We've gotten used to new technology that comes along and renders obsolete the old tech it displaces. But there are also plenty of instances where the new meshes nicely with the old, changing the world in amazing and unforeseen ways.
That's what I thought when I stumbled across an article from BusinessWeek about a five-employee startup company in Maine called Advanced Infrastructure Technologies (AIT). This outfit unites innovative new materials with one of humanity's hoariest engineering accomplishments: the construction of the arched bridge.
Specifically, the company has designed a system that allows for the building of a new bridge in as few as 10 days, with no heavy equipment involved. What's more, these structures—because they offer greater protection from corrosive factors like weather and salt—are projected to have a longer life than those made with traditional construction techniques. Although materials are a bit costlier, that's more than offset by savings in labor.
AIT's technique involves using concrete-filled, carbon fiber-reinforced polymer composite tubes.
Many people probably still think of carbon as the stuff that makes up the human body or the end of a graphite pencil, or what is left over after you burn paper. OK, most know that it also makes diamonds. But turning it into a fiber that's strong enough to replace steel in bridge arches? That doesn't seem possible.
Yet it is. Here's how the process—"Bridge in a Backpack," as it's known—works:
Whether AIT will be able to convince a sizeable chunk of the notoriously conservative construction industry that this is in fact a better approach remains to be seen. But so far, it has been involved in the construction of 13 bridges, mostly in Maine, Massachusetts, and Michigan.
In any event, the unlikely image of bridge supports made out of fiber got me to wondering just what other uses there might be for this miracle material. I knew that my golf club shafts use it, for example, and that it's in some car parts which used to be metal. But where else do we find it? Well, turns out that it's just about everywhere.
First, though, just what is it anyway?
Carbon fiber, or CF, is a material made up of carbon atoms bonded together in crystals along the long axis into filaments about 5-10 μm (micrometers) in diameter. This is what one such filament looks like; it's laid atop a human hair for comparison purposes.
CF is possible because of one of the peculiarities of carbon is that it can exist in a number of different forms (allotropes), depending on the way the atoms bond together. Each of these allotropes—which can be fashioned by nature into coal and by man into buckyballs and nanotubes—will have very different properties.
For example, each carbon atom in a diamond is covalently bonded to four other carbons in a tetrahedron. These tetrahedrons together form a three-dimensional network of six-membered carbon rings.
Graphite, on the other hand, consists of sheets of carbon atoms ("graphene" sheets) arranged in a regular hexagonal pattern.
The structure of CF is similar to graphite, with the difference being in the way the graphene sheets interlock.
One surprising fact is that while carbon fibers are generally thought of as a space-age material, their lineage actually dates back to the late 1800s. Thomas Edison used carbon fibers in his early light bulb filaments, which required the ability to conduct electricity while remaining fire resistant and capable of enduring the intense heat needed to create incandescence.
In order to make the fibers, you start with a raw material, or precursor. Edison took a cellulose-based precursor such as bamboo and baked it at high temperature in a controlled atmosphere in a carbonization process known as "pyrolysys." It's similar to what we still do today.
The technology took a long time to evolve. Bamboo and other such materials were not replaced as precursors until the introduction of rayon into the process in the late 1950s. That yielded the first high-tensile-strength fibers. Shortly thereafter, in the early 1960s, modern CF arrived with the discovery that polyacrylonitrile, derived from petroleum, was the ideal precursor. However, this early manufacturing process produced a fiber that was only 55% carbon.
At present, polyacrylonitrile is still the source of 90% of the world's carbon fiber, but purification has improved dramatically, with standardization of quality coming in 1990. The precursor is now stretched into long strands, and then heated to a very high temperature without allowing it to come in contact with oxygen. Without oxygen, the fiber cannot burn. Instead, the high temperature causes the atoms to vibrate violently until most of the non-carbon atoms are expelled. This method of carbonization leaves a fiber that's nearly 100% carbon.
Carbon fibers are relatively expensive when compared to similar products such as glass or plastic fibers, due to the manufacturing process being slow and energy intensive. But their properties—high stiffness, high tensile strength, low weight, high chemical resistance, high temperature tolerance, and low thermal expansion—make them desirable for particular applications, especially when combined with resins and molded. (If perchance you have some DIY home projects that might benefit from carbon fiber molding, you can have a go at it, beginning with this tutorial.)
That is to say, CF by itself is an interesting material, but alone it's of little value in structural applications. What really kicked its usage into high gear was what happened when it was added to different kinds of resins to create composites, generally termed "carbon-fiber-reinforced polymers" (CFRPs).
You may remember the first composite tennis racquets, which revolutionized the game in the early 1980s. And as soon as they could, golfers of a certain age began choosing carbon fiber (usually mischaracterized as "graphite") shafts instead of steel for their clubs, because the former are more forgiving and much easier on older bodies.
Even before that, though, governmental and private aerospace efforts had been quick to embrace the possibilities. Carbon fiber composites' favorable strength-to-weight ratio means weight savings of 20-30% over heavier metals. Thus it began to replace steel and aluminum—wherever possible consistent with safety—in airplanes and helicopters… a godsend for the Air Force. But commercial interests weren't far behind. Weight reduction is everything in the airline business. A modern jet aircraft is apt to have carbon fiber all over the place: in its fairings, landing gear, engine cowls, rudder, elevators, flaps, fin boxes, doors, floorboards, and many other components.
Much the same happened in extraterrestrial craft. CF has gone into space with NASA and on to the moon. Again, weight considerations are paramount when lifting off from the earth. But equally important is a lower ablation rate (i.e., the speed at which a material is stripped away by the friction of reentry), along with higher bulk density, superior mechanical strength, and high modulus (inelasticity). Carbon fiber composites—including carbon-carbon, which consists of CF-reinforced graphite—that have been densified fill the bill, and are used in nose tips and heat shields.
The space shuttle was largely dependent on CF materials. CF/epoxy composites made up the payload bay doors and the shuttle's remote manipulator arm. Likewise for satellites, which require high specific stiffness and dimensional stability to combat the large temperature swings in space. Thus similar composites are employed in fabricating antenna ribs and struts.
Lately, there has also been much publicity about unmanned aerial vehicles (UAVs), or drones, as they are more commonly called. UAV bodies are likely to be made of CF materials. So are the gondolas and tail fins used in blimps.
But carbon fiber is not only found in such esoteric arenas. It's very much a part of the more grounded aspects of life. For example:
All of this merely scratches the surface.
The fact of the matter is that carbon fiber has in a relatively short time become an integral part of modern life. New applications are popping up literally on literally a daily basis. Usage is expected to drive a $13+ billion/year business by 2015. That figure will be amplified a great deal as cheaper, more efficient manufacturing techniques are developed.
If carbon fibers were suddenly to disappear, we'd be up the proverbial creek without a (CF) paddle…
[Doug Hornig is a senior editor for Casey Extraordinary Technology.]
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