Piezoelectric Pressure Energy Harvesters Using Circular Diaphragms with Concentric Ring-boss Structures

This paper describes the design, fabrication and testing results of aluminum nitride piezoelectric micromachined circular diaphragm energy harvesters with concentric ring-boss structures to convert pulsed pressure to electrical power with high efficiency. Compared to the state-of-the-art, the concentric-ring-boss harvesters (CRBH) produce multiple strained regions for: (1) more effective electrode areas; (2) high charge extraction rates; and (3) high power outputs. Finite element modeling is used to evaluate the effectiveness of the proposed design by comparing stress and stretching energy of various designs. The conventional single-boss and concentric ring-boss harvesters have been fabricated and tested under a mean pressure of 1.5 kPa with 1 kHz frequency. Experimental results show the ring-boss and the conventional single-boss harvesters can generate power of 1.3 and 0.82 μW, respectively. As such, the CRBH is 1.6X higher in energy conversion efficiency and power density as compared to single-boss design of similar dimensions. INTRODUCTION Battery replacements have become a practical problem for long-term sustainable implantable systems such as pacemakers, deep brain stimulators, and telemetry systems for epilepsy and chronic pain treatments. In general, patients have to undergo a surgical procedure for battery replacement in every two or three years (depending on personal usage), which is not only a medical risk but also an economic burden. Therefore, it could be desirable to develop an implantable power source which can generate electrical power within the body continuously. Recent advances in device designs of microelectronics and low power circuit have reduced the power requirements for the implantable devices considerably, making energy harvesting within human body even more attracting as a viable solution. Recent efforts to harvest energy within human body include a vibration based energy harvester utilizing eardrum vibrations [1]; a glucose fuel cell to convert glucose content of cerebrospinal fluid to electricity [2]; and a flexible ZnO-based piezoelectric sheet to convert heart and lung deflections to electricity [3]. We aim to make use of pressure fluctuations of body fluids as a power source; thus, in this study we propose a novel circular diaphragm energy harvester design to enhance the power output. Micro-scale pressure energy harvesters generally utilize a circular diaphragm to convert incoming pressure waves to mechanical energy, and the stored mechanical energy is then converted to electrical energy using the piezoelectric effect. Various researchers have proposed pressure harvesters for different applications. Lai et al. proposed a SiC/AlN harvester having electrode covering majority of the surface for high temperature environments and operation frequencies of 1-2 kHz [4]. This harvester has a single-boss structure to increase the stress on the diaphragm and decrease the system natural frequency. Horowitz et al. have proposed a harvester design utilizing the Helmholtzresonance phenomena to convert acoustic waves between 1-4 kHz, and a power density of 0.34 μW/cm2 u under 149 dB has been reported [5]. Kimura et al. developed a circular diaphragm harvester for audible sound and demonstrated improved power density when operating in the third resonance mode [6]. Figure 1: a) The proposed harvester with a concentric ring-boss structure (a single ring in this example); b) the conventional circular single-boss harvester; c) dimensions of the structure shown on axisymmetric cross-sectional view; d) simulation results of stress on the top surface versus radius plot for various ro and w combinations under an applied pressure of 1 kPa. CONCENTRIC RING-BOSSED HARVESTER (CRBH) In this study, our aim is to develop a harvester which is capable of converting pressure fluctuations in fluid medium with increased power density and higher efficiency. The majority of the designs for pressure energy harvesting utilize either a single-boss structure or multiple electrodes. The boss structure can reduce the natural frequency of the whole system and increase the stress around the boss and membrane interface regions. However, the area on top of the boss itself becomes a zero-strain region without the possibility to generate extra charges. On the other hand, the placement of multiple electrodes around the membrane can cover larger stressed regions to increase harvested energy. However, the electrodes need to be placed at regions that have large strain under the applied pressure to be effective. However, as the high strain areas generated under the fundamental vibration mode are fully covered by electrodes, the second or higher vibration modes will be required for larger areas with high strain regions and the structural frequency will increase considerably. The concentric ring-boss design together with multiple electrodes can increase the high stress regions on diaphragm while operating under the fundamental vibration mode. Figure 1 illustrates the design of CRBH with a circular diaphragm and a concentric ring and boss underneath. Table 1: Structural material properties used for simulations [10]. The concentric ring-boss structures provide several high stress regions on top of the diaphragm in operation under the first mode. For example, the single ring structure as shown in Figure 1a produces two high stress regions as compared to the single high stress region from the single-boss structure. Depending on the application and operating frequency, it is also possible to increase the number of the ring structures to increase the number of strained regions on the diaphragm. Figure 1b shows critical dimensions of the CRBH, such as overall radius of the diaphragm, R1, outer radius of the ring, R2; width between the ring and boss, w; and the distance from the center of the diaphragm to the midpoint between the edge of the boss and the inner radius of the ring, ro. In order to show the effect of the proposed design, Comsol finite element program was used to simulate stress generated on the harvester diaphragm upon an applied pressure. Figure 1d shows the stress generated on the CRBH for different combinations of ro and w. Compared to the conventional design shown in Figure 1c, the proposed design has larger stress concentration regions while preserving the amplitude of the maximum stress. By utilizing a piezoelectric layer as a diaphragm, the enhanced stress regions can provide additional charge extractions for higher power outputs. In order to show the effect of this design, we investigated mechanical energy of the diaphragm upon applied pressure [7, 8]. The work done on the diaphragm when a pressure is applied consists of stretching (due to extension of the middle plane of the diaphragm) and bending (due to transverse deflection) components. Because stretching energy, Vs, is orders of magnitude higher than the bending energy, Vb; we focus on Vs generated on the diaphragm which is calculated as: