Carbon nanotubes are the most recent addition to the growing list of electroactive actuator materials. These materials have exciting electrical and mechanical properties derived from their structure that consists of hollow cylinders of covalently bonded carbon just one atomic layer thick. Recent reports have described several different actuation mechanisms for carbon nanotubes, including actuation by double-layer charge injection [Baughman, Cui et al., 1999], electrostatic actuation [Kim and Lieber, 1999], and photo-thermal actuation [Zhang and Iijima, 1999]. Double-layer charge injection is a particularly promising mechanism for nanotube actuation, wherein supercapacitor charging results in relatively large changes in covalent bond lengths. The achievable actuator strain, coupled with the excellent mechanical properties of carbon nanotubes, mean that these materials potentially offer actuator performance exceeding that of all other actuator materials in terms of work-per-cycle and stress generation.
The accessibility of nanoscale structural features in nanotubes also makes these materials very exciting âbuilding blocksâ for nanotechnology. The extremely high stiffness, strength, and damage tolerance of carbon nanotubes makes them ideal structural members for nano-devices. The actuation and sensing capabilities of nanotubes [Kong, Franklin et al., 2000] adds multifunctionality, so that the entire structure may become like a living cell: responding in a pre-determined way to changes in the surrounding environment. Breakthroughs in nanotube processing and nano-assembly [Yu et al., 1999] will provide the platform for integration of nanotubes into existing micro-electromechanical systems (MEMS) devices and, in the future, into entirely new nano-electro-mechanical systems (NEMS) applications.
Realization of the potential of nanotube actuators requires a thorough understanding of their synthesis, assembly, and properties aspects. The synthesis of and theoretical and experimental characterization of nanotubes are reviewed, with special emphasis on nanotube mechanical properties. Emerging techniques for the controlled assembly of nanotubes into macroscopic structures, such as fibers, are described. The theory of actuation in carbon nanotubes arising from double-layer charge injection is also discussed. Finally, the experimental results obtained for carbon nanotube actuators are summarized, along with suggested future research directions.