Graphene: the “shuffler” in the field of new materials

Abstract Researchers Andrew Gem and Costia Novosev of the University of Manchester first discovered that when the graphite layer is as thin as one carbon atom, its toughness and electrical properties are outstanding. This discovery caused nothing in the scientific community...

Researchers Andrew Gem and Costia Novosev of the University of Manchester first discovered that when the graphite layer is as thin as one carbon atom, its toughness and electrical properties are outstanding. This discovery has caused quite a stir in the scientific community, not only because it breaks the theoretical prediction that two-dimensional crystals cannot exist, but more importantly, many properties of graphene may change our lives. Andre Gamm and Costia Novosev also won the Nobel Prize for their outstanding contributions in the field of graphene.

"Soil method" to produce new materials

Scientists have recently unveiled a contact lens manufacturing program that gives users "super vision". The research team from the University of Michigan said that the contact lens is embedded in graphene, which has excellent photosensitivity and can detect visible and invisible light. , such as infrared, with night vision. This news caused the graphene to be covered with a magical halo.

The birth of the first single layer of graphene does not have much "technical content". Andre Gamm uses a scotch tape to stick it on the graphite, so that a graphite layer is stuck to the tape. Then fold the tape in half, stick it and pull it apart. Thus, there are graphite layers on both ends of the tape, and the graphite layer becomes thinner. Repeatedly, when the graphite layer on the tape is as thin as only one carbon atom, the graphite layer becomes graphene.

Scientists seem to have a hard time defining graphene. Tan Pingheng, a researcher at the Institute of Semiconductors of the Chinese Academy of Sciences, told reporters that in the review of graphene papers published by Professor Gaim in 2007, the two-dimensional crystal structure of how many layers of carbon atoms can be called graphene is clearly defined: Double-layer, multi-layer (3 to 10 layers) materials can be called graphene because they still retain two-dimensional crystal properties."

The China Graphene Industry Technology Innovation Strategic Alliance gave a standard for the definition of graphene materials in 2013. It is considered that graphene is a single-layer graphene, two-layer graphene and a small layer of graphene (3 to 10 layers) defined by the standard. Collectively. Tan Pingheng believes that how to identify the generalized graphene layer is an urgent problem to be solved in the research and industrialization of graphene.

Although the "name" is still not correct, countries have shown enough enthusiasm. In January 2013, the EU listed graphene as one of the “Future Emerging Technology Flagship Projects” and plans to provide 1 billion euros in funding in 10 years; the UK announced at the end of 2012 that it will invest an additional 21.5 million pounds to finance the commercialization of graphene and establish A national research institute; the United States, South Korea, and Japan are among the world's leaders in the field of graphene-made computer chips and transistors, flexible touch screens and flexible organic electroluminescent devices, and transparent conductive films and heat-dissipating films.

According to incomplete statistics, there are more than 50 companies doing scale preparation in the world, among which there are more than a dozen in China, and many research results have been at the forefront of the world.

Challenge all kinds of "impossible"

With its special physical structure and traits, graphene can revolutionize many fields.

If it is successfully used in the electrode materials of supercapacitors or lithium-ion batteries, the current application is more popular, "Mom no longer have to worry about my battery." According to Dr. Chen Chengmeng from the Shanxi Institute of Coal Chemistry, Chinese Academy of Sciences, there are two key indicators for energy storage materials. One is conductivity. Graphene is the world's smallest resistivity material. The electrons move freely on the two-dimensional plane of graphene. For example, the car is generally unimpeded on the highway; the second is the specific surface area, which directly affects the specific capacity of the energy storage material, that is, the battery life. All the carbon atoms that make up the graphene are on the surface, which is effective for energy storage and is not wasted at all. "It takes only 5 minutes to complete the charging, and it can be used for 3 days or even longer." Chen Chengmeng said that this application may open a new window for electric vehicle power batteries, and it is expected to break through in energy conservation and environmental protection.

Electronic products such as mobile phones and computers will be heated for a long time, because the heat generated by continuous work cannot be exchanged with the outside world through the heat sink in time. With the replacement of high-power electronic products, the requirements for heat dissipation are getting higher and higher, and this is precisely the opportunity for graphene.

According to the information provided by Chen Chengmeng, the theoretical thermal conductivity of graphene at room temperature is as high as 5,300 watts/meter. Kelvin, the United States has measured the thermal conductivity of monolithic graphene over 4000 watts/meter Kelvin in the laboratory, which goes beyond carbon nanotubes. , diamond and highly oriented graphite, and far superior to the best thermal metal copper up to an order of magnitude. "But these properties are based on a very microscopic nanoscale, invisible and intangible. It is more difficult to apply it to reality." Chen Chengmeng said that according to the high thermal conductivity of graphene, their research institute has successfully used graphene and carbon fiber. Composite into a new film, the thermal conductivity can exceed 1100 W / m · Kelvin.

Graphene oxide is a "close relative" of graphene, and the film made thereof has various excellent properties. In 2012, Prof. Wu Hengan from the Department of Modern Mechanics of the School of Engineering Science, University of Science and Technology of China, in collaboration with Prof. Andre Gamm, made a new type of gas-permeable membrane material using graphene oxide. They did an interesting experiment, using this film to seal a bottle of vodka, and as the water evaporates, the wine tastes stronger. Wu Hengan said: "The special interaction between water and graphene surface accelerates the speed of water molecules passing through the nanometer wide channel."

In 2014, this collaborative research group further produced a graphene oxide film with superior ion screening function. According to Wu Hengan, the graphene oxide film in the water environment can prevent the passage of ions or molecules with a radius greater than 0.45 nm in water under the action of water. These findings have broad application prospects in the fields of seawater desalination and purification, sensing technology and energy conversion.

Carbon family rookie too "noble"

Graphene, which has many excellent genes for the carbon family, will take time to go to market. Chen Chengmeng told reporters that the vast majority of graphene manufacturers are still in the transition stage from laboratory to pilot test, or pilot test to industrialization.

"In fact, the core problem in the development of graphene industry is that downstream applications are not opened, graphene can not be productized, and the production process cannot be shaped and cured." Chen Chengmeng said. At present, graphene is “priceless and has no market” and the price is too high. According to different tastes, the price of graphene ranges from 1,000 yuan to 8,000 yuan per gram. The high price suppresses the market demand, and the existing demand is limited to research institutions and research institutes.

The immaturity of the preparation technology also hinders the large-scale application of graphene. Chen Chengmeng introduced that the most mature methods for preparing graphene are physical method and chemical method. The typical representative of physical method is obtained from graphite crystal by tape bonding. This method can obtain graphene crystal with perfect structure. More suitable for the development of electronic devices or physical research, but not mass production.

Mass production relies mainly on chemical methods, a method known as "bottom-up" in the academic world, that is, assembly of graphene sheets from small molecules or carbon atoms by chemical vapor deposition, epitaxial growth or organic synthesis. The scale of this method is relatively easy to enlarge, and the crystal structure of the obtained graphene is relatively complete, and is more suitable for a transparent conductive film of a touch Screen or a solar cell. Another way is the "top-down" method, sometimes called chemical stripping, which involves disassembling a three-dimensional layer of graphite crystals and stripping them into a single layer or a few layers of graphene.

"The process route of each manufacturer and the intercalant used are different. Some use salt intercalation and then liquid phase stripping. The graphene prepared by this method has low oxygen content and good electrical conductivity, but the degree of peeling is relatively low. The specific surface area is not too large, and it is more suitable for conducting conductive agents; some are first oxidized and then stripped. This method has high degree of peeling and large specific surface area, but has more graphene defects, large oxygen content, and surface activity. Strong, more suitable for catalysis, energy storage and other applications." Chen Chengmeng said. Advanced process development, equipment manufacturing and process control for graphene materials remain a major issue.

From the current technological development, the downstream industry most likely to achieve industrial use of graphene is in the field of composite materials and display technology. Adding graphene to a matrix of plastics, rubber, paint, etc., the product that is "tuned" will be stronger in performance. For graphene produced by vapor deposition, Chen Chengmeng believes that its application fields are mainly concentrated in the fields of touch screens and solar cells.

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