ABSTRACT
The direct conversion of different types of energy to mechanical energy is of prime importance in a wide variety of actuation applications such as robotics, arti„cial muscles, valves, optical displays, sensors, optical telecommunication, micro-electro-mechanical systems (MEMS), and microopto-mechanical systems (MOMS), spanning various engineering disciplines. Materials that have
6.1 Introduction .......................................................................................................................... 177 6.2 Carbon Nanotubes ................................................................................................................ 179
6.2.1 Physical Properties ................................................................................................... 179 6.2.2 Electronic Properties of Carbon Nanotubes ............................................................. 182 6.2.3 Thermal Properties of Carbon Nanotubes ................................................................ 185 6.2.4 Mechanical Properties of Carbon Nanotubes ........................................................... 185 6.2.5 Optical Properties of Carbon Nanotubes .................................................................. 186
6.3 Photomechanical Actuators .................................................................................................. 190 6.3.1 Photostrictive Actuators ............................................................................................ 191 6.3.2 Polarized Photomechanical Actuators ...................................................................... 191 6.3.3 Liquid-Crystal-Based Photomechanical Actuators .................................................. 192 6.3.4 Photomechanical Actuators Based on Optothermal Transitions .............................. 193 6.3.5 Charge-Induced Photomechanical Actuators ........................................................... 193 6.3.6 Photomechanical Actuators Based on Radiation Pressure ....................................... 194 6.3.7 Photon-Induced Actuation of Shape-Memory Polymers .......................................... 195 6.3.8 Photomechanical Actuation of Carbon Nanotubes .................................................. 196
6.3.8.1 Photomechanical Actuation of Pristine Carbon Nanotubes ...................... 196 6.3.8.2 Photomechanical Actuation of Nanotube-Polymer Composites ............... 201
6.4 Mechanisms of Photomechanical Actuation ........................................................................206 6.4.1 Electrostatic Effects .................................................................................................. 211 6.4.2 Polaronic Effects ....................................................................................................... 213 6.4.3 Localized Thermal Effects ....................................................................................... 213
6.5 Applications of Photomechanical Actuation of Carbon Nanotubes ..................................... 214 6.5.1 Nanotube Micro-Opto-Mechanical Actuators ......................................................... 215 6.5.2 Nanotube Micro-Opto-Mechanical Grippers ........................................................... 219
6.6 Summary ..............................................................................................................................226 Acknowledgment ........................................................................................................................... 227 References ...................................................................................................................................... 227
the capability of changing their physical dimensions in response to external stimuli such as heat, electrical „eld, magnetic „eld, and light are used as actuators. The best-known materials used today for actuators are piezoelectrics, electrostrictive materials, conducting polymers, and shape-memory alloys (SMA) (Kaneto et al. 1995; Smela et al. 1995; Kovacs 1998), which are primarily driven by electrical or thermal stimuli. Piezoelectric and electrostrictive materials are limited by the high driving voltages, low work density per cycle, low maximum allowable operational temperatures, and low strain output. Various polymer-based actuation materials exhibit excellent actuation performance with stroke, force, and ef„ciency similar to that of human muscles. They are also low cost and have a wide variety of choices of materials; however, either they suffer from Faradaic processes involving ionic diffusion, which present limitations on the actuation rate and cycle life (Baughman et al. 1999), or they require a high electrical „eld in operation. A further limitation for many conducting polymer-based actuators is the need for a liquid environment for their operation, presenting dif„culties in dry and vacuum applications. SMA-based actuation has been widely used in many applications and can be con„gured to give high strain and stress responses; however, a cyclic deformation mechanism is needed for repeatable operation, which presents limitations for reversible actuation. It must be appreciated that all these actuation technologies have already been successfully employed in a wide variety of practical applications demonstrating that different actuation materials match speci„c application requirements while their drawbacks are not a critical issue in such applications. Most of the actuation technologies mentioned earlier, however, are not suitable for fabricating low-cost, high-performance miniature actuators in micro-or nanoscales for future nanotechnology, bio-nanotechnology, and biomedical applications where batch fabrication capability, scalability into nanoscales, ease of operation, and high performance are all critical issues. Therefore, actuators based on new material systems need to be developed to meet such future application requirements.
