COMPOSITION- AND ION-DEPENDING ACTUATION MECHANISM OF CARBON NANOTUBE ARCHITECTURES

Abstract

Future adaptive applications such as morphing surfaces require materials featuring low density, high mechanical stiffness, high active strain but low energy consumption. These properties are well combined by carbon nanotubes (cnts)-based actuators which are typically made of paper-like mats. Two carbon nanotube-papers are arranged towards to each other within an electrolyte. The charging of these papers results in an electrical field which forms a double-layer of the electrolyte ions at the surface of the electrodes. During charging a deflection of the papers can be detected. Although various cnt-architecures have been extensively analysed there is still no generally accepted theory for the actuation mechanism. This paper focuses on investigations of the actuation mechanisms of cnt-papers in comparison to cnt-arrays. The cntpapers represent architectures of randomly oriented and entangled cnts. The sample geometry is formed by micro-sized cnts. In contrast cnt-arrays are based on multi-walled cnts whose length is as long as the sample length. Both sample types are tested within an actuated tensile test set-up under different conditions. The cnt-papers are tested in water-based electrolytes while the hydrophobic cnt-arrays need to be tested in ionic liquids. Furthermore different ions are used to investigate the influence of the ion-radius on the properties. Regarding the cnt-papers, these tests are completed by in-plane actuation tests measuring the free strain. During the actuated tensile tests three parameters were indentified to influence the mechanical performance of cnt-papers decisively: the humidity of the conditions under which the papers are tested, a preloading of the sample and the electrical charging. All these influencing factors point at the weak inter-tube linking which seem to influence not only the actuation but also the mechanical properties of cnt-papers. Finding a general trend of influencing conditions on the mechanical behaviour of cnt-arrays turns out to be dfficult because of varying results. The mounting of the samples seems to be an important influence. However, the quadratic voltage-strain correlation suggests an electrostatic effect maybe on a quantum-chemical level of the carbon structure.