Figure 1. Four seamed CNT panels. Image: Nanocomp Technologies, Inc.
by Christie Martin, Sector Manager, Electrical and Conductive Products, Nanocomp Technologies, Inc., Concord, N.H.
Fabrication techniques can produce lighter, flexible conductive cables.
With steady advances in electronic components, advanced conductive materials will be necessary to provide analogous improvements in electromagnetic interface (EMI) shielding, weight reduction, strength, and durability.
Carbon nanotubes (CNTs) have gained attention as a way to deliver some of these advancements. However, in order to access their attractive electrical properties for real-world applications, carbon nanotubes must be fashioned into an easily integrated product form, preferably a non-woven textile-like sheet or spun yarn that functions as wiring.
Applications that could leverage the combined properties of CNT sheets and yarns are broad, and include lightweight conductors/cables, EMI shielding, ground planes, lightning protection, and microwave reflectors. Many potential applications for these materials have yet to be discovered.
These properties help make CNT materials attractive:
- Conductivity is equivalent to or better than copper, especially in high frequency domains.
- Finished material handles like cloth or yarn.
- Material has the strength of steel.
- Material is lighter than aluminum.
The raw strength of CNT sheets ranges between 200 MPa to 1 GPa with electrical conductivity greater than 2 x 106 S/m. This level of conductivity is sufficient for applications such as EMI shielding and electrostatic discharge (ESD) lightning strike protection in aerospace and high-end antennas. It also suggests the potential to be used as a replacement for copper shielding in high-end data cables such as coaxial, twisted pair, USB, and others.
Sheets (Figure 1) are used for various EMI/ESD shielding applications, including cables. The material can also be enhanced through various doping methods using acid, inorganic salts, or halogens to increase electrical conductivity by more than 80% and enable conductor and EMI shielding applications where high conductivity is desired. In recent testing, CNT sheets also underwent flexural testing of more than one million cycles without failing; due to time restrictions, the test had to be stopped before reaching a failure point.
Figure 2. Working Cat 5, Coax, and USB cables made from CNT nanomaterials. All images: Nanocomp Technologies, Inc.
The electrical conductivity of the nanotube material also allows it to be used as a substitute for copper braid in single- or multiple-conductor shielded cables in aerospace and other weight-sensitive applications. Spooled CNT yarns represent these primary conductors.
The key benefit here is 30% to 50% weight savings as compared to traditional copper. The new Boeing 787 has 61 miles of cabling and a Boeing 777 has 100 miles of cabling. Aircraft weight can be reduced significantly by replacing traditional copper conductors with CNT conductors. Also, similar to sheet format, CNT yarns yield improved cabling flexibility and durability, as seen in Figure 2. Because the resistivity of CNT conductive yarns can be lower than copper, another potential application is the replacement of high-frequency copper conductors.
But how does one go about turning a product that has traditionally been delivered in the form of a powder (like black talcum) into a large-scale format? The first step is to grow scalable quantities of very long CNTs (approximately 1 mm long), which can then be harvested in these sheet or yarn configurations directly from the reactor. These products are different from traditional Bucky papers in that they contain non-dispersible, long CNTs resulting in much better electrical and strength properties at the macro scale. When only available in powder form, nanotubes are very difficult to incorporate into final products or manufacturing processes.
Currently, conterminous 4-foot by 8-foot CNT sheets are being fabricated in this fashion by Nanocomp Technologies, Inc., Concord, N.H., in large closed systems. Nanocomp’s engineers have also devised a technology to seam these panels into rolls of to facilitate further surface modification or infiltration with resins on industrial equipment.
The company’s 11,000-ft2 pilot scale production facility produces tens of kilometers of conductive yarns and hundreds of square meters of sheets per week and will be expand capacity in a larger manufacturing facility within the year.
Published in R & D magazine: Vol. 52, No. 3, June, 2010, p. 40.