Pb52(Zr,Ti)48O3 Ferroelectric Dipole Electret Exploiting Surface Pillar Array Structure for Electrostatic Vibration Energy Harvesters

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


Introduction
Vibration energy harvesters (VEHs) have been considered as potential self-sustainable power sources for the Internet of Things (IoTs). (1)(2)(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 of electret, named ferroelectric dipole electrets (FDEs), which was a ferroelectric material with a relative permittivity (ε r ) of more than 1000, much higher than that of conventional CYTOP polymer electrets (CPEs, ε r ~ 2.1). (4,6,7) As a result, the output power of FDE-based VEHs was increased threefold that of CPE-based energy harvesters. (4) However, such an increase was limited 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 the effective 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 of electrets by fabricating a patterned pillar structure on commercial Pb 52 (Zr,Ti) 48 O 3 (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.

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 size of 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/m 2 for both FDEs. The positive charges were placed on the upper surface, while the negative charges were set on the lower surfaces. The surface charges were fixed and could not move on the surface. The simulated y-component of electric fields (E y ) obtained for both the flat (unpatterned) and patterned 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 the electric field was 1.5 × 10 6 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 for the two FDEs. The magnitude of the electric field of both FDEs decreased as the distance increased. However, the electric field of the patterned sample degraded much faster than that of 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 the energy harvester device should move close to the FDE surface. These simulation results served as a basis for the design of experimental devices.

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 mm 3 ). In this study, these hard-type ferroelectric materials were selected because they show longer stability of their surface potential. (9) Patterned structures with three different pitch sizes (0.2, 0.5, and 1.0 mm) were prepared using a dicer (DISCO, 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 determined using a noncontact electrostatic voltmeter (Trek Inc., Model 344).
After removing the upper electrode and silicon oil, the output power generated by the patterned FDE was measured using the setup shown in Fig. 2. The FDE was placed on a translational 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 electrode was 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)

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. The surface charge density of the FDEs was calculated as σ = (ε r × ε 0 × V s )/t, where σ is the surface charge density, ε r and ε 0 are the relative permittivity of the material and the vacuum permittivity, respectively, V s is the surface potential, and t is the electret thickness. By using the patterned pillar structure, we obtained the highest σ of 6.9 mC/m 2 with a d p of 2.0 mm. This σ increased 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 an electret, the development of a high-performance electret is required. In this study, we show that the surface charge density of a CYTOP electret is significantly improved by the addition of terminal 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 measurements of surface potential and thermally stimulated discharge (TSD) showed that they were 1.3-fold higher than those of the flat one. For comparison, we summarized the ferroelectric properties of the FDEs in Table 1. The Curie temperature (T c ) and electromechanical coupling factor (k p ) were provided by the manufacturer (Fuji Ceramics Corp.) for the hard PZT.

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  Table 1 Poling electric field and material characteristics of FDEs and CYTOP. (4) The Curie temperature (T c ) and electromechanical coupling factor (k p ) were provided by the manufacturer for the hard PZT.
E poling (kV/mm)     electrode for the FDE with a d p of 2.0 mm (FDE1). The acceleration and load resistance were firstly 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 acceleration of 0.5 g was 303 µm; therefore, when the gap became narrower (< 320 µm), the FDE hit the upper 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 the acceleration. 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 the upper 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 of the 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.

Conclusions
We successfully developed the pillar array structures of PZT FDEs. The electrets showed a superior 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   (4,7,(12)(13)(14)(15)(16)(17)(18)(19)(20) 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 in electrostatic VEHs. Lab between France and Japan at Tohoku University. His research interests are mainly in the understanding of multiphysics coupling in materials, and their application to low-power energy harvesting from temperature and vibration, as well as electrocaloric and electrocaloric cooling materials 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, and over 50 patents in microelectromechanical systems and particle beam processing. His research interests 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)