Its density is lower than that of air, so it is a magical tool for keeping warm and insulating!

In the process of human history development, the discovery and invention of new materials have played a vital role in promoting human civilization. At present, a rough way to divide human history is to divide it according to the names of materials: the Stone Age, the Bronze Age and the Iron Age.

Since the beginning of the 20th century, the rapid development of human science and technology has led to the emergence of new materials in an endless stream. Recently, a material has set 15 "world records". Its most dazzling performance is ultra-lightness and heat insulation , and therefore it has shown amazing application potential in various industrial fields. This material is aerogel.

A piece of silica aerogel (Image source: Wikipedia)

One of the most important applications of aerogels recently is in aerospace. When the Zhurong rover landed on Mars in 2021, it experienced the test of "ice and fire". During the landing phase of the rover, the heat generated by the engine caused the surrounding temperature to reach over 1000°C; during the patrol phase, the rover had to work in an environment of -130°C. The researchers laid a large area of ​​aerogel panels on the surface of the rover to protect it in such an extreme environment. Such excellent performance also makes aerogel materials shine.

Aerogel, the lightest solid

Aerogel is a solid material with very low density. Because of its low density, it is also known as "frozen smoke", "solid air", "blue smoke", etc. It is a nano-scale porous structure gel whose pores are filled with gas. It has the characteristics of extremely low density, high transparency and excellent thermal insulation performance.

So how is aerogel made? To get it, you first need to prepare a wet gel (the jelly we loved to eat when we were young is a common wet gel), then extract all the solvent inside while ensuring that its shape does not change, and you can get an aerogel. **Of course, the most difficult part is how to remove the liquid and only keep the solid skeleton.

Aerogel and wet gel (Image source: author)

In 1931, Professor Kistler of the Pacific College in California first used supercritical drying technology to produce the first recorded aerogel in human history. The key to this technology lies in the unique properties of supercritical fluids. When the solvent reaches the critical point between liquid and gas at a specific temperature and pressure, it becomes a supercritical fluid. The solvent in the supercritical state has no obvious surface tension, so that the gel can maintain an intact skeleton structure during the drying process.

Supercritical drying is a technical means to carry out the entire drying process under the critical pressure and critical temperature conditions of the drying medium. Usually, the wet gel is first placed in a high-pressure container. Under the supercritical state, the medium fluid (usually carbon dioxide) penetrates into the interior of the object to be dried, gently and quickly exchanges with the solvent molecules, replaces the solvent, and then changes the operating parameters to change the fluid from the supercritical state to the gaseous state and release it from the dried raw material. Since the solvent has no gas-liquid phase separation, it can leave the gel pores very peacefully without destroying the structure of the gel, thereby achieving the drying effect.

Aerogel has a three-dimensional network structure, similar to a porous sponge, except that the pores of a sponge are visible to the naked eye, while the pore structure of aerogel is nanometer-sized. The porosity of aerogels currently produced is generally between 80% and 99.8%, and the typical pore size is in the range of 50 nm. Its density is smaller than any other substance, and even smaller than air in a vacuum state. If the lightest aerogel is placed on a blooming flower, the stamens will not be bent.

Chinese scientists have recently successfully prepared all-carbon aerogel using an aqueous solution containing graphene and carbon nanotubes. It is currently the world's lowest density solid material. The density of this aerogel is 0.16mg/cm3, which is only 1/6 of the density of air. Aerogel also has a three-dimensional nano-skeleton structure, and its anisotropic three-dimensional network connection ensures its high elasticity. Because of this, it can withstand great pressure without being damaged.

Microscopic characterization of different aerogels (Image source: author)

How does aerogel insulate? Three weapons

To understand the thermal insulation method of aerogel, we first need to review the knowledge of junior high school physics: heat transfer. There are three ways of heat transfer: convection, radiation, and conduction.

Thermal convection occurs in fluids, and is a phenomenon in which hot particles transfer heat energy from one place in space to another through a flowing medium; thermal radiation is a phenomenon in which an object with temperature radiates electromagnetic waves. The higher the temperature, the higher the energy of the radiated electromagnetic waves; in heat conduction, particles in an object collide with each other, causing heat to be transferred from a high temperature area to a low temperature area.

Three mechanisms of heat transfer Image source: Wikipedia

Aerogel insulation starts from these three aspects. Its unique structure enables it to have the following three insulation "weapons":

"Zero convection" effect : Heat conduction mainly transfers energy through the continuous collision of gas molecules. Scientifically, the distance that gas molecules can move without colliding with surrounding molecules is called molecular free path. The average free path of air is 70 nm. The average pore size inside aerogel is about 20-50 nm, which is much smaller than the free path of air. Therefore, the convective heat transfer is very small and the gas heat conduction is greatly reduced.

"Infinite thermal baffle" effect : The pore size of aerogel is at the nanometer level. Although there are a lot of pores, the staggered pores make it impossible to have "through holes". The infinite pore walls are equivalent to heat shields, which minimizes the heat conducted by thermal radiation. Combined with special anti-radiation materials, it can more effectively block radiation heat transfer.

"Infinite path" effect : Heat can only be transferred along the pores. Infinite pores make the heat transfer path infinitely long, so that the transferred heat tends to be minimal. When aerogel materials are used to block heat, its slender skeleton makes heat conduction very difficult, which is equivalent to letting heat walk along a maze on a three-dimensional network along a narrow path; and a large number of nanopores are like separate rooms, locking up individual gas molecules, so that the gas molecules can neither flow nor contact each other, thereby eliminating convective heat transfer.

When a flame gun is used to heat aerogel, the crayons placed on it will not melt, which shows the excellent thermal insulation ability of aerogel (Image source: NASA)

What can aerogel be used for? Just imagine!

Aerogel has developed from the original SiO2 aerogel into a large aerogel family. Aerogel materials can be divided into single-component and multi-component aerogels according to their components. Single-component aerogels include oxide aerogels, carbide aerogels and other types of aerogels, while multi-component aerogels refer to aerogel composite materials composed of two or more single-component aerogels or reinforced by fibers, whiskers, nanotubes, etc.

The continuous three-dimensional network structure of aerogel makes it show unique properties in thermal, mechanical, acoustic, optics, electrical, adsorption and other aspects, and has broad application prospects in various fields.

Among them, the most eye-catching is in the aerospace field . Due to its excellent physical and chemical properties such as high porosity, high specific surface area, low density, and good insulation performance, it has been successfully used in the Long March rocket series, Mars rovers, and space suits.

(Image source: author)

Recently, a research team from Zhejiang University invented a "polar bear sweater" that is warmer than a down jacket and was published in the top magazine Science. Scientists simulated the porous structure of polar bear hair and prepared a super-warm artificial fiber that encapsulates aerogel with composite elastic thermoplastic materials, truly realizing the wearing of warm aerogel on the body.

However, it should be noted that many businesses in the market have also labeled their clothes "aerogel" as winter approaches, claiming their ability to keep out the cold, but their actual thermal insulation ability is often unsatisfactory. This is mainly due to the inherent brittleness of aerogel. The current "aerogel" winter clothing is mixed with some aerogel particles in the clothing, but a large number of gaps in the clothing still transmit heat, and the thermal insulation effect is far less than expected, even worse than ordinary down jackets. In addition, there are a series of problems such as the material cannot be machine washed and it is easy to lose its thermal insulation ability in a humid environment. So, you know whether the current "aerogel down jacket" is a waste of money.

In addition, in terms of adsorption and separation , aerogel has a strong adsorption capacity due to its internal structure similar to that of a sponge. The "carbon sponge" can adjust its shape at will, has excellent elasticity, and can still return to its original shape after being compressed by 80%. All-carbon aerogel can absorb oil up to 900 times its own weight. Every year, a large amount of crude oil pollution occurs on the sea surface and cannot be handled, so it can be used to make tools for adsorbing oil in the future.

In addition, aerogels also have important applications in electricity and chemistry . The three-dimensional network structure of porous carbon aerogels can not only form high-end conductive channels for charge transport, but also serve as a skeleton for doping or coating various organic or inorganic active materials. Once used as a catalyst fixed bed in chemical production, it can greatly increase the catalytic reaction area, thereby increasing the chemical reaction rate.

Finally, due to its high porosity, biocompatibility and biodegradability, aerogels are widely used in the medical field.

Application areas of aerogel (Image source: author)

Of course, the field of aerogel is not mature enough, and a lot of research work needs to be done. For example, the physical and chemical changes that occur during the aging (liquid solidification) process of the gel are not clear, and the influence of various preparation parameters on its properties needs further exploration, and the plasticity is not strong. But I believe that in the future, this magical material of aerogel will create a healthier, smarter, more reliable and better living environment for mankind.

References:

[1] PIERRE AC, PAJONK GM. Chemistry of aerogels and their applications [J]. Chemical Reviews, 2002,102: 42434265.

[2] BLASZCZYNSKI T, SLOSARCZYK A, MORAWSKI M. Synthesis of silica aerogel by supercritical drying method [J]. Procedia Engineer, 2013, 57: 200206.

[3] BAG S, KANATZIDIS MG. Chalcogels: porous metal-chalcogenide networks from main-group metal ions.effect of surface polarizability on selectivity in gas separation [J]. Journal of the American Chemical Society, 2010, 132: 1495114959

[4] Zhang Ting, Zhao Chunlin, et al., Research progress of aerogels, Advanced Ceramics, 1005-1198 (2018) 01-0001-39

Author: Wang Zhen

Author unit: Institute of Solid State Physics, Hefei Institutes of Physical Science, Chinese Academy of Sciences

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