well. They changed the temperature of the airgel between −198 and 900 degrees Celsius at speeds up to 275 degrees per second. After 500 such temperature jumps, the morphology of the material did not change, and its ultimate strength did not practically decrease. Finally, the authors measured one of the main parameters of aerogels and found that the material has a thermal conductivity of 20 milliwatts per meter-kelvin in air and 2.4 milliwatts per meter-kelvin in vacuum. For comparison, the thermal conductivity of air is 22-24 milliwatts per meter-kelvin.
The low density of the aerogels led the researchers to speculate that some lightweight and durable materials could allow an airgel to float in air due to its low density. However, in 2017, American scientists showed that in reality it is impossible to create a graphene airgel with a density lower than air, because in this case, external pressure would simply compress the material.
