Page 287 - Carbon Nanotube Fibres and Yarns

276 Carbon Nanotube Fibers and Yarns

Fig. 11.4 Ions of opposite charges are attracted to the CNT electrodes in an electro-
lyte when a voltage is applied, resulting in a change in the C–Cbond length and
actuation [43]. (Source: T. Mirfakhrai, J. Oh, M. Kozlov, E.C.W. Fok, M. Zhang, S. Fang, R.H.
Baughman, J.D.W. Madden, Electrochemical actuation of carbon nanotube yarns, Smart
Mater. Struct. 16 (2) (2007) S243.)

yarn in such a way that one end of the yarn can slide without losing its twist
(i.e., T is constant) and we swell the yarn (i.e., yarn radius r increases), the
twist angle θ will increase and the yarn will contract in length, resulting in
an increase of actuated strain.
Although the actuated strains are small, the stress generated can be sig-
nificant due to the high modulus of the CNT structures.

11.3.2 Torsional actuators—Twist spun yarns
If a twist-spun yarn is tethered at both ends and thus both the total number
of twist and the length of the yarn are kept constant and the yarn is swelled,
the twist angle will increase and tension in the yarn will increase to com-
pensate the required yarn contraction due to the increase of twist angle, and
consequently, the torque in the yarn will increase according to Eq. (11.3).
In Fig. 11.5, the two ends of a twist-spun CNT yarn are tethered, and
one-half of the yarn is immersed in an electrolyte [44]. The output of the
device is via a paddle attached to the middle of the yarn. The internal pres-
sure associated with ion insertion drives the contraction of the twist-spun
yarn, causing the torque in the immersed part to be higher than that in
the unimmersed part. This torque imbalance drives twist redistribution in
the two parts of the yarn, which causes the paddle in the middle to rotate,
the torque in the unimmersed part increases. The inertia of the paddle
means that the rotation of the paddle will continue even when the electrical