RESEARCH OF THE ACTUATORS CHARACTERISTICS BASED ON DIELECTRIC ELASTOMERS
The results of experimental studies of the characteristics affecting the movement of actuators based on dielectric elastomers under the action of an external electric field are given. The influence of the concentration of fillers: barium titanate, burnt and unburned quartz on the modulus of elasticity and deformation of the dielectric elastomer at high control voltage was revealed.
Actuators based on "intelligent" or "smart" materials are used for precise movement of objects (machined items, tools) in micro- and nanotechnology equipment (lithography facilities, scanning probe and electron microscopes, etc.). These include, among others, various hydraulic devices based on controlled fluids capable of rapid and controlled transformation from liquid to plastic and solid states in external fields , actuators based on magnetostrictive materials, which are compact and powerful , actuators using magnetorheological elastomers  and piezoelectric materials . The latter ones are most commonly used as micro- and nano-movement mechanisms. In their operating principle, piezoelectrics are similar to dielectric elastomers (DE), a subgroup of electroactive polymers. Dielectric elastomer actuators are electromechanical transducers that function by deforming DE when an external electric field is applied to them. Due to such characteristics as low modulus of elasticity, wide range of created displacements and fast response time, the use of DE makes it possible to develop fundamentally new motion devices with improved characteristics.
The main indicator that characterises effectiveness of DE application is the range of deformations created under the influence of an external electric field. It was found that one of the key characteristics influencing the deformation is the dielectric permittivity which can be controlled by the amount of dielectric filler in the DE composition. For example, in  it was found that addition of graphene nanoplates to DE significantly improved the electrical and mechanical characteristics of the sample, compared to a pure rubber sample, by increasing deformation from 1.3% to 2.4%. The concentration of the filler was 23%, but the technology of mixing and manufacturing the components is rather difficult to implement, requires special equipment and takes a long time [6, 7].
Acrylic-based polymers are often used as a matrix because they exhibit large deformations of up to 380% and have a relatively high dielectric constant (≈4.7). However, due to the high viscosity, acrylic exhibits a slow response and a long recovery time. To circumvent these problems, low-viscosity materials such as silicone rubber are often used instead of acrylic. The dielectric constant of such materials is lower (≈2.8), so it is necessary to select a filler with the highest dielectric constant . One of the highly effective fillers are carbon nanotubes, whose modification is capable of increasing dielectric permittivity and reducing the elastic modulus, but the manufacturing technology for such DE is extremely expensive . Barium titanate is considered to be well compatible with the elastic matrix due to its piezoelectric properties, high dielectric constant and low dielectric loss [10, 11].
In this work, barium titanate was used as a base filler to reveal the effect of the composition on the DE movements created under the action of an external electric field. Quartz is also similar in piezoelectric properties and mechanical characteristics but is more accessible filler which was used as an alternative to the selected filler. Thus, 10 batches of samples with different mass concentrations of the filler – burnt and unburnt quartz and barium titanate – were made. These samples had the composition shown in Table 1. Set No. 10 had no filler. Silicone of SIEL 159–322A grade was used as a matrix.
The elasticity modulus has a significant influence on the characteristics of the DE. Increase of the stiffness contributes to reduction of deformation under the influence of electric field. In order to find out the influence of elasticity of the samples on the displacements created, it is necessary to know the modulus of elasticity of each of them. In order to study the elasticity modulus of DE, two identical samples were made in each batch. Tests were performed on an Instron DX600 hydraulic press. Before stretching, the sample was placed in a special vice where it was held in a free state. Tensioning was carried out at a speed of 10 mm/min under a fixed load.
The measurements resulted in the plots illustrated dependence of the generated stress σ on the relative elongation ε. Fig.1 shows the obtained tensile stress dependences for samples with barium titanate, which showed the greatest increase in elastic modulus when the concentration was increased.
Since 2 compression and 2 tension samples were produced in each batch, their average modulus of elasticity was considered to be more reliable. The results with tensile and compression modulus values are presented in Table 2.
It is evident that the stiffness of elastomers increases with increasing filler concentration. When studying the samples subject to tension at the filler concentration increasing from 33% to 66%, it was recorded that the quartz-modified samples exhibited elasticity modulus by 5.75 times, the samples modified with burntquartz exhibited 4.2 times and the barium titanate-modified samples exhibited 1.7 times higher elasticity modulus.
For compression samples with quartz, an increase in concentration from 33% to 66% increased the modulus of elasticity by a factor of 3.5, with burnt quartz by a factor of 4, with barium titanate by a factor of 2. Thus, modification with quartz increases the modulus of elasticity by on average of 2 times more effectively than barium titanate.
From the previously mentioned samples for each batch, 4 actuators with different number of layers were made by bonding elastomers with electrodes: one-layer, two-layer, four-layer and eight-layer. These samples are shown in Fig.2.
With the help of the developed laboratory bench at the Bauman Moscow State Technical University, displacement measurements of manufactured actuators based on DE with different concentrations and different number of layers were carried out. A voltage of up to 3,000 V from a high voltage power supply was applied to the samples.
The diagram of the test bench is shown in Fig.3. Voltage was applied by means of a high voltage power supply (HVPSU) to a voltage divider (VD) which was connected to the electrodes of the test sample. The sample itself was placed on an insulated surface over which a capacitive position sensor connected to the position transmitter control unit (PTCU) was installed. The sensor recorded the position of the test object while the voltage supplied from the unit was increased in steps from 0 to 3,000 V in 170 V increments.
Fig.4 shows the movement of the quartz-filled actuators as a function of the applied voltage at a 50% concentration. When the quartz concentration was increased from 33% to 66%, the maximum possible displacement doubled from 5 µm to 10 µm.
Increasing the burnt quartz concentration in DE from 33% to 66% increases the displacement twofold from 3.5 µm to 7 µm, indicating a positive effect of filler concentration on strain. The dependencies for samples with barium titanate as a filler do not show a similar trend. The displacements of the single-layer actuator vary only slightly with concentration. Most likely, the reason is inevitable air gaps between electrodes and DE disks which distort the experimental data and introduce uncertainty. However, there is a tendency for the displacement to increase as the number of DE layers in the actuator increases.
Deformation of DE in an external electric field is the result of various effects: electrostatic attraction of electrodes, orientation and displacement of dipole filler particles in an external field. The study of the influence of these factors on the characteristics of actuators is the task of further research.
The characteristics of dielectric elastomers depend primarily on the composite composition, the characteristics of the elastic medium – matrix and filler. A key factor is the concentration of the filler, which affects deformation and modulus of elasticity of dielectric elastomers.
It was found that for samples with quartz, when the concentration of the filler was increased (hereafter mass concentration) from 33% to 66%, the created strain increased by a factor of two. For samples with barium titanate as a filler, a similar trend was not observed. In tensile and compression tests and at increasing the filler concentration from 33% to 66%, the elasticity modulus of the quartz modified samples increased by a factor of 5.75 and 3.5, accordingly, with the burnt quartz modified samples the increase equaled 4.2 and 4 times and with the barium titanate modified samples it was 1.7 and 2 times accordingly.
The 66% mass concentration for the tested fillers is the most effective in terms of maximum deformation of the dielectric elastomer in an external electric field.
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Declaration of Competing Interest. The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.