Getting to know aerogels

I. Aerogels: Definition and a Century of Development

When aerogels are mentioned, many people associate them with the label of "the lightest solid material," but their connotation goes far beyond that. Initially, aerogels specifically referred to solid materials with a hierarchical pore structure prepared by supercritical drying. However, with the enrichment of material systems (such as carbon-based, metal oxide-based, polymer-based, etc.) and innovations in preparation technologies (such as freeze-drying, ambient pressure drying, etc.), their definition has been continuously expanded. Today, the authoritative definition given by the International Union of Pure and Applied Chemistry (IUPAC) — "a gel composed of a microporous solid in which the dispersion medium is a gas" — accurately summarizes its core characteristics: in the microporous structure constructed by the solid framework, gas occupies most of the space, which is also the source of its many excellent properties.

The story of aerogels began in 1931, when American scientist Professor Kistler successfully prepared SiO₂ aerogel for the first time using the sol-gel method, breaking people's perception of traditional solid materials. In the nearly 100 years since then, researchers have continuously broken through technical bottlenecks: from being difficult to mass-produce in the early days to now being able to stably prepare aerogel products of different forms and functions; from a single SiO₂ aerogel to multi-system materials including carbon aerogels, alumina aerogels, polymer aerogels, etc. Among them, aerogel blankets, as one of the most practical forms, relying on excellent thermal insulation performance, have derived segmented products such as high-temperature aerogel blankets (capable of withstanding thousands of degrees Celsius), aerogel fire blankets (with both flame retardant and thermal insulation properties), nano-aerogel blankets (improving performance relying on nano-scale frameworks), and ultra-thin aerogel blankets (meeting the application needs of narrow spaces), laying the foundation for their industrialization and proximity to daily life.

II. Microstructure of Aerogels: The "Nanoscale Code" Determining Properties

To understand the "magic" of aerogels, one must delve into their microscopic world. Taking the most classic SiO₂ aerogel as an example, its microstructure is like a three-dimensional network woven from nanoparticles: SiO₂ particles with a diameter of only 1-5nm serve as basic units, connected to form an intricate chain structure, and the gaps between particles form a large number of micropores and mesopores. This structure is not unique; although aerogels of different systems (such as nanofiber aerogels, nanosheet aerogels) have different basic unit forms (one-dimensional fibers, two-dimensional nanosheets), they all take the "three-dimensional porous network" as the core feature.

It is this unique microstructure that endows aerogels with a series of amazing properties: first, extremely low density. Due to the minimal proportion of the solid framework's mass, the density of aerogels is usually between 0.001-0.3g/cm³, even lower than the density of air, so they are also called "frozen smoke"; second, ultra-high specific surface area. The specific surface area of each gram of aerogel can reach several hundred or even thousands of square meters, equivalent to the area of a football field condensed into a material the size of a fingertip; most importantly, excellent thermal properties. The high porosity greatly inhibits the thermal conduction of gas molecules, and the nanoscale structure of the solid framework reduces the heat transfer of phonons. Therefore, the thermal conductivity of aerogels is extremely low, and high-quality aerogel thermal insulation materials can have a thermal conductivity as low as 0.012W/(m·K) or less, far superior to traditional thermal insulation materials such as glass wool and rock wool.

However, early aerogels also had obvious performance shortcomings: the chain three-dimensional network structure led to weak mechanical properties, high brittleness, low strength and modulus, and they were easy to break during processing and use. This is why early SiO₂ aerogels were mostly used in powder form or needed to be compounded with other materials (such as glass fibers, organic polymers) to improve toughness. With the advancement of technology, researchers have successfully opened up two important directions for improving the mechanical properties of aerogels through structural design and process optimization: one is the development of highly flexible and resilient aerogels. By regulating the flexibility of the framework and the elasticity of the network structure, aerogels can quickly return to their original shape after being subjected to external forces such as compression and bending. Such aerogels are especially suitable for fields such as flexible electronics and wearable devices; the second is the preparation of high-strength and high-modulus rigid aerogels. By enhancing the bonding force between particles and optimizing the network density, the compressive strength and rigidity of aerogels are improved, enabling them to be used independently as structural materials, expanding application scenarios in aerospace, building structures and other fields.

III. Applications of Aerogels: From High-End Fields to "Safety Guardians" in Daily Life

The excellent properties and designability of aerogels have continuously expanded their application fields, gradually penetrating from initially high-end fields such as aerospace to construction, environmental protection, medical care, daily safety protection, etc., becoming "super materials" that silently guard people's lives.

(1) Thermal Management Materials: Guarding Temperature Safety with Extreme Thermal Insulation

The extremely low thermal conductivity of aerogels makes them ideal thermal insulation materials, playing an irreplaceable role in the fields of "temperature control" and "fire prevention". In the aerospace field, when spacecraft enter the atmosphere, they will face high temperatures of thousands of degrees Celsius. Aerogels (such as SiO₂ aerogels and alumina aerogels) with light weight and good heat resistance can be used as thermal insulation layers to effectively block the damage of high temperatures to the cabin and internal equipment, ensuring the safety of astronauts; in the industrial field, high-temperature aerogel blankets are widely used in the heat preservation of high-temperature pipelines and kilns, which can not only reduce heat loss and energy consumption but also prevent scalding accidents caused by excessively high temperatures on the outer walls of pipelines.

In the construction industry, aerogel thermal insulation materials are becoming the "new favorite" for energy-saving and thermal insulation. Traditional building insulation materials (such as polystyrene boards, rock wool) either have limited thermal insulation performance or have fire hazards. However, aerogel blankets (especially ultra-thin aerogel blankets) not only have extremely low thermal conductivity, which can significantly improve the thermal insulation performance of buildings, reduce energy consumption for winter heating and summer cooling, but also have excellent flame retardant performance, which can effectively curb the spread of fire and add a line of defense for residents' living safety. In addition, aerogels can also be used as encapsulation carriers for phase change materials (such as paraffin) — their ultra-large specific surface area can provide more interfaces, promote heat transfer between phase change materials and the external environment, and improve the efficiency of the phase change process. This composite system can be used for passive temperature control of buildings, making indoor temperatures more stable and comfortable.

(2) Adsorbent Materials: "Nanoscale Sponges" for Environmental Purification

The high porosity and large specific surface area of aerogels make them like "nanoscale sponges", which can efficiently adsorb harmful substances in gases and liquids, showing great potential in environmental protection and daily purification fields. In terms of water purification, aerogels (especially hydrophobically modified aerogels) can quickly adsorb oil pollutants in water, with an adsorption capacity of dozens or even hundreds of times their own weight. Moreover, by endowing them with resilience or magnetism, aerogels after adsorption saturation can be easily recovered, desorbed and regenerated, realizing recycling and effectively solving water pollution problems caused by oil spills; in terms of air purification, aerogels can adsorb harmful gases in the air such as formaldehyde, benzene, sulfur dioxide, nitrogen oxides, etc. Especially when compounded with photocatalytic materials (such as titanium dioxide), they can catalytically degrade these harmful gases into harmless substances under light conditions, achieving the dual functions of "adsorption + purification" and protecting people's respiratory health.

In daily life, this adsorption performance can be extended to more scenarios: for example, making aerogels into refrigerator deodorants, whose adsorption capacity is far superior to traditional activated carbon, and can remove odors more persistently; applying aerogels to shoe insoles can absorb moisture and odors generated by feet, keeping feet dry; in the automotive field, aerogel adsorbent materials can be used for in-car air purification, reducing the impact of formaldehyde and VOCs (volatile organic compounds) on the health of drivers and passengers.

(3) Catalyst Carriers: "Efficient Platforms" for Promoting Chemical Reactions

The ultra-large specific surface area and porous structure of aerogels make them ideal catalyst carriers — they can uniformly load catalyst particles (such as metal nanoparticles, metal oxide catalysts) and provide sufficient diffusion channels for reactants, significantly improving the efficiency of catalytic reactions. In the field of environmental protection, aerogel-based catalysts can be used for advanced treatment of industrial wastewater, such as degrading refractory organic substances in wastewater such as dyes and pesticide residues; in the energy field, aerogel carriers can be used for electrode materials of fuel cells and catalyst carriers for CO₂ reduction reactions, promoting the development of clean energy; in the field of organic synthesis, aerogel-supported catalysts can improve the selectivity and conversion rate of reactions, reduce the generation of by-products, in line with the development concept of green chemistry.

It is worth mentioning that aerogels themselves can sometimes play a synergistic role of "adsorption + catalysis": for example, when treating sulfur dioxide and nitrogen oxides in industrial waste gas, aerogels first capture and concentrate harmful gases through adsorption, and then convert them into recyclable substances such as ammonium sulfate and ammonium nitrate through the loaded catalysts, which not only solves the pollution problem but also realizes the recycling of resources. This "killing two birds with one stone" characteristic makes aerogels highly competitive in the field of environmental protection catalysis.

(4) Sensing and Detection Devices: "Sensitive Antennas" for Perceiving the Environment

Compared with traditional foam materials, aerogels (especially nanofiber aerogels) have better mechanical properties — they can withstand greater compressive deformation, and have excellent compression rebound and fatigue resistance. Even after tens of thousands of compression cycles, they can still maintain stable performance. This characteristic makes them ideal substrates for high-performance sensors: for example, pressure sensors made by compounding aerogels with conductive materials (such as carbon nanotubes, graphene) not only have high sensitivity (can sense the pressure of a few grams of weight) but also have good stability and repeatability, and can be used in smart wearable devices (such as heart rate monitoring, gait analysis), electronic skin, industrial pressure detection and other fields; in terms of humidity sensing, the porous structure of aerogels can quickly adsorb or release water molecules, leading to changes in their electrical properties (such as resistance, capacitance). Humidity sensors made based on this have fast response speed and wide detection range, and can be used in indoor humidity monitoring, food preservation humidity control and other scenarios.

In the field of safety detection, aerogel sensors can also play an important role: for example, using the adsorption sensitivity of aerogels to specific gases (such as natural gas, carbon monoxide), gas leakage detectors can be made. Once the concentration of harmful gases in the environment exceeds the standard, the sensor can quickly respond and issue an alarm, ensuring the safety of families and industrial places; in terms of fire detection, the thermal sensitivity of aerogels enables them to quickly sense temperature changes. Combined with their flame retardant performance, they can be made into early fire warning devices, gaining valuable time for personnel evacuation and fire fighting.

(5) Other Fields: Multi-Functional Expansion, Close to Every Aspect of Life

In addition to the above core fields, the multi-functionalization of aerogels has also enabled them to play a role in more scenarios, gradually integrating into people's daily lives. In terms of anti-electromagnetic interference, aerogels compounded with conductive materials can be made into lightweight and flexible electromagnetic shielding materials, used for internal shielding of electronic devices such as mobile phones and computers, reducing the impact of electromagnetic radiation on the human body; in the field of seawater desalination, the porous structure and hydrophilic/hydrophobic regulation performance of aerogels can be used for the support layer of seawater desalination membranes or pre-treatment adsorption materials, improving desalination efficiency and reducing energy consumption; in the field of tissue engineering, polymer aerogels with good biocompatibility (such as gelatin aerogels, chitosan aerogels) can be used as cell scaffolds, providing a suitable microenvironment for cell growth and differentiation, promoting the development of regenerative medicine; in terms of sound insulation, the porous structure of aerogels can effectively absorb sound energy, especially high-frequency sound waves, and can be used for building sound insulation and automotive interior sound insulation, creating a quieter and more comfortable living and travel environment for people.

It is particularly worth mentioning aerogel fire blankets, which, with their characteristics of light weight, softness and excellent flame retardant performance, have gradually entered families and public places: in families, aerogel fire blankets can be used as fire-fighting equipment in the kitchen. In case of oil pan fire, simply covering the fire blanket on the oil pan can quickly cut off the air and put out the flame, avoiding the expansion of the fire; in public places (such as shopping malls, hotels), aerogel fire blankets can be used for personal protection during fire escape. Wrapping the body can effectively block high temperatures and flames, gaining time for escape and becoming a "personal guard" to protect life safety.

IV. Multi-Functional Design of Aerogels: "Super Materials" Moving Towards the Intelligent Era

With the development of science and technology, materials with a single function can hardly meet the needs of complex scenarios. The multi-functional design of aerogels has become an important direction of their development — by integrating multiple materials and introducing responsive components, aerogels can not only achieve multiple functions at the same time but also make adaptive adjustments according to changes in the external environment, gradually transforming into intelligent materials.

The core of the multi-functional design of aerogels lies in the "synergistic effect": for example, compounding aerogels with electromagnetic shielding materials (such as metal meshes, conductive polymers) can produce materials with both thermal insulation and electromagnetic protection functions. Such materials can be used for the外壳 of aerospace equipment — not only blocking external high temperatures but also preventing electromagnetic interference from affecting internal electronic equipment; in the field of environmental protection, aerogels can integrate adsorption, catalysis and magnetic response functions at the same time — capturing pollutants through adsorption, degrading pollutants through catalysis, and realizing material recovery through magnetic response. The entire process is efficient, environmentally friendly and recyclable; in the construction field, transparent aerogel films (such as SiO₂ transparent aerogel films) integrate hydrophobic self-cleaning, thermal insulation, sunlight reflection, flame retardancy and other functions: hydrophobic performance makes the film surface not easy to be contaminated with dust, and rainwater can achieve self-cleaning; thermal insulation performance reduces heat exchange between inside and outside the building; sunlight reflection performance reduces solar radiation heat gain of buildings in summer; flame retardant performance improves the fire safety level of buildings. This "one material with multiple functions" characteristic can not only replace traditional materials such as glass, insulation layers, and fireproof coatings but also reduce construction difficulty and building weight, providing a new direction for the development of green buildings.

With the rise of intelligent materials, "responsive aerogels" have become a research hotspot — by introducing components sensitive to external stimuli such as temperature, humidity, pH value, light, and magnetic fields, aerogels can automatically adjust their own properties according to environmental changes to achieve "intelligent control and self-regulation". For example, in the field of building energy conservation, temperature-responsive aerogels can automatically increase thermal resistance when the temperature rises (such as at noon in summer) to reduce heat entering the room; automatically reduce thermal resistance when the temperature drops (such as at night in winter) to prevent indoor heat from being easily lost, realizing "adaptive optimization" of building energy consumption; in terms of humidity regulation, humidity-responsive aerogels can absorb moisture when indoor humidity is high and release moisture when humidity is low, automatically maintaining indoor humidity within a comfortable range, reducing the use of humidifiers and dehumidifiers; in the medical field, pH-responsive aerogels can be used as drug carriers — remaining stable in the normal human environment (neutral pH) and automatically releasing drugs when entering the lesion site (such as the acidic environment of tumor tissue), improving the targeting and therapeutic effect of drugs.

V. Future Outlook: Aerogels, Making Life Safer and More Comfortable

From "novel materials" in laboratories to "performance benchmarks" in industrial fields, and then to "safety guardians" in daily life, the development process of aerogels has confirmed the truth that "technology changes life". Today, with the continuous maturity of preparation processes (such as the large-scale application of ambient pressure drying which reduces production costs), continuous optimization of performance (such as the improvement of mechanical properties solving the brittleness problem), and continuous expansion of application scenarios (from high-end fields to civil markets), aerogels are approaching people's lives at a faster pace.

In the future, we have reason to believe that aerogels will guard our safety and comfort in more fields: in families, aerogel thermal insulation materials will make houses more energy-efficient and fireproof, aerogel deodorants will make the air fresher, and aerogel fire blankets will become a "standard configuration" for family safety; in travel, aerogel sound insulation materials will make cars and high-speed trains quieter, and aerogel thermal insulation materials will make batteries safer (preventing overheating and fire of new energy vehicle batteries); at work, aerogel adsorption materials will make office air cleaner, and aerogel anti-electromagnetic interference materials will make electronic equipment more stable; in emergency rescue, aerogel fire blankets and aerogel thermal insulation blankets will become "powerful assistants" for rescuers, guarding life safety.

The story of aerogels is an exploration journey about "nanoscale power". With its extreme performance, diverse functions and unlimited potential, it is moving from the "laboratory" to the "life circle", guarding everyone's safety and comfort with the warmth of technology. In the future, when we enjoy energy-efficient houses, safe travel and a clean environment, we may not realize that the silent guardianship comes from aerogels — the nanoscale care brought by this "super material".