May 2008 Mechanical Engineering Departments: Tech Focus, Pt. 2

This section was edited by Associate Editor Alan S. Brown.

Technology Focus part 2:
Materials and Assembly

Sleep on a Nanotube Sheet

A three-year-old New Hampshire start-up, Nanocomp Technologies Inc., has produced the largest cohesive nanotube structures ever, a series of 3-foot-by-6-foot sheets made entirely of carbon nanotubes. Not only is the structure big, but the way Nanocomp made it opens the door to new types of nanotube composites and fibers as well as nanotube wiring, aircraft lightning protection, and high-performance heat sinks.

Nanocomp's breakthrough lies in its ability to make nanotubes about 1 millimeter long. This may not sound like a lot, but given the small size of most nanotubes (a few nanometers in diameter, perhaps 100 to 200 micrometers long), it is equivalent to building a 10-foot-diameter tube nearly 180 miles high. Production takes place in a tube furnace. Nanocomp injects a hydrocarbon gas and iron catalyst on one end and pulls off nanotubes on the other.

A new nanoscale: This 3-foot-by-6-foot sheet is the largest coherent nanotube structure ever created. The technology could be used for lightweight electrical wires in aircraft and satellites.

To make large sheets, it simply captures the exiting nanotubes on a rotating belt. The mutual attraction of the nanotubes, long a problem when it came to dispersing them evenly in other materials, proves an advantage here. The randomly arranged nanotubes stick together so strongly, they exhibit tensile strengths of 200 to 500 megapascals.

"Aluminum breaks at 500 megapascals," said company CEO Peter Antoinette. "We can also relax and partially align the tubes, which raises the breaking strengths for these sheets to 1.2 gigapascals, which is around that of carbon steel. What is most significant is that our sheets have densities of 0.2 to 0.5 grams per cubic centimeter. Aluminum is 2.8 grams per cubic centimeter and steel 7.8 grams per cubic centimeter. The strength-to-weight ratio for our sheets is a key benefit."

Of course, nanotubes are not ready to take on traditional metals in most applications. But Antoinette says if he can scale the process, he should be able to get costs down to about $400 per kilogram or less. That sounds like a lot, but on a volume basis it is well within the range of high-end carbon fibers.

Antoinette has his markets picked out as well. "When you say 'aircraft' and 'nanotubes,' most people think fuselages or wings. I say, 'Think about lightning strike protection, wiring, or ground planes for electrical equipment.'"

His argument is simple. When he aligns nanotube sheets, he turns them into outstanding conductors of electricity and heat along their length. The millimeter-long nanotubes are also large enough to wrap around each other to form conductive yarns.

"The aerospace industry is very interested in these applications, as our products are so much lighter-weight than the copper-based wiring harnesses used today," he explained. "The wires actually weigh less than the insulation around them."

Copper, he notes, accounts for about one-third the weight of most satellites. A large satellite that weighs 10,000 pounds would contain more than 3,000 pounds of copper wiring. He claims that nanotube wiring could save more than 1 ton of weight. "It costs between $10,000 and $30,000 per pound to launch to orbit, thus the interest," said Antoinette.

The same reductions in weight might interest airframe builders. For example, the new Airbus A380 jumbo jet uses 350 miles of copper wiring. Slashing cabling weight would lower fuel costs, especially when multiplied over the 30-year service life of the aircraft. Airframers might also consider conductive nanotube sheets to protect composites from lightning strikes and for lightweight ground planes for electrical devices.

Antoinette also sees applications for heat sinks in cell phones and other mobile devices. "It outperforms any metal," he said. Oriented sheets would also transfer heat in the direction engineers want it go. Nanocomp is getting ready to announce nanotube yarns for reinforcement, ballistics, and other applications. Meanwhile, Antoinette will focus on getting production rates up and cost down.

Stainless Tools Take on Corrosion

Anyone who works with stainless-steel screws eventually finds out that, yes, they do rust. Corrosion may occur after a single season on boats that sail the salty seas, or after several years on window and door frames.

The reason, according to Wera Tools of Wuppertal, Germany, is that assemblers almost always use carbon steel or other non-stainless-steel tools to tighten stainless hardware. Simply turning a screw or a hex nut generates enough force to break off microscopic particles of tool steel. These particles will corrode. Left in a damp-enough environment, corrosion will eventually attack the stainless fastener. At best, the screw will bleed rust; at worst, it could start to pit and deteriorate.

Contamination from carbon steel tools can cause corrosion on stainless-steel fasteners such as these.

The solution sounds like it should be obvious: Switch to stainless-steel tools, which generate stainless particles that resist corrosion. The problem is that stainless, by itself, is too soft for high-volume factory work. Past attempts to harden stainless steel make tools too brittle for regular use. This is why stainless screwdrivers are usually reserved for medical and small-scale hardware.

Wera spent two years attacking the problem. Its solution includes a final cryogenic tempering step that supercools its stainless-steel tools in a vacuum chamber. The resulting material achieves a Rockwell hardness of 58, hard enough to compete with carbon steel. The company scores its screwdriver tip with a laser to improve its grip and uses oversize contact zones to reduce notching on its hex drivers.

One caveat, though: Stainless-steel tools are only for stainless-steel fasteners. Tighten a carbon steel tool with them, and it will require autoclaving to remove the contamination.