Large-Format Carbon Nanotube Sheets Fabricated With Chemical Vapor Deposition
- Tuesday, 25 August 2009
Non-woven, textile-like sheets of carbon nanotubes can be created without post-processing.
Carbon nanotube (CNT) sheets, yarns, and their derivative products are beginning to emerge in the marketplace. A productive and scalable manufacturing method utilizes a chemical vapor deposition process for making very-large-format CNT non-woven, textile-like sheets directly from the reactor without post-processing.
These materials are different from similar “bucky papers” in that they contain non-dispersible, long CNTs. Raw sheet strength is high (200 MPa to 1 GPa) and electrical conductivity (>2 × 106 S/m) is sufficient for good EMI shielding and for replacing copper wiring for some applications. It is also possible to impregnate CNT rolls on commercial equipment with a wide variety of commercial resins including Bismaleimide toughened epoxy (BMI) and the cyanate ester family. An example of a roll of these sheets is shown in Figure 1. Yarns have been plied on commercial wire braiding machines to produce CNT wires ranging from 33 gauge to 22 gauge or lower. The material can be further doped to increase electrical conductivity to enable conductor and EMI shielding applications that require high conductivity.
Today, sheets are fabricated in large, closed systems capable of producing panels that are 52 inches wide by about 8 feet long. Technology to seam these panels into rolls of indefinite length also exists. This process facilitates handling for surface modification or for infiltration with resins on industrial equipment. Figure 2 shows an example of such a seamed panel.
Applications for these materials are broad, and leverage the following properties:
- Electrical — For lightweight conductors, EMI shielding, ground plane, and microwave reflectors. The shielding quality of the CNT material allows it to be used as a substitute for copper braid in single- or multiple-conductor shielded cable. Weight savings from this step alone may range from 30 to 50 percent as compared to conventional materials used for shielding. Another application is to replace copper conductors at very high frequencies where the impedance of CNT conductive yarns can be better than copper.
- Thermal — For heat straps, thermal interfaces for IC cooling, and thermal interface materials. The thermal conductivity of individual tubes can be very high, exceeding 2000 W/°K at the nano-scale. Conductivity at the macro-scale, as seen in CNT sheets, is generally around 120 W/°K. However, CNT sheets have a density of 0.4 g/cc, which is about seven times better thermal conductivity than copper on a per-weight basis. The material acts like a black body from near-UV to 12 microns in the infrared, and strips of this material can be used for Joule heaters at very high specific power.
- Mechanical — For armor, composites, and bonding improvements with graphite epoxy.
Scale-up plans from these processes envision a four-metric-ton capacity for sheet and yarn material by 2013.
This work was done by Dr. David S. Lashmore, VP and CTO of Nanocomp Technologies Inc. For more information, visit http://info.hotims.com/22926-124.