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Energy Harvesting from Vibration Using Polymer Electret Yuji Suzuki Dept. of Mechanical Engineering, The University of Tokyo, Tokyo, Japan ABSTRACT:
A vibration-driven electret generator has been developed for energy harvesting applications. Perfluorinated polymer electret with extremely-high surface charge density is employed for large power output. By using parylene as the spring material, a low-resonant-frequency MEMS generator is realized. Large in-plane amplitude of 0.8 mm at the resonant frequency as low as 37 Hz has been achieved. With our early prototype, output power of 0.28 FiW has been obtained. 1. INTRODUCTION
Energy harvesting from environmental vibration attracts much attention in view of its potential to replace button batteries used for low power applications such as RFIDs and automotive sensors [1-3]. Various prototype devices using piezoelectric, electro-magnetic, and electrostatic principles are proposed. Figure 1 shows a simplified model of vibration-driven power generator, where m, k, and C respectively represent the mass, the spring constant, and the damping coefficient. In this velocity-damped resonant generator (VDRG) , the maximum power output Pmax with the maximum travelling distance of the mass Zlim can be written as: Pmax = 2-1 MYO23 Zlimim (1) *t
2. POLYMER ELECTRET
Electret is a dielectric material with quasi-permanent charge. M. Eguchi  first developed carnauba wax electret using a thermal polarization method. Charge in carnauba wax thus prepared is stable after 36 years . Since then, various applications of electrets such as acoustic/mechanical transducers and air filter have been proposed . For conventional applications of electrets, fluorinated polymer materials such as PTFE and FEP are often used. However, they are not compatible with the MEMS processes, since they are insoluble in solvents. In addition, the surface charge density should be improved for higher performance of the devices. Hsieh et al.  and Boland et al.  employed Teflong AF (Du Pont) as their electret film, which is perfluorinated amorphous polymer. Lo and Tai  found that fluorinated parylene (parylene HT®) provide very
When a specific application is chosen, the angular frequency w and the amplitude of external vibration yo are given. Therefore, the output power of VDRG is dependent only on its mass and the travelling distance; in principle, large/heavy generator is necessary for large output power. Figure 2 shows power spectra of vibration acceleration in a car cabin. Since high-frequency vibrations are damped out with the suspension system, vibration energy is concentrated below 100 Hz. Thus, the use of electromagnetic induction is problematic due to its low output voltage and power. On the other hand, electret-based electrostatic induction can produce high voltage even at low frequencies [4, 5].
More recently, MEMS generator using electrets are
prototyped [6-1 1]. Boland et al.  microfabricated rotational electret power generator, and obtained 25 itW with a rotor 8 mm in diameter. Tsutsumino et al. [9, 10] developed a seismic electret generator with 20x20 mm2 grid electrets/electrodes, and demonstrated 0.28 mW power generation at an oscillation frequency of 20 Hz. In this report, out recent efforts on MEMS electret generator are introduced.
10 100 Frequency [Hz] Figure 2. Power spectra of vibration in a car cabin.
high surface charge density. We recently reported that CYTOP CTL-M (Asahi Glass Co., Ltd.), MEMS-friendly amorphous perfluorinated polymer, can provide 3 times higher surface charge density than Teflon AF [9, 10]. Since theoretical power output of electret generators is proportional to the square of the surface charge density , electret generators with CYTOP CTL-M should have potential to produce nine times larger power than similarly built generators with Teflon AF. Figure 3 shows the molecular structure of CYTOP. There are three different types of commercial grade CYTOP CF2
Atype: -COOH M type: -CONH-C3H6-Si(OC2H5)3
Figure 3. The molecular structure and the end groups of CYTOP. 2.0
0 11.5 1E E
3. ELECTRET POWER GENERATOR
For electret generators, power output is increased with
decreasing the gap between the electret and the counter
electrode. However, since electrostatic attraction force in the vertical direction is also creased, the gap control is crucial to avoid pull-in. Tsurumi et al.  found that electrostatic repulsion force can be obtained between opposed patterned as shown in Fig. 6. Thus, in the present design, interdigitized electrets are formed both on the seismic mass the bottom substratearrangement to keep the constant. In the two-phase is employed to addition, gap reduce the horizontal force.
= (aminosilane 0%) oXCTL-A
Figure 4. Time trace of the surface charge density of aminosilane-doped CYTOP (aminosilane 0.6 wt%, and 3.0 wt%; film thickness:15 im) . CTL-M CTL-A CTL-A
depending upon their end group, i.e., trifluoromethyl (CTLS), carboxyl (CTL-A), and amidosilyl (CTL-M). Corona discharge is employed for the charging, and a series of measurements of surface potential was made. Sakane et al.  found that the functional end group, especially amidosilyl in CTL-M, significantly enhances the surface charge density of CYTOP. Thus, they doped aminosilane into the CTL-A to increase the concentration of amidosilyl functional group. Figure 4 shows the surface charge density of 15 rim-thick CYTOP CTL-A film for different concentration of aminosilane. By doping only 0.63.0% of aminosilane into CTL-A, the surface charge density has been doubled; surface charge density as high as 1.5 MC/M2 has been achieved, which is 3.5 times larger than that of Teflon AF. Note that the charges are stable and the surface voltage remains constant over 4,000 hours. Figure 5 shows the thermally-stimulated-discharge (TSD) spectra of aminosilane-doped CTL-A in comparison with that of CTL-A and CTL-M. The peak temperature of TSD spectra of aminosilane-doped CTL-A is shifted to much higher temperature of 184°C. Therefore, not only the surface charge density but also the thermal stability are improved by doping of aminosilane into CYTOP.
16.0 14.0 12.0
II I I~~~~~~~~~~~~~. 0 100 200 300 400 500 Gap [jm] Figure 6. Repulsive electrostatic force between two patterned electret plates. Surface voltages of upper and lower electrets are respectively -1170 V and -980 V .
150 200 Temperature (0C)
| overlap: 0%
Figure 5. TSD spectra of CTL-A, CTL-M, and aminosilane-doped CYTOP (aminosilane 3.0 wt% ; film thickness:15 iim) [171.
Fgure 8. Fabrication process of te electret generator.
Figure 10. Backside of the top Si structure, a)Overview,
generator designed in the present study. The top substrate consists ofen ranSi pr
Fabrcaton rocss f te eectrt gnertoris how in Fig.8. or heop ubstatewit a eisic mss,theproess * wit a 40 r-tic 4" Si wae wit 2 tic starts thermal oxide. Using an oxide mask, 350 rim-deep trenches DRIB (Fig. 8a). The are etched into the substrate with ' trenches are used as the parylene molds. Then, bottom Cr/Au/Cr electrodes are evaporated on the backside and patre,floe
(CTL-809M) films (Fig. 8b). Cr/Au/Cr electrodes on the top of CYTOP layer and a metal mask for CYTOP etching are
b)Patterned electret on the seismic Si mass, c) Parylene high-aspect-ratio spring, d)SEM image of the parylene
evap orated and pnatterned Fi z. g 8c . 15 in-thick nar lene-C is then deposited and etched back with 02 plasma. Then, the second parylene layer 15 Fim in thickness iS deposited to fully refill the trenches (Fig. 8d). The parylene films and the CYTOP films are etched with 02 plasma with the metal mask (Figs. 8ef). Finally, the Si substrate surrounding the mass iS etched away with XeF2 (Figs. 8gh). For the bottom Pyrex substrate, Cr/Au/Cr electrodes and CYTOP film are patterned (Fig. 8i). Then a PDMS spacer is bonded after 02 ~~~~~plasma treatment (Fig. 8j). After these processes, charges are implanted into CYTOP electret using corona charging for 3 minutes at 120 °C. The needle and grid voltages are respectively -8 kV and -600 V. Figures 9 and 10 shows photographs of the generator thus fabricated. The di;mension of the device fis 30 ~~~~~~prototype x 0m2(Fg9)whltesieotemass146x6 m2 Th sesi mss sspotdb h ayeesrns o hc it s2 Fg.IOd.Tewdho h electrode is 150 im (Fig. lOc). The patterned electret sraevlaeo lcrto h o isbtaei 50V
and that of on the bottom Pyrex substrate is -450 V. The gap between the electret and the electrode is 170 tm. The micro electret generator is fixed on the shaker, and the device is oscillated in the in-plane direction at its resonant frequency of 37 Hz. Each phase is connected to an external load of 100 MQ. The total output power is 0.28 tW, which is much smaller than the designed value of 12 tW . This is partially due to the fact that the alignment between the top and bottom electrodes is poor, so that the velocity of overlapping area is much smaller than the designed value. 5. CONCL1USIONS
Vibration-driven electret generator for energy harvesting applications has been developed. Parylene high-aspect-ratio spring is successfully microfabricated to support an in-plane seismic mass. Patterned CYTOP electret is formed on both the top and bottom substrates in order to keep the gap using electrostatic repulsive force. Resonant frequency as low as 37Hz has been achieved with a large in-plane amplitude of 1 mm. With our early prototype, we have obtained output power of 0.28 FtW with an external road of 100 MQ. We thank Messrs Y. Sakane, K. Kashiwagi, T. Tsutsumino and M. Edamoto for their extensive corporation during the course of this work. This work is supported by the New Energy and Industrial Technology Development Organization (NEDO) of Japan, and Ministry of Internal Affairs and Communications (MIC) of Japan.
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