This paper presents an interferometric measurement of the out-of-plane deflections produced by a piezoelectric transducer, manufactured by thick-film deposition of a ceramic paste over an alumina substrate, when is subjected to a DC electric voltage. It is shown that a digital speckle pattern interferometer with an incorporated phase-shifting facility allows the measurement of nanometer displacements generated by the piezoelectric device. These measurements are used to evaluate the effective piezoelectric charge constant along the polarization direction (d33)eff that characterizes the thick-film transducer.Fil: Tendela, Lucas Pedro. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Rosario. Instituto de Física de Rosario (i); Argentina. Universidad Nacional de Rosario; ArgentinaFil: Federico, Roque Alejandro. Instituto Nacional de Tecnologia Industrial; ArgentinaFil: Kaufmann, Guillermo Hector. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Rosario. Instituto de Física de Rosario (i); Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Rosario. Centro Internacional Franco Argentino de Ciencias de la Información y Sistemas; Argentina. Universidad Nacional de Rosario; Argentin

Federico, Roque Alejandro

Kaufmann, Guillermo Hector

Tendela, Lucas Pedro

Optics and Lasers in Engineering

CONICET Digital

Optics and Lasers in Engineering 49 (2011) 281–284Contents lists available at ScienceDirectOptics and Lasers in Engineering0143-81doi:10.1 CorrE-mjournal homepage: www.elsevier.com/locate/optlasengEvaluation of the piezoelectric behaviour produced by a thick-film transducerusing digital speckle pattern interferometryLucas P. Tendela a,, Alejandro Federico b, Guillermo H. Kaufmann ca Instituto de Fı́sica Rosario (Consejo Nacional de Investigaciones Cientı́ficas y Técnicas – Universidad Nacional de Rosario), Blvd. 27 de Febrero 210 bis, S2000EZP Rosario, Argentinab Electrónica e Informática, Instituto Nacional de Tecnologı́a Industrial, P.O. Box B1650WAB, B1650KNA San Martı́n, Argentinac Instituto de Fı́sica Rosario and Centro Internacional Franco Argentino de Ciencias de la Información y de Sistemas (Consejo Nacional de Investigaciones Cientı́ficas y Técnicas –Universidad Nacional de Rosario), Blvd. 27 de Febrero 210 bis, S2000EZP Rosario, Argentinaa r t i c l e i n f oArticle history:Received 23 August 2010Received in revised form29 September 2010Accepted 4 October 2010Keywords:Digital speckle pattern interferometryPiezoelectric transducersThick filmsScreen printed ceramics66/$ - see front matter & 2010 Elsevier Ltd. A016/j.optlaseng.2010.10.002esponding author. Tel.: +54 3414853200; faxail address: tendela@ifir-conicet.gov.ar (L.P. Ta b s t r a c tThis paper presents an interferometric measurement of the out-of-plane deflections produced by apiezoelectric transducer, manufactured by thick-film deposition of a ceramic paste over an aluminasubstrate, when is subjected to a DC electric voltage. It is shown that a digital speckle patterninterferometer with an incorporated phase-shifting facility allows the measurement of nanometerdisplacements generated by the piezoelectric device. These measurements are used to evaluate theeffective piezoelectric charge constant along the polarization direction (d33)eff that characterizes thethick-film transducer.& 2010 Elsevier Ltd. All rights reserved.1. IntroductionLead Zirconate Titanate (PZT) ceramics are typical ferroelectricmaterials with outstanding piezoelectric properties which arewidely used in nondestructive testing and ultrasound for medicalimages. Its popularity is mainly due to the high piezoelectricresponse that they exhibit, and also because of their high acousticand electrical impedance. To manufacture piezoelectric devices ona millimeter scale capable of producing large deflections and highactuation forces, screen printed ceramics can be used, thus allow-ing to obtain relatively thick-film layers on a ceramic substrate.This process, known as thick-film technology, allows a greatgeometrical flexibility design [1]. However, thick films exhibitdistinct physical behaviour characteristics when they arecompared with bulk ceramics. This is mainly due to the effectsof the ceramic substrate, as well as the different contributions thatresult from the composition and sintering process of the film, suchas glass and porosity. Therefore, many efforts are currently investedto find adequate choices of active materials, bonding agents andprocessing conditions to incorporate large piezoelectric effects inthick films. Due to the difficulties presented to control this wide setof choices, there exists a great interest in the use of real-timemeasurements of static displacement fields as a way of optimizingthe manufacture process of thick-film piezoelectric devices.ll rights reserved.: +54 3414808584.endela).Atomic force microscopes and scanning near-field optical micro-scopy have been used for the inspection of microcomponents [2].However, they are very expensive, require delicate adjustment, useshort working distances and measurements are time consuming whentypical components need to be qualified in production. Whole-fieldoptical methods provide a promising alternative to the previouslymentioned microscopic techniques. The main advantages of theseoptical methods are their noncontact, nondestructive and whole-fieldinspection nature. Other important characteristics are their highsensitivity and accuracy, and also the automatic analysis of results.This paper presents an interferometric evaluation of the staticdeflections produced by a piezoelectric transducer, manufactured bythe deposition of a thick PZT layer over an alumina substrate, when issubjected to a DC electric voltage. It is shown that a digital specklepattern interferometer with an incorporated phase-shifting facility [3]allows the measurement of nanometer displacements generated by thepiezoelectric device. It is also demonstrated that the measured out-of-plane deflections do not replicate the classical piston movementproduced by a free sensor due to the clamping effect generated bythe substrate. Finally, these measurements are used to evaluate theeffective piezoelectric charge constant along the polarization direction(d33)eff that characterizes the thick-film transducer.2. Preparation of the thick-film transducerA typical paste for screen printing using thick-film technologycontains three basic components: an active element thatLLPHPHDCPZT sensorPTMCCDBSBSLaserMFig. 2. Optical arrangement of the out-of-plane digital speckle patterninterferometer.L.P. Tendela et al. / Optics and Lasers in Engineering 49 (2011) 281–284282determines the main electromechanical characteristics of thesintered layer, a support or organic vehicle, and a glass binderthat acts as a bonding agent among the particles themselves andthe substrate. The active element of the paste was a PZT powdermanufactured by Ferroperm (PZ27) [4], which was dispersed in acommercial vehicle to obtain rheological properties suitable for thescreen-printing process [5]. The connection between the PZTparticles was improved by adding a lead borosilicate glass thatalso helped to attach the film to the substrate due to the relativelylow sintering temperature that was used in the manufactureprocess. It is important to note that although a low porosity isusually preferred, it is not possible to add a large quantity of glassbinder because of the risk of inhibiting excessively the piezoelectricproperties of the transducer.The thick-film ceramic elements were printed on an aluminasubstrate of 0.6 mm thickness by means of a screen-printingprocedure which used a stainless steel screen [1]. Eachpiezoelectric element had a circular shape with a diameter of8.5 mm and a thickness t¼ 130 mm. A lower electrode of Au wasprinted on the alumina substrate before applying the piezoelectricpaste and then an upper electrode was made on the fired PZT paste.At the back surface of the transducer, an absorbent block made withepoxy and ferrotungsten was glued to approach the acousticimpedance of the alumina. Each layer was dried at 140 1C during10 min and then sintered in a belt furnace at 850 1C. As the filmthickness diminishes about 30% during the sintering process, thefinal thickness of the film must be designed taking into account thedifference between dry and fired film.Finally, the PZT film was polarized by heating the sample at110 1C and an electric field of 3 MVm1 was applied between theelectrodes for 30 min. The temperature was lowered to roomtemperature before turning off the electric field.Fig. 1 shows a thick-film transducer sintered over an aluminasubstrate with the external electrical connection that is guided toan inner electrode between the thick film and the substrate.3. Experimental system and data analysisThe experimental setup used to measure the normal displace-ment of the thick-film transducer was a conventional out-of-planeinterferometer, as shown in Fig. 2. The light beam of a Nd:Yag laserwith a wavelength l¼ 0:5328 mm was first divided into the objectand reference beams by a beam splitter (BS). The reference beamFig. 1. Thick-film piezoelectric transducer.was directed to a mirror (M) linked to a piezoelectric transducer(PT), which was controlled by an electronic unit. This phase-shifting facility enables the introduction of a constant phaseshift between both beams in order to calculate the wrappedphase map. The reference beam was expanded by themicroscope objective (L) and directed through another beamsplitter into the CCD camera (Pulnix TM-720), where it wasrecombined with the light scattered by the object surface. Inorder to obtain a uniform illumination intensity, a pin hole wasused in both object and reference beams. The video camera had azoom lens (CL) which allows to image a small region of thespecimen of 12 12 mm2 in size. Finally, the camera output wasfed to a frame grabber (Matrox Pulsar) located inside a personalcomputer which digitized the images in grey levels with aresolution of 512512 pixels and 256 grey levels (8-bit). Togenerate the speckle patterns, the surface of the transducer wascovered with a very thin layer of white paint.The experimental procedure used to record the pair of speckleinterferograms to be correlated was as follows. First, the thick- filmtransducer was subjected to a selected DC electric voltage and a setof four phase-shifted speckle interferograms was acquired andstored in the computer. Then, the same DC electric voltage but withan inverse polarity was applied to the thick-film device and a newset of four phase-shifted speckle interferograms was acquired andstored in the computer. Afterwards, the corresponding wrappedphase distribution Df was calculated using the Carré phase-shifting technique [3], which is based on the processing of thetwo sets of four phase-shifted speckle interferograms. Thistechnique evaluates the difference of phase and gives asignificant improvement in the quality of the reconstructedphase maps. In order to improve the signal-to-noise ratio ofpixels having very low-intensity modulation, a local smoothingkernel of 3 3 pixels was also used.It is important to note that the pair of speckle interferograms tobe correlated generated less than a fringe because the out-of-planedisplacement produced by the thick-film transducer lies in thenanometer scale. Therefore, as the wrapped phase distribution didnot present the usual 2p phase discontinuities, it was not necessaryto apply a phase unwrapping algorithm.Then, the out-of-plane displacement component w was com-puted from the wrapped phase map Df by means of [3]w¼l4pDf ð1ÞL.P. Tendela et al. / Optics and Lasers in Engineering 49 (2011) 281–284 2834. Experimental resultsAs a typical result, Fig. 3 displays the wrapped phase mapproduced by a thick-film transducer for an applied voltage of 314 V.Fig. 4 shows the 3D plot of the out-of-plane displacementcomponent w evaluated from Eq. (1) using the wrapped phasemapDfdisplayed in Fig. 3. As the transducer edges remain fixed bythe substrate, it is observed that the out-of-plane displacementcomponent does not replicate the classical piston movementproduced by a free device.Fig. 5 shows a plot of the out-of-plane displacement componentalong a line crossing the center of the transducer and determinedfrom the values displayed in Fig. 4.As previously mentioned, in a thick-film PZT transducerobtained by screen printing there are various factors that modifythe elastic and piezoelectric characteristics from those of the bulkmaterial. We can mention three main causes: the glass binderphase that was added when the paste was prepared to achievebetter consistency in the sintered film, the porosity resulting fromthe relatively low temperature of sintering and the low quantity ofFig. 3. Wrapped phase map produced by the thick-film transducer for an appliedvoltage of 314 V.0051015−0.0200.020.040.060.080.10.12y (mm)w (µm)Fig. 4. Out-of-plane displacement component determinglass in the film, and finally the clamping introduced by thesubstrate which is a characteristic of this technology. Taking intoaccount that from the properties of the bulk materials it is quitedifficult to determine separately the effect of these factors on theelectromechanical behaviour of the transducer, the manufactureprocess can be optimized through the determination of theeffective piezoelectric charge constant along the polarizationdirection (d33)eff given by the sintered device. It should be notedthat the poling electric field can also affect the (d33)eff value becausethe electric field must go through the pores inside the film duringthe polarization process and some regions could remain partiallypolarized. Therefore, for all these reasons the d33 value of the bulkmaterial will always be much higher than the effective piezo-electric charge constant measured from the thick-film transducer.If ek is the strain tensor written in matricial form due itssymmetry, being k¼1,y,6, and Ei with i¼1,2,3 is the electric fieldcomponent, the equation describing the piezoelectric effect can beexpressed as [6]ek ¼ dikEi ð2Þwhere dik is the piezoelectric charge constant.Therefore, the effective piezoelectric charge constant (d33)efffrom a clamped thick-film disc, polarized along the three directionwhich is parallel to the normal to the disc surface, can be obtainedfrom Eq. (2) asðd33Þeff ¼ e3=E3 ð3Þwhere e3 ¼w=t and E3¼V/t being V the DC voltage applied to thetransducer.As from Eq. (3) it is seen that the effective piezoelectric chargeconstant (d33)eff does not depend on the thickness of the transducer,its value can be determined fromðd33Þeff ¼w=V ð4ÞEvaluating the difference between the maximum out-of-planedisplacement component obtained from Fig. 5 by means of acurve-fitting algorithm and the deflection measured at thetransducer edge, and taking into account the applied DC voltageV¼314 V, the effective piezoelectric charge constant calculatedfrom Eq. (4) resulted ðd33Þeff ¼ 270 1012 N=C. This resultcompares quite well with the value ðd33Þeff ¼ 297 1012 N=Cwhich was determined in other thick-film transducers producedusing the same piezoelectric material and similar manufacturingconditions, through the measurement of the permittivity and otherelectromechanical parameters [5]. It is noted that due to thepreviously mentioned reasons, the thick-film transducer is lessactive piezoelectrically than the bulk ceramic material, as from thePZ27 data sheet d33 ¼ 425 1012 N=C [4].2 46 810 12x (mm)ed from the wrapped phase map shown in Fig. 3.0 2 4 6 8 10 1200.010.020.030.040.050.060.070.080.090.1x (mm)w (µm)Fig. 5. Out-of-plane displacement component along a line crossing the center of the thick-film transducer, obtained from Fig. 3.L.P. Tendela et al. / Optics and Lasers in Engineering 49 (2011) 281–2842845. ConclusionsThis paper presents the application of the digital speckle patterninterferometry technique to the evaluation of the electromecha-nical properties produced by a thick-film piezoelectric devicemanufactured with screen-printing deposition over a ceramicsubstrate. The experimental work presented here clearly demon-strates that the mentioned whole-field optical technique, with theaddition of a phase shifting facility for automating data analysis,allows the measurement of displacement fields in the nanometerscale if in-plane displacements or decorrelation are not introducedby the object deformation. As expected, it is also demonstrated thatthe measured out-of-plane deflections produced by the piezo-electric transducer do not replicate the classical piston movementproduced by a free sensor due to the clamping effect generated bythe substrate. Evaluating the maximum out-of-plane deflectiongenerated by the transducer for a known DC voltage, it is shownthat digital speckle pattern interferometry enables quite easily thedetermination of the effective piezoelectric charge constant (d33)effthat characterizes the thick-film ceramic. As known, this constanthas a much lower value than the one generated by a bulk ceramic ofthe same material due to the glass binder used to achieve betterconsistency in the sintered film, the porosity resulting from therelatively low sintering temperature and the clamping introducedby the substrate.The screen-printing technology to manufacture thick-filmpiezoelectric transducers has great flexibility, allowing batchproduction and facilitating the fabrication with different formatsand geometries including several substrates. Therefore, the inter-ferometric measurement of the effective piezoelectric chargeconstant presented in this work would enable the optimizationof the different parameters used in the fabrication procedure of thethick-film ceramic to obtain transducers with good electromecha-nical responses. In this way it will be possible to analyze theinfluence of different glasses, variations in the composition of thepaste, the residual porosity, the sintering temperature and variousother fabrication parameters, which will ultimately define theutility of the piezoelectric device for each application. Finally, theDSPI testing applied in this work will also enable the evaluation ofthe electromechanical response of individual thick-film piezo-electric transducers to form bidimensional arrays with differentgeometries.AcknowledgementsThe authors express their gratitude to SN Gwirc for proving thethick-film transducer used in this work. They would also like tothank the financial support provided by Agencia Nacional dePromoción Cientı́fica y Tecnológica of Argentina.References[1] Prudenziati M. Handbook of sensors and actuators: thick film sensors, vol. 1.Amsterdam: Elsevier; 1994.[2] Osten W, editor. Optical inspection of microsystems. Boca Raton: Taylor &Francis; 2007.[3] Rastogi PK, editor. Digital speckle pattern interferometry and related techni-ques. Chichester: Wiley; 2001.[4] Ferroperm piezoceramics data. /www.ferroperm-piezo.comS.[5] Gwirc SN, Negreira CA. Characterization of porous thick film PZT composite forbilayer ultrasonic transducers. Ferroelectrics 2005;321:41–52.[6] Kino GS. Acoustic waves: devices imaging and analog signal processing.Englewood Cliffs: Prentice-Hall; 1987.

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Evaluation of the piezoelectric behaviour produced by a thick-film transducer using digital speckle pattern interferometry

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Evaluation of the piezoelectric behaviour produced by a thick-film transducer using digital speckle pattern interferometry

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