Carbon nanotubes have been around for centuries. They are basically micron-size cylinders of carbon that scientists have claimed could do amazing things—if they could ever be successfully used.
The properties they hold include exceptional strength, excellent electrical conductivity, high heat resistance, and an innate resistance to electromagnetic interference. The problem has always been that nanotubes are so tiny—only tenths of a micron long—that the end result is basically powder, which makes them extremely difficult to transform into any usable format for manufacturing.
Other drawbacks have been expense and material impurity. Modern attempts to produce nanotubes in volume tend to generate significant amounts of impurities, which requires an added, expensive purification step. But, even with all the problems, because of the benefits of the material, many forms of processing have been attempted. Until recently, all results have been disappointing.
The tide turned for carbon nanotubes with an initial $2 million contract awarded to Concord, NH-based Nanocomp Technologies, Inc. by the U.S. Army’s Natick Soldier Systems Center. The Army was looking for advanced materials for body armor and other applications, and saw the inroads that Nanocomp had achieved in processing this material very promising.
Peter Antoinette, President, CEO, and cofounder of Nanocomp, said that this grant allowed them to develop methods to continuously produce longer carbon nanotubes (millimeter in length) in high growth rates. These longer nanotubes deliver all the properties expected of the material, but in a safer and easier-to-handle format. Because of the proprietary technology developed to produce these longer nanotubes, the end result is a very pure product that does not require post-growth purification processes.
Without breaching proprietary details, David Lashmore, Chief Technology Officer and first developer of the process, explained that they are growing the nanotubes in a heated gas environment to the desired length. This process is done using a carbon-containing fuel such as ethanol or methane, heating it up, and flowing it past a catalyst—a nanoparticle that can be made from any number of materials, including oxides of nickel, cobalt, or iron.
“Heat causes the flowing fuel to react with the catalyst, breaking off the carbon atoms, which build up on the catalyst, atom by atom, into a nanotube. The size of the catalyst determines the diameter of the nanotube,” Lashmore said.
Antoinette added that Nanocomp’s technical challenge was to figure out a way to maintain the catalyst particle at the desired size and hold it stable long enough for the nanotube to grow to millimeter length. Achieving this goal required dedicating a computer to controlling about 30 different parameters in the process, including temperature, temperature gradient, gas flow rates, and the chemistry of the mix. This process allows the builders to control the properties of the tubes. One setting gives them single-walled tubes, and another gives multi-walled versions, with one cylinder inside another, which provide different properties.
Nanocomp next developed a way to spin these tubes into fiber. Lashmore said that after growing the individual nanotubes to the length desired, they spin them on a rotating spindle that is electronically controlled. “We did hundreds of thousands of experiments until we found what worked,” he said.
These spun conductive yarns exhibit breaking strengths up to 3 GPa, with fracture toughness that is higher than aramids (such as Kevlar or Twaron). They also show enhanced electrical conductivity with the ability to carry more current with better conductivity than copper at high frequencies.
“In principle, there is no limit in how long we can make the yarn, how wide a diameter we can make it, or how much power they can carry. It is all just a matter of cost. We are working to get our efficiency levels up and cost down,” Lashmore said.
The Army has been sponsoring this development, which is divided into two programs. One is to make the yarns as tools to conduct electricity. The second part is making sheet materials.
For making sheet material, the nanotubes are deposited onto a larger belt system in many layers. When it is finished, the resulting material is cut into large sheets. One application for this large sheet is lightning protection; another is electromagnetic interference (EMI) shielding at high frequencies.
The nanotube-based sheets are extremely lightweight, and their tensile strength ranges from 200 to 500 MPa. A sheet of aluminum of equivalent thickness, for comparison, has strength of 500 MPa. By aligning nanotubes in a specific manner, the strength potentially jumps to 1200 MPa.
Antoinette said that because of composite materials’ lack of conductivity, many aerospace designers are working on trying to develop new ways for lightning strike protection and grounding of electrical circuits. Nanotechnology can offer a feasible way to make composites conductive.
“Large nanotube sheets are extraordinarily conductive, very lightweight, and since based on a carbon system, work with the resins they are already using to lay up current systems,” he said. “You could just lay [the sheet] out and put it right into existing systems—carbon-fiber wing systems, fuselage, and so forth.”
“In the short run, just taking advantage of the conductivity is where the Air Force started looking at it for EMI shielding applications. Plus, it clearly lends itself to classified applications with radar signature questions,” Antoinette added.
Spun nanotube yarn is currently being evaluated by major aerospace companies for the replacement of copper wire in aircraft. It is already much better for high-frequency applications than copper wire, but is under test for lower-frequency applications.
“A third of the weight of a satellite is copper wiring harnesses,” Antoinette said. “We can reduce that from 25 to 50%. The copper wiring in a 747 is upwards of 4000 lb. If nanotube cable could cut the weight of copper wire in a 747 in half, we’re talking literally millions of dollars of savings in fuel costs over the life of an airplane.”
Specific evaluations are currently in process with the U.S. Air Force, the U.S. Navy, and commercial companies, but Antoinette said that since all are under non-disclosure agreements, no details can be divulged at this time.
“As we go forward, we will be making products in more formats that manufacturers can easily use,” he said. “We are trying to fit as closely as possible into existing manufacturing technology. Even with something this advanced, we certainly do not expect that any major company is going to retool their entire production system just for us. So as much as we can, we will try to create sizes, shapes, and formats that fit right into their existing production to make this as easy as possible for them to use.”