January seemed an appropriate month to discuss the high-tech material called carbon nanotubes (CNT). CNT have probably existed longer than we are aware, but the invention of the transmission electron microscope (TEM) allowed them to be seen for the first time. The initial buzz associated with CNT resulted after the 1991 discovery of multi-walled CNT in the insoluble material of arc-burned graphite rods.

This probably gave researchers the idea for how to make CNT using an arc process. Although there are a number of manufacturing methods, the two most common are arc discharge and chemical vapor deposition (CVD). These two methods operate at temperatures in excess of 1830˚F (1000˚C). ORNL has found that laser ablation is one of the best ways to produce high-quality single-walled nanotubes (pictured). As a cost-reduction measure, one of the efforts under way among researchers is to produce CNT at lower temperatures. Registered patents indicate these temperatures are as low as 660-750˚F. In either case, metal catalysts are used to grow the CNT. The temperature stability of CNT is estimated to be higher than 5000˚F in vacuum and nearly 1400˚F in air.

Nanotubes belong to the fullerene family with their sister, the buckyball. The name comes from their size, as the diameter of a CNT is a few nanometers. They are about 50,000 times thinner than a human hair. CNT can be either single-walled nanotubes (SWNT) or multi-walled nanotubes (MWNT). SWNT are important because of their unique electric properties. Double-walled carbon nanotubes (DWNT) – a variant of the MWNT – are important because their morphology and properties are similar to SWNT, but their resistance to chemicals is significantly improved. MWNT nested within one another slide and rotate almost without friction. This property has already been used to create the world’s smallest rotational motor.

Based on their tensile strength and elastic modulus, CNT are the strongest and stiffest materials yet discovered. This extraordinary strength along with their unique electrical properties and their efficiency in heat conduction have resulted in new applications. CNT have been used in a variety of applications, including structural – clothes, concrete, sports equipment, bridges, etc. Electromagnetic applications include chemical nanowires, magnets, solar cells and more. Other chemical and mechanical applications include nanotube membranes, water filters, infrared detectors and a slick waterproof surface – slicker than Teflon.

The CNT applications go well beyond the more conventional products. Researchers are working on a number of innovative applications that will take advantage of the unique properties of this material. NASA researchers are combining nanotubes with other materials into composites to be used for lightweight spacecraft construction. Medical researchers are utilizing the tubular shape by attaching molecules that are attracted to cancer cells to nanotubes to deliver drugs directly to the diseased cells. CNT are also being used to assist in the healing process for broken bones.

Bayer is one of the companies producing this unique material. Their MWNT product is called Baytubes®. Reinforced composite materials based on Baytubes and aluminum powder have resulted in material with strength similar to that of steel but weighing only half as much. This material is a viable alternative to steel for many applications, but its temperature properties will limit structural applications of this material (e.g., I-beams).

Researchers at Georgia Institute of Technology are developing more efficient ion thrusters utilizing CNT. Ion propulsion systems have been utilized on Earth-orbiting and interplanetary spacecraft. Using CNT in this application results in a 10% improvement in the amount of propellant available for the actual mission. The greatest benefit of the technology will be for low-power spacecraft and small satellites.

University of Cincinnati researchers have created an antenna from CNT thread. The electrical properties of CNT are due to the “skin effect.” Electrons transfer well because they travel across the skin of the material instead of through a bulk mass as in copper wire. CNT thread, a fraction of the weight of copper conductors, could significantly benefit aerospace activities because there are several hundred pounds of copper wiring on any aircraft.

Utilizing the electrical properties of CNT to reduce the weight of aerospace vehicles is one way this technology will help save energy. The production of stronger and lighter composites for many types of vehicles is another. Larger and lighter wind-turbine rotor blades, for energy-generation optimization, can also be made with CNT technology. Whether assisting the green movement or making life better for cancer sufferers, now you know how carbon nanotubes will impact our lives in the years to come. IH