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JOURNAL OFC O M P O S I T EM AT E R I A L SArticle

Flexible ceramic-elastomer compositepiezoelectric energy harvester fabricatedby additive manufacturing

Jae-Il Park1, Gil-Yong Lee2, Jinkyu Yang2, Chung-Soo Kim1,3 andSung-Hoon Ahn1,3

Abstract

As a renewable energy harvesting method, interest in piezoelectric energy harvesting has increased significantly. Despite

the piezoelectric energy-harvesting technology expanding its area to the flexible (elastic, amendable) energy harvesting,

striking use or application of the technology is hardly found in the market. Here, we report a novel flexible piezoelectric

energy harvester fabricated by using an additive manufacturing process, which enables both effective and customized

manufacturing of energy harvesting devices. By advantages of additive manufacturing, further application of the piezo-

electric energy-harvesting technology is highly expected. Particles of BaTiO3, a ceramic with a large piezoelectric con-

stant, were mixed with polyether block amide elastomer to form a flexible piezoelectric composite. The energy

harvester was fabricated using an additive manufacturing process, by printing the piezoelectric composite on a laser-

patterned flexible Indium-tin-oxide–coated polyethylene terephthalate substrate. Performance of fabricated energy har-

vester was evaluated by applying a mechanical stress to the energy harvester; voltage and current output were 2 V and 40

nA, respectively. An analytical model of the piezoelectric energy harvester was developed and discussed to explain the

form of the voltage waveforms in response to the applied stress.

Keywords

Flexible piezoelectric energy harvesting, additive manufacturing, barium titanate, polyether block amide

Introduction

Various renewable energy-harvesting methods havebeen proposed as alternatives to fossil fuels, includingdriving turbines using hydropower1,2 or wind power,3,4

converting chemical energy from ‘green fuels’ to elec-trical energy,5,6 and solar energy.7–9 Energy harvestingusing piezoelectric materials can convert mechanicalenergy directly into electrical energy. By its simple prin-ciple of harvesting energy, piezoelectric energy harvest-ing is easily applicable in numerous fields wheremechanical energy exists.

Recently, interest in flexible (elastic, amendable)piezoelectric energy harvester has been increased glo-bally. A flexible piezoelectric energy harvester may beused to increase the number of applications of piezo-electric energy harvesting, due to its potential advan-tage of easy integration to the personalized devices.Numerous fabrication methods have been employedto achieve this.10–18 Swallow et al.11 embedded lead zir-conium titanate (PZT) fibers in a polymer matrix

placed on metal electrodes; Kim et al.12 synthesizedZnO nanorods on a paper-based substrate12; andPark et al.13 spin-coated a BaTiO3/carbon nanotube/reduced grapheme oxide/PDMS 1composite materialonto a flexible substrate to create a deformable piezo-electric film.

Despite the progress of piezoelectric energy harvest-ing technology, there has not been a prominent appli-cation or market growth offered through the

1Department of Mechanical and Aerospace Engineering, Seoul National

University, Seoul, Republic of Korea2William E. Boeing Department of Aeronautics & Astronautics,

University of Washington, WA, USA3Institute of Advanced Machinery and Design, Seoul National University,

Seoul, Republic of Korea

Corresponding author:

Sung-Hoon Ahn 4, Seoul National University, School of Mechanical and

Aerospace Engineering, Seoul National University, Seoul, South Korea,

SEOUL, 151742, Korea.

Email: [email protected]

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technology. To open up the new stage of piezoelectricenergy harvesting technology, there is a need to developa piezoelectric energy harvester fabricated by themanufacturing technique which enables proper custom-ization of piezoelectric energy harvesters to variousfields.

Here, we describe a flexible piezoelectric energyharvester fabricated by novel and simple methodusing a nanocomposite deposition system(NCDS),23,24 which is an additive manufacturing pro-cesses involving fused deposition methods. NCDS is atechnology that enables free-form objects of polymerand polymer-based composite to be fabricated with-out sub-processes. As additive manufacturing has sig-nificant advantages in terms of a rapid design-to-production times and customization,19–25 NCDS pro-vides rapid fabrication of customized piezoelectricenergy harvester by printing piezoelectric compositematerial.

In the presented research, flexible piezoelectricenergy harvester was fabricated by printing a flexiblepiezoelectric composite material on a flexible polymersubstrate using NCDS. To form a piezoelectric com-posite material, BaTiO3 particles were mixed withpolyether block amide (PEBA). BaTiO3 is a ceramicmaterial which is easily found as particle with a highpiezoelectric constant, and PEBA is an elastomer witha low-density, high-mechanical strength, high-elasti-city, and high-adhesive characteristics on any surfacewhen it is heated. Due to those properties of BaTiO3

and PEBA, piezoelectric composite with high piezo-electric and mechanical performance could easily beformed by a simple method of blending BaTiO3 par-ticle in molten PEBA. Also, the composite can beformed in low temperature and the melting andhardening processes of the BaTiO3-PEBA compositeis compatible with substrate materials including poly-mer because of the low melting point of PEBA(�135�C). The piezoelectric composite was directlyprinted using NCDS on a flexible polymer substrate,to form the desired structure. Performance of fabri-cated piezoelectric energy harvester was evaluated bymeasuring the output voltage and current, and a

simple model of the energy harvester was developedand discussed.

Fabrication of the flexible piezoelectricenergy harvester

Figure 1 presents schematic diagram of the fabricationprocedure. The BaTiO3-PEBA composite was preparedusing a simple mixing method. BaTiO3 particles(<3 mm, Sigma-Aldrich) were dispersed in moltenPEBA (Pebax�, Arkema) using mechanical stirring at140�C. Indium-tin-oxide (ITO)-coated polyethylene ter-ephthalate (PET) was used as the flexible substrate.Two electrodes were fabricated on the one side of thesubstrate to fabricate energy harvester by one step ofprinting process of PEBA-BaTiO3 composite. Tolengthen the length of the electrodes, interdigitated elec-trodes were devised. Fabrication of the electrodes wascarried out using a direct writing process, whereby sub-tractive laser patterning of the ITO layer was involvedusing a CO2 laser cutter in a 12mm� 30mm area. Sincethe area in which two electrodes should be placed waslimited, interdigitated configuration could providelonger electrodes which can lead better output perform-ance of the energy harvester. The length of interdigi-tated electrode was 12mm, and the width of each fingerof the electrode was about 600 mm. The milling width oflaser, which defined the gap between the electrodes, was300 mm.

A 300-mm-thick layer of the BaTiO3-PEBA compos-ite was deposited on the substrate using NCDS.Figure 2(a) presents the NCDS and Figure 2(b) illus-trates the principle of composite deposition by NCDS.The BaTiO3-PEBA composite was a dense liquid,melted using heat applied at the barrel and nozzle, asshown in Figure 2(b). After the composite had melted,compressed air was applied to the nozzle to form aliquid-state composite, which could be squeezed out.The composite was then deposited on the substrateand hardened into a solid state by allowing it to coolto room temperature. During deposition of the com-posite, a three-axis stage translated the substrate to fab-ricate desired features. Three different compositions of

Figure 1. Fabrication process of flexible piezoelectric energy harvester.

2 Journal of Composite Materials 0(0)


BaTiO3-PEBA composite were used: BaTiO3 wt 5%,10%, and 15%. The nozzle diameter was 500 mm, thetemperature was 150�C, and the pressure of the com-pressed air was 50 kPa.

Finally, the structure was poled using an externalelectric field to align polarization of the BaTiO3 par-ticles. An electric field of 4 kV/cm was applied for 10hours at 120�C, which is the Curie point of BaTiO3.

26

Figure 3(a) shows the interdigitated electrodes pat-terned on the substrate and Figure 3(b) shows theBaTiO3-PEBA composite material deposited on thepatterned ITO/PET film. Figure 3(c) presents a cross-sectional view of the structure.

Experiments and results

The flexible piezoelectric energy harvester was analyzedby measuring the output voltage and current inresponse to a force of 20N with 0.5m/s2 of acceleration

applied over the 12mm� 20mm area of the deviceusing a one-axis translation stage to deform the struc-ture. Kistler Type 9265C dynamometer (Kistler, USA)was used to measure applied force. Electrical measure-ments were performed using a Keithley 6514 electrom-eter (Keithley, USA).

Figure 5(a) shows the measured open-circuit voltagein response to deformation, and Figure 5(b) shows theshort-circuit current. Both the voltage and currentexhibited positive and negative peaks induced by therepetitive mechanical stressing of the piezoelectricenergy harvester. Positive peaks were generated whenthe stress was applied and negative peaks were gener-ated when the stress was released. Actual energy har-vesting was carried out to confirm energy generation byapplying force to energy harvester. By human foot step-ping, �600N of force was applied on the energy

Figure 3. Piezoelectric energy harvester: (a) laser patterning of

the ITO-PET substrate, (b) BaTiO3-PEBA composite on the ITO-

PET film and (c) cross-section of the structure.

Figure 2. (a) nanocomposite deposition system (NCDS),

together with a magnified view of deposition system; (b) a

schematic diagram showing the BaTiO3-PEBA composite

deposition process.

Park et al. 3


harvester for 20minutes (�1000 times). Generatedenergy was captured in a capacitor using rectifier circuitsystem which is presented in Figure 5(c), and poweredLED successfully for �0.5 second.

Figure 6(a) and (b) compare the voltage and currentbetween structures formed with different fractions ofBaTiO3, presented as the half of peak-to-peak valuein a single stressing cycle. Voltage and current increasedas BaTiO3 content increased: with a BaTiO3 content ofwt 15%, the average peak voltage was 2V and the aver-age peak current was 40 nA; the maximum peak valuesobserved were 3V and 50 nA. The peak power andRMS power of the energy harvester were 0.072 mW(0.030 mW per cm2) and 0.008 mW (0.003 mW per cm2),respectively.

Error bars in Figures 6(a) and (b) indicate irregular-ity of output voltage and current. Irregular magnitudeof the output signals might be occurred by some rea-sons. BaTiO3 particle could not be distributed perfectlyin the PEBA matrix and the force impacts on the energyharvester were not perfectly same every time. Also,there were some outliers in signals generated by meas-urement errors of electrometer. Since we used data withno artificial processing, outliers which were supposed tobe measurement failures in the data made the error barslarger.

Discussion

Figure 7 presents a magnified voltage trace duringsingle impact based on the data shown in Figure 5.During the rising part of the peak, the potential differ-ence results from the piezoelectric effect of the BaTiO3

particles in the composite when the stress was applied.This causes a current to flow in the circuit of the piezo-electric energy harvester and the measurement system,which in turn causes the voltage to drop, resulting inthe decaying part of the curve, as shown in Figure 7.

Figure 5. Measured (a) voltage and (b) current of the flexible

piezoelectric energy harvesters, (c) circuit set-up for energy-

harvesting experiment, together with the image of powered LED

by harvested energy.

Figure 4. Schematic diagram of experimental setup3 .



The potential difference is induced between electrodesby the piezoelectric effect in response to an appliedmech-anical stress, which defines the amplitude of the peak.Then, the current induced by piezoelectric voltage can bedescribed by the equivalent circuit shown in Figure 8(a).The voltage between the electrodes can be derived byapplying Kirchhoff’s circuit laws

V�Qg

Cg� iRg � iRL ¼ 0, ð1Þ

where Qg is the charge between the electrodes, Cg is theequivalent capacitance of the energy harvester, Rg is theequivalent resistance of the energy harvester, and RL isthe resistance of the load, i.e. the measurement system.

Then the voltage applied to the load, VL, can beexpressed as

VL ¼RL

Rg þ RLVe� tðRgþRLÞCg : ð2Þ

The decaying part of the voltage peak was mathemat-ically compared with the calculated form in equation(2). The data fit well to an exponential decay model, asshown by the regression curve in Figure 8(b), where thecoefficient of determination was R2

¼ 0.99554.

Applying this model, the electric energy produced bypiezoelectric energy harvester during a single voltagepeak can be calculated by integrating the power con-sumption. By ignoring the rise time, the electric energyin a single peak can be expressed as

W �1

2CgV

2 RL

Rg þ RL: ð3Þ

From the above equation, the electric energy generatedincreases as Cg or V increases, or as Rg decreases. Inother words, a short distance between the electrodes,high BaTiO3 content, or a low resistivity shouldimprove the efficiency of the piezoelectric energy har-vester. However, it is not always possible to controlthese three parameters independently, and furthermore,we are constrained by the manufacturing processes.

In particular, a large BaTiO3 content increases thechances of structural problems with the fused-deposi-tion-based additive manufacturing process, as it mayinterrupt the flow of molten composite. Additionally, itmay reduce the mechanical strength and elasticity of thecomposite. ITO was used for electrode material on flex-ible PET substrate in this work. As the printing processof the BaTiO3-PEBA composite is compatible on vari-ous substrate materials including polymer, electrode andsubstrate material with higher conductivity can beselected depending on use, which can lead to improve-ment of internal resistance Rg. In future work, the par-ameters should be analyzed further to find optimizedconditions for fabrication, and thereby enhance the per-formance of the flexible piezoelectric energy harvester.

Conclusions

A flexible piezoelectric energy harvester was success-fully fabricated by novel and simple method by apply-ing an additive manufacturing process. A piezoelectric

Figure 6. Comparison of the half of peak-to-peak (a) voltage

and (b) current in the piezoelectric energy harvesters with dif-

ferent BaTiO3 contents.

Figure 7. Magnified view of the voltage trace in response to an

applied stress.

Park et al. 5


composite was fabricated easily by dispersing piezoelec-tric BaTiO3 ceramic particles into a PEBA elastomermatrix in heating condition at low temperature. Thenflexible piezoelectric energy harvester was fabricatedusing NCDS, by printing BaTiO3-PEBA compositeon a flexible polymer substrate. The flexible piezoelec-tric energy harvester showed reasonable performancefor potential use as an actual energy harvesting deviceby measuring the output voltage and current, whichhad amplitudes of 2V, 40 nA when a stress was applied.An analytical model was developed to describe thepiezoelectric energy harvester, and the measured datawere described using regression to the analyticalexpressions.

Presented method using additive manufacturing canprovide very short design-to-production time and pro-duction of complex, customized structures including2.5D and 3D structures potentially. Also, excellentmechanical stability is expected compared to previousflexible energy harvesters due to outstanding mechan-ical properties of PEBA. By its advantages of easy fab-rication and customization, the energy harvesterintroduced in this work can realize actual application

on various fields including personalized device. Also, itshowed possibility of application by actual energy har-vesting. Further improvement of the performance ofthe energy harvester is expected by carrying out para-metric study to find optimized parameter sets includingthe material properties of the BaTiO3-PEBA compositeand electrode configurations.

Acknowledgements

J Yang and G–Y Lee acknowledge the support of SamsungResearch America through Think Tank Team Award.

Conflict of interest

None declared.

Funding

This work was supported by National Research Foundation

of Korea (NRF) grants funded by the Korea government(MEST), [NRF-2010-0029227].

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