• Mahrholz, Thorsten
• Geier, Sebastian
• Sinapius, Michael
• Wierach, Peter

Abstract

Successful actuators have to meet a typically mechanical profile combining high flexibility and stiffness for bearing high structural loads. Current smart materials suffer from low or unstable mechanical properties. That is why these actuators are additionally fixed on structures and are used for the deflection process. In passive/uncharged status the actuators represent additional weight. In contrast to the mostly used piezo ceramic actuators (PZT) the new class of carbon nanotube actuators show promising electromechanical properties combined with low weight. Young’s modulus of 640GPa and comparable high active strains of 1% seem to fulfill the requirements of a structural actuator. For characterisation paper-like architectures made of CNTs are tested in capacitor mode, two electrodes with an electrically insulating but ionically conductive electrolyte in between. An in-plain strain of the electrodes can be detected. However, the actuation-mechanism is still unknown. Different experiments point out different physical effects, ranging from electrostatic effects to quantum-mechanical effects. Actually it seems that the found results are a matter of the specimen, its composition and the type of experiment. The presented paper focuses on the results found during actuated tensile tests addressing dependencies between specimen composition and possible physical effects. Architectures made of single walled CNTs, also called CNT-papers and multi walled CNT-arrays, which feature single, continuous CNTs in one dimension, are compared. The tensile tests are conducted in dry, wet and wet/actuated condition to determine swelling effects. Furthermore tensile tests were conducted at different actuation potentials to identify an electrostatic effect. Different electrolytes are used too to be able to find a correlation between strain and ion radius. It can be found that the mechanical performance of the CNT paper strongly depends on the conditions. The Young’s modulus is reduced by 40 %. In the case of actuation it is not as significant. However, reproducible results can be found by testing the CNT-paper sample in its elastic regime. As soon as charging begins an irreversible degradation of the stiffness can be detected. This points out a mechanical dependency to the charging maybe an effect of the ion radii. Also CNT-arrays seem to depend on the conditions. In terms of complete wetting CNT-arrays require ionic liquids due to their hydrophobic character while CNT-papers were tested in an aqueous electrolyte. According to the experimental conditions, the sample composition and the found electromechanical results a quantum-mechanical effect might probably be the reason for the array actuation.

Topics

• impedance spectroscopy
• Carbon
• experiment
• nanotube
• ceramic