Material of the Day: Carbon Fiber By Mingzhou xu

Today's material of the day is carbon fiber. Carbon fiber is a new fiber material that mainly used in sports equipments, aerospace and general industrial. You will learn that carbon fiber has low density, high stiffness, high temperature tolerance and low thermal expansion, high chemical resistance and other excellent features. Moreover its property and its structure are related.

My introduction to carbon fiber happened in the beginning of 2016, the day I bought my car at The Collection. The sales man explained a lot about the carbon fiber to me and I started to get interested in carbon fiber.

What is Carbon Fiber?

Carbon fiber is a fiber material contains more than 85% of carbon, it's a new generation of fiber material not only has excellent properties as carbon materials, but also has properties of soft and machinability as textile fiber.

Carbon fiber material (http://baike.baidu.com/subview/82595/82595.htm)

Structure and Composition of Carbon Fiber

Carbon fiber is also called graphite fiber or graphite fibre, which is a kind of fiber about 5-10 micrometer in diameter and composed mostly of carbon atoms. It is a fiber material made of graphite crystallite organic fiber like in the figure below along the fiber axial direction by carbonization and graphitization.

Carbon fiber is usually used in the form of a continuous tow wound onto a reel, and tow is a beam composed of thousands of seperate carbon filament.

Atomic structure of carbon fiber is similar to graphite atomic structure, and it's consist of sheets of carbon atoms arranged in a regular hexagonal pattern and the difference is the way these sheets interlock. Graphite is soft and brittle because the sheets in graphite are stacked parallel to one another, which causes the weak Van der Waals forces between two sheets.

Mechanical Properties of Carbon Fiber

There are lots of mechanical properties of carbon fiber. The focus here will be on low density, high stiffness, high temperature tolerance and low thermal expansion, high chemical resistance.

(1) Carbon fiber has low density and high stiffness:

Carbon fiber has a high tensile strength which is about 2-7 Gpa, and it causes carbon fiber has low density. The density of carbon fiber (1.5~2.9g/cm3) is smaller than aluminum and its even less than 1/4 of steel's density.

Since the carbon atoms in carbon fiber are arranged neatly, carbon fiber has high stiffness and the thinner the fiber is , the larger stiffness it has. (Carbon fiber's stiffness is about 20 times of iron)

(2) Carbon fiber has high temperature tolerance and low thermal expansion:

Carbon fiber is a fiber material contains more than 85% of carbon atoms, and it like the element - carbon, can tolerate temperatures above 3000 degrees, has outstanding heat resistance.

Small thermal expansion coefficient, and thermal conductivity decrease when temperature rise. Carbon fiber will not never crack even suddenly from quenching temperature to room temperature.

(3) Carbon fiber has high chemical resistance:

The properties of carbon fiber and carbon are similar. Thus carbon fiber can only be oxidized by strong oxidant and it's inert to normal alkaline.

Carbon fiber has good corrosion resistance to general organic solvent, acid and alkali. Insoluble and not expanding, the corrosion resistance is excellent and there is no rust problem completely.

Conclusion

As a new generation of fiber material, carbon fiber can be used in many fields because of its properties. Such as light weight for sports cars and sports equipments, high temperature tolerance and low thermal expansion for aerospace field. Carbon fiber's properties are mostly related to its structure. Thus understanding the relationship between carbon fiber's properties and structure is the key point to exploring more uses of carbon fiber to make the world better.

References:

1. Structure and properties from Wikipedia (https://en.wikipedia.org/wiki/Carbon_fibers)

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