Electro Active Materials for Flat Panel Transducers

L. Ehrig, D. Beer, S. BrixFraunhofer IDMT, 98693 Ilmenau, Germany

Correspondence should be addressed to Daniel Beer (beer@idmt.fraunhofer.de)

IntroductionMaterials changing their geometry due to the presenceof an electrical field are mentioned as electro activematerials [1]. Two different material inherent effects suchas piezoelectricity and electrostriction cause mechanicalstress or strain. While piezoelectricity is idealized asa linear coupling of electrical and mechanical field andoccurs only in materials having no center of symmetry,electrostriction can be observed in all dielectrics but it isusually weak and the coupling is quadratic and thereforeindependent of the direction of the electrical field.

The potential to use these materials as sound reproducingstructures is enormous. The material itself can be re-garded as diaphragm and drive system of a loudspeaker.An advantage for their use is the extremely simple, flatand lightweight loudspeaker design. Furthermore theuse of piezoelectrical elements helps to solve the maindisadvantage of current flat panel speakers [2]: the airbehind the diaphragm causing an undesired increase ofresonance frequency [3]. Assuming a material oscillatingonly in thickness direction, it can be attached directly ona boundary surface, e.g. a wall, having no interfering airvolume in the rear, as depicted in Figure 1. Moreover the

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Figure 1: Thickness oscillator mounted on a wall

force that yields a deformation is uniformly distributedover the whole surface of the diaphragm, which is thematerial itself. Thus partial oscillations can be decreased.

This paper presents electro active materials in orderof their historic development. Advantages and dis-advantages regarding their use for sound reproducingstructures are discussed and approaches for loudspeakerdesigns are presented.

Piezoelectric CrystalsPiezoelectricity was discovered in several mono-crystalsby Pierre and Jaque Curie in 1880. A force applied ona crystal led to a measurable electrical potential on itssurface which is known as the direct piezoelectric effect.More interesting for actuator applications is the inverseeffect: An electrical potential causing a displacement ofthe ionic crystal lattice, and thus a deformation of thecrystal itself. This was predicted by Gabriel Lippmannone year later and verified experimentally by the Curie’sas well [4].

The piezoelectric effect remained a scientific curiosityuntil the First World War, when Paul Langevin wascontracted to develop a system for detecting submarines.In this context Langevin demonstrated electroacoustictransducers in the audible frequency range based onpiezoelectric quartz [5]. At the same time Alexander M.Nicolson applied for a patent using piezoelectric crystalsfor different applications [6]. A crystal loudspeaker isshown in Figure 2.

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Figure 2: Cross section view of a crystal speaker [6]

A significant step for transducer constructions withhigher sensitivity, was an idea by Charles B. Sawyerto use piezoelectric crystals analog to bi-metallic strips.Thus a relatively small mechanical strain in longitudinaldirection of a disk or plate can be transformed in alarger deflection. Sawyer presented several applicationsincluding record cutters and loudspeakers as shown inFigure 3.

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Figure 3: Crystal loudspeaker consisting of a piezoelectricbi-morph and a conical diaphragm attached at the free end[7]

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At the end of the 1940s crystal materials have beenreplaced by piezoelectric ceramics having much strongercoupling of electrical and mechanical field.

Piezoelectric CeramicsIn order to possess piezoelectric properties ceramics haveto be ferroelectric. Therewith they can be poled in astrong constant electrical field by rotating some domains180 in a preferred direction. When the electrical fieldis turned off a remanent polarization remains and thematerial is piezoelectric. Above a specific temperature,the Curie point, the material looses piezoelectric andferroelectric properties due to phase transition.

Besides higher sensitivity compared to piezoelectric crys-tals ceramics have the advantage that the base materialscan be formed arbitrarily before sintering and that thedirection of polarization can be chosen ad libitum as well.

For electroacoustic transducers similar constructions asfor piezoelectric crystals have been used. Due to thehigher piezoelectric constants simple uni-morph benderscan be built consisting of a ceramic disc between twoelectrodes having different thicknesses. The mechan-ical strain of the ceramic disc due to a voltage istransformed in a deflection. Attached on a panel itworks like distributed mode loudspeakers (DML) and canfor example be found in greeting cards making sound.For applications where space is limited these simplestructures can be used as tweeters in front of conventionalcone speakers as shown in Figure 4.

Figure 4: A classic 8” electrodynamic loudspeaker with acone diaphragm combined with a coaxially mounted ceramicpiezoelectric tweeter, Quam-Nichols Company, type 8C10CO[8]

Electro Active PolymersPolymers changing their shape due to an electrical fieldare called electro active polymers (EAP). Since polymershave some unique properties they might be preferred overcrystals and ceramics. Usually they have a lower Young’smodulus making them much more flexible and they canbe formed arbitrarily.

Ferroelectric PolymersBesides ceramics there are also polymers showing fer-roelectric properties. Polyvinylidene fluoride (PVDF orPVF2) is one example. It is a transparent, semicrystalineplastic material. After a poling process under high directcurrent voltage it has piezoelectric properties, which wasdiscovered by Heiji Kawai in 1969 [9].

Poled PVDF is available in various film thicknessesranging from 28 µm up to over 100 µm. Thus it isobvious that the absolute variation in thickness is verysmall and not capable for a sufficient contribution inthe excitation of sound waves. However the strain inlongitudinal direction can be transformed in a pulsatingmovement by curving the film as shown in Figure 5.

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Figure 5: Transforming the longitudinal motion to apulsating motion [10]

This principle has been used in many experimentaland also commercial available tweeters. An exampleis the omni-directional polymer tweeter presented in1974 by Masashiko Tamura, as it is seen in Figure 6[10]. The tweeter was used in the multi-way loudspeaker

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Figure 6: Polymer tweeter using a PVDF film [10]

system HPM-150 by the Japanese company Pioneer [11].Nowadays it is hard to find any loudspeaker manufacturerusing this technology. Some years ago the tweeter HD3Pfrom the French company Audax was available but is notbuilt anymore.

With the decreasing size of electronic components andthe increasing demand for mobile devices like cell phonesand laptop computers the interest in this material isrising again. Combined with transparent electrodesbased on carbon nanotubes or polyaniline PVDF can beused as a sound emitting structure on displays, at thesame time as a sensor for touchscreens or as a vibrationabsorber on windows [12, 13].

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Polymer Foams as ElectretsMaterials having properties of a battery or a chargedcapacitor are referred to as electrets. The reason for thevoltage is a polarization that retains after the material issubjected to a strong electrical field. First electrets weremade on the basis of resin around the 1920s [14].

Polymer foams consisting of charged voids show theseproperties and can therefore be classified as electrets[15] or as ferroelectrets [16]. During fabrication gas isinjected in the polymer forming small spherical voids.Subsequently the polymer is biaxially stretched and thevoids become lens shape with a thickness around 1 µm.The film is subjected to a high electrical field causingelectrical breakdowns in the voids. Therewith the innerfaces have an opposite charge as shown in Figure 7a andthe material has piezoelectric properties. Thus, applying

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Figure 7: Schematic view of a polarized polymer foam; (a)the inner surfaces of the voids are opposite charged leadingto free charges on the materials surface; (b) a force is appliedon the material, the number of free charges on the surface isincreased (sensor application)

a force on the faces of the film changes the distancebetween surface and voids and the number of surfacecharges is increased, as depicted in Figure 7b or decreaseddepending on how the force is applied. This can be usedfor sensor applications.

For actuator applications two different options are pos-sible. It can be used as a diaphragm for an electrostaticloudspeaker or as a thickness oscillator. The first optionis commercially used in loudspeaker panels by the Finnishcompany Panphonics [17]. Two layers of polymer film areplaced between electrodes as shown in Figure 8. Due tothe small gap between the film and the electrodes it canoscillate freely. The material of stator and electrodes isporous for the sound waves.

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Figure 8: cross section view of an electrostatic loudspeakerelement using an polarized polymer foam as diaphragm [18]

As a thickness oscillator the electrodes are attacheddirectly on the films surface and the thickness changes

according to an applied voltage alike the inverse piezo-electric effect. The unique feature of this material isthe outstanding weak coupling in longitudinal directioncombined with a relatively strong coupling in trans-verse direction. Thus it can be regarded as a truethickness oscillator and attached on a bounding surfacethe constricted strain in longitudinal direction does notinterfere with the change in thickness. A major drawbackfor sound reproduction is the small amplitude of thethickness oscillation particularly for reproduction of lowfrequencies.

Electrostrictive PolymersAs mentioned before the piezoelectric effect which is alinear coupling of electrical and mechanical field can onlyoccur in materials having no center of symmetry. Bycontrast the electrostrictive effect, which is the quadraticcoupling of those fields, is present in all dielectrics butusually very weak. Nevertheless some materials showa rather strong coupling, like P(VDF-TrFE) which is acopolymer of PVDF and can be used for electroacoustictransducers [19].

As seen in Figure 9 electrodes are attached on the faceon a curved polymer film. An electrical field yields adeformation particularly in longitudinal direction. Thusthe curved partition performs a pulsating movement.

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Figure 9: Electrostrictive loudspeaker element [19]

Due to the quadratic coupling, some specialties have tobe considered. To avoid frequency doubling caused bythe independence of the direction of strain and electricalfield a direct current voltage as a bias voltage is necessary.

Dielectric Polymers

Insulators having a high specific resistance (> 1010 Ωcm)are referred to as dielectrics. The concentration of freecharge carriers is low as well as the carrier mobility [14].Dielectric polymers having high dielectric constants andadditionally low stiffness can be used as actuators. Apolymer film with electrodes on opposite faces shrinksin thickness and expands in area when voltage is ap-plied. The shrinking in thickness is primarily due toelectrostatic forces that attract or repel the electrodesdepending if the charges are opposite or alike. Theelectrodes have to be compliant to allow the film to strain[20].

Strictly speaking dielectric polymers are no electro activematerials since the force origins not from the materialitself. They are mentioned here anyway since the sameconstruction principles for transducers as already statedfor the other materials can be realized. The materialdoes not need to be treated in a special way, thus it isinexpensive and very high strains can be achieved [20].

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ConclusionAs this overview and a closer investigation of the specificmaterial properties reveals there is no solution at hand,that is capable of replacing conventional loudspeakertechnology, neither one of the presented materials norone of the design principles. However the increasingdemand for flat panel speakers that can be operateddirectly on boundary surfaces like walls needs uncon-ventional solutions. The presented class of materialsin particular polymers offer some unique features forrealizing thickness oscillators, which might be the firstchoice for the design of ultra flat speakers. Neverthelessamplitudes in oscillations are far below the requirementsfor achieving sufficient sound pressure levels especially forlow frequencies. Thus material properties have to be im-proved or adequate solutions might be found improvingsound reproducing properties of such structures.

References[1] Hecht, H.: Die Elektroakustischen Wandler. 5.

Leipzig : Johann Ambrosius Barth, 1961

[2] Beer, D.: Flachlautsprecher - ein Uberblick. DAGA,Dresden, Germany 2008

[3] Beer, D. et al: The Air Spring Effect ofFlat Loudspeakers, presented at the 124th AESconvention, Amsterdam, The Netherlands, 2008

[4] Mason, W. P.: Piezoelectric Crystals and TheirApplication to Ultrasonics. 5. New York : Nostrand,1950

[5] Sawyer, C. B.: The Use of Rochelle Salt Crystalsfor Electrical Reproducers and Microphones. In:Proceedings of the Institute of Radio Engineers 19(1931), No. 11, p.2020–2029

[6] Nicolson, A. M. (Inventor): US 2212845 (August1940). Western Electric Company (Applicant). Pr.:10th April 1918

[7] Sawyer, Charles B. (Inventor): US 1802782(April 1931). Pr.: 6th May 1927

[8] Reference to the Quam-Nichols Company productcatalogue 2009 URL: http://www.quamspeakers.com/documents/catalogue/QNC_BINDER.pdf

[9] Kawai, H.: The Piezoelectricity of Poly(vinylideneFluoride). In: Japanese Journal of Applied Physics 8(1969), July, No. 7, p.975–976

[10] Tamura, M. et al: Electroacoustic Transducerswith Piezoelectric High Polymer Films. In: Journalof the Audio Engineering Society 23 (1975), Jan-uary/February, No. 1, p.21–26

[11] Locanthi, B. et al: Development of a LoudspeakerSystem with Omni-Directional Horn Loaded HighPolymer Tweeter. 58th AES Convention, November1977

[12] Yu, X. et al: Carbon nanotube based transparent

thin film acoustic actuators and sensors. In: Sensorsand Actuators A: Physical 132 (2006), November, No.2, p.626–631

[13] Sugimoto, T. et al: Loudspeakers for FlexibleDisplays. 123rd AES Convention, October 2007

[14] Nitzsche, K. ; Ullrich, H.-J.: Funktion-swerkstoffe der Elektrotechnik und Elektronik. 2.Deutscher Verlag fur Grundstoffindustrie, 1993. –ISBN 3–342–00524–6

[15] Gerhard-Multhaupt, R.: Less can be More– Holes in Polymers lead to a New Paradigmof Piezoelectric Materials for Electret Transducers.In: IEEE Transactions on Dielectrics and ElectricalInsulation 9 (2002), October, No. 5, p.850–859

[16] Bauer, S. et al: Ferroelectrets: Soft ElectroactiveFoams for Transducers. In: Physics Today 57 (2004),February, No. 2, p.37–43

[17] Reference to the Panphonics homepage. URL: http://www.panphonics.com

[18] Paajanen, M. ; Lekkala, J. ; Kirjavainen,K.: ElectroMechanical Film (EMFi) – a newmultipurpose electret material. In: Sensors andActuators A: Physical 84 (2000), p.95–102

[19] Heydt, R. et al: Design and Performance of anElectrostrictive-Polymer-Film Acoustic Transducer.In: Journal of Sound and Vibration 215 (1998),August, No. 2, p.297–311

[20] Kornbluh, R. et al: Application of DielectricElastomer EAP Actuators. In: Bar-Cohen, Y.(Editor): Electroactive Polymer (EAP) Actuatorsas Artificial Muscles: Reality, Potential, andChallenges. Washington : SPIE - The InternationalSociety for Optical Engineering, 2001. – ISBN0–8194–4054–X

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