Piezoelectric electret transducer for ultrasonic generation and detection up tomicrowave frequenciesC. Alquié, J. Lewiner, and C. Friedman Citation: Applied Physics Letters 29, 69 (1976); doi: 10.1063/1.88969 View online: http://dx.doi.org/10.1063/1.88969 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/29/2?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Finite element predictions of guided ultrasonic wave fields generated by piezoelectric transducers AIP Conf. Proc. 557, 829 (2001); 10.1063/1.1373842 Nonbonded piezoelectric ultrasonic transducer J. Acoust. Soc. Am. 79, 1204 (1986); 10.1121/1.393716 Highly directional ultrasonic electret transducer J. Acoust. Soc. Am. 68, 388 (1980); 10.1121/1.384499 Piezoelectrically driven ultrasonic transducer J. Acoust. Soc. Am. 67, 1418 (1980); 10.1121/1.384105 Electrostatic ultrasonic transducers and their utilization with foil electrets J. Acoust. Soc. Am. 53, 1663 (1973); 10.1121/1.1913516

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The absence or weakness of parallel orienting action observed for some lower complex homologues (see Table I) may be explained by failure in the alignment of methylene chains parallel to the substrate surface, which alignment is sterically hindered in case of the complexes of lower dicarboxylic acids with shorter methylene chains.

It should be noted that a given boundary surface will not always have the same orienting action for all LC materials. For instance, as seen from Table I, none of the homologous chromium complexes of aliphatic dicarboxylic acids could produce parallel alignment of RCB having a biphenyl structure, For a preliminary study of the influence of chemical structure on the ori­enting action, chromium complexes of p -phenylenedi­acetic and p -phenylenedipropanoic acids, which are aromatic dicarboxylic acids, were prepared and their orienting effect on RCB molecules was examined. The results showed that both these complexes successfully achieve parallel orientation of RCB molecules. From the foregoing, it may be inferred that the orienting action strongly depends on the speCific chemical struc­tures of both the boundary surface and LC molecule,

1M. Scharlt and W. HeUrich, Appl. Phys. Lett. 18, 127 (1971).

2M. F. Schiekel and K. Fahrenschon, Appl, Phys. Lett. 19, 391 (1971); J. Borel, J. Robert, and F. Charadjedaghi, In­ternational Conference on Alpha-numerical Display Devices and Systems, PariS, 1973 (unpublished).

:IG.H. Heilmeier, L.A. Zanoni, and L.A. Barton, Proc. IEEE 56, 1162 (1968).

lC.R. Stein and R.A. Kashnow, Appl, Phys. Lett. 19, 343 (1971).

'F .J. Kahn, G. N. Taylor, and H. Schonhorn, Proc. IEEE 61, 823 (1973), and references therein.

6L. T. Creagh and A.R. Kmetz, SID International S.vmposium, San Francisco, 1972 (unpublished)..

7R. A. Kashnow, 4th International Liquid Crystal Conference, Kent, Ohio, 1972 (unpublished).

8J. E. Proust, L. Ter-Minassian-Saraga, and E. Guyon, Solid State Commun. 11, 1227 (1972).

9J. L. Janning, Appl, Phys. Lett. 21, 173 (1972), 10F.J. Kahn, Appl, Phys. Lett. 22, 386 (1973). 11S. Matsumoto, M. Kawamoto, and N. Kaneko, Appl, Phys.

Lett. 27, 268 (I 975). 12The polynuclear dicarboxylatochromium complexes are not

commercially available. 11Further details of the synthetic method will be published

elsewhere. lip. Chatelain, Bull. Soc. Franc. Min. Crist. 66, 105 (1943), I,I. Haller and H.A. Huggins, U.S. Patent No. 3656834

(1972).

Piezoelectric electret transducer for ultrasonic generation and detection up to microwave frequencies

C. Alquie, J. Lewiner, and C. Friedman

Laboratoire d'Electricite Generale. Ecole Superieure de Physique et de Chimie 10. rue Vauquelin. 75005 Paris. France (Received 2 April 1976; in final form 29 April 1976)

In this paper we report some observations of ultrasonic generation and detection at frequencies up to 9360 MHz using a piezoelectric electret transducer. The transducers were made with biaxially stretched polyvinylidene fluoride films.

PACS numbers: 43.85.+f, 77.60.+v, 06.70.Mx, 81.50.Nv

Since the discovery by Sessler and West1 of the poly­mer electret microphone, many theoretical and experi­mental works have been reported2 on the use of elec­tromechanical polymer electret transducers, As is well known now, a polymer electret can be used either as an electrostatic device or as a piezoelectric device. In the

J 5u I~ Iv ,. 2 , I~ iv

A , II \Ij ~~ ~ ala.

FIG. 1. Ultrasonic echos generated and detected by two 13-1l­thick PVF2 films at 110 MHz and 77 K.

69 Applied Physics Letters. Vol. 29. No.2, 15 July 1976

first case the electret, which creates a static electric field, acts as the biasing element of an electrostatic transducer. Such systems, highly developed in the audiofrequency range (microphones and earphones), have been reported to operate up to 200 MRz. 3

corundum

FIG. 2. The transducer, bonded at the bottom of the corun­dum, is placed in the strong electric field region of a reen­trant microwave cavity.

Copyright © 1976 American Institute of Physics 69

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Su I iv

5 , / iv

i

, i

1.1 .Ie .Ie

FIG. 3. Ultrasonic echos generated and detected by a 12-1-' PVF2 film at 9360 MHz and 1. 3 K. The propagation medium was a 19-mm-Iong 3-mm-diam cylinder of corundum

In the second case, the piezoelectric properties of polymer electrets are being used. Such transducers are now appearing in the audiofrequency range (micro­phones and earphones). In the ultrasonic range they have been reported to operate satisfactorily up to 500 MHz,4 and recently the present authors have observed operation in the X-band range. 5

In this letter we report the use of piezoelectric elec­tret transducers to generate and detect ultrasonic waves up to a frequency of 9360 MHz. The electrets were made according to the method described by Kawai6 from biaxially stretched polyvinylidene fluoride (PVF2) film, 7

Two series of measurements were carried out: at low frequencies (10-500 MHz) and at microwave frequen­cies (around 10 000 MHz). A standard pulse -echo tech­nique was used for all measurements, either in trans­mission mode with two transducers (in the low-frequen­cy range) or in reflection mode with one transducer (in the microwave range). The pulse width was of the order of 1 /lS. The propagation media were either germanium or corundum single crystals. In the latter case, when the crystals were not metallized, a thin layer of gold was vacuum deposited on both Sides of the transducer. In all other cases, such coating was not required. The electrets, which had thicknesses varying from 12 to 100 /l, were bonded to the propagation medium either with grease or with an epoxy mixture.

In the low-frequency range, measurements were made at room and liquid-nitrogen temperatures. In Fig, 1 a typical echo system is shown at a frequency of 110 MHz, obtained with a 13-/l-thick film. For thin transducers ("" 12 /l), the response was more or less independent of frequency below the first resonance. By measuring the positions of the fundamental and odd harmoniC resonances, the sound velocity of com pres -sional waves in these films was determined to be v, = 2600 m/s with an uncertainty of ± 10%. This figure leads to a value of the elastic modulus C33 of 1. 2 x 1010

N/m2• The Q factor of the resonances, strongly damped

by the bonding of the transducer to the propagation medium, was of the order of 3 at room temperature and of 10 at 77 K.

In the microwave frequency range the experiments were performed at 9360 MHz. The propagation medium was corundum, at low temperature (10 3 K). The elec­tret was bonded to a 3 -mm -diam rod with optically flat

70 Appl. Phys. Lett., Vol. 29, No.2, 15 July 1976

TABLE 1. Comparison of the acoustical impedances of corun­dum and PVF2 films.

Material Density Velocity of sound Acoustical impedance (kg/m3) mls (kgm-2 S-l)

corundum 3960

PVF2 1800

10900

2600

surfaces (to within 1/10 of a wavelength of sodium). The transducer, not metallized, was placed in the strong electric field region of a reentrant microwave cavity, as is shown in Fig. 2. Since at 9360 MHz the wavelength of ultrasonic waves in PVF2 is 2600 A, the parallelism between transducer and sample should be within 300 A, which is of course almost impossible to achieve. Thus one can expect the phase of the emitted ultrasonic wave not to be exactly constant in planes parallel to the end surfaces. This can make the detection difficult, since the transmitted electrical Signal is proportional to the integrated amplitude of the wave over the surface of the transducer; if the phase is not constant over this sur­face, the integral will vanish.

In order to approach the ideal requirements, the bonding was done with epoxy, by pressing the electret between two optically polished surfaces (within 1/10 of a wavelength of sodium) in a V -shaped block to ensure good alignment. The thickness of the bond obtained by this technique was evaluated to be in the range 0,5-5 /l, depending on the pressure exerted during bonding.

In most experiments the input peak power in the microwave cavity was of the order of 3 W. In Fig. 3, many echoes are visible (the first two are missing be­cause the receiver, saturated by the exciting pulse, has a recovery time of about 10 /ls). These echos may seem of small amplitude (the associated output power for the first echo could be evaluated to be of the order of 10-10 W); however, one must take into account the following facts:

(i) The sensitivity of the receiver was only 80 dBm above noise,

(ii) There is a loss of signal during the detection due to the small wavelength of the sound waves as compared with the imperfection of the geometric assembly.

(iii) The impedance matching between the electret and the corundum is not very good, as is shown in Table L

These experiments show that piezoelectric electret transducers can be used up to very high frequencies particularly when emission of ultrasonic waves is in­volved or when the medium in which the waves are to be propagated is of low acoustical impedance.

The authors would like to thank Dr. G. Dreyfus and S. Abdin for their assistance during this work.

1G.M. SesslerandJ.E. West, J. Acoust. Soc. Am. 34, 1787 (1962).

2See, for instance, G.M. Sessler and J. E. West, J. Acoust. Soc. Am. 53, 1589 (1973).

Alquie, Lewiner, and Friedman 70

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3D. Legros and J. Lewiner, J. Acoust. Soc. Am. 53, 1663 (1973).

4H. Sussner, D. Michas, A. Assfalg, S. HunkUnger, and K. Dransfeld, Phys. Lett. A 45, 475 (1973).

5C. Alquie, J. Lewiner, and C. Friedman, Compt. Rend. C.R. Acad. Sci. (paris) B282, 389 (1976).

6H. Kawai, Jpn. J. Appl. Phys. 8, 975 (1969). TObtained from Kureha Chemical Industry.

Piezoelectric effect in a cholesteric liquid crystal layer subjected to shear vibration

Yukio Kagawa and Toyomasa Hatakeyama Department of Electrical Engineering, National University Toyama, Takaoka, Toyama 933, Japan (Received 22 January 1976; in final form 3 May 1976)

Piezoelectric effects are observed in a mixed cholesteric liquid crystal layer. It is sandwiched by a pair of nesa-coated glass plates one of which is subjected to lateral motion to develop the shear vibration in the layer. It is found that the electric potential is generated from the layer, the frequency of which is the same as that of the exciting vibration. Three cases of the molecular orientation process for the liquid crystal layer are tested: (i) previous impression of dc voltage, (ii) lecithin application on the surfaces of the substrate glass plates, and (iii) previous static shear strain on the layer. 10-20 mY peak-to-peak (P-p) are generated for the vibratory displacement of 1 J-Lm (p-p) of 25 kHz.

PACS numbers: 61.30.+w, 81.SS.+x, 77.60.+v

The possibility of the presence of the piezoelectric effect in liquid crystals has theoretically been predicted and is termed piezoelectricity or flexoelectricity. 1,2

The present paper reports our finding that a cholesteric liquid crystal layer subjected to shear vibration gener­ates electric potential of the same frequency as the exciting vibration.

Crystals and polycrystalline materials like crystal quartz and barium titanate are well known for their piezoelectric or electrostrictive effect. Soft and flex!­ble materials such as PVF are also proven piezoelec­tric. Liquid crystals are viscoelastic liquids with crys­tal-like characteristics. To the best of our knowledge, we have made the first direct observation of the piezo­electric effect in a fluid material.

The optical and dielectric effects in a nematic liquid crystal layer subjected to shear vibration have been investigated. 3 With a similar arrangement, the experi­ment is repeated for a mixed cholesteric liquid crystal layer of the electric field effect type (cholesteryl oleyl carbonate: cholesteryl Chloride: cholesteryl nonanoate = 3 : 3 : 2), for which generation of electric potential is observed. This effect is not observed for nematic liquid crystals.

The experimental setup is illustrated in Fig. 1. The liquid crystal layer is sandwiched between a pair of glass plates with wire spacers for proper layer thick­ness and with nesa -coated electrodes. One of the plates is fixed while the other is connected to the vibrator by which shear vibration is developed in the liquid crystal layero The layer thickness is 100 /lm and the effective area is 1 cm2

• The capacitance is about 60 pF, and the dc resistance is 3.2 X 1012 O. The output from the nesa­coated electrodes is connected to the oscilloscope through the probe (10: 1, the input capacitance 15 pF, and the input impedance 10 MO). The frequency of the

71 Applied Physics Letters, Vol. 29, No.2, 15 July 1976

vibration is 25 kHz and the displacement is measured by the noncontact displacementmeter. The liquid crys­tal cell is housed in a box for shielding, and its temper­ature is electronically controlled. The temperature of the liquid crystal layer is mOnitored by the thermo­couple plugged in the glass plate.

The molecules of the liquid crystal must be properly oriented. Three tests for orientation are carried out.

Test 1. The mixed cholesteric liquid crystal has a transition point at a temperature of 53 "C, above which it turns isotropic. For the molecular orientation, the dc voltage is applied to the liquid crystal cell for 30 min at 80°C. The temperature is then decreased at a rate of 1°C for 2 min until it reaches the temperature of the measurement when the applied voltage is re­moved. The results obtained are shown in Fig. 2.4 The reSidual dc voltage remains for a considerable period after the applied voltage is removed. The residual volt­age is measured by an electrometer with an input im­pedance of 1014 0 which is shown in Fig. 2(a) by the dotted lines. The ac voltage generated in the layer sub-

FERRITE VIBRATffi LH,)j In CRYSTAL LAYER

FIG. J. Experimental setup.

SHI8.D OOX Willi ElfCTR!JHC

IDIDAlm: COO1ULER

Copyright © 1976 American Institute of Physics 71

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