What’s Carbon Fiber?

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Carbon fiber is a fabric made primarily of carbon. It is made by spinning various carbon-based polymers into fibers, treating them to remove most other substances, and weaving the resulting material into a fabric. This is usually incorporated into plastics, typically epoxy, to form carbon fiber reinforced plastic or carbon fiber composite. The most notable characteristics of the material are its high strength-to-weight ratio and relative chemical inertness. These properties give it a wide range of applications, but its use is limited by the fact that it is quite expensive.

Production

The production of this material is usually based on polyacrylonitrile (PAN), a plastic used in synthetic fabrics for clothing, or pitch, a tar-like substance obtained from petroleum. The rosin is first spun into strands, but the PAN is normally in fibrous form to begin with. They are converted into carbon fiber by strong heating to remove other elements, such as hydrogen, oxygen and nitrogen; this process is known as pyrolysis. Stretching the fibers during this procedure helps remove any irregularities that could weaken the final product.

The raw fibers are initially heated to approximately 590°F (300°C) in air and under tension, in a stage known as oxidation or stabilization. This removes the hydrogen from the molecules and converts the fibers into a more mechanically stable form. They are then heated to approximately 1,830°F (1,000°C) in the absence of oxygen in a stage known as charring. This removes additional non-carbonaceous material, leaving mostly carbon.

When high-quality, high-strength fibers are required, a further step occurs, known as graphitization. The material is heated to between 1,732 and 5,500°F (1,500 to 3,000°C) to convert the formation of carbon atoms into a graphite-like structure. This also removes most of the residual non-carbon atoms. The term “carbon fiber” is used for material with a carbon content of at least 90%. Where the carbon content is greater than 99%, the material is sometimes called graphite fiber.

The resulting raw carbon fiber does not bind well with the substances used to make the composites, so it is slightly oxidized by treatment with suitable chemical products. The oxygen atoms added to the structure allow it to form bonds with plastics, such as epoxy resin. Having received a thin protective coating, it is woven into yarn of the required size. These in turn can be woven into fabrics, which are then typically incorporated into composite materials.

structure and properties
A single fiber has a diameter of about 0.0002 to 0.0004 inch (0.005 to 0.010 mm); yarn is made up of many thousands of these threads twisted together to form an extremely strong material. Within each filament, the carbon atoms are arranged in a similar way to graphite: hexagonal rings joined together to form sheets. In graphite, these sheets are flat and only loosely bonded to each other, so they slide easily. In a carbon fiber, the sheets are folded and crumpled and form many tiny interconnected crystals, known as crystallites. The higher the temperature used in manufacturing, the more these crystallites are oriented along the axis of the fiber and the higher the resistance.
Within a composite, the orientation of the fibers themselves is also important. Depending on this, the material may be stronger in one direction or equally strong in all directions. In some cases, a small piece can withstand an impact of many tons and still deform minimally. The complex intertwined nature of the fiber makes it very difficult to break.

In terms of strength-to-weight ratio, carbon fiber composite is the best material that civilization can produce in appreciable quantities. The strongest are about five times stronger than steel and considerably lighter. Research is underway into the possibility of introducing carbon nanotubes into the material, which could improve the strength-to-weight ratio by 10 times or more.
Other useful properties it has are its ability to withstand high temperatures and its inertness. The molecular structure is, like graphite, very stable, which gives it a high melting point and makes it less likely to react chemically with other substances. It is therefore useful for components that may be subject to heat and for applications that require corrosion resistance.

it is used
Carbon fiber is used in many areas where a combination of high strength and low weight is required. These include public and private transportation, such as cars, airplanes and spacecraft; sports equipment, such as racing bicycles, skis and fishing rods; and construction. The relative inertness of the material makes it suitable for applications in the chemical industry and in medicine: it can be used in plants as it does not react with substances in the body. In civil engineering, it has been established that old bridges can be spared from destruction and reconstruction through simple carbon fiber reinforcements, which are relatively cheaper.
Economy
As of 2013, the uses and demand for carbon fiber have been limited by its cost. A bicycle made of composite material typically costs around a few thousand US dollars (USD). Formula 200 race cars, traveling at speeds in excess of 320 mph (1 km/h), can cost over $100 million to build and maintain, a cost largely driven by the lavish use of this material. Demand has increased significantly, however, mainly due to increased production of large commercial aircraft. If the cost can be significantly reduced, it can become a universal material for vehicles and small products designed for extreme durability and light weight.

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