High Permittivity CaCu3Ti4O12 Particle‐Induced Internal ...nesel.skku.edu/paper files/250(2).pdf · 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim = ⋅ ⋅ (2) −12−1, BMF–)

www.advenergymat.de

1903524 (1 of 8) © 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Full PaPer

High Permittivity CaCu3Ti4O12 Particle-Induced Internal Polarization Amplification for High Performance Triboelectric Nanogenerators

Jihye Kim, Hanjun Ryu, Jeong Hwan Lee, Usman Khan, Sung Soo Kwak, Hong-Joon Yoon, and Sang-Woo Kim*

DOI: 10.1002/aenm.201903524

polymers are limited to several types of polymers, including poly(vinylidene dif-luoride) copolymers and nylon.[25] In order to overcome the material constraints for ferroelectric polymers, we propose the strategy of incorporating the high permit-tivity particles into triboelectric dielectric materials.

Here, we develop a butylated mela-mine formaldehyde (BMF)–CaCu3Ti4O12 (CCTO) composite material as a tribo-electric dielectric material for stable high output TENGs. CCTO particles have a high permittivity of 7500,[26] potentially leading to strong internal polarization into the triboelectric dielectric material under the electric field from triboelec-tric charges. Strong internal polariza-tion of CCTO particles enhances the charge induction in the bottom electrode,

thereby increasing the triboelectric output performance. The method of incorporating CCTO particles can be applied to any polymer matrix. In this work, BMF with high abrasion resist-ance and positive triboelectric property was selected as the polymer matrix to avoid being worn by the strong frictional force and heat of the rotation-type TENG.[27]

First, we investigated the relationship between the per-mittivity and internal polarization. Polarization–electric field (P–E) curves were measured to confirm that high permittivity CCTO particles are able to amplify the internal polarization of the BMF matrix. The permittivity effect on the triboelectric output was studied using various particles (Al2O3, TiO2, and CCTO) with different permittivity; high permittivity enhances the triboelectric output performance. The investigation of the CCTO concentration into the BMF matrix reveals that excessive CCTO concentrations lead to leakage currents of composite material thereby reducing the triboelectric output power. The strategy for incorporating high permittivity par-ticles is applicable to any triboelectric polymer matrix, such as BMF, poly(methyl methacrylate) (PMMA), polydimethylsi-loxane (PDMS), and poly(vinylidenefluoride-co-trifluoroeth-ylene) [P(VDF-TrFE)], etc. As a result, the composite material based on high permittivity particles, CCTO, can form a strong internal polarization within triboelectric dielectric materials, enabling stable high output TENG without constraints of polymer matrix materials.

Here, a composite material based on the butylated melamine formaldehyde (BMF) and high permittivity CaCu3Ti4O12 (CCTO) particles as a triboelec-tric dielectric material for stable high output triboelectric nanogenerators (TENGs) is proposed. CCTO particles, which have the high permittivity of 7500, can potentially result in the formation of strong internal polarization into the dielectric material under the electric field from triboelectric charges. As a consequence, the charge induction on the bottom electrode is enhanced thereby increasing the triboelectric output performance. A rotation-type freestanding mode TENG based on BMF–CCTO 1 wt% composite material demonstrates high performance power output of a root-mean-square voltage and current density with 268 V and 25.8 mA m−2, respectively. The strategy of incorporating the high permittivity CCTO particles can be universally applied to any triboelectric polymer matrix in order to enhance the output perfor-mance of TENGs.

J. Kim, Dr. H. Ryu, Dr. J. H. Lee, Dr. U. Khan, Dr. S. S. Kwak, Dr. H.-J. Yoon, Prof. S.-W. KimSchool of Advanced Materials Science and EngineeringSungkyunkwan University (SKKU)Suwon 16419, Republic of KoreaE-mail: [email protected]

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/aenm.201903524.

1. Introduction

Triboelectric nanogenerators (TENGs) follow a mechanism in which two different triboelectric materials come in contact and then separate or slide against each other. Electron transfer between two materials occurs on the surface of the triboelectric materials.[1–6] On the basis of this mechanism, studies on sur-face phenomena of TENGs have been intensively conducted, and various surface treatment methods of triboelectric mate-rials have been proposed for high triboelectric output perfor-mance.[7–16] However, there is a problem of aging in which the triboelectric output decreases over time due to the low mechanical durability of surface treatment. Ferroelectric polymer based TENGs with strong internal polarization have been devel-oped for stable high triboelectric output,[17–24] but ferroelectric

Adv. Energy Mater. 2020, 10, 1903524

http://crossmark.crossref.org/dialog/?doi=10.1002%2Faenm.201903524&domain=pdf&date_stamp=2020-01-29

www.advenergymat.dewww.advancedsciencenews.com

© 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim1903524 (2 of 8)

2. Results and Discussion

CCTO with a huge permittivity of 7500 is of a cubic perovskite structure with slightly tiled TiO6 octahedra facing each other (Figure 1a).[28–30] The permittivity of material is related to the electrical polarization within the material. When exposed to an electric field, positive and negative charges within individual atoms and molecules try to separate from one another. In here, the degree of electrical charge separation within a material is represented by the electric polarization. As shown in Figure 1b, the relationship between the permittivity and the electrical polarization can be expressed as[31]

P E

10 rε ε( )= − (1)

where

P is the electrical polarization within a material, ε0 is the permittivity of free space (8.854 × 10−12 F m−1), εr is the relative permittivity, and

E is the electric field. According to Equation (1), the permittivity defines how strongly a material becomes electrically polarized under the electric field. There-fore, high permittivity CCTO particles form strong internal polarization under the electric field from triboelectric charges. The strong polarization of CCTO particles increases the tri-boelectric output by improving the charge induction in the bottom electrode. To demonstrate this theory experimentally, we produced the pure polymer and polymer–CCTO composite material. As the polymer matrix, BMF was selected due to its mechanical durability and highly positive triboelectric prop-erty, which originated from its functional group containing hydrogen atoms (Figure S1, Supporting Information). Figure 1c shows a cross-sectional view field emission-scanning electron

microscopy (FE-SEM) image of BMF–CCTO 1 wt% composite material. In order to investigate the elemental composition of CCTO particle in BMF matrix, energy dispersive spectroscopy (EDS) was performed as shown in Figure 1d; Ca, Cu, Ti, and O elements were detected strongly at the position of CCTO particle. The constituent elements of BMF such as C, N, and, O were detected at the matrix position.

In order to verify that high permittivity CCTO particles form strong polarization into the dielectric layer under the electric field, we measured P–E curves for pure BMF and BMF–CCTO 1 wt% composite material.[32] As shown in Figure 1e, BMF–CCTO 1 wt% composite material forms three times stronger polarization (0.19 mC m−2) than that (0.06 mC m−2) of pure BMF under an electric field of 60 MV m−1 at a frequency of 1 Hz. These results reveal that high permittivity particles can form strong internal polariza-tion within dielectric matrix.

Since TENG is driven by friction on the surface of two dif-ferent materials, the important factors affecting the triboelectric output are surface potential and surface morphology. In order to consider only the permittivity factor in this work, pure BMF and BMF–CCTO composite material should have same surface potential and surface morphology. For this reason, BMF–CCTO composite material are prepared in three coating steps; bottom is pure BMF thin layer, middle is BMF–CCTO composite mate-rial layer and top is pure BMF thin layer (Figure S2, Supporting Information). Due to the three-layer structure, CCTO particles are not located on the surface of BMF–CCTO composite mate-rial. Figure 2a shows a top view FE-SEM image of BMF–CCTO 1 wt% composite material. Element mapping was performed by EDS measurements to confirm the absence of CCTO particles

Adv. Energy Mater. 2020, 10, 1903524

Figure 1. a) Chemical structure of CCTO. b) Relationship between εr and

P under the

E . c) Cross-sectional view FE-SEM image of CCTO particle into BMF matrix. d) Element mapping profiles of CCTO particle into BMF matrix measured EDS. e) P–E curves of pure BMF and BMF–CCTO 1 wt% composite material.



on the surface of BMF–CCTO 1 wt% composite material (Figure 2b). C, N, and O elements of BMF were detected, however Ca, Cu, and Ti were not detected; the element com-position ratios on surface of BMF–CCTO 1 wt% composite material are listed in Table S1 (Supporting Information). For pure BMF, top view FE-SEM image and EDS results were in Figure S3a,b (Supporting Information), respectively. This result shows that CCTO particles are not located on the surface of BMF–CCTO composite material.

We investigated the surface potential and morphology of pure BMF and BMF–CCTO 1 wt% composite material by using Kelvin probe force microscopy (KPFM).[33] Figure 2c shows the contact potential difference (CPD) values (which means the sur-face potential) of pure BMF and BMF–CCTO 1 wt% composite material were 0.360 and 0.359 V, respectively. Figure 2d shows the surface morphology images of pure BMF and BMF–CCTO 1 wt% composite material; the root mean square (RMS) rough-ness (Rq) for pure BMF and BMF–CCTO 1 wt% composite material were 0.351 and 0.395 nm, respectively. Due to the three-layer structure of BMF–CCTO composite material, pure BMF and BMF–CCTO 1 wt% composite material have similar values of surface potential and morphology. When the CCTO particles are embedded in the polymer matrix, the hardness of the composite materials increases, exerting greater frictional force on the areas where the particles are present. Thus, CCTO particles can form more triboelectric charges on the surface. In order to exclude the hardness effect on triboelectric output, we investigate the permittivity effect on triboelectric output using

various particles with same size and different dielectric con-stant, as shown in Figure 3.

Figure 3a schematically describes BMF–CCTO composite material based rotation-type freestanding mode TENGs. The stator is comprised of the dielectric layer with segmented bottom electrodes (Au); each segment is divided into two groups (A and B). On the other hand, the rotor also consists of patterned Au sectors on the epoxy substrate, corresponding to a single group of the Au electrode of the stator. As shown in Figure 3b,c, triboelectrification between the rotator and stator results into charges on their surfaces and, as the rotator departs from one electrode group to another (for instance, from A to B), opposite charges are electrostatically induced in the corresponding bottom electrode thereby generating a cur-rent between the two electrodes (A and B).[4] The electric field from triboelectric charges applied to BMF–CCTO composite material allows CCTO particles to form strong polarization that enhances the charge induction in the bottom electrodes. There-fore, high permittivity particles can result into high triboelectric output performance.

In order to investigate the permittivity effect on the tribo-electric output, we realized BMF based composite materials using three different particles, such as Al2O3, TiO2, and CCTO, having permittivity of 9, 85, and 7500, respectively.[26,34,35] The three types of particles have similar size of 1–2 µm. We fabri-cated 10 samples each for pure BMF, BMF–Al2O3, BMF–TiO2, and BMF–CCTO composite materials (with 1 wt%) and deter-mined their capacitances by measuring the impedance curve

Adv. Energy Mater. 2020, 10, 1903524

Figure 2. a) Top view FE-SEM image of BMF–CCTO 1 wt% composite material. b) Element mapping profiles on the surface of BMF–CCTO 1 wt% composite material measured by EDS. c) CPD values and d) RMS Rq for pure BMF and BMF–CCTO 1 wt% composite material measured by KPFM.



from 1 MHz to 10 kHz (Table S2, Supporting Information). The relative permittivity can be expressed by[36]

r0

εε

= ⋅⋅

C d

A (2)

where εr is the relative permittivity, C is the capacitance, d is the thickness of dielectric layer, ε0 is the permittivity of free space, and A is the effective electrode area; d is 1.25 × 10−4 m, ε0 is 8.854 × 10−12 F m−1, and A is 10−4 m2. Figure 3d shows the average relative permittivity of pure BMF, BMF–Al2O3, BMF–TiO2, and BMF–CCTO composite materials (with 1 wt%), that are 6.77, 7.07, 7.95, and 21.74, respectively (Table S3, Sup-porting Information). Section S1 (Supporting Information) describes the theoretical permittivity of the composite mate-rials in detail. Figure 3e shows the RMS voltage and current density of rotation-type freestanding mode TENGs based on pure BMF, BMF–Al2O3, BMF–TiO2, and BMF–CCTO 1 wt% composite materials (friction area of 7853 mm2, 96 bottom electrode segments, and a rotation speed of 640 rpm). RMS voltages (VRMS) for pure BMF, BMF–Al2O3, BMF–TiO2, and BMF–CCTO 1 wt% composite materials based TENG are 113, 129, 146, and 268 V, respectively. RMS current density (JRMS) for pure BMF, BMF–Al2O3, BMF–TiO2, and BMF–CCTO 1 wt% composite materials based TENG are 8.7, 9.5, 12.5, and 25.8 mA m−2, respectively. These results demonstrate that high permittivity of triboelectric dielectric materials enhances the triboelectric output voltage and current density (see Figure S4 in the Supporting Information). The amount of transferred

charges for pure BMF and BMF–CCTO 1 wt% composite materials based TENG are shown in Figure S5 (Supporting Information). CCTO particles increased the amount of trans-ferred charges on the bottom electrode. Figure S6 (Supporting Information) shows the triboelectric output performance of BMF–CCTO 1 wt% composite material-based rotation-type freestanding mode TENG under various conditions (the number of electrode segmentation, rotation speed, and load resistance). Figure S7 (Supporting Information) shows the durability test of BMF–CCTO 1 wt% based rotation-type free-standing mode TENG for 8400 s.

To study the effect of CCTO concentration on triboelectric output voltage and current density, we fabricated BMF–CCTO composite materials with the CCTO concentration of 0, 0.5, 1, 1.5, 3, and 5 wt%. Figure 4a shows cross-sectional view FE-SEM images of the BMF–CCTO composite materials depends on CCTO concentration; the thickness is ≈125 µm (Figure S8, Sup-porting Information). As the CCTO concentration increases, the CCTO particles aggregate with each other, forming a leakage current path within the composite layer.[37] Leakage currents due to the excessive CCTO concentration cancel out the tribo-electric charges on the surface of dielectric material and the induced charges on the bottom electrode, ultimately reducing the triboelectric output performance. Figure 4b explains the process of leakage current generation in the BMF–CCTO com-posite materials as the CCTO concentration increases from 0 to 5 wt%.

Figure 5a shows VRMS and JRMS of the rotation-type free-standing mode TENG as function of the CCTO concentration

Adv. Energy Mater. 2020, 10, 1903524

Figure 3. a) Schematic illustrations of BMF–CCTO composite material based rotation-type freestanding mode TENG. Working mechanism of BMF–CCTO composite material based TENG for b) initial state and c) final state. d) Relative permittivity of pure BMF, BMF–Al2O3, BMF–TiO2, and BMF–CCTO composite materials (with 1 wt%). e) VRMS and JRMS of pure BMF, BMF–Al2O3, BMF–TiO2, and BMF–CCTO (with 1 wt%) composite materials based rotation-type freestanding mode TENGs.



ranging from 0 to 5 wt%; the results indicate that the triboelectric output increases until the concentration of CCTO particles reaches 1 wt%, and then decreases as CCTO particles concentration increases above 1 wt%. Leakage cur-rents occurring at excessive CCTO concentration greater than 1 wt% reduce the triboelectric charges on the surface of dielectric material and the induced charges on the bottom electrode, thereby reducing the triboelectric output perfor-mance. Figure 5b indicates the leakage current density of the BMF–CCTO composite materials under an external elec-tric field of 0–8 MV m−1 at the room temperature. When the CCTO concentration increases from 0 to 5 wt%, the leakage current density increases from 7.39 × 10−6 A m−2 to 1.23 × 10−4 A m−2 under the external electric field of 8 MV m−1; excessive CCTO concentration causes higher leakage current through BMF–CCTO composite material. Figure 5c (which is a magnified version of Figure 5b) shows the leakage current den-sity as a function of the external electric field up to 1.5 MV m−1.

The triboelectric field strength is around 0.5 MV m−1 (see the theoretical calculation of the triboelectric field in Sec-tion S2, Supporting Information). In the range of the triboelec-tric field, there is no leakage current up to the CCTO concentra-tion of 1 wt%, however, the leakage current occurs at excessive CCTO concentration (greater than 1 wt%), which reduces the triboelectric output performance.

The strategy of incorporating high permittivity CCTO parti-cles is general and applicable to any polymer. Indeed, CCTO particles enhance the polarization strength within the dielectric material regardless of the polarization direction, which is deter-mined by the triboelectric properties of the polymer matrix. In order to demonstrate experimentally, we selected four different polymer matrices, BMF, PMMA, PDMS, and P(VDF-TrFE), and then CPD values of four polymer matrices were measured by KPFM (Figure S9, Supporting Information). Based on these results, triboelectric series are listed in Figure 6a; BMF has positive triboelectric property and P(VDF-TrFE) has negative

Adv. Energy Mater. 2020, 10, 1903524

Figure 4. a) Cross-sectional view FE-SEM image of BMF–CCTO composite material with the CCTO concentration from 0 to 5 wt%. b) Schematic illustrations of leakage current generation in the BMF–CCTO composite material with the CCTO concentration from 0 to 5 wt%.


© 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim1903524 (6 of 8)Adv. Energy Mater. 2020, 10, 1903524

triboelectric property. Because of the low abrasion resistance of PMMA, PDMS, and P(VDF-TRFE), we fabricated the con-tact–separation mode TENGs instead of the rotation-type free-standing mode TENG. Opposite friction material is Au and active area is 0.0016 m2. Figure S10 (Supporting Information) shows the schematic illustrations of the working mechanism in which CCTO particles in the positive or negative polymer matrix enhance the output performance of contact–separation mode TENG. Figure 6b–e shows the peak output voltage and current density of the TENGs based on BMF, PMMA, PDMS, and P(VDF-TrFE) without and with CCTO 1 wt% (see Figure S11 in the Supporting Information). These results show that the incorporation of CCTO particles to any polymer matrix

greatly increases the output voltage and current of the contact–separation mode TENGs.

3. Conclusion

In summary, we developed an effective method to improve the triboelectric output performance by incorporating high per-mittivity CCTO particles into the triboelectric polymer matrix. It was found that the CCTO particles were able to amplify the internal polarization within the dielectric polymer mate-rial under the electric field from triboelectric charges. The BMF–CCTO 1 wt% composite material formed three times higher internal polarization than pure BMF under the same electric field condition. BMF–CCTO 1 wt% composite material based a rotation-type freestanding mode TENG generated high RMS voltage and current density with 268 V and 25.8 mA m−2, respectively. Finally it was demonstrated that the incorpora-tion of high permittivity CCTO particles into a polymer matrix could dramatically and effectively enhance power-generating performance of TENG without constraints of polymer matrix materials.

4. Experimental SectionFabrication of the BMF–CCTO Composite Material: BMF solution

and CCTO particles were blended using stirring, and then an ultrasonication process was carried out for 1 h in an ultrasonication bath. BMF bottom layer was coated on Au coated on an FR-4 epoxy resin substrate by using the automatic bar coater. Immediately following the coating process, BMF bottom layer was annealed at 180 °C for 2 h in the oven and was then naturally cooled down. And then, BMF–CCTO solution was bar-coated and then underwent an annealing process at 180 °C for 2 h in the oven and a cooling process. As a last step, BMF top layer was prepared by the automatic bar-coating, and then the film underwent an annealing process at 180 °C for 2 h in the oven and a cooling process.

Material Characterizations: For FE-SEM and EDS (Jeol Ltd., JSM-7500F at the MEMS Sensor Platform Center of SKKU) measurements, the sample was prepared on the glass substrate. For the P–E curve measurements (RADIANT TECHNOLOGIES, Inc., PLC1104-772), the sample was coated on copper plate. For noncontact mode KPFM analysis (Park Systems, XE100), the sample was prepared on indium tin oxide (ITO)/glass substrate and measured under a set point of 13 nm, at scan rate of 0.5 Hz (temperature = 21 °C, humidity = 17%). KPFM measurements were performed with a Pt/Cr-coated silicon tip; a lock-in amplifier (Stanford Research, SR830) and a 2 Vac signal at a frequency of 17 kHz were employed. For impedance curve measurement (Bio-Logic, VMP3), the metal–insulator–metal (MIM) structure was fabricated, i.e., Au/dielectric/Au on silicon substrate. For the measurement of the leakage current density, the probe station (MSTECH, MST5000) and a semiconductor parameter analyzer (Keithley SCS-4200) were utilized at room temperature; the sample was fabricated with metal–insulator–metal structure (Au/dielectric material/Au on silicon substrate).

Measurement of Triboelectric Output Performance: For the TENG characterizations, a rotator (SIMTECH, SMR-10), a pushing tester (Z-Tech, ZPS-100), a digital phosphor oscilloscope (Tektronix, DPO 3052 Digital Phosphor), and low-noise current preamplifier (Stanford Research Systems Inc., SR570) were used. The amount of charge was measured by using an electrometer (KEITHLEY, 6514 SYSTEM ELECTROMETER).

Figure 5. a) VRMS and JRMS of the rotation-type freestanding mode TENG as a function of CCTO concentration varying from 0 to 5 wt%. b,c) Leakage current density of the BMF–CCTO composite material with the CCTO concentration from 0 to 5 wt% as a function of E.



Supporting InformationSupporting Information is available from the Wiley Online Library or from the author.

AcknowledgementsThis work was financially supported by the Center for Advanced Soft Electronics (CASE) under the Global Frontier Research Program (2013M3A6A5073177) through the National Research Foundation (NRF) of Korea Grant funded by the Ministry of Science and ICT, a project no. SI1802 (Development of One patch Device for HMI Based on 3D Device Printing) of the Korea Research Institute of Chemical Technology (KRICT), and the Korea Basic Science Institute (KBSI) National Research Facilities & Equipment Center (NFEC) grant funded by the Korea Government (Ministry of Education) (No. 2019R1A6C1010031).

Conflict of InterestThe authors declare no conflict of interest.

KeywordsCaCu3Ti4O12, composite, high permittivity, internal polarization, triboelectric nanogenerators

Received: October 28, 2019Revised: December 5, 2019

Published online: January 29, 2020

[1] R. Hinchet, H.-J. Yoon, H. Ryu, M.-K. Kim, E.-K. Choi, D.-S. Kim, S.-W. Kim, Science 2019, 365, 491.

[2] F.-R. Fan, Z.-Q. Tian, Z. L. Wang, Nano Energy 2012, 1, 328.[3] S. Niu, S. Wang, L. Lin, Y. Liu, Y. S. Zhou, Y. Hu, Z. L. Wang, Energy

Environ. Sci. 2013, 6, 3576.[4] G. Zhu, J Chen, T. Zhang, Q. Jing, Z. L. Wang, Nat. Commun. 2014,

5, 3426.[5] T. Jiang, X. Chen, K. Yang, C. Han, W. Tang, Z. L. Wang, Nano Res.

2016, 9, 1057.[6] R. Hinchet, W. Seung, S.-W. Kim, ChemSusChem 2015, 8,

2327.[7] X.-S. Zhang, M.-D. Han, R.-X. Wang, B. Meng, F.-Y. Zhu, X.-M. Sun,

W. Hu, W. Wang, Z.-H. Li, H.-X. Zhang, Nano Energy 2014, 4, 123.

[8] H. Y. Li, L. Su, S. Y. Kuang, C. F. Pan, G. Zhu, Z. L. Wang, Adv. Funct. Mater. 2015, 25, 5691.

[9] S. Wang, Y. Xie, S. Niu, L. Lin, C. Liu, Y. S. Zhou, Z. L. Wang, Adv. Mater. 2014, 26, 6720.

[10] H.-J. Choi, J. H. Lee, J. Jun, T. Y. Kim, S.-W. Kim, H. Lee, Nano Energy 2016, 27, 595.

[11] K.-E. Byun, Y. Cho, M. Seol, S. Kim, S.-W. Kim, H.-J. Shin, S. Park, S. Hwang, ACS Appl. Mater. Interfaces 2016, 8, 18519.

[12] S. Wang, Y. Zi, Y. S. Zhou, S. Li, F. Fan, L. Li, Z. L. Wang, J. Mater. Chem. A 2016, 4, 3728.

[13] W. Shang, G. Q. Gu, F. Yang, L. Zhao, G. Cheng, Z.-L. Du, Z. L. Wang, ACS Nano 2017, 11, 8796.

[14] S.-H. Shin, Y. H. Kwon, Y.-H. Kim, J.-Y. Jung, M. H. Lee, J. Nah, ACS Nano 2015, 9, 4621.

[15] L. Zhao, Q. Zheng, H. Ouyang, H. Li, L. Yan, B. Shi, Z. Li, Nano Energy 2016, 28, 172.

Figure 6. a) Triboelectric series of BMF, PMMA, PDMS, and P(VDF-TrFE). Peak output voltage and current density of the contact–separation mode TENGs based on b) pure BMF and BMF–CCTO 1 wt% composite material, c) pure PMMA and PMMA–CCTO 1 wt% composite material, d) pure PDMS and PDMS–CCTO 1 wt% composite material, and e) pure P(VDF-TrFE) and P(VDF-TrFE)–CCTO 1 wt% composite material.



[16] H. Ouyang, Z. Liu, N Li, B. Shi, Y. Zou, F. Xie, Y. Ma, Z. Li, H. Li, Q. Zheng, X. Qu, Y. Fan, Z. L. Wang, H. Zhang, Z. Li, Nat. Commun. 2019, 10, 1821.

[17] W. Seung, H.-J. Yoon, T. Y. Kim, H. Ryu, J. Kim, J.-H. Lee, J. H. Lee, S. Kim, Y. K. Park, Y. J. Park, S.-W. Kim, Adv. Energy Mater. 2017, 7, 1600988.

[18] X. Chen, M. Han, H. Chen, X. Cheng, Y. Song, Z. Su, Y. Jiang, H. Zhang, Nanoscale 2017, 9, 1263.

[19] X. Wang, B. Yang, J. Liu, Y. Zhu, C. Yang, Q. He, Sci. Rep. 2016, 6, 36409.

[20] K. Y. Lee, S. K. Kim, J.-H. Lee, D. Seol, M. K. Gupta, Y. Kim, S.-W. Kim, Adv. Funct. Mater. 2016, 26, 3067.

[21] S. Cheon, H. Kang, H. Kim, Y. Son, J. Y. Lee, H.-J. Shin, S.-W. Kim, J. H. Cho, Adv. Funct. Mater. 2017, 28, 1703778.

[22] J. W. Lee, H. J. Cho, J. Chun, K. N. Kim, S. Kim, C. W. Ahn, I. W. Kim, J.-Y. Kim, S.-W. Kim, C. Yang, J. M. Baik, Sci. Adv. 2017, 3, e1602902.

[23] Y. S. Choi, Q. Jing, A. Datta, C. Boughey, S. Kar-Narayan, Energy Environ. Sci. 2017, 10, 2180.

[24] J. Kim, J. H. Lee, H. Ryu, J.-H. Lee, U. Khan, H. Kim, S. S. Kwak, S.-W. Kim, Adv. Funct. Mater. 2017, 27, 1700702.

[25] Q. M. Zhang, V. Bharti, X. Zhao, Science 1998, 280, 2101.

[26] A. P. Ramirez, M. A. Subramanian, M. Gardel, G. Blumberg, D. Li, T. Vogt, S. M. Shapiro, Solid State Commun. 2000, 115, 217.

[27] S. S. Kwak, S. M. Kim, H. Ryu, J. Kim, U. Khan, H.-J. Yoon, Y. H. Jeong, S.-W. Kim, Energy Environ. Sci. 2019, 12, 3156.

[28] C. C. Homes, T. Vogt, S. M. Shapiro, Phys. Rev. B 2003, 67, 092106.[29] G. Deng, N. Xanthopoulos, P. Muralt, Appl. Phys. Lett. 2018, 92,

172909.[30] T.-T. Fang, C. P. Liu, Chem. Mater. 2005, 17, 5167.[31] A. Kuchler, High Voltage Engineering: Fundamentals-Technology-Appli-

cations, Springer Vieweg, Berlin, Germany 2018, p. 79.[32] L. Yang, W. Li, E. Allahyarov, P. L. Taylor, Q. M. Zhang, L. Zhu,

Polymer 2013, 54, 1709.[33] H. Kang, H. T. Kim, H. J. Woo, H. Kim, D. H. Kim, S. Lee,

S. M. Kim, Y. J. Song, S.-W. Kim, J. H. Cho, Nano Energy 2019, 58, 227.

[34] P. K. Park, E.-S. Cha, S.-W. Kang, Appl. Phys. Lett. 2007, 90, 232906.[35] S. K. Kim, W.-D. Kim, K.-M. Kim, C. S. Hwang, J. Jeong, Appl. Phys.

Lett. 2004, 85, 4112.[36] S. K. Bhattacharya, J. Y. Park, R. R. Tummala, M. G. Allen, J. Mater.

Sci.: Mater. Electron. 2000, 11, 455.[37] P. Thomas, S. Sataphthy, K. Dwarakanath, K. B. R. Varma, Express

Polymer Lett. 2010, 4, 632.

Documents

High Permittivity CaCu3Ti4O12 Particle‐Induced Internal ...nesel.skku.edu/paper files/250(2).pdf · 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim = ⋅ ⋅ (2) −12−1, BMF–)