Composites Part B 203 (2020) 108476

Available online 17 October 20201359-8368/© 2020 Elsevier Ltd. All rights reserved.

Piezoelectric BaTiO3 microclusters and embossed ZnSnO3 microspheres-based monolayer for highly-efficient and flexible composite generator

Hongbeom Park a,1, Dong Yeol Hyeon b,1, Minwoo Jung a, Kwi-Il Park b,*, Jinsub Park a,**

a Department of Electronics and Computer Engineering, Hanyang University, 222 Wangsimni-ro, Seongdong-gu, Seoul, 04763, Republic of Korea b School of Materials Science and Engineering, Kyungpook National University, 80 Daehak-ro, Buk-gu, Daegu, 41566, Republic of Korea

A R T I C L E I N F O

Keywords: Piezoelectric BaTiO3 cluster Microsphere Energy harvesting Self-powered

A B S T R A C T

As a permanent power source for self-powered electronics, piezoelectric energy harvesters (PEHs), which convert waste mechanical energy into electrical energy, have attracted considerable interest. We herein developed a high-performance PEH by employing a piezoelectric BaTiO3 microclusters (MCs) composite and a ZnSnO3 mi-crospheres (MSs)-based pressure concentrator. The piezoelectric composite film and an embossed pressure concentrator were fabricated by optimized bar-coating and unidirectional rubbing processes, respectively. The final energy device, fabricated by stacking a ZnSnO3 MSs-based embossed pressure concentrator onto a BaTiO3 MCs-based piezoelectric composite, harvested output signals of ~206 V and ~24 μA under an applied pressure of 0.27 MPa, which are significantly improved results compared to previously reported composite-type PEHs. Furthermore, multiphysics-based finite element analysis was performed to support the hypothesis of effective piezo-potential distribution by adopting the BaTiO3 MCs embedded in polymeric matrix and attaching the ZnSnO3 MSs-monolayer onto the piezoelectric composite. This technology represents a new approach with significant advantages for fabricating high-output composite-based PEHs.

1. Introduction

The penetration of the Internet of Things (IoT) into both industry and daily life is leading us into massive IoT, which provide the ubiquitous connectivity of a large number of devices. Robust network coverage, high network capacity scalability, performance diversity according to service/device requirements, and long battery life are key challenges for promoting the massive IoT usage. Although the development of fifth- generation (5G) mobile networks has made substantial improvements to communication technology, there are still unresolved issues such as sustainable and stable power source to realize the massive IoT tech-nology [1–4].

These limitations can be alleviated by energy harvesting technolo-gies that can effectively involuntarily scavenge useful electrical energy from wasted ambient energy such as that of heat [5,6], magnetic fields [7–9], and mechanical deformation [10–12]. Because energy harvesting devices can work as an advanced power source for self-powered sensors

[10,13] and standalone-powered electronics [8,14] without additional energy storage systems, they have attracted considerable attention as alternatives to conventional batteries. In particular, the conversion of mechanical and vibrational energies to electricity by piezoelectric en-ergy harvesters (PEHs) is considered a promising candidate as a power source of wireless microelectronics, pressure and vibration monitoring systems, and implantable devices [15,16] because of their simple structure, excellent durability, and biocompatibility [17,18].

After the ZnO single nanowire-based PEH was proposed, studies have reported various types of PEHs fabricated from diverse materials by various processes such as nanostructure array growth [12,19,20], aerosol-deposition [14], laser lift-off [21], inkjet-printing [22], elec-trospinning [23], and spin-casting [24–26]. The new forms of PEH using piezoelectric composites, known as nanocomposite generators (NCGs), fabricated by dispersing piezoelectric ceramics in an elastomer, have been developed by multiple researchers with various piezoelectric nanomaterials because they have the advantages of a simple fabrication

* Corresponding author. ** Corresponding author.

E-mail addresses: kipark@knu.ac.kr (K.-I. Park), jinsubpark@hanyang.ac.kr (J. Park). 1 These authors contributed equally to this work.

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Composites Part B

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https://doi.org/10.1016/j.compositesb.2020.108476 Received 14 September 2020; Received in revised form 7 October 2020; Accepted 15 October 2020

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process, excellent mechanical flexibility, and high performance [27–29]. Furthermore, NCGs can be easily customized by selecting the dispersed active nanomaterials and/or polymeric matrix. Park et al. developed a piezoelectric nanoparticles (NPs)-based NCG with filling materials such as carbon nanotubes [28,30], copper nanorods [24], and silver nanowires [31] to well-distributed NPs inside a polymeric matrix; this strategy enables improved output performance of NCG devices. To avoid the aggregation of active nanomaterials, some researchers used piezoelectric nanowires as energy-generating materials without non-piezoelectric dispersing agents [29,32]. Jeong et al. [17] used an inorganic-organic hybrid piezoelectric composite comprising poly (vinylidene fluoride-co-trifluoroethylene) and perovskite BaTiO3 (BTO) nanowires to fabricate a highly-efficient NCG device. Sun et al. reported a high-temperature PEH composed of piezoelectric (Bi,La)FeO3–PbTiO3 ceramic powders having a high Curie temperature and a heat-resistant polymeric matrix [33].

A structural approach using surface clusters can simply and cost effectively improve the output performance of composite-based PEHs. Shin et al. [25,34] reported a piezoelectric composite film composed of hemispherically aggregated BTO NPs and poly (vinylidene fluo-ride-co-hexafluoropropene), generating high electrical outputs up to ~110 V and ~22 μA under a compressive force of ~0.23 MPa. Although they demonstrated the feasibility of achieving the improved NCG device in terms of performance by forming novel surface structures, poor controllability and reproducibility of cluster formation remain a chal-lenge for developing simple and scalable composite-based high--performance PEHs [35,36].

Herein, we demonstrate a highly-efficient PEH using BTO micro-clusters (MCs)-embedded piezoelectric composite and a uniformly or-dered ZnSnO3 (ZTO) microspheres (MSs)-based pressure concentrator. The MCs-based piezoelectric composite as an energy generating source was fabricated by a scalable bar-coating process of BTO NPs-dispersed polydimethylsiloxane (PDMS) polymer. The embossed ZTO monolayer-based pressure concentrator was prepared using simple unidirectional rubbing of ZTO MSs on a PDMS seed layer and subsequent die-casting of polyvinyl alcohol (PVA) solution. The fabricated BTO MCs-based PEH exhibited the output signals of up to ~35 V and ~7 μA at an applied pressure of ~0.13 MPa, which are much higher performance compared to those of BTO NPs-dispersed NCG devices [28,37]. By adopting the ZTO MSs-based pressure concentrator, the composite-based PEH generated an open-circuit voltage (Voc) of ~80 V and a short-circuit current (Isc) of ~15 μA at the same pressing condi-tions; which are ~2.5 times higher than the piezoelectric outputs of the PEH without an embossed ZTO monolayer. Furthermore, under a peri-odically applied pressure of ~0.27 MPa, the ZTO MSs-monolayer stacked BTO MCs-PEH converted the output signals of up to 206 V and 24 μA; these values were higher than those of the previously re-ported BTO clusters-NCG device [25,34]. As per these results, we re-ported that effective morphological treatments with the forming BTO MCs and adopting a ZTO MSs-based monolayer can improve the stress confinement effect and maximize the total dipole moment [25]. Sub-sequently, the finite element analysis (FEA) with multiphysics simula-tion software was conducted to theoretically confirm the improvement of output performance using the proposed BTO MC and the embossed ZTO monolayer-based pressure concentrator. Finally, the generated power source from our PEH driven by hand slapping successfully was used to operate the commercial electronic devices.

2. Experimental section

2.1. Preparation of BTO NPs

Piezoelectric BTO NPs were synthesized by a facile hydrothermal method [38]. NaOH (0.02 mol; 100 ml) was dissolved in deionized (DI) water in a Teflon-lined autoclave. A mixture of 0.05 mol anatase TiO2 and 0.05 mol BaCl2 solution was poured into an autoclave. The

autoclave was heated to 210 ◦C for 12 h, and then cooled to room temperature. The products were then filtered and cleaned twice with DI water and dried at 80 ◦C for 12 h in a convection oven.

2.2. Preparation of ZTO MSs

ZTO MSs were fabricated using a two-step reaction. First, zinc tin hydroxide (ZTH) MSs were synthesized using an ethanol precipitation method [39]. The mixture of ethanol and DI water was then divided between beakers filled with powdered Zn(CH3COO)2⋅2H2O, SnCl4⋅5H2O, and NaOH (purity ≥98%). Then, the SnCl4⋅5H2O solution was added to the Zn(CH3COO)2⋅2H2O solution and stirred; subse-quently, the NaOH solution was added and stirred for an additional 30 min without heating. After reaction completion, the solution was centrifuged several times with pure ethanol and DI water. The collected ZTH MSs were then annealed at 400 ◦C for 3 h to convert their phases to ZTO MSs.

2.3. Material characterization

Field-emission scanning electron microscopy (FE-SEM, S-4800, Hitachi) was performed on the powder and film samples at an acceler-ating voltage of 15 kV to investigate their morphological and dimen-sional properties. The crystal structures of the micro/nanomaterials were characterized using X-ray diffractometer (XRD, miniFlex 600, Rigaku) with a Cu Kα source (λ = 1.5406 Å) operated at 40 kV and 15 mA.

2.4. Fabrication steps for the BTO MCs-based piezoelectric composite and PEH

The BTO MCs-based piezoelectric nanocomposite was prepared using an optimized bar-coating technique. The synthesized BTO NPs were dispersed inside a PDMS (Sylgard 184, Dow Corning, base to curing agent ratio 10:1) matrix in specific proportions (from 5 to 35 wt%). The piezoelectric composite was coated onto a polyethylene terephthalate (PET) substrate with multiple coatings (3, 5, 7, and 9 times) and bar shift speeds (3, 5, 7, and 9 cm/s) using a bar-coater (KP–3000H, Kipae E&T Co. Ltd.). Next, the piezo-polymeric layers were cured in an oven at 80 ◦C for 2 h. Piezoelectric composites cut into a size of 2 cm × 2 cm were peeled off from the PET substrate and sandwiched between two aluminized PET substrates (thickness = 125 μm; Sigma-Aldrich). Finally, the top and bottom electrodes were connected with Cu wires using an Ag-based conductive epoxy (CW2400, Chemtronics) and poled under a high electric field of 100 kV/cm at 100 ◦C for 24 h to improve its piezoelectric performance.

2.5. Fabrication steps for ZTO monolayer stacked BTO MCs-based PEH

The ZTO MSs-based monolayer was achieved by adopting the simple unidirectional rubbing process. ZTO MSs loaded onto the PDMS sub-strate were arranged by unidirectional rubbing using a PDMS pad. Subsequently, the PVA solution was poured onto the well-aligned ZTO MSs on the PDMS substrate and dried in a vacuum chamber for two days to fix the ZTO particles. The piezoelectric composites and monolayers sliced into a size of 2 cm × 2 cm were detached from their substrates, and stacked on top of each other. The bumpy surface of the ZTO monolayer was attached to the clusters-based piezoelectric composites. For forming an adhesive layer, PDMS was coated on the backside of the monolayer and semi-cured at 80 ◦C for 10 min; simultaneously, the stacks of piezoelectric composite-ZTO monolayer were sandwiched be-tween the two aluminized PET substrates. To detect electrical signals from the device, Cu wires were attached to the top and bottom elec-trodes using a conductive epoxy; finally, the poling process was per-formed under a high electric field.

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2.6. Measurement of the converted electrical signals

To measure the piezoelectric electricity generated from the composite-based PEH during periodic pushing and releasing motions, a customized pushing system (PS01–37 × 120F-HP-C, LinMot) was used for applied forces of 52 and 108 N at a frequency of 2.5 Hz. The alter-nating electrical signals produced from the energy device during the repeated mechanical deformation was measured using an oscilloscope (TBS1022, Tektronix) and a picoammeter (Picoammeter 6485, Keith-ley), and recorded in real-time by a computer.

3. Results and discussion

3.1. Fabrication and piezoelectric output performance of BTO MCs-based PEH

Fig. 1a shows the fabrication process for preparing BTO MCs- embedded piezoelectric composites using a simple and scalable bar- coating process. The Experimental Section shows the detailed fabrica-tion process. Figs. 1b and c and shows an SEM image and XRD pattern of the synthesized BTO NPs. The BTO NPs have polydisperse angular shapes with a size of 200–500 nm and typical XRD patterns of tetragonal perovskite crystalline BTO. Fig. 1d and the inset show the top surface and cross-sectional SEM images of 50 μm thick piezo-composite with

BTO clusters formed on the PET substrate by coating with nine layers at a bar shift speed of 5 cm/s. As shown in Figs. S1a–S1h, the resulting thickness and top surface morphology according to bar-coating condi-tions, such as the number and speed of coating, were characterized. These results indicate that the size and number of BTO clusters depends on the number of coating and speed of bar-shifting. Fig. 1e shows a photograph of the piezoelectric composite with aggregated BTO NPs- based MCs with an actual size of 10 cm × 10 cm. The fabricated BTO MCs-embedded piezoelectric composite exhibit high flexibility and stretchability, as shown in Figs. S2a and S2b.

To select the optimum composition of BTO NPs embedded in the polymeric matrix and thickness of the piezo-composite, the output performance of BTO MCs-based PEHs with various mixture ratios of NPs and thicknesses of composite film were evaluated using a precise pushing machine with the repeated pressure (Figs. 2a–c). To measure the output voltage and current signals, each piezo-composite film was inserted between two aluminized PET substrates. Fig. 2b shows the electrical output signals generated by the periodic pushing of BTO MCs- based PEHs with the range of weight ratios of piezoelectric BTO NPs of 5–35 wt% embedded in the polymeric matrix. The harvested Voc and Isc from the stressed PEHs rise with increase in weight fraction of BTO NPs, up to 20 wt% at a fixed thickness of 50 μm. When the weight fraction of BTO NPs exceeded 20 wt%, the output voltage and current decreased. These behaviors are consistent with previously reported results [17,24].

Fig. 1. (a) Schematic illustrations of the fabrication process for BTO MCs-based piezoelectric composites. (b, c) SEM image (b) and XRD pattern (c) of the perovskite- BTO NPs. (d) Top surface SEM image and cross-section (inset) of BTO MCs-based piezoelectric composites. (e) Photograph of the BTO MCs-embedded translucent piezo-composite.

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The enhancements of the generated output signals from 5 to 20 wt% achieved by the improved polarization because of substantial change in dielectric constant inside piezoelectric composites, while the excess content of piezoelectric NPs led to the inferior output performance due to the degradation of electromechanical coupling introduced from the overly high dielectric constant. Fig. 2c shows measurement results of BTO MCs-based PEHs with various thicknesses of active layer at fixed BTO weight fraction of 20 wt%. The generated Voc from a metal-insulator-metal (MIM) structured-piezoelectric device under an external stress (σ) can be expressed as Voc = σ⋅g⋅L, where g is the piezoelectric voltage constant and L is the distance between the

electrodes (corresponding to the thickness of the piezoelectric material). From the abovementioned relationship, the output voltage increases with the thickness of the piezo-composite because of the increased dis-tance between the electrodes (blue line in Fig. 2c). However, the current signals fall owing to the drop of surface charge density because of the increased thickness of the piezo-composite (red line in Fig. 2c). There-fore, we selected 50 μm-thickness of piezo-composite as the optimal thickness of a BTO MCs-based piezo-composite.

Fig. 2. (a) Photographs of the BTO MCs-based PEH in the original/released (left panel) and compressed (right panel) states. The inset shows a schematic of the BTO MCs-based PEH structure. (b) Generated output signals of BTO MCs-based PEHs with different weight ratios of BTO NPs inside polymeric matrix. (c) Variations of Voc and Isc harvested from BTO MCs-based PEHs with various thickness of the piezoelectric layer.

Fig. 3. (a) Schematic diagrams of the fabrication process for the embossed ZTO MSs-based monolayer. (b) SEM image and XRD pattern (inset) of the synthesized ZTO MSs. (c) Photograph of the ZTO MSs monolayer-based transparent pressure concentrator. The inset shows a tilted-view of the well-aligned ZTO MSs with PVA binder.

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3.2. Fabrication and performance of embossed ZTO monolayer-stacked BTO MCs-based PEH

Fig. 3a shows the fabrication process of the ZTO MSs-based mono-layer [40], which were detailed in the Experimental Section. As shown in Fig. 3b, the synthesized ZTO MSs show a spherical shape with a size of ~800 nm. The inset XRD pattern indicates that amorphous ZTO MSs act as a pressure concentrator without an energy generating effect. Fig. 3c shows a photograph of the MSs-based embossed pressure concentrator with a size of 4 cm × 4 cm fabricated using simple unidirectional rub-bing of particles on an elastomer. The fabricated ZTO MSs-pressure

concentrator can then provide an effective structure for piezoelectric energy harvesting and enhance the stress concentration effects over the entire surface of the piezoelectric composite layers without requiring complicated and expensive additional processes such as nanofabrication techniques [35,36]. The inset of Fig. 3c shows a tilted top surface SEM image of the well-aligned ZTO MSs with PVA binder.

Fig. 4a shows a schematic of the fabrication process for the embossed ZTO monolayer stacked BTO MCs-based PEHs (see the Experimental Section for details). Fig. 4b shows a photographic image of the BTO MCs- based piezoelectric composite (top sample) and ZTO MSs-based pressure concentrator (bottom sample) attached to the aluminized PET substrate,

Fig. 4. (a) Schematic of the fabrication process for the embossed pressure concentrator-stacked BTO MCs-based PEH. (b) The photographic image of the BTO MCs- based piezo-composite (top) and ZTO MSs-based monolayer (bottom). (c, d) Output voltage (c) and current signals (d) generated by BTO MCs-based PEH with and without the ZTO MSs-based pressure concentrator.

Fig. 5. (a, b) Open-circuit voltage (a) and short-circuit current (b) measured from the embossed pressure concentrator-stacked PEH in forward (left) and reverse (right) connections under an applied force of ~0.27 MPa.

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respectively, before fabricating the energy device. Fig. S3 shows the cross-sectional SEM image of the bent embossed ZTO monolayer-stacked PEH consisted with BTO MCs-piezoelectric layer and ZTO monolayer between the two aluminized plastic substrates: There is no separation of the piezoelectric composite and plastic substrates under mechanical deformation owing to inherently sticky property of a PDMS elastomer. Figs. 4c and d shows the roles of the stacked pressure concentrator based on the ZTO MSs-monolayer on the output performance of the piezo- composite PEHs. The generated electrical signals from the BTO MCs- based PEH were ~35 V and ~7 μA (corresponding to an average voltage of 25.7 V and current of 5.8 μA, respectively), under an applied pressure of 0.13 MPa (left panel of Figs. 4c and d). These values are encouraging compared to those of the previously reported composites- based PEHs based on piezoelectric NPs. Thus, BTO MCs provided an effective structure for mechanical energy harvesting. Moreover, we investigated the dependence of the ZTO MSs-pressure concentrator on the power generation of composite-based PEHs (Figs. 4c and d). As shown in the right panel of Figs. 4c and d, when the embossed ZTO MSs- based pressure concentrator was laminated to BTO MCs-based piezo-electric composite layers, the piezoelectric power source dramatically improved. The embossed ZTO monolayer stacked BTO MCs-based PEH harvested the Voc of ~80 V and Isc of ~15 μA (corresponding to an average voltage and current of 79.62 V and 13.39 μA, respectively) from mechanical deformation with at the same pressure; these values were approximately 2.5 times higher than those of the BTO MCs-based PEHs.

From these results, we found that the unique embossed structure of ZTO monolayer onto piezoelectric composites can maximize total dipole moments inside piezoelectric composite and improve the introduced stress concentration of BTO MCs.

To clarify the low piezoelectricity of the ZTO MSs-based monolayer, we characterized the electric signals detected from the pure PDMS, PVA, and ZTO monolayer, which are components of the ZTO MSs-pressure concentrator by mechanical pushing at 0.13 MPa (Fig. S4). Moreover, by introducing a compressive force of ~0.27 MPa, the embossed ZTO monolayer-stacked PEH produced a high output voltage of 206 V (cor-responding to an average voltage of 168.8 V) and a current pulse of 24 μA (corresponding to an average current of 18.3 μA) (see Figs. 5a and b), which are higher than those of previously reported PEHs made of BTO MCs [25,34]. Moreover, the pressure concentrator-stacked PEH shows superior mechanical stability during 5000 pushing cycles without the appearance of a significant change of output signals (Fig. S5). To verify the genuineness of the measured output signals, we conducted a switching-polarity test by connecting the energy device with the mea-surement unit in forward and reverse (left and right panels of Figs. 5a and b). The improved power generation of our PEH was obtained by employing unique morphological treatments such as the employing piezoelectric BTO MCs and the well-aligned ZTO MSs-based monolayer [25].

Fig. 6. Simplified multiphysics simulation models with boundary conditions (left panel) and calculated piezo-potential distributions (right panel) of the PEH for a flat-surfaced model (a), the BTO MCs-embedded model (b), and an embossed ZTO monolayer-stacked model (c) under mechanical pushing deformation.

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3.3. Multiphysics analysis of PEH simulation models

To theoretically investigate the roles of BTO MC and the embossed pressure concentrator, we conducted a case-control study by FEA simulation using COMSOL v5.5 multiphysics software (Figs. 6a–c). As shown in the left panels of Figs. 6a–c, the simplified geometry models, which are composed of the same number of BTO NPs, PDMS matrix, and Al top/bottom electrodes, were initially designed with the standard material parameters from the material browser of the COMSOL package. The bottom electrodes of the simulation models were fixed and the top electrodes were stressed by a boundary load of 0.13 MPa along the counter direction of the +y-axis. The right panels of Figs. 6a–c shows the calculated simulation results, which provide the piezoelectric potential distribution within the geometry illustrated by color-code. Based on the calculated results of Figs. 6a and b, forming the piezoelectric MCs inside polymeric matrix is advantageous to improve the induced potential difference between the two electrodes under the given conditions. Furthermore, the synergetic effect of combining the BTO MCs and the embossed ZTO MSs-based monolayer can efficiently stimulate piezo-electric potential generation within composites composed of inorganic piezo-ceramic particles and non-piezoelectric polymeric matrix (Fig. 6c); these high piezo-potential differences lead to superior power generation. The multiphysics simulation well supported the compara-tive test results of the characterized output signals of the PEHs with and without ZTO MSs monolayer-pressure concentrators (Figs. 4c and d).

3.4. Energy applications of the fabricated energy device

To explore practical applications of the harvested power from the ZTO MSs monolayer stacked BTO MCs-based PEH, we demonstrated the operation of two types of commercial electronic devices. The energy device was integrated with rectifiers and used for charging capacitors (Figs. 7a–i). As shown in Figs. 7a-ii and a-iii, a commercial digital timer was connected to the charged capacitors and successfully operated. Moreover, 200 commercial blue light emitting diodes (LEDs) were turned-on by the energy device without any external circuits (Fig. 7b). During repeated hand tapping of the BTO MCs-PEH, bulbs in series connection were simultaneously operated by the generated alternating electricity, as shown in the bottom panel of Fig. 7b–ii and Video S1: This conclusively demonstrates that the fabricated PEH can act as a sufficient power source to drive commercial electronic devices.

4. Conclusions

In summary, we developed a high-performance composite-based PEH by adopting an energy generation source of BTO NPs-based clusters and an embossed pressure concentrator of well-ordered ZTO MSs-based monolayer. The fabricated BTO MCs-based PEH harvested a Voc of ~35 V and Isc of ~7 μA under a compressive force of ~0.13 MPa, which is encouraging compared to the performance of previously reported composites-based PEHs based on BTO NPs. Embossed ZTO monolayer- stacked BTO MCs-based PEHs generated electric signals up to 206 V and 24 μA under a periodic compressive force of 0.27 MPa, which is considerably higher than the performance of previously reported results in the pushing mode. To theoretically support the measurement results, we performed the FEA analysis with COMSOL multiphysics software and verified that the BTO MCs and ZTO MSs-based monolayer are advan-tageous to maximize the total dipole moment and to concentrate the applied pressure onto the piezoelectric layer, respectively. Furthermore, we demonstrated the operation of commercial electronic devices such as digital timer and LEDs by using a power source converted from the ZTO MSs monolayer-stacked BTO MCs-based PEHs. These results indicate that the excellent output performance of PEHs can be realized using simple and cost-effective morphological treatments without expensive materials and complicated procedures. Our technology provides a new approach for fabricating high-performance composite-based PEHs, which can help the development of future stand-alone electronics and wearable devices.

Author statement

Hongbeom Park: Conceptualization, Methodology, Investigation, Formal analysis, Writing – original draft. Dong Yeol Hyeon: Methodol-ogy, Investigation, Formal analysis, Visualization, Writing – original draft. Minwoo Jung: Methodology, validation. Kwi-Il Park: Conceptu-alization, Writing – review & editing, Supervision, Resources, Funding acquisition, Project administration. Jinsub Park: Conceptualization, Writing – review & editing, Supervision, Resources, Funding acquisition.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Fig. 7. (a) Photographs of a commercial digital timer driven by the embossed pressure concentrator-stacked PEHs. (b) A captured photograph showing simultaneous power up of 200 blue LEDs when the high-output energy device was stressed by human fingers without an external circuit.

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Acknowledgements

This research was supported by the Basic Science Research Program of the National Research Foundation of Korea, funded by the Ministry of Education (NRF-2018R1D1A1B07048382, NRF-2019R1I1A2A0 1057073) and the Ministry of Science and ICT (NRF-2018 R1A4A1022260). This research was also supported by the Dongil Cul-ture and Scholarship Foundation.

Appendix A. Supplementary data

Supplementary data related to this article can be found at https://do i.org/10.1016/j.compositesb.2020.108476.

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H. Park et al.

1

Supplementary Material

Piezoelectric BaTiO3 microclusters and embossed ZnSnO3

microspheres-based monolayer for highly-efficient and flexible

composite generator

Hongbeom Park a,, Dong Yeol Hyeonb,, Minwoo Jung a, Kwi-Il Parkb,*, and Jinsub Parka,*

a Department of Electronics and Computer Engineering, Hanyang University, 222 Wangsimni-

ro, Seongdong-gu, Seoul 04763, Republic of Korea

b School of Materials Science and Engineering, Kyungpook National University, 80 Daehak-

ro, Buk-gu, Daegu 41566, Republic of Korea

This PDF file includes:

Figures S1 to S5

Other Supplementary Online Material for this manuscript:

Video S1

2

Fig. S1. (a-d) Top-view images of BTO MCs-based piezoelectric composite with different

numbers of bar-coatings: 3, 5, 7, and 9 times at a fixed coating speed of 5 cm/s. (e-h) Cross-

sectional SEM images of BTO MCs-based piezoelectric composite with different coating

speeds: 3, 5, 7, and 9 cm/s for nine coatings.

3

Fig. S2. Photograph of the BaTiO3 MCs-based piezoelectric composite wrapped round a glass

tube (a) and stretched by tweezers (b).

4

Fig. S3. The cross-sectional SEM image of the bent embossed ZTO monolayer-stacked PEH.

5

Fig. S4. (a-c) Output voltage (i) and current pulse (ii) generated from the devices fabricated

using only PDMS (a), PVA (b), and embossed ZTO monolayer (c).

6

Fig. S5. The mechanical stability test result of the ZTO monolayer-stacked PEH.

7

Video S1. Lighting up of the 200 blue LEDs by electricity generated from embossed pressure

concentrator-stacked BTO MCs-based PEH.

Live video includes:

A commercial LEDs powered up by the electricity generated from the embossed ZTO

monolayer-stacked PEH