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wee-Leong K, Neil W and Nick H, Fabrication and characterization of free-standing thick-film
piezoelectric cantilevers for energy harvesting, J. Meas. Sci. Technol. 20 -124010 ,13pp (2009)
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[3]
e-mail: [email protected]
[4]
Keywords: Energy harvesting, Organic micro-beam, Trapezoid, Comsol simulation
[5]
[6]
1"mm"
500"µm"
700"µm"
1"mm"
700"µm"
1"mm"
Anchor"pad"
150"µm"
Top"view"
10"µm"
Anchor'Pad' Tip2mass'
Anchor'pad' Tip2mass'
500"µm"
1"mm"
500"µm"
1"mm"
Ag+epoxy'
a)#
Tip+mass"
SU28'
Ag+epoxy'
Microcan-lever'
500'µm'
b)#
SU28'
Microcan-lever'
500'µm'
Substrate)
Cross"sec9on"
Figure 1. Dimensions of the designed micro-beams
Figure 2. Optical images of the fabricated microbeams
8E-6
0.8
0.7
0.6
0.5
7E-6
Experimental
FEM
Trapezoidal
6E-6
Rectangle
0.4
0.3
Strain
Displacement (µm)
With the miniaturization, the systems consume less energy allowing the elaboration of autonomous systems
where an energy harvesting device is directly integrated. In the case of vibration energy harvesting, energy
sources are characterized by a relatively low acceleration (< 1 g) [1] and a low vibration frequency
(< 1kHz) [2]. In accordance with the environmental sources of mechanical vibrations, the majority of
vibrational energy harvesters developed to date are based on a mechanical spring-mass system providing a
maximum power at the resonance frequency. The main techniques for the processing of organic MEMS are
molding or printing using piezoelectric conversion [3,4]. In this work, an original mechano-electrical
conversion based on electrostriction is selected, where a strain dependent permittivity induces changes in
the capacitance of the electrostrictive material [5]. In case of a vibrating micro-beam, the capacitance
variation will depend on the micro-beam’s strain under vibration [6]. In this context, flexible micro-beams
based on organic materials are of particular interest for this application, with a trapezoidal shape and a
seismic mass [6]. Indeed, finite element simulations using Comsol confirm that trapezoidal micro-beams
appear to be the most relevant because of their low resonance frequency and a uniform strain profile under
vibration. Based on these simulations, two types of beams with a tip-mass are fabricated and characterized:
a rectangular one used as reference, and the optimized trapezoidal one. Their dimensions are specified on
Fig. 1. An original method of fabrication combining photolithography and screen-printing has been
developed for the fabrication of these micro-structures. Concretely, the micro-beams are patterned by
photolithography. The photosensitive epoxy-based resist SU-8 3050 is selected because of its low Young's
modulus to achieve large strain under deflection (E = 3GPa). Then the tip-mass is screen-printed using an
epoxy paste loaded with Ag (ESL1901-SD). This paste presents a relatively high density (ρ ≈ 4700kg/m3)
allowing an important decrease of the resonance frequency as desired. A thick and rigid anchor pad is also
screen-printed using this Ag-epoxy based paste. Figure 2 shows examples of successfully fabricated
organic beams. The dynamic behavior of the fabricated micro-structures is characterized thanks to a laser
doppler vibrometer (Polytec MSA-500). From one hand, the measurements show clearly the influence of
the trapezoidal geometry of the structure with a tip-mass, with a low resonance frequency in the range of
environmental vibrations (Fig. 3), compared to the rectangular shape. On the other hand, the experimental
beam’s strain profiles are also obtained at resonance and compared to the simulated ones. We see in figure
4 that the strain is nearly constant along x for the trapezoidal structure, a clear benefit for an efficient
mechano-electrical transduction. A good correlation with finite element simulations is observed, in terms of
both frequency values and strain profiles. The calculations of the theoretical harvesting power, based on
previous results, show that the power generated by the trapezoidal structure is 2.2 times higher than the one
generated by the rectangular shape. This is due to the uniform strain along the surface in the optimized
trapezoidal structure. This geometry allows moreover a remarkable decrease in the resonance frequency,
making this design clearly suitable for the fabrication of efficient mechanical energy harvesters. Work is
now under progress to integrate the electrostrictive material and validate the resulting devices as a new
generation of vibration energy harvesters.
500"µm"
Université de Bordeaux, Laboratoire IMS, UMR 5218, Talence Cedex, 33405, France
50"µm"
[2]
H. Nesser , H. Debéda, I. Dufour, C. Ayéla
1"mm"
[1]
100"µm"
Design and fabrication of trapezoidal organic micro-beams for mechanical energy
harvesting from environmental sources
4E-6
2E-6
0.1
1E-6
250
300
350
400
450
Frequency (Hz)
500
Figure 3. Measured and simulated resonance
spectra of the rectangular and trapezoidal microbeams.
Rectangle
3E-6
0.2
0
Trapezoidal
5E-6
Experimental
FEM
0E+0
0
100
200
300 400 500
Profile x (µm)
600
700
Figure 4. Measured and simulated strain profiles
along x for the two designs.