Actuation-mechanisms of CNT-bucky papers and CNT-arrays

Abstract

In the fields of smart materials there is still a demand for a material featuring high modulus, low density and large strain. Carbon materials catch enormous scientific attention not only since carbon fibers were used for highperformance composite structures. But more and more the scientific attention moves from the macroscale to the nanoscale. This paper focuses with a adaptive point of view on one of the carbon allotropes: carbon nanotubes (CNTs). Beside excellent electromechanical properties another interesting feature was first mentioned 1999 -the active behavior of paper-like mats (bucky-papers) made of CNTs. CNT-papers are electrically activated using a double-layer interaction of ions provided by an electrolyte and the charged, high specific surface area of the paper formed by carbon nanotubes. Until now the detailed mechanism behind the strain/force generation of CNT-based architectures is unknown. A clarification of this principle reveals the potential of carbon tubes to be or not to be a resilient smart material in order to use their strong covalent carbon bonds instead of weak van der Waals force as tube-linking. This paper presents further investigations about the composition of CNT-papers and their performance in contrast to vertical aligned CNT-arrays using an actuated tensile test set-up. For better comparison the experiments of both specimen-types are carried out in dry, wet and wet/charged conditions. Especially in the case of CNT-arrays it is essential to preload the specimens because the curly CNT-structure superimposes the vertical orientation. While the CNT-paper is tested in an aqueous solution of one molar sodium chloride, the hydrophobic character of CNT-arrays requires an ionic liquid (IL) as electrolyte. It is found that the mechanical properties of CNT-papers drop significantly by wetting and can be controlled by charging what indicates an electrostatic dominated effect. In contrast the CNT-arrays show identical results regardless of the test conditions and furthermore an active, reversible behavior of tube elongation by charging. These results indicate strongly a quantum mechanical effect as reason of a CNT-array actuation.