Carbon Nanotube Strain Measurements via Tensile Testing

Discovering and characterizing new smart materials is an urgent need to close the performance-gaps of standard active materials. There is still the demand for a material providing high modulus, low density and large strain. Carbon materials catch scientific attention since a while but one sort among these is of special interest, carbon nanotubes (CNTs). Beside excellent material properties another interesting feature was first mentioned 1999 — the active behavior of paper-like mats made of CNTs. The CNT-papers are electrical activated using a double-layer interaction of ions provided by an electrolyte and the charged high surface area of the paper formed by carbon tubes. Until now the mechanism behind the strain generation of CNT-based architectures is unknown. The principle of the mechanism reveals the potential of carbon tubes to be or not to be a resilient smart material in order to use the strong carbon bonds instead of weak van der Waals force as linking between the tubes. This paper presents further investigations about the mechanical composition of CNT-papers as well as vertical aligned CNT-arrays using an actuated tensile test set-up. For better comparison the experiments are conducted in dry, wet and wet/charged conditions. Especially in the case of CNT-arrays it is essential to preload the specimens because their curly CNT-structure superimposes the vertical orientation of the arrays. While the CNT-paper is tested in an one molar sodium chloride solution, the hydrophilic character of CNT-arrays requires an ionic liquid (IL). It is found that the mechanical properties of CNT-papers drop significantly by wetting and further more by charging what indicates an electrostatic dominated effect. In contrast the CNT-arrays show similar results independent of their test-conditions and an active, reversible behavior of tube-elongation by charging. These results indicate strongly a quantum-mechanical effect of the single tubes.