Fast Carbon Nanotube Charging and Actuation

Abstract

A number of new and established technologies employ electrical stimuli to induce mechanical deformation in materials. Carbon nanotube (CNT) actuators have recently attracted attention due to their low operation voltage (∼ 1 V), high operating load (26 MPa to date), and high tensile strengths (1.8 GPa, and possibly reaching 37 GPa in individual tubes). However, CNT actuators tend to exhibit a full response on a timescale of seconds rather than the milliseconds typical of electromagnetic actuators, piezoceramics, and muscle. A key objective of this work is to push the limits of CNT actuation rates. A second objective is to observe charging rates. CNT papers have been shown to exhibit specific capacitances on the order of 10 F kg and higher. If they can also be shown to charge and discharge rapidly, they will demonstrate particular promise for use as high-power energystorage devices. Previous work presented the use of resistance compensation to increase the rate of actuation in CNT devices. The rate of actuation was found to be limited to 3 % s. At high acceleration rates the inertia of the apparatus appeared to be the limiting rate. In the present work, the rate of change in force is measured while CNT films are held at a fixed length (isometric conditions). The advantage of measuring force changes under isometric conditions is that there are no moving parts, and hence inertia does not limit the peak rates that can be observed. CNTs are formed into porous sheets or fibers which are immersed in electrolyte to form electrodes in an electrochemical cell. The application of potential causes the formation of an electrical double layer at the nanotube/electrolyte interface in which the electronic charge stored in the carbon atoms is balanced by the ionic charge in the electrolyte. The accumulated nanotube charge and the resulting change in band structure is believed to cause a change in the C–C bond length during electron injection due to a number of factors, including Coulomb repulsion. Assuming that these mechanisms are correct, it is the rate at which charge can be injected that determines the strain rate. The key to increasing rate is to increase the current injected per unit volume. Achieving rapid charging rates is a challenge in fibers and sheets composed of CNTs because of the enormous capacitance of CNT electrodes, which is a consequence of the enormous internal surface area. This capacitance is more than six orders-of-magnitude higher than is observed in polyester capacitors, and is similar to that of commercial superand ultracapacitors. Internal resistances and ion-transport rates prevent rapid charging and discharging, limiting the effectiveness of CNTs both as actuators and supercapacitors. Work by Niu and colleagues has shown that it is possible to obtain fast response times by using a combination of relatively thin porous CNT electrodes and high-conductivity electrolytes. Charge and discharge times of 7 ms and specific powers of 8 kW kg were reported. In this paper, it is shown that fast response times can be obtained both for actuation and charging of CNT papers. In order to increase the rates of charging and actuation in thin, porous CNT films, millisecond-long voltage pulses of up to ± 35 V in amplitude are applied. Potentials are much higher than the ∼ 1 V used in charging of CNT paper and fiber actuators and supercapacitors, where such large potential magnitudes would typically lead to parasitic reactions and degradation. However, if the solution resistance and any contact and lead resistance are significantly greater than the CNT paper impedance, most of the voltage drop will initially be across these rather than the double layer within the nanotube films. The use of short pulses ensures that no overcharging of the nanotube double layer will occur, allowing rapid charging without unwanted reactions. The maximum allowable pulse duration is estimated by multiplying the desired potential change at the electrode (for example, 1 V) by the elecC O M M U N IC A TI O N S