A series of carbon fiber products such as carbon fiber laptops, carbon fiber bicycles, and carbon fiber tennis rackets have entered the homes of ordinary people, making "carbon fiber" one of the most familiar new materials. In addition to frequent appearances in daily necessities, carbon fiber is also used in high-tech industries such as automobiles, aviation, aerospace, ships, and energy.
What is carbon fiber?
Before answering this question, let us understand what carbon fiber is. Carbon fiber (carbon fiber), as its name implies, is a fiber composed of carbon, with a carbon content of more than 90%. Microscopically, the structure of carbon fiber is similar to that of graphite, both of which are composed of layered carbon flake crystals. These carbon flake crystals are called graphene. It has only one layer of atomic thickness, and the carbon atoms in the plane are arranged in hexagons, similar to the shape of a honeycomb.
This unique arrangement of atoms makes graphene the toughest material. In graphite, these carbon lamellar crystals are stacked very regularly; however, in carbon fibers, these carbon lamellar crystals are not always so regular, and sometimes carbon lamellar crystals may intersect with each other.
The microstructure of carbon fiber makes it have many excellent properties. It can obtain great axial strength and Young's modulus while maintaining low density. At the same time, carbon fiber has very good resistance to ultra-high temperature, fatigue and corrosion. And good electrical and thermal conductivity.
How is such a good carbon fiber made?
Although carbon fiber is mainly composed of carbon and its microstructure is very close to graphite, people have not yet been able to make carbon fibers directly from carbon or graphite. Only carbon-containing organic fiber raw materials can be used to combine organic fibers with plastic resins. Carbonization produces carbon fibers.
The organic fibers selected to prepare carbon fibers mainly include polyacrylonitrile fibers, pitch fibers, viscose yarns, or phenolic fibers. Among them, polyacrylonitrile carbon fibers and pitch carbon fibers are more commonly used. Selecting different organic fibers as raw materials will have different requirements for specific process parameters, and the properties of the prepared carbon fibers will also have certain differences.
However, in this process, fiber spinning, thermal stabilization, oxidation, and carbonization are generally required to undergo four processes, which are accompanied by chemical changes such as dehydrogenation, cyclization, pre-oxidation, oxidation, and deoxidation. After these processes, we can obtain the carbon fiber precursor.
However, the production of raw silk does not mean that the process of making carbon fibers is over. After the raw silk is prepared, it needs to go through a sizing and curing process. The so-called sizing is the process of mixing raw silk and epoxy resin. There are many methods that can be used in the sizing process. Carbon fibers can be soaked in epoxy resin through automatic placement machinery, or epoxy resin and other polymers can be artificially mixed with carbon fiber.
After the sizing is finished, you need to go through a final procedure, that is, fixed words. Common methods of curing are heating, or putting epoxy resin under vacuum. The curing is mainly to harden the carbon fiber wrapped with epoxy resin. In addition, during the curing process, the carbon fiber may undergo a chemical reaction to generate a substance that does not contain carbon in the raw silk material, resulting in stronger agglomeration between the fiber particles.