Nano-cluster-enhanced High-performance Perfluoro-polymer Electrets
for Micro Power Generation
Kimiaki Kashiwagi1, Kuniko Okano1, Yoshitomi Morizawa1, and Yuji Suzuki2
1
Asahi Glass Co., Ltd. Research Center,
2
Dept. of Mechanical Engineering, The University of Tokyo
Abstract: Development of high-performance electret materials is required to achieve high-power vibration-driven
energy harvesters. We previously found that perfluorinated polymer CYTOPTM shows high surface charge density,
which is larger than that of any other fluorine containing materials such as Teflon® AF. In this study, we have
found that formation of nano-clusters in the perfluorinated polymer films is effective to improve the properties of
electrets. By adding aminosilane derivatives into CYTOP, surface charge density and thermal stability of trapped
charges are significantly enhanced. Using small-angle X-ray scattering (SAXS) analysis and tapping mode AFM
analysis, we have also revealed existence of nano clusters in the CYTOP film, which should work as charge traps
for excellent electrets properties. Furthermore, we have demonstrated a record-high surface charge density of 2
mC/m2 as polymer electrets as well as higher thermal stability of charges.
Keywords: electret, energy harvesting, nano cluster, surface charge density, fluorinated polymer, CYTOPTM
INTRODUCTION
In recent years, energy harvesting from
environmental vibration attracts much attention from
the perspective of its application to low-power
electronic devices such as sensor network nodes [1, 2].
Since the frequency range of vibration existing in the
environment is below 100Hz, electrostatic induction
power generators using electrets [3-7] should have
higher performance than electromagnetic ones.
In the electrostatic induction power generation
using electrets, theoretical power output is proportional
to the square of the surface charge density of the
electrets. We previously found that perfluorinated
polymer CYTOPTM shows higher surface charge
density than that of any other fluorine containing
materials such as Teflon® AF [6, 7].
It is well known that introduction of elongated
voids and/or additives as a charge trap is effective to
improve the charge storage and the charge stability for
electrets with hydrocarbon polymers, for example,
polypropylene [8, 9]. We have already found that the
introduction of aminosilane into the CYTOP
significantly enhances the surface charge density [10].
This study proposes a new strategy to engineer
nano-size domains (nano-clusters) embedded in
perfluorinated polymer films for high surface charge
density and high thermal stability. We also investigate
the mechanisms of the improved electrets properties
and the fundamental characteristics of the nano-cluster.
high-aspect-ratio soft springs, which enable large
oscillation amplitude and low resonant frequency. [12]
This type of electrostatic MEMS energy harvesters
using electrets is expected to realize compactness
(small size, light weight and thin) as well as excellent
durability.
EXPERIMENTAL
Manufacturing of polymer solution
composition and thin films for electrets
To prepare CYTOP solution with additives,
CYTOP having carboxyl end group (CTL-A in Fig. 2)
was dissolved in perfluorinated solvent such as
CT-Solv180 (Asahi Glass Co., Ltd.). A small amount
of aminosilane such as 3-aminopropyl (triethoxy)silane,
3-aminopropyl(diethoxy)methylsilane,
N-(2-aminoethyl)-3-aminopropyl(triethoxy)silane, etc.
was then added to the CYTOP solution resulting to
make the polymer solution composition for electrets.
Above mentioned polymer solution composition
was spin-coated on copper substrates or silicon wafers
to make 15 �m-thick thin films for evaluation as
electrets and AFM analysis. The same composition
was casting on the PTFE film, drying and peeling off
to make about 100�m-thick films for SAXS analysis.
ELECTRET POWER GENERATOR
Figure 1 shows our prototype of MEMS electret
generator for energy harvesting applications [11, 12].
Induced charge on the counter electrode is changed
due to the variation in the overlapping area during
seismic operation, resulting in electric current in the
external circuit. Seismic mass is supported by parylene
Figure 1 Schematic of micro seismic electret power
generator [11, 12].
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CYTOPTM(CTL-A) +Aminosilane
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Evaluation of electrets properties
In order to evaluate performance of CYTOP as an
electret material, temporal change of the surface
charge density was examined. The above mentioned
electret samples were charged by a corona charging
technique for 3 minutes as shown in Fig. 3(a). The
needle voltage is kept constant at -8 kV, while the grid
voltage is changed between -600 and -2500V. The
samples were kept at 120 �C during charging, which is
slightly higher than the glass transition temperature
(Tg=108�C) of CYTOP. The samples were stored at 20
�C and 60 % humidity. Surface potential was measured
with a surface voltmeter (Model 279, Monroe
Electronics).
Open-circuit thermally-stimulated-discharge (TSD)
measurements [13] were also performed to examine
the thermal stability of charged electrets. Figure 3(b)
shows the set-up for the TSD measurement. The
electret sample was placed against a facing probe, and
heated up at the rate of 1 �C/min. When the
temperature increased, the trapped charges were
released due to the thermal energy. The discharged
current was measured with an electrometer (Model
6517A, Keithley Instruments) set into the circuit. In
the present study, the TSD peak temperature where the
current has a peak was used as the charge stability
index and compared to different materials.
Size of nonuniformities�
� � about 20nm �D=2�/q�
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CYTOPTM(CTL-A, no additive)
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Figure 4 The small-angle X-ray scattering (SAXS)
analysis of aminosilane-dopoed CYTOP, compared
with pure CYTOP.
aminosilane doped
CYTOPTM
(a)
100nm
4. RESULT AND DISCUSSION
Pure CYTOPTM
Direct detection of nano-clusters using SAXS and
AFM measurements
X
CF2
CF
F2C
CF
CF2
X
O
C
F2
X=CO2H
n
Figure 2 Molecular structure and the end groups of
CYTOP (CTL-A).
Needle voltage
(-8 kV)
Grid voltage
(controllable)
electret sample
A
(a)
i
probe
A
electret
electrode
(b)
Heating at a rate of 1 �/min
Figure 3 Experimental setup of the corona charging
method (a) and thermally-stimulated-discharge (TSD)
measurement (b).
(b)
500nm
Figure 5 The surface phase image of aminosilane
doped CYTOP (a) and pure CYTOP (b) analyzed by
tapping-mode AFM.
Small-angle X-ray scattering (SAXS) and AFM
(tapping mode) measurements were applied to the
analysis of the CYTOP films, which could be effective
for the detection of nano-structure in the polymer film
without any destroy of the components. Figure 4
shows the result of SAXS for a 100�m-thick CYTOP
film containing aminosilane, compared with pure
CYTOP film (CTL-A, no additive). The SAXS spectra
clearly show the nonuniformity formation of about 20
nm into the CYTOP film.
Tapping-mode AFM analysis was also applied to
15�m-thick CYTOP films on silicon substrates. In the
tapping-mode AFM analysis, the surface of samples
are scanning by tapping cantilever, and the phase
images are detected by the difference of viscoelasticity
or strength of adsorption. The surface phase images by
the tapping-mode AFM in Fig. 5 show that there are
Performances of nano-cluster-enhanced polymer
electrets
The CYTOP films including aminosilane-based
nano-clusters on copper substrates were evaluated by
the above-mentioned corona charging method and the
TSD measurement. As a comparison, Teflon® AF (Du
Pont), PTFE (poly-tetrafluoroethylene), and two types
of pure CYTOP with CO2H or CF3 end group were
examined. Figure 6 shows the results of the surface
charge density which was measured 200 hours after
corona charging versus the temperature of TSD spectra
peak. Among the materials without additive, CYTOP
with the CO2H end group has the highest surface
charge density, and the Teflon AF has the best thermal
stability of electrets. Introducing aminosilane-based
nano-clusters into CYTOP, the highest surface charge
densities more than 1.5 mC/m2 have been achieved. In
addition, the thermal stability is also enhanced. TSD
spectra of two types of pure CYTOP and aminosilanedoped CYTOP electret films are shown in Fig.7. It is
clear that the TSD peak is shifted to higher
temperature by introducing of aminosilane.
Figure 8 shows the charge trapping model in the
CYTOP film, where the nano-clusters work as the
charge trapping sites by their polarization, and their
thermal motion is restricted by the connection with
polymer end groups.
The surface voltages were controllable by grid
voltage of the corona charging. A punching metal
electrode with thickness of 1 mm and about 70%
aperture rate was used as the grid electrode to generate
a uniform electric field during the corona charging.
Temperature�of�TSD�peak�top�(� )
200
Teflon AF
160
CYTOP
(CO2H end group)
150
140
130
CYTOP
(CF3 end group)
120
0.0�
PTFE
�0.5�
�1.0�
�1.5�
Surface�charge�density�after�200�hours(mC/m2)
�����������
CYTOP
(CF3 end
group)
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Figure 7 Thermally-stimulated-discharge
spectra of CYTOP electret films.
Corona electric field
(negative polarity)
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� �
�
�
�
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� � �
� �
�
�
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(TSD)
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Figure 8 Charging trapping mechanism in
aminosilane-based nano-cluster-enhanced CYTOP.
Nano-clusters are polarized by the negative corona
electric field.
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Film thickness: 15�m
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Figure 9 Surface charge density of nano-clusterenhanced CYTOP versus the grid voltage.
180
170
CYTOP
(CO2H end
group)
CYTOP (CO2H end group)
+ Aminosilane
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CYTOP (CO2H end group)
+ Aminosilane (3 types)
190
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nanometer-size domains in the amonosilane-doped
CYTOP film, but no domains are detected in the pure
CYTOP film (CTL-A). The sizes of the domains were
nearly the same as the nonuniformities obtained with
SAXS. From these results, the formation of the
nano-sized domains (nano-clusters) of aminosilane
into the perfluorinated polymer matrixes has been
revealed.
�2.0�
Figure 6 The properties of electrets using CYTOP and
other materials.
As shown in Fig. 9, using above-mentioned
nano-cluster-enhanced CYTOP electrets, higher
surface charge density can be obtained by increasing
the grid voltage. Especially, when the grid voltage up
to -2.0 kV was imposed, the record-high surface
charge density up to 2 mC/m2 has been obtained.
Figure 10 shows long-term stability of the
above-mentioned electrets. Surface charge density of
the nano-cluster-enhanced CYTOP is unchanged over
2,500 hours. Furthermore, as shown in Fig. 11, it has
high thermal stability at 100�. On the other hand,
surface voltages of other materials (PTFE and Teflon
AF) are deteriorated even at 20�.
REFERENCES
[1]
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�����
Optimized CYTOP
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�nano-cluster enhanced�
�����
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Pure CYTOP (CTL-A)
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�����
Teflon AF
����
��
�����
�����
�����
�����������
Figure 10 The time dependence of the surface charge
density of nano-cluster-enhanced CYTOP. Very high
surface charge density has been demonstrated.
����������������������������������
����
����
����
����
CYTOP nano-cluster enhanced(20� )
CYTOP nano-cluster enhanced(100� )
PTFE (20� )
TeflonAF(20� )
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����
����
��
���
����
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Figure 11 Comparison of the thermal stability of
electrets. The normalized surface potential of
nano-cluster-enhanced CYTOP under two different
temperature conditions at 20� and 100�, and other
materials at 20� are compared.
CONCLUSION
In conclusion, we have proposed a new strategy
for high-performance perfluorinated polymer electrets
using aminosilane-based nano-clusters. By adding
animosilane into CYTOP electrets, nano-clusters of
about 20 nm are formed in the film. It is clearly shown
that the nano-clusters are critically important to obtain
higher surface charge density and better thermal
stability of electrets, thus higher power output and
reliability of electrostatic power generation for energy
harvesting.
ACKNOWLEDGEMENT
This work is partially supported through the New
Energy and Industrial Technology Development
Organization (NEDO) of JAPAN.
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