Solid Electroytes for CNT based Actuators

Abstract

Actuators based on carbon nanotubes (CNT) have the potential to generate high forces at very low voltages. The density of the raw material is just 1330 kg/m3, which makes them well applicable for lightweight applications. Moreover, active strains of up to 1% can be achieved - due to the CNTs dimensional changes on charge injection. Therefore the nanotubes have to be arranged and elec-trically wired like electrodes of a capacitor. In previous works the systems response of the Nanotubes comprising a liquid electrolyte was stud-ied in detail. The major challenge is to redo such experiments with solid electrolyte, which is a pre-requisite for structural integration. Most experiments dealing with solid electrolyte based CNT ac-tuators have been performed with bimorph benders, only. Unfortunately, there is no evidence, that resulting displacements which were measured, are caused by a displacement of the nanotubes themselves. There are several other mechanisms that could cause such a structure to bend. Among those are change in mass/volume distribution by mass transport and thermal expansion. In this paper a method is proposed to make sure the expansion is not based on thermal expansion. This is done by analysing the electrical system response. As thermal expansion is dominated by ohmic resistance the CNT based actuators show a strong capacitive behaviour. This behaviour is due to the constitution of the electrochemical double layer around the nanotubes, which causes the tubes to expand. Also a novel test setup is described, which guarantees that the displacement which is measured is not caused by bending of a bimorph but due to expansion of a single layer of nanotubes. This paper also presents experimental results demonstrating both the method of electrical charac-terisation of CNT based actuators based on solid electrolytes and the novel test setup which is used to measure the needed data. The actuators which were characterized are hybrids of CNT and the solid electrolyte Nafion which is supplying the ions needed to constitute the electrochemical double layer. The manufacturing and processing of these actuators is also A general trend of the actuatoric behaviour is shown. Unfortunately, the results are not as clear as those for liquid electrolytes, which is due to the hybrid character of the analysed devices. In the liquid electrolyte based case the CNTs are the only source of stiffness, whereas in the solid electro-lyte case electrodes and electrolyte contribute to the overall stiffness and damping as well. Since the introduction of solid electrolytes is a major stumbling block in the development of such actuators, this work is of particular importance.