When you go to a sports shop you are inundated with new “graphite” based materials for sports equipment: golf clubs, tennis racquets, bicycles (frames and wheel disks), ultralight airframes, and even America’s Cup yachts feature these new lightweight materials. But we are also familiar with graphite as being a very common and mundane substance. Graphite has long been a component of pencil lead, and is used as a basic lubricant. How is it that graphite is both a hi-tech and low-tech material? Could we take a bunch of pencil leads and epoxy them together into a cutting edge tennis racquet? Anyone who has used mechanical pencils knows that the leads break far too easily to provide a strong frame. It would seem as if there are two different kinds of graphite. In fact, this is true. When vendors market “graphite fiber” products they are usually selling a “carbon fiber” product. The correct name for the fibers used in all strengthening and reinforcing applications is carbon fibers. But, there is more to the story than just a general misconception over the term “graphite fibers.” Surprisingly, if we look at a small section of graphite and carbon fibers on the atomic level they appear to be identical.
How are carbon fibers and graphite produced?
Carbon fibers are made from organic polymers such as poly(acrylonitrile). Although a full description of polymers is not really appropriate at this point, it should be noted that polymers are giant molecules comprised of repeating units. Poly(acrylonitrile) is a polymer with chains of carbons connected to one another. To make carbon fibers, the polymer is stretched into alignment parallel with what will eventually be the axis of the fiber. Then, an oxidation treatment in air between 200 and 300 °C transforms the polymer into a non-meltable precursor fiber. This precursor fiber is then heated in a nitrogen environment. As the temperature is raised, volatile products are given off until the carbon fiber is composed of at least 92% carbon. The temperature used to treat the fibers varies between 1000 °C and 2500 °C depending on the desired properties of the carbon fiber. The process used to make carbon fibers is summarized in Figure 3.2. Each carbon fiber is very thin; the total diameter of a carbon fiber is 6-10um or about five times thinner than an average human hair (Figure 16). When carbon fibers are used in industry, they are woven into sheets, tubes, or other desired shapes (Figure 3.3). Epoxy resins or other binders are often added to the carbon fibers. The resulting composite of epoxy and carbon fibers is stronger than either component individually.
Figure 3.2 Schematic showing the process for making carbon fibers
Figure 3.3 Braiding of carbon fibers
If during the treatment process the temperature is raised above 2500 °C, graphite will be formed instead of carbon fibers! We will return to why the higher temperature leads to graphite later. Graphite can also be found in nature as flakes mixed with clay and other impurities. While graphite can be mined or formed through the carbon fiber process, most of the graphite used in industry is manufactured by heating petroleum byproducts to about 2800 °C. The petroleum byproducts are similar to the polymers used in the carbon fiber process in that both contain chains of carbon atoms.
What are the structures of graphite and carbon fibers, and what can they tell us?
The atomic structure of graphite has been determined by x-ray diffraction and other analytical techniques and is shown in Figure 3.18. Parallel sheets of hexagonal rings are spaced 3.35 Å apart. Bonds within the chicken wire-like sheets are very strong, but interactions between the sheets are weaker and can be broken easily. Given this atomic arrangement, we can begin to explain some of the properties of graphite. When the interactions between sheets break, the planes slide over one another. It is this sliding that makes graphite such a good lubricant, and also explains why it is a soft brittle substance.
Figure 3.18 Structure of graphite
The great stability of graphite can be explained in terms of its bonding. From the interlocking hexagonal rings we see that each carbon is bonded to three other carbon atoms. We know that the bonding rules for carbon call for a total of four bonds. This suggests that there are two single and one double bond from each carbon. It is observed from the x-ray structure that all of the carbon bond lengths are equivalent. We encountered a similar situation with benzene As with benzene, we can explain why the bond lengths do not alternate between single and double bond lengths by saying the bonds are in resonance That is, the bonds are all a blending of single and double bonds . Having the double bond character spread evenly throughout the entire structure makes the sheets of atoms in graphite very stable.
So, why does carbon fiber convert to graphite at high temperatures?
Recall that graphite is an extremely stable form of carbon, due to the extensive resonance of single and double bonds. Carbon fibers are also stabilized by resonance, but because the structure is irregular, the effect is not as extensive. Ultimately, graphite is more stable than carbon fibers. The figure below shows a depiction of the reordering from carbon fibers to graphite as temperature is increased.
Figure 3.4 The transition from amorphous carbon to graphite
Why is temperature related to the reordering process? You may not have given much thought to what temperature physically represents. In a formal sense, temperature is a measure of the average kinetic energy in a system. So, when higher temperatures are applied to the carbon fibers, eventually enough energy is present to break the bonds in the carbon fibers, allowing them to reorganize to the more stable graphite form.
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