Fabrication and stiffness optimization of carbon-based composite double polymer compliant electrode

WU Xiaojun, WEN Binhua, TONG Xin, ZHANG Ying

(School of Mechanical and Electrical Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, China)

Abstract： A manufacturing method is proposed for carbon based composite double polymer compliant electrode. The stiffness of this compliant electrode is changed by adjusting the mass fraction of carbon black and the ratios between Ecoflex20 and RT625. Tensile machine is used to test its ductility and hardness. The conductivity is measured through the source table. Finally, it is printed on the dielectric elastomers (DE) film, and the high-voltage amplifier is used for dielectric elastomers actuators (DEAs) dynamics testing. The results show that the compliant electrode has high tensile properties (>200%), low stiffness (<300 kPa) and well conductivity (0.049 3 S/cm). It is proved that the DEAs displacement output is up to 1.189 mm by this compliant electrode under dynamic response, which is 1.64 times and 1.32 times of the same type. Moreover, this formula extends the curing time of the original compliant electrode ink. It can provide a reference for the production of compliant electrode and DEAs in the future.

Key words： dielectric elastomer actuators (DEA); carbon-based composite double polymer compliant electrode; stiffness；conductivity

0 Introduction

Dielectric elastomer (DE), one of the electroactive polymers, is a functionalized rubber material. DE materials have many unique properties, such as large deformation (>100%), high energy density (>3.4 MJ·mm-3) and millisecond response speed. Low production costs and relative mature fabrication processes make DE have a wide range of application prospects[1-2]. Dielectric elastomers actuator (DEA) is a typical “sandwich” structure, which is composed of a layer of DE film and compliant electrodes sandwiched on both sides. The working principle is shown in Fig.1. It is assumed that the DE film is an incompressible material. When a voltage is applied, the DE film expands to the surroundings with shrinking in thickness due to Maxwell’s stress. The most widely used DE materials are acrylic (such as 3 M VHB4910[3-4]) and silicone (Wacker ELASTOSIL2030[5-6], Wacker RT625[7]), in which the acrylic has lower modulus of elasticity, high electrical breakdown strength and dielectric constant, so electro-strictive large scale. However, its high viscosity makes the response speed slow. On the contrary, silicone rubber material is low viscosity and fast response speed. However, it has high elastic modulus, low dielectric constant and electric breakdown strength. The electric deformation is small, and the dynamic performance is obvious. At present, the peak output power density (≈600 W/kg) is higher than the natural muscle reported[8]. DEAs have a widely range of application scenarios, such as compliant pneumatic pumps[9-10], soft crawling robots[11], bionic aircraft[12]and multifunctional grippers[13].

Fig.1 Schematic diagram of DEA working principle

In fact, one of the main factors that determine the performance of DEA is the characteristics of the two-sided compliant electrode (CE). The CE requires to have strong conductivity and low elastic modulus, so as not to hinder the movement of DEA. At the same time, it should be attached to the DE film and have good adhesion to prevent slipping between CE and DE during the movement process. Thus, it will affect the output performance of DEA. Commonly used compliant electrodes are carbon-based electrodes[14]. Although a metal powder has a better conductivity than the conductive carbon blacks, but the metal is easily oxidized to form an insulating layer on the surface with poor ductility. Carbon nanotubes and graphene have special structures than carbon materials. Carbon black has a cheaper price, simple manufacturing process (no need for ultrasound and dispersion) and easy surface characterization[15]. The mutual agglomeration leads to a decrease in the mechanical properties of the composite compliant electrode, such as ultimate tensile strength and strain capacity[16]. Carbon powder is used to spray directly on the DE film[17]. The life of DEAs will be limited because these carbon powder electrode particles have the possibility of detaching from the DE film under DEA resonance. However, this type of electrode is mainly used for stronger viscous acrylates (such as 3 M VHB4910 and 4905). Carbon grease is used to solve the problem of toner particles easily falling off[18-19]. Experiments show that the carbon grease is difficult to cure. The silicone oil will penetrate into the silicone rubber, thus it is difficult to model and analyze the dynamic of DEA. This kind of uncured electrode is easily affected by the environment, and its applications is limited. However, the rubber-based compliant electrode can overcome, the above limitations and be completely cured, and it has a good adhesion to the DE film. Most of the literatures[20-21]only reported the content and stretchability of conductive carbon black in compliant electrodes. Rosset[22]studied the dissipation of conductive particles in a compliant electrode and reported that the compliant electrode was basically unusable after several thousand cycles, and the resistance was more than tens of megaohms. At the same time, Saint-Aubin[23]studied the life of the compliant electrode and found that the life of the electrode was independent of the driving frequency by observing the resistance and output displacement. Schlatter[24]developed a low-resistance and low-stiffness carbon-based compliant electrode. The study found that this compliant electrode showed a non-linear decrease in resistance as the number of pad printing increased. Zhang studied the effects of ambient temperature[25], humidity[26], electrode thickness and conductivity[27]on DE performance.

So far, there has not been a systematic study of CE to develop a suitable silicone electrode for DEAs dynamic application scenarios. Here, a method is reported for fabricating a compliant electrode with variable softness and hardness, and its electromechanical properties (stiffness stretchability and conductivity) are explored comprehensively. The characteristics of the compliant electrode can be changed by adjusting the ratio of RT625 to Ecoflex20 and the content of carbon powder.

1 Materials and methods

1.1 Materials

ELASTOSIL2030 silicone membrane (WACKER, Germany) is a widely used commercial DE film. The base material of the compliant electrode is mainly RT625 (WACKER, Germany), which has low viscoelasticity and high elongation (>500%), so it has a wide range of DEA applications[28-29]. However, RT625 has a high stiffness (tensile strength of 6.50 N/mm2, while ELASTOSIL2030 has a tensile strength of 6.00 N/mm2). In order to reduce the rigidity of the compliant electrode, a softer Ecoflex20 (Smooth-on, USA) is added to the base material. Ecoflex20 has relatively low viscoelasticity and little effect on dynamic performance. Its tensile strength is 1.10 N/mm2. The conductive material uses conductive carbon black with a diameter of 35 nm-45 nm (XFI15, Pioneer Nano Corporation, China). Isopropanol (AR, Dongjiang, China) is added to dissolve the conductive carbon black powder to ensure that the conductive carbon black can be uniformly dispersed. Silicone rubber adhesive (Smooth-on, USA) is used to adhere the silicone membrane to the metal membrane support. Among them, the DE film and its support are bonded with tape (5302A, Nitto, Japan).

1.2 Stretching sample preparation

In this tensile experiment, the sample shape of the experiment adopts the international standard ASTM D412 protocol[30]. A total length of selected sample shape of “Mold C” is 115 mm, and the narrow section has a length of 33 mm and a width of 6 mm. Among them, the mass fraction of conductive carbon black is 10%-25%, and the step size is 5%. Ecoflex20-to-RT625 is equal to 0, 0.2∶1, 0.4∶1, 0.6∶1, 0.8∶1. A total of 20 combinations are obtained. For each combination, three samples are made and tested to eliminate random errors. A sample with a conductive carbon black mass fraction of 10% and Ecoflex20-to-RT625=0.2∶1 as an example illustrates the manufacturing process in detail. The general scheme for fabricating the sample is described below and illustrated in Fig.2.

Fig.2 Schematic diagram of production process

a) 0.5 g conductive carbon black is added to 5 g of isopropanol, and then mixed with a stirrer (DACC150R, Speed-Mixer, China) at a speed of 3 000 r/min for 10 min.

b) After pre-mixing of carbon black, 3.75 g RT625AB is added, where A/B is 1∶9 according to the instruction and 0.75 g Ecoflex20AB (1A∶1B) to the beaker, then mixing again for another 8 min at the same speed.

c) A laser cutting machine (VLS2.30, Universal, USA) is used to cut PET (Polyethylene terephthalate) as a mold. The thickness of PET is 0.1 mm.

d) The PET molds is put on the acrylic board through adding the alcohol in order to prevent gaps between the PET and the acrylic board. And then the uncured mixing compliant electrode is added to the PET molds. The obtained mixture is scraped onto the PET molds by using a spatula to scratch.

e) The acrylic board with the uncured sample is placed in constant temperature oven (XGQ-200, JLG, China) at 60 ℃ for 12 h.

f) The sample production is successful when the compliant electrodes is completely cured.

1.3 DEA sample preparation

The preparation process of DEA is shown in Fig.3. Detailed steps are described as a)-f).

a) First of all, the DE film are cut into required size of the wafer by hand. And then it is glued in advance with double-sided adhesive tape (5302A, Nitto, Japan) taped on the stretcher. The DE film is stretch to specify multiple position with circle where the stretch ratio is 1.1 × 1.1.

Fig.3 DEA sample production process

b) The stretched DE film is posted in iron ring holder which has adhesive with silicone rubber (90 g, Smooth-on, USA) in advance. Moreover, the iron rings are introduced to keep tension in the DE film.

c) Then it is adjusted to its proper position for the printing plate which is full of the compliant electrodes of a pad printer. Meanwhile, this step is in preparation for the next pad printing electrodes.

d) An iron ring with a pre-stretched DE film is placed on a prepared pad. Then it is making its compliant electrode adhere to the positive of DE film.

e) The DE film is printed the compliant electrode on the reverse side when the positive of DE film is succeed.

f) Finally, the DE film is removed from iron ring. The entire DEA sample production is completed.

2 Mechanical performance testing

2.1 Uniaxial tensile testing setup

The tensile test environment is carried out at an average temperature of 24 ℃ and a humidity of 45% RH. A tensile machine (ESM303, MARK-10, USA) is used with the displacement range (450 mm), force range (25 N) and an accuracy of 0.01 N. It is connected to the computer by USB to collect the experimental data. In the measurement process, the sample is pre-fixed with a pre-fixture in order to ensure that the sample is not damaged and facilitate the test. Then it will be fixed in the inherent fixture of the stretching machine (as shown in Fig.4).

In the test process, in order to ensure the reliability of the experiment, it should be paid attention to 1)-5) when placing the samples.

Fig.4 Stress-strain experiment platform

1) The surface area of the clamped part should be large enough, and it must be ensured that 70% of the sample end is clamped. If the clamped part is too small, the sample will fall off before it is broken during the stretching stage.

2) In order to better ensure that the sample can be fixed, a laser cutting machine is used to engrave fine zigzag patterns crisscrossing the fixture to specifically improve the friction and ensure the reliability of the experiment.

3) Adding sandpaper to the surface of the sample should be avoided, because it cannot improve the friction between the sample and the fixture. In some case may also cause the sample to break.

4) When the sample is fixed on the stretching machine, making sure that the force direction of the sample is in the same straight line as the stretching direction. Firstly, the upper end of the sample is fixed, and then it sags naturally. The clamp is adjusted to the initial position to clamp the clamp.

5) In order to prevent the sample from being stretched before the test. The limit load value is set in advance to ensure that the sample will not be stretched before the test. the limit load value is set as 0.05 N in this experiment.

2.2 Repeatability of tensile testing

(1)

andSis the revised sample standard,

(2)

In the next test, three samples of each ratio are made. The average curve and sample standard deviation obtained from three tensile tests of the three samples are used for plotting. The measurement results show that the materials based on this manufacturing method have good consistency and repeatability. Taking Ecoflex20∶RT625=0.2∶1 as an example, the actual stress-strain tensile test data of three samples are measured and plotted (as shown in Fig.5).

Fig.5 Cubic stress-strain curves at a tensile speed of 49.9 mm/min

2.3 Electrical performance testing setup

In addition to ductility, a key characteristic of DE electrodes is their electrical resistance, which depends on the volume resistivity of the material. Based on the method of making the above-mentioned stretched sample, the conductivity is measured. The sample is a rectangular strip with a length of 100 mm and a width of 10 mm. The high aspect ratioL/wis of the order of 10∶1 (at least 5∶1) to ensure a uniform current flow, so that the current can be in the center of the sample flow past. To avoid measuring contact resistance, a four-point probe method is used[13]. The laser-cut rectangular slender strips are put in DuPont wires at both ends to contact the strip electrodes scratched on the acrylic plate. The constant currentIis injected into the sample through these two DuPont wires, and then the source meter (Keysight E4980AL-102, USA) is used to measure the surface resistance (as shown in Fig.6). The voltage dropVis measured at a distanceLbetween any two points. Excessive force should be avoided during measurement, so as not to damage the sample and cause measurement error.

Fig.6 Four-point surface resistance measurement

The volume resistivity of an electrode changes uniformly with a thicknessd, a lengthLand a widthw. The volume resistanceRalong the length is

(3)

The electrode surface (or sheet) resistanceRsis defined as

(4)

Eqs.(3) and (4) can be combined to calculate the surface resistance of the part of the specimen with a length ofL, that is

(5)

2.4 DEA dynamic testing setup

A conical structure is designed in the testing to study the output performance of DEA. A linear motor is used to connect the linear spring to deform the center of the DEA film 4 mm out of the plane. DAQ is connected through MATLAB (MathWorks, USA) to generate a sinusoidal signal with a frequency range of 0 Hz to 150 Hz (with a speed of 1 Hz/s) to a high-voltage amplifier (10/40A-HS-H, TREK, USA). The output voltage amplitude is 4 132 V, and the output displacement of DEA is measured with a laser displacement sensor (LK-G153, Keyence, Japan). The laser displacement sensor records the DEA position in real time at a sampling frequency of 2 kHz, where the schematic of testing setup is shown in Fig.7. The real testing setup is shown in Fig.8.

Fig.7 Schematic of setup

Fig.8 Experimental testing setup

3 Results and discussion

3.1 Mechanical properties

According to pre-experimental judgments, the hardness of the compliant electrode increases with the increased mass fraction of conductive carbon black for the specified ratio of Ecoflex20 and RT625. But the ductility of compliant electrode decreases. The tensile performance deteriorates (as shown in Fig.9). Both changes are non-linear. The tensile performance is well, and it can be stretched to more than four times of the original when the mass fraction of conductive carbon black is 10%. At this time the performance of the compliant electrode is basically similar to that of the elastomer.

Fig.9 Stress-strain curves of different Ecoflex20 content (mass fractions of conductive carbon black are 10%, 15%, 20% and 25%, respectively)

The hardness is the lowest compared with other mass fractions. However, the hardness is the highest and the ductility is very poor when the mass fraction of carbon black is 25%. It will stretch to about twice as mush as when it breaks. As the proportion of Ecoflex20 increases, the hardness of the compliant electrode decreases, but the elongation is unpredictable.

Fig.10 depicts the tensile modulus of each electrode formulation combination (defined as the average slope before strain-stress is 40%). The tensile strength of Ecoflex20 is lower than that of RT625, and its modulus increases with the increase of the content of carbon black. It is obviously shown the tensile strength of compliant electrode decreases with the increase of Ecoflex20 content in the Fig.9.

Fig.10 Tensile modulus under different electrode formulations

3.2 Electrical performance

It can be clearly seen from Fig.11 that the conductivity of the compliant electrode becomes better with the increasing of the carbon black. In particular, the conductivity of the compliant electrode obviously increases when the mass fraction of the carbon black is greater than 15%. Simultaneously test different samples from three uniform batches with very little error.

Fig.11 Conductivity under different mass fractions of carbon black

3.3 DEA output performance

There are many different formulations of compliant electrode in the above experiments. According to the test results, there are some conditions for the selection of two compliant DEA electrodes. First of all, the choice is greater than 100%, so the percent of carbon black is less than 25%. Secondly, the next factor is conductivity, thus the formulation of compliant electrode chooses the mass fraction of conductive carbon black is 20%. As Ecoflex20 and DE film do not bond, the content of Ecoflex20 should be reduced as far as possible. Ecoflex20 is introduced to change the stiffness of the compliant electrode (Ecoflex20 is much less rigid than RT625), so the ratio of Ecoflex20 to RT625 is 0.2∶1. From what has been discussed above all, the percent of carbon black is 20%, Ecoflex20∶RT625=0.2∶1. The DEA is selected for dynamic study (The experimental platform is shown in Fig.8). The results in Fig.12(a) show that there is a displacement output of 1.189 mm under the excitation of 4 132 V high voltage with sine wave. The resonance frequency at that time is 130.9 Hz.

The comparison is made to better illustrate the clear advantages of DEA based on this approach. In order to have a clear comparison with other types of DEA output, a nominal displacement (displacement/radius) is used to define. Here the average peak amplitude of the three DEA samples is taken for comparison.A=1.118/15=0.074, other DEAs areA1=0.5/11=0.045 (as shown in Fig.12(b))[31]andA2=(2.56/2)/22.5 =0.056 (as shown in Fig.12(c))[32]. Thus, they are 1.64 times and 1.32 times higher than the other research performance, respectively.

Fig.12 Dynamic output and compare

The production of DEA gradually tends to automate the production of equipment (e.g. pad printing, screen printing). It is necessary to develop a kind of compliant electrode with long-curing time. So the study of compliant electrode is particularly important. Compliant electrodes made based on this method can be used as ink formulations, this method can extend the curing time. Two inks with the same mass fraction of carbon black (pure RT625 and Ecoflex20+RT625) are compared after curing for two hours at the same humidity and temperature. It is shown clearly in Fig.13.

It is found that the ink added with Ecoflex00-20 can still be the same as the beginning, the solution is uniform, and the brightness is obvious (Fig.13(a)). However, pure RT625 is about to solidify while taking on the shape of bean paste (Fig.13(b)). It has obvious advantages for screen printing, spray coating, spin coating, pad printing and casting processes. It has the characteristics of improving manufacturing efficiency and reducing costs for comparing with carbon paste in the production process of DEA.

Fig.13 Physical drawing of ink

4 Conclusions

A method is proposed for manufacturing a carbon-based composite double polymer compliant electrode. It conducts stress-strain test and conductivity test on the manufactured electrode. And they are printed on DE film for electrokinetic test. The experimental results show the hardness and conductivity of the compliant electrode increase non-linearly with the increase of the conductive carbon black mass fraction. On the contrary, the ductility decreases. Among all the compliant electrode formulations, the ductility of conductive carbon black is the best at 10% mass fraction and the worst at 25%. The ductility decreases sequentially, but the stiffness increases sequentially. As the proportion of Ecoflex20 increases, the hardness of the compliant electrode gradually decreases. The compliant electrode is attached to the DE film by pad printing. The DEA power-up dynamic experiment is carried out, which can produce a large displacement output (1.189 mm). Therefore, it is 1.64 times and 1.31 times the output of the same type of DEAs. These results show that the compliant electrode made based on this method has a higher application prospect. Furthermore, the manufactured method provides a reference for the future research of DEAs. It also provides an ink production method for the automated production of DEAs in the future. All in all, the curing electrode based on this method can be used to improve production efficiency. Meanwhile, the production costing can be reduced during the production of multi-layer DEAs.