Page 291 - Carbon Nanotube Fibres and Yarns

280 Carbon Nanotube Fibers and Yarns

F z F t F z

Current Current

F t
Fig. 11.8 Schematic illustration of the lengthwise contraction and rotary torsion of a
CNT yarn upon the passage of electrical current [49]. (Source: W. Guo, C. Liu, F. Zhao, X.
Sun, Z. Yang, T. Chen, X. Chen, L. Qiu, X. Hu, H. Peng, A novel electromechanical actuation
mechanism of a carbon nanotube fiber, Adv. Mater. 24 (39) (2012) 5379–5384.)

11.4.1 Electromechanical
When subjected to a direct current of several microamperes, a twist-spun
CNT yarn shrinks along the axial direction, and the two ends rotate in the
opposite directions to generate a torque (Fig. 11.8). Guo et al. [49] used
Ampere’s law to explain the generation of electromagnetic forces between
the helically aligned CNTs.

11.4.2 Swelling by solvent and vapor
CNT yarn-based actuators activated by solvent and vapor can be divided
into two groups. One is pure CNT yarn (or modified CNT yarn) and the
other is CNT yarns infiltrated with active guest materials.
Actuations of pure CNT yarn actuators can be achieved as a result of
the solvent and vapor infiltration through capillary forces [50]. The channels
formed between CNTs in a CNT yarn provide the capacity for solvent
and vapor infiltration. When the contact angles of solvent and vapor against
modified CNT are well below 90 degree, the CNT yarn can be rapidly
wetted. The energy for the contractive and rotary actuations originate from
the surface free energy that is released during the wetting process, in which
air-solid interfaces are rapidly replaced by liquid-solid interfaces. The ki-
netic energy thus generated may be expressed as follows:

W =−∆ G = γ vs + γ vl − γ ls (11.5)
k
where ∆G represents the change in the Gibbs free energy during the wet-
ting process and γ vs , γ ls , and γ vl correspond to the surface free energies (sur-
face tensions) of the vapor-solid, liquid-solid, and vapor-liquid interfaces,
respectively [46].
In the case of CNT yarns filled with active guest materials, the ac-
tuations of the actuators are realized from the volume expansion of the
solvent-responsive guest materials in the CNT yarns [51]. For example,