Boron Nanoribbons
January 11, 2012 As I remarked in an earlier article (Sound and Heat, August 23, 2011 ), thermal conductivity of materials is mediated by electrons and phonons. For electrical insulators, only the phonons matter, so it's not surprising that the thermal conductivity of alumina (al2O3), about 18 W/m-K at room temperature, is much smaller than that of aluminum, 237 W/m-K; and the thermal conductivity of titania, about 12 W/m-K, is less than that of titanium, 21.9 W/m-K. metals are always better thermal conductors than non-metals. Diamond, an electrical insulator, has the extremely high thermal conductivity of about a thousand, which makes it an excellent substrate for power electronics. Metal alloys, which have poor electron transport properties, have relatively low thermal conductivity. For example, stainless steels have relatively low thermal conductivity. Type 316 stainless steel (nominal Fe 72%, Cr 16%, Ni 10%, Mo 2%, by weight) has a thermal conductivity of only about 15 W/m-K at room temperature. Although there's no hard rule about what makes a better thermal conductor, there seems to be a rule that nanoscale versions of materials behave differently than their bulk counterparts. A recent paper in Nature Nanotechnology by a large, multidisciplinary research team reports on some unusual behavior in the thermal conductivity of boron nanoribbons.[1-2] Members of this research team were from the Department of Mechanical Engineering, Vanderbilt University (Nashville, Tennessee), the School of Mechanical Engineering, Southeast University (Nanjing, China), the Department of Mechanical Engineering and Engineering Science, the University of North Carolina at Charlotte (Charlotte, North Carolina), the Advanced Technology Center, Lockheed Martin Space Systems Company, (Palo Alto, California), the Advanced Research Projects Agency-Energy (Washington, DC) and the School of Engineering Matter, Transport, and Energy, Arizona State University (Tempe, Arizona). Boron is a metalloid, one of such elements that populate a thin, diagonal line in the rightmost portion of the periodic table. Its thermal conductivity, 27.4 W/m-K at room temperature, is not quite as high as that of most metals, but it's still higher than most other materials. When materials, including boron, are prepared on the nanoscale, their thermal conductivity is reduced. The reason is that there is very little interaction between these nanostructures. They interact primarily through van der Waals forces.[1] The unusual effect found by the authors of this study is that bundles of boron nanoribbons have a thermal conductivity that's significantly higher than what is expected from a collection of non-interacting nanoribbons.[1-2] Not only that, but this enhanced thermal conductivity can be switched on and off by washing the nanoribbon assemblage to add or remove surface contamination.[1] It was always assumed that phonons, the heat carriers in these materials, were not able to move through the interface between such nanostructures. In the case of nanoribbons, however, the flat surfaces seem to allow a tighter contact between individual nanostructures and improved transmission of phonons between them.[2] The research team found that when the nanoribbons were washed in isopropyl alcohol, there was a remaining residue that clung to the nanoribbons, prevented intimate contact, and resulted in lower thermal conductivity. A wash in reagent-grade alcohol did not leave any residue, and the thermal conductivity was high. The washing effect is reversible, and it depends on which solution was used last.[2] Terry T. Xu, an associate professor of mechanical engineering at Vanderbilt, says that this ability to control the thermal conductivity may be useful.
"If you want more heat dissipated, but only in certain conditions, you can apply a solution to create a bundle structure with tight bonds and higher thermal conductivity. It could similarly be reversed by adding a residue between the nanoribbons and reducing the thermal conductivity to that of an individual ribbon."[2]This research was funded by the National Science Foundation and Lockheed Martin.[2]
References:
- Juekuan Yang, Yang Yang, Scott W. Waltermire, Xiaoxia Wu, Haitao Zhang, Timothy Gutu, Youfei Jiang, Yunfei Chen, Alfred A. Zinn, Ravi Prasher, Terry T. Xu and Deyu Li, "Enhanced and switchable nanoscale thermal conduction due to van der Waals interfaces," Nature Nanotechnology, doi:10.1038/nnano.2011.216, December 11, 2011.
- James Hathaway, "Boron Nanoribbons Reveal Surprising Thermal Properties in Bundles," The University of North Carolina at Charlotte Press Release, Dec. 20, 2011
- Nanomaterials Lab at the University of North Carolina at Charlotte.
- James Hathaway, "Boron Nanoribbons Reveal Surprising Thermal Properties in Bundles," The University of North Carolina at Charlotte Press Release, Dec. 20, 2011