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Piezoelectric effect in polyvinylidene fluoride at high frequencies
Volume 45A, number 6
5 November 1973
PHYSICS LETTERS
PIEZOELECTRIC EFFECT IN POLYVINY LIDENE FLUORIDE AT HIGH FREQUENCIES H. SUSSNER*, D. MICHAS, A. ASSFALG, S. HUNKLINGER*
and K. DRANSFELD*
Physik-Department E IO der Technischen Universitiit Miinchen, D-8046 Garching. Germany
Received 13 September 197 3 A strong piezoelectric effect in polyvinylidene fluoride was observed at frequencies as high as 500 MHz using polarized films as ultrasonic transducers. In contrast to the low frequency behaviour, the transducer efficiency showed a remarkable increase with decreasing temperature.
A large piezoelectric effect in drawn and polarized films of polyvinylidene fluoride (PVF2) was observed by Kawai and Fukuda [ 1,2], when applying a sinusoidal stress of 20 Hz in the drawing direction. Although the piezoelectric constant at room temperature is about three times larger than in quartz, it decreases by orders of magnitude when the films are cooled down to nitrogen temperatures [2]. The temperature behaviour and the origin of the low frequency piezoelectricity in PVF, is not understood so far. Whether PVF2 is still piezoelectric at higher frequencies was completely unknown, because higher frequency investigations could not be performed by the previous methods [3,4]. Here we present the first experimental data using PVF, films excited in their resonance thickness mode up to 500 MHz demonstrating a strong piezoelectric effect at temperatures down to 80 K. In our experiments we used 251.1-and 50@hick films obtained from Kureha Chemical Industries. By infrared absorption measurements we found that in our samples the polymers were unoriented and that more than 90% were in their helical form and not in the planar zigzag conformation. The crystallinity was determined by small angle X-ray scattering [5] to be 65%. The PVF2 films were coated with a thin silver layer and made piezoelectric by the poling procedure already described previously [ 11. Subsequently one PFV2 film was cemented to an optically polished zcut quartz and ultrasonic measurements could be performed by the conventional pulse-echo method [6]. *Present address: Max-Planck-Institut fTlrFestkGrperforschung, Hochfeld-Magnetlabor Grenoble, B.P. 166, Centre de Tri, F-38042 Grenoble-Cedex, France.
Fig. 1. Ultrasonic echoes generated with a 25 p PVFz transducer at 180 MHz and 200K (1 cm = 5~s).
When applying an RF pulse to the PVF2 transducer very clear ultrasonic signals are detected by the same film (fig. 1). Varying the frequency from 10 MHz to 500 MHz characteristic resonances appear, whenever the film vibrates in a resonant thickness mode. When we observe the received signal at resonance as a function of the temperature, the signal rises considerably with decreasing temperatures (fig. 2). This effect was similar, whether we used the PVF, transducer at 20 MHz or at higher resonances as for example 250 MHz. Thus, at low temperatures ultrasound could be generated up to 480 MHz with a 25~ transducer. Ultrasonic attenuation measurements which we performed on bulk PVF, indicate that the signal increase with lower temperatures might be due to the steep de475
Volume 45A, number 6
PHYSICS LETTERS
5 November 1973
completely different from low frequency measurements. This implies that only small scale oscillations are responsible for the piezoelectric effect in PVF, at high frequencies. We are indebted to W. Arnold for his experimental support in the early stage of the measurements. I
100
I 200 TEMPERATURE
300 [K1
Fig. 2. The relative increase of the received signal at resonance is shown as a function of the temperature. The resonance frequency was 250 MHz at room temperature.
crease in attenuation at the glass transition temperature (233 K). These results show that PVF, is still strongly piezoelectric at frequencies as high as 500 MHz even below the glass temperature. The temperature dependence of the high frequency piezoelectric effect is
476
References [l] H. Kawai, Japan. J. Appl. Phys. 8 (1969) 975. [2] E. Fukada and S. Takashita, Japan. J. Appl. Phys. 8 (1969) 960. [ 31 K. Nakamura and Y. Wada, J. Polym. Sci. A-2 9 (197 1) 161. [4] J. Cohen and S. Edelman, J. Appl. Phys. 42 (1971) 3072. [S] C.G. Vonk and G. Kortleve, Kolloid.-Z. und Z. Polym. 220 (1967) 19. [6] W.P. Mason, Physical acoustics and the properties of solids (Van Nostrand, 1958).
5 November 1973
PHYSICS LETTERS
PIEZOELECTRIC EFFECT IN POLYVINY LIDENE FLUORIDE AT HIGH FREQUENCIES H. SUSSNER*, D. MICHAS, A. ASSFALG, S. HUNKLINGER*
and K. DRANSFELD*
Physik-Department E IO der Technischen Universitiit Miinchen, D-8046 Garching. Germany
Received 13 September 197 3 A strong piezoelectric effect in polyvinylidene fluoride was observed at frequencies as high as 500 MHz using polarized films as ultrasonic transducers. In contrast to the low frequency behaviour, the transducer efficiency showed a remarkable increase with decreasing temperature.
A large piezoelectric effect in drawn and polarized films of polyvinylidene fluoride (PVF2) was observed by Kawai and Fukuda [ 1,2], when applying a sinusoidal stress of 20 Hz in the drawing direction. Although the piezoelectric constant at room temperature is about three times larger than in quartz, it decreases by orders of magnitude when the films are cooled down to nitrogen temperatures [2]. The temperature behaviour and the origin of the low frequency piezoelectricity in PVF, is not understood so far. Whether PVF2 is still piezoelectric at higher frequencies was completely unknown, because higher frequency investigations could not be performed by the previous methods [3,4]. Here we present the first experimental data using PVF, films excited in their resonance thickness mode up to 500 MHz demonstrating a strong piezoelectric effect at temperatures down to 80 K. In our experiments we used 251.1-and 50@hick films obtained from Kureha Chemical Industries. By infrared absorption measurements we found that in our samples the polymers were unoriented and that more than 90% were in their helical form and not in the planar zigzag conformation. The crystallinity was determined by small angle X-ray scattering [5] to be 65%. The PVF2 films were coated with a thin silver layer and made piezoelectric by the poling procedure already described previously [ 11. Subsequently one PFV2 film was cemented to an optically polished zcut quartz and ultrasonic measurements could be performed by the conventional pulse-echo method [6]. *Present address: Max-Planck-Institut fTlrFestkGrperforschung, Hochfeld-Magnetlabor Grenoble, B.P. 166, Centre de Tri, F-38042 Grenoble-Cedex, France.
Fig. 1. Ultrasonic echoes generated with a 25 p PVFz transducer at 180 MHz and 200K (1 cm = 5~s).
When applying an RF pulse to the PVF2 transducer very clear ultrasonic signals are detected by the same film (fig. 1). Varying the frequency from 10 MHz to 500 MHz characteristic resonances appear, whenever the film vibrates in a resonant thickness mode. When we observe the received signal at resonance as a function of the temperature, the signal rises considerably with decreasing temperatures (fig. 2). This effect was similar, whether we used the PVF, transducer at 20 MHz or at higher resonances as for example 250 MHz. Thus, at low temperatures ultrasound could be generated up to 480 MHz with a 25~ transducer. Ultrasonic attenuation measurements which we performed on bulk PVF, indicate that the signal increase with lower temperatures might be due to the steep de475
Volume 45A, number 6
PHYSICS LETTERS
5 November 1973
completely different from low frequency measurements. This implies that only small scale oscillations are responsible for the piezoelectric effect in PVF, at high frequencies. We are indebted to W. Arnold for his experimental support in the early stage of the measurements. I
100
I 200 TEMPERATURE
300 [K1
Fig. 2. The relative increase of the received signal at resonance is shown as a function of the temperature. The resonance frequency was 250 MHz at room temperature.
crease in attenuation at the glass transition temperature (233 K). These results show that PVF, is still strongly piezoelectric at frequencies as high as 500 MHz even below the glass temperature. The temperature dependence of the high frequency piezoelectric effect is
476
References [l] H. Kawai, Japan. J. Appl. Phys. 8 (1969) 975. [2] E. Fukada and S. Takashita, Japan. J. Appl. Phys. 8 (1969) 960. [ 31 K. Nakamura and Y. Wada, J. Polym. Sci. A-2 9 (197 1) 161. [4] J. Cohen and S. Edelman, J. Appl. Phys. 42 (1971) 3072. [S] C.G. Vonk and G. Kortleve, Kolloid.-Z. und Z. Polym. 220 (1967) 19. [6] W.P. Mason, Physical acoustics and the properties of solids (Van Nostrand, 1958).