1. Introduction
In recent years, vibration energy harvesters have drawn more attention in the world. It can be used in many fields ranging from implanted devices and wearable electronic devices to mobile electronics and self-powered wireless network nodes[1-3]. There are three ways to convert vibration energy into electrical energy, namely electromagnetic, piezoelectric and electrostatic[4, 5]. Among these three harvesters, the electromagnetic harvester cannot produce a larger current but relative low voltage, so the design of management power circuit is more difficult. Because of the need for an external voltage source, the electrostatic harvester is impossible to be used in practical applications. Therefore most of the researchers have been working on the piezoelectric harvester, which has a high output voltage and a high efficiency of electromechanical energy conversion of piezoelectric.
As the key part of the piezoelectric vibration harvester, the piezoelectric materials mainly include lead zirconate titanate (PZT) piezoceramic, polyvinylidene fluoride (PVDF) polymer, polycrystalline zinc oxide (ZnO), aluminum nitride (AlN) and so on. AlN has proven to be the superior piezoelectric ceramic when it comes to energy harvester devices owing to its high energy density, moderate voltage levels, low dielectric constant and especially its compatibility with CMOS processes[6, 7]; furthermore it can be deposited easily at low temperatures and has already been widely used in the MEMS, microelectronic and microsensor industries[4].
A number of works have been devoted to fabricating the single beam harvester based on the AlN film[4-6, 8-15], but the harvester array based on the AlN film was rarely reported, the max output was about 1
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2. Energy harvester cantilever design and fabrication
Based on the piezoelectric effect, the vibration energy harvester array converts the environmental vibration energy in the surrounding to electric energy. The vibration energy harvester array consists of five cantilevers ended with one attached mass. The three advantages of the design are that the grooves between cantilevers allow the air to flow smoothly and reduce the air damping ratio, the cantilever arrays connected in series make the currents increase greatly, one attached mass should guarantee the consistency of the five cantilevers resonant frequency. Equation (1) in Ref. [16] shows that the air damping ratio decreases with decreasing the width of cantilever when keeping other parameters constant. The currents increasing can be explained by Kirchhoff's current law in Ref. [17]. The one attached mass ensures that the 5 cantilevers can vibrate in phase and the piezoelectric elements can be connected directly in parallel or series. The structure in this paper is different from the single cantilever structure reported in reference[4, 5, 10-13, 18, 19], but alike to the structure in Ref. [3]. The schematic structure is shown in Fig. 1. The fabricating process of the vibration piezoelectric energy harvester arrays is shown in Fig. 2.
ξsq=νb22ρag30hω, |
(1) |
where
N-type 100 mm diameter, double-sided-polished, (100) CZ silicon (Si) of resistivity 2-4
3. Results and discussion
The power out of piezoelectric vibration piezoelectric energy harvester arrays is mainly dependent on the properties of AlN and the structure of the harvester array. So the X-ray diffraction (XRD) is utilized to characterize the AlN film, as shown in Fig. 4. The X-ray rocking curve measurement indicates the film has the crystal orientation (002) and the intensity reaches 1.1
The vibration piezoelectric energy harvester can be equivalent to the two-port network as shown in Fig. 5. While
P0=V20Zl,thatisp0=V2sZl(Zs+Zl)2. |
(2) |
From Eq. (1), an conclusion can be drawn that the max power dissipated on the load can be attained when and only when
The power out and open circuit voltage of the harvester arrays were detected when a series of different load impedances were applied, as shown in Fig. 6. The varying of the resonant frequency and the output voltage with the applied acceleration were obtained as shown in Fig. 7. The equivalent impedance of 80 k
4. Conclusions
In this work the fabrication of piezoelectric vibration energy harvester arrays based on AlN thin film deposited by pulsed-DC magnetron sputtering is presented. The maximum power output is about 30.4