piezoelectric flexible energy harvester based on...

Piezoelectric Flexible Energy Harvester Based on BaTiO3Thin Film Enabled by Exfoliating the Mica Substrate

Dong Yeol Hyeon and Kwi-Il Park*

Flexible piezoelectric energy harvesters (f-PEHs) have exhibited significantpotential as long-lasting self-powered sources or sensor devices, as they cangenerate reliable and repeatable electricity under harsh and tiny mechanicalbending cycles without restraints anywhere and anytime. Herein, a new approachfor transferring piezoelectric ceramic thin films onto a single flexible substrate viapiecemeal elimination of the sacrificial mica substrates is proposed. The crys-tallized piezoelectric BaTiO3 thin film on a rigid mica substrate with electrodesand passivation layers is peeled off by means of a physical delamination processusing sticky tape as the remover. The film is then transferred onto a flexiblepolyimide substrate using a polymer elastomer as a support. The fabricatedBaTiO3 thin-film f-PEH successfully converts an open-circuit voltage of �0.5 Vand a short-circuit current of �30 nA from repeated mechanical bendingdeformations. The energy generation mechanism and performance of theperovskite BaTiO3 thin-film f-PEH is supported using finite-element analysis(FEA) with multiphysics simulations. The novel transfer techniques adopting theremoval of sacrificial mica layers opens the door to various high-performanceflexible and wearable applications based on all-inorganic materials.

Energy harvesting technologies that can provide the permanentelectricity from various renewable energy resources have gainedattention in the scientific society, as they enable the demonstra-tion of the implantable, wearable, and Internet of Things (IoT)system.[1–5] Among the various technologies, adopting themechanical energy, such as bending, pressure, vibration, andeven body activities, is more attractive than all others for feasibleenergy generation and applications anywhere and anytime with-out constraints.[6–8] One way of power generation from mechan-ical energy sources is to use the piezoelectric effect, whichproduces the electric potential difference inside the ferroelectricmaterials due to the polarization change of dipole moments. Thefabrication of nanogenerator made of flexible piezoelectric nano-materials is particularly attracting interest because it can convertthe electric energy from tiny movements.[6,9]

The advent of a flexible piezoelectric energy harvester (f-PEH)will be a great innovation in a self-powered system, which enables

the operation of commercial electronicdevices and the sensing/monitoring of bio-mechanical motions.[10–12] To demonstratethe mechanically flexible and robust energydevice, a variety of piezoelectric nanomateri-als, such as 0D nanoparticles, 1D nanowires/nanotubes/arrays, 2D nanosheets, and thinfilms, have been used based on ZnO,[10,12–14]

BaTiO3,[15–20] PbZrTiO3 (PZT),[21–24]

Pb(Mg1/3Nb2/3)O3–PbTiO3 (PMN-PT),[1,25,26]

(1�x)Pb(Mg1/3Nb2/3)O3–(x) Pb(Zr,Ti)O3

(PMN-PZT),[27] 0.5(Ba0.7Ca0.3)TiO3–0.5Ba(Zr0.2Ti0.8)O3 (BCTZ),[28] MoS2,

[29,30] andboron nitride (BN)[31–33] by many researchersin the past few years. Especially, theperovskite-structured thin film on a singleplastic substrate enables the highly effi-cient and lightweight f-PEH with energygeneration efficiency during tiny mechan-ical and vibrational movements.[11,24,34,35]

Many researchers have used the varioustransfer technologies, such as the stampingwith sticky polymer,[11,34] laser lift-off(LLO),[24,36,37] exfoliation with thick Ni

film,[1] and mechanical polishing,[27] to realize the thin-film-basedf-PEHs. Park et al.[11] demonstrated the piezoelectric BaTiO3 thinfilm onto flexible substrate using the complicated microfabrica-tion processes and soft-lithographic transfer from bulk Si sub-strate. By the LLO process, an entire area of the PZT thinfilm on sapphire substrates can be transferred onto flexible poly-ethylene terephthalate (PET) substrate.[24,37] A single crystallinepiezoelectric PMN-PT thin film on mother Si substrates wasexfoliated and was transferred onto a flexible substrate by theresidual stress of thick electroplated Ni film-based stressor.Hwang et al. transferred the inherently excellent piezoelectricPMN-PZT thin film on bulk plate grown by a solid-state singlecrystal growth method onto a PET substrate by polishing anddelamination processes.[27] Themajor issue in the demonstrationof f-PEH made of perovskite thin films is to transfer thecrystallized ceramic thin film, which inevitably requires ahigh-temperature process for achieving high-quality thin filmonto a single plastic substrate. The aforementioned complicatedand unstable fabrication processes can lead to microcrack andlimit the activation area, which is directly related to the electricoutput performance of f-PEHs.

In this study, a new approach for piezoelectric ceramic thinfilm onto plastic substrates is proposed by adopting the exfolia-tion of 2D sacrificial mica substrates. A perovskite-structuredBaTiO3 thin film deposited on a mica substrate by radio fre-quency (RF) magnetron sputtering with a sintered ceramic target

D. Y. Hyeon, Prof. K.-I. ParkSchool of Materials Science and EngineeringKyungpook National University80 Daehak-ro, Buk-gu, Daegu 41566, Republic of KoreaE-mail: [email protected]

The ORCID identification number(s) for the author(s) of this articlecan be found under https://doi.org/10.1002/ente.201900638.

DOI: 10.1002/ente.201900638

COMMUNICATIONwww.entechnol.de

Energy Technol. 2019, 7, 1900638 1900638 (1 of 7) © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

was annealed at 700 �C for the crystallized piezoelectric ceramicsmaterial. The interdigitated electrodes (IDEs) were depositedonto piezoelectric thin film by means of sputtering depositionand conventional microfabrication process; subsequently, a pas-sivation layer (SU-8) was stacked on an energy device. The pie-zoelectric BaTiO3 thin-film-based PEH onto mica substrate waspeeled off by the physical delamination process with sticky tapesfor remover and then moved onto a flexible polyimide (PI)substrate by utilizing polymer elastomer for a supporter. Toinvestigate the ferroelectric properties of BaTiO3 thin film, wemeasured polarization-electric field (P-E) hysteresis loop and pie-zoelectric response. The final BaTiO3 thin-film f-PEH harvestedan open-circuit voltage of �0.5 V and a short-circuit current of�30 nA from periodically mechanical deformations. Thefinite-element analysis (FEA) with multiphysics simulation soft-ware was also conducted to support the output performance gen-erated from the fabricated energy device.

Figure 1 shows a schematic diagram of the fabrication pro-cesses for preparing the BaTiO3 thin-film f-PEH through theexfoliation of sacrificial mica substrates, which were detailedin the Experimental Section. The inset of Figure 2a shows theperovskite BaTiO3 thin-film f-PEH with an activation area of2 cm� 2 cm onto a rigid 2D mica substrate. By a simple andlow-cost exfoliation process of sacrificial layers, the perovskiteBaTiO3 thin film on a mica substrate became flexible andbendable (see Figure 2a for a BaTiO3 thin-film f-PEH on a curvedball pen). This simple one-step process assisted by peeling off a2D mica substrate enabled the crystallized BaTiO3 thin film ontoa flexible substrate without cracks, thereby yielding a thin-filmf-PEH based on perovskite-structured piezoelectric ceramics.Figure 2b shows an IDE-type f-PEH device obtained by transfer-ring a BaTiO3 thin film with residual mica layers (�25 μm inthickness) onto a flexible PI substrate (3 cm� 3 cm). The final

f-PEH (see the inset of Figure 2b) connected with Cu wires usinga conductive epoxy was poled at an external high electric field toenhance the output performance. Furthermore, the crystal struc-tures of the sputtered BaTiO3 thin film were characterized viaX-ray diffraction (XRD) using CuKα radiation, and the XRDanalysis results were shown in Figure 2c. The BaTiO3 thin filmannealed for 1 h at 700 �C was well crystallized and indicated bythe peaks associated with the polycrystallized perovskite struc-tures, which were well agree with previously reported analysisresults.[11,17] Although the obtained diffraction patterns fromBaTiO3 thin film do not show the peak splitting between the(200) and (002) peaks, the relatively low intensity and the slightshift to higher angle of peak at 45� compared with cubic phase(JCPDS No.31-0174, Pm3m space group) confirm the existenceof tetragonal phase.[38] The inset of Figure 2c shows the cross-sectional scanning electron microscopy (SEM) image of �500 nmthick BaTiO3 thin film on a mica substrate. To further investigatethe phase characterization of our BaTiO3 thin film, we performedthe Raman analysis with a 514.5 nm Arþ laser line as the excita-tion source. Figure 2d shows the Raman spectra of the piezoelec-tric BaTiO3 thin film with the peaks at �305 and �720 cm�1,which indicated the E, B1(transverse optical [TO]þ longitudinaloptical [LO]) and E, A1(LO) modes, respectively: These modesrevealed that the BaTiO3 thin film subjected to high temperatureannealing consists of a perovskite tetragonal phase.[19,20,39]

To investigate the ferroelectric properties of perovskite thinfilm, we conducted the deposition of the crystallized BaTiO3 ontoPt electrode-coated Si substrate and subsequently measured theP-E hysteresis curve (Figure 3a) with ranging from 100 to500 kV cm�1 at room temperature using a precise measurementunit and a device structure with Pt dots (diameter of 100 μm)-based top electrodes (see the inset of Figure 3a). We confirmedthat the BaTiO3 thin film showed ferroelectric properties with the

Figure 1. Schematics of the process for fabricating a piezoelectric BaTiO3 thin-film f-PEH by adopting a 2D rigid mica substrate. a) A BaTiO3 thin filmdeposited onto a mica substrate: This piezoelectric thin film is annealed at high temperature for crystallization and piezoelectricity. b) IDE-type electrodespatterned onto a piezoelectric thin film. c) Epoxy layer for passivation formed on a device: This passivation layer is patterned through a standard micro-fabrication process for forming the contact holes. d) Metal pads for connecting Cu wires deposited onto an energy device. e) Exfoliation process of backside of mica substrate: Sacrificial thin mica layers are peeled off piece by piece. f ) Final f-PEH obtained by transferring a thin device onto a flexiblesubstrate: This energy device is connected with Cu wires and poled at applied electric fields.

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remnant polarization (Pr) of 63.7 μC cm�2 at an electric field of500 kV cm�1. Figure 3b shows the piezoresponse force micro-scope (PFM) analysis results of the perovskite BaTiO3 thin filmon a Si substrate. A PFM technique is a widely used method todetermine the piezoelectric charge constants of thin films ornanostructures (nanoparticles and nanowires).[11,22,28] As sche-matically shown in the inset of Figure 3b, by applying the exter-nal voltage by a conductive tip, the selected spot of a piezoelectricthin film showed the piezoelectric displacement along theout-of-plane direction; consequently, the plot of the piezores-ponse amplitude versus the bias voltage was obtained. Because

the direction of introduced displacement due to inverse piezo-electric effect is parallel to the electric field, piezoelectric chargeconstants is d33, which were calculated from the slope of the lin-ear fitting lines. Table 1 shows the d33 values obtained from theBaTiO3 thin film by means of the PFM technique: The averaged33 value of ten data points measured from the randomly selectedareas is 128� 18 pmV�1.

The schematics presented in Figure 4a-i–iv show the detailedenergy generation mechanism of the IDE-type f-PEH by the pie-zoelectric potential introduced between each pair of adjacentelectrodes. The randomly distributed charge dipoles in the top

Figure 2. a) A bendable and flexible BaTiO3 thin film with a thin mica substrate around a ball pen after the delamination process of the back side ofsacrificial mica substrate. The inset shows a photograph of the BaTiO3 thin film prior to peeling off from a rigid substrate (scale bar: 5 mm). b) A fabricatedBaTiO3 thin-film f-PEH on a plastic substrate bent by human fingers. The inset shows the final energy device after the poling process (scale bar: 5 mm).c) XRD analysis results and a cross-sectional SEM image (inset) obtained from the crystallized BaTiO3 thin film on mica substrate. d) Raman spectra ofperovskite BaTiO3 thin film after high-temperature annealing process at 700 �C.

Figure 3. a) The P-E hysteresis loop obtained from the BaTiO3 thin film on a bulk substrate at room temperature under various electric fields. The insetshows the device structure composed of the BaTiO3 thin film deposited on a Pt electrode (bottom electrode)-coated Si substrate with the Pt dots(top electrode) to characterize the ferroelectric properties. b) Piezoelectric displacement versus applied voltage curve of a BaTiO3 thin film obtainedby means of PFM analysis: The slope of curve consisted with the dots indicates the piezoelectric charge constant (d33). The inset shows the schematicillustration of PFM analysis.



surface of the polycrystalline piezoelectric BaTiO3 thin film(Figure 4a-i) can be aligned along the direction of the externalelectric field. By the poling process with high temperature andvoltage, as shown in Figure 4a-ii, the polarization directionbecomes parallel to the surface of the piezoelectric thin film.The tensile force resulting from mechanical bending alongthe direction perpendicular to the IDEs can generate the piezo-electric potentials at the electrode pairs, leading to the electronflow through an external load (see Figure 4a-iii). By releasingof introduced stress, the piezoelectric potential disappears, allow-ing electric signals in the opposite direction (see Figure 4a-iv). Asa result, the alternative electric peaks are observed from the IDEs-based BaTiO3 thin-film f-PEH during periodic bendings.

Figure 4b,c presents the FEA simulation models and the sim-ulated results according to the same dimensions as the actual

device with the piezoelectric BaTiO3 thin film with a thicknessof 500 nm, the residual mica substrate with a thickness of25 μm, and the electrode gap of 100 μm. A single IDE model withthe two lateral electrodes on the piezo-ceramic was bent with abending radius (Rc) of 0.83 cm, which was corresponding to thedisplacement of 5mm from an original 3 cm long plastic sub-strate. We used the each material parameters of piezoelectricBaTiO3 (an elastic modulus of E¼ 67 GPa, a piezoelectric chargeconstant of d31¼ 78 pC N�1, a mass density of ρ¼ 5700 kgm�3,and a dielectric constant of KT¼ 1450) provided by the simula-tion software (COMSOL v5.4).[17–19,40–42] To determine the effec-tive strain induced in the BaTiO3 layer, we calculated the distance(hneutral) from the top surface of the f-PEH to the mechanical neu-tral plane by Equation (1) as follows

hneutral ¼PN

i¼1 Ei � ti

�Pij¼1 tj � ti

2

�PN

i¼1 Ei � ti(1)

where N, ti, and Ei are the total number of layers, thickness ofthe ith layer from the top surface, and effective elasticmodulus, respectively. Ei can be calculated from equationfEi ¼ Ei=ð1� vi2Þg. where Ei and vi are, respectively, elasticmodulus and Poisson’s ratio of the ith layer. The mechanicalparameters and thickness of each layer are 1) SU-8 epoxy:ESU-8¼ 4.02 GPa, vSU-8¼ 0.22, and tSU-8¼ 5 μm; 2) BaTiO3 thinfilm: EBaTiO3

¼ 67 GPa, vBaTiO3¼ 0.25, and tBaTiO3

¼ 500 nm;3) residual mica layer: Emica¼ 178 GPa, vmica¼ 0.25, andtmica¼ 25 μm; and 4) flexible PI substrate: EPI¼ 2.5 GPa,vPI¼ 0.4, and tPI¼ 125 μm. From Equation (1) and the materialparameters, we found that the mechanical neutral plane of theBaTiO3 thin-film f-PEH locates about 24 μm from the top sur-face, which means hneutral¼ 24 μm; moreover, a value of18.7 μm was calculated for the distance (δ) between the neutral

Table 1. Piezoelectric charge constant of BaTiO3 thin film measured byPFM analysis.

Data pointPiezoelectric charge

constant [d33, pm V�1] Std. Error R2

Spot 1 139 0.00217 0.9413

Spot 2 119 0.00243 0.9034

Spot 3 106 0.00278 0.8502

Spot 4 123 0.00297 0.8696

Spot 5 147 0.00283 0.9133

Spot 6 103 0.00233 0.8684

Spot 7 145 0.00323 0.8871

Spot 8 114 0.00245 0.8939

Spot 9 125 0.00224 0.9237

Spot 10 158 0.00255 0.9372

Figure 4. a) Schematic illustration showing working mechanism of an IDE-type f-PEH. i) The randomly distributed dipoles are aligned along the surface ofthe BaTiO3 thin film by ii) poling process. iii) Under bending motions, a piezoelectric potential is introduced at the adjacent electrodes, resulting in theelectron flow and the electric signals. iv) The releasing motions of f-PEH reverses the electric peaks. b) Schematics showing the f-PEH deformed with anRc of 0.83 cm. c) FEA simulation results calculated from a simple IDE-simulation model under tensile stress.



plane and the center of the piezoelectric thin film. In addition,the perovskite BaTiO3 thin film on a residual mica/plasticsubstrate was completely deformed by the only tensile strain(ε) of 0.225%, which is obtained from simple equation{strain (ε)¼ δ/Rc} and the given Rc of 0.83 cm. The potential dis-tribution indicated by the color codes in Figure 4b is introducedinto a BaTiO3 layer when the f-PEH is subjected to horizontaltensile deformation. Consequently, the IDE-based BaTiO3

f-PEH enabled by a mica substrate can harvest an electric poten-tial difference (voltage) of �0.6 V between adjacent electrodesunder mechanical bending that yields a displacement of 5 mmfrom the original 3 cm long sample.

We characterized the converted electric signals by operatinga perovskite BaTiO3 thin-film f-PEH under periodicallymechanical bendings. An energy device connected with a cus-tomized bending stage was deformed with an Rc of 0.83 cm,corresponding to a displacement of 5 mm from the original3 cm long sample. Afterward, the harvested electricity wasmeasured using a precise electrometer and then recordedon a computer; these measurement results are shown inFigure 5a,b. The positive output voltage and current pulse weredetected by bending of BaTiO3 thin-film f-PEH, whereasunbending motions led to the reverse negative signals. As aresult, the fabricated f-PEH subjected to the tensile stress bythe bending motions produced the alternative electric peaks,which showed the open-circuit voltage of �0.5 V and theshort-circuit current of�30 nA (see Figure 5a-i,ii); these valuesare higher than that of BaTiO3 thin-film-based f-PEH.[11] Theoutput performance can be enhanced using the fabricationoptimization, the material selection, and the dual-structuredintegration.[37] The measurement process for characterizingthe output performance of f-PEH was conducted using a pre-cise measurement instrument in Faraday cage and well-grounded tables to exclude any artifact signals. Furthermore,

we performed a switching-polarity test in forward and reverseconnections to confirm the measured electric signals obtainedfrom the f-PEH. Connecting a BaTiO3 thin-film f-PEH to theopposite polarity of an electrometer resulted in an inversion ofthe electric peaks, as shown in Figure 5b. This behavior indi-cated that the recorded electrical yields were truly originated bythe piezoelectric effect introduced between adjacent electrodeson BaTiO3 thin film.

In summary, a new approach for transferring piezoelectricceramic thin films onto plastic substrates is proposed by per-forming piece-by-piece exfoliation of mica substrates with stickytapes. The deposition of piezoelectric BaTiO3 thin film onto arigid mica substrate was carried out by means of an RF magne-tron sputtering process with a sintered ceramic target.Subsequently, the film was then annealed to obtain the high-quality ceramic thin film. The IDEs were deposited onto piezo-electric thin film to harvest the electricity from the piezo-thinfilm, and an epoxy layer was then formed on an IDEs-depositedBaTiO3 thin film for packing an energy device. The BaTiO3 thin-film-based PEH onto thick mica substrate was peeled off by aphysical delamination process and transferred onto a flexiblesubstrate by utilizing a polymeric stamp. The perovskiteBaTiO3 thin film showed the ferroelectric properties with a Pr

of �63.7 μC cm�2 and a piezoelectric charge constant of128� 18 pmV�1. The maximum output voltage and current pro-duced from a fabricated BaTiO3 thin-film f-PEH were�0.5 V and�30 nA, respectively, during mechanical bendings. The FEAwith multiphysics simulation results corresponded closely tothe generated output signals. This novel transfer technique usingthe removal of a sacrificial mica layer can be expanded to varioushigh-performance flexible and wearable applications based on all-inorganic materials. We are currently exploring the high-outputf-PEH by adopting dual-structured stacking with excellent piezo-electric ceramics.

Figure 5. The measured open-circuit voltage i) and short-circuit current ii) from a piezoelectric BaTiO3 thin-film f-PEH in the a) forward and b) reverseconnection with a precise electrometer.



Experimental Section

Materials: A sintered BaTiO3 target (3 in.) for depositing perovskite-structured piezoelectric thin films onto a rigid mica substrate was pur-chased from the Taewon Scientific Co. (Seoul, Korea). A mica film for asacrificial substrate (size: 2 cm� 2 cm, thickness: 100 μm) was from Hi-Solar Co. (GwangJu, Korea). For adhesion and passivation layers, the SU-8 photoresist (PR) solution was purchased from MicroChem Co.(Westborough, MA, USA). The flexible PI substrate with a thicknessof 125 μm was purchased from Isoflex Co. (Ansan, Korea).Furthermore, the conductive epoxy (CW2400) for connecting the elec-trode pads and copper (Cu) wire was prepared from Chemtronics Co.(Seongnam, Korea).

Material Characterization: A cross-sectional image of the BaTiO3 thinfilm on a mica substrate was observed using field-emission SEM (JEOLJSM-6701F). The crystal structures of the annealed piezoelectric BaTiO3

thin film were determined by means of XRD (Rigaku D/Max-2500) per-formed at 40 kV and 300mA with Cu Kα radiation (λ¼ 1.5406 Å).Moreover, further phase characterization of the perovskite thin filmswas performed via Raman analysis (LabRAM HR UV/Vis/NIR, HoribaJobin Yvon, France) using a 514.5 nm Arþ laser line as the excitationsource. To characterize the P-E hysteresis loop of the perovskite layer,the BaTiO3 thin film was deposited onto a Pt electrode-coated Si sub-strate and annealed for crystallization. Pt dots-based top electrodes witha diameter of 100 μm were formed onto the top surface of piezoelectricthin film. The hysteresis behavior was measured using a precision mate-rials analyzer (Radiant Technologies INC., Precision LC) with rangingfrom 100 to 500 kV cm�1 at room temperature. Piezoelectric charge con-stant of annealed BaTiO3 thin film was determined using PFM(Park Systems, NX20) modified from atomic force microscope with aconductive tip.

Fabrication of the BaTiO3 Thin-Film-Based f-PEH: A 500 nm BaTiO3 thinfilm was deposited at room temperature through a standard route of RFmagnetron sputtering process with a sintered piezoelectric ceramic target(Figure 1a). The deposited BaTiO3 thin film on a rigid mica substrate wasannealed at 700 �C for 1 h in an electric furnace for crystallization and fer-roelectric property. After deposition of thin chromium (Cr) layer, a 100 nmthick Au layer was deposited on a perovskite thin film through a sputteringprocess. The layer was then patterned via standard microfabricationincluding photolithography, PR patterning, and wet etching for the forma-tion of IDEs with an interelectrode gap of 100 μm and 35 finger pairs, afinger length of 12mm, and an electrode width of 100 μm (Figure 1b):These values that determine the output performance of the IDE-typePEH were selected from our pervious study.[24] An SU-8 epoxy as a pas-sivation layer was coated and patterned with contact holes, as shown inFigure 1c. The additional metal lines were formed on a passivation layer forelectric contact pads, which were connected with Cu wires (correspondingto Figure 1d). A BaTiO3 thin-film-based PEH was attached to a sticky poly-dimethylsiloxane (PDMS) stamp as a support; subsequently, the sacrificiallayers (bottom sheets) of a rigid mica substrate were exfoliated piece bypiece using Scotch tapes (3M Co., US) until the mica layers were suffi-ciently bendable and flexible with a thickness of about 25 μm(Figure 1e). After delamination of substrate, the perovskite thin-film-basedf-PEH was transferred onto a PI substrate and fixed with an SU-8 epoxy(Figure 1f ). A settled energy device was sealed using poly(methyl meth-acrylate) and connected with Cu wire with a conductive epoxy. Finally, anf-PEH was poled with an external electric field of 100 kV cm�1 for 2 h at120 �C to improve the piezoelectricity of the BaTiO3 thin film.

Measurement of Output Signals Generated from f-PEH under MechanicalBendings: A programmable bending machine (SnM, Gumi, Korea) wasused to periodically deform the fabricated BaTiO3 thin-film f-PEH throughbending that yields a displacement of 5 mm from an original 3 cm longplastic substrate at a deformation rate of 15 cm s�1. The electric signals,such as voltage and current, produced from the f-PEH during repeat bend-ings were measured by a precise electrometer (Keithley 6514/E, US) andrecorded in real time by a computer. The measurement process was con-ducted in a Faraday cage on an optical table at room temperature toremove the noise sources by any external charges.

AcknowledgementsThis research was supported by Basic Science Research Program throughthe National Research Foundation of Korea (NRF) funded by the Ministryof Education (NRF-2019R1I1A2A01057073) and the Ministry of Scienceand ICT (NRF-2018R1A4A1022260).

Conflict of InterestThe authors declare no conflict of interest.

KeywordsBaTiO3, energy harvester, flexible, mica, piezoelectric

Received: May 30, 2019Revised: July 17, 2019

Published online: August 20, 2019

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