It was invented by a group of scientists led by Chinese professor Gao Chao from Zhejiang University and it created a sensation in the scientific world. Graphene, an incredibly light material in itself, is widely used in modern nanotechnology. And scientists managed to obtain a porous material from it - the lightest in the world.
Graphene airgel was made in the same way as other aerogels - by sublimation drying. A porous sponge made of carbon-graphene material almost completely copies any shape, which means the amount of airgel depends only on the volume of the container.
In terms of chemical properties, airgel has a density lower than hydrogen and helium. Scientists have confirmed its high strength and high elasticity. And this is despite the fact that graphene airgel absorbs and retains volumes of organic substances almost 900 times its mass! 1 gram of airgel can literally absorb 68.8 grams of any substance insoluble in water in a second. This is amazing and perhaps very soon all bars on poeli.ru and all hotels will use this material for some of their own purposes to attract visitors.
Another property of the new material was of great interest to the environmental community - the ability of the graphene sponge to absorb organic substances, which will help in eliminating the consequences of man-made accidents.
The potential property of graphene as a catalyst for chemical reactions is intended to be used in storage systems and in the manufacture of complex composite materials.
Since 2011, scientists have developed several innovative materials, which in turn held the title of “the lightest material on the planet.” First, an airgel based on carbon nanotubes (4 mg/cm3), then a material with a micro-lattice structure (0.9 mg/cm3), then aerographite (0.18 mg/cm3). But today the lightest material is graphene airgel, whose density is 0.16 mg/cm3.
This discovery, belonging to a group of scientists from Zhejiang University (China) under the leadership of Professor Gao Chao, caused a real sensation in modern science. Graphene itself is an unusually light material that is widely used in modern nanotechnology. First, scientists used it to create one-dimensional graphene fibers, then two-dimensional graphene ribbons, and now a third dimension was added to graphene, resulting in a porous material that became the lightest material in the world.
The method for producing porous material from graphene is called freeze drying. Other aerogels are prepared in the same way. The porous carbon-graphene sponge is capable of almost completely repeating any shape given to it. In other words, the amount of graphene airgel produced depends solely on the volume of the container.
Scientists boldly declare such qualities as high strength and elasticity. At the same time, garfen airgel is capable of absorbing and retaining volumes of organic substances up to 900 times its own weight! So, in a second, 1 gram of airgel can absorb 68.8 grams of any substance that does not dissolve in water.
This property of the innovative material immediately interested environmentalists. After all, in this way you can quickly eliminate the consequences of man-made accidents, for example, using airgel in oil spill sites.
In addition to environmental benefits, graphene airgel has enormous potential for energy, in particular, it is planned to be used in storage systems. In this case, the airgel can be a catalyst for certain chemical reactions. Also, graphene airgel is already beginning to be used in complex composite materials.
The combination of graphene and carbon nanotubes made it possible to obtain a carbon airgel without the disadvantages of aerogels made only of graphene or only of nanotubes. The new carbon composite material, in addition to the usual properties of all aerogels - extremely low density, hardness and low thermal conductivity - also has high elasticity (the ability to restore its shape after repeated compression and stretching) and excellent ability to absorb organic liquids. This latter property may have applications in oil spill response.
Let's imagine that we heat a closed vessel with liquid and vapor of this liquid. The higher the temperature, the more liquid will evaporate, turning into the gas phase, and the higher the pressure will be, and with it the density of the gas phase (in fact, the number of evaporated molecules). At a certain pressure and temperature, the value of which will depend on what kind of substance is in the vessel, the density of molecules in the liquid will be the same as in the gas phase. This state of liquid is called supercritical. In this state there is no difference between the liquid and gas phases, and therefore there is no surface tension.
Even lighter (less dense) aerogels are obtained by chemical deposition of a substance that will act as the solid phase of the airgel onto a previously prepared porous substrate, which is then dissolved. This method allows you to regulate the density of the solid phase (by regulating the amount of deposited substance) and its structure (by using a substrate with the required structure).
Due to their structure, aerogels have a set of unique properties. Although their strength approaches that of solids (Fig. 1A), their density is similar to that of gases. Thus, the best samples of quartz airgel have a density of about 2 mg/cm 3 (the density of the air included in their composition is 1.2 mg/cm 3), which is a thousand times less than that of non-porous solid materials.
Aerogels also have extremely low thermal conductivity (Fig. 1B), since heat must travel a complex path through a branched network of very thin chains of nanoparticles. At the same time, heat transfer through the air phase is also difficult due to the fact that these same chains make convection impossible, without which the thermal conductivity of air is very low.
Another property of the airgel - its extraordinary porosity - made it possible to deliver samples of interplanetary dust to Earth (see Stardust collector returns home, "Elements", 01/14/2006) using the Stardust spacecraft. His collection device was a block of airgel, falling into which, dust particles stopped with an acceleration of several billion g, without collapsing (Fig. 1C).
Until recently, the main disadvantage of airgel was its fragility: it cracked under repeated loads. All aerogels obtained at that time - from quartz, some metal oxides and carbon - had this drawback. But with the advent of new carbon materials - graphene and carbon nanotubes - the problem of obtaining elastic and fracture-resistant aerogels was solved.
Graphene is a sheet one atom thick in which the carbon atoms form a hexagonal lattice (each cell of the lattice is a hexagon), and a carbon nanotube is the same sheet rolled into a cylinder with a thickness of one to tens of nanometers. These forms of carbon have great mechanical strength, elasticity, a very high internal surface area, as well as high thermal and electrical conductivity.
However, materials prepared separately from graphene or separately from carbon nanotubes also have their drawbacks. Thus, a graphene airgel with a density of 5.1 mg/cm 3 did not collapse under a load exceeding its own weight by 50,000 times, and restored its shape after being compressed by 80% of its original size. However, due to the fact that graphene sheets have insufficient bending rigidity, a decrease in their density worsens the elastic properties of graphene airgel.
Airgel made from carbon nanotubes has another disadvantage: it is more rigid, but does not recover its shape at all after removing the load, since the nanotubes under load are irreversibly bent and entangled, and the load is poorly transferred between them.
Let us recall that deformation is a change in the position of particles of a physical body relative to each other, and elastic deformation is a deformation that disappears along with the disappearance of the force that caused it. The “degree” of elasticity of a body (the so-called elastic modulus) is determined by the dependence of the mechanical stress that arises inside the sample when a deforming force is applied on the elastic deformation of the sample. Stress in this case is the force applied to the sample per unit area. (Not to be confused with electrical voltage!)
As a group of Chinese scientists demonstrated, these shortcomings are completely compensated for if graphene and nanotubes are used simultaneously in preparing the airgel. The authors of the article discussed in Advanced Materials used an aqueous solution of nanotubes and graphene oxide, from which the water was removed by freezing and sublimating ice - lyophilization (see also Freeze-drying), which also eliminates the effects of surface tension, after which the graphene oxide was chemically reduced to graphene. In the resulting structure, graphene sheets served as a framework, and nanotubes served as stiffeners on these sheets (Fig. 2A, 2B). As shown by electron microscope studies, the graphene sheets overlap each other and form a three-dimensional framework with pores ranging in size from tens of nanometers to tens of micrometers, and carbon nanotubes form an entangled network and tightly adhere to the graphene sheets. This is apparently caused by the nanotubes being pushed out by growing ice crystals as the initial solution freezes.
The density of the sample was 1 mg/cm3 excluding air (Fig. 2C, 2D). And according to calculations in the structural model presented by the authors, the minimum density at which the airgel from the used starting substances will still retain the integrity of the structure is 0.13 mg/cm 3, which is almost 10 times less than the density of air! The authors were able to prepare a composite airgel with a density of 0.45 mg/cm 3 and a graphene-only airgel with a density of 0.16 mg/cm 3 , which is less than the previous record held by a ZnO airgel deposited on a substrate from the gas phase. Reducing the density can be achieved by using wider graphene sheets, but this reduces the stiffness and strength of the resulting material.
During testing, samples of such a composite airgel retained their shape and microstructure after 1000 repeated compressions to 50% of the original size. The compressive strength is approximately proportional to the airgel density and in all samples increases gradually with increasing strain (Figure 3A). In the range from –190°C to 300°C, the elastic properties of the resulting aerogels are almost independent of temperature.
The tensile test (Figure 3B) was carried out on a sample with a density of 1 mg/cm3, and the sample withstood a stretch of 16.5%, which is completely unthinkable for oxide aerogels, which crack immediately when stretched. In addition, the rigidity during tension is higher than during compression, that is, the sample is easily crushed, but stretched with difficulty.
The authors explained this set of properties by the synergistic interaction of graphene and nanotubes, in which the properties of the components complement each other. Carbon nanotubes covering graphene sheets serve as a bond between adjacent sheets, which improves load transfer between them, as well as stiffeners for the sheets themselves. Due to this, the load does not lead to the movement of the sheets relative to each other (as in an airgel made of pure graphene), but to elastic deformation of the sheets themselves. And because the nanotubes adhere tightly to the sheets and their position is determined by the position of the sheets, they do not experience irreversible deformation and entanglement and do not move relative to each other under load, as in an inelastic airgel made only of nanotubes. An airgel consisting equally of graphene and nanotubes has optimal properties, and with an increase in the content of nanotubes, they begin to form “tangles”, as in an airgel made only of nanotubes, which leads to a loss of elasticity.
In addition to the described elastic properties, composite carbon airgel also has other unusual properties. It is electrically conductive, and the electrical conductivity changes reversibly upon elastic deformation. In addition, the graphene and carbon nanotube airgel repels water, but at the same time perfectly absorbs organic liquids - 1.1 g of toluene in water was completely absorbed by a piece of airgel weighing 3.2 mg in 5 seconds (Fig. 4). This opens up excellent opportunities for eliminating oil spills and purifying water from organic liquids: just 3.5 kg of such airgel can absorb a ton of oil, which is 10 times more than the capacity of a commercially used absorbent. At the same time, the absorbent made from composite airgel is regenerable: thanks to its elasticity and thermal resistance, the absorbed liquid can be squeezed out like from a sponge, and the remainder is simply burned off or removed by evaporation. Tests have shown that the properties are maintained after 10 such cycles.
The variety of forms of carbon and the unique properties of these forms and materials obtained from them continue to amaze researchers, so we can expect more and more discoveries in this area in the future. How many things can be made from just one chemical element!
The combination of graphene and carbon nanotubes made it possible to obtain a carbon airgel without the disadvantages of aerogels made only of graphene or only of nanotubes. The new carbon composite material, in addition to the usual properties of all aerogels - extremely low density, hardness and low thermal conductivity - also has high elasticity (the ability to restore its shape after repeated compression and stretching) and excellent ability to absorb organic liquids. This latter property may have applications in oil spill response.
Let's imagine that we heat a closed vessel with liquid and vapor of this liquid. The higher the temperature, the more liquid will evaporate, turning into the gas phase, and the higher the pressure will be, and with it the density of the gas phase (in fact, the number of evaporated molecules). At a certain pressure and temperature, the value of which will depend on what kind of substance is in the vessel, the density of molecules in the liquid will be the same as in the gas phase. This state of liquid is called supercritical. In this state there is no difference between the liquid and gas phases, and therefore there is no surface tension.
Even lighter (less dense) aerogels are obtained by chemical deposition of a substance that will act as the solid phase of the airgel onto a previously prepared porous substrate, which is then dissolved. This method allows you to regulate the density of the solid phase (by regulating the amount of deposited substance) and its structure (by using a substrate with the required structure).
Due to their structure, aerogels have a set of unique properties. Although their strength approaches that of solids (Fig. 1A), their density is similar to that of gases. Thus, the best samples of quartz airgel have a density of about 2 mg/cm 3 (the density of the air included in their composition is 1.2 mg/cm 3), which is a thousand times less than that of non-porous solid materials.
Aerogels also have extremely low thermal conductivity (Fig. 1B), since heat must travel a complex path through a branched network of very thin chains of nanoparticles. At the same time, heat transfer through the air phase is also difficult due to the fact that these same chains make convection impossible, without which the thermal conductivity of air is very low.
Another property of the airgel - its extraordinary porosity - made it possible to deliver samples of interplanetary dust to Earth (see Stardust collector returns home, "Elements", 01/14/2006) using the Stardust spacecraft. His collection device was a block of airgel, falling into which, dust particles stopped with an acceleration of several billion g, without collapsing (Fig. 1C).
Until recently, the main disadvantage of airgel was its fragility: it cracked under repeated loads. All aerogels obtained at that time - from quartz, some metal oxides and carbon - had this drawback. But with the advent of new carbon materials - graphene and carbon nanotubes - the problem of obtaining elastic and fracture-resistant aerogels was solved.
Graphene is a sheet one atom thick in which the carbon atoms form a hexagonal lattice (each cell of the lattice is a hexagon), and a carbon nanotube is the same sheet rolled into a cylinder with a thickness of one to tens of nanometers. These forms of carbon have great mechanical strength, elasticity, a very high internal surface area, as well as high thermal and electrical conductivity.
However, materials prepared separately from graphene or separately from carbon nanotubes also have their drawbacks. Thus, a graphene airgel with a density of 5.1 mg/cm 3 did not collapse under a load exceeding its own weight by 50,000 times, and restored its shape after being compressed by 80% of its original size. However, due to the fact that graphene sheets have insufficient bending rigidity, a decrease in their density worsens the elastic properties of graphene airgel.
Airgel made from carbon nanotubes has another disadvantage: it is more rigid, but does not recover its shape at all after removing the load, since the nanotubes under load are irreversibly bent and entangled, and the load is poorly transferred between them.
Let us recall that deformation is a change in the position of particles of a physical body relative to each other, and elastic deformation is a deformation that disappears along with the disappearance of the force that caused it. The “degree” of elasticity of a body (the so-called elastic modulus) is determined by the dependence of the mechanical stress that arises inside the sample when a deforming force is applied on the elastic deformation of the sample. Stress in this case is the force applied to the sample per unit area. (Not to be confused with electrical voltage!)
As a group of Chinese scientists demonstrated, these shortcomings are completely compensated for if graphene and nanotubes are used simultaneously in preparing the airgel. The authors of the article discussed in Advanced Materials used an aqueous solution of nanotubes and graphene oxide, from which the water was removed by freezing and sublimating ice - lyophilization (see also Freeze-drying), which also eliminates the effects of surface tension, after which the graphene oxide was chemically reduced to graphene. In the resulting structure, graphene sheets served as a framework, and nanotubes served as stiffeners on these sheets (Fig. 2A, 2B). As shown by electron microscope studies, the graphene sheets overlap each other and form a three-dimensional framework with pores ranging in size from tens of nanometers to tens of micrometers, and carbon nanotubes form an entangled network and tightly adhere to the graphene sheets. This is apparently caused by the nanotubes being pushed out by growing ice crystals as the initial solution freezes.
The density of the sample was 1 mg/cm3 excluding air (Fig. 2C, 2D). And according to calculations in the structural model presented by the authors, the minimum density at which the airgel from the used starting substances will still retain the integrity of the structure is 0.13 mg/cm 3, which is almost 10 times less than the density of air! The authors were able to prepare a composite airgel with a density of 0.45 mg/cm 3 and a graphene-only airgel with a density of 0.16 mg/cm 3 , which is less than the previous record held by a ZnO airgel deposited on a substrate from the gas phase. Reducing the density can be achieved by using wider graphene sheets, but this reduces the stiffness and strength of the resulting material.
During testing, samples of such a composite airgel retained their shape and microstructure after 1000 repeated compressions to 50% of the original size. The compressive strength is approximately proportional to the airgel density and in all samples increases gradually with increasing strain (Figure 3A). In the range from –190°C to 300°C, the elastic properties of the resulting aerogels are almost independent of temperature.
The tensile test (Figure 3B) was carried out on a sample with a density of 1 mg/cm3, and the sample withstood a stretch of 16.5%, which is completely unthinkable for oxide aerogels, which crack immediately when stretched. In addition, the rigidity during tension is higher than during compression, that is, the sample is easily crushed, but stretched with difficulty.
The authors explained this set of properties by the synergistic interaction of graphene and nanotubes, in which the properties of the components complement each other. Carbon nanotubes covering graphene sheets serve as a bond between adjacent sheets, which improves load transfer between them, as well as stiffeners for the sheets themselves. Due to this, the load does not lead to the movement of the sheets relative to each other (as in an airgel made of pure graphene), but to elastic deformation of the sheets themselves. And because the nanotubes adhere tightly to the sheets and their position is determined by the position of the sheets, they do not experience irreversible deformation and entanglement and do not move relative to each other under load, as in an inelastic airgel made only of nanotubes. An airgel consisting equally of graphene and nanotubes has optimal properties, and with an increase in the content of nanotubes, they begin to form “tangles”, as in an airgel made only of nanotubes, which leads to a loss of elasticity.
In addition to the described elastic properties, composite carbon airgel also has other unusual properties. It is electrically conductive, and the electrical conductivity changes reversibly upon elastic deformation. In addition, the airgel made of graphene and carbon nanotubes repels water, but at the same time perfectly absorbs organic liquids - 1.1 g of toluene in water was completely absorbed by a piece of airgel weighing 3.2 mg in 5 seconds (Fig. 4). This opens up excellent opportunities for eliminating oil spills and purifying water from organic liquids: just 3.5 kg of such airgel can absorb a ton of oil, which is 10 times more than the capacity of a commercially used absorbent. At the same time, the absorbent made from composite airgel is regenerable: thanks to its elasticity and thermal resistance, the absorbed liquid can be squeezed out like from a sponge, and the remainder is simply burned off or removed by evaporation. Tests have shown that the properties are maintained after 10 such cycles.
The variety of forms of carbon and the unique properties of these forms and materials obtained from them continue to amaze researchers, so we can expect more and more discoveries in this area in the future. How many things can be made from just one chemical element!
Aerogels (from lat. aer- air and gelatus- frozen) - a class of materials that are a gel in which the liquid phase is completely replaced by a gaseous phase, as a result of which the substance has a record low density, only one and a half times the density of air, and a number of other unique qualities: hardness, transparency, heat resistance, extremely low thermal conductivity and lack of water absorption.
General view of the airgel
Airgel is also unique in that it consists of 99.8%... air!
Aerogels based on amorphous silicon dioxide, alumina, and chromium and tin oxides are common. In the early 1990s, the first samples of carbon-based airgel were obtained.
Airgel is a very unusual creation of human hands, a material awarded 15 positions in the Guinness Book of Records for its unique qualities.
Aerogels belong to the class of mesoporous materials, in which cavities occupy at least 50% of the volume. The structure of aerogels is a tree-like network of clustered nanoparticles 2–5 nm in size and pores up to 100 nm in size.
To the touch, Aerogels resemble light but hard foam, something like polystyrene foam. Under heavy load, the airgel cracks, but in general it is a very durable material - an airgel sample can withstand a load of 2000 times its own weight. Aerogels, especially quartz ones, are good heat insulators.
Quartz Aerogels are the most common, they also hold the current record for the lowest density in solids - 1.9 kg/m³, which is 500 times less than the density of water and only 1.5 times the density of air.
Quartz Aerogels are also popular due to their extremely low thermal conductivity (~0.017 W/(m.K) in air at normal atmospheric pressure), less than the thermal conductivity of air (0.024 W/(m.K)).
Application of Airgel
Aerogels are used in construction and industry as heat-insulating and heat-retaining materials for the thermal insulation of steel pipelines, various equipment with high- and low-temperature processes, buildings and other objects. It can withstand temperatures up to 650°C, and a 2.5cm thick layer is enough to protect the human hand from direct exposure to a blowtorch.
The melting point of quartz Airgel is 1200°C.
Airgel production
The process of producing aerogels is complex and labor-intensive. First, the gel polymerizes using chemical reactions. This operation takes several days and the output is a jelly-like product. Then the water is removed from the jelly with alcohol. Its complete removal is the key to the success of the entire process. The next step is “supercritical” drying. It is produced in an autoclave at high pressure and temperature, using liquefied carbon dioxide.
The pioneer in the invention of airgel is credited to the chemist Steven Kistler from the College of the Pacific in Stockton, California, USA, who published his results in 1931 in the journal Nature.
Kistler replaced the liquid in the gel with methanol, and then heated the gel under pressure until the critical temperature of methanol (240°C) was reached. Methanol left the gel without decreasing in volume; Accordingly, the gel “dried up”, almost without shrinking.
