The Birth of Aerogel
In 1931, Kistler separated the liquid from the solid by using an alcoholic supercritical drying technique and kept the structure of the gas from shrinking and collapsing, resulting in aerogel in the world.

Definition of Aerogel
Aerogel is a lightweight nano-porous amorphous solid material with nanometer-scale micro-particles aggregated with each other to form a nanometer-scale porous network structure and filled with a large number of gaseous dispersion media in the network skeleton.

Aerogels are not only a functional material, but also a new state of matter. The aerogel state is fundamentally different from other states of matter in many properties. It has a fixed size and shape like solid matter, yet its apparent density can span from a maximum of 1 g/cm³ to 0.001 g/cm³ (lower than the density of air); at the same time, it has a sponge-like porous structure, with multiple properties of microscopic (nano-scale skeleton), macroscopic (condensed matter) and mesoscopic (multi-level and fractal structure) characteristics, making aerogels possess many unique properties. It exhibits very low thermal conductivity, very low modulus of elasticity, very low phonon rate, very low refractive index, low dielectric constant, very low speed of sound, high specific surface area, very wide density and refractive index distribution, etc.

Classification of Aerogels
There is a wide range of aerogels, covering almost all material systems. Generally, according to the chemical composition, they can be divided into organic aerogels, inorganic aerogels and composite aerogels, etc. Aerogels can also be classified into silicon, carbon, sulphur, metal oxide and metal systems according to their main components.

The Status & Role of Aerogels
Scientists say aerogels will change the world.

Aerogel was listed as one of the top 10 hot science and technology issues in the 250th issue of the US journal Science. On 20 August 2007, The Times reported that aerogel is the world’s lightest solid that can withstand the explosive power of 1kg of dynamite and keep you out of the heat of a blowtorch above 1300°C. Scientists are exploring the use of aerogel in everything from super-insulated space suits to supercapacitors to catalysts to the next generation of tennis rackets. Aerogel promises to rival generations of legendary materials such as phenolic resin from the 1930s, carbon fiber from the 1980s and silicone from the 1990s.

Aerogel has been listed in the Guinness Book of World Records several times as the world’s lightest solid. Aerogel also has a very low thermal conductivity and refractive index and is 39 times more insulating than the best glass fibers. Aerogel has been used to thermally insulate the Russian Mir space station and the US Mars Pathfinder rover, and has already played a vital role in the aerospace industry.

History of Aerogel Development
1931, aerogels were first prepared by Kistler at Stanford University, USA.
1985, the first symposium on aerogels, known as ISA, was organized by the Institute of Physics at the University of Würzburg, Germany.
1993, aerogels are used to insulate spacesuits, spacecraft and space shuttles.
2002, Aspen Aerogel, a NASA company, was established to develop and produce aerogels.
2004, aerogel research conducted by domestic Chinese research institutes.
2006, domestic aerogel companies in China started to develop aerogels.
2017, China promulgated, GB/T 34336-201, Nonporous Aerogel Composite Insulation Products
2021, the State Council issued, “Action Plan for Carbon Reaching the Peak by 2023” on accelerating the research and development of carbon fiber, aerogel, special steel and other basic materials.

The Key Properties of Aerogels
(1) Ultra-light. Aerogel is the world’s least dense solid, with a minimum density of one-sixth of the mass of air. The range of density variation is generally 0.001~1g/cm³.

(2) High porosity and high specific surface area. The porosity is 80%~99.9%, the specific surface area is 200~1000m2/g, and the typical size of the pores is 1~100nm.

(3) Super thermal insulation. The aerogel’s slim nano-network structure effectively limits the propagation of local thermal excitation, and its solid-state thermal conductivity is 2~3 orders of magnitude lower than that of the corresponding glassy material.

The principle of aerogel thermal insulation
Reduced thermal transfer: The infinite number of nanoparticles in aerogel forms a mesh skeleton, and heat can only flow along an infinite number of nanoparticles. The heat flow can only be conducted along an infinite number of nanoparticles, resulting in a nearly infinite path of heat transfer in the solid. The heat transfer in the solid is reduced to the lowest possible level.

Inhibition of thermal convection: The average pore size of aerogels is 20-40 nm, which is smaller than the average free range of gas molecules of 69 nm, and the gas molecules in the pores lose their ability to move freely, making convection almost impossible.

Reduced thermal radiation: The pore walls formed by the nanoparticles in aerogels are infrared shields, and the infinite number of shields greatly reduces thermal radiation.

(4) Catalysis. The small particle size, high specific surface area and low density of aerogel make the activity and selectivity of aerogel catalysts much higher than conventional catalysts, and the active components can be very uniformly dispersed in the carrier, and it also has excellent thermal stability, which can effectively reduce the occurrence of side reactions.

(5) Sound insulation. The low sound velocity properties of aerogels make them an ideal material for acoustic delay or high temperature sound insulation. The variable range of acoustic impedance of aerogel is large [103 ~ 107kg/(㎡-s)], which is a more ideal acoustic resistance coupling material for ultrasonic detectors. The acoustic resistance of piezoelectric ceramics used for ultrasonic generators and detectors is 1.5×107kg/(㎡-s), while the acoustic resistance of air is only 400kg/(㎡-s). Using silicon aerogel with a thickness of 1/4 wavelength and a density of about 0.3g/cm³ as the acoustic resistance coupling material between piezoelectric ceramics and air can increase the sound intensity by 30dB, improve the transmission efficiency of acoustic waves and reduce the signal-to-noise ratio in device applications.

(6) Filtration performance. Nanostructured aerogels can also be used as a new type of highly efficient gas filtration material. Because of its exceptionally large specific surface area, aerogels are referred to by scientists as “super sponges” and are ideal for adsorbing pollutants in water, sucking out lead and mercury from water, making them an excellent material for dealing with ecological disasters.

(7) Optical properties. Pure SiO₂ aerogel is transparent and colorless, and its refractive index (1.006~1.06) is very close to that of air, which means that SiO₂ aerogel has almost no reflection loss to incident light and can effectively transmit sunlight. Therefore, it can be used to make insulation and noise reduction glass.

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