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Carbon nanotube yarn actuators Mirfakhrai, Tissaphern

Abstract

The first demonstration of electromechanical actuation in carbon nanotubes (CNTs), aligned in the form of a twisted yarn, is presented in this thesis. Sheets of CNTs have been known to actuate when charged electrochemically. When an electric potential is applied between a sheet of CNTs and another electrode, both submersed in an electrolyte, the sheet expands. Actuation loads and stresses are low (< 33 MPa); this is partly due to the random orientation of CNTs in those sheets. When actuated under similar conditions, the yarns show significantly higher stress (2.0±0.4 x 10⁸ Pa) with similar strain (0.6 %). However, unlike the sheets, the yarns contract when the electric potential is applied; this is quite unexpected. The mechanism of this contraction is studied and is related to the insertion of ions in the yarn and the structural changes that take place in the yarn due to its helical twisted structure. A model is presented, relating the actuation strain with yarn geometry (diameter, twist rate, internal packing), electrolyte properties such as ion size, and applied potential. The actuation and mechanical properties of the yarn are studied in various electrolytes. Characterization techniques such as cyclic voltammetry and electrochemical impedance spectroscopy are employed to study the charge storage behaviour of the yarn, showing gravimetric capacitances between 10-60 F/g. It is also shown that the yarns can convert mechanical energy into electrical energy, and are thus capable of measuring mechanical forces. Their behaviour as mechanical force sensors is characterized, showing sensitivities about 2.0±0.6 x 10−⁵ V/MPa. The yarns are superior in work density to piezoelectric actuators when normalized by active material. Creep, can still be an issue, while a tensile modulus of about 16±5 GPa and a tensile strength of about 900 MPa mean that creep is much less than other types of artificial muscles at similar loads. Torsional actuation is also shown to take place if one end of the yarn is free to rotate. Thus, the CNT yarn actuators show the highest achieved stress among polymer artificial muscles and are the first to actuate torsionally, opening a wide range possibilities for applications.

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Attribution-NonCommercial-NoDerivatives 4.0 International

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