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A finely tuned carbon nanotube thin film has the potential to act as a thermoelectric power generator that captures and uses waste heat, according to researchers at the Energy Department's National Renewable Energy Laboratory (NREL). The research could help guide the manufacture of thermoelectric devices based on either single-walled carbon nanotube (SWCNT) films or composites containing these nanotubes. Because more than half of the energy consumed worldwide is rejected primarily as waste heat, the idea of thermoelectric power generation is emerging as an important part of renewable energy and energy-efficiency portfolios.

"There have not been many examples where people have really looked at the intrinsic thermoelectric properties of carbon nanotubes and that's what we feel this paper does," said Andrew Ferguson, a research scientist in NREL's Chemical and Materials Science Center and co-lead author of the paper with Jeffrey Blackburn.

The research, "Tailored Semiconducting Carbon Nanotube Networks with Enhanced Thermoelectric Properties," appears in the journal Nature Energy, and is a collaboration between NREL, Professor Yong-Hyun Kim's group at the Korea Advanced Institute of Science and Technology, and Professor Barry Zink's group at the University of Denver. The other authors from NREL are Azure Avery (now an assistant professor at Metropolitan State University of Denver), Ben Zhou, Elisa Miller, Rachelle Ihly, Kevin Mistry, and Sarah Guillot.

Nanostructured inorganic semiconductors have demonstrated promise for improving the performance of thermoelectric devices. Inorganic materials can run into problems when the semiconductor needs to be lightweight, flexible, or irregularly shaped because they are often heavy and lack the required flexibility. Carbon nanotubes, which are organic, are lighter and more flexible.

How useful a particular SWCNT is for thermoelectrics, however, depends on whether the nanotube is metallic or a semiconductor, both of which are produced simultaneously in SWCNT syntheses. A metallic nanotube would harm devices such as a thermoelectric generator, whereas a semiconductor nanotube actually enhances performance. Furthermore, as with most optical and electrical devices, the electrical band gap of the semiconducting SWCNT should affect the thermoelectric performance as well.

Fortunately, Blackburn, a senior scientist and manager of NREL's Spectroscopy and Photoscience group, has developed an expertise at separating semiconducting nanotubes from metallic ones and his methods were critical to the research, Ferguson said.