Actuation mechanisms of carbon nanotube-based architectures

摘要

State of the art smart materials such as piezo ceramics or electroactive polymers cannot feature both, mechanicaludstiffness and high active strain. Moreover, properties like low density, high mechanical stiffness and high strainudat the same time driven by low energy play an increasingly important role for their future application. Carbonudnanotubes (CNT), show this behavior. Their active behavior was observed 1999 the first time using paper-likeudmats made of CNT. Therefore the CNT-papers are electrical charged within an electrolyte thus forming a doublelayer.udThe measured deflection of CNT material is based on the interaction between the charged high surface areaudformed by carbon nanotubes and ions provided by the electrolyte. Although CNT-papers have been extensivelyudanalyzed as well at the macro-scale as nano-scale there is still no generally accepted theory for the actuationudmechanism. This paper focuses on investigations of the actuation mechanisms of CNT-papers in comparison toudvertically aligned CNT-arrays. One reason of divergent results found in literature might be attributed to differentudtypes of CNT samples. While CNT-papers represent architectures of short CNTs which need to bridge each otherudto form the dimensions of the sample, the continuous CNTs of the array feature a length of almost 3 mm, alongudwhich the experiments are carried out. Both sample types are tested within an actuated tensile test set-upudunder different conditions. While the CNT-papers are tested in water-based electrolytes with comparably smalludredox-windows the hydrophobic CNT-arrays are tested in ionic liquids with comparatively larger redox-ranges.udFurthermore an in-situ micro tensile test within an SEM is carried out to prove the optimized orientation of theudMWCNTs as result of external load. It was found that the performance of CNT-papers strongly depends onudthe test conditions. However, the CNT-arrays are almost unaffected by the conditions showing active responseudat negative and positive voltages. A micro alignment as result of tensile stress can be proven. A comparison ofudboth results point out that the actuation mechanism strongly depends on the weakest bonds of the architectures: Van-der-Waals-bonds vs. covalent C-bonds