Carbon-nanotube strips harness waste heat

Where work is done there is waste heat, but research is making progress to develop ways of harnessing heat so that this energy need not be wasted. Devices for converting heat into mechanical energy and then into electrical energy have attracted particular interest because they produce higher voltages that can outperform ‘Seebeck thermoelectric’ devices, which convert heat directly into electrical energy.

However, so far thermal-to-mechanical energy-conversion devices have been large, heavy and required bulky heat baths and operating temperatures significantly above room temperature. Now Takashi Ikuno, Tatsuo Fukano, Kazuo Higuchi and Yasuhiko Takeda have developed a simple ‘bimorph’ strip just millimetres in length size, which converts heat into mechanical energy at temperatures below 100℃, and under a temperature difference of as little as 5℃.

The bimorph strip is a freestanding film (FSF) with a composite of multiwalled carbon nanotubes (MWNT) on one side and nickel on the other. The nickel expands much more readily than the MWNT composite, and this causes the strip to bend when in contact with a hot surface. When the strip bends the nickel moves away from the hot surface, where it rapidly cools due to the strip’s low heat capacity. On cooling, the strip stretches towards the hot plate where it is heated again and so it continuously bends and stretches, generating mechanical energy.

The bimorph strip’s low mass and small size are key to its high heat sensitivity, and also make it suitable for integration into microdevices. The heat sensitivity may be improved further still by aligning the carbon nanotubes in the composite layer. The researchers conclude, “We believe that the MWNT-FSFs developed in this study could be one of the building blocks for energy conversion nanodevices.”