ALIGNED CARBON NANOTUBE/NAFION NANOCOMPOSITE IONIC ELECTROACTIVE - PDF document

18 TH INTERNATIONAL CONFERENCE ON COMPOSITE MATERIALS ALIGNED CARBON NANOTUBE/NAFION NANOCOMPOSITE IONIC ELECTROACTIVE POLYMER ACTUATORS Yang Liu 1,* , Sheng Liu 1 , Roberto Guzman de Villoria 3 , Hülya Cebeci 3 , Jun-Hong Lin 2 , Brian L. Wardle 3 and Q. M. Zhang 1,2 1 Department of Electrical Engineering, 2 Department of Materials Science and Engineering The Pennsylvania State University, University Park, PA 16802 U.S.A. 3 Department of Aeronautics and Astronautics, Massachusetts Institute of Technology 77 Massachusetts Avenue, Cambridge, MA 02139 U.S.A. * Corresponding author (yul165@psu.edu) Keywords : carbon nanotubes, actuators, electroactive polymers enhances the actuation strain (>8% strain under 4 1 Introduction volts). The data here show the promise of optimizing Ionic electroactive polymers ( i -EAPs) are the electrode morphology in IPCNCs via the ultra- attractive because relatively large electromechanical high volume fraction VA-CNTs for ionic polymer actuations can be generated under low voltage (~ a actuators to achieve high performance. The low few volts) [1-6]. Hence, they can be directly operation voltage, high strain level, and fast integrated with microelectronic controlling circuits, actuation speed make the IPCNCs with ultra-high which have operation voltage of several volts, to volume fraction VA-CNTs suitable for applications perform complex actuation functions and low such as artificial muscles, robots, micro- applied voltage also makes them safe to use. These electromechanical devices, and even PEM fuel cells i -EAP actuators hold promise for applications [7]. A transmission line model will be shown to aid including artificial muscles, robots, micro- in understanding how the ions interact within the electromechanical systems (MEMS) and nano- actuator based on observed resistive and capacitive electromechanical systems (NEMS), and energy behavior in the experimental impedance spectra. harvesting. A critical issue in applying the i -EAPs for these applications is how to significantly 2 Experimental improve the electromechanical performance, 2.1 VA-CNTs fabrication including the actuation speed, actuation strain level and efficiency. VA-CNTs were grown using a modified chemical vapor deposition (CVD) method on silicon Recent advances in fabricating controlled- substrates using an Fe-on-alumina catalyst system. morphology aligned carbon nanotube (VA-CNTs) The resulting aligned CNTs have been characterized with ultra-high volume fraction create unique previously for alignment, CNT diameter, opportunities for markedly improving the distribution, and spacing. By varying the inter-tube electromechanical performance of ionic polymer distance via mechanical densification, variable conductor network composite actuators (IPCNCs). densities can be obtained [8]. The experimental results show that the continuous paths through inter-VA-CNT channels for fast ion 2.2 Composite Fabrication transport and low electrical conduction resistance In the fabrication process of the VA-CNT due to the continuous CNTs in the composite forest/Nafion composites, the alcohol solvent in a electrodes of the IPCNC lead to fast actuation speed commercial Nafion dispersion purchased from Ion- (>10% strain/second).[7] A design challenge in Power was replaced by dimethylformamide (DMF). developing advanced actuator materials is how to The DMF/Nafion solution is infiltrated into CNT suppress or eliminate unwanted strains generated arrays under vacuum for several hours to remove under electric stimulation, which reduce the any trapped air. After removing the solvent, the actuation efficiency and may also lower the composite is annealed at 130 °C under vacuum for actuation strains. The experimental results 1h to increase crystallinity of the Nafion. The demonstrate that the VA-CNTs create non-isotropic fabricated VA-CNTs/Nafion nanocomposites were elastic modulus in the composite electrodes which embedded in an epoxy and then sectioned using a suppresses the unwanted strain and markedly

finesse microtome with VA-CNTs perpendicular to while enhancing the strain in the perpendicular the cutting direction. Excess epoxy at the edges was direction, both characteristics highly desirable for removed by manually trimming the edges with a the actuators developed here. razor blade. The ionic actuators developed with CNT/Nafion composites exhibit an actuation strain 2.3 Actuator Fabrication of more than 8% under 4 volts with fast actuation The VA-CNT/Nafion CNC layers were speed τ =0.82s, as shown in Fig. 2. (a) and (b). The bonded to the Nafion film by an ultrathin layer of strain level and actuation speed of the ionic polymer Nafion dispersion (< 0.1 μ m), which was deposited actuators developed here are much improved on the neat Nafion film surfaces by ultrasonic compared with that of the bimorph actuators with spraying. CNC layers were laminated on the neat RuO 2 /Nafion CNC electrodes, which have been Nafion film surfaces and the CNC/Nafion/CNC investigated extensively and have shown the highest actuator stack was then clamped by two Kapton strain response (3% strain) among the IPCNC films under pressure. The stacks were dried and then actuators studied earlier. [9, 10] annealed at 130 °C to further improve the bonding. To understand the interaction between ions 40wt% of 1-ethyl-3-methylimidazolium and electrodes in VA-CNTs/Nafion based bending trifluoromethanesulfonate (EMI-Tf) is soaked into actuators, a transmission line model is employed to the actuator. 50 nm thick gold electrodes are bonded analyze the impedance of the actuators. To illustrate on composite surfaces to increase surface the physical meaning of each component and conductivity. The fabricated actuator is shown in Fig. minimize the fitting parameters, the complicated de 2. (a) and (b). Levie transmission line model [11] in Fig. 4(a) for composites is simplified into a capacitors and 3 Results and Discussion resistors network in Fig. 4(b). Since this device has For the bending actuators investigated here, two phases (electrical conductor phase and ionic the strains generated in the two electrodes have the conductor phase), with resistance within each phase same magnitude with opposite sign. As a result, the and capacitances between the two phases. C 1 is the anisotropic strain generation property of the VA- interface resistance between the ionic liquids in CNT/Nafion composite could not be easily CNC layer and the gold external electrodes; C 2 measured. Hence, this unique property is represents the capacitance between ionic liquids in demonstrated by directly measuring the strains CNC layer and the electrical conductor (CNTs) in generated along and perpendicular to the CNT the CNC layer; C 3 and C 4 are the interface alignment direction (i.e., the thickness direction or z- capacitance between the electrical conductor (CNTs) direction) of VA-CNTs/Nafion composites from the and the ionic liquids in the ionomer layer (Nafion). absorption of Imidazolium ionic liquids (ILs). For For the conduction of ionic liquids, three resistances the comparison, pure Nafion films were also represent, respectively, the ionic resistance in the fabricated under the same condition and the strains composite (R 1 ), on the interface (R 2 ) and in the bulk due to the absorption of ILs were also characterized Nafion middle layer (R). The resistance of the highly along and perpendicular to the thickness direction. conductive CNTs is neglected. The data and fitting Imidazolium based IL, EMI-Tf, was chosen for the are shown in Fig. 4(c) and the fitting parameters are study in this paper. summarized as follows for the device studied: After soaking with IL, the CNCs filled with C 1 =40µF, C 2 =80µF, C 3 =16µF, C 4 =3µF, R 1 =3k Ω , VA-CNTs exhibit a very different anisotropic R 2 =500 Ω and R=617 Ω . It can be seen that the model deformation from the Nafion films. The pure Nafion fits the experimental data quite well. films, upon absorption of EMI-Tf, exhibit a large 4 Conclusions thickness strain and a much smaller lateral strain. In contrast, the VA-CNTs/Nafion nanocomposite films In conclusion, we show that the ionic polymer exhibit much smaller thickness strain, while the nanocomposite actuators with VA-CNTs/Nafion lateral strain is enhanced. These results demonstrate electrodes of controlled morphology exhibit three that the high volume fraction VA-CNTs reduce the distinct advantages compared to the traditional ionic strain in the composites along its alignment direction polymer actuators with the CNCs with randomly