novel heat durable electromechanical film: processing for...

IEEE Transactions on Dielectrics and Electrical Insulation Vol. 13, No. 5; October 2006

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Novel Heat Durable Electromechanical Film: Processing for Electromechanical and Electret Applications

Eetta Saarimäki, Mika Paajanen, Ann-Mari Savijärvi and Hannu Minkkinen

VTT Technical Research Centre of Finland Sinitaival 6

33720 Tampere, Finland

Michael Wegener, Olena Voronina, Robert Schulze, Werner Wirges, and Reimund Gerhard−Multhaupt

University of Potsdam Department of Physics Am Neuen Palais 10

14469 Potsdam, Germany

ABSTRACT New ferroelectrets were developed on the basis of foams from cyclo-olefin polymers and copolymers. The results obtained on the cyclo-olefin polymer foam demonstrate a significant improvement of the service temperature for ferroelectret transducer materials. Suitable compounding and preparation led to cyclo-olefin ferroelectrets with an electromechanical activity of around 15 pC/N, which is thermally stable at least up to 110°C. The properties in sensor and actuator applications are strongly dependent on the processing parameters related to film-making, sensor and actuator preparation, gas content and electric charging. The processing window for the film stretching was very narrow compared to the earlier developed polypropylene ferroelectrets. The film porosity, softness and thus the electromechanical activity are adjusted by gas-diffusion expansion. The activity of the electromechanically operating sensors and actuators was increased by stacking several layers of cellular cyclo-olefin film. For applications such as flat loudspeakers, the foamed films are tuned by tensioning them on a support frame. Correct tensioning was essential also for reducing the distortion levels.

Index Terms — Electromechanical effects, electrets, ferroelectrets, cyclo-olefin polymers, cyclo-olefin copolymers, plastic films, actuators, transducers, stability.

1 INTRODUCTION

ELECTROMECHANICAL transducers, including those that are based on electrostatics, cover a wide range of sensor and actuator applications. In the past, electrically poled ferroelectric polymers as well as electrically charged (non-voided) space-charge electrets were mostly used as soft transducer materials in these applications. However, within the last decade or so, a new class of transducer materials was discovered and developed. These so-called ferroelectrets can be implemented as functional materials in electromechanical sensors and actuators or as flexible, electrically charged layers in electrostatic transducers.

Ferroelectrets are voided space-charge electrets that show electromechanical properties after suitable electric charging. The workhorse in the field of ferroelectrets is cellular polypropylene (PP) film. The development of cellular PP film

for electromechanical-transducer purposes started in the late eighties and accelerated in the late nineties [1-3]. Since then, the basic working mechanisms, the optimization of foam preparation, as well as the resulting electromechanical properties of ferroelectret transducers were explored in more detail, as described in recently published review articles [4, 5]. Some details are: (i) the PP foam preparation by extrusion of filler-loaded polymers [6], (ii) the optimization of void structures by various procedures of gas-diffusion expansion (GDE) [7-11], (iii) the internal void charging by micro-plasma discharges within the voids [12], (iv) the required threshold electric fields that must be applied in order to initiate the internal discharges [13-15], as well as (v) the dependence of the electromechanical properties on the elastic stiffness of the foam [16]. The electromechanical properties connected with high piezoelectric coefficients are described in the above-mentioned articles too. However, several other applications-relevant properties were also investigated. Examples are Manuscript received on 31 March 2006, in final form on 28 June 2006.

E. Saarimäki et al.: Novel Heat Durable Electromechanical Film: Processing for Electromechanical and Electret Applications 964

piezoelectric thickness resonances [17], thickness-extension resonance frequencies [9], applications in microphones or loudspeakers that operate either in the electromechanical (or (quasi)-piezoelectric) mode [18-22] or in the electret mode of a classical electrostatic transducer [2]. The quasi-piezoelectric or electromechanical operation is based on changes in the film thickness, while the electret operation relies on the movement of the film with respect to an air gap. Due to the larger range of electret and thus also charge motion, transducers working in the electret mode show higher transducer coefficients [23]. However, their preparation and handling is often more difficult than that of a single (electromechanical) transducer film.

PP ferroelectret films are implemented in applications such as pick-ups for musical instruments, motion sensors for health-care applications or sensors for touch panels (see the review [5] and references therein). However, the use of PP-ferroelectret transducers is limited because of their maximum service temperature in the range between 60 and 80 °C, depending on preparation and required electromechanical activity [24, 25].

The experience with PP ferroelectrets was also the basis for developing ferroelectrets from other polymers such as polyethylene terephthalate (PETP) [26-28] or cyclo-olefin polymers (COP) and copolymers (COC) [29-31]. With cellular PETP ferroelectret films, the service temperature is slightly enhanced [26]. A much larger improvement is, however, expected if thermally more stable base materials such as suitable cyclo-olefin polymers and copolymers are prepared and employed in ferroelectret transducers.

Thus, the aim of this study is the development of heat-durable sensors and actuators from novel electromechanical cyclo-olefin based polymer films. Here, we discuss the manufacturing and voiding of several cellular COP and COC films as well as their electromechanical properties. We focus on the improvement of the stability of the electromechanical activity by means of short- and long-term thermal treatments and on its evaluation. Furthermore, we describe in detail the prototype manufacturing of electromechanical (eight-layer stack) and electrostatic transducers and their characterization in the audible frequency range. Novel heat-durable cellular electromechanical films have wide potential to be used in electromechanical and electret applications because of their good heat resistance up to 110 °C.

2 EXPERIMENTAL DETAILS

2.1 POLYMER MATERIALS, COMPOUNDING, AND CASTING OF FILMS

A cyclo-olefin copolymer (COC) grade Topas 6015 from Ticona and a cyclo-olefin polymer (COP) grade Zeonor 1420 from Zeon Chemical Company were used as base polymers for the manufactured compounds. Both base polymers are high-temperature grades with glass-transition temperatures (Tg) of 150 and 140 °C, respectively.

These cyclo-olefin polymers were blended with lower-Tg olefin polymers (labelled as 1 and 2 in the following). The blend compositions are shown in Table 1. All compounds contain 5 wt.% of a suitable mineral filler, with an average particle size of 2 μm. The polymers and the filler were compounded before film casting in order to insure proper mixing of the components. The cyclo-olefin compounds were prepared by a twin-screw extruder (Berstorff ZE25 x 48D). The following processing parameters were chosen for the compounding: an output rate of 6 kg/h; a rotation speed of the screw of 200 revolutions per minute (rpm); a temperature of 260 °C. Vacuum de-gassing was used during compounding, as the materials were not pre-dried. The filler was dosed into the molten polymer mixture.

Table 1. Manufactured compounds.

SampleCOC COP Polymer 1 Polymer 2

ID Topas 6015 Zeonor 1420R(%) (%) (%) (%)

1 25 752 50 503 75 254 90 10

5 25 756 50 507 75 258 90 10

Polymer contents in the various blends

Cast films were extruded on a Brabender Extrusiograph

PL 2100-6 extruder having a conical compression of 3:1. Cast films were driven to a conveyor belt and cooled with pressurized air. The screw-rotation speed was 50 rpm, and the screw heating-zone temperatures were between 260 and 290 °C. Sample 6 was additionally extruded on a bigger pilot-scale Dolci single-screw extruder with a D/L ratio of 27 (L = 45 mm, D = 1210 mm). The screw was specifically designed for the cyclo-olefin polymers. Casting was performed on a metal casting drum heated to 60 °C. A screw-rotation speed of 50 rpm was employed and the extrusion heating-zone temperatures varied between 250 and 270°C. The temperature was held at 260 °C for all compounds. Before casting, the compounds were pre-dried for 2 h at 90 °C and at a low pressure.

Dynamical mechanical thermal analyses for the cast films were performed with a PolymerLabs DMTA model MKII by heating the samples from 0 to 200 °C, while the tensile deformation frequency was kept constant at 10 Hz.

2.2 FROM CAST FILMS TO VOIDED FILMS The COC-based cast films were drawn using a Brückner

laboratory-scale stretcher model Karo IV. Biaxial drawing was performed simultaneously in both directions (machine direction “MD” and transverse direction “TD”) to a draw ratio of 3.2 × 3.2. During stretching, a preheating time of 120 s, a strain rate of 0.5 m/min and a fan power of 100% were employed. The voided structures were achieved at a


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particular drawing temperature, which varied between 165 and 185 °C, depending on the blend composition, i.e. the amount of the lower-Tg polymer.

The COP-based samples were drawn sequentially in MD on a pilot-scale stretching line and in TD using an Instron model 4500 extensometer equipped with a hot box. Drawing temperatures in both directions were between 145 and 170°C. Maximum draw ratios (at selected temperatures and draw rates) were varied between 2 and 3.5 in MD and between 3.0 and 5.0 in TD. A draw rate of about 5m/min was used for MD stretching. The TD stretching was performed by using a preheating time of 2 min and moderate strain rates (0.5 – 1 m/min). The cast film sample 6 manufactured by means of the Dolci extruder and drawn in MD on a pilot-scale unit was also drawn off-line 3.6 times in TD on a pilot-scale line using a line speed of 3.5 m/min. The temperatures in the preheating and drawing zones were adjusted to 140 and 144 °C, respectively. Optimization of void dimensions in the film-thickness direction is performed by gas-diffusion expansion (GDE). The GDE procedure contains two preparation steps: (i) the saturation phase and (ii) the expansion phase. During the saturation phase (i), gas (here N2) with a high pressure was diffused inside the cellular structure. During the expansion phase (ii), the increase of the film temperature reduces the film stiffness accompanied by a gas release and an expansion of the film. For gas penetration and saturation (i), the samples were treated in a high-pressure chamber, for expansion (ii), the samples were heat-treated in a fan oven. The GDE step can be varied by the adjustment of at least five parameters, such as saturation time, temperature and pressure, as well as expansion temperature and time.

Before the GDE treatment, the samples were tightened between two frames in order to avoid shrinkage during the expansion phase. Gas saturation of the samples was usually achieved with a treatment at 6 MPa (60 bar) for 30 min at 20 °C. The expansion temperatures were varied between 110 and 150 °C, depending on the particular cyclo-olefin compounds. At these temperatures, a treatment lasted 30 s.

Scanning-electron-microscope (SEM) images were recorded with a JEOL scanning electron microscope model T100. Before the SEM investigation, the samples were fractured in liquid nitrogen in MD as well as in TD.

2.3 ELECTRICAL AND ACOUSTICAL METHODS FOR CHARACTERIZATION

Foamed COP films were electrically charged either in a point-to-plane corona discharge or in direct contact. Corona charging is performed in different ambient gases such as air, nitrogen (N2, with pressures up to 0.7 MPa (7 bar)) or sulfur hexafluoride (SF6, with pressures up to 0.15 MPa (1.5 bar)). The N2 or SF6 gas surrounding the film allows charging with high electric fields, which leads to high internal charging levels and thus also to high electromechanical activity, as already demonstrated for PP

ferroelectrets [7, 15]. Here, tip-voltages of up to −60 kV and charging times of 60 s were used during corona charging. Charging of cellular cyclo-olefin polymer films in direct contact was performed in air on samples metallized on both sides. The charging time was typically 10 s. After charging and before characterisation, the samples were stored between aluminium foils in order to connect both surfaces and to allow for fast relaxation processes (that are not relevant here).

Before electromechanical characterisation, all samples were metallized on both outer surfaces with aluminium or gold layers. Furthermore, the samples were stored for well-defined times under laboratory conditions in order to achieve stable charge trapping within the sample.

The piezoelectric d33 coefficients are determined by means of a dynamic mechanical excitation of the sample at a force amplitude of 1 N at a frequency of 2 Hz with a Brüel & Kjaer model 4810 shaker. In addition, a bias force of 3 N is applied to the sample. The resulting electric response of the sample is amplified and recorded with an oscilloscope. The direct electromechanical d33 coefficient is calculated from the applied force and the resulting electric signal. The reported piezoelectric coefficients are average values from five separate measurements.

In order to investigate the thermal stability some of the samples were heat treated for one hour at different temperatures in the range from 50 to 120 °C in an oven. After heat treatment the samples were cooled down to room temperature. The piezoelectric coefficients were measured at room temperature with the dynamical method. For investigation of long-term thermal stability samples were heat treated at 90, 100 and 110 °C for different times until finally up to 200 h. In between the samples were cooled down to room temperature in order to determine the piezoelectric coefficient. After that, the heat treatment was continued until the next d33 measurement.

The acoustical measurements were performed with two different setups. The large-area loudspeaker (15 cm x 15 cm), operating in the electret mode, was investigated in the audio-frequency range up to 5 kHz, and the sound-pressure level (SPL) was recorded for 1, 3 and 5 % distortion levels at a distance of 30 cm above the actuator in a semi-anechoic room. The actuator stack operating in the electromechanical mode was investigated in a frequency range between 1 kHz and 17.5 kHz. The sinusoidal electrical signal with an amplitude of 80 V was applied to each single film in the stack. The fundamental sound and the harmonic distortion of the films and the stack were measured with a microphone (Microtech Gefell, MK 301), connected to a conditioning amplifier (Brüel & Kjaer Nexus), and recorded with a spectrum analyzer (Stanford Research SR 770 FFT). The power supply consisted of a custom-build system with a high-voltage amplifier and a function generator (Hewlett Packard HP33120a). The measurements were performed in an anechoic room in the acoustic far field at a distance of 0.5 m between the transducer and the microphone.


2.4 PREPARATION OF SENSORS AND ACTUATORS

After foaming and electrical charging, transducer films were prepared by depositing suitable metal electrodes. Electrical connections were applied with conductive tape and secured with conductive paint. Electrical connections on the film surface have been made flexible by cutting the conductive tape into a zigzag shape. Flexible connection ensures good adhesion between film and tape during film movements by avoiding the possible peeling of the metallization from the polymer film. Finally wires can be soldered to the conductive tape in order to connect the transducer film to other pieces of equipment.

Figure 1. Manufacturing of multi-layer sensors (or actuators) from heat-resistant electromechanical cyclo-olefin polymer films.

Figure 2. Geometry and operating principle of the electromechanical sensor (or actuator).

Multi-layer transducers were made by stacking several cyclo-olefin polymer films as demonstrated in Figures 1 and 2. Double-sided adhesive tape from 3M (Minnesota Mining and Manufacturing Co.) was employed for joining the individual transducer films. The same adhesive tape was also used for maintaining a fixed distance inside the loudspeaker (by means of stripes). Electrical connections to the films were prepared with 3M conductive aluminium

tape and SPI (Structure Probe, Inc.) conductive carbon paint.During stack preparation, problems arise from the gluing of the individual cyclo-olefin transducer films. Gluing created short circuits between the films, which were not noticed when checking any single layer. When short circuits were noticed in multi-layer sensors based on PP, large currents were applied to the system in order to “burn out” the short-circuiting points. This method did not work with the cyclo-olefins, instead volcano-like holes appeared in the film. The problems were solved by stacking multiple layers together with thin double-sided insulating tape, which, however, increased the overall stack thickness.

For the COP-based transducers, the transducer constants of each individual film were measured before stacking and after soldering the connections of the stack. Stacking with double-sided tape reduced the transducer constant slightly from 162 pC/N to 150 pC/N. The films were mounted onto a steel plate (90 mm × 100 mm × 6 mm) with an active sensor area of 60 mm × 60 mm. The flat loudspeaker actuators were tensioned on a stainless-steel frame with pore sizes of 2 mm and porosity of 40 %. The size of the porous steel plate was 175 mm × 175 mm × 1.5 mm and the active area of the actuators was 150 mm × 150 mm.

Flat loudspeakers were also manufactured for operation as electrostatic transducers, in the following called “electret mode”. In the electret mode, the operation was based on the movement of the ferroelectret film relative to an electrode across an air gap. For this purpose, the cyclo-olefin films were first negatively charged by contact charging and then metallized on one surface with an aluminium electrode by means of cathode sputtering. The films were prepared as loudspeakers by mounting them on a frame. Beside providing mechanical support, the frame functions also as the second electrode. Distance plates were used to enable the film movements in the gap between the electrodes. The perforated steel plate allows for air movements from and to the gap and thus reduces the build-up of counter pressures. Figures 3 and 4 show the preparation steps and the working mechanism of the “electret-mode” loudspeaker.

The most critical part of the loudspeaker manufacturing is the tensioning of the film. Tensioning is essential in order to avoid distortion effects that would be caused when the films touch the backing structure. Low tensioning of the film enabled high sound-pressure levels because of an optimal movement of the film in the air gap. Tensioning of the film had to be stronger than the electric forces between the electrodes, but the tensioning forces should not too much attenuate the electric forces moving the films. The optimal tensioning strain was calculated to be about one third of the values found for PP-based loudspeakers. In this study, the air gap between the electrodes (film and backing structure) was not optimized, since the real manufacturing of the electret-mode transducers is a compromise between the distance to the electrode, the cavity length and the tensioning force, even though the electric forces would increase when the distance to the electrode were decreased.

signal base

1 1 2 2 3 3 4 4 5 6 5 6 7 7 8 8

electrodes on the

reverse side

metallis_ _ _

metallis

metallis

metallis_ _ _

adhesive layer

+ similar layers with adhesive layer and film

1

1

2

2

Adhesives

Metallised films Backing

structure

Conductive paint

Conductive tape


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The best theoretical optimizations have generated too high distortion levels in real manufactured structures. Finally, the electrical connections to the films were produced with conductive tape and conductive paint.

Figure 3. Manufacturing steps for the electret loudspeaker.

Figure 4. Geometry and operating principle of a flat electret loudspeaker.

3 RESULTS AND DISCUSSION

3.1 CELLULAR STRUCTURE By increasing the content of the lower-Tg olefin polymer (polymer 1 or 2) in the manufactured compounds, the ductility of the COC- and COP-based compounds was improved. Figure 5 shows DMTA results of the tensile storage modulus E’ as a function of temperature for the COC-based compounds and the plain commercial grades Topas 6015 and polymer 1. In Figure 5, it can be seen that an increase of polymer-1 content in the COC compounds results in a decrease of the dynamical Tg and of the thermal stability of the compounds . At the same time, an increase of the polymer-1 content in the COC compounds improves the processability. A similar result is found for several compounds of polymer 2 with COP grade Zeonor 1420R. The voided microstructure of these cyclo-olefin films is obtained by drawing the initial cast films. Figure 6 shows the dependences of the DMTA-based loss-factor (tan δmax) maxima and processing temperatures of each COP-based compound on the content of COP grade Zeonor 1420R. The MD drawing temperature, which is about 15 °C higher than the peak temperature for tan δmax in the DMTA tests,

increases linearly with the content of COP grade Zeonor 1420R. The COP drawing temperatures on the Instron extensometer in TD also depend linearly on the content of the COP grade Zeonor 1420R and are close to the DMTA-based tan δmax peak temperatures also shown in Figure 6.

4

5

6

7

8

9

10

75 100 125 150 175temperature (°C)

tens

ile lo

g (E

') (P

a)

Sample 1Sample 2Sample 3Sample 4100% Topas 6015100% Polymer 1

Figure 5. DMTA measurements of the tensile storage modulus (E’) as a function of temperature for the compounds based on Topas grade 6015 and polymer 1. From left to right, the curves are for polymer 1, compound samples 1, 2, 3, and 4, and COC grade Topas 6015.

Figure 6. Temperatures of the tan δ maxima from DMTA experiments and processing temperatures of the COP-based compounds as functions of the content of COP grade Zeonor 1420R.

The biaxially drawn voided films contain flat lens-like voids, as seen in Figure 7 (top). With the subsequent GDE treatment, the film thicknesses were expanded typically by 50 to 100% (one GDE step). Figure 7 (bottom) shows a 56% expanded cyclo-olefin foam film. The starting point of the expansion was the same film, shown in Figure 7 (top).

A suitable foam preparation and thus an optimal cavity formation during extrusion and stretching are crucial for the possibility to later expand the films with GDE. When the films were stretched sequentially, it is important to achieve cavity formation inside the films already during the first MD stretching. If needle-like cavities were not already formed during MD stretching, the subsequent TD stretching did not form an adequate cellular structure that would enable gas diffusion and thus also expansion.

_ _ _ _ _

adhesive layer

distance plate perforated

backing structure

metallis.

_ _ _

Metallized film

Tensioned film

Distance plates

Perforated backing structure attached by gluing

Framed sensor/actuator


Figure 7. SEM images in MD of COC sample 4 after biaxial drawing (top) and after biaxial drawing plus a 56% gas-diffusion expansion (bottom). It was also noticed that all five expansion parameters affected the final expansion result. In particular, a suitable parameter set for the saturation rate, the gas pressure and the expansion temperature is required in order to achieve good expansion results. An expansion will not occur if the film is not properly saturated with the process gas, either because of a treatment at gas pressures below 5 MPa (50 bar) or because of too short treatment times or if the gas release occurs at too low a temperature.

The present series of GDE tests led to the conclusion that the saturation temperature could actually be decreased by increasing the saturation time. However, it was also noticed that it is not possible to decrease the saturation temperature by increasing the treatment time during expansion, since a permanent non-elastic expansion of the polymer matrix is necessary.

3.2 ELECTROMECHANICAL PROPERTIES Electric charging of voided COP (50% Zeonor 1420)

films with high corona voltages led to electromechanical activities of typically 10 to 20 pC/N, as shown in Figure 8. These values are quite comparable to those of ferroelectric PVDF and of non-inflated PP ferroelectrets. Some of the samples showed significantly higher electromechanical activity of around 43 pC/N. The variation in the observed piezoelectric d33 coefficients probably stems from structural differences that result from the fabrication of the cellular

sheets and from the GDE inflation process. However, the electromechanical activity was at least around 15 pC/N.

Figure 8. Electromechanical activity of cellular COP (50% Zeonor 1420) films as a function of the corona-point voltage during charging.

Charging of voided COP (50% Zeonor 1420) films in direct contact with electrode voltages of about 4 kV led to significant electromechanical activities of about 9 pC/N, as shown in Figure 9. During these first investigations, it was not possible to charge the cellular COP (50% Zeonor 1420) films in air with much higher voltages in direct contact without destroying them because of electrical breakdown across the sample thickness. The breakdown may be facilitated by the lack of non-voided skin layers on the outer surfaces and by the lower homogeneity of the cellular microstructure in comparison with the PP ferroelectrets which were developed over more than 15 years. Because only relatively low electric fields could be applied so far, a definite threshold for micro-plasma discharges could not yet be determined. However, the electromechanical activity of cellular COP films after charging with relatively low electric fields was proven experimentally.

Figure 9. Electromechanical activity of cellular COP (50% Zeonor 1420) films as a function of the electrode voltage during direct contact.

Further attempts to improve the electromechanical activity of COC and COP ferroelectrets are based especially on the optimization of the expansion process in order to decrease the elastic stiffness. However, the main aim of this study was the development of ferroelectrets that are thermally more stable than the PP and PETP ferroelectrets.


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3.3 THERMAL STABILITY OF FOAMED CYCLO-OLEFIN FILMS

Cellular COP (50% Zeonor 1420) samples were annealed in an oven for one hour at the temperatures indicated in Figure 10, in order to investigate their short-term thermal stability. Before the heat treatment, the samples had been corona-charged in SF6 at 0.15 MPa (1.5 bar) for 60 s, which lead to an electromechanical activity of 15 pC/N. After the heat treatment, the samples were stored at room temperature for 30 min in order to cool them before the measurement of the d33 coefficient. The electromechanical activity decreased only slightly to a value of 11 pC/N after a thermal treatment at 110 °C. Treatment of COP (50% Zeonor 1420) at 120°C led to slight changes of the sample dimensions (shrinking). Because of the irreversible changes, the electromechanical activity that remained after the treatment at 120 °C was not taken into account. Thus, a maximum short-term thermal stability of 110 °C was concluded for this kind of COP (50% Zeonor 1420) compound.

Figure 10. Remaining electromechanical activity in cellular COP (50% Zeonor 1420) film as a function of the treatment temperature. The line is only a guide for the eyes.

Figure 11. Remaining electromechanical sensitivities of the samples 2, 4, 7, and 8 after aging at the temperatures and for the time periods indicated.

For the evaluation of the short-term thermal stability, the cellular films had been treated for seven hours at various temperatures. The long-term stability at temperatures well

below the maximum possible temperature is, however, much more relevant for device applications. As an example, shown in Figure 11, heat treatment of sample 4 reduced the electromechanical activity from the value of 14.4 pC/N (after 24 h of heat treatment) only slightly to about 12.6 pC/N after 9 days at 110 °C. This demonstrates the enhanced thermal stability of the newly developed cyclo-olefin-based ferroelectret films. A similar thermal stability was found for several samples at 90 °C, as also shown in Figure 11. The lower-Tg compounds 1, 5 and 6 were stable only up to 70 °C. The reasons for the decrease, possibly an aging of the elasticity or a loss of trapped charge, are currently under investigation.

4.0 CHARACTERIZATION OF COC AND COP ACTUATORS

4.1 EIGHT-LAYER STACK OF COP FILMS FOR ELECTROMECHANICAL OPERATION

For stack preparation, voided COP films were charged, both sides then metallized and glued together with a double-sided adhesive tape. The electrical connections allowed the application of a voltage to each individual film separately or to several layers in the stack simultaneously. Voided COP films showed a relatively flat frequency dependence of the sound-pressure level (SPL), as shown in Figure 12. As expected, the SPL increases when more layers in the stack are driven simultaneously.

Figure 12. Frequency-dependent SPL radiated from a single COP (50% Zeonor 1420) film and from different numbers of layers in a stack.

The increase of the radiated sound-pressure level as a function of the number of simultaneously driven layers is shown in Figure 13 for the fundamental and the quadratic distortion. The SPL increase is not quite linear for a larger number of layers in the stack. The quadratic distortion lies, however, at least 20 dB below the fundamental sound.

As recently observed on stacks of PP ferroelectrets [22], the less than linear increase of the SPL with the number of layers originates from the strong additional mechanical load that is caused by the adhesive-tape films and by the added mass of the active layers themselves.


Figure 13. Selected SPL values of the fundamental sound and the quadratic distortion for different numbers of active layers in the COP (50% Zeonor 1420) stack during application of a sinusoidal electric voltage at 10 kHz.

4.2 COC AND COP ACTUATORS FOR AIR-GAP ELECTRET LOUDSPEAKERS

Cellular COP and COC films were also employed in electrostatic transducers. Here, the cellular film did not operate in the electromechanical mode. As discussed above, the charged cellular electret films were mounted over an air gap. The mechanical arrangement may be described as a device (“black box”) with electromechanical properties. The electrostatic actuators can also be characterized in terms of the radiated sound-pressure level. For loudspeakers made with charged COP and COC films, the maximum sound-pressure levels were achieved at 2.5 kHz. A cyclo-olefin-polymer-based actuator showed the best performance (SPL of 97.3 dB), and a cyclo-olefin-copolymer-based actuator produced 70.4 dB at 5 % distortion, as can be seen in Figures 14 and 15.

Two major factors, film tensioning and stiffness, lead to the different SPLs of the COP and COC actuators. The first demonstration loudspeaker with a heat-durable foamed electret was made with the cyclo-olefin copolymer. Thus, the film tensioning was higher than the design value (in order to avoid electrical discharges to the backing structure). In comparison to the COC films, the cyclo-olefin-polymer films are more rigid.

Figure 14. Maximum sound-pressure levels at three different distortions for a cyclo-olefin-polymer-based actuator.

Figure 15. Maximum sound-pressure levels at three different distortions for a cyclo-olefin-copolymer-based actuator.

Lower frequencies and higher SPLs could be attained with larger active loudspeaker areas and by optimizing the tensioning, the distance between the electrodes and the cavity length. Here, the active area of the loudspeakers was only 15 cm x 15 cm, which is less than 10 % of a typical PP-based electret loudspeaker. A larger area could lead to a 10 to 15 dB increase of the SPL.

5 CONLUSIONS

Cellular ferroelectret films with higher thermal stability for electromechanical and electret applications were made on a laboratory scale or on a pilot line.

For electromechanical applications, the average void heights inside the foams and thus their elastic stiffnesses were adjusted by means of gas-diffusion expansion. Electromechanical transducers were prepared from single or multiple layers of different cyclo-olefin polymers and copolymers. Some of the new cyclo-olefin ferroelectrets show good thermal stability up to 110 °C. This is a clear advantage in comparison to already existing PP and PETP ferroelectrets. However, the COP and COC ferroelectrets that were developed so far exhibit lower electromechanical activities than optimised PP and PETP ferroelectrets.

For classical electret applications with an air gap, such as electrostatic loudspeakers, the films did not undergo a GDE treatment. A flat electret loudspeaker with an air gap was built from cellular cyclo-olefin films. The loudspeaker produces a SPL of 97.3 dB with 5 % distortion.

There are still several challenges in the optimization of the film-making process, which should be fully adaptable to industrial production, because of the narrow temperature windows and since several parameters are critical for the micro-voiding during film stretching. Research and development will continue with new materials modified on a nano-scale and with the optimization of the parameters for large-scale film processing.

ACKNOWLEDGMENTS The authors would like to thank all partners of the

European Union FP5 Growth project G1RD00653 DURASMART for the fruitful cooperation in the project over three years and the EU for partial financial support. The group at the University of Potsdam gratefully acknowledges additional financing from the German Research Foundation within project GE 945/8-2.


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[4] S. Bauer, R. Gerhard-Multhaupt, and G.M. Sessler, “Ferroelectrets: Soft electroactive foams for transducers”, Physics Today, Vol. 57, No. 2, pp. 37-43, 2004.

[5] M. Wegener and S. Bauer, “Microstorms in cellular polymers: A route to soft piezoelectric transducer materials with engineered macroscopic-dipoles”, ChemPhysChem, Vol. 6, pp. 1014-1025, 2005.

[6] J. Raukola, N. Kuusinen, and M. Paajanen, “Cellular electrets – from polymer granules to electromechnically active films”, IEEE 11th International Symposium on Electrets (ISE 11), IEEE Service Center, Piscataway, NJ, pp. 195-198, 2002.

[7] M. Paajanen, M. Wegener, and R. Gerhard-Multhaupt, “Understanding the role of the gas in the voids during corona charging of cellular electret films – A way to enhance their piezoelectricity,” J. Phys. D: Appl. Phys., Vol. 34, pp. 2482-2488, 2001.

[8] M. Paajanen, H. Minkkinen, and J. Raukola, “Gas diffusion expansion – increased thickness and enhanced electromechanical response of cellular polymer electret films”, IEEE 11th Intern. Symposium on Electrets (ISE 11), IEEE Service Center, Piscataway, NJ, pp. 191-194, 2002.

[9] M. Wegener, W. Wirges, R. Gerhard-Multhaupt, M. Dansachmüller, R. Schwödiauer, S. Bauer-Gogonea, S. Bauer, M. Paajanen, H. Minkkinen, and J. Raukola, “Controlled inflation of voids in cellular polymer ferroelectrets: Optimizing electromechanical transducer properties”, Appl. Phys. Lett., Vol. 84, No. 3, pp. 392-394, 2004.

[10] M. Wegener, W. Wirges, J. Fohlmeister, B. Tiersch, and R. Gerhard-Multhaupt, “Two-step inflation of cellular polypropylene films: Void−thickness increase and enhanced electromechanical properties”, J. Phys. D: Appl. Phys., Vol. 37, No. 4, pp. 623-627, 2004.

[11] X. Zhang, J. Hillenbrand, and G.M. Sessler, “Improvement of piezoelectric activity of cellular polymers using a double-expansion process”, J. Phys. D: Appl. Phys., Vol. 37, pp. 2146-2150, 2004.

[12] M. Lindner, S. Bauer-Gogonea, S. Bauer, M. Paajanen, and J. Raukola, “Dielectric barrier microdischarges: mechanism for the charging of cellular piezoelectric polymers”, J. Appl. Phys., Vol. 91, No. 8, pp. 5283-5287, 2002.

[13] M. Wegener, M. Paajanen, W. Wirges, and R. Gerhard-Multhaupt, “Corona-induced partial discharges, internal charge separation and electromechanical transducer properties in cellular polymer films”, in Proceedings, 11th International Symposium on Electrets (ISE 11), IEEE Service Center, Piscataway, NJ, pp. 54-57, 2002.

[14] R. Gerhard-Multhaupt, M. Wegener, W. Wirges, J.A. Giacometti, R.A.C. Altafim, L.F. Santos, R.M. Faria, and M. Paajanen, “Electrode poling of cellular polypropylene films with short high-voltage pulses”, IEE Conf. Electr. Insul. Dielectr. Phenomena (CEIDP), IEEE Service Center, Piscataway, NJ, pp. 299-302, 2002.

[15] X. Qiu, M. Wegener, W. Wirges, X. Zhang, J. Hillenbrand, Z. Xia, R. Gerhard-Multhaupt and G. M. Sessler, “Penetration of sulfur hexafluoride into cellular polypropylene films and its effect on the electric charging and electromechanical response of ferroelectrets”, J. Phys. D: Appl. Phys., Vol. 38, pp. 649-654, 2005.

[16] M. Wegener, E. Tuncer, W. Wirges, R, Gerhard-Multhaupt, M. Dansachmüller, S. Bauer-Gogonea, R. Schwödiauer, and S. Bauer, “Ferroelectrets: Highly anisotropic electrically charged polymer foams for electromechanical transducer applications”, IEEE Ultrasonics Symposium, IEEE Service Center, Piscataway, NJ, pp. 1138-1141, 2004.

[17] G.S. Neugschwandtner, R. Schwödiauer, S. Bauer-Gogonea, S. Bauer, M. Paajanen and J. Lekkala, “Piezo- and pyroelectricity of a polymer-foam space-charge electret”, J. Appl. Phys., Vol. 89, No. 8, 4503-4511, 2001.

[18] J. Backman, “Audio applications of electrothermomechanical film (ETMF)”, J. Aud. Eng. Soc., Vol. 38, pp. 364-371, 1990.

[19] R. Kressmann, “New piezoelecric polymer for air-borne and water-borne sound transducers”, J. Acoust. Soc. Am., Vol. 109, No. 4, pp. 1412-1416, 2001.

[20] M. Wegener, S. Bergweiler, W. Wirges, A. Pucher, and Reimund Gerhard-Multhaupt, “Voided space-charge electrets – Piezoelectric transducer materials for electro-acoustic applications”, Proc. 116th Convention of the Audio Engineering Society, Convention Paper 6149, Audio Engineering Society, New York, 8 pp., 2004.

[21] J. Hillenbrand and G.M. Sessler, “High-sensitivity piezoelectric microphones based on stacked cellular polymer films (L)”, J. Acoust. Soc. Am., Vol. 116, No. 6, pp. 3267-3270, 2004.

[22] M. Wegener, S. Bergweiler, W. Wirges, A. Pucher, E. Tuncer, and R. Gerhard-Multhaupt, “Piezoelectric two-layer stacks of cellular polypropylene ferroelectrets: Transducer response at audio and ultrasound frequencies”, IEEE Trans. Ultras. Ferroelectr. Freq. Contr., Vol. 52, pp. 1601-1607, 2005.

[23] J. Hillenbrand and G.M. Sessler, “Quasistatic and dynamic piezoelectric coefficients of polymer foams and polymer film systems”, IEEE Trans. Dielectr. Electr. Insul., Vol. 11, pp. 72-79, 2004.

[24] D. M. Taylor and O. Fernández, “Thermal instability of electromechanical films of cellular polypropylene”, IEEE Trans. Dielectr. Electr. Insul. Vol. 12, pp. 768-778, 2005.

[25] A. Mellinger, M. Wegener, W. Wirges, R.R. Mallepally, and R. Gerhard-Multhaupt, “Thermal and temporal stability of ferroelectret films made from cellular polypropylene/air composites”, Ferroelectrics, accepted for publication.

[26] M. Wegener, W. Wirges, and R. Gerhard-Multhaupt, “Piezoelectric polyethylene terephthalate (PETP) foams − Specifically designed and prepared ferroelectret films”, Adv. Eng. Mat., Vol. 7, pp. 1128-1131, 2005.

[27] M. Wegener, W. Wirges, J.P. Dietrich, and R. Gerhard-Multhaupt, “Polyethylene terephthalate (PETP) foams as ferroelectrets”, IEEE 12th Intern. Sympos. Electrets (ISE 12), IEEE Service Center, Piscataway NJ, pp. 28-30, 2005.

[28] W. Wirges, M. Wegener, O. Voronina, L. Zirkel, and R. Gerhard-Multhaupt, “Foaming with supercritical carbon dioxide: A new way of producing elastically soft, highly piezoelectric, cellular ferroelectrets from non-porous polyethylene-terephthalate films”, Adv. Fun. Mat. accepted for publication.

[29] A.-M. Savijärvi, M. Paajanen, E. Saarimäki, and H. Minkkinen, “Novel heat durable electromechanical films: Cellular film making from cyclic olefin polymers”, IEEE 12th Intern. Sympos. Electrets (ISE 12), IEEE Service Center, Piscataway, NJ, pp. 75-78, 2005.

[30] E. Saarimäki, M. Paajanen, A.-M. Savijärvi, and H. Minkkinen, “Novel heat durable electromechanical film processing: Preparations for electromechanical and electret applications”, IEEE 12th Intern. Sympos. Electrets (ISE 12), IEEE Service Center, Piscataway, NJ, pp. 220-223, 2005.

[31] M. Wegener, M. Paajanen, O. Voronina, R. Schulze, W. Wirges, and R. Gerhard-Multhaupt, “Voided cyclo-olefin polymer films: Ferroelectrets with high thermal stability”, IEEE 12th Intern. Sympos. Electrets (ISE 12), IEEE Service Center, Piscataway, NJ, pp. 47-50, 2005.

Eetta Saarimäki was born on 12 April 1965 in Kuopio (Finland). She received the M.Sc. degree from the Tampere University of Technology, Tampere, Finland in 1993. In 1994 she worked in VTT Technical Research Centre of Finland in the field of natural fibre filled thermoplastic composites. During 1994-1995 she worked in Institut for Composites in Kaiserslautern (Germany) researching impregnation of continuous glass fibers with thermoplastics. In

1995 she continued in VTT Technical Research Centre of Finland studying plastic processing technologies in different industrial projects (i.e. foaming, electrical properties, high temperature resistant films etc.). Since 2006 she has been a team leader of the “Applied Plastic Materials”.


Mika Paajanen was born on 22 August 1969, in Jyväskylä, Finland. He studied physics and electrical engineering in Tampere University of Technology from 1989 to 1995. He received his Master and doctoral degrees in technology in 1995 and 2001, respectively, both at the Tampere University of Technology. He has worked with VTT Technical Research Centre of Finland as a research scientist from 1996 to 2003 and as a senior research scientist

from 2003. He has also been leading a research group related to functional polymer materials since 2004. His present research topics include electroactive polymer dielectrics and novel thermally more stable cellular ferroelectrets

Hannu Minkkinen was born in August 1959 in Kuru, Finland. He studied mechanical engineering and materials science at the Tampere University of Technology and graduated from the Institute of Material Science in 1985. In 1985 he worked as a research scientist in the university and at the same year he started to work as a research engineer in Karhu-Titan sport equipment company. From 1987 to 1989 he did research related to metal and ceramic injection moulding technology in Elatec Oy and

Levanto Oy. At 1989 he started as a research scientist at VTT Technical Research Centre of Finland. The main research fields have been composite materials, plastic compounding and extrusion as well as processing of electromechanical films.

Ann-Mari Savijärvi was born in Valkeakoski, Finland in 1978. She received the M.Sc. degree from the Tampere University of Technology, Finland in 2001. After graduation she worked as a research scientist in the fields of PET films at DuPont Teijin Films in Middledborough, UK. During 2002-2005 she studied cellular. electromechanical films at VTT Technical Research Centre in Tampere, Finland. Currently she is working as a research engineer at

Kiiltoplas Oy in Virrat, Finland. Kiiltoplast Oy produced mainly PVC coated paper, board and textiles.

Michael Wegener was born on 24 September 1968 in Neuruppin, Germany. He studied physics and mathematics at the University of Potsdam from 1990 to 1995. In February 2000, he received the Ph.D. degree in physics. From 1997 until 2000, as Ph.D. student, he was admitted to the DFG-sponsored joint Research Training Group on "Polymer Materials" of the Berlin and Potsdam universities and received a merit-based scholarship. He also completed an industry internship at Huber & Suhner AG in

Herisau, Switzerland during July and August 1998. Since March 2000 he is a senior member of the scientific staff and a project manager in the Applied Condensed-Matter Physics group at the University of Potsdam. Main topics of his present research are the investigation of novel voided space-charge electrets (ferroelectrets) as well as of ferroelectric polymers, their electrical and electromechanical properties as well as their applications in piezoelectric sensors and actuators.

Olena Voronina was born on 29 November 1978, in Potsdam, Germany. She studied physics at the Kiev Taras Schevchenko University, Ukraine, where she obtained the Master degree (Diplom-Physics) in 2002. Since 2003 she is a Ph.D. student in the Applied Condensed-Matter Physics group (head Prof. Dr. R. Gerhard-Multhaupt) at the University of Potsdam, Germany. Her main research topic is “Structure-property relations in polymer electrets”, with particular emphasis on the electrical and

electromechanical properties of novel ferroelectrets. As a Ph.D. student she was admitted to the Graduate School “Multifunctional Polymer Composites” of the University of Potsdam and received a scholarship. She is a member of the German (DPG) Physical Society.

Robert Schulze was born on 15 August 1983, in Potsdam, Germany. Since September 2003 he has been studying physics at the University of Potsdam, Potsdam, Germany, and working as student assistance in the Applied Condensed-Matter Physics group (head Prof. Dr. R. Gerhard-Multhaupt) at the University of Potsdam. His present research topics are investigations of acoustical properties of ferroelectric polymers, their

electrical and electromechanical properties as well as the visualization of flows at organ pipes. Mr. Schulze is a member of the German Physical Society (DPG).

Werner Wirges was born on 23 January 1962 in Bonn (Germany). He was trained as a plumber before studying physical engineering at the Aachen University of Applied Sciences. He graduated as Dipl.-Ing. in April 1988. Between 1988 and 1992 he worked at the Heinrich Hertz Institute für Nachrichtentechnik, where he was in charge of the production and coating of viscoelastic control layers. From 1992 to 1997 he worked on the technology of thin film polymer systems for non-

linear optical applications. This work focused on the production and poling of polymer multilayers. At the same time he started his work on plasma deposition of new fluoropolymers for optical communication. Until April 2000, he was employed in two industrial projects on passive optical components (thermo-optical switches and polymeric waveguide filter elements). His work is being continued in Potsdam since May 2000 in the group of Prof. R. Gerhard-Multhaupt, where he focuses on production and characterization of non-polar fluoropolymers.

Reimund Gerhard-Multhaupt (S’80–M’84–SM’85–F’93) studied mathematics and physics at the Darmstadt University of Technology in Germany from 1972 until 1978. After graduating as Diplom-Physiker, he spent one year as research fellow at the Collège Militaire Royal in St-Jean, Québec, Canada. From 1979 until 1985, he did his Ph.D. thesis with Professor Gerhard M. Sessler in Darmstadt. From 1985 until 1994, he was research scientist and project manager at the Heinrich-Hertz Institute for

Communications Technology in Berlin. Since 1994 he has been an associate professor and since 1997 a full professor in the Physics Department of the University of Potsdam in Germany. The main research areas of Prof. Gerhard-Multhaupt are polymer electrets and ferroelectrets, in particular the mechanisms of space-charge storage and dipole polarization in dielectric polymers and polymer composites, their ferro-, pyro- and piezoelectrical properties, and their applications in sensors and actuators, as well as the nonlinear optical properties of polymers, and more recently also the physics of musical instruments. From 1974 until 1979, he was a fellow of the Studienstiftung des Deutschen Volkes. In 1988, he was awarded an ITG-Preis by the Informationstechnische Gesellschaft im VDE. In 1989, he received a Silver Medal from the Stiftung Werner-von-Siemens-Ring. In 2001, he was awarded the first Technologietransfer-Preis by the Technologie-Stiftung Brandenburg and the Prof.-Adalbert-Seifriz-Preis by the Verein Technologie-Transfer Handwerk for his technological collaborations with small industrial companies. Reimund Gerhard-Multhaupt is a member of the American, European and German Physical Societies. Since 2002, he serves as Digest Editor of the IEEE Dielectrics and Electrical Insulation Society.

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