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TABLE 8.4 Comparative Properties of EAPs, Shape Memory Alloy, and Piezoceramic Actuators
Characteristic Property EAP Shape Memory Alloy Piezoelectric Ceramics
Achievable strain more than 10% up to 8% up to 0.3%
Young’s modulus (GPa) 0.114 (wet) 75 89
Tensile strength (MPa) 34 (wet) 850 76
Response time msec–min sec–min µsec–sec
3
Mass density (g/cm ) 2.0 6.5 7.5
Actuation voltage 1–10 V N/A 50–1000 V

FIGURE 8.9 Polymer metal composite actuator.

FIGURE 8.10 Two-step Pt plating process.

of an ion-exchange membrane covered with a conductive layer as illustrated in Fig. 8.9(a). Upon appli-
cation of a potential difference at points A and B the composite bends towards the anodic side as shown
in Fig. 8.9(b). Among the numerous ion-exchange polymers, perfluorinated sulfonic acid (Nafion Du
Pont, USA ) and perfluorinated carboxylic acid (Flemion, Asahi, Japan) are the most commonly used in
actuator applications. The chemical formula of a unit chain of Nafion is
( [ CF 2 –CF 2 ) –CF–CF 2 –] m
n
(8.51)
− +
O–CF–CF 2 –O–CF 2 –SO 3 M
+ + + +
where M is the counterion (H , Na , Li , …). The ionic clusters are attached to side chains, which
according to transmission electron microscopy (TEM) studies, segregate in hydrophilic nano-clusters
with diameters ranging from 10 to 50 Å [Xue 1989]. In 1982, Gierke proposed a structural model [Gireke
1982] according to which, the clusters are interconnected via narrow channels. The size and distribution
of these channels determine the transport properties of the membrane and thus the mechanical response.
Metal-polymer composites can be produced by vapor or electrochemical deposition of metal over the
surface of the membrane. The electrochemical platinization method [Fedkiw 1992], used by the author,
is based on the ion-exchange properties of the Nafion. The method consists of two steps: step one—ion
2+
2+
+
exchange of the protons H with metal cations (e.g., Pt ); step two—chemical reduction of the Pt ions
in the membrane to metallic Pt using NaBH 4 solution. These steps are outlined in Fig. 8.10 and an SEM
microphotograph of the resulting composite is shown in Fig. 8.11. The electrode surfaces are approximately
0.8 µm thick Pt deposits. Repeating the above steps several times results in dendritic growth of the
electrodes into the polymer matrix [Oguro 1999] and has been shown to improve the actuation efficiency.

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