Page 286 - Carbon Nanotube Fibres and Yarns

Carbon nanotube yarn-based actuators 275

Fig. 11.3 SEM images of (A) single-ply twisted yarn, (B) two-ply twisted yarn, (C) single
coiled yarn, (D) two-ply coiled yarn [41], and (E) five-ply coiled left-handed yarn [42].
(Source of (A–D): J.A. Lee, Y.T. Kim, G.M. Spinks, D. Suh, X. Lepró, M.D. Lima, R.H. Baughman,
S.J. Kim, All-solid-state carbon nanotube torsional and tensile artificial muscles, Nano Lett.
14 (5) (2014) 2664–2669. Source of (E): P. Chen, S. He, Y. Xu, X. Sun, H. Peng, Electromechanical
actuator ribbons driven by electrically conducting spring-like fibers, Adv. Mater. 27 (34)
(2015) 4982–4988.)
Obviously, several single yarns can be combined to form a torque-free
multiply yarn. Coiled yarns (cylindrical snarls) can also be reverse-twisted
into a torque-free multiply yarn. Fig. 11.3 shows these different structures
formed from CNT single yarns and coils.

11.3 Actuator architectures
11.3.1 Tensile actuators—Twist spun yarns

Electrodes made from sheets of single-walled carbon nanotubes (SWCNT)
[3] and multiwalled CNTs [14] filled with electrolytes was used to generate
contraction, as illustrated in Fig. 11.4. Changing the applied voltage injects
electronic charge into the CNT electrode, which is compensated at the
nanotube-electrolyte interface by electrolyte ions (forming the so-called
double layer). The charge injection originated from quantum chemical and
double-layer electrostatic effects cause changes of the CC bond length.
The strain generated on CNT sheets is quite small, in the order of 0.2%.
When the CNT sheets are replaced by twisted CNT yarns, strains of up
to 0.5% can be obtained in response to an applied potentials of 2.5 V [43]. As
shown by Eqs. (11.1) and (11.2), if we tether the two ends of a twist-spun