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The effect of gold electrode thicknesses on electromechanical performance of Nafion-based Ionic Polymer Metal Composite actuators
Composites Part B 165 (2019) 747–753
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Composites Part B journal homepage: www.elsevier.com/locate/compositesb
The effect of gold electrode thicknesses on electromechanical performance of Nafion-based Ionic Polymer Metal Composite actuators
T
Ozgun Cem Yılmaza, Ibrahim Senb, Baris Oguz Gursesc, Okan Ozdemird, Levent Cetine, Mehmet Sarıkanatc,∗, Yoldas Sekif, Kutlay Severg, Emine Altinkayah Graduate School of Natural and Applied Sciences, Department of Mechatronics Engineering, Dokuz Eylül University, İzmir, Turkey Department of Biocomposites, İzmir Katip Çelebi University Graduate School of Natural and Applied Sciences, İzmir, Turkey c Department of Mechanical Engineering, Ege University, İzmir, Turkey d Department of Mechanical Engineering, Dokuz Eylul University, İzmir, Turkey e Department of Mechatronics Engineering, İzmir Katip Çelebi University, İzmir, Turkey f Department of Chemistry, Dokuz Eylul University, İzmir, Turkey g Department of Mechanical Engineering, İzmir Katip Çelebi University, İzmir, Turkey h Department of Bioengineering, Çanakkale Onsekiz Mart University, İzmir, Turkey a
b
A R T I C LE I N FO
A B S T R A C T
Keywords: Ionic polymer metal composite Actuator Electroactive properties Mechanical properties
The effect of gold electrode thickness (10, 27, 45, 67 and 80 nm) on the electromechanical performance of Nafion-based Ionic Polymer Metal Composite (IPMC) actuators was investigated in this study. The mechanical, morphological, electrical properties and electroactive behaviors of IPMC under direct current voltage (DC) and alternating voltage (AC) were examined. The tip displacement and maximum blocking force of actuators under various electrical stimulations were measured. In order to define transient response characteristics and quasisteady state value of the actuators, DC excitations of 1, 3, 5, 7 and 9 V were used. Besides, to define bandwidth of the actuator samples, square wave excitation with magnitudes of 3, 5, and 7 V and frequencies of 0.1, 0.25, 0.5, 1, and 2 Hz were applied to actuator samples. The actuator having a gold electrode thickness of 45 nm produced maximum tip displacement among all actuators for all excitation DC voltages. In the square wave experiments, the higher cutoff frequencies were observed for the actuators with 27 and 45 nm electrode thicknesses. The blocking force of IPMC increased with increasing gold electrode thickness from 10 nm to 45 nm and decreased with increasing gold electrode thickness from 45 nm to 80 nm.
1. Introduction Actuation and sensing technology of electroactive polymers have advanced in the past decade because of the unique properties of polymers: large deformation in response to a low applied voltage, soft actuation, high power to weight ratios, ease of processing, high sensitivity to stimulus, low power consumption and built-in sensing capabilities [1–4]. Ionic polymer-metal composite (IPMC) shows remarkable flexural response to an applied electric potential (< 5 V) and produces voltage in the order of millivolts when deformed mechanically [5–7]. IPMC consists of an ionic polymer membrane sandwiched by two conductive electrodes [8]. The electrode serves as a medium to maintain the moisture content in the polymer, while the moisture level in the IPMC profits as a medium for ion migration [9]. The soft and compliant nature of the IPMC actuators make them good candidates for use in ∗
bionics engineering applications, such as biomimetic robots, artificial muscles, dynamic sensors and soft actuators [3,10–21]. IPMC actuators have also potential applications in medical, mechanical, electric, and aerospace engineering, space-effective manipulators [22–24], and energy harvesters [17,25]. IPMC can produce a wide range of bending motion when an electric field is implemented between two electrodes [26]. However, IPMC generates a relatively low blocking force, which limits its further application [27]. Therefore, there are increasing demands to improve the electromechanical performance of IPMCs affected by many parameters such as counter ions of polymer, surfaceelectrode resistance, the geometrical dimension of IPMC, the thickness of the membrane, water content in the membrane, and the surface area of the membrane [28–35]. The effect of membrane thickness at various thicknesses 200, 400, 600, and 800 μm on the tip force measurement of IPMCs was studied [36]. It was obtained that the shorter and thicker IPMCs led to higher actuation force but lower actuation displacement
Corresponding author. E-mail addresses: [email protected], [email protected] (M. Sarıkanat).
https://doi.org/10.1016/j.compositesb.2019.02.050 Received 18 November 2018; Received in revised form 11 February 2019; Accepted 13 February 2019 Available online 15 February 2019 1359-8368/ © 2019 Published by Elsevier Ltd.
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[36]. The existence of electrode causes anisotropic swelling in IPMC and the strain state was significantly affected by the electrode volume proportion and the elasticity ratio of the electrode and polymer [37]. However, to our knowledge, the effect of gold electrode thickness, which is also an important parameter for electromechanical performance of IPMC actuator, on the tip displacement and maximum blocking force of IPMC actuators has not been studied in details up to now. Therefore, in this paper, we focused on the effects of the electrode thickness on the electromechanical performance of IPMC actuators which were coated by different thicknesses of gold electrode from 10 nm to 80 nm. As it is known that among the noble metals, gold exhibits an excellent electrical conductivity, which is about 4 times greater than platinum. Moreover gold is more flexible and resistant to cracks related to repetitive loading and mechanical stress [38]. The mechanical, morphological, electrical properties and electroactive behavior of IPMC under DC and AC voltages were investigated.
Fig. 1. The experimental setup for measuring actuation of IPMC actuator.
2. Materials and method
displacement of actuators under various electrical stimulations. For this purpose, a computer based experimental setup was established as shown in Fig. 1. The experimental setup consists of two subsystems: an electrical actuation system and a vision system. Both of the systems work on standalone NIPXIe-9133pc. The electrical actuation system consists of a data acquisition card (NI-PXI 7854R) to generate user defined signal form. The output of the data acquisition card is applied to actuator through a buffer (TDA2040) Op amp circuit. The vision system is a single camera (Basler acA2040180 km camera and Computer M0814-MP – 8 mm – F1.4 lens) connected to a frame grabber (NI PXIe-1435). The user interface for control of the experimental setup is developed in LabView. The two subsystems are synchronized to enable accurate data collection in time scale. The reason to use a vision setup for data collection is to observe exact tip position of the sample because the IPMC actuators change their curvature when excited and as a result of this, tip displacement occurs on a curve on the plane. In this case, using a laser displacement measurement system causes errors because of the fact that it can only measure linear tip displacements. The vision system calibrated using a calibration sample with known geometry placed in the measurement plane as depicted in Fig. 2. A scale for image plane to real world coordinates is defined using the calibration results.
2.1. Materials Nafion 117 (274674-1 EA-thickness: 0.007 in) was supplied from Sigma-Aldrich Corp. Hydrogen Peroxide (purity of 35%) and hydrochloric acid (purity of 37%) were purchased from Sigma-Aldrich Corp. Gold in a purity of 99.5% was used as-received. 2.2. Nafion surface treatment Nafion membranes of 60 × 60 mm, as a base material, were used in this study. First step, Nafion membranes were dipped into 3% H2O2 solution for 1 h at room temperature and then washed with distilled water. After immersion of the membranes into 2 M HCl solution for 1 h at room temperature, the membranes were washed with distilled water for several times. Finally, the membranes were dried at 60 °C for 12 h. 2.3. Fabrication of actuator IPMC actuators were composed of Nafion membrane whose planar surfaces were coated with gold in nanoscale. Gold was deposited on the surfaces of Nafion membrane via physical vapor deposition method using direct thermal evaporation. After Gold was placed in vacuum chamber of NANOVAK Thermal Evaporator, the coating was carried out at a current of 55 A under the base pressure of 6–8x10−6 torr. The thicknesses of the coatings were set to 10, 27, 45, 67 and 80 nm by the help of quartz crystal microbalance (QCM) sensor.
2.5.2. Experimental method Two set of experiments were designed to determine the effect of the electrode thickness on actuator dynamics: DC and square wave (AC) input experiments. In the experiments, it is assumed that actuator shape change is planar and the straightening-back phenomenon does not occur. These assumptions were held for strip-shaped samples and relatively short experiment duration. Nafion membranes used in this study were 50 × 5 mm and the electrode thicknesses were 10, 27, 45, 67, and 80 nm. Although it is known that 1.2 V is critical in terms of life expectancy of the actuators, there are plenty of researches and applications that use a larger voltage range to test their actuators because of the increase in power generation capabilities of the actuators. In literature, it is frequently mentioned that typical operation voltage range for IPMC actuators are 0–5 V [39–43]. Therefore we carried out our experiments in a larger voltage range (1V-9V). The first set of examples uses DC excitation values of 1, 3, 5, 7 and 9 V. Using this set of experiments; it is aimed to define transient response characteristics and quasi-steady state value of the actuators. In the second set of examples square wave excitation with magnitudes of 3, 5, and 7 V and frequencies of 0.1, 0.25, 0.5, 1, and 2 Hz were applied to actuator samples. Using this set of experiment, it is aimed to define bandwidth of the actuator samples.
2.4. Characterization of IPMC actuators Roughness analyses of Nafion and gold coated Nafion membranes were conducted through Atomic force microscopy (AFM). AFM analyses of samples were performed via Multimode SPM (AFM/STM) Nanoscope IV from Digital Instrument in a tapping mode with Si cantilever. The analyses were made thrice on different zones of each sample. The tensile strength, Young modulus and elongation at break were determined by a tensile testing machine with a 50 N load cell at a crosshead speed of 0.1 mm/min. The resistance of IPMCs was measured by using two-probe setup. The setup for two-probe resistance measurement was connected to Keithley 2612 System Source Meter. For all measurements, the distance between two probes was carefully maintained to be 2 mm. 2.5. Determination of electroactive properties of actuators 2.5.1. Experimental setup One of the main focuses of the study is to monitor the tip 748
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Fig. 2. The vision system for measuring actuation of IPMC actuator.
3.2. Mechanical properties
2.5.3. Determination of blocking force Besides of conventional actuators, distributed force generation exists in IPMCs. When voltage is applied across the electrodes of IPMC, a bending moment is generated on the lateral surface of sample which creates movement as a curvature. If the tip of sample is fixed on load cell, force is transferred to load cell by a line contact and the middle of sample starts to buckle and rises because there is no constraint to inhibit such a motion on IPMC. Overall, this unwanted motion reduces the force output. Despite the fact that contact type measurement technique is common for force characterization of IPMCs, there is an error in force measurements due to these motions. A new experimental method is considered in this study, in which a surface contact is created between sample and balance as shown in Fig. 3. Length of surface contact is fixed as 30 mm for the sample length of 50 mm. Maximum blocking forces of samples were measured by Precisa 225SM-DR precision balance. Blocking force data were logged on a PC via RS-232 protocol.
Fig. 5 shows tensile testing results, in terms of tensile stress versus strain, for IPMCs with the electrode thicknesses of 10, 27, 45, 67 and 80 nm (relative to Nafion 117). As was also seen in Fig. 5, the tensile strength values for Nafion membranes and IPMCs with the electrode thicknesses of 10, 27, 45, 67 and 80 nm were determined to be about 37.20, 37.32, 37.57, 37.71, 37.82 and 40.25 MPa, respectively. Considering that Nafion is the adopted base material for these IPMCs, this comparison is useful. There is a small increase in mechanical strength of IPMCs with the electrode thicknesses of 10, 27, 45 and 67 nm (both stiffness and the modulus of elasticity), but it still follows the intrinsic nature of Nafion itself. This means that, in tensile strength, the stress/ strain behavior is predominated by the Nafion rather than electrode materials. Tensile strength value of IPMC with the electrode thickness of 80 nm increased by about 7%, when compared to that of Nafion since electrode materials become predominated. 3.3. Electrical properties
3. Results and discussion Resistance of IPMCs was measured to investigate the effect of the electrode thickness on electrical properties. The resistance values of IPMCs with the electrode thicknesses of 10, 27, 45, 67 and 80 nm are shown in Fig. 6. The resistance values of IPMCs with the electrode thicknesses of 10, 27, 45, 67 and 80 nm were obtained to be about 40, 30, 24, 19 and 10 Ω/cm, respectively. The IPMC samples exhibited a decrease in the resistance with increasing electrode thickness.
3.1. AFM analysis Root-mean- square (RMS) and arithmetic mean value (Ra) were utilized to determine the surface roughness of the samples. These parameters were estimated for Nafion and gold coated Nafion membranes by averaging the results obtained from nine separate areas. AFM images of samples with a scan area of 10 μm × 10 μm were shown in Fig. 4. From analysis of Fig. 4, The RMS and Ra values were obtained. The vertical height scale of all images was set to 300 nm. Untreated Nafion has RMS and Ra roughness values of 2.95 ± 0.36 and 2.13 ± 0.27 nm, respectively. RMS and Ra roughness values increased when the thickness of gold coating increased from 10 nm to 27 nm. When the thickness of gold coating was increased to 45 nm, RMS and Ra values decreased. Afterwards the thickness of gold coating was raised to 67 nm, RMS and Ra values continued to decrease. But with the increase in thickness to 80 nm, RMS and Ra values increased to 9.85 ± 1.14 nm and 7.93 ± 0.90 nm, respectively. The higher RMS and Ra roughness values were obtained as 9.85 ± 1.14 nm and 7.93 ± 0.90 nm for the thickness of 80 nm and 7.65 ± 0.28 and 6.06 ± 0.19 nm for the thickness of 27 nm, respectively.
3.4. Electroactive properties 3.4.1. Actuator response to DC excitations The samples do not make any significant displacement when 1 V is applied and the 9 V causes the actuator sample to shrivel and not to function any further (Fig. 7a). The experiments were carried out by applying 3, 5 and 7 V DC voltages for a time span of 10 s. The time responses of the sample actuators were recorded during experiments. We observed a generic response form for all actuator samples. In order to represent this generic form of the actuator response, the captured images at different instants during experiment were shown in Fig. 7b. Fig. 8 shows the time evolution of the tip displacement (time response of the actuator). As can be seen from Fig. 8, actuator reached the tip displacement of 20 mm within 10 s. When actuator response is analyzed with reference to time scale, it is seen that increment in tip displacement in every proceeding time span becomes smaller. Therefore, the tip displacement vs time curves can be approximated to be in exponential form: t
x dc (t ) = Xmax (1 − e− T )
(1)
where Xmax is the maximum tip displacement and T is time constant [44]. Following this assumption, the time constant and maximum tip displacement values can be defined as characteristic parameters of time response of the actuators. The maximum tip displacement values were found using the experimental data for Nafion actuator samples with the electrode thicknesses of 10, 27, 45, 67 and 80 nm when the input voltage is 3, 5 and 7 V. As can be seen from Fig. 9, the maximum tip displacement
Fig. 3. The experimental setup for blocking force characterization of IPMCs. 749
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Fig. 4. AFM cross-sectional images of Nafion and Gold coated Nafion samples. a) Nafion, b) 10 nm gold coated Nafion c) 27 nm gold coated Nafion, d) 45 nm gold coated Nafion, e) 67 nm gold coated nafion f) 80 nm gold coated Nafion. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 5. Tensile stress-strain curves of Nafion and IPMCs with the electrode thicknesses of 10, 27, 45, 67 and 80 nm.
Fig. 6. Resistance of IPMCs with the electrode thicknesses of 10, 27, 45, 67 and 80 nm.
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Fig. 7. a) The deformed actuator sample after it is excited with DC of 9V, b) Captured images from the 5 VDC excitation experiment of the nafion with electrode thickness of 45 nm.
Fig. 10. Time constant of Nafion with the electrode thickness of 10, 27, 45, 62 and 80 nm when excited with DC of 3, 5 and 7V
Fig. 8. The time evolution of the tip displacement (time response) in the same experiment.
versa. The time constants were calculated using the experimental data for Nafion actuator samples with the electrode thicknesses of 10, 27, 45, 67 and 80 nm when the input voltage is 3, 5 and 7 V. The results are presented in Fig. 10. In Fig. 10, it is seen that there are two types of patterns in time response vs. excitation voltage curves. The first pattern is that the observed motion becomes faster as voltage increases. This pattern occurs in actuator samples with electrode thicknesses of 10 and 27 nm. The second pattern is that the observed motion becomes faster as voltage is increased from 3 V to 5 V and it becomes slower or remains approximately the same as voltage is increased from 5 V to 7 V. This pattern occurs in actuator samples with electrode thicknesses of 45, 67 and 80 nm. Therefore, no clear result can be deduced since there is no a generic pattern that describes actuator time constant change with respect to excitation voltage or electrode thickness. Besides that, it is observed that the fastest time response is observed with Nafion with the electrode thickness of 27 nm under DC excitation of 7 V. When the results are considered together, the actuator with electrode thickness of 45 nm has the maximum tip displacement value and the second fastest response when excited with 5 V. The observations on DC characteristics show that selection of any IPMC actuator with a certain electrode thickness and an excitation voltage requires optimization in terms of prospective application.
Fig. 9. Maximum tip displacement of Nafion with the electrode thickness of 10, 27, 45, 62 and 80 nm when excited with DC of 3, 5 and 7V
increases from 3 V to 5 V, but when voltage rises to 7 V, tip displacement reduces slightly. This is an unexpected result because the more electrical potential energy is provided to actuator and hence the more mechanical work is expected to be done but the results show the contrary situation that tip displacement reduces when a threshold of input voltage value is exceeded (5 V in our experiments). When the values for different electrode thicknesses were compared, two results arose:
3.4.2. Actuator response to square wave excitations In the second set of experiments, the maximum tip displacement was observed with respect to increase in excitation frequencies and voltages. The results were presented as a group of maximum thickness vs. excitation frequency curves for constant excitation voltages of 3, 5 and 7 V, as shown in Fig. 11a–c. In each of the experiments, it is observed that the maximum tip displacement decreased as the excitation frequency is increased. But the decrease in maximum tip displacement per change in frequency is not linear. This observation is compatible with conclusions from DC excitation experiments because the time constant of a time response also defines the frequency range at which actuators can be used effectively.
• The first deduction is that the actuator with electrode thickness of •
45 nm produces maximum tip displacement when all actuators for all excitation voltages are taken into consideration. The other observed result is that as voltage increases, tip displacement does not increase always for a given electrode thickness. There is a nonlinear relationship in quasi quadratic form. Therefore, there is one optimal thickness value suitable for operating voltage or vice
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Fig. 11. The effect of the excitation frequency on maximum tip displacement for excitation voltages of a) 3V, b) 5V, c) 7V
3.4.3. Determination of blocking force In the third group of experiments, the blocking force values of Nafion actuator samples with the electrode thicknesses of 10, 27, 45, 67 and 80 nm were measured when the input voltage was 3, 5 and 7 V. The results are presented in Table 2. It was observed that as the input voltage increases the maximum value of the blocking force also increased for constant electrode thickness. When the electrode thickness is considered, measured results are compatible with displacement characteristics. The blocking force for the actuator samples increased with varying electrode thicknesses from 10 to 45 nm and decreased with varying electrode thicknesses from 45 to 80 nm.
Table 1 Corner frequencies calculated from time response curves. Excitation Voltage in [V]
3 5 7
Electrode Thickness in [nm] 10
27
45
67
80
0.23 0.29 0.50
0.33 0.80 1.33
0.40 0.83 0.50
0.31 0.50 0.56
0.33 0.50 0.33
Table 2 Blocking force values of IPMC sample at 3, 5 and 7 V. Electrode thickness (nm)
Blocking force (gf)
4. Conclusion 10 27 45 67 80
3V
5V
7V
2.21 2.65 3.16 2.32 1.14
2.65 2.95 3.62 2.94 1.51
2.85 3.15 3.82 3.14 1.81
The observations on electromechanical characteristics show that there is a nonlinear relationship between electrode thickness and electroactive characteristics of IPMC actuators: the maximum tip displacement, time constant and the blocking force values. All three properties increased as electrode thickness increases from 10 to 45 nm and decreased as electrode thickness increases from 45 to 80 nm. The bending moment leading to motion of the Nafion-based IPMC takes place due to cation motion which is the result of applied electrical field. The applied electrical field is directly related to electrode thickness as it affects spatial density of electrical flux. On the other hand, the bending motion and bending moment are related to each other via stress-strain interactions which are dominated with elastic modulus when other structural parameters are constant. The experimental results showed that the optimization in terms of excitation voltages and electrode thickness is the result of the trade-off between electrical force acting on cations and stiffness of actuators. The square wave experiments demonstrated the maximum tip displacement decreased as the excitation frequency was increased. Based on the maximum tip displacements, it can be noted that the actuators with electrode thicknesses of 27 and 45 nm exhibited better frequency characteristics due to their wider bandwidths. The blocking force for the actuator samples enhanced with increasing electrode thicknesses from 10 to 45 nm and decreased with increasing electrode thicknesses from 45 to 80 nm. From electrical conductivity measurements, it was seen that Nafion based IPMC actuator showed a decrease in the resistance with increasing electrode thickness.
The approximated generic time response function (Equation (2)) represents first order dynamics in system theory and the first order systems have low pass filter characteristics. This means that the high frequency signals are attenuated by the strongly decreasing amplitude. The limit frequency, in which this strong attenuation occurs, is defined as bandwidth of the system or the corner frequency (fc). Calculated corner frequencies for IPMC samples are given in Table 1.
fc =
1 T
(2)
The maximum tip displacement vs. excitation frequency curves in Fig. 11 show also the effect of the bandwidth of the actuators on displacement behavior of IPMCs. In the experiment of excitation voltage of 3 V (Fig. 11a), it is seen that only the actuator with electrode thickness of 45 nm moves significantly when excited with 0.25 Hz. This is due to its corner frequency of 0.4 Hz. The maximum tip displacements of all other actuators are very small in magnitude. In 5 and 7 V excitation voltage experiments (Fig. 11b, and c), the similar results were observed. The actuators with electrode thicknesses of 45 and 27 nm showed significant displacement when excited with the frequencies below corner frequency approximately of 1 Hz. The maximum tip displacement of the other actuators reduces very fast when they have corner frequencies of 0.25 and 0.5 Hz. The square wave experiments exhibited the effect of the corner frequency (bandwidth) on displacements of the actuators. The observed results are compatible with DC experiments. The actuators with electrode thicknesses of 27 and 45 nm have better frequency characteristics. The results also showed that selecting an excitation voltage determines the frequency range at which the actuator can be used effectively. Therefore, it should also be considered in optimization procedures.
Acknowledgements Financial support for this study was provided by TUBITAK-The Scientific and Technological Research Council of Turkey, Project Number: 111M643. References [1] Shahinpoor M. Mechanoelectrical phenomena in ionic polymers. Math Mech Solid 2003;8(3):281–8.
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Composites Part B journal homepage: www.elsevier.com/locate/compositesb
The effect of gold electrode thicknesses on electromechanical performance of Nafion-based Ionic Polymer Metal Composite actuators
T
Ozgun Cem Yılmaza, Ibrahim Senb, Baris Oguz Gursesc, Okan Ozdemird, Levent Cetine, Mehmet Sarıkanatc,∗, Yoldas Sekif, Kutlay Severg, Emine Altinkayah Graduate School of Natural and Applied Sciences, Department of Mechatronics Engineering, Dokuz Eylül University, İzmir, Turkey Department of Biocomposites, İzmir Katip Çelebi University Graduate School of Natural and Applied Sciences, İzmir, Turkey c Department of Mechanical Engineering, Ege University, İzmir, Turkey d Department of Mechanical Engineering, Dokuz Eylul University, İzmir, Turkey e Department of Mechatronics Engineering, İzmir Katip Çelebi University, İzmir, Turkey f Department of Chemistry, Dokuz Eylul University, İzmir, Turkey g Department of Mechanical Engineering, İzmir Katip Çelebi University, İzmir, Turkey h Department of Bioengineering, Çanakkale Onsekiz Mart University, İzmir, Turkey a
b
A R T I C LE I N FO
A B S T R A C T
Keywords: Ionic polymer metal composite Actuator Electroactive properties Mechanical properties
The effect of gold electrode thickness (10, 27, 45, 67 and 80 nm) on the electromechanical performance of Nafion-based Ionic Polymer Metal Composite (IPMC) actuators was investigated in this study. The mechanical, morphological, electrical properties and electroactive behaviors of IPMC under direct current voltage (DC) and alternating voltage (AC) were examined. The tip displacement and maximum blocking force of actuators under various electrical stimulations were measured. In order to define transient response characteristics and quasisteady state value of the actuators, DC excitations of 1, 3, 5, 7 and 9 V were used. Besides, to define bandwidth of the actuator samples, square wave excitation with magnitudes of 3, 5, and 7 V and frequencies of 0.1, 0.25, 0.5, 1, and 2 Hz were applied to actuator samples. The actuator having a gold electrode thickness of 45 nm produced maximum tip displacement among all actuators for all excitation DC voltages. In the square wave experiments, the higher cutoff frequencies were observed for the actuators with 27 and 45 nm electrode thicknesses. The blocking force of IPMC increased with increasing gold electrode thickness from 10 nm to 45 nm and decreased with increasing gold electrode thickness from 45 nm to 80 nm.
1. Introduction Actuation and sensing technology of electroactive polymers have advanced in the past decade because of the unique properties of polymers: large deformation in response to a low applied voltage, soft actuation, high power to weight ratios, ease of processing, high sensitivity to stimulus, low power consumption and built-in sensing capabilities [1–4]. Ionic polymer-metal composite (IPMC) shows remarkable flexural response to an applied electric potential (< 5 V) and produces voltage in the order of millivolts when deformed mechanically [5–7]. IPMC consists of an ionic polymer membrane sandwiched by two conductive electrodes [8]. The electrode serves as a medium to maintain the moisture content in the polymer, while the moisture level in the IPMC profits as a medium for ion migration [9]. The soft and compliant nature of the IPMC actuators make them good candidates for use in ∗
bionics engineering applications, such as biomimetic robots, artificial muscles, dynamic sensors and soft actuators [3,10–21]. IPMC actuators have also potential applications in medical, mechanical, electric, and aerospace engineering, space-effective manipulators [22–24], and energy harvesters [17,25]. IPMC can produce a wide range of bending motion when an electric field is implemented between two electrodes [26]. However, IPMC generates a relatively low blocking force, which limits its further application [27]. Therefore, there are increasing demands to improve the electromechanical performance of IPMCs affected by many parameters such as counter ions of polymer, surfaceelectrode resistance, the geometrical dimension of IPMC, the thickness of the membrane, water content in the membrane, and the surface area of the membrane [28–35]. The effect of membrane thickness at various thicknesses 200, 400, 600, and 800 μm on the tip force measurement of IPMCs was studied [36]. It was obtained that the shorter and thicker IPMCs led to higher actuation force but lower actuation displacement
Corresponding author. E-mail addresses: [email protected], [email protected] (M. Sarıkanat).
https://doi.org/10.1016/j.compositesb.2019.02.050 Received 18 November 2018; Received in revised form 11 February 2019; Accepted 13 February 2019 Available online 15 February 2019 1359-8368/ © 2019 Published by Elsevier Ltd.
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[36]. The existence of electrode causes anisotropic swelling in IPMC and the strain state was significantly affected by the electrode volume proportion and the elasticity ratio of the electrode and polymer [37]. However, to our knowledge, the effect of gold electrode thickness, which is also an important parameter for electromechanical performance of IPMC actuator, on the tip displacement and maximum blocking force of IPMC actuators has not been studied in details up to now. Therefore, in this paper, we focused on the effects of the electrode thickness on the electromechanical performance of IPMC actuators which were coated by different thicknesses of gold electrode from 10 nm to 80 nm. As it is known that among the noble metals, gold exhibits an excellent electrical conductivity, which is about 4 times greater than platinum. Moreover gold is more flexible and resistant to cracks related to repetitive loading and mechanical stress [38]. The mechanical, morphological, electrical properties and electroactive behavior of IPMC under DC and AC voltages were investigated.
Fig. 1. The experimental setup for measuring actuation of IPMC actuator.
2. Materials and method
displacement of actuators under various electrical stimulations. For this purpose, a computer based experimental setup was established as shown in Fig. 1. The experimental setup consists of two subsystems: an electrical actuation system and a vision system. Both of the systems work on standalone NIPXIe-9133pc. The electrical actuation system consists of a data acquisition card (NI-PXI 7854R) to generate user defined signal form. The output of the data acquisition card is applied to actuator through a buffer (TDA2040) Op amp circuit. The vision system is a single camera (Basler acA2040180 km camera and Computer M0814-MP – 8 mm – F1.4 lens) connected to a frame grabber (NI PXIe-1435). The user interface for control of the experimental setup is developed in LabView. The two subsystems are synchronized to enable accurate data collection in time scale. The reason to use a vision setup for data collection is to observe exact tip position of the sample because the IPMC actuators change their curvature when excited and as a result of this, tip displacement occurs on a curve on the plane. In this case, using a laser displacement measurement system causes errors because of the fact that it can only measure linear tip displacements. The vision system calibrated using a calibration sample with known geometry placed in the measurement plane as depicted in Fig. 2. A scale for image plane to real world coordinates is defined using the calibration results.
2.1. Materials Nafion 117 (274674-1 EA-thickness: 0.007 in) was supplied from Sigma-Aldrich Corp. Hydrogen Peroxide (purity of 35%) and hydrochloric acid (purity of 37%) were purchased from Sigma-Aldrich Corp. Gold in a purity of 99.5% was used as-received. 2.2. Nafion surface treatment Nafion membranes of 60 × 60 mm, as a base material, were used in this study. First step, Nafion membranes were dipped into 3% H2O2 solution for 1 h at room temperature and then washed with distilled water. After immersion of the membranes into 2 M HCl solution for 1 h at room temperature, the membranes were washed with distilled water for several times. Finally, the membranes were dried at 60 °C for 12 h. 2.3. Fabrication of actuator IPMC actuators were composed of Nafion membrane whose planar surfaces were coated with gold in nanoscale. Gold was deposited on the surfaces of Nafion membrane via physical vapor deposition method using direct thermal evaporation. After Gold was placed in vacuum chamber of NANOVAK Thermal Evaporator, the coating was carried out at a current of 55 A under the base pressure of 6–8x10−6 torr. The thicknesses of the coatings were set to 10, 27, 45, 67 and 80 nm by the help of quartz crystal microbalance (QCM) sensor.
2.5.2. Experimental method Two set of experiments were designed to determine the effect of the electrode thickness on actuator dynamics: DC and square wave (AC) input experiments. In the experiments, it is assumed that actuator shape change is planar and the straightening-back phenomenon does not occur. These assumptions were held for strip-shaped samples and relatively short experiment duration. Nafion membranes used in this study were 50 × 5 mm and the electrode thicknesses were 10, 27, 45, 67, and 80 nm. Although it is known that 1.2 V is critical in terms of life expectancy of the actuators, there are plenty of researches and applications that use a larger voltage range to test their actuators because of the increase in power generation capabilities of the actuators. In literature, it is frequently mentioned that typical operation voltage range for IPMC actuators are 0–5 V [39–43]. Therefore we carried out our experiments in a larger voltage range (1V-9V). The first set of examples uses DC excitation values of 1, 3, 5, 7 and 9 V. Using this set of experiments; it is aimed to define transient response characteristics and quasi-steady state value of the actuators. In the second set of examples square wave excitation with magnitudes of 3, 5, and 7 V and frequencies of 0.1, 0.25, 0.5, 1, and 2 Hz were applied to actuator samples. Using this set of experiment, it is aimed to define bandwidth of the actuator samples.
2.4. Characterization of IPMC actuators Roughness analyses of Nafion and gold coated Nafion membranes were conducted through Atomic force microscopy (AFM). AFM analyses of samples were performed via Multimode SPM (AFM/STM) Nanoscope IV from Digital Instrument in a tapping mode with Si cantilever. The analyses were made thrice on different zones of each sample. The tensile strength, Young modulus and elongation at break were determined by a tensile testing machine with a 50 N load cell at a crosshead speed of 0.1 mm/min. The resistance of IPMCs was measured by using two-probe setup. The setup for two-probe resistance measurement was connected to Keithley 2612 System Source Meter. For all measurements, the distance between two probes was carefully maintained to be 2 mm. 2.5. Determination of electroactive properties of actuators 2.5.1. Experimental setup One of the main focuses of the study is to monitor the tip 748
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Fig. 2. The vision system for measuring actuation of IPMC actuator.
3.2. Mechanical properties
2.5.3. Determination of blocking force Besides of conventional actuators, distributed force generation exists in IPMCs. When voltage is applied across the electrodes of IPMC, a bending moment is generated on the lateral surface of sample which creates movement as a curvature. If the tip of sample is fixed on load cell, force is transferred to load cell by a line contact and the middle of sample starts to buckle and rises because there is no constraint to inhibit such a motion on IPMC. Overall, this unwanted motion reduces the force output. Despite the fact that contact type measurement technique is common for force characterization of IPMCs, there is an error in force measurements due to these motions. A new experimental method is considered in this study, in which a surface contact is created between sample and balance as shown in Fig. 3. Length of surface contact is fixed as 30 mm for the sample length of 50 mm. Maximum blocking forces of samples were measured by Precisa 225SM-DR precision balance. Blocking force data were logged on a PC via RS-232 protocol.
Fig. 5 shows tensile testing results, in terms of tensile stress versus strain, for IPMCs with the electrode thicknesses of 10, 27, 45, 67 and 80 nm (relative to Nafion 117). As was also seen in Fig. 5, the tensile strength values for Nafion membranes and IPMCs with the electrode thicknesses of 10, 27, 45, 67 and 80 nm were determined to be about 37.20, 37.32, 37.57, 37.71, 37.82 and 40.25 MPa, respectively. Considering that Nafion is the adopted base material for these IPMCs, this comparison is useful. There is a small increase in mechanical strength of IPMCs with the electrode thicknesses of 10, 27, 45 and 67 nm (both stiffness and the modulus of elasticity), but it still follows the intrinsic nature of Nafion itself. This means that, in tensile strength, the stress/ strain behavior is predominated by the Nafion rather than electrode materials. Tensile strength value of IPMC with the electrode thickness of 80 nm increased by about 7%, when compared to that of Nafion since electrode materials become predominated. 3.3. Electrical properties
3. Results and discussion Resistance of IPMCs was measured to investigate the effect of the electrode thickness on electrical properties. The resistance values of IPMCs with the electrode thicknesses of 10, 27, 45, 67 and 80 nm are shown in Fig. 6. The resistance values of IPMCs with the electrode thicknesses of 10, 27, 45, 67 and 80 nm were obtained to be about 40, 30, 24, 19 and 10 Ω/cm, respectively. The IPMC samples exhibited a decrease in the resistance with increasing electrode thickness.
3.1. AFM analysis Root-mean- square (RMS) and arithmetic mean value (Ra) were utilized to determine the surface roughness of the samples. These parameters were estimated for Nafion and gold coated Nafion membranes by averaging the results obtained from nine separate areas. AFM images of samples with a scan area of 10 μm × 10 μm were shown in Fig. 4. From analysis of Fig. 4, The RMS and Ra values were obtained. The vertical height scale of all images was set to 300 nm. Untreated Nafion has RMS and Ra roughness values of 2.95 ± 0.36 and 2.13 ± 0.27 nm, respectively. RMS and Ra roughness values increased when the thickness of gold coating increased from 10 nm to 27 nm. When the thickness of gold coating was increased to 45 nm, RMS and Ra values decreased. Afterwards the thickness of gold coating was raised to 67 nm, RMS and Ra values continued to decrease. But with the increase in thickness to 80 nm, RMS and Ra values increased to 9.85 ± 1.14 nm and 7.93 ± 0.90 nm, respectively. The higher RMS and Ra roughness values were obtained as 9.85 ± 1.14 nm and 7.93 ± 0.90 nm for the thickness of 80 nm and 7.65 ± 0.28 and 6.06 ± 0.19 nm for the thickness of 27 nm, respectively.
3.4. Electroactive properties 3.4.1. Actuator response to DC excitations The samples do not make any significant displacement when 1 V is applied and the 9 V causes the actuator sample to shrivel and not to function any further (Fig. 7a). The experiments were carried out by applying 3, 5 and 7 V DC voltages for a time span of 10 s. The time responses of the sample actuators were recorded during experiments. We observed a generic response form for all actuator samples. In order to represent this generic form of the actuator response, the captured images at different instants during experiment were shown in Fig. 7b. Fig. 8 shows the time evolution of the tip displacement (time response of the actuator). As can be seen from Fig. 8, actuator reached the tip displacement of 20 mm within 10 s. When actuator response is analyzed with reference to time scale, it is seen that increment in tip displacement in every proceeding time span becomes smaller. Therefore, the tip displacement vs time curves can be approximated to be in exponential form: t
x dc (t ) = Xmax (1 − e− T )
(1)
where Xmax is the maximum tip displacement and T is time constant [44]. Following this assumption, the time constant and maximum tip displacement values can be defined as characteristic parameters of time response of the actuators. The maximum tip displacement values were found using the experimental data for Nafion actuator samples with the electrode thicknesses of 10, 27, 45, 67 and 80 nm when the input voltage is 3, 5 and 7 V. As can be seen from Fig. 9, the maximum tip displacement
Fig. 3. The experimental setup for blocking force characterization of IPMCs. 749
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Fig. 4. AFM cross-sectional images of Nafion and Gold coated Nafion samples. a) Nafion, b) 10 nm gold coated Nafion c) 27 nm gold coated Nafion, d) 45 nm gold coated Nafion, e) 67 nm gold coated nafion f) 80 nm gold coated Nafion. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)
Fig. 5. Tensile stress-strain curves of Nafion and IPMCs with the electrode thicknesses of 10, 27, 45, 67 and 80 nm.
Fig. 6. Resistance of IPMCs with the electrode thicknesses of 10, 27, 45, 67 and 80 nm.
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Fig. 7. a) The deformed actuator sample after it is excited with DC of 9V, b) Captured images from the 5 VDC excitation experiment of the nafion with electrode thickness of 45 nm.
Fig. 10. Time constant of Nafion with the electrode thickness of 10, 27, 45, 62 and 80 nm when excited with DC of 3, 5 and 7V
Fig. 8. The time evolution of the tip displacement (time response) in the same experiment.
versa. The time constants were calculated using the experimental data for Nafion actuator samples with the electrode thicknesses of 10, 27, 45, 67 and 80 nm when the input voltage is 3, 5 and 7 V. The results are presented in Fig. 10. In Fig. 10, it is seen that there are two types of patterns in time response vs. excitation voltage curves. The first pattern is that the observed motion becomes faster as voltage increases. This pattern occurs in actuator samples with electrode thicknesses of 10 and 27 nm. The second pattern is that the observed motion becomes faster as voltage is increased from 3 V to 5 V and it becomes slower or remains approximately the same as voltage is increased from 5 V to 7 V. This pattern occurs in actuator samples with electrode thicknesses of 45, 67 and 80 nm. Therefore, no clear result can be deduced since there is no a generic pattern that describes actuator time constant change with respect to excitation voltage or electrode thickness. Besides that, it is observed that the fastest time response is observed with Nafion with the electrode thickness of 27 nm under DC excitation of 7 V. When the results are considered together, the actuator with electrode thickness of 45 nm has the maximum tip displacement value and the second fastest response when excited with 5 V. The observations on DC characteristics show that selection of any IPMC actuator with a certain electrode thickness and an excitation voltage requires optimization in terms of prospective application.
Fig. 9. Maximum tip displacement of Nafion with the electrode thickness of 10, 27, 45, 62 and 80 nm when excited with DC of 3, 5 and 7V
increases from 3 V to 5 V, but when voltage rises to 7 V, tip displacement reduces slightly. This is an unexpected result because the more electrical potential energy is provided to actuator and hence the more mechanical work is expected to be done but the results show the contrary situation that tip displacement reduces when a threshold of input voltage value is exceeded (5 V in our experiments). When the values for different electrode thicknesses were compared, two results arose:
3.4.2. Actuator response to square wave excitations In the second set of experiments, the maximum tip displacement was observed with respect to increase in excitation frequencies and voltages. The results were presented as a group of maximum thickness vs. excitation frequency curves for constant excitation voltages of 3, 5 and 7 V, as shown in Fig. 11a–c. In each of the experiments, it is observed that the maximum tip displacement decreased as the excitation frequency is increased. But the decrease in maximum tip displacement per change in frequency is not linear. This observation is compatible with conclusions from DC excitation experiments because the time constant of a time response also defines the frequency range at which actuators can be used effectively.
• The first deduction is that the actuator with electrode thickness of •
45 nm produces maximum tip displacement when all actuators for all excitation voltages are taken into consideration. The other observed result is that as voltage increases, tip displacement does not increase always for a given electrode thickness. There is a nonlinear relationship in quasi quadratic form. Therefore, there is one optimal thickness value suitable for operating voltage or vice
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Fig. 11. The effect of the excitation frequency on maximum tip displacement for excitation voltages of a) 3V, b) 5V, c) 7V
3.4.3. Determination of blocking force In the third group of experiments, the blocking force values of Nafion actuator samples with the electrode thicknesses of 10, 27, 45, 67 and 80 nm were measured when the input voltage was 3, 5 and 7 V. The results are presented in Table 2. It was observed that as the input voltage increases the maximum value of the blocking force also increased for constant electrode thickness. When the electrode thickness is considered, measured results are compatible with displacement characteristics. The blocking force for the actuator samples increased with varying electrode thicknesses from 10 to 45 nm and decreased with varying electrode thicknesses from 45 to 80 nm.
Table 1 Corner frequencies calculated from time response curves. Excitation Voltage in [V]
3 5 7
Electrode Thickness in [nm] 10
27
45
67
80
0.23 0.29 0.50
0.33 0.80 1.33
0.40 0.83 0.50
0.31 0.50 0.56
0.33 0.50 0.33
Table 2 Blocking force values of IPMC sample at 3, 5 and 7 V. Electrode thickness (nm)
Blocking force (gf)
4. Conclusion 10 27 45 67 80
3V
5V
7V
2.21 2.65 3.16 2.32 1.14
2.65 2.95 3.62 2.94 1.51
2.85 3.15 3.82 3.14 1.81
The observations on electromechanical characteristics show that there is a nonlinear relationship between electrode thickness and electroactive characteristics of IPMC actuators: the maximum tip displacement, time constant and the blocking force values. All three properties increased as electrode thickness increases from 10 to 45 nm and decreased as electrode thickness increases from 45 to 80 nm. The bending moment leading to motion of the Nafion-based IPMC takes place due to cation motion which is the result of applied electrical field. The applied electrical field is directly related to electrode thickness as it affects spatial density of electrical flux. On the other hand, the bending motion and bending moment are related to each other via stress-strain interactions which are dominated with elastic modulus when other structural parameters are constant. The experimental results showed that the optimization in terms of excitation voltages and electrode thickness is the result of the trade-off between electrical force acting on cations and stiffness of actuators. The square wave experiments demonstrated the maximum tip displacement decreased as the excitation frequency was increased. Based on the maximum tip displacements, it can be noted that the actuators with electrode thicknesses of 27 and 45 nm exhibited better frequency characteristics due to their wider bandwidths. The blocking force for the actuator samples enhanced with increasing electrode thicknesses from 10 to 45 nm and decreased with increasing electrode thicknesses from 45 to 80 nm. From electrical conductivity measurements, it was seen that Nafion based IPMC actuator showed a decrease in the resistance with increasing electrode thickness.
The approximated generic time response function (Equation (2)) represents first order dynamics in system theory and the first order systems have low pass filter characteristics. This means that the high frequency signals are attenuated by the strongly decreasing amplitude. The limit frequency, in which this strong attenuation occurs, is defined as bandwidth of the system or the corner frequency (fc). Calculated corner frequencies for IPMC samples are given in Table 1.
fc =
1 T
(2)
The maximum tip displacement vs. excitation frequency curves in Fig. 11 show also the effect of the bandwidth of the actuators on displacement behavior of IPMCs. In the experiment of excitation voltage of 3 V (Fig. 11a), it is seen that only the actuator with electrode thickness of 45 nm moves significantly when excited with 0.25 Hz. This is due to its corner frequency of 0.4 Hz. The maximum tip displacements of all other actuators are very small in magnitude. In 5 and 7 V excitation voltage experiments (Fig. 11b, and c), the similar results were observed. The actuators with electrode thicknesses of 45 and 27 nm showed significant displacement when excited with the frequencies below corner frequency approximately of 1 Hz. The maximum tip displacement of the other actuators reduces very fast when they have corner frequencies of 0.25 and 0.5 Hz. The square wave experiments exhibited the effect of the corner frequency (bandwidth) on displacements of the actuators. The observed results are compatible with DC experiments. The actuators with electrode thicknesses of 27 and 45 nm have better frequency characteristics. The results also showed that selecting an excitation voltage determines the frequency range at which the actuator can be used effectively. Therefore, it should also be considered in optimization procedures.
Acknowledgements Financial support for this study was provided by TUBITAK-The Scientific and Technological Research Council of Turkey, Project Number: 111M643. References [1] Shahinpoor M. Mechanoelectrical phenomena in ionic polymers. Math Mech Solid 2003;8(3):281–8.
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