Nafion actuators

Sensors and Actuators A 170 (2011) 164–171

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Sensors and Actuators A: Physical journal homepage: www.elsevier.com/locate/sna

Nonlinear strain–electric field relationship of carbon nanotube buckypaper/Nafion actuators P.-J. Cottinet, C. Souders, D. Labrador, S. Porter, Z. Liang ∗ , B. Wang, C. Zhang High-Performance Materials Institute, Florida State University, United States

a r t i c l e

i n f o

Article history: Received 13 May 2011 Received in revised form 15 June 2011 Accepted 19 June 2011 Available online 24 June 2011 Keywords: Smart material Electroactive composites Nanotube Deformation properties Actuator

a b s t r a c t Due to their large electric field-induced strains and flexibility, carbon nanotube thin film or buckypaper/Nafion actuators can be used in various applications, such as lightweight, low power consuming electroactive materials for morphing structures. However, this material is very new, and only limited studies have been devoted to characterizing and understanding their electromechanical couplings. This paper reports on the study of the conversion mechanism and determination of the current flowing through buckypaper/Nafion actuators driven by electric fields. Experimental results revealed that the saturation of the electrically induced strains of the actuator were due to nonlinear characteristics between the electric field and electric charge behaviours. Moreover, by modeling the electric charge changes versus electric fields, we are able to calculate the strain and current versus electric fields of the actuators. The predicted values were in good agreement with the experimental data. The efficiency of the conversion was calculated and demonstrated to determine the potential of CNT buckypaper/Nafion actuators for smart structure applications. Published by Elsevier B.V.

1. Introduction Due to the increasing demand in multifunctionality and system efficiency, smart materials have gained attention over the past decades, although the physical effects of many typical candidate materials have been known for over 50 years [1]. These materials have demonstrated the characteristics of both sensors and actuators. Most of these materials are also capable of reversibility, changing their mechanical properties (viscosity, stiffness, shape) due to the influence of temperature changes or an electric or magnetic field. These so called active materials, include traditional piezoelectric ceramics and the comparatively new and novel materials like electro active polymers (EAPs) [1,2]. EAPs have come to the forefront as a popular family of active materials because of the numerous advantages they can offer [3]. EAPs are lightweight, flexible, ductile, possess high strength to weight ratio, good processability, large actuation stroke, superior sensing response, low acoustic and mechanical impedance [4]. These materials hold a distinct advantage over other smart materials such as lead zirconium titanate (PZT) and shape memory alloys (SMAs) and can be useful in future space structure applications [5]. Therefore, they are envisioned for use as inexpensive, lightweight

∗ Corresponding author. E-mail address: [email protected] (Z. Liang). 0924-4247/$ – see front matter. Published by Elsevier B.V. doi:10.1016/j.sna.2011.06.013

actuators that consume low power and are able to actuate at low voltages [6]. Electrochemical and electromechanical properties of ionomeric polymer–metal composites (IPMCs) have attracted attention due to their ability to provide effective mechanical actuation under low electric fields (several volts), high strain with respect to ferroelectric polymers, such as PVDF [7], and relatively fast response time compared to ionic gels [8] and conductive polymers [9]. The most studied IPMC materials are Nafion-based actuators, membranes with electrochemically plated Pt electrodes on both sides [10–13]. The actuation mechanism of this composite was described in terms of electro-osmotic water transport driven by solvated cations and charging of the double layer at the interface between Nafion and Platinum [14]. The discovery of carbon nanotube (CNT) electromechanical actuation introduced a unique technology enabling the conversion of an electrical stimulus to mechanical displacement due to a novel quantum mechanical mechanism [15]. The activation mechanism was based on a double-layer charge injection working in a liquid electrolyte [16]. Together with the quantum chemical effect, this double layer of ions causes large dimension changes in the covalent bonds of the nanotubes. For real world engineering applications, the challenge is to achieve these remarkable properties of individual CNTs in macroscopic nanotube assemblies through sheets or thin films that will operate in an open air environment [17,18]. These films are called buckypapers.

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This paper reports on the electromechanical properties of a composite material prepared by sandwiching a Nafion membrane between two buckypapers (BP). These BP/Nafion actuators can be operated in the open air with relatively low voltages. When the displacement of BP/Nafion actuator was measured under different electric excitations, nonlinearity was observed between the strain and the electric field strengths. Based on these results, the conversion efficiency and mechanical energy density were calculated while demonstrating the exceptional properties of this new type of smart material or actuator. 2. Experimental procedure 2.1. Actuators preparation In this study, randomly oriented multi-walled nanotube (MWNT) BPs were used to make BP/Nafion actuators. The BPs were produced specifically for this study. The MWNT used in this study was produced by CNano Technology (San Francisco, CA). The BP fabrication procedures were documented in detail by the groups of Smalley [19] and Wang [20]. The procedure of the BP fabrication can be summarized as: (1) prepare the CNT suspension; and (2) conduct filtration process. CNTs were randomly dispersed in the BP during the filtration process. Fig. 1 shows an SEM image of MWNT BP. To realize the potential real-world applications of nanotube actuators, the liquid electrolytes should be replaced by solid electrolytes in the nanotube actuators design. Following this thought, we used DuPont Nafion NRE-212 Membranes as solid state electrolyte for BP actuators [21], due to its high electrical conductivity (10−2 S/cm) and

Fig. 1. SEM image of surface of MWNT buckypaper.

commercially availability. In fact, inside the Nafion polymer chain exists a network of fixed anionic clusters along with mobile cations [22]. Once an electric potential is applied at a given frequency, the H+ cations are attracted to the electrode carrying a negative potential. The conglomeration of H+ ions and water molecules within the Nafion structure caused swelling in areas where the electrode is negatively charged. In this case, the size and number of ions presented dictate the magnitude of the electromechanic response. Both Nafion swelling and C–C bond expansion induced by

Fig. 2. Fabrication process of BP/Nafion actuator.

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Fig. 3. Test bench; (a) scheme of the experimental test bench for actuation measurement (b) photo of the test bench.

electrical excitation can possibly promote mechanical deformation of BP actuators [21]. To prepare the BP/Nafion solid electrolyte composite actuator, a sheet of the BP was cut into two strips of 30 mm × 5 mm, and then a few drops of Nafion solution were applied onto the two BP layers to increase the interfacial bonding between the BP and Nafion layers (Fig. 2). After the BP dried, they were adhered to each side of a Nafion NR-212 membrane, which was measured to be about 50 ␮m thick. The assembly was hot-pressed at 120 ◦ C for 10 min at a pressure of 120 psi. Table 1 gives the major characteristic of the BP/Nafion actuator, where YNafion is the Young’s modulus of Nafion, YBP is the Young modulus of BP, L and w are the length and the width of the actuator respectively, tNafion and tBP are the thickness values of Nafion and BP. The actuator fabrication process is relatively simple compared to Electro Active Polymer (EAP) actuators [23–25]. 2.2. Experimental set-up of actuation test

The tip displacement was converted into strains (S1 ) using geometry parameters and Eq. (1), where ı was the tip displacement (zero-to-peak), t was the sample thickness and L was the free length of the sample (see Fig. 4). This equation assumes that the actuator deforms with a uniform curvature [26]. S1 =

ı·t L2

(1)

The sample was driven by a harmonic (single frequency) voltage signal supplied by an Agilent 2310A function generator. As in previous experiments [17], the tip displacement was measured using the displacement laser sensor (MICROTRAK II; MTI Instrument, Inc.). The clamp that holds actuators was mounted to a breadboard from ThorLabs. The breadboard had a Plexiglas enclosure with a door mounted on top to prevent air flow from disturbing the results. The breadboard is mounted on top of the Iso-Plate Passive Isolation System to dampen vibrations in the room. The current consumed by the actuators was also measured during the course of this work and used to compute the capacitance of the membranes. In order to

BP/Nafion actuators were tested in a cantilevered configuration and would bend when stimulated by an applied voltage. One end was fixed by a rigid clamp fitted with copper foil electrodes that was connected to BP surface of the sample, and the deflection of the free end was measured with a displacement laser sensor, as shown in Fig. 3.

Table 1 Characteristic of the BP/Nafion actuators. YNafion (MPa)

YBP (MPa)

L (mm)

w (mm)

tNafion (␮m)

tBP (␮m)

248

470

30

5

50

27

Fig. 4. Principle of calculation of the strain.

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167

0.02 0.015

Strain (%)

0.01

Fig. 5. Circuit used to measure the current through a BP/Nafion actuator when driven with a voltage input.

0.005 0 -0.005 -0.01 -0.015

determine the electrical impedance and the efficiency of the electromechanical conversion, the current drawn by the actuator for a given voltage input was measured using the circuit shown in Fig. 5. This circuit operates by amplifying the voltage drop across a 1  resistor placed in series with the polymer sample: the gain of the circuit was V/i = 110 mA/V (with V as the output voltage and i as the input current). The value of this resistor was chosen to be small enough that the effect on the current through the polymer was negligible. The time history of the tip displacement, the input voltage and the current consumed were measured using an oscilloscope.

-0.02 -100

-50

Fig. 6 shows the mechanical responses of the BP/nafion/BP bimorph cantilever or actuator under different electric fields. The actuation mechanism presumably can be associated with the electromechanical properties of both the BP and the Nafion, as previously mentioned in Section 2.1. For low electric fields, typically E3 = 20 V/mm, there was a linear relationship between the voltage and the displacement. However, when the electric field was higher than Esat = 45 V/mm, a nonlinear characteristic existed between the strain and the electric field. The transported charge was the key for governing the electromechanical conversion of electroactive actuators [18]. Fig. 7 clearly shows this is also true for BP/Nafion actuators, where the 4

0

0

Strain (%)

Electric field (V/m)

-4

x 10

x 10

-5 0

5

10

15

20

25

30

35

40

time (s)

-4

x 10 2

60

1

40

0

20

-1

0

-2

-20 5

10

15

20

25

30

35

-3

40

time (s) Fig. 7. Measured the strain versus the applied electric field (a) and the charge density (b).

Strain (%)

Charge density (C/m²)

b 80

0

100

temporal evaluation of both the electric field relation to strain (Fig. 7(a)) and charge transported relation to strain (Fig. 7(b)) are respectively plotted. The amount of transported charge was calculated by the time-integration of the measured current. For case (a) there was a phase-shift between the strain and the electric field, but it did not exist between the charge and strain [27]. This result confirmed that the charge transport dictated the electromechanical conversion in the samples. Cyclic electric field current measurements provided information regarding electrochemical reactions occurring at the interface between the BP and Nafion [27], which is helpful for understanding the saturation of the strain within the electric field. In Fig. 8, the peak at ±45 V/mm can be assigned to the oxidation of the oxygen-containing functional groups located on the carbon nanotube surface of the BP, as reported in literature [28,29], or nanotubes of different chirality analogous to the chemoselectivity, as demonstrated by Strano et al. [30]. The cycle appears nonlinear for an electric field of 45 V/mm, which was the same as for the strain saturation (Fig. 6). A link appears to exist between the oxidation in BP and the

3.1. Experimental measurement of strain and charge

5

50

Fig. 6. Strain versus electric field at 0.1 Hz.

3. Results and discussion

a

0

Electric field (V/mm)

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where Q is the electric charge, V is the applied voltage across the sample, Vsat is the saturation voltage, which is the limit that can be reached by the electric field within the sample, and C0 is the capacity of the BP/Nafion. The current flowing through the sample has two contributions: one from the charge variation versus time and another that originates from electrical loss in Nafion,

40 30

Current density (A/m²)

20 10 0

itotal (t) =

-10 -20 -30 -40 -50

0

50

Voltage (V/mm) Fig. 8. Cyclic electric field versus current density of the BP/Nafion actuator at 0.1 Hz.

3.2. Modeling of the electromechanical conversion of BP/Nafion actuator In order to understand the observed saturation, an electromechanical convention model of BP/Nafion actuator was developed. The electric field-induced charge within the Nafion film was assumed to be equal to

 V 

(2)

Electric field (V/mm)

Vsat

V (t) = V (t).sgn(V )[1 (V ) + 2 (V 2 ) + 3 (V 3 )] R(V )

itotal (t) = C0



∂V (t) 1 − th2 ∂t

 V  Vsat

+

V (t) R(V )

0

0

5

10

15

20

25

30

35

time (s) -3

Current (A)

5

experimental

0 -5

0

5

10

15

20

25

30

35

time (s) -3

x 10 5

model

0 -5

0

5

10

(5)

With C0 is the electrical double layer, V is the voltage, Vsat is the voltage of saturation and R(V) is a function that takes into account the electrical loss. By using this model and adjusting Vsat and the parameters of R(V) function, it becomes possible to accurately model the experimental curves of the current versus the voltage and time using the data from Table 2. Fig. 9 depicts the experimental current along with its calculated counterpart. A good agreement can be seen between the experimental results and simulated curves, indicating that the model reflects the major conversion mechanism.

50

-50

(4)

Note that iloss is supposed to be an odd function of V. This is why sgn(V) appears in (4) [32]. By combining Eqs. (2) and (3), the expression of the current becomes:

x 10

Current (A)

Q = C0 · Vsat · th

(3)

With itotal representing the total current, iloss is the current due to the loss in the material and ∂Q(t)/∂t is the current due to the electric charge. In fact the current response under a step voltage input will not vanish at the steady state [31] because of the DC resistance of the polymer. One can approximate the current iloss by a series of polynomial functions (V) [32]. In this modeling work, a three-order polynomial function was used: iloss (t) =

electromechanical response. However, Nemat-Nasser and Wu [27] showed that the charge accumulation for the Flemion-based IPMCs is linearly related to their tip displacement over the entire actuation process. However, for BP/Nafion actuators, we observed a very different phenomenon. In fact, the current or electric charge density increased, but the strain did not increase proportionally in the BP/Nafion actuators.

∂Q (t) + iloss (t) ∂t

15

20

25

time (s) Fig. 9. Measured and modeled currents of BP/Nafion actuator.

30

35

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a

2

x 10

169

-3

total charge loss

Current (A)

1 0 -1 -2

0

5

10

15

20

25

30

35

40

45

time (s)

Current (A)

b

5

x 10

-3

total charge loss 0

-5

0

5

10

15

20

25

30

35

40

45

time (s) Fig. 10. Measured current and the decomposition currents of the model for two applied electric field (a) 20 V/mm and (b) 50 V/mm.

S1 = ˛31 · Q

(6)

where ˛31 is the coupling constant between the electric charge (Q) and the strain S1 . Using Eq. (2), the dependence of the strain versus voltage becomes: S1 = ˛31 · C0 · Vsat · th

 V 

(7)

Vsat

The effective coefficient ˇ31 can be calculated from the curves S1 = f(E3 ) by assuming that S1 = ˇ31 ·E3 before the saturation is reached. In fact, experimentally, it is easier to measure the coefficient ˇ31 than ˛31 because there is no problem of electric loss. The coefficient ˛31 can be calculated from ˇ31 using ˛31 = ˇ31 /C0 . Using Table 2 data and Fig. 1, ˛31 is equal to 16 × 10−5 (m/C). By employing the calculated effective electromechanical coefficient ˛31 and by using the relationship between the strain and voltage-induced electric charge (Eq. (7)), we can calculate the strain–voltage relationship and compare with the experimental values, as shown in Fig. 11. A good agreement between the model results and experimental data was obtained, which confirmed that

the saturation of the strain is mainly due to the saturation of the voltage-induced electric charge. 3.3. Efficiency of the conversion The efficiency of an electromechanical actuator is determined by the mechanical and electrical losses within the material [33]. In order to achieve high efficiency, the actuator must convert a large percentage of applied electrical energy into mechanical work. Fig. 12 addresses the major contributions to the efficiency conversion of the BP/Nafion actuators. The global efficiency total of the actuator is given by the following equation: total = electric · transduction · mechanic

(8)

where electric is the efficiency of the electrical conversion, which is equal to electric =

V · Icharge V · Itotal

;

(9)

0.02 0.015 0.01

Strain (%)

Fig. 10 displays the variation in the measured current versus time for low electric fields: 20 V/mm (Fig. 10(a)) and 50 V/mm (Fig. 10(b)) and also the current decomposition using the model. For the BP/Nafion driven by a Sinusoidal electric field, E, two contributions to the total current flowing through the Nafion were expected: a resistive contribution corresponding to the losses in phase with the electric field; and a capacitive one corresponding to the electric field-induced charge transport Q = C0 ·V, which shifted the applied electric field forward by a phase angle of 90◦ . The induced strain is proportional to the charge density [27], which can be expressed by the transverse strain (S1 ) by:

0.005 0

Data

Modeling

-0.005 -0.01 -0.015

Table 2 Experimental measurements and parameters for the current model.

-0.02 -100

C0 (␮F)

Vsat (V)

 1 (S)

 2 (S2 )

 3 (S3 )

150

4

4 × 10−4

2 × 10−5

1.6 × 10−5

-50

0

50

Electric field (V/mm) Fig. 11. Measured and modeled actuation strain.

100

170

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Fig. 12. Schematic representation of the energy conversion.

transduction corresponds to the mechanical output power on the electric input power (V·Icharge ); mechanic is related to the loss tangent of the material, tan ı. The strain–energy density (J/m3 ) of the BP/Nafion actucator can be defined by [34], 1 · Y · S12 2

Wmechanical =

1 · Y · S12 · f · Vol 2

(11)

where Pmechanical is the mechanical power output to the BP/Nafion actuator; f is the frequency of actuation, and Vol is the volume of the actuator. Therefore, the efficiency of the electrical to mechanical energy conversion becomes: transduction =

0.5 · Y · S12 · f · Vol

(12)

V · Icharge

The efficiency of the mechanical conversion can be expressed as: mechanical =

Type

S31 (%)

E3 (V/mm)

Frequency (Hz)

Reference

IPMC + Pt PMMA + SWNT Nafion + BP

0.008 0.012 0.016

15 20 100

0.5 0.5 0.1

[41] [42] –

(10)

where Wmechanical is the strain–energy density, Y is the Young’s modulus, and S1 is the bending strain. The mechanical power output, Pmechanic , is Pmechanical =

Table 4 Comparison with other electroactive polymer.

1 1 +  · tan ı

(13)

with tan ı, a measure of energy lost to energy stored in a cyclic deformation, is a material property defined by the ratio of the loss modulus to the storage modulus and is a function of frequency, temperature, environment, and residual stresses and strains [35]. Dynamic mechanical analysis (DMA) is a technique used to study and characterize thermal dynamic properties of materials. It is most useful for studying the viscoelastic behavior of polymer materials [35]. For example, during a DMA test, a sinusoidal stress is applied and the strain in the material is measured as temperature is ramped up, allowing one to determine the complex modulus (elastic modulus and viscous modulus), tan ı (damping) and glass transition. In this study, DMA tests were used to determine tan ı at 0.1 Hz frequency to find the damping property during the actuation. The peak tan ı value of the test sample was 0.2. Table 3 provides the values of the efficiency ratios for an electric field of E3 = 80 V/mm at 0.1 Hz. The efficiency of the conversion was low, below 1%. However, a previous study concerning (IMPC Nafion) showed the value of energy conversion factor as much less than 1% [36], and its maximum value was about 0.1–0.2% [37]. Moreover, in those reports, the mechanical losses were not taken into Table 3 Electromechanical conversion efficiency of the BP/Nafion actuator at an electric field E3 = 80 V/mm at 0.1 Hz. electric (%)

transduction (%)

mechanic (%)

total (%)

20%

0.32%

83%

0.052%

account and the frequency was higher, which should resulted in an improvement of the conversion [38]. Our results also show that the biggest part of the energy was lost during the electrical to mechanical transduction. Hence, in the electrical conversion, only 20% of total energy was used, with the other part dissipated by the Joules effect. The global efficiency of the conversion can be improved by using another type of BP with higher Young’s modulus, for example, and by doping the Nafion to enhance the ionic conduction to charge the transport [39]. Table 4 gives a comparison among actuator configurations, which consist of solid electrolytes sandwich between two electrodes (Pt or carbon nanotubes). The Nafion/BP actuator can be seen to have a higher strain performance, since it is possible to apply a higher electric field than other cases. High actuation strains, combined with attractive mechanical energy densities, positions buckypaper actuators (6.4 J/cm3 ) comparable to piezoelectric polymers (PVDF) for overall actuation performance [40].

4. Conclusion The strain versus electric field measurements were performed on BP/Nafion actuators. The measurements of current versus electric field were also conducted, showing that the current had two components: a resistive part due to electrical losses and a capacitive part related to the voltage induced electric charge. The results in particular showed that the amplitude of the capacitive current did not follow a linear relation versus electric field. In order to explain this phenomenon, the voltage induced electric charge behavior was modeled using a function which was saturated for a given threshold of voltage. The current and voltage-induced transverse strain relationships were calculated showing very good agreement with the experimental data. The electromechanical conversion efficiency of the actuator was calculated and showed a relatively low value. Further improvements to enhance the BP/Nafion actuation efficiency are feasible, for instance, using higher Young’s modulus buckypaper and improve ion conduction electrolytes to improve the free strain and blocking force. Moreover, future research is required to determine the magnitude of improvements that can be achieved and the essential tradeoffs between strain and force to optimize the actuator’s performance for different applications. The advantages of lightweight and low driving voltage of BP/Nafion actuators show great promise as alternatives for use in robotics, biotechnology and industrial applications.

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