Dielectric elastomer and ferroelectret films combined in a single device: how do they reinforce each other?
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- Wirges, W., Raabe, S. & Qiu, X. Appl. Phys. A (2012) 107: 583. doi:10.1007/s00339-012-6833-6
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Dielectric elastomers (DE) are soft polymer materials exhibiting large deformations under electrostatic stress. When a prestretched elastomer is stuck to a flat plastic frame, a complex structure that can be used as an actuator (DEA) is formed due to self-organization and energy minimization. Here, such a DEA was equipped with a ferroelectret film. Ferroelectrets are internally charged polymer foams or void-containing polymer-film systems combining large piezoelectricity with mechanical flexibility and elastic compliance. In their dielectric spectra, ferroelectrets show piezoelectric resonances that can be used to analyze their electromechanical properties. The antiresonance frequencies (fp) of ferroelectret films not only are directly related to their geometric parameters, but also are sensitive to the boundary conditions during measurement. In this paper, a fluoroethylenepropylene (FEP) ferroelectret film with tubular void channels was glued to a plastic frame prior to the formation of self-organized minimum-energy DEA structure. The dielectric resonance spectrum (DRS) of the ferroelectret film was measured in-situ during the actuation of the DEA under applied voltage. It is found that the antiresonance frequency is a monotropic function of the bending angle of the actuator. Therefore, the actuation of DEAs can be used to modulate the fp of ferroelectrets, while the fp can also be taken for in-situ diagnosis and for precise control of the actuation of the DEA. Combination of DEAs and ferroelectrets brings a number of possibilities for application.
Electromechanically active polymers are extensively studied and are widely used in various applications. Among them, ferroelectrets (i.e., internally charged polymer foams or void-containing polymer-film systems) have attracted considerable interest in research and industry due to their large piezoelectricity combined with mechanical flexibility and elastic compliance [1, 2]. Voids in ferroelectrets can be charged by means of dielectric barrier discharges (DBDs) under high electric fields . After charging, the voids carry space charges of opposite polarities on their top and bottom inner surfaces, respectively, and thus can be considered as macroscopic dipoles. The man-made dipoles can be easily deformed under mechanical or electrical stress, resulting in large piezoelectricity.
Extensive studies on ferroelectrets were initiated about two decades ago by colleagues from Finland using cellular polypropylene (PP) . Since then, cellular PP has become the workhorse of ferroelectret research and technology mainly due to its ease of processing. However, the piezoelectricity of cellular PP ferroelectrets is stable only up to 60°C, hindering their use in more demanding applications . Therefore, polymers with better charge stability have been employed for ferroelectrets. Recently, fluoroethylenepropylene (FEP) ferroelectrets with well defined and uniform voids have been produced by means of a straightforward lamination process . The piezoelectricity is stable at least up to 130°C when the sample is charged at suitable elevated temperatures. Thus, the application range of ferroelectrets is greatly broadened. The lamination process is quite promising with advantages of simplicity and suitability for a wide range of polymers.
The large deformation of DEs under applied E is the basis for a wide range of potential applications such as artificial muscles, microrobots, energy harvesters, adaptive optical elements, etc. As indicated by (1), the actuation of a DE film is dependent on the applied electric field. For more precise control and for possible further optimization of the device performance, an in-situ detection of the actuation is highly desired. One approach to this issue is to employ self-sensing technique in which the electrical variables such as capacitance and leakage current of the DEA are measured directly during operation for characterization of the performance of the device [11–13].
Recently, a variety of operation schemes are explored with DE films for improved and/or novel properties. For example, it has been shown that applying a certain level of prestretch to DE actuators can further improve their performance [14, 15]. When a prestretched DE film is fixed to a flexible plastic frame, the frame changes its shape upon release of the DE film from the prestretch, and finally a complex structure with minimum of free energy is formed [16, 17]. The structure deforms toward the initial planar state when a voltage is applied, and thus can be used as bending actuator . In this paper, the possibility of combining DE and ferroelectret films in a single device is explored. An FEP ferroelectret film was mounted to a DE bending actuator with minimum-energy structure. It is demonstrated that the electromechanical parameters of the FEP ferroelectret film such as the anti-resonance frequency fp exhibit monotropic dependence on the bending angle of the DEA. Therefore, combination of DEs and ferroelectrets brings a variety of possibilities for application.
Ferroelectret sample with well-defined tubular channels was prepared by means of a thermal lamination technique described in . In this process, two polymer electret films are laminated around a template between them. The template, which can be made of metal foils or of polymers whose melting temperature is higher than that of the electret films, contains regular openings through which the electret films can be fused with each other. Lamination is performed at a temperature substantially higher than the melting temperature of the electret films yet lower than that of the template. After the outer layers have been fused, the template is removed, resulting in a polymer-film system with open void channels. Here, 25 μm thick FEP films were laminated around a polytetrafluoroethylene (PTFE) template at 300°C. The template was fabricated from a 50 μm thick Teflon PTFE film with an area of 62 mm×40 mm. Six parallel rectangular openings (1.5 mm×50 mm) were cut by means of a computer-controlled laser system. The PTFE ridges between adjacent openings also had a width of 1.5 mm. After cooling down under laboratory conditions, the FEP layers are permanently fused with each other through the openings of the template. The PTFE template was then removed after cutting it open at one end, and the sample was metallized on both sides in the central areas with aluminum electrodes 10 mm in width (in the direction parallel to the channels) and 20 mm in length (in the direction perpendicular to the channels).
In order to charge the FEP film system with tubular channels, a dc voltage of 4 kV was applied to the Al electrodes for 10 s by a high voltage power supplier (Trek model 610D). The piezoelectric d33 coefficient was determined by means of dynamic mechanical excitation of the sample with a sinusoidal force with an peak-to-peak amplitude of 1 N at a frequency of 2 Hz (Brüel&Kjaer model 4810 shaker), superimposed with a static force of 3 N. The resulting electrical response of the sample was amplified with a Brüel&Kjaer model 2635 charge amplifier. From the applied force and the resulting electrical signal, the piezoelectric d33 coefficient was calculated. Dielectric resonance spectra (DRS) of the FEP ferroelectret film were recorded in situ using a Novocontrol ALPHA high-resolution dielectric analyzer. The sample was connected to the equipment via thin copper wires.
3 Results and discussion
3.1 Piezoelectricity of FEP ferroelectret film with tubular channels
In (2), εr, A, h, kt, and fp are the relative permittivity of the sample, the electroded sample area, sample thickness, the complex electromechanical coupling factor, and the complex anti-resonance frequency of the fundamental TE mode, respectively.
3.2 Performance correlation between dielectric elastomer and ferroelectret films combined in a single device
For each applied voltage, the DRS of the FEP ferroelectret film was measured. It is found that the DRS is sensitive to the bending angle. The inset of Fig. 6 shows the imaginary part of the capacitance of the ferroelectret film when the film is flat and when it is bending with the DEA at an applied voltage of 0 V or 2 kV. As can be seen from the figure, the fp of the ferroelectret film clearly shifts with the voltage applied to the actuator. At V=0, an fp of 93 kHz is determined, 22.5 kHz higher than the value obtained when the ferroelectret film is flat. With increasing applied voltage, the bending actuator unfolds towards the flat state, and accordingly, fp decreases to about 80 kHz at V=2 kV.
As mentioned in the previous subsection, the FEP ferroelectret film exhibits extremely low c33 coefficient during the DRS measurement. With increasing external stress, the c33 increases sharply, leading to substantial decrease of the d33 coefficient (cf. Fig. 4). When the DEA bends, the stress exerted to the ferroelectret film is more complex. In the curved part of the ferroelectret film, there is a so-called neutral surface where the stress is zero, while the outer and the inner part of the film are subjected to tensile and compressive stress, respectively . The bigger the bending angle, the larger are the stresses. Our results show that the c33 coefficient obtained by means of DRS is quite sensitive to the stresses generated during bending. The c33 coefficient increases with increasing bending angle. At an bending angle of 60∘ (corresponding to an applied voltage of 2 kV), a c33 coefficient of 0.41 MPa was measured, while at 101∘ (V=0), a c33 of 0.55 MPa was determined from the DRS, which is about 70% higher than the value obtained when the ferroelectret film is flat. Consequently, fp increases with increasing bending angle, as can be seen from Fig. 7.
Such a monotropic dependence of the fp on the bending angle of the actuator is very promising for potential applications. After calibration, the fp of the ferroelectret film can be taken for in-situ diagnosis and for precise control of the DE actuators. This approach has the advantages of low cost, simplicity in manufacture and ease of operation, as compared with the self-sensing method which normally requires an oscillating driving signal carried out using usually expensive high voltage switching. On the other hand, fp is one of the most important parameters for the application of ferroelectrets because it directly affects the frequency response of devices. Aided by a DE bending actuator, a single piece of ferroelectret film can have an fp covering a certain frequency range. Such a ferroelectret film can be used as a filtering sensor, detecting signals only around the designed frequency range. Also, the fp of ferroelectrets can be adjusted according to the requirements of a given practical application by applying a suitable voltage to the combined DEA, so that the devices can be operated either around or far away from the fp.
The possibility of combining dielectric elastomer actuators (DEA) and ferroelectrets into a single device has been demonstrated. A self-organized minimum-energy structure that can be used as bending actuator was produced by sandwiching a prestretched elastomer between a flexible plastic frame and rigid frame pieces. Such an actuator was equipped with an FEP ferroelectret film containing tubular channel voids. The bending angle of the DEA was controlled by applying a voltage to the electrodes of the DE films, and the dielectric resonance spectrum (DRS) of the ferroelectret film was measured in-situ during the actuation of the DEA. It turns out that the antiresonance frequency is a monotropic function of the bending angle of the actuator. Therefore, combination of DEAs and ferroelectrets opens up various new possibilities for application. The fp of ferroectrets can be taken for in-situ diagnosis and precise control of the actuation of DEA devices. Also, the actuation of DEAs can be used to modulate the fp of ferroelectrets in order to meet the requirements of given applications. The concept proposed here has the obvious advantages of low cost, simplicity in manufacture, and ease of operation.
The authors are indebted to Mr. Matthias Kollosche and Mr. Nicolas Marroquin Jacobs (both University of Potsdam) for stimulating discussions and to the European Union for cofounding some of the equipment used in their work.