Nanoporous-Gold-Polypyrrole Hybrid Materials for Millimeter-Sized Free Standing Actuators

This work studies the actuation of hybrid materials made from nanoporous gold, polypyrrole, and aqueous electrolyte. The deposition protocol affords a conformal polypyrrole coating on the entire internal interface of millimeter-sized nanoporous metal specimens made by dealloying. The hybrid material emerges when the remaining pore space is filled with perchloric acid. The metal serves as load-bearing and electronically conductive substrate, the polypyrrole as functional active component for actuation, and the aqueous electrolyte provides a pathway for fast ionic conduction. In this way, an actuator material is obtained that can be manufactured as monolithic bodies that can be subjected to compressive loads. Actuators with dimensions of millimeters in each spatial direction achieve characteristic times for actuation in the order of seconds and their work density is comparable to that of state-of-the-art piezoceramics. As a remarkable finding, the actuation strain scales with the square of the polypyrrole phase fraction. This is surprising in view of the linear scaling predicted by conventional micromechanical models. A micromechanical model that explains this behavior as the result of polypyrrole bridges that link adjacent struts of the metal skeleton is presented.