1056 IEEE SENSORS JOURNAL, VOL. 10, NO. 6, JUNE 2010

Opacity Sensor Based on Photovoltaic Effect ofFerroelectric PLZT Ceramic With Pt Wire Implant

Ernesto Suaste-Gómez, José de Jesús Agustín Flores-Cuautle, and Carlos Omar González-Morán

Abstract—This work reports the formation of a ferroelectricpoled bulk ceramic structure as opacity sensor (OPS) and theimproved photovoltaic output of the lead lanthanum zirconatetitanate (PLZT) employed. This ceramic was implanted duringits fabrication with a platinum wire (Pt-wire) which works as aninternal electrode. Photovoltaic current have been measured inPLZT with Pt-wire under chopped LASER beam illumination.This photovoltaic current was measured on the upper face of asensor in order to obtain a characterization curve. Different thinmaterials were put on the OPS in order to determine its opacityaccording to their thickness and viscosity. Opacity is a conditionin which a material partially or completely impedes the passageof light beam. The results indicated that the photovoltaic currentresponse was less than 700 pA at 160�� ��

� maximum powerof illumination; 2-D scans were obtained from fruit tissues, vegetaloils and thin materials at 160�� ��

� of illumination in orderto get a representation of opacity images.

Index Terms—Opacity images, opacity sensor, photovoltaic ef-fect, PLZT-Pt wire implanted ceramics.

I. INTRODUCTION

T HERE are several kinds of ferroelectric materials thatexhibit photovoltaic effects under near-ultraviolet light.

When the material is illuminated after poling, voltage andcurrent can be generated due to the separation of photo in-duced electron and holes caused by its internal electric field.This is considered an optical property of the material itselfwhich has potential applications for supplying energy transferin microelectromechanical systems and optoelectronic de-vices [1], [2]. The steady current in the absence of appliedvoltage, called photocurrent, is considered the result of photocarriers and the asymmetric electromotive force induced bynear-ultraviolet radiation [3]. Therefore, photocurrent is a veryimportant parameter for optical detection [4]. The behavior ofthe photovoltaic effect in ferroelectrics is similar to that of thephotovoltaic effect in semiconductor p-n junctions.

The semiconductor p-n junction is an interface; in order toextract an electrical output, it is indispensable to apply a biasvoltage to it; however, this electrical stimulus is unnecessary forproducing the photovoltaic effect on ferroelectrics [5].

Manuscript received October 12, 2009; revised December 08, 2009; acceptedJanuary 27, 2010. Current version published April 02, 2010. This work was sup-ported in part by ICTP, Trieste-Italy of the Latin-American Network of Ferro-electric Materials (NET-43). The associate editor coordinating the review of thispaper and approving it for publication was Dr. M. Abedin.

The authors are with the Centro de Investigación y de Estudios Avanzados delIPN (CINVESTAV), Electrical Engineering, Bioelectronics, MEX (e-mail: es-uaste@cinvestav.mx; coglez@gmail.com; jflores_cuautle@hotmail.com; cgon-zale@cinvestav.mx).

Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/JSEN.2010.2042953

There are three types of photovoltaic samples [6], namely, abulk single plate [7], a bulk bimorph [8] and film [9]. The singleplate exhibit high voltage. The bimorph has a large degree ofdistortion, fast mechanical response, and is suitable for mecha-tronics. The film shows a high current output and is a good cur-rent source for the optical sensor of the MEMS. Thus, the outputpower of the film is still one order of magnitude lower than thatof the bulk structures reported here.

The photovoltaic effect of many samples (as fruit tissues, veg-etal oils, and thin materials) on the OPS cause photovoltaic cur-rent when exposed to LASER beam.

This new type sensor has some interesting advantages suchas free upper face (it is not necessary to add any electrodes),it is a better voltage source than multilayer film PLZT and theoutput from the photovoltaic effect of bulk PLZT is controllableby only varying the light intensity [5].

Among such photovoltaic materials, lead lanthanum zir-conate (PLZT) is the most popular. PLZT is a ferroelectricsolid solution with wide-ranging material properties whichdepend on its composition [10]. Recently, a new PLZT bulksingle plate, called opacity sensor (OPS), has been built. Thissensor has a Pt-wire implant to get a free upper face in orderto measure the opacity percentage of thin materials and liquidsamples. This structure design includes a modified electrodeconfiguration: it has a Pt-wire introduced into a bulk PLZT.The Pt-wire was chosen as implant because it possesses highresistance to chemical attack, excellent high-temperature(melting point 1768.3 C) characteristics, stable electricalproperties, and thermal conductivity with small variations [11].The sample preparation and its characteristics are described inthe following sections [12].

The bulk PLZT has many applications as thermal, mechan-ical, and optical sensors [13]. It was proposed to verify if itsoptical property could be used as opacity sensor. Therefore,two experimental setups were used in order to get the pho-tovoltaic current in 1-D way, and 2-D scans were performedusing a LASER source of 650 nm with a maximum power of160 mW in order to obtain opacity percentage and opacity im-ages, respectively.

II. METHODS

Among several kinds of photovoltaic material, a PLZT, whichis a ceramics of withand generally denoted as (9/65/35) was chosen. Itwas made by the oxide-mixing technique. The raw materialswere mixed by ball-milling with an electronic mill (Pulverisette2, Fritsch) for 20 minutes; some polyvinyl alcohol drops wereadded at a rate of 1.5 drops per each gram of mixture. Powders

1530-437X/$26.00 © 2010 IEEE

SUASTE-GÓMEZ et al.: OPACITY SENSOR BASED ON PHOTOVOLTAIC EFFECT OF FERROELECTRIC PLZT CERAMIC WITH Pt WIRE IMPLANT 1057

Fig. 1. OPS and its configuration. (a) Schematic OPS implanted with Pt wire.(b) Graphical symbol of OPS.

TABLE IAVERAGE THICKNESS OF DIFFERENT THIN MATERIAL SAMPLES

then were pressed into discs of 10 mm diameter and 2 mm ofthickness; the pressure applied was 3500 .

During this process, a Pt-wire of 0.32 mm diameter was im-planted in the middle of the ceramic in transversal position, thus,a metallic electrode in totally immersed into the ceramic wascreated. These discs were sintered in air with a heater ramp rateof 5 C/min from room temperature to 600 C and a secondheater ramp rate of 10 C/min from 600 C to 1200 C for1 hour into a platinum crucible. After sinterization, silver elec-trodes were deposited on lower side face and Pt-wire of theOPS. Finally, the discs were electrically poled, at 1.5 kV/mmfor 1 hour at 60 C in a silicone oil bath, to be used in photo-voltaic measurements.

Fig. 1 shows the schematic of the OPS and the graphicalsymbol of the schematic applications, respectively.

The thickness of each thin material was measured and thevalues obtained are show in Table I. Two types of oil were usedas liquid samples: soy oil and linseed oil. The thin materialsand liquids were analyzed on the first setup in a 1-D way. Othermaterials like plant leaves, a steel nut and a cross made withsingle-sided adhesive copper conducting tape 3M Code 1181were recorded in a bidimensional scan.

III. EXPERIMENTAL

Photovoltaic currents from translucent samples were obtainedby the 1-D experimental setup described below; a standardmethod was used in order to estimate the photovoltaic effect.Fig. 2 shows the first setup used in order to obtain photovoltaiccurrent from translucent samples. In this case, a fiber-coupledLASER system BWF1 model BWF-650-15E/55369 with awavelength of 650 nm and maximum power of 160 mW wasused [14]. This LASER was modulated to a frequency of 3 Hzwith a Beckman Industrial Function Generator; the distancebetween sample and LASER beam was 1 cm. The electricalcontacts (on the lower side face electrode and Pt-wire) of thesensor were connected to an SRS-Low Current Noise Pream-plifier SR570 in order to obtain photovoltaic current. These

Fig. 2. One-dimensional experimental setup of OPS system.

Fig. 3. Bidimensional experimental setup of OPS system.

signals were registered by an Oscilloscope Agilent DSO3062Awhich measured their peak to peak voltage.

The second experimental setup used to obtain 2-D opacityimages from a plant leaf (Myrtus communis), steel nut and acopper cross is shown in Fig. 3. A fiber-coupled LASER systemBWF1 mod, BWF-65015E/553369 was modulated at the samefrequency of the first setup but in this case, a set of lenses whichreduce the LASER beam spot to a 0.5 mm of diameter size wasused. The sample was placed on the OPS and both were placedon an X-Y translation microscope stage.

The analysis of each sample was done millimeter by mil-limeter with a total of 100 points recording for each sample. Theelectrical contacts from OPS were connected to an SRS-LowCurrent Noise Preamplifier SR570 in order to obtain 2-D scans;these signals records were registered by an Oscilloscope Agi-lent DSO3062A which measured their peak to peak voltage inorder to construct a 3-D graphics in which the axis representsthe magnitude of the photovoltaic current of each record.

IV. RESULTS

In the 1-D setup, the OPS showed satisfactory results be-cause it reacted to a wide range of detection which allowedto have higher resolution at a higher intensity of illumination.Photovoltaic current was 688 pA, in the region of highest illu-mination intensity, 160 for the OPS without sample.The graphic of OPS shows a clear difference among thin ma-terials and liquids, Fig. 4. The OPS recording without samplesdetermines 0% of opacity; while for the recordings with sam-ples, this percentage increased according to the characteristicsof each of them. Table II shows the percentage of opacity at

1058 IEEE SENSORS JOURNAL, VOL. 10, NO. 6, JUNE 2010

Fig. 4. The OPS detected curves for different liquids and thin solids.

Fig. 5. OPS response without sample in a Bi-dimensional scan. (a) 3-D repre-sentation of OPS. (b) Top view of OPS in 2-D scan.

91.43 of illumination; this value was chosen as ex-ample of the opacity percentage of each sample.

In the bidimensional setup, firstly a 2-D scan responsewithout sample was obtained; the records were from 1 to 10 in

axis and from to in axis, this graphic shows a greatincrement in the Pt-wire zone because the ferroelectric domainsare very close to each other. The maximum photovoltaic currentresponse was 3.28 pA at 160 of illumination; thephotovoltaic current response was lower because the LASER

TABLE IIOPACITY PERCENTAGE OF DIFFERENT LIQUIDS AND THIN MATERIALS AT 91.43

����� OF ILLUMINATION USING ONE-DIMENSIONAL SETUP

Fig. 6. OPS response with a steel nut, to the right a little picture of the steelnut. (a) 3-D Graphic. (b) 2-D scan.

beam spot area was smaller. In this bidimensional experimentalsetup the 2-D scans were performed using a plant leaf, steel nutand a cross made of copper. The maximum photovoltaic currentresponse was 0.1 pA at 160 of LASER illumination,as shown in Figs. 6–8.

Fig. 6 shows the magnitude of the photovoltaic current of thesteel nut registered by the OPS. The edges associated with thegraphic are due to the curved form of the nut which dispersed thelight and caused that the signal was detected in an area differentto the one expected.

The records obtained from a plant leaf gave good results and achange of opacity in its different regions, Fig. 7. The magnitude

SUASTE-GÓMEZ et al.: OPACITY SENSOR BASED ON PHOTOVOLTAIC EFFECT OF FERROELECTRIC PLZT CERAMIC WITH Pt WIRE IMPLANT 1059

Fig. 7. OPS response with a plant leaf (Myrtus communis). (a) 3-D Graphic.(b) 2-D scan.

of the signal increases in the periphery because the plant leafdid not cover all the OPS, as shown in the little picture in themiddle.

Finally, the acquisition of the signal of the copper cross, onthe OPS, was obtained in order to verify each point of the sampleso that its image could be constructed, Fig. 8.

These results demonstrated that the OPS allowed to perform2-D scanning and thus generated 3-D graphics or images ofpacity which varied according to the percentage of opacity ofeach sample in the bidimensional setup.

Quantitative sensitivity comparison with other types ofopacity/transmission would be relative due to the chemicalcompositions of each sensor and because finally whatever thetype of sensor, they only give a percentage measure parameter.The most significant differences among ferroelectric sensorsand Si-based sensors are: 1) Si-based sensors require electricalexternal supply and do not have domains; in contrast, ferro-electric sensors do not require an external electrical supply[15]; 2) the Si-based sensors have greater sensitivity than

Fig. 8. OPS response with a copper cross. (a) 3-D graphic. (b) 2-D scan.

ferroelectric sensors but these last offer a good response atextreme temperatures, up to 570 , according to its chemicalcomposition (ferroelectric phase transition) [16].

V. CONCLUSION

It was shown that the PLZT bulk with Pt-wire has the fol-lowing advantages: (1) The increase in surface analysis is supe-rior because of the role of Pt-wire that works as a third electrode.(2) The OPS has a useful working range of photovoltaic currentbecause it can capture a power of 330 of LASER illu-mination and give a photovoltaic current response of 1280 pA.Furthermore, the range of operating temperature of the OPSreached the Curie temperature of 365 C, while a standard sil-icon photo detector operating up to 125 C maximum; due tothose parameters, the sensor proposed in this work can be usedat much higher temperatures and higher illumination intensity ascompared to conventional Si-based OPS. (3) The OPS is easy tomanufacture and its size can be modified. The production costis lower than that of any other semiconductor photo detector.(4) It allows to perform 2-D scanning and generate 3-D graphicsor images of opacity depending on the percentage of opacity ofthe sample in the bidimensional setup. In general, the OPS offer

1060 IEEE SENSORS JOURNAL, VOL. 10, NO. 6, JUNE 2010

good versatility as a sensor of opacity due to its ferroelectricityand could have novel applications such as before transplant 2-Dcornea scan in order to verify its opacity, 2-D scan opacity ofinsects (Entomology), before transplant 1-D skin scan in orderto analyze its color, 1-D scan on real time of CO2 car emissions[17]–[20].

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[3] K. Tonooka, P. Poosanaas, and K. Uchino, “Mechanism of the bulkphotovoltaic effect of ferroelectrics,” in Proc. SPIE., 1998, vol. 224, p.3324.

[4] M. Qin, K. Yao, Y. C. Liang, and S. Shannigrahi, “Thicknesseffects on photoinduced current in ferroelectric ��� �� ���� �� thin films,” J. Appl. Phys., vol. 101, pp.014104-1–014104-4, 2007.

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[9] M. Ichiki, R. Maeda, Y. Morikawa, Y. Mabune, T. Nakada, and K.Nonaka, “Photovoltaic effect of lead lanthanum zirconate titanate in alayered film structure design,” Appl. Phys. Lett., vol. 84, pp. 395–397,2004.

[10] G. H. Haertling and C. E. Land, “Hot-pressed �������������ferroelectric ceramics for electrooptic applications,” J. Amer. Ceram.Soc., vol. 54, p. 1, 1971.

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[12] E. Suaste and A. Flores, “Behavior of the temperature dependence ofdielectric constants and Curie temperature of pt-implanted modifiedBaTiO3, KNbO3, PbZrO3, Pb0.88LN0.08Ti0.98Mn0.02O3 ( �� ��, Eu) ceramics,” in Proc. XVII IMRC and VII Congress of NACEInt., Can-Cun, México, Aug. 17–20, 2008, (S7-P1).

[13] E. Suaste-Gómez, C. O. González-Morán, and J. J. A. Flores-Cuautle,“Developed and applications of a novel ceramic-controlled piezoelec-tric due to an implant of Pt-wire into the body of sigle disk of���ceramic,” in Proc. 25/XII, WC2009, IFMBE, O. Dössel and W. C.Schlegel, Eds., 2009, pp. 89–92.

[14] E. Suaste-Gómez and C. O. González-Morán, “Photovoltaic effect oflead-free piezoelectric ceramics, �� �� � �� � and�� ���� �� � ( �� ��, Eu),” Ferroelectrics.,vol. 386, pp. 70–76, 2009.

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[16] F. Jona and G. Shirane, Ferroelectric Crystals. New York: Oxford,1993, pp. 389–391.

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[18] H. M. Dekking, “Opacity meter for cornea and lens,” Ophthalmologica,vol. 115, no. 4, pp. 219–226, 1948.

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Ernesto Suaste-Gómez received the M.Sc. degree inbioelectronics and the Doctorate degree in biomed-ical engineering in 1997 from the Centro de Inves-tigación y Estudios Avanzados (CINVESTAV-IPN),México D.F.

He was a Visiting Researcher from the Universityof California, Berkeley, the California State Univer-sity, Long Beach, NIH Bethesda, MD, and the Fac-ulty of Informatics and Automation of the TechnicalUniversity of Ilmenau, Germany, He has publishedover 100 research papers and patents in this area. His

interest areas are ferroelectric, piezoelectric and pyroelectric materials, ultra-sound and human vision instrumentation (eye behavior).

José de Jesús Agustín Flores-Cuautle was bornin México City, México. He majored in bionicsfrom the Professional and Interdisciplinary Unitof Engineering and Advanced Technologies, Insti-tuto Politécnico Nacional (UPIITA-IPN) in 2006,and received the M.S. degree in electrical engi-neering in 2008 from the Centro de Investigacióny Estudios Avanzados (CINVESTAV-IPN). He iscurrently working towards the Doctoral degree atCINVESTAV.

His interest areas are ferroelectric, piezoelectricand pyroelectric materials, ultrasound and human vision.

Carlos Omar González-Morán received the B.Eng.degree from Benemérita Universidad Autónoma dePuebla (BUAP), Puebla, Pue. México, in 2003 andthe M.Sc. degree from the Centro de Investigacióny Estudios Avanzados (CINVESTAV-IPN), MéxicoD.F., in 2004, both in electronics and bioelectronics,respectively. He is currently working towards theDoctorate degree at the Centro de Investigación yEstudios Avanzados (CINVESTAV-IPN), México,D.F.

His research interests include ferroelectrics ce-ramics, PVDF, and sensors.