Gas diffusion expansion-increased thickness and enhanced electromechanical response of cellular polymer electret films

Paajanen, Mika; Minkkinen, Hannu; Raukola, J.

doi:10.1109/ise.2002.1042976

Cited by 45 publications

(27 citation statements)

References 5 publications

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“…In a second process step after coextrusion and biaxial stretching, the size of the voids can be varied by an inflation process, based on a high-pressure treatment and a subsequent heat-setting process. [12][13][14][15][16][17][18] For the controlled inflation of the voids in the cellular polymer, the external gas pressure is usually raised and kept at a high value for a certain time (Figure 4). …”

Section: à3mentioning

confidence: 99%

“…The polymer is inflated by a subsequent sudden release of the external gas pressure (Figure 4). [13][14][15][16][17][18] The inflated structure needs to be stabilized by a thermal treatment above ambient temperature. [15] The thermal treatment increases the crystallinity of the polymer matrix, as revealed by differential scanning calorimetry, thereby stiffening and stabilizing the foam structure.…”

Section: à3mentioning

confidence: 99%

“…[49] Piezoelectric coefficients between 100 and 1000 pC/N are now routinely achieved, depending on the preparation technique, thereby increasing the microphone sensitivity by a similar level. [13][14][15][16][17][50][51][52] Cellular polypropylene films can also be used as microphones in the superaudio frequency range up to 40 kHz. [52] A bandwidth exceeding 315 kHz was demonstrated, thereby opening routes for the detection (and generation) of airborne ultrasound.…”

Section: Microphones Musical Pickups and Hydrophonesmentioning

confidence: 99%

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Microstorms in Cellular Polymers: A Route to Soft Piezoelectric Transducer Materials with Engineered Macroscopic Dipoles

2005

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Cellular polymers can be internally charged by "microstorms" (silent or partial discharges) within the voids of the polymer foam. The resulting material, which carries positive and negative charges on the internal void surfaces, is called a ferroelectret. Ferroelectrets behave like typical ferroelectrics, hence they provide a novel class of ferroic materials. The soft foams are strongly piezoelectric and can be used, in a wide range of applications, as transducers for interconverting mechanical and electrical signals. Herein, an overview is provided on the preparation of cellular polymers by physical foaming (extrusion, biaxial stretching, and controlled inflation by pressure treatments), on their charging by "microstorms", on their piezo- and pyroelectricity, and on analogies to ferroelectrics. Finally, a survey of selected applications is presented.

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Section: à3mentioning

confidence: 99%

Section: à3mentioning

confidence: 99%

Section: Microphones Musical Pickups and Hydrophonesmentioning

confidence: 99%

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Microstorms in Cellular Polymers: A Route to Soft Piezoelectric Transducer Materials with Engineered Macroscopic Dipoles

2005

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“…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][8][9][10][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][14][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 abovementioned articles too.…”

Section: Introductionmentioning

confidence: 99%

Novel heat durable electromechanical film: processing for electromechanical and electret applications

Saarimäki

Paajanen

Savijärvi

et al. 2006

IEEE Trans. Dielect. Electr. Insul.

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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.

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“…For instance, the void heights may be adjusted with controlled inflation [8], [12]- [14], electrical charging is achieved with fields well above the threshold for microplasma discharges [15]- [17], charging during or after treatment in electronegative gas environments allows for higher charge densities [18], etc. We know from previous investigations that an optimized cellular structure and suitable electrical charging lead to piezoelectric activities of several hundred pico Coulomb per Newton, depending on the excitation frequency.…”

Section: Introductionmentioning

confidence: 99%

Piezoelectric two-layer stacks of cellular polypropylene ferroelectrets: transducer response at audio and ultrasound frequencies

Wegener

Bergweiler

Wirges

et al. 2005

IEEE Trans. Ultrason., Ferroelect., Freq. Contr.

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Piezoelectric cellular polypropylene films, socalled ferroelectrets, are assembled in a stack with two active transducer layers. The stack is characterized with respect to its linear and quadratic response in a frequency range from 1 kHz to 80 kHz. A relatively smooth frequency response in the sound-pressure level is found for the individual layers as well as for both layers driven in phase. The piezoelectric response of the two-layer stack is twice the response of an individual layer over a rather broad frequency range. Furthermore, the influence of the preparation conditions on the resonance frequency and the effect of the quadratic distortion on the radiated sound are investigated both for the individual transducer films in the stack and for the stack system as a whole.

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Gas diffusion expansion-increased thickness and enhanced electromechanical response of cellular polymer electret films

Cited by 45 publications

References 5 publications

Microstorms in Cellular Polymers: A Route to Soft Piezoelectric Transducer Materials with Engineered Macroscopic Dipoles

Microstorms in Cellular Polymers: A Route to Soft Piezoelectric Transducer Materials with Engineered Macroscopic Dipoles

Novel heat durable electromechanical film: processing for electromechanical and electret applications

Piezoelectric two-layer stacks of cellular polypropylene ferroelectrets: transducer response at audio and ultrasound frequencies

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