Carbon nanotube actuators based on solid electrolytes

Carbon nanotubes (CNT) show interesting properties with respect to applications in actuator technologies. Mats made of randomly oriented CNTs (so-called buckypapers), when in contact with an electrolyte, require relatively low applied potentials (few volts) for exhibiting dimensional changes. This actuation capability combined with the mechanical properties of an individual tube and the high electrical conductivity makes CNTs an attractive material. This direct conversion of electrical energy into mechanical energy through a material response is crucial actuator based applications like robotics, microscopic pumps, optical displays, optical fiber switches, or medical prosthetic devices. In order to achieve reliable CNT actuator devices, the two main goals of the thesis are the following: The first goal is the characterization of macroscopic carbon nanotube structures. Material properties such as Young’s modulus, tensile strength, electrical conductivity, electrical capacitance, specific surface area and morphology are investigated and put in relation to the length (in-plane) actuation performance in a liquid aqueous electrolyte (1M NaCl). A dynamic mechanical analyzer is adapted for actuation strain measurements on the samples under various tensile prestress levels. The Young’s modulus of the CNT structure was found to have a significant impact on actuation performance. The second goal is the replacement of the liquid electrolyte by a solid one in order to develop a self-supported and non-volatile CNT actuator device. For this purpose, two gel electrolytes, based on ionic liquid (IL) and a polymer matrix are investigated. The gels differ in the nature of the polymers: The cross-linked HEMA is a thermoset requiring no solvent for processing of the gel electrolyte, whereas the linear PVdF-HFP is a thermoplastic and processed from solution. For both gel electrolytes, film processing for obtaining manageable electrolyte membranes is presented. Membranes are characterized using Differential Scanning Calometry (DSC), Thermal Gravimetric Analysis (TGA), cyclic voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS). The manufacturing process of in-plane actuator devices is presented and their electromechanical actuation ability characterized. The actuation strain measurements under various tensile prestress levels have shown that the obtained maximum strains are comparable for both electrolyte types, aqueous liquid and gelled ionic liquid, respectively. Finally, two other gel electrolyte actuator designs are presented, namely a stacked (out-of-plane) and a membrane actuator (in-plane). Only actuators based on the in-plane actuation could be made to perform reproducibly whereas the stacked out-of-plane actuators showed erratic behavior due to undesired bending.