Energy Harvesting Using a Lead Zirconate Titanate (PZT) Thin Film on a Polymer Substrate Thibault Dufay, Benoit Guiffard, Raynald Seveno, Jean-Christophe Thomas

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Energy Harvesting using a Lead Zirconate Titanate (PZT) Thin Film on a Polymer Substrate Thibault Dufay, Benoit Guiffard, Raynald Seveno, Jean-Christophe Thomas

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Thibault Dufay, Benoit Guiffard, Raynald Seveno, Jean-Christophe Thomas. Energy Harvesting using a Lead Zirconate Titanate (PZT) Thin Film on a Polymer Substrate. Energy Technology, Wiley, 2018, 6 (5), pp.917-921. 10.1002/ente.201700732. hal-01757007

HAL Id: hal-01757007 https://hal.archives-ouvertes.fr/hal-01757007 Submitted on 24 Oct 2019

HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Energy Harvesting using aLead Zirconate Titanate (PZT) Thin Film on aPolymer Substrate Thibault Dufay,*[a] BenoitGuiffard,[a] RaynaldSeveno,[a] and Jean-Christophe Thomas[b]

Acomposite structure with an highly flexible polymersub- performed to confirm the ferroelectric characteristics of the strate and athin film of lead zirconate titanate, Pb(Zr,Ti)O3 composite.Finally,energy harvestingmeasurements are real- (PZT),isrealized using an all-chemical process.The fabrica- ized with interdigitated electrodes structure.Amaximal tion of the structure comprises three steps:first, PZT is de- energy density of 20 mJcmÀ2 is obtainedwith manual me- posited on an aluminum thin foil, then the PZT thin film is chanical excitation and an output voltage up to 35 Vunder bondedtoa polymer, and, finally,aluminum foil is removed free oscillations conditions in bendingmode.This demon- by selectivechemical etching. Structural characterization strates that the recently developed PZT/polymer thin films techniques are used to ensurethe quality of the PZT/poly- are very promising for low-frequency vibrating energy-har- mer composite structure.Electrical measurements are also vestingapplications.

Introduction

Nowadays,energy harvesting from ambientand renewable the piezoelectric material from aprimary substratetothe sources is an objective so that we can become independent polymer substrate. from fossils energies.This objective is now close to be Our research team has recently developed reliable tech- reached because many research teams are working toward niquesfor the fabrication of thin films of lead zirconatetita- this goal. Forexample,when the wind is taken as an energy nate (PZT) on aflexible metallic substrate,acommercial source,there are already various scales of power that could aluminum(Al) foil with thickness less than 30 mm.[2] Thefab- be generated. Giant offshore windmills that are being devel- rication process is based on chemical solution deposition oped will soon provideenough energytopower cities.Ona (CSD) method and is cost effective.The light weight and the smaller scale,apersonal windmill could be used to power a weak stiffness of the micro-generator make it sensitive to air single house.Inaddition, the designofthe windmill could be flow.This PZT/Alstructure has been thoroughly character- modified in order to integrate it into the city landscape.[1] ized[3–5] and shows promising results for energyharvesting However, when the power needed is very small, for example under mechanical stress.[6] However, the metal-insulator- for powering small sensorsrequiring few hundreds of mW in metal (MIM) structure with full electrodes that has been operation, it would not be interesting to usethose kinds of tested implies ahuge capacitance,which limits the obtained wind energy harvesters whichwould waste part of the har- electrical energy density.Amore efficient design for energy vested energy.That is why some researchers are working on harvestingisthe interdigitated electrodes (IDE) structure low power energyharvesters that are capable of harvesting (without ground plane), whichalso presents the advantage of energy from low speed wind. working in the 33 (longitudinal) piezoelectric mode whose

Piezoelectric vibrating energy harvesters are apart of the piezoelectric coefficient d33 is roughly twice as large as the family of the mechanical energyharvesters and they could d31 in transversemode. be efficientenough to scavenge energy from low speed wind such as abreeze.Realizationofthis type of generators, which are able to withstandlarge wind flow induced deflec- [a] T. Dufay,Prof. B. Guiffard, R. Seveno tions,requires the developmentofanactive thin-layer on IETR UMR CNRS 6164, FacultØ des Sciences et Techniques flexible and insulating substrates.The useofpiezoelectric UniversitØ Bretagne Loire, UniversitØ de Nantes 2rue de la Houssinire,BP92208,44322 Nantes Cedex 3(France) polymers was not possible due to their low piezoelectric E-mail: [email protected] properties and the weakelectro-mechanical coupling. Thus, [b] J.-C. Thomas the challenge is to obtain apiezoelectric ceramic material, GeM (Institute for Research in Civil and mechanical Engineering), UMR which requires high crystallization temperatures (>6008C), CNRS 6183 on apolymer substrate that cannot withstand such high tem- UniversitØ de Nantes-Ecole Centrale Nantes 2rue de la Houssinire,BP92208, 44322, Nantes Cedex 3(France) peratures.Inthis case,two main methods may be envisaged: to develop acomplete low temperature process or to transfer

1 To obtain an IDE structure,PZT films must be deposited realizedtostudy the energy harvesting properties of the on an insulating substratewith good flexibility.Specific poly- transferred PZT thin film. mer materials including some thermoplastics are good candi- Structural characterizations are given in Figure 2: panel dates because they present the required properties.However, (a) X-raydiffraction (XRD) patterns of PZT on aluminum the direct synthesis of PZT ontothe polymeric substrate is and transferred to PET,and panel (b) cross-section SEM avoided by the low melting temperature of the soft polymers. image of PZT transferred on PET.XRD patterns of PZT/Al One solution is to transfer the PZT thin film, obtained by and PZT/PET are roughly the same except for the substrates classicalmethods on rigid substrates,onto the polymeric sub- peaks.All the peaks of PZT remain at their positionsbut the strate.This solution was used to realizePZT on apolymer, peak of aluminum disappearsinthe pattern of PZT/PET and but the method employed—laserlift-off—is expensiveand is replaced by the peak of the polymer substrate.The slight could be difficult to transfer to the industry.[7–9] This process difference in 2V angle observed is aconsequence of the flexi- is based on chemical solution depositionofPZT on high-cost ble substrates used. They induce different base surfaces for rigid and transparentsubstrate (e.g.,sapphire). After this, the two cases,leadingtoshifts in the 2V angle. the polymer substrate is attached to PZT.The flexible piezo- Thecross-sectionSEM image of transferred PZT (Fig- electric structure is released from the rigid substrate by mul- ure 2b)has been realized to estimate the bondstrengthbe- tiple laser shots (squared spots of 500 mm500 mm) to cover tween PZT and PU and between the two layers of polymer. all the centimetric surface of the sample. Thesteps require SEM observations reveal good adhesion between the oxide high cost equipmentand are not easily adaptablefor serial layer and the stack of polymers. production. This is why the current study is devoted to the Thetransferred structure (PZT/PET) must be poled development of asimplemethod to achieve aPZT/polymer before any electric tests. In order to induce macroscopic pie- structure. zoelectric properties to the transferred PZT,astatic electric Here,wefocus on the processdeveloped to transfer apie- field is applied through the interdigitated electrodes.Apho- zoelectric PZT thin film from aluminumfoil to aflexible polymersubstrate.Photographs of PZT/Aland PZT/polymer bilayers are presented in Figure 1. Polyethyleneterephtalate (PET) is the most used polymerasinsulating and flexible substrate. Thechemical process is cheap and simple,and would be easily transferred to an industrial scale.Inaddition, with this method, the metallic substrateisremoved from the complete surface (~6 cm2)ofPZT thin film in one step whereas the laser lift-off methodimplies severallaser shots to separate PZT from the first substrate.Structural and electrical characterizations of the PZT/polymer thin film are also presented to confirm the good quality of the transferred PZT thin film. Finally,energy harvestingmeasurements with IDE structures after the transfer to the polymer substrateare realized to explore the possibilities of this new generator.

Figure 1. Photographs of a) PZT/Al structure and b) PZT/PET IDE structure.

Results and Discussion Themethod described in Experimental Section demonstrates how to transfer aPZT thin layer ontoaflexible substrate. Theinteresting pointisthat it is possible using asimple Figure 2. a) XRD patternsofPZT before and after transfer from the aluminum chemical method. Theobtained piezoelectric thin film on a substrate to the PET substrate. b) Cross-sectionSEM imagesofPZT trans- polymer substrate has been characterized.IDE design was ferredonto polymer.

2 Figure 3. PZTpoling bench for IDE samples. tograph of the polingbench is presented in Figure 3. The sampleispreheated at atemperature of 1008Cbyusing a hot plate.Thisisrealized to preventthe risks of electric breakdown during the poling step.ADCfield of 50 kV cmÀ1 is applied for 2hours and the electric field is maintained while the temperature of the PZT/PET returns to ambient. With higher electric field values,many breakdowns occur and damage the PZT sample or the IDE. Bending mechanical excitationsaround 2Hzare applied manually on aPZT/PETbeam clamped at one end. The order of magnitude of the deflection at the free end is 10 mm. Thedelivered voltage is recorded using an oscillo- scope with three different load resistances.The sampleex- hibits avery weak capacitance around 1.6 pF,which leads to ahigh optimal load resistance,close to 49 GW.Inthe labora- tory,the maximal load resistance available is only 100 MW, which allowsextrapolation of the maximum power that a micro-generator PZT/PET could deliver. Theoretical curves were calculated for harmonic deformations of 2Hzwith a Figure 4. Voltage, current, and power delivered by PZT/PETunder mechanical piezoelectric current of 115 nA. They areplotted in Figure 4 excitation at 2Hz: a) experiment and theory,b)zoom on power and voltage curves. with the three experimental measurements realized at load resistances of 1, 10, and 100 MW.Good agreement between the experimentand the theory is observed. micro-generator made of PZT on polymer reached amaxi- In theory,the open-circuit voltage may be of 5.6 kV and mal energy density of 259 mJcmÀ2.[7] This value is 10 times the maximal power could reach 160 mWatthe optimal load larger than the harvestedenergy with PZT transferred onto resistance.The energy density, Emax,ofthe micro-generators polymer presented in this article but the excitation is still not is calculatedusing the followingformula where I0 is the pie- exactly the same.Aquasi-harmonic deformationwas used zoelectric current, C is the capacitance, S is the active sur- for the measurements presented here,while the Park team face,and f is the mechanicalexcitation frequency: applied punctual stress in both the upward and downward di- rections. In the two cases,the voltage signals appear to be si- I2 ¼ 0 nusoid, but when the method employed by Park is used, Emax 8pCSf 2 higher output voltages are obtained, as it is demonstrated in the following paragraph. À Amaximal energy density of 20 mJcm 2 is obtained with Other promising resultswere obtained with bendingPZT/ an active surface of 4 cm2 (red perimeter in Figure 7). These PET samples when they are punctually deflected and subse- theoretical results are promising but need to be confirmed by quently released. Thethree curves presentedinFigure 5cor- measurements. respondtothe signal delivered by the micro-generator with Theexperimental set-up is an importantpoint in the the 1, 10, and 100 MW load resistances.High voltage peak output power measurements because of the characteristics of amplitudes around 35 Vwere recorded with aload resistance the external stress (magnitude,frequency if harmonic excita- of 100 MW.This is promising for future applicationsthat will tion) applied to micro-generators.Thus,itisdifficult to com- be developed to harvest energy from low frequencymechani- pare results from the literature data directly. Theclosestme- cal excitations. chanicalexcitation features we have found in the literature Thepiezoelectric current produced with the IDE structure are those used by Park et al.[7] With their high-cost process,a is smallerthan the one already obtained with the samePZT

3 products.Initially,lead acetate was dissolved in asolution of acetic acid using areflux technique.Zirconium n-propoxide and titanium n-propoxide were mixed in the desired proportions to obtain aZr/Tiratio of 54/46 and added to the lead solution. The final precursor solution was obtained by adding ethylene glycol, which limited the crack formations in PZT thin film during its thermal treatment.

Theprecursor solution was then deposited using aspin-coating technique on asacrificial substrate,that is,analuminum thin foil, which could be etched later using asimple chemical process.A stainless steel support was used in order to facilitate the spin- coating deposition. This technique allowed the formation of a piezoelectric film on the complete surface of the aluminum substrate.The rotation speed (6000 rpm) and spinning duration (20 s) were chosen according to the desired thickness of the thin layer (300 nm). After each spin-coating step,the material was 8 Figure 5. Voltage deliveredbyPZT/PETfollowing apunctual deflection and subjected to athermal treatment at temperature of 650 Cfor subsequent release. the duration of 2min to crystallize the PZT thin layer. In prac- tice,the thickness of the PZT thin layer obtained after the exe- cution of one time spin-coating step was about 300 nm. Thus, to form athicker PZT thin film, deposition step could be repeated [4] with an MIM structure ( 2 mA ). That means that the pie- as many times as necessary to obtain athin film of several micro- zoelectric properties of PZT samples with IDE structure are meters.Figure 6provides schematic illustrations of the next certainly lower than those with MIM electrodes structure.A three steps of the PZT transfer on plastic substrate. solution to improve the piezoelectric properties of the PZT with IDE is to optimize the polingprocedure.

Conclusions Achemical process for transfer of PZT to apolymer substrate is described in this article.The realization of the PZT by CSD on asacrificial substrateiscomplementary with chemical techniques used to transfer the piezoelectric layer onto the polymer. Thedeveloped process could be easy to industrialize. Structural characterizations were realized to ensure good adhesionbetween the different layers after the transferand to check the crystalline quality of the PZT. It appearsthat the transfer is effective and that the crystalline quality is preserved after transfer. Finally,energy harvesting tests were made after implementing an IDE structure on the top surface of the PZT thin film with apolymer substrate. Only three load resistances were tested due to the very low capacitanceofthe generators,which implies ashift of the op- Figure 6. Schematic illustration of the process of PZT transfer. timal load towards the high resistance values.The three experimental valuesofoutput current,voltage,and harvested To ensure good adhesion between the piezoelectric layer and the power are in good agreementwith the theoretical curves.A further substrate,athin adhesive layer of polyurethane (PU, theoretical maximum value of 5600 Vwas found for the NOA81, Norland Optics)was added onto PZT by spin-coating. open-circuit voltage.Amaximum powerof160 mWand a Theadhesive layer was exposed to UV light for 2h,toevaporate maximum energy density of 20 mJcmÀ2 were calculated by solvents and reach the final mechanical properties of the adhesive layer (UV curing). Polyurethane NOA81 has elastic nature, extrapolation with the theory.Those interesting values which is required for the fabrication the flexible piezoelectric should be weighted by the fact that they are only obtained structure and it ensured good bond strength between the PZT using theoretical calculations. An experimentwith higher and the polymeric materials.The separation from the aluminum load resistance is alreadyenvisioned to confirm the results ground plane needed to occur before the realization of IDE. presented in this article. However,PZT thin films are not enough rigid to stand alone, so they must be fixed on the new polymer substrate. Experimental Section Athick elastic polymer layer of 75 mmwas attached on the PU adhesive using athermofusing technique:asmall pressure and PZT thin layer fabrication began with the preparation of apre- heat were applied to create good adhesion between the two poly- cursor solution, which was obtained by mixing different chemical mer layers.The permanent substrate of the final flexible piezo-

4 electric structure was then formed. Commercial polyethylene ter- 300 nm. Gold and aluminum electrodes of 200 nm of thickness ephthalate (PET) was asuitable thermoplastic material for the were tested. Thebest results were obtained with gold because of new substrate because it is an elastically deformable and low- its better flexibility.With aluminum, electrode discontinuity cost polymer. Furthermore,several PET thicknesses were avail- could happen during experiments. able to tune the mechanical properties of the final multilayer in- XRD patterns were obtained using aBruker D8 diffractometer l= q= 8 cluding stiffness and flexibility. with CuKa radiation ( 1.5406 )and scanning from 2 20 to At this point, the obtained structure still comprised the alumi- 2q=608 at a0.038 scan rate.ACarl Zeiss Merlin SEM was used num sacrificial substrate.This structure was immersed in an iron to realize cross-section images with electron energy set to 3keV. chloride solution of formula FeCl3 to undergo aselective chemical etching of the aluminum foil. Thechemical agent iron chloride FeCl3 etched aluminum in less than 5min when apure solu- Acknowledgements tion was used at room temperature.Itappeared to be inactive in the presence of piezoelectric and polymeric materials (PZT,PU, The authors would like to thank Jean-Emmanuel LechÞne and PET). from Cookson SAS (Cholet, France) for his receptiveness to Abasic flexible piezoelectric structure was finally obtained after supplythe shadow masks required for electrode deposition. the total dissolution of aluminum. Thecrystalline piezoelectric This work was supported by the French region Pays de la thin film was fixed on astack of elastically deformable polymeric Loire throughthe 2014 07965 contract. layers,which were used as the flexible permanent substrate.This structure was free of ground plane and completely flexible. To finalize the interdigitated structure,electrodes were evaporat- Conflict of interest ed through ashadow mask. Thedimensions of the IDE design were given in Figure 7. The authorsdeclarenoconflictofinterest.

Keywords: chemical transfer · energy conversion · interdigitated electrodes · polymers · PZT thin films

[1] L’USINE Nouvelle, http://www.usinenouvelle.com/editorial/l-eton- nant-arbre-a-vent-de-new-wind-agite-ses-feuilles-et-electrise-la- cop21.N367355, accessedinFebruary 2016,inFrench. [2] R. Seveno, D. Averty, J. Sol-Gel Sci. Technol. 2013, 68,175–179. [3] T. Dufay,B.Guiffard, J.-C.Thomas, R. Seveno, J. Appl. Phys. 2015, 117,204101. [4] R. Seveno, J. Carbajo,T.Dufay,B.Guiffard, J. C. Thomas, J. Phys.D 2017, 50,165502. [5] R. Seveno, B. Guiffard, J.-P.Regoin, Funct. Mater.Lett. 2015, 08, 1550051. [6] R. Seveno, B. Guiffard, T. Dufay,J.C.Thomas, 2015 Jt.IEEE Int. Symp.on the Appl.Ferroelectr.(ISAF), Int. Symp.onIntegr.Funct.(I- SIF), and PiezoelectricForce Microsc. Workshop (PFM), 2015,IEEE CFP15ISA-POD,ISBN 978-1-4799-9975-0, pp.94–97. [7] K.-I. Park, J. H. Son, G.-T.Hwang, C. K. Jeong,J.Ryu, M. Koo, I. Choi, S. H. Lee,M.Byun, Z. L. Wang, K. J. Lee, Adv.Mater. 2014, 26, 2514–2520. [8] Y. H. Do,W.S.Jung, M. G. Kang, C. Y. Kang, S. J. Yoon, Sens.Actua- tors A 2013, 200,51–55. [9] Y. H. Do,M.G.Kang, J. S. Kim, C. Y. Kang, S. J. Yoon, Sens.Actua- Figure 7. Scheme of the IDE design. tors A 2012, 184,124 –127.

Thewhole active surface was around 4 cm2 including all the digits and the connexions lines.The length of one digit was 1.5 cm, its width was 200 nm, and the gap between two digits was