Double-layer electret transducer

Journal of

ELECTROSTATICS ELSEVIER

Journal of Electrostatics 39 (1997) 33-40

Double-layer electret transducer R. Kacprzyk a'*, A. Dobrucki b, J.B. Gajewski c Institute of Electrical Engineering and Technology, Technical University of Wroclaw, Wybrzeke S. Wyspiahskiego 27, 50-370 Wroclaw, Poland bInstitute of Telecommunications and Acoustics, Technical University of Wroclaw, Wybrze~ S. Wyspiahskiego 27, 50-370 Wroclaw, Poland cInstitute of Heat Engineering and Fluid Mechanics, Technical University of Wroclaw, Wybrze~e S. Wyspiahskiego 27, 50-370 Wroclaw, Poland

Received 19 February 1996; accepted after revision 12 June 1996

Abstract A simplified analysis of the double-layer electret with a frozen space charge leads to the conclusion that such a structure can show a piezoelectric effect when the elasticity coefficients of the layers differ. Results of experiments carried out on the P T F E foil-PP non-woven fabric sandwich confirm the possibility of construction of a simple and relatively sensitive transducer using this effect. Keywords: Electret; Piezoelectric transducer; Electrical double-layer; Corona charging; P T F E

foil

1. Introduction Monitoring the natural rhythms of the human body can sometimes require application of a transducer in the form of a thin, pliable sheet with a relatively large surface area. Piezosensitive foils [1] or double- and/or multi-layer electrets [2-4] can be utilized in the design of transducers. It has been shown [3-7] that a double-layer electret having layers with different mechanical and electrical properties should exhibit piezosensitivity when an electric charge is deposited in their interface. In the case when a perpendicular force is applied to the surface of an electret-sandwich and the resulting deformations are very small, one can write [3, 6] ~le2xlx2 d3s = - qs (e2xl + e l X z ) 2

El

E2 E1E-----~" -

-

* Corresponding author. 0304-3886/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. Pll S 0 3 0 4 - 3 886 ( 9 6 ) 0 0 0 3 0 - 7

(1)

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R. Kacprzyk et al./Journal of Electrostatics 39 (1997) 33-40

where d33 is the piezoelectric coefficient defined as the total change of the surface charge density caused by the force acting uniformly on the sample surface; el, e2, and xl, x2 are the relative permittivities and thicknesses of both layers, respectively; Ex, E2 are the coefficients of elasticity for both layers, and q~ is the surface charge density of the charge deposited in the interface between layers 1 and 2. Eq. (1) shows that d33 5 0 if and only if E1 v6E2 . Since the x and e values are functions of an external force, the value of the coefficient d33 must also depend on the stress. On the assumption that only xl depends on the stress p and xl = Xlo(1 - p/E1), where x l 0 is the value of x l for p = 0, while el,/~2 and x2 are left constant, and E1 < 0), which is denoted by d33p, should satisfy the following relation: qs glg2X2Xlo(1 -- p/EO d33p ~-- E1 [elx2 + g2xlo(1 - p/E1)] 2'

(2)

where d33p is the piezoelectric coefficient existing only under the action of a stress (pressure) p which is the pressure exerted on the transducer by a force acting perpendicularly to its surface. Taking account of the condition elX2 = 52 Xlo = a, for which a m a x i m u m value of the piezoelectric coefficient d33 is obtained, and after suitable rearrangements, Eq. (2) is now of the form (1 -

p/EO

d33p ~---4d33 (2 - p / E 1 ) 2'

(3)

where 4d33 = qJE~, as can easily be shown. According to Eq. (3), a decrease ( > 1%) in the value of d33 p shall appear for p/E1 > 0.2.

2. E x p e r i m e n t a l

The transducer consists of two sandwiches, as shown in Fig. 1. The electret layer of the sandwich was made of the teflon (PTFE) foil (1) of 100 tam thickness that has one side metalized (2) - a first electrode - and is polarized using a corona discharge from a string electrode (30 ~tm in diameter) polarized with - 1 0 kV. An electret with dimensions 30 m m x 90 m m was polarized within a period of 600 s at the elevated temperature (420 K) and then cooled down (under the polarizing field) to r o o m temperature. The elastic layer (3) was made of thick non-woven polypropylene (PP) fabrics, whose thicknesses are 150 and 350 lam and which are covered with a latex layer (4) and an aluminium foil (5), as a second electrode. The whole structure was bent at mid-length, equipped with two pins (6) and inserted into a P E T envelope (7), whose thickness is 60 ~tm, giving the required mechanical strength to the transducer. After the preparation of pins (the outer electrode works as a screen and totally covers the inner one), the envelope with the structures was closed at a temperature of 383 K within 10 s under a slight pressure. The final dimensions of the sensitive part of the transducer were 25 m m x 47 m m and its thickness was 0.6 or 0.8 mm. The characteristic capacitances Cc (the capacitance of 1 m 2 of the sensitive surface of the transducer

R. Kacprzyk et al./Journal of Electrostatics 39 (1997) 33-40

35

Fig. 1. Section of a double-layer electret transducer: 1 - polarized PTFE foil; 2 - evaporated aluminium inner electrode; 3 PP non-woven fabric; 4 latex layer; 5 - aluminium foil; 6 - metal foil pins, and 7 - PET envelope covering the whole structure.

±

Fig. 2. Schematic diagram of the measuring system used in the determination of the piezoelectric coefficient of the double-layer electret transducer.

d33 p

as m e a s u r e d at 1 k H z ) were 68 a n d 33 n C m - 2 for t r a n s d u c e r s with the P P layer thicknesses of 150 a n d 350 pm, respectively. T h e piezoelectric coefficient d33 p was d e t e r m i n e d by m e a s u r i n g the electric charge u n d e r static c o n d i t i o n s a n d using a typical m e a s u r i n g system (see e.g. [7]) is s h o w n in Fig. 2. T h e system consists of the following parts: a l o a d (weight) (1), an e l e c t r o m a g netic screen (2), a n u p p e r e l e c t r o d e (3), the d o u b l e - l a y e r electret t r a n s d u c e r (4) u n d e r test, a lower e l e c t r o d e (5), an i n s u l a t o r (6), a d y n a m o m e t e r (high precision spring

36

R. Kacprzyk et al./Journal of Electrostatics 39 (1997) 33-40

I

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Fig. 3. Schematic diagram of the system used in measurement of the double-layer electret transducer sensitivity.

scales) or a digital balance (type WPT-5) (7) used in the measurements of stresses, a standard capacitor (8) which capacitance was 1 nF, and a vibrating-capacitor electrometer STATRON 6350 (9) employed for the charge measurements. The frequency characteristics of sensitivity S (S = da3p/Cc) for the transducers with different PP layers were determined using a measuring system shown in Fig. 3. The characteristics were obtained for the double-layer transducers mounted in an anechoic chamber (1) and excited with infrasound from a loudspeaker GDN 40/200 (2) connected with a signal generator ROBOTRON 03020 (6). The following devices and instruments were also used in the measuring system: a measuring standard microphone MK 102 (3) with a preamplifier ROBOTRON MV 102 and a loudness level meter ROBOTRON 00017 (7), the transducer tested (4) put on an insulating base (5) and connected to the input of a preamplifier 2619 along with an adapter JJ2612 and measuring amplifier 2609 made by Briiel & Kjaer (8), a spectrum analyzer ROBOTRON 01021 (9) with a voltmeter 02022 and a band filter 01013, and a loudness level recorder ROBOTRON 02060 (11). A frequency meter ROBOTRON 51039 (10) was connected directly to the generator. The accuracy of measurements performed using the above instrumentation is estimated to be of the order of 5-10%.

3. Results of measurements

Each measurement, performed during the research upon two double-layer electret transducers, was repeated some 10 times, after the input quantity (e.g. a load F acting on the transducer under test) had been changed, to have a certain statistics and eliminate random errors.

R. Kacprzyk et al./Journal of Electrostatics 39 (1997) 33-40 28

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Results of the measurements of the piezoelectric coefficient d3 3p as a function of the pressure p and storage time t for the transducers described above are shown in Figs. 4 and 5, respectively. The practically permanent decrease of the value of the coefficient daap for stresses exceeding 5 kPa is observed.

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R. Kacprzyk et al./Journal of Electrostatics 39 (1997) 33-40 10

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Frequency f [Hz] Fig. 6. Frequency characteristics of the PTFE-PP transducers.

Stability of the properties of transducers was characterized by the measurement of dssp as a function of the storage time. The measurement results shown in Fig. 5 and carried out for the transducers stored under normal conditions (room temperature, 40-55% RH) in an open circuit, reveal that the sensitivity of the structure varies with the storage time. Within the first 30-50 h, a permanent decrease of dssp down to 30-70% of the initial value is observed and then it becomes practically constant. Transducers with a thicker P P layer (350 ~tm) showed generally higher values of the piezoelectric coefficient dssp and its stronger dependence on the stress/pressure, but better time stability was observed for structures with thinner P P layers. Both transducers presented here had, as described in Section 2, two layered structures made of mechanically soft (the P P non-woven fabric) and hard (PTFE foil) dielectrics. All the measurement results, as presented in Figs. 4-6, were obtained for the values of dssp and S that were reckoned for only one sandwich structure. The low-frequency characteristics of sensitivity S for the transducers with different P P layers are given in Fig. 6. Low-frequency 3 dB limits for both kinds of transducers appear at a frequency of about 5 Hz. The characteristics were obtained for the transducers mounted in an anechoic chamber and excited with infrasound on a level of 110 dB. For higher frequencies, the transducers showed flat characteristics up to 2000 Hz within the 3 dB bandwidth - here is shown a range of very low frequencies, i.e. only up to 20 Hz. The values of the piezoelectric coefficient d33p were determined on the basis of the values of sensitivity S (S = dssp/C¢) of both transducers presented in Fig. 6, and they were considerably higher (several times) than those presented in Figs. 4 and 5. It was caused likely by the application of a basically lower stress (20 Pa)

R. Kacprzyk et al./Journal of Electrostatics 39 (1997) 33-40

39

during sensitivity measurements. The permanent decrease with pressure of the coefficient d33p results from the relatively low value of the coefficient of elasticity E1 for the P P non-woven fabric.

4. Conclusions The results of the measurements have borne out the possibility of preparation of transducers working on the basis of a double-layer electret with layers made of dielectrics with distinctly different mechanical properties (elasticity). The low-frequency limits of frequency dependences of sensitivities (see Fig. 6) showed that they are not influenced by the thickness of the elastic layer. The application of the elastic layer made of a non-woven fabric allows one to get a transducer whose sensitivity is comparable with that of the P V D F piezoelectric foils [8], P ( V D F - T r F E ) copolymers [9], or other composites [10], The time instability of the sensitivity (see Fig. 4) was probably caused by uncontrolled changes in the geometry of the structure and a surface distribution of the sensitivity of the transducer - the coefficient d33p was determined by the measurements made on a small part of the transducer surface, i.e. only about 2 x 10 -4 m 2 while a measuring sensor used had a sensing surface of about 12 × 10 -4 m 2. In comparison with the previous work [3] in which the sensitivity of a layer transducer was about 100 gV/Pa over the range of frequencies 20 Hz to 2 kHz, the sensitivity of 5 mV/Pa, i.e. 50 times higher, was obtained for the same frequencies using the non-woven fabric. This resulted from the use of a good insulating material of small value of the coefficient of elasticity El. No limits are imposed on the preparation of multistructures (more than 2) and large surface transducers work on the basis of the principle presented above.

Acknowledgements This work was partly financed by the Committee of Scientific Research, Warsaw, Poland, under contract No. 3 P401 051 06.

References [1] [2] [3] [4] [5] [6]

R.A. Betz, Proc. 6th Int. Symp. on Electrets (ISE 6), Oxford, England (1988) 214. C. Hennion and J. Lewiner, J. Acoust. Soc. Am., 62 (1978) 279. C. Hennion and J. Lewiner, J. Acoust. Soc. Am., 63 (19781 1229. J. Lewiner and C. Hennion, US Patent No. Re, 32 180 (1986). R, Hayakawa and Y. Wada, Adv. Polymer Sci., 11 (1973) 1. R. Kacprzyk, E. Motyl, J.B. Gajewski and A. Pasternak, Conf. Rec. IEEE/IAS Annual Meeting, Toronto, Canada (1993) 1743-1745 in: J. Electrostat., 35 (1995) 161.

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R. Kacprzyk et aL/Journal of Electrostatics 39 (1997) 33-40

[7] G.A. Lu~ejkin, Polimiernyje elektriety, Izd. Chimia, Moskva (1976). [8] G.M. Sessler, ed, Electrets, Topics in Applied Physics, Vol. 33, Springer, Berlin, 1987. [9] H.L.W. Chan, Z. Zhao, K.W. Kwok and C.L Choy, Proc. 8th Int. Symp. on Electrets (ISE 8), Paris, France (1994) 583. [10] C.J. Dias, M.P. Wenger, Y. Kaminorz, U. Hopfnor and D.K. Das-Gupta, Proc. 8th Int. Syrup. on Electrets (ISE 8), Paris, France (1994) 589.