Fluoroethylenepropylene ferroelectrets with patterned microstructure and high, thermally stable piezoelectricity
- First Online:
- Cite this article as:
- Zhang, X., Hillenbrand, J., Sessler, G.M. et al. Appl. Phys. A (2012) 107: 621. doi:10.1007/s00339-012-6840-7
- 279 Views
Layered fluoroethylenepropylene (FEP) ferroelectret films were prepared from sheets of FEP films by template-patterning followed by a fusion-bonding process and contact charging. The layered ferroelectret films show consistency and regularity in their void structures and good bonding of the layers. For films composed of two 12.5 μm thick FEP layers and a typical void of 60 μm height, the critical voltage necessary for the built-up of the “macro-dipoles” in the inner voids is approximately 800 V. At room temperature, Young’s modulus in the thickness direction, determined from dielectric resonance spectra of the fabricated films with a typical thickness of 85 μm, is about 0.21 MPa. Initial quasistatic piezoelectric d33 coefficients of samples contact charged at a peak voltage of 1500 V are in the range of 1000–3000 pC/N. From these, ferroelectrets with high quasistatic and dynamic (up to 20 kHz) d33 coefficients of up to 1000 pC/N and 400 pC/N, respectively, which are thermally stable at 120°C, can be obtained by proper annealing treatment. This constitutes a significant improvement compared to previous results.
Some non-polar space-charge electrets based on polymer foams exhibit a significant piezoelectric effect. These foams are now named ferroelectrets or piezoelectrets [1–3]. Since the first such films, made of polypropylene (PP), were developed in Finland about 15 years ago , the polarization, charge storage, mechanical properties, and piezoelectricity have been intensively investigated. Owing to their unique features, such as large piezoelectric d33 coefficients, flexibility, light weight, and low cost, many applications have been suggested [1–9]. Some devices based on PP ferroelectret films, such as movement monitors and non contact vital monitors , are commercially available right now. Many other applications, such as microphones , ultrasonic transducers , flexible ferroelectret field-effect transistors , and ferroelectret accelerometers  are being developed in some laboratories. Unfortunately, the relatively poor stability of d33 in PP ferroelectret films, with working temperatures normally lower than 60°C, limits their applications when the devices are required to work at elevated temperatures.
Recently, the development of thermally stable ferroelectrets has been pursued in several laboratories and much progress has been made. Since the thermal stability of d33 in ferroelectrets is dependent on the retention of the internal space charges, two approaches are possible to achieve this goal. One is the development of new ferroelectrets based on thermally stable electret materials, such as cross-linked PP (XPP) , cycloolefines (COC) , polyethylene-terephthalate (PET) , polyethylene-naphthalate (PEN) , and fluorocarbon polymers [14–25]; the other is chemical modification, such as surface modification of cellular PP by using fluorination . Of these two approaches, the former one, in particular the one using fluorocarbon films, has shown the most promise. For instance, the very recently developed fabrication process for ferroelectrets based on thermally stable fluorocarbon polymers with tailored void structure is very attractive. Particularly, template-based methods allow one to control the microstructure of the films. In addition, the thermal stability can be optimized by controlling the charge distribution during charging .
In this article, an improved process for preparing mechanically stable fluoroethylenepropylene (FEP) ferroelectrets with highly regular, patterned microstructure and good bonding of the layers is described. This process also leads to piezoelectric d33 coefficients which are of equal magnitude, but thermally more stable, than the best results  reported before. The thermal stability, pressure dependence, and spatial dependence of piezoelectric d33 coefficients in such FEP ferroelectret films will be discussed.
2 Experimental details
2.1 Sample preparation
The polymer layers used were 12.5 μm thick FEP films provided by the DuPont Company. The preparation process consisted of two steps: Patterning of the FEP layers by pressing at a given temperature and fusion bonding of the film stack.
Most of the experimental results have been obtained for two-layer samples. Therefore, in this paper only results for such samples will be discussed. However, three- and multi-layer samples are also of interest. Studies on such films will be continued and results will be reported in a later paper.
2.2 Charging process
In order to render the fabricated films piezoelectric, contact charging was performed on the films at room temperature. The samples were first metalized by evaporation on both sides with ∼100 nm thick aluminum electrodes. The electrodes have a diameter of 20 mm. A triangular-shaped voltage pulse of 20 ms duration (rising and falling parts 10 ms each) with peak amplitudes between 100 and 1500 V, supplied from a Radiant Precision Workstation Materials Analyzer was applied to the samples. The resulting electrode charge was measured and from these measurements the remanent charge on the interior walls of the voids was calculated (see Sect. 2.3). Charging can also be performed with pulses of longer duration, up to hundreds of seconds. In this case, higher remanent charges and thus higher d33 coefficients are observed. If the triangular voltage pulse is followed by another such pulse of opposite polarity, a hysteresis curve can be determined. This will be discussed in Sect. 3.1.
2.3 Calculation of remanent charge in the voids from the electrode charge
For the void structure shown in Fig. 3(b), there is no induction charge in the areas connecting adjacent voids. Therefore, a correction factor equal to the ratio of the total sample area to the area covered by the voids, which equals 1.53, has to be introduced. Thus σ0=−1.81σi.
These results for the charge density on the inner walls of the voids will be further discussed in Sect. 3.1.
2.4 Mechanical properties
2.5 Piezoelectric properties
2.5.1 Quasistatic measurements
2.5.2 Acoustic measurements
2.5.3 Interferometric measurements
Interferometric measurements of the d33 coefficient were performed by utilizing the inverse piezoelectric effect. During the measurements the ferroelectret films are glued to a sample holder and are electrically excited by a sinusoidal AC-voltage. The resulting movement of the free surface of the film is detected by the laser beam of a Michelson interferometer (SIOS SP-S). AC-voltages of 20 Vpp are supplied by a PC-controlled function generator (M&R Systems WG-820). Since the interferometer utilizes a focused laser beam, the spatial resolution is in the 10 μm range. The exact measuring position x and the focus of the beam on the film sample can be manually adjusted by means of three micrometer screws, which are moving the sample holder.
2.6 Isothermal decay of the piezoelectric d33 coefficient
Measurements of the isothermal decay of the quasistatic piezoelectric d33 coefficient were carried out in order to investigate the thermal stability of the ferroelectrets. Samples were annealed at a temperature of 120°C for a specified amount of time before measurements of d33 were taken at room temperature. A sequence of such measurements on a sample yields an isothermal decay curve which characterizes the sample with respect to thermal stability.
3 Results and discussion
3.1 Electric polarization of laminated FEP-PTFE films
In order to evaluate the possible contributions of charge injection from the electrodes into the polymer and of interfacial polarization to the quasi-permanent trapping of charge , a reference experiment was performed on a single-layer FEP sample . An electric field of 80 MV/m, which is much higher than that experienced by the FEP layers in the laminated films, was applied to the single-layer FEP sample in the reference experiment. A quasi-permanent charge density less than 0.01 mC/m2 was observed, much smaller than the value found in the two-layer sample. Therefore, under the given experimental conditions, charge injection and an interfacial polarization between electrodes and polymer surfaces are negligible, and the Paschen breakdown in the gas volume of the voids is the dominant charging mechanism [37, 38].
3.2 Mechanical properties of laminated FEP-PTFE films
3.3 Thermal stability of the piezoelectric d33 coefficients
3.4 Pressure dependence of d33
3.5 Local dependence of d33
3.6 Acoustic measurements of d33
The main advantage of the acoustic method when measuring laminated films is the proper averaging of the d33 coefficients obtained , since the applied sound pressure is identical everywhere on the sample for low frequencies, i.e. when the wavelength of the sound is much larger than the sample. The acoustic method therefore supplements quasistatic and interferometric methods for the determination of d33.
Fluorocarbon ferroelectrets with well-controlled microstructures were successfully fabricated from compact FEP layers by patterning with templates and subsequent fusion bonding. The ferroelectrets show consistency and regularity in their void structures and good bonding of the layers. The critical voltage across the laminated films, which leads to electrical breakdown in the inner voids, is around 800 V. At room temperature, Young’s modulus in the thickness direction of the fabricated films with a thickness of 85 μm, determined from the dielectric resonance spectra, is about 0.21 MPa. The typical initial quasistatic piezoelectric d33 coefficients of the ferroelectrets are in the range of 1000–3000 pC/N. By proper annealing samples with very high d33 coefficients (up to 1000 pC/N quasistatic and up to 400 pC/N in the audio range), thermally stable at 120°C, can be obtained. This constitutes a significant improvement compared to previous results .
The authors want to dedicate this article to Professor Reimund Gerhard at the occasion of his 60th birthday. In addition, the authors gratefully acknowledge financial support by the “Hessische Ministerium für Wissenschaft und Kunst”, the “Deutsche Forschungsgemeinschaft” (DFG), the Natural Science Foundation of China (NSFC, No. 50873078), and the State Key Laboratory of Electrical Insulation and Power Equipment, Xi’an Jiaotong University (EIPE11203).