Page 246 - Carbon Nanotube Fibres and Yarns

236 Carbon Nanotube Fibers and Yarns

and tensile motion. By stretching and relaxing tightly twisted yarns made of
CNT yarns, Kim et al. [83] devised a way to harvest and store the motion
as electrical energy. The production of electrical energy can be optimized
by controlling the twist intensity and a combination of homochiral and
heterochiral coiled yarns.
This electrochemical procedure is a reversible process in the sense
that the electrochemical charge could be used for actuation. CNT yarns
could be used as an electrochemically powered, all-solid-state torsional
and tensile artificial yarn muscles that provide attractive performance.
Recently it has been shown that highly twisted MWCNT yarns pro-
duce a unique mechanical actuation involving coupled rotation and
axial contraction [84]. The torsional and tensile actuation is achieved
by reversible yarn volume changes driven by electrolyte ion influx/
release during electrochemical charge/discharge, or thermal expansion/
contraction of a guest material, like paraffin wax. Fig. 9.13 shows the
structures of MWCNT yarns for torsional and tensile actuation. Single
yarns are plied and twisted to form coiled yarns. From Fig. 9.13, the sur-
face of a single-ply non-coiled yarn used for the torsional muscle exhibits
high porosity that allows the electrolyte to infiltrate the yarn, as well as a
highly oriented fibrous structure which contributes to the yarn direction
strength and electrical conductivity.

Fig. 9.13 (A, B) SEM images of neat, single and two-ply yarns, respectively, used in two-
ply form for torsional actuation when electrolyte filled. (C, D) SEM images of neat, single
and plied, coiled yarns, respectively, used in two-ply form for tensile actuation when
electrolyte filled. (E) A magnified SEM image of the lateral surface of a neat, twist-spun
yarn, where the arrow indicates the fiber direction. (F) SEM image of a plied, coiled yarn
that is fully infiltrated with PVA/H 2 SO 4 solid gel electrolyte [84]. (Source: J.A. Lee, Y.T. Kim,
G.M. Spinks, D. Suh, X. Lepró, M.D. Lima, et al., All-solid-state carbon nanotube torsional and
tensile artificial muscles, Nano Lett. 14 (2014) 2664–2669.)