Electrothermally-driven actuators

Carbon is a distinctive electrode material for actuators, as it is available in a wide variety of forms, ranging from monoliths to powders, fibers, and yams. The diversity in the properties of different carbonaceous materials is also expressed in a variety of actuation mechanisms. This chapter considers two classes of actuators - electrochemically and electrothermally driven actuators - which both make use of carbonaceous materials as active elements. In both of the listed types of actuators, carbon is especially advantageous because of its chemical and... 

Actuators Electrochemical impedance spectroscopy (EIS) Electrochemically driven actuators Voltage v. current control Electromechanical characterization Electrothermally driven actuators Ionic liquids (IL) Pulse-width-modulated (PWM) control signal... 

The selection of an appropriate waveform for driving electrochemically and electrothermally driven actuators is essential to achieve their full actuation capability and to prevent them from degradation. To date, boffi types of actuators under consideration in this chapter lack practice of use in industrial appUcations, even at fire prototype level. Consequently, standard control methods have not yet been established for these types of actuators. The control signals that have been used in the scientific works on the smart electroactive actuators to date are selected primarily with the purpose to facilitate convenient comparison of the newly prepared actuators with the previously available ones. 

The equivalent electrical circuit is the primary concern in the selection of an appropriate waveform to drive any type of actuator. This chapter is to give an overview of the electrical control methods for electrochemically and electrothermally driven actuators, in consideration of their equivalent electrical circuits. 

Joule heating of the host material - possibly carbon nanotube fibers - governs fire actuation response of the electrothermally driven actuators. The electrical equivalent circuit for an electrothermally driven actuator is a resistor, which undergoes linear or rotary motion, when energy is dissipated on it. 

Electric conductivity along the carbon filament in case of the electrothermally driven actuators, and along individual electrodes in case of the electrochemically driven actuators, is an important characteristic that has a large impact on the speed of the actuators, as it is explained in Sect. 2.2. Conductivity of an electrode can be measured using four-point measurement setup. Alternatively, voltage drop along an eleetrode during actuation can be used to estimate its conductivity (Kruusamae etal. 2012). 

Polymer composite actuators with a bilayer structure of different coefficients of thermal expansion (CTE) can generate bending displacement. When one side of the film is introduced with conductive fillers, the actuator can be electrothermally driven. PDMS/CNT composite was reported to be driven by the thermal expansion of the PDMS with CNTs as... 

The twisting nanotube yam actuators based on first principle mentioned above enable fully dry torsional actuation as the main driving mechanisms are based on electrothermal and/or photothermal effects. These actuators do not require electrolyte, counter electrode, or extra package as it is needed for electrochemically driven actuators (Chun et al. 2014). Indeed, the electroactivity of nanotube yams were firstly described in setup where bundled fibers were immersed in electrolyte the overall capability of twisting of actuators was demonstrated later without ion... 

Fig. 10 Electrothermal tensile actuation for two-end-tethered, homochiral, wax-filled coiled CNT yams, (a) Tensile actuation strain versus time after 1,400,000 reversible cycles for an 11.5 (rai diameter, coiled Fermat yam having -25,000 tums/m twist when driven by a 18.3 V/cm, 20 Hz symmetric square-wave voltage while lifting a load that provided a 14.3 MPa stress, (b) Tensile actuation for the yam of (a) with 109 MPa pUed tensile stress when driven at 3 % duty cycle by 15 ms, 32 V/cm square-wave voltage pulses having a period of 500 ms. (c) Tensile strain versus time for a 150 pm diameter, diial-Aichimedean yam with 3,990 tums/m of inserted twist per preciusor sheet stack length, when supporting a 5.5 MPa tensile stress and driven by a 15 V/cm square wave having 50 ms pulse duration and 2.5 s period (From Lima et aL (2012). Reprinted with permission from AAAS)...

Mechanical properties of the actuator materials can be eharacterized without any electrical signal applied. The typical stress range for eleetrothermally and eleetro-chemically driven actuators is different. The electrothermally driven aetuators are preferably operated in the stress range that approaches to the ultimate tensile strength of the actuating fibers, which makes their load-bearing capacity as an essential... 

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