In this blog post, we will look at the principles and structure of aerogel made using supercritical drying technology, as well as its potential applications in various industries.
Can a solid material that is as light as air exist? Can a material consisting of 99% air and 1% solid exist? Imagine breaking all the concrete in a building, leaving only the steel reinforcement, or creating a gel mass that is 99% liquid and then removing all the liquid. However, if you try to remove the liquid inside this solid by drying it like a wet towel or applying heat like boiling water in a pot, the soft and squishy mass will dry up into a handful of powder.
In 1931, in a laboratory in California, Steve Kistler succeeded in creating a substance filled with 99% air, which had seemed impossible, using a special drying method. The reason why a large lump cannot maintain its shape and turns into a handful of powder is that when the liquid inside turns into gas, it cannot withstand the surface tension and capillary force acting on the solid skeleton and breaks apart. Supercritical drying is a drying method that has essentially overcome this limitation.
Just as water freezes at 0 degrees Celsius and boils at 100 degrees Celsius, substances reach a supercritical state when they exceed a certain temperature and pressure called the supercritical temperature. A substance in a supercritical state, which is both a liquid-like gas and a gas-like liquid, cannot be defined as either one, and the substance no longer feels any change in state between the two. In other words, if a liquid inside a solid is first converted to a supercritical state and then converted to a gas, the solid skeleton remains unaffected, and all the liquid inside can be replaced with gas. Using this supercritical drying method, a new material called aerogel, nicknamed “frozen air,” was created, leaving only the skeleton of the solid.
Imagine a soft, bear-shaped jelly (wet gel) that has been soaked in water all day long. Now, imagine removing all the water from it, leaving only a light, hollow skeleton that barely holds its shape. Aerogel, which is mostly empty space, is so light that you cannot feel it when you hold it in your palm. In addition to its lightness, aerogel has other interesting properties.
One of the most notable characteristics of aerogel, which has a very unique structure consisting of a solid skeleton made up of 90-99.8% air pockets, is its very low thermal conductivity. This is because it has very small air holes (mesopores) of 2-40 nm, so air particles passing through the holes can no longer freely transfer heat, and only the skeleton remains, which is very low in density, so almost no radiant heat is transferred. An area where these advantages are being maximized and commercialized is the use of silica (SiO2) aerogel as a super-insulating material.
In fact, when renovating historic buildings, it is used as insulation inside walls, and aerogel mixed with other organic materials is made into a blanket (aerogel blanket) and used as insulation in various parts of the building. NASA first used silica aerogel as a super-insulating material to withstand the extreme temperatures of Mars in 1996 on the Sojourner rover, a terrain explorer on the Mars Pathfinder spacecraft. Later, in 2003, silica aerogel mixed with graphite was used in the Mars Exploration Rover Spirit and Opportunity to further maximize the insulation effect.
Aerogel, which consists of extremely small particles and air pores at the nanometer level, has the advantage of having many reaction sites compared to other solid materials because its surface area in contact with the outside is maximized structurally. Many scientists have taken notice of this and are rushing to conduct research on highly reactive metal oxide aerogels. Representative examples include Al2O3 aerogel, which can be used as a supercapacitor utilizing the charging phenomenon caused by surface chemical reactions, and YSZ (Y2O3 doped ZrO2) aerogel, which is used as an anode material for fuel cells.
In addition, research is actively underway on functional aerogels that can be used in various fields, such as cardiovascular implantable devices, biomaterials such as drug delivery systems, and ultra-high-speed nuclear particle absorbers for spacecraft. The versatility of aerogels is leading to innovative changes in various industries.
Despite its recent discovery, aerogel has attracted numerous researchers as one of the dream materials that will change the future due to its unique structural characteristics, such as ultra-low density, maximized surface area, and high porosity of over 95%, which cannot be found in any existing solid materials. Based on advances in synthesis and processing technologies, aerogel is expected to bring about major innovations in an even wider range of applications in the future.
In particular, it is expected to contribute greatly to environmental issues and energy efficiency. The excellent thermal insulation performance of aerogel can play a major role in reducing energy consumption in buildings and heating and cooling costs. In addition, due to its potential for use in various industries, aerogel will establish itself as an important material for sustainable development.
