Page 293 - Carbon Nanotube Fibres and Yarns

282 Carbon Nanotube Fibers and Yarns

of one to few tens of nanometers, can be estimated from the Debye length
(κ−1), which is inversely proportional to the square root of the molar con-
centration of the electrolyte. Electric double layer creates repulsive forces
between the charged surfaces, which act over distances that are comparable
to the Debye length [1]. In CNT yarns, formation of the double layer leads
to an increase in the diameter of the yarn, which translates to torsional and
linear actuation [57].

11.5 Performance metrics for actuators
Due to the different fibers and actuator geometries adopted in experiments,
some metrics used in publications to characterize the performance of actu-
ators are not always comparable to metrics used in other publications. For
example, the statement that an actuator can lift “x times its own weight”
conveys little useful information because it neglects both actuator length
and stroke. By cutting such an actuator in half, it would weigh half as much,
but still be able to lift the same weight, and thus arbitrarily double the met-
ric. A comparable metric should not change with the length or weight of
actuators. Some common metrics for measuring the performance of actu-
ators are discussed here.

11.5.1 Output strain (ε)
Definition: the change in length upon excitation normalized to the initial
length of the actuator, as expressed in Eq. (11.6)
∆L
ε % () = ×100 % (11.6)
L 1
where ∆L (m) is the change in length upon excitation and L 1 (m) is the
initial length of the actuator.
Within limits, the change in length for coiled yarn actuators can in-
crease with the pre-strain applied to the actuator. This is because under a
low tension there is small or even no space between neighboring turns of
the coil and thus little or no space for the actuator to contract. Pre-straining
increases the gap between neighboring turns of the coil so that the actuator
can contract to a greater extent [22].
For torsional actuators, the output strain upon excitation is an angular
movement, which is usually normalized to the initial length of the actuators
(e.g., degree/mm, radian/mm, or turns/mm), rather than normalized to the
number of initial twist turns in the whole length of the actuator. Obviously,
increasing the degree of twist in the actuator and tension on the yarn can
increase the output torsional strain.