Did you know that carbon is the third element created after the Big Bang? And that carbon is the fourth most abundant element in the universe after hydrogen, helium, and oxygen? And, did you also know that carbon is the most important element in all of what are called “advanced composites”? In this post, I’m going to give a brief introduction to the fibers that are made from carbon, and why they are as strong, light, and stiff as they are.
Pure carbon comes in several different forms which are collectively called “allotropes”. These forms are the arrangements of the carbon atoms when they are bound together. The two most common forms of carbon that all of us know about are graphite (pencil lead) and diamond (the hardest naturally occurring gem stone). Graphite is made up of layers of a substance called graphene which is just a whole bunch of 6-carbon ring structures all attached together in a flat sheet. These flat sheets can slip across each other, which is what makes your pencil write. Graphite is actually “rock that writes”. And it is pretty amazing stuff. It is used for the electrodes in carbon arc furnaces for making steel because it conducts electricity and can withstand temperatures in excess of 3000° C.
These 6-carbon rings connected together to form graphene are extremely strong because carbon forms covalent bonds – meaning that it does not dissolve in water to form ions like common table salt does. It is this ring structure that makes things made out of carbon so strong and stiff. The bonds between the carbon atoms in these rings are extremely hard to break. And this leads our story to carbon fiber or more colloquially known as graphite fiber, which is the most important fiber in use in composites today.
Carbon fiber was originally developed by Union Carbide at the Parma Technical Center in Cleveland. In 1958, Roger Bacon(*) was trying to measure the triple point of carbon (the temperature and pressure at which the solid, liquid, and gas are in equilibrium) in a carbon arc furnace and discovered these stalagmite-like carbon whiskers forming on his negative electrode. When he examined them under a microscope and tried to pull them to their breaking point, he found that they were extremely strong and stiff for their size. In fact, they were stronger and stiffer than any fiber known at the time.
At the same time at the Parma Technical Center, two other researchers, Curry Ford and Charles Mitchell, were experimenting with rayon fibers and cloth. They found that if they heated rayon in a vacuum at temperatures up to 3000° C they could drive off almost everything but the carbon and what they had left was fairly pure carbon fiber. This discovery spawned the “advanced composites” industry as we know it today.
Of course, the technology to make carbon fiber has advanced significantly since the early days, and the manufacturing costs have come down dramatically. This is both because of the advancing manufacturing technology for making carbon fiber as well as the fact that the market for carbon fiber is ballooning very rapidly. Whereas carbon fiber used to be as much as $200-$400 per pound, today’s carbon fiber is on the order of $20 per pound. These fibers come in both high strength and high modulus fibers and they are sold either in multiple fiber tows or in fabrics of nearly any weave. Carbon fiber is used not only for golf clubs and motorcycle helmets, but also tennis rackets, bike frames, skis and snowboards, and a whole host of other commercial and industrial products. And of course, the Boeing 787 Dreamliner is largely made out of carbon fiber.
I will be talking much more about carbon fiber, and all fibers in general in future posts, but I wanted to introduce you to carbon here because carbon is such a useful and interesting element, and is the basis of most if not all composites, as well as life itself.