Modern industrial processes use strong acids, alkalis and even plasma, but according to the new method of American scientists, only an acetylene cylinder, an oxygen cylinder and a spark are needed.
Left to right: Justin Wright, Chris Sorensen, Arjun Nepal
Graphene - a layer of carbon one atom thick - has suddenly become one of the most coveted materials in the high-tech world. It is perceived by many as a panacea for solving the problems of medicine and electronics. It is believed that with graphene, batteries will get a higher capacity, neural interfaces will become a reality, and doctors will learn how to make unique prostheses.
Now the production of graphene on an industrial scale is a very energy-intensive, complex and expensive process. This is either the peeling of layers, which is done manually in laboratories and cannot become an industrial solution. Or the use of chemistry, catalysts and heating up to 1000 degrees Celsius, which is energy-intensive.
Most often it is obtained from natural material- pyrolytic graphite, which is reduced to pure carbon, and then mechanically and by chemical means ensure that individual graphene particles are no thicker than several layers. In the production process, strong acids, alkalis are used, very high temperatures and pressures are created. Therefore, the emergence of a cheap method for obtaining this material is important.
Scientists from Kansas State University announced the discovery of a cheap way to mass-produce graphene. This requires only a few available components: hydrocarbon gas, oxygen, a spark plug and a combustion chamber.
To obtain graphene, it is enough to fill the combustion chamber with acetylene or gaseous ethylene and oxygen, and then use a car spark plug to cause the gas mixture to detonate. In this case, graphene will be formed, which will only be collected from the walls of the combustion chamber. Thus, the process of obtaining graphene consists in the explosion of materials with a high carbon content.
This method was discovered by scientists quite by accident. The researchers were developing a method for producing a carbonaceous aerosol gel. To do this, they used the above process. After the detonation, soot was formed, which, after studying, turned out to be graphene. Scientists say that they did not plan to obtain this material, they were just lucky.
The new way of making graphene has a number of advantages over currently used methods. It does not require the use of harmful chemicals and a lot of energy. It also makes it possible to produce graphene in large quantities and easily scale production. Finally, this method is more profitable from an economic point of view.
Graphene is a two-dimensional allotropic modification of carbon, in which all atoms are arranged on a plane in rows of regular hexagons.
First obtained in 2004, graphene has proven to be an extremely useful material for electronics and energy. It is very strong, very thermally conductive, and some of its properties are generally unique: for example, graphene is a material with the highest electron mobility of all known to science. It is this property that made it indispensable in electronics, catalysts, batteries and composite materials.
Subscribe to Qibble on Viber and Telegram to keep abreast of the most interesting events.
The invention can be used in the manufacture of electronic and optoelectronic devices, as well as solar cells. The original graphite is dispersed by needling to obtain a dispersion product containing graphene and graphite elements. Then, graphene is isolated from the resulting dispersion product by using its hydrophobic properties, using a liquid having a density of 1.80-2.3 g/cm 3 . After that, the graphite elements are split by abrasion in a drum containing abrasive elements made in the form of abrasive rollers, the length of which is equivalent to the length of the generatrix of the drum. The inner surface of the drum and the surface of the rollers have a roughness of not more than 0.32 microns. The productivity of the graphene production process increases, the harmfulness of production decreases. 2 w.p. f-ly, 1 pr.
The invention relates to the field of obtaining graphene and, in particular, to a method for obtaining graphene in the form of films, nanosized particles, flakes, etc. from starting materials based on graphite and other layered graphite compounds.
Graphene, which is a layer of carbon, the atoms of which are connected by sp 2 bonds into a hexagonal two-dimensional lattice, is one of the strongest materials known today. Graphene structures can be formed from several superimposed graphite sheets, so-called "small-layer graphenes", and thicker structures, called "nanoscale graphene sheets". Each of these materials differs significantly from each other in terms of physical and mechanical characteristics and can be used in various fields of technology, in particular, in electronic devices, solar cells, optoelectronic devices, etc.
Currently, graphene is obtained in several ways:
Destruction (splitting) of graphite by its chemical intercalation with halogen compounds, metal salts, etc., followed by exposure to thermal shock, ultrasound, shear machining. (KR no. 20110089625, B82B 3/00, 2011; KR no. 20100116399, B82B 3/00, 2010; US no. 2005271574, C01B 31/00 2005; US no. .; US No. 2009155578, B82Y 30/00, 2009; US No. 3885007, C04B 35/536 2005). The methods described in the patents are complex, inefficient, harmful in terms of use. chemical substances, high temperatures and pressures, require the use of complex equipment: ultrasonic units, supercentrifuges.
Obtaining graphene through oxidized graphite by treating graphite with strong acids, followed by reduction of oxidized graphite with strong reducing agents - hydrozine, NaBH 4 , hydroquinone, etc. (US 2013197256, class B82B 3/00, 2013 US No. 2010).
These methods involve the use of strong acids and large quantity water, which is necessary for washing off unreacted products and neutralizing the suspension.
Given the shortcomings of the above known methods can be used in industrial production to a limited extent. In addition, graphene obtained by these methods has a defective crystalline structure, which reduces the performance of the material in terms of electrical and thermal conductivity, wear resistance, and other characteristics.
The most promising methods for obtaining graphene for industrial production are methods for obtaining graphene by micromechanical exfoliation of graphite layers.
One of the first methods of mechanical exfoliation is the Novoselov method (Scotch tape method). The method, seemingly simple, is actually difficult to reproduce today and allows you to get only very small (no more than 0.001 mg) amounts of graphene, and requires the use of special graphites - natural, highly oriented pyrolytic graphite of the HOPG brand (Gubin SP., Tkachev SV "Graphene and related nanoforms of carbon", M., Book house "Librokom", 2012, pp. 38-39).
A known method for producing graphene, including grinding graphite into graphite powder in a ball mill in the presence of an organic solvent with a surface tension of 30-45 mNm -1 and grinding balls coated with a soft polymer. The polymer coating on the balls reduces damage to the graphite crystalline structure from hard collisions with the grinding balls. Grinding in a ball mill significantly improves the productivity of graphene production, the graphene product can be obtained with a uniform thickness of 1-2 carbon atoms, the method can be easily implemented in industrial production (WO 2011054305, SW 31/04, 2011). However, this method regrinds the fragments of the resulting graphene, because balls work more on the principle of grinding the source material.
A method is known for exfoliating a layered material, which includes dispersing graphite in a liquid medium containing a surfactant, exposing said suspension or suspension to ultrasound at an energy level for a period of time sufficient to obtain separated nanoscale flakes. The sonication is followed by a mechanical shear treatment, such as air grinding, a ball mill, a rotating shear blade, or a combination thereof (US 2008279756, SW 31/04, 2008). The disadvantage of this method lies in the uncontrolled grinding of the material in almost the entire volume, the subsequent separation of graphene from a solution containing surfactants is an extremely time-consuming operation, and, in addition, the use of ultrasound in production is harmful.
A known method of obtaining graphene particles or flakes by abrasion of solid graphite on a rough rough surface, such as a glass surface having a roughness of 0.01 to 10 pm. During friction, graphite is transferred to a rough surface, leaving traces that represent a graphene material. The specified surface is then subjected to ultrasonic treatment to separate the graphene material from it (WO 2011055039, class B82Y 30/00, 2011). The disadvantage of this method lies in its low productivity, tk. the method requires a constant interruption of the process to separate the layers of graphene from the abrasion surface to restore roughness and continue the process of abrasion of hard graphite.
A known method for producing graphene particles, including the dispersion of the original graphite material by cutting graphite blocks having nanosize, from the original graphite ultramicrotome; as a tool, a diamond knife with an edge radius of 1 to 5 nm is used, while the blocks are cut with separate cutter passes in two mutually intersecting directions. After cutting out the blocks, they are split into a plurality of graphene particles using chlorosulfonic, sulfuric acids, or mixtures thereof. When graphene elements are dispersed in a liquid, they are separated from the liquid by filtration (US 2012272868, B82Y 30/00, 2012).
The disadvantage of this method lies in the low productivity of the process, due to the fact that in order to obtain the required number of blocks with one cutter, it is necessary to make many passes. In addition, obtaining graphene known way is a harmful production, tk. the subsequent splitting of blocks into graphene elements is carried out by strong acids. The method is quite expensive due to the high cost of microtome diamond cutters, the cutting of which must be carried out on precision expensive equipment.
The closest technical solution to the claimed method is a method that includes dispersing the starting graphite material by needle milling to obtain a product containing graphene and graphite elements, and separating graphene from the resulting product by centrifugation in a liquid, contributing to the separation of the product into graphite and graphene (application CN No. 102602914, C01B 31/04, 2012). According to the application, graphite elements remain after separation of the graphene. The authors of the application do not indicate the possible further use of graphite elements.
The technical objective of the proposed method is to create a simple, inexpensive, high-performance method for producing graphene with an increased yield using high-performance mechanical means without using complex instrument and equipment.
The technical solution of the task is that in the method for obtaining graphene, including the dispersion of the original graphite by needling to obtain a dispersion product containing graphene and graphite elements, and the separation of graphene from the resulting dispersion product, after the separation of graphene from the dispersion product, the graphite elements are split by abrasion in a drum containing abrasive elements made in the form of abrasive rollers, while the separation of graphene from the dispersion products is carried out by using the hydrophobic properties of graphene using a liquid having a density of 1.80-2.3 g/cm 3 .
The length of the abrasive rollers is equivalent to the length of the drum generatrix.
The inner surface of the drum and the surface of the rollers have a roughness of not more than 0.32 microns.
The essence of the invention lies in the fact that the process of dispersion of the original graphite material is carried out with a multi-bladed tool - a needle cutter, which has many cutting edges, allowing a large amount of a dispersion product containing graphene and graphite elements to be obtained in one pass of the tool. After isolating the graphene obtained by needle milling, the graphite elements are split in the drum in the abrasion-rolling mode of the abrasive elements made in the form of rollers. The process of splitting by abrasion-rolling makes it possible to obtain graphene in the form of sufficiently large elements, almost completely eliminating the crushing of graphene.
The method is carried out as follows.
As a source of graphite, materials such as natural graphite, electrode graphite, pyrolytic graphite, HOPG, KIS, etc., can be used, which are a material consisting of a plurality of carbon layers oriented relative to each other. The starting graphite material is dispersed by a needling process. The needle cutter is a disk, on the periphery of which a large number of densely spaced cutting elements are fixed in the idea of needles or wires made of high-strength steel material. Each needle or wire simultaneously removes chips from the surface of the graphite material. Depending on the needle milling conditions, chips are obtained - a dispersion product, which is a mixture of graphene and graphite elements. The conditions for needle milling are cutting modes, the number of cutting elements per unit of the working surface of the needle cutter, the rigidity of the cutting elements, depending on the modulus of elasticity of the materials from which the cutting elements are made, geometric parameters: the diameter of the cutting elements, their overhang above the fixing surface, the density of the cutting elements, etc. By varying the needle milling conditions, it is possible to obtain graphene and graphite elements of the sizes that are required. To obtain dimensional graphite elements, the following cutting conditions are recommended: cutting speed - up to 35 m/s, transverse feed ≤0.03 m/s, longitudinal feed - 0-1 m/min. More severe cutting conditions cause high temperatures in the cutting zone, leading to the oxidation of the dispersion products.
Needle milling is carried out mainly on grinding machines that provide high cutting speeds. In the process of needle milling, graphene is obtained, which is separated from the graphite elements. The separation of graphene is necessary so that the resulting graphene is not subjected to subsequent processing, leading to damage to the crystal lattice, as well as possible regrinding. Ultimately, the isolation of graphene makes it possible to increase the yield of graphene with a minimally damaged crystal lattice.
The separation of graphene from dispersion products is carried out by using the hydrophobic properties of graphene using a liquid having a density of 1.80-2.3 g/cm 3 .
Due to its crystallinity, graphene has hydrophobic properties that allow it to float in liquids with a density higher than that of graphene. To isolate graphene from dispersion products, a suspension is prepared from water and a substance that dissolves in water and increases its density in the range of 1.80-2.3 g/cm 3 . Preferably, salts such as, for example, NaCl, NaNO 3 KaCl, etc., can be used as such substances. The needle milling product is introduced into the suspension, shaken and left for some time alone until the liquid-precipitate interface appears. Floated graphene in the form of a film, graphene fragments, is removed from the surface of the suspension. The suspension density of more than 1.80 g/cm 3 allows graphene to float, separating it from the hafite material, which has a higher density and hydrophilic properties. At a lower density of the suspension, the dispersion product does not float up and does not separate. The maximum density of the suspension is determined by the density of graphite. When the density of the suspension is greater than 2.3 g/cm 3 the dispersion product is not separated, because both graphene and graphite float to the surface.
After the separation of graphene, the settled precipitate containing graphite elements is decanted from the liquid, dried, and placed in a drum, in which rollers serve as abrasive elements. The inner surface of the drum and the surface of the rollers must have a roughness of not more than 0.32 microns. Such roughness provides efficient production of 1-3-layer graphene. With greater roughness, multilayer graphene with worse physical and mechanical properties is obtained.
The length of the abrasive rollers must be equivalent to the length of the drum generatrix. In this case, a more uniform rolling-sliding of the rollers along the wall of the drum and the rollers against each other will be carried out. During the processing in the drum in the rolling-sliding mode, abrasion of graphite elements occurs, causing the splitting of graphite into graphene fragments without their reprocessing. The number of abrasive rollers loaded into the drum should not exceed one third of the drum diameter.
The proportionality of the length of the rollers and the length of the generatrix of the drum is necessary to obtain a dense packing of the rollers along the wall of the drum, at which the abrasion process occurs. Violation of the packing density of the rollers leads to the appearance of shock loads, i.e. to the appearance of graphene with a distorted crystal lattice.
After completion of the processing of the graphite material in the drum, a suspension is again prepared from the resulting product to separate the graphene fragments. Depending on the results, the precipitate in suspension can be repeatedly processed in a drum with intermediate separation of graphene fragments from the product of the next attrition.
Graphite electrodes grade E 16 mm in diameter were cut with a needle cutter ⌀120 mm, 20 mm wide at a cutting speed V=25 m/s, longitudinal feed S prod =0.5 m/min, transverse feed S pop =1.0 mm/stroke . in a closed container without refrigeration. The resulting milling product was placed in a suspension consisting of common salt and distilled water. The density of the suspension was 2.0 g/cm 3 . The suspension was shaken for 30 min and left alone until the liquid-precipitate interface was obtained. The floating product was removed from the surface of the liquid. The product was graphene in the form of a film, fragments, and individual elements. The yield of graphene was 11 wt%. The settled product was decanted, dried and placed in a drum with roller abrasive elements. The inner diameter of the drum was 240 mm, the diameter of the rollers was 12 mm. The length of the drum was 360 mm, the length of the rollers was 12 mm. Loading rollers was a third of the diameter of the drum. The inner surface of the drum and the surface of the rollers had a roughness of 0.32 µm. Drum rotation speed 200 rpm, processing time - 5 hours.
After abrasion, the resulting product was again processed in suspension to isolate graphene. The removed product in the form of a film was dried and weighed. The yield of graphene was 14% wt. For the cycle was obtained 25% wt. graphene.
The remaining sediment after decanting and drying was sent to additional attrition in the drum.
Thus, by using the needle milling process to obtain graphene and the subsequent splitting of graphite elements in a drum with abrasive rollers, the yield of graphene is increased, and the cost of the process is reduced. The method is carried out without the use of strong acids, which significantly reduces the harmfulness of production.
1. A method for obtaining graphene, including dispersion of the original graphite by needle milling to obtain a dispersion product containing graphene and graphite elements, and the separation of graphene from the resulting dispersion product, characterized in that after the separation of graphene from the dispersion product, the graphite elements are split by attrition in a drum containing abrasive elements made in the form of abrasive rollers, while the separation of graphene from the dispersion products is carried out by using the hydrophobic properties of graphene using a liquid having a density of 1.80-2.3 g/cm 3 .
2. The method according to p. 1, characterized in that the length of the abrasive rollers is equivalent to the length of the generatrix of the drum.
3. The method according to p. 1, characterized in that inner surface the drum and the surface of the rollers have a roughness of not more than 0.32 microns.
Similar patents:
The invention relates to the field of creation and production of carbon materials with high physical and mechanical characteristics, in particular carbon-carbon composite materials based on woven reinforcing fillers from high-modulus carbon fiber and a carbon matrix formed from pitches during carbonization and subsequent high-temperature treatments.
The invention can be used in the manufacture construction materials. The method of stacking carbon fired large-sized blanks of fine-grained graphite of isostatic pressing during graphitization includes their arrangement vertically and horizontally across the core in columns separated from each other by layers of core filling with a thickness of approximately 0.2 of the diameter of the blank.
The invention can be used for the manufacture of thermally expanded graphite (TEG) and fire-retardant materials. The original powdered graphite is treated with an oxidizing solution containing the following components in the ratio, g/g of graphite: sulfuric acid 2.0-5.0; ammonium nitrate 0.04-0.15; carbamide 0.04-0.15.
The invention can be used in medicine, biology and agriculture as chemical containers for storing and transporting substances. Graphite is fluorinated with fluoroxidants - chlorine or bromine trifluoride in a solvent inert to the indicated fluoroxidants, which is carbon tetrachloride or freon.
The inventions relate to nanotechnology and can be used in the manufacture of catalysts and sorbents. Graphene pumice consists of graphenes arranged in parallel at distances greater than 0.335 nm, and amorphous carbon as a binder along their edges, with a ratio of graphene and binder from 1:0.1 to 1:1 by weight.
The group of inventions can be used in the manufacture of materials for the electrical and chemical industries. The graphite-containing component is mixed with a kaolin-based filler, dry mixing is carried out with simultaneous dispersion sequentially in drum and centrifugal mixers.
The invention relates to carbon-silicon carbide composite materials. The technical result of the invention is to improve the performance of products.
Graphene fibers under a scanning electron microscope. Pure graphene is recovered from graphene oxide (GO) in a microwave oven. Scale 40 µm (left) and 10 µm (right). Photo: Jieun Yang, Damien Voiry, Jacob Kupferberg / Rutgers University
Graphene is a 2D modification of carbon formed by a layer one carbon atom thick. The material has high strength, high thermal conductivity and unique physical and chemical properties. It exhibits the highest electron mobility of any known material on Earth. This makes graphene an almost ideal material for a wide variety of applications, including electronics, catalysts, batteries, composite materials, etc. The point is small - to learn how to obtain high-quality graphene layers on an industrial scale.
Chemists from Rutgers University (USA) have found a simple and fast method for producing high-quality graphene by processing graphene oxide in a conventional microwave oven. The method is surprisingly primitive and effective.
Graphite oxide is a compound of carbon, hydrogen and oxygen in various proportions, which is formed when graphite is treated with strong oxidizing agents. To get rid of the remaining oxygen in the graphite oxide, and then get pure graphene in two-dimensional sheets, requires considerable effort.
Graphite oxide is mixed with strong alkalis and the material is further reduced. As a result, monomolecular sheets with oxygen residues are obtained. These sheets are commonly referred to as graphene oxide (GO). Chemists have tried different ways to remove excess oxygen from GO ( , , , ), but GO (rGO) reduced by such methods remains a highly disordered material, which is far from real pure graphene obtained by chemical vapor deposition (CVD) .
Even in its disordered form, rGO has the potential to be useful for energy carriers ( , , , , ) and catalysts ( , , , ), but in order to take full advantage of the unique properties of graphene in electronics, one must learn how to obtain pure high-quality graphene from GO.
Chemists at Rutgers University propose a simple and fast way reduction of GO to pure graphene using 1-2 second microwave pulses. As can be seen from the graphs, graphene obtained by “microwave reduction” (MW-rGO) is much closer in its properties to the purest graphene obtained using CVD.
Physical characteristics of MW-rGO compared to pristine graphene oxide GO, reduced graphene oxide rGO, and chemical vapor deposition (CVD) graphene. Shown are typical GO flakes deposited on a silicon substrate (A); X-ray photoelectron spectroscopy (B); Raman spectroscopy and the ratio of crystal size (L a) to the peak ratio l 2D /l G in the Raman spectrum for MW-rGO, GO and CVD.
Electronic and electrocatalytic properties of MW-rGO compared to rGO. Illustrations: Rutgers University
The technical process for obtaining MW-rGO consists of several stages.
- Oxidation of graphite by the modified Hummers method and its dissolution to single-layer flakes of graphene oxide in water.
- GO annealing to make the material more susceptible to microwave irradiation.
- Irradiation of GO flakes in a conventional 1000W microwave oven for 1-2 seconds. During this procedure, the GO quickly heats up to high temperature, desorption of oxygen groups and excellent structurization of the carbon lattice occur.
On images from a translucent electron microscope the structure of graphene sheets is shown with a scale of 1 nm. On the left is a single layer rGO with many defects, including oxygen functional groups (blue arrow) and holes in the carbon layer (red arrow). In the center and on the right is a perfectly structured two-layer and three-layer MW-rGO. Photo: Rutgers University
The excellent structural properties of MW-rGO when used in field effect transistors allow the maximum electron mobility to be increased to about 1500 cm 2 /V·s, which is comparable to the outstanding performance of modern high electron mobility transistors.
In addition to electronics, MW-rGO is useful in the production of catalysts: it showed an exceptionally low value of the Tafel coefficient when used as a catalyst in the oxygen evolution reaction: about 38 mV per decade. The MW-rGO catalyst also remained stable in the hydrogen evolution reaction, which lasted over 100 hours.
All this suggests an excellent potential for the use of microwave-reduced graphene in industry.
Research Article "High-quality graphene via microwave reduction of solution-exfoliated graphene oxide" published September 1, 2016 in the magazine Science(doi: 10.1126/science.aah3398).
Graphene is the thinnest material known to mankind, only one carbon atom thick. It entered physics textbooks and our reality in 2004, when researchers from the University of Manchester, Andre Game and Konstantin Novoselov, managed to obtain it using ordinary adhesive tape to sequentially separate layers from ordinary crystalline graphite, familiar to us in the form of a pencil rod (see . Application). Remarkably, a graphene sheet placed on an oxidized silicon substrate can be viewed with a good optical microscope. And this is despite its thickness of only a few angstroms (1Å = 10 -10 m)!
The popularity of graphene among researchers and engineers is growing day by day as it has unusual optical, electrical, mechanical and thermal properties. Many experts predict in the near future a possible replacement of silicon transistors with more economical and high-speed graphene ones.
So how do you make graphene at home?
- To create and observe the thinnest material on our planet, you will need clean conditions (for example, a physical and chemical laboratory, although an ordinary room with good ventilation will do), clean hands, preferably with gloves, and clean thoughts ☺.
- First, prepare the substrate on which you will place the graphene for observation under the microscope. To do this, you need to take a silicon substrate with a natural oxide on the surface, which should be cleaned before the study. The best solution for this is a solution of hydrochloric acid and hydrogen peroxide in a ratio of 1: 3. Place the plate in the solution for 30 seconds and then dry with compressed nitrogen.
- Attach the peeled piece of graphite to the tape using tweezers. Gently fold the tape in half, covering the graphite with the sticky side. Gently press the tape against the graphite on both sides and unhurriedly open the tape so that you can observe the delamination of the graphite into two parts.
- Repeat the previous step ten times. The thinner the layers of graphite become, the more difficult it will be to do this.
- Very carefully place the adhesive tape with graphite on the surface of the silicon substrate. Using plastic tweezers, remove air bubbles between the tape and backing. Walk over the surface of the sample with tweezers, gently pressing it against the substrate for ten minutes. Then very slowly remove the tape while holding the backing.
- Place your sample under a 50x or better 100x microscope lens. You will see a lot of graphite "flakes" different sizes and shapes shimmering with all the colors of the rainbow. If you're lucky, you'll notice graphene: an almost transparent, crystalline 'flake' that is very different in color from bright colors"thick" graphite counterparts.
- And here is the link. where the Russian scientist, Nobel laureate Konstantin Novoselov shows how to get graphene at home yourself
Until last year, the only way known to science to produce graphene was to apply the thinnest layer of graphite on adhesive tape and then remove the base. This technique is called the "scotch tape technique". Recently, however, scientists have discovered that there is a more efficient way to obtain a new material: as a base, they began to use a layer of copper, nickel or silicon, which is then removed by etching (Fig. 2). In this way, rectangular sheets of graphene 76 centimeters wide were created by a team of scientists from Korea, Japan and Singapore. Not only did the researchers set a kind of record for the size of a piece of a single-layer structure of carbon atoms, they also created sensitive screens based on flexible sheets.
Figure 2: Obtaining graphene by etching
For the first time, graphene "flakes" were obtained by physicists only in 2004, when their size was only 10 micrometers. A year ago, the team of Rodney Ruoff at the University of Texas at Austin announced that they had managed to create centimeter-sized "scraps" of graphene.
Ruoff and colleagues deposited carbon atoms on copper foil using chemical vapor deposition (CVD). Researchers in the laboratory of Professor Bunya Hee Hong from Sungkyunkhwan University went further and enlarged the sheets to the size of a full-fledged screen. The new “roll” technology (roll-to-roll processing) makes it possible to obtain a long ribbon from graphene (Fig. 3).
Figure 3: High-resolution transmission electron microscopy image of stacked graphene layers.
A layer of an adhesive polymer was placed on top of the graphene sheets of physics, the copper substrates were dissolved, then the polymer film was separated - a single layer of graphene was obtained. To give the sheets greater strength, scientists in the same way "grew up" three more layers of graphene. At the end, the resulting “sandwich” was treated with nitric acid to improve conductivity. A brand new graphene sheet is placed on a polyester substrate and passed between heated rollers (Fig. 4).
Figure 4: Roll technology for obtaining graphene
The resulting structure transmitted 90% of the light and had an electrical resistance lower than that of the standard, but still very expensive, transparent conductor, indium tin oxide (ITO). By the way, using sheets of graphene as the basis of touch displays, the researchers found that their structure is also less fragile.
True, despite all the achievements, the commercialization of technology is still very far away. Transparent carbon nanotube films have been trying to supplant ITO for quite some time, but manufacturers can't seem to get around the problem of "dead pixels" that appear on film defects.
The use of graphenes in electrical engineering and electronics
The brightness of pixels in flat panel screens is determined by the voltage between two electrodes, one of which is facing the viewer (Fig. 5). These electrodes must be transparent. Currently, tin-doped indium oxide (ITO) is used to produce transparent electrodes, but ITO is expensive and not the most stable material. Besides, the world will soon exhaust its reserves of indium. Graphene is more transparent and more stable than ITO, and a graphene electrode LCD has already been demonstrated.
Figure 5: Brightness of graphene screens as a function of applied voltage
The material also has great potential in other areas of electronics. In April 2008, scientists from Manchester demonstrated the world's smallest graphene transistor. A perfectly correct layer of graphene controls the resistance of the material, turning it into a dielectric. It becomes possible to create a microscopic power switch for a high-speed nano-transistor to control the movement of individual electrons. The smaller transistors in microprocessors, the faster it is, and scientists hope that graphene transistors in computers of the future will be the size of a molecule, given that modern silicon microtransistor technology has almost reached its limit.
Graphene is not only an excellent conductor of electricity. It has the highest thermal conductivity: atomic vibrations easily propagate through the carbon mesh of a cellular structure. Heat dissipation in electronics is a serious problem because there are limits to the high temperatures that electronics can withstand. However, scientists at the University of Illinois have found that graphene-based transistors have an interesting property. They manifest a thermoelectric effect, leading to a decrease in the temperature of the device. This could mean that graphene-based electronics will make heatsinks and fans a thing of the past. Thus, the attractiveness of graphene as a promising material for microcircuits of the future further increases (Fig. 6).
Figure 6: An atomic force microscope probe scanning the surface of a graphene-metal contact to measure temperature.
It was not easy for scientists to measure the thermal conductivity of graphene. They invented an entirely new way to measure its temperature by placing a 3-micron-long graphene film over exactly the same tiny hole in a silicon dioxide crystal. The film was then heated with a laser beam, causing it to vibrate. These vibrations helped to calculate the temperature and thermal conductivity.
The ingenuity of scientists knows no bounds when it comes to using the phenomenal properties of a new substance. In August 2007, the most sensitive of all possible sensors based on it was created. It is able to respond to one gas molecule, which will help to detect the presence of toxins or explosives in a timely manner. Alien molecules peacefully descend into the graphene network, knocking out electrons from it or adding them. As a result, the electrical resistance of the graphene layer changes, which is measured by scientists. Even the smallest molecules are trapped by the strong graphene mesh. In September 2008, scientists from Cornell University in the United States demonstrated how a graphene membrane, like the thinnest balloon, inflates due to a pressure difference of several atmospheres on both sides of it. This feature of graphene can be useful in determining the course of various chemical reactions and in general in studying the behavior of atoms and molecules.
Getting large sheets of pure graphene is still very difficult, but the task can be simplified if the carbon layer is mixed with other elements. At Northwestern University in the United States, graphite was oxidized and dissolved in water. The result was a paper-like material - graphene oxide paper (Fig. 7). It is very tough and quite easy to make. Graphene oxide is suitable as a durable membrane in batteries and fuel cells.
Figure 7: Graphene oxide paper
The graphene membrane is an ideal substrate for objects of study under an electron microscope. Flawless cells merge in images into a uniform gray background, against which other atoms stand out clearly. Until now, it was almost impossible to distinguish the lightest atoms in an electron microscope, but with graphene as a substrate, even small hydrogen atoms can be seen.
The possibilities of using graphene are endless. Recently, physicists at Northwestern University in the US figured out that graphene can be mixed with plastic. The result is a thin, super-strong material that can withstand high temperatures and is impervious to gases and liquids.
The scope of its application is the production of light gas stations, spare parts for cars and aircraft, durable wind turbine blades. Plastic can be used to pack food products, keeping them fresh for a long time.
Graphene is not only the thinnest, but also the most durable material in the world. Scientists at Columbia University in New York have verified this by placing graphene over tiny holes in a silicon crystal. Then, by pressing the thinnest diamond needle, they tried to destroy the graphene layer and measured the pressure force (Fig. 8). It turned out that graphene is 200 times stronger than steel. If we imagine a graphene layer with a thickness of cling film, he would have withstood the pressure of the tip of a pencil, at the opposite end of which an elephant or a car would have been balanced.
Figure 8: Pressure on graphene diamond needle