Page 292 - Carbon Nanotube Fibres and Yarns

Carbon nanotube yarn-based actuators 281

the drive mechanism for actuation of twisted and coiled CNT fiber infil-
trated with silicone rubber is rubber swelling during exposure to a nonpo-
lar solvent. Silicone rubber-infiltrated CNT yarn actuation can generate a
large tensile stroke (up to 50%) during contraction when absorbing solvents
such as hexane and ether [52]. Similarly, water-driven actuators have also
been demonstrated by infiltration with water-absorbing polymers, such as
poly(diallyldimethylammonium chloride) (PDDA). The PDDA/CNT ac-
tuator provides a large tensile stroke up to 78% and a high gravimetric work
−1
capacity up to 2.17 kJ kg during contraction, which is over 50 times that
of human muscle for the same weight [53].

11.4.3 Thermoexpansion by Joule heating
Although pure CNT yarns also provide anisotropic thermal expansion,
this dimensional change is small and it can only generate useful actuation
when the yarn is heated across extreme temperature ranges. Modifying or
combining the CNT yarn with a volume expanding guest is an effective
method to transform a large isotropic guest expansion into an anisotropic
yarn expansion, thereby producing actuation. The CNT yarn can act as a
Joule heating source.
For example, a phase-change material (PCM) can be introduced as guest
materials in a CNT yarn to make a composite yarn that induces volume
expansion upon the change of temperature. Suh et al. [54] fabricated an ac-
tuator that could convert environmental temperature variation into a useful
form of mechanical work making use of the drastic volume change of PCM
icosane. The electrically conductive CNT yarn acts as a heater to induce
melting of PCM. Shang et al. [55] reported a helical wax-filled CNT yarn
actuator actuated by Joule heating. A large deformation (90% tensile con-
traction) could be produced when a 2 V power source was applied.
Polymer threads were also combined with CNT yarns to make compos-
ite yarns to induce volume expansion upon the change of temperature. For
example, Inoue et al. [56] fabricated thermally driven metal-free soft ac-
tuators consisting of poly(ethylene terephthalate) (PET) threads and CNT
yarns. The actuation performance of actuators containing a homogeneous
arrangement of PET threads and CNT yarns in their cross-sectional profile
was the highest due to homogeneous thermal distribution in the actuators.

11.4.4 Electrochemistry

An electrolyte-filled CNT yarn can be used as an electrode. Electrical dou-
ble layer forms at the CNT fiber-electrode interface when a voltage is ap-
plied to the yarn [3]. The thickness of the double layer, which is in the order