ABSTRACT
Beyond individual CNT level, the direct conversion of electrical energy into mechanical energy was also realized through CNT films, CNT bundles/fibers, and CNT composites. Different actuation mechanisms including electrostatic actuation, electrochemical actuation, and shape-memory composite actuation, have been studied. 15.3.1 CNT Electrostatic Actuators Electrostatic attraction and repulsion between individual CNTs were used for cantilever-based nanotweezers [10], and mechanical-based switches and logic elements [60]. This mechanism was also utilized to develop CNT electrostatic actuators from CNT aerogel sheets and CNT bundles. Baughman’s group [61]produced centimeter-wide, meter-long transparent CNT aerogel sheets, which are highly oriented, freestanding, and also have superior gravimetric strength compared with the sheet of high-strength steel. These CNT aerogel sheets can be easily stacked and can support liquid droplets that are 50,000 times more massive than the supporting sheet region in contact with the droplet. They used these CNT aerogel sheets to study the electrostatic actuation [17], and obtained giant stroke (220%) and high actuation speed (3.7 × 104%/s) [17]. As shown in Figure 15.10, when applying a voltage to the CNT sheet with respect to ground, the CNT sheet can show “ballooning” effect in the width direction due to electrostatic repulsive force. By tuning this voltage, the width of CNT sheet can change continuously from 0% (Figure 15.10A) to 220% (Figure 15.10B). Detail study showed the actuation in the width direction of CNT sheet is voltage dependent, with V2 dependence at low voltages and V2/3 dependence at high voltages. Large-stroke actuation by expansion in width (as well as in thickness) due to charge injection is also accompanied by the contraction in length. As a result, the actuator possesses large stress-generation capability and large work capability per cycle in length direction, with an isometric specific stress up to 4.0 MPa cm3/g and the work per cycle about ~30 J/kg. More important, such electrostatic actuation can be realized at a wide temperature range, from room temperature
to 1500 K (Figure 15.10C), and the stroke does not have significant change upon increasing temperature.
