It is impervious to water and fire and can capture comet particles. What is the origin of this material?

Aerogel is the lightest solid material in the world (graphene aerogel is the lightest solid in the world recognized by Guinness), because it contains 97% air and 3% solid structure, which is only 1.5 times the density of air.

In addition to being very light, aerogels also have very good thermal insulation capabilities, mainly due to the "Knudsen effect." Since aerogels are mostly composed of silica and air, silica solids have average thermal conductivity, while air has a very low thermal conductivity.

In addition, aerogels have many nanoscale pores, which make it difficult for air to diffuse through the aerogel to transfer convective heat.

Therefore, aerogels often appear in environments that require high temperature resistance. For example, aerogels are used for heat preservation on the Mars rover.

In addition, the water repellency of aerogel mainly comes from modification. By converting the surface polar -OH into non-polar -OR, hydrophobic aerogel can be obtained.

Such a material that is "impervious to water and fire" seems to be the latest product of modern technology, but in fact, aerogel has appeared in the 1930s and was first produced by chemist Samuel Keesler.

The birth of the first aerogel

Gel substances are actually very common. The jelly we eat is a gel substance, which is a combination of solid and liquid.

It happened that Samuel and his colleague Charles Learned made a bet with jelly. Charles believed that the reason why jelly becomes a gel is due to its liquid properties, but Samuel believed that the solid structure in the gel is also the key to the formation of gel.

In order to verify who was right, Samuel began experiments to prove that wet gels contain continuous solid networks of the same size and shape . The purpose of the experiment was very simple, which was to remove the liquid in the gel and retain the solid structure, thus proving that the gel had nothing to do with the liquid in it.

But it is easier said than done. If the liquid in the gel is simply allowed to evaporate, the corresponding solid structure will inevitably shrink.

Because after the liquid molecules are removed, they will attract each other, thereby pulling on the surrounding solid structure, the gel will "collapse" from the inside until it shrinks to 1/10 of its original volume.

This method definitely wouldn't work. After much thought, Samuel concluded that only by replacing the liquid inside the gel could the integrity of the solid structure be ensured.

If we want to replace it, we must use gas, because the gel already contains both solid and liquid states of matter.

However, normal gas certainly cannot replace the gel liquid, so Samuel chose to take a roundabout way to save the situation. He pressed and heated the liquid to make it break through the critical point, so that the liquid became a supercritical fluid (there is basically no difference between liquid and gas), and the molecules no longer have mutual attraction.

Samuel selected sodium silicate as the raw material, used hydrochloric acid to promote hydrolysis, and used water and ethanol as solvent exchangers to transform it into alcohol gel. He then placed the alcohol gel in a high temperature and high pressure environment. After the ethanol in it became a supercritical fluid, he continued to maintain the critical temperature while reducing the pressure on the gel. As the pressure decreased, the ethanol molecules were released as gas.

Samuel in the lab

The gel is then removed from the heat source, and after cooling, the ethanol liquid originally in the gel turns into gas and evaporates, leaving only a solid structure filled with gas. This is the birth of the first aerogel.

Improvement of aerogel preparation method

There is no doubt that this research is epoch-making, but strangely, in the following 30 years, research on aerogels was almost stagnant, mainly because the preparation conditions at the time were relatively difficult and took a particularly long time.

It was not until 1970 that the University of Lyon, in search of a porous material that could store oxygen and rocket fuel, dug out aerogel from more than 30 years ago and improved the preparation method based on Samuel's.

The new preparation method uses alkoxysilane (TMOS) instead of sodium silicate and formaldehyde instead of ethanol. The gel alcohol gel produced in this way can produce higher quality silica aerogel and the time is also much faster. This method directly leads to a major advancement in aerogel science.

As the methods were improved, more and more researchers joined the aerogel field.

In 1983, the Microstructure Materials Group at Berkeley Lab discovered that the highly toxic compound TMOS could be replaced by the safer tetraethyl orthosilicate (TEOS), and used the sol-gel method to hydrolyze and polycondense TEOS.

Furthermore, the Microstructured Materials Group found that the alcohol in the gel could be replaced with liquid carbon dioxide before supercritical drying without damaging the aerogel.

This represents a major advance in safety, as CO2 does not present the same explosion hazard as alcohols .

Other Applications of Aerogel

As research on aerogels continued to deepen, particle physicists realized that this nanoscale material could be used to collect elusive Cherenkov radiation particles, because these particles have difficulty passing through the complex structure of the aerogel and thus remain inside the aerogel.

Capturing particle traces

In addition to collecting particles, silica aerogel prepared by NASA's Jet Propulsion Laboratory has also taken a "flight" to space and undertaken the task of collecting comet particles.

Having said so much, I believe everyone has understood the various properties of aerogel and the constantly improved preparation methods. No matter from which aspect, it is very excellent, but why has it not been popularized in people's life?

The first thing is production. Even though the preparation method has undergone many improvements, the most critical supercritical conditions still set a threshold.

Secondly, there is another severe challenge in the industrial production of aerogels, which is that aerogels are very brittle. Although they have a strong load-bearing capacity, unfortunately, their tension is very small and they can be "split in two" with a slight pinch, so other additives are generally needed.

There is also the issue of price that cannot be ignored. The price of about 6 cubic centimeters of aerogel is around RMB 350, so cost is also a factor that restricts production and application.

But these problems are not enough to outweigh its merits. From its birth to now, aerogel still seems to be a "future material" that is many years ahead of its time and has great potential.