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Sensors and Materials, Vol. 32, No. 7 (2020) 2517–2525
MYU Tokyo
S & M 2279

Pb52(Zr,Ti)48O3 Ferroelectric Dipole Electret
Exploiting Surface Pillar Array Structure forElectrostatic Vibration Energy Harvesters
Hung Hoang Nguyen,1,2* Haruhiko Asanuma,3 Hiroyuki Oguchi,1,4Gaёl Sebald,5 and Hiroki Kuwano1,2,5
1

New Industry Creation Hatchery Center (NICHe), Tohoku University,
6-6-10 Aramaki-Aoba, Aoba-ku Sendai, Miyagi 980-8579, Japan
2
Sendai Smart Machine Co., Ltd. (SSM), 6-6-40 Aza-Aoba, Aramaki, Aoba-ku, Sendai, Miyagi 980-8579, Japan
3
Institute of Science and Engineering, Faculty of Frontier Engineering, Kanazawa University,
Kakuma-machi, Kanazawa 920-1192, Japan
4
Advanced Institute for Materials Research, Tohoku University,
2-1-1 Katahira, Aoba-ku, Sendai, Miyagi 980-8577, Japan
5
ELyTMaX UMI 3757, CNRS – Université de Lyon – Tohoku University, International Joint Unit,
Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, Miyagi 980-8577, Japan(Received March 9, 2020; accepted May 18, 2020)

Keywords: ferroelectric dipole electrets, FDE, PZT, vibration energy harvesters, electrets

In this paper, we present fer roelectric dipole electrets (FDEs) with patter ned
Pb52(Zr,Ti)48O3 (PZT) pillar arrays. Their output power generation capability was higher
than that of flat FDEs when used for electrostatic vibration energy harvesters (e-VEHs). This
excellent capability was achieved since the patterned pillar structure increased the electricfield near the electret surface. The surface potential (Vs) became as high as 1035 V when
the surface charge density was 6.9 mC/m2. This surface charge density was 1.3-fold higher
than that of flat FDEs and sixfold higher than that of conventional CYTOP polymer electrets(CPEs). Consequently, VEHs (20 × 20 × 1 mm3) showed a maximum power of 102 µW at anacceleration of 0.6 g (1 g = 9.8 m/s2) and a frequency of 20 Hz. These results will open a new
strategy for the use of FDEs in e-VEHs.

1.

Introduction

Vibration energy harvesters (VEHs) have been considered as potential self-sustainable powersources for the Internet of Things (IoTs).(1–3) In this aspect, among various types of VEH,
electrostatic energy harvesters have attracted attention since they can operate under ambient
vibration conditions (frequency and acceleration are below 100 Hz and 1.0 g, respectively).(4,5)
Their compatibility with MEMS processing(6) is another advantage regarding miniaturization.
One of the main challenges for electrostatic VEHs is to increase their output power
generation capability. Responding to this challenge, Asanuma et al. developed a new type ofelectret, named ferroelectric dipole electrets (FDEs), which was a ferroelectric material with*

Corresponding author: e-mail: hung@nanosys.mech.tohoku.ac.jp
https://doi.org/10.18494/SAM.2020.2854
ISSN 0914-4935 © MYU K.K.
https://myukk.org/

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Sensors and Materials, Vol. 32, No. 7 (2020)a relative permittivity (εr) of more than 1000, much higher than that of conventional CYTOP
polymer electrets (CPEs, εr ~ 2.1).(4,6,7) As a result, the output power of FDE-based VEHs wasincreased threefold that of CPE-based energy harvesters.(4) However, such an increase waslimited because strong electric fields were still trapped inside the FDE; hence, these prototype
VEHs effectively used only the fringe electric field at the corner of the FDE.(8) To increase theeffective electric field strength, one possible approach is to increase the number of edges.
On the basis of this idea, in this study, we attempted to increase the boundary area ofelectrets by fabricating a patterned pillar structure on commercial Pb52(Zr,Ti)48O3 (PZT). The
effects of the pitch size of the structure on remnant polarization, surface potential, and surface
charge density were investigated. We achieved a high surface charge density, which increased
sixfold that of CPEs and 1.3-fold that of flat FDEs. The output power generated by the pillar
array FDE was measured to confirm the validity of using this structure.

2.

Design of Electrets

We firstly simulated the electric fields generated by the patterned and flat (unpatterned)
FDEs. The simulation was conducted using COMSOL Multiphysics. For the patterned
structure, the dimensions for the simulation were set at t = w = g = 1 mm, where t is the electret
thickness, w and g are the width of the pillars and the gap between them, respectively. The sizeof the air domains was set to be much larger than electret dimensions as 10(w + g) × 10t.
Surface charge density was set as the remnant polarization of 20 mC/m2 for both FDEs.
The positive charges were placed on the upper surface, while the negative charges were seton the lower surfaces. The surface charges were fixed and could not move on the surface.
The simulated y-component of electric fields (Ey) obtained for both the flat (unpatterned) andpatterned FDEs are compared in Fig. 1(a). Such a comparison indicated that patterning could
enhance the electric field. On the surface of the patterned structure (d ~ 0), the strength of theelectric field was 1.5 × 106 V/m, 2.5-fold that on the surface of the flat FDE.
Figure 1(b) shows the magnitude of the y-component of electric fields near the center forthe two FDEs. The magnitude of the electric field of both FDEs decreased as the distanceincreased. However, the electric field of the patterned sample degraded much faster than thatof the flat FDE. When moving far from the FDE surface, the contribution of the fringe field
became negligible.(8) The electric fields of the pillar FDEs were higher up to 922 µm. On
the basis of this result, to exploit the advantage of the patterned structure, the oscillator in theenergy harvester device should move close to the FDE surface. These simulation results served
as a basis for the design of experimental devices.

3.

Experimental Procedure

The pillar arrays were fabricated on unpoled hard PZT ceramics (Fuji Ceramics Corp.; No.
C-2: L × W × t = 20 × 20 × 1.0 mm3). In this study, these hard-type ferroelectric materials wereselected because they show longer stability of their surface potential.(9) Patterned structureswith three different pitch sizes (0.2, 0.5, and 1.0 mm) were prepared using a dicer (DISCO,

Sensors and Materials, Vol. 32, No. 7 (2020)

(a)

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(b)

Fig. 1. (Color online) (a) Electric fields along y-direction (Ey) generated by flat and patterned FDEs and (b)magnitude of Ey at x = 0.

DAD3240). We also prepared unpatterned samples (flat FDEs) for the comparison of the FDE
performance characteristics.
The pillar PZTs were polarized by applying an external electric field to a pair of Au
electrodes in a silicon oil bath.(4) The applied poling electric field and treatment time were
4.0 kV/mm and 1.0 h, respectively. The permittivities of the pillar structures were measured
using an impedance analyzer (HP-4194A). The surface potentials of the FDEs were determinedusing a noncontact electrostatic voltmeter (Trek Inc., Model 344).
After removing the upper electrode and silicon oil, the output power generated by thepatterned FDE was measured using the setup shown in Fig. 2. The FDE was placed on atranslational stage with the lower electrode and vibrated vertically using a standard shaker(G-Master APD-200FCG). The initial air gap between the FDE surface and the upper electrodewas precisely controlled using a micrometer. To measure the displacement of the electret, we
vibrated the FDE without the upper electrode and used a laser Doppler vibrometer (Ono-Sokki
LV-1710) focused on the surface of the FDE.
The output power was obtained at a frequency of 20 Hz with various initial air gaps, load
resistances (R L), and accelerations (a). The waveform of the output voltage generated from the
FDE was obtained using an oscilloscope (Iwatsu, DS-5552). As shown in the inset in Fig. 2,
the waveform of the output voltage is not sinusoidal because the capacitance between the upper
electrode and the FDE does not change linearly with the air gap during vibration.(4,10,11)

4.

Result and Discussion

4.1

FDEs characteristics

We successfully fabricated the pillar arrays with different pitch sizes on FDEs by dicing.
Typical SEM images of the pillar arrays fabricated using a 0.5-mm-thick blade are shown in Fig. 3.

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Fig. 2. (Color online) Experimental setup for output power evaluation and the waveform of output voltage
generated by our FDEs (inset).

The sizes of the pillar and the pitch (dp) were designed to be 1.0 × 1.0 × 1.0 mm3 and 1.5 mm,
respectively.
The P–E hysteresis loops of FDEs with different dp values of 1.2, 1.5, and 2.0 mm and a flat
FDE with a thickness of 1.0 mm, are plotted in Fig. 4. We obtained a similar Pr of 216 mC/m2
at the applied electric field of 4.0 kV/mm for all samples, indicating that the FDEs are fullypolarized. Their surface potential (Vs) and corresponding surface charge density are shown in
Fig. 5. The surface potential measured at around 200 µm above the FDE surface was 1035 V
for a dp of 2.0 mm (FDE1). This value was almost twofold higher than that of the flat one (FDE4),
consistent with the simulation results.
The surface charge density of the FDEs was calculated as σ = (εr × ε 0 × Vs)/t, where σ is thesurface charge density, εr and ε 0 are the relative permittivity of the material and the vacuum
permittivity, respectively, Vs is the surface potential, and t is the electret thickness. By using thepatterned pillar structure, we obtained the highest σ of 6.9 mC/m2 with a dp of 2.0 mm. This σincreased sixfold that of conventional CYTOP.(4,12) Micropower generation using electrets has
attracted much attention owing to their large power output in the low frequency range. Since
the theoretical power output is proportional to the square of the surface charge density of anelectret, the development of a high-performance electret is required. In this study, we showthat the surface charge density of a CYTOP electret is significantly improved by the addition ofterminal groups. On the basis of this finding, a novel high-performance polymer electret has
been developed by doping a silane-coupling reagent into the polymer. A series of measurementsof surface potential and thermally stimulated discharge (TSD) showed that they were 1.3-foldhigher than those of the flat one. For comparison, we summarized the ferroelectric propertiesof the FDEs in Table 1. The Curie temperature (Tc) and electromechanical coupling factor (k p)were provided by the manufacturer (Fuji Ceramics Corp.) for the hard PZT.
4.2

Output power measurement

By using the pillar structures, we successfully enhanced the output power of the FDE.
Figure 6 shows the results of initial air gap optimization between the electret and the upperSensors and Materials, Vol. 32, No. 7 (2020)

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Fig. 3. (Color online) (a) SEM image of FDE composed of PZT pillar arrays with pitch (dp) of 1.5 mm and (b)enlarged image at cutting lines.

Fig. 4. (Color online) P–E hysteresis loops of
FDEs with different pitches (dp). The polarization
was determined on the effective area of ferroelectricmaterials. Our FDEs obtained a remnant polarizationof 216 mC/m 2.

Fig. 5. (Color online) Surface potentials and charge
densities of FDEs with different pitches (dp) [(FDE1 (2.0mm), FDE2 (1.5 mm), FDE3 (1.2 mm), FDE4 (flat, 0mm)], and conventional CYTOP.

Table 1Poling electric field and material characteristics of FDEs and CYTOP.(4) The Curie temperature (Tc) andelectromechanical coupling factor (k p) were provided by the manufacturer for the hard PZT.
Epoling
kp
Vs
σ
Tc
Pr
Ec
εr
(kV/mm)(V)(mC/m 2)(%)(°C)
(mC/m 2)
(kV/mm)
FDE1
4.0
1035
6.9
740
59.6
300
216
1.83
FDE2
4.0
408
3.4
853
59.6
300
210
1.99
FDE3
4.0
395
4.2
1233
59.6
300
212
2.36
FDE4
4.0
394
5.2
1480
59.6
300
209
2.24
—
559
1.0
2.1
—
—
—
—
CYTOP(4)2522

Fig. 6. (Color online) Results of air gap optimizationfor output power of FDE1. The optimized gap was in
the range of 320–350 µm.

Sensors and Materials, Vol. 32, No. 7 (2020)

Fig. 7. (C olor on l i ne) Re s u lt s of i mp e d a nce
matching for FDE1. The optimal load resistancewas 106 MΩ and the output power and voltage were
93.9 µW and 99.3 V, respectively.

electrode for the FDE with a dp of 2.0 mm (FDE1). The acceleration and load resistance werefirstly set at 0.5 g and 30 MΩ, respectively. The highest output power was obtained with an air
gap in the range of 320–350 µm. The zero-to-peak displacement of the FDE at an accelerationof 0.5 g was 303 µm; therefore, when the gap became narrower (< 320 µm), the FDE hit theupper electrode and the output power saturated.
Figure 7 shows the dependence of output power on the total load resistance (R L). The
optimized load resistance for the harvester was 106 MΩ. We achieved a high output power of
93.9 µW with a voltage of 99.3 V at an acceleration of 0.5 g, which was almost twofold higher
than that of the flat FDE previously reported.(4)We investigated the dependences of the output power and FDE displacement on theacceleration. The obtained results are plotted in Fig. 8. The initial air gap between the FDE
and the upper electrode was set to 350 µm. The displacement of the FDE was measured without
the upper electrode. The output power increased monotonically to 102 µW as the acceleration
increased up to 0.6 g. At an acceleration of 0.6 g, the zero-to-peak displacement of the FDE was
346 µm. At an acceleration of more than 0.6 g, the output power saturated since the FDE hit theupper electrode.
To evaluate the performance of a VEH, the output power density should be considered.
The output power density is defined as the output power normalized by the total volume ofthe electret and air gap. Figure 9 shows the power densities of VEHs using electrets for power
generation. Our VEH with an FDE provided a power density of 189 µW/cm−3, which is higher
than most of the power densities of the VEHs, and close to the power densities of the best CPEs
or polytetrafluoroethylene (PTFEs) using in-plane structures.(12,13) Possibly, the output power
density can be further improved by our FDEs if the in-plane structures are used.

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Fig. 8. (Color online) Dependences of output power and FDE displacement on acceleration. The FDE generated amaximum output power of 102 µW at 0.6 g with a load resistance of 106 MΩ.

Fig. 9. (Color online) Power densities of this work and state-of-the-art electret-based VEHs. The volume forpower density calculation is the total volume of the electret and air gap.(4,7,12–20)

5.

Conclusions

We successfully developed the pillar array structures of PZT FDEs. The electrets showed asuperior performance to a flat FDE and CPEs. The FDEs with patterned pillar arrays showed
three- and sixfold increases in surface charge density in comparison with the flat FDE and
CPEs, respectively. Consequently, the output power of VEHs with a patterned pillar-based FDE
was almost 200% higher than that of the VEH with a flat FDE and comparable to the highest

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value reported for CPEs/PTFEs. The result obtained in this work confirmed the validity of our
microstructuring approach and will facilitate the use of ferroelectric materials as electrets inelectrostatic VEHs.

Acknowledgments
This work was supported by the JKA Social Action Program. We would like to thankDr. LE VAN MINH of New Industry Creation Hatchery Center (NICHe), Tohoku University,
for his supports in establishing the fabrication technique of patterned FDEs.

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About the Authors
Hoang Hung Nguyen received his B.Sc. and M.Sc. degrees from Hanoi University of Scienceand Technology, Vietnam, in 2010 and 2012, respectively. He obtained his Ph.D. degree fromTohoku University, Japan, in 2018. From 2018 to 2019, he was a postdoctoral fellow at Tohoku
University, Japan. Since 2019, he has been a researcher in Sendai Smart Machine Co., Ltd. His
research interests are in MEMS and sensors. (hung@nanosys.mech.tohoku.ac.jp)
Haruhiko Asanuma received his B.Sc. and M.Sc. degrees in physics from the Faculty of
Science and the Graduate School of Science, Tohoku University, Japan, in 2005 and 2007,
respectively. After five years of service in industry, he joined the Graduate School ofEngineering, Tohoku University, and received his Ph.D. degree in 2015. He is currently an
assistant professor at Kanazawa University, Japan. His research interests include appliedmaterial physics, VEHs, and semiactive vibration control. (h-asanuma@se.kanazawa-u.ac.jp)
Hiroyuki Oguchi was born in Shiga Prefecture, Japan in 1975. He obtained his M.Eng. degreefrom Tokyo University (2002) and his Ph.D. degree from University of Maryland (2008). He
was an assistant professor (2008–2012) and has been an associate professor (2012–2020) at
Tohoku University. He has worked on growth of films with energy-related functions and theirapplication to energy devices. One representative recent work is the growth of hydride films
with fast Li-ion conduction, which will be applied to Li-ion batteries in the future. Anotherrepresentative work is the growth of AlN-based films and their application to high-power
generation VEHs. (oguchi@nanosys.mech.tohoku.ac.jp)Gaёl Sebald received his M.Sc. degree in electrical engineering and his Ph.D. degree inacoustics from INSA Lyon, France, in 2001 and 2004, respectively. He then received aJapan Society for Promotion of Science (JSPS) fellowship for a postdoctoral position at
Tohoku University (Sendai, Japan). He became an associate professor in 2005 at INSA Lyon,
and a professor in 2016. In 2010, he was a JSPS Invited Researcher in the Department of
Nanomechanics at Tohoku University. Since 2016, he has been based in an International Joint
Lab between France and Japan at Tohoku University. His research interests are mainly in theunderstanding of multiphysics coupling in materials, and their application to low-power energy
harvesting from temperature and vibration, as well as electrocaloric and electrocaloric coolingmaterials and applications. He is also working on nonlinear dynamics applied to energy
harvesting, ferroelectric/ferromagnetic modeling, and fractional calculus applied to hysteresis
dynamics and electromagnetic nondestructive testing. (gael.sebald@insa-lyon.fr)
Hiroki Kuwano received his B.Eng. and M.Eng. degrees in mechanical engineering, and his
Ph.D. degree in electrical engineering from Tohoku University, Sendai, Japan, in 1975, 1977,
and 1990, respectively. He was a member of the Electrical Communication Laboratories of
Nippon Telephone and Telegraph Public Corporation. Since 2003, he has been a professor at
Tohoku University. He has authored or co-authored over 120 technical papers and books, andover 50 patents in microelectromechanical systems and particle beam processing. His researchinterests are in nanoenergy systems, including energy harvesting systems and sensor networks,
particularly those used for safety and medical applications.
(hiroki.kuwano@nanosys.mech.tohoku.ac.jp)