Boron nanoribbons reveal surprising thermal properties in bundles

 Size matters… but apparently so does shape -- when it comes to conducting heat in very small spaces.

Researchers looking at the thermal conductivity of boron nanoribbons have found that they have unusual heat-transfer properties when compared to other wire/tube-like nanomaterials. While past experiments have shown that bundles of non-metallic nanostructures are less effective in conducting heat energy than single nanostructures, a new study shows that bundling boron nanoribbons can have the opposite effect and "the thermal conductivity of a bundle of boron nanoribbons can be significantly higher than that of a single free-standing nanoribbon," according to a report in Nature Nanotechnology, published online on December 11.

The finding is the result of work by a multidisciplinary team headed by Ravi Prasher of the Advanced Research Projects Agency, Terry Xu of the University of North Carolina at Charlotte, and Deyu Li of Vanderbilt University (see a complete list of authors below).

Additionally, the researchers found that the unusual heat-transfer properties of boron nanoribbon bundles can be modified, allowing the higher thermal conductivity to be switched on and off through relatively simple physical manipulation. The study concludes that the ribbon structure of the nanomaterials is strongly related to the unusual thermal conductivity of the bundles.

Boron-based nanostructures are a promising class of high temperature thermoelectric materials -- substances that can convert waste heat to useful electricity -- and thermal conductivity is related to other thermoelectric properties. Physicists describe the transmission of heat energy in materials like boron as happening through the conduction of "phonons," quasi-wave-particles that carry energy through excitations of the material's atoms.

"What we found was largely unexpected," said Xu. "When two nanoribbons were put together, the thermal conductivity was found to rise significantly rather than staying the same or going down, as has been the case in previous measurements. It has been assumed that phonons were hampered by the interface between the individual nanostructures in similar materials.

"That seems to mean that the phonon can pass effectively through the interface between two boron nanoribbons," she said. "The question is whether or not this result is due to the weak van der Waals interactions between two nanostructures of ultra-flat geometry."

The team suspects that the reason for the enhanced thermal conductivity is due in large part to the flat surface structure of the nanoribbons, based on another experimental result that the group discovered by accident.

The nanoribbon bundles exhibiting the unexpectedly higher thermal conductivity were originally prepared in a solution of reagent alcohol and water, which was then allowed to evaporate, leaving some nanoribbons drawn together by van der Waals force (the weak attraction that non reactive and uncharged substances can have for each other). When other members of the team attempted to duplicate this result, however, the experiment failed and the bundles only had the lower thermal conductivity of single ribbons. The researchers then noted that a significant difference between the two attempts was that the second experiment had used isopropyl alcohol rather than reagent alcohol in the solution. Since isopropyl alcohol was known to leave minute residue following evaporation, the researchers suspected that a residue was forming on the ribbons surfaces -- a fact that microscopy confirmed -- and the residue apparently prevented tight contact between two nanoribbons. Further tests were made on the lower-conducting bundles, where the ribbon interfaces were washed with reagent alcohol to remove the isopropyl residue, and in this experiment the higher thermal conductivity was achieved.

The results point to the conclusion that boron nanoribbons form better heat-conducting bundles because the ribbons flat surfaces allow for tighter, more complete contact between the individual structures through van der Waals interaction and improved transmission of phonons overall.

"The result implies that achieving a tight van der Waals interface between the ribbons is important in thermal conductivity, something their geometry encourages," Xu said. "It is possible that this result may have implications for other materials with ribbon-based nanostructures."

Xu notes that there are potential engineering applications for the finding come not just from the improved thermal conductivity of boron nanoribbon bundles, but also from the reversible nature of the effect.

"This may lead to a simple way to switch the thermal conductivity of the bundle on and off," she said. "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."

The finding appears in a letter to Nature Nanotechnology. The authors are Juekuan Yang, Yang Yang, Scott Waltermire and Deyu Li from Vanderbilt University; Xiaoxia Wu, Haitao Zhang, Timothy Gutu, Youfei Jiang, and Terry Xu from UNC Charlotte; Yunfei Chen from Southwest University in Nanjing, China; Alfred Zinn from Lockheed Martin Space Systems and Ravi Prasher from the Advanced Research Projects Agency in the US Department of Energy. This research was funded by the National Science Foundation and Lockheed Martin.

Story Source:

The above story is reprinted from materials provided by University of North Carolina at Charlotte.

Note: Materials may be edited for content and length. For further information, please contact the source cited above.

Journal Reference:

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, Deyu Li. Enhanced and switchable nanoscale thermal conduction due to van der Waals interfaces. Nature Nanotechnology, 2011; DOI: 10.1038/nnano.2011.216

Nano bundles pack a powerful punch: Solid-state energy storage takes a leap forward

Rice University researchers have created a solid-state, nanotube-based supercapacitor that promises to combine the best qualities of high-energy batteries and fast-charging capacitors in a device suitable for extreme environments.

A paper from the Rice lab of chemist Robert Hauge, to be published in the journal Carbon, reported the creation of robust, versatile energy storage that can be deeply integrated into the manufacture of devices. Potential uses span on-chip nanocircuitry to entire power plants.

Standard capacitors that regulate flow or supply quick bursts of power can be discharged and recharged hundreds of thousands of times. Electric double-layer capacitors (EDLCs), generally known as supercapacitors, are hybrids that hold hundreds of times more energy than a standard capacitor, like a battery, while retaining their fast charge/discharge capabilities.

But traditional EDLCs rely on liquid or gel-like electrolytes that can break down in very hot or cold conditions. In Rice's supercapacitor, a solid, nanoscale coat of oxide dielectric material replaces electrolytes entirely.

The researchers also took advantage of scale. The key to high capacitance is giving electrons more surface area to inhabit, and nothing on Earth has more potential for packing a lot of surface area into a small space than carbon nanotubes.

When grown, nanotubes self-assemble into dense, aligned structures that resemble microscopic shag carpets. Even after they're turned into self-contained supercapacitors, each bundle of nanotubes is 500 times longer than it is wide. A tiny chip may contain hundreds of thousands of bundles.

For the new device, the Rice team grew an array of 15-20 nanometer bundles of single-walled carbon nanotubes up to 50 microns long. Hauge, a distinguished faculty fellow in chemistry, led the effort with former Rice graduate students Cary Pint, first author of the paper and now a researcher at Intel, and Nolan Nicholas, now a researcher at Matric.

The array was then transferred to a copper electrode with thin layers of gold and titanium to aid adhesion and electrical stability. The nanotube bundles (the primary electrodes) were doped with sulfuric acid to enhance their conductive properties; then they were covered with thin coats of aluminum oxide (the dielectric layer) and aluminum-doped zinc oxide (the counterelectrode) through a process called atomic layer deposition (ALD). A top electrode of silver paint completed the circuit.

"Essentially, you get this metal/insulator/metal structure," said Pint. "No one's ever done this with such a high-aspect-ratio material and utilizing a process like ALD."

Hauge said the new supercapacitor is stable and scaleable. "All solid-state solutions to energy storage will be intimately integrated into many future devices, including flexible displays, bio-implants, many types of sensors and all electronic applications that benefit from fast charge and discharge rates," he said.

Pint said the supercapacitor holds a charge under high-frequency cycling and can be naturally integrated into materials. He envisioned an electric car body that is a battery, or a microrobot with an onboard, nontoxic power supply that can be injected for therapeutic purposes into a patient's bloodstream.

Pint said it would be ideal for use under the kind of extreme conditions experienced by desert-based solar cells or in satellites, where weight is also a critical factor. "The challenge for the future of energy systems is to integrate things more efficiently. This solid-state architecture is at the cutting edge," he said.

Co-authors of the paper include graduate student Zhengzong Sun; James Tour, the T.T. and W.F. Chao Chair in Chemistry as well as a professor of mechanical engineering and materials science and of computer science, and Howard Schmidt, adjunct assistant professor of chemical and biomolecular engineering, all of Rice; Sheng Xu, a former graduate student at Harvard; and Roy Gordon, the Thomas Dudley Cabot Professor of Chemistry at Harvard University, who developed ALD.

Story Source:

The above story is reprinted (with editorial adaptations) from materials provided by Rice University.

Journal Reference:

Cary L. Pint, Nolan W. Nicholas, Sheng Xu, Zhengzong Sun, James M. Tour, Howard K. Schmidt, Roy G. Gordon, Robert H. Hauge. Three dimensional solid-state supercapacitors from aligned single-walled carbon nanotube array templates. Carbon, 2011; 49 (14): 4890 DOI: 10.1016/j.carbon.2011.07.011