A High Performance Piezoelectric Micro Energy Harvester Based on Stainless Steel Substrates

Abstract

Abstract. This paper presents a high performance piezoelectric micro energy harvester (PMEH) fabricated on stainless-steel substrate with metal MEMS process. The PMEHs fabricated in this study are with simple unimorph or bimorph cantilever structure with one or two layers of high quality lead zirconate titanate (PZT) piezoelectric films deposited by aerosol deposition method (ADM) and a glued tungsten proof mass (4 mm* 6 mm* 1 mm). The thickness of the PZT active layer are 18μm for unimorph PMEH and 10μm thick on both sides for bimorph PMEH. The length and width of the cantilever structure is 9mm and 6mm. With all the experimental process optimized, the results show that unimorph PMEH has a maximum output power of 122μW tested with optimal load under 0.5 g acceleration vibration level in resonant frequency around 120Hz. The corresponding value of bimorph PMEH is 304μW. The normalized power density (NPD) for unimorph PMEH and bimorph PMEH are 18.9mW·cm -3 ·g -2 and 46.81mW·m -3 ·g -2 respectively, which outperformed all previous published PMEHs based on either silicon or stainless steel substrates. IntroductionThe output performance of piezoelectric micro energy harvesters (PMEH) with area smaller than 1 cm 2 has great improvement over the past decade. The performance of the piezoelectric materials, the device structure design, the fabrication process, and the substrate materials are all crucial factors to optimized the output perfomance of PMEHs. Early studies of PMEHs are mostly based on silicon substrate with conventional MEMS processes [1][2][3][4], the chip area of these PMEHs are usually small and with higher resonant frequencies in around kHz range. Although the normalized power density (NPD) of silicon based PMEHs can be high, the devices are difficult to find real field applications because the mechanical vibration frequencies are usually in low frequencies. Furthermore, silicon and ceramic piezoelectric material are both brittle material which tends to break easily under strong vibration levels. There are growing studies of PMEHS based on metal substrates, especially stainless steel [5][6][7][8]. Metal substrate materials like stainless steel are ductile materials show not only much better mechanical strength over silicon but can be pre-stressed to avoid tensile cracks of ceramic piezoelectric materials. The metal MEMS processes of cantilever stucture are also simpler and cheaper because the metal substrates are usually chosen as the final beam thickness and buck micromachining backside etching for silicon is not required. The output power of metal substrate based PMEHs therefore showed better performance in lower frequency but the NPD are in general smaller than silicon based devices with high performance thin piezoelectric layers. In this study, high performance unimorph and bimorph PMEHs with around 10 μm and 20 μm high quality lead zirconate titanate (PZT) deposited by aero deposition method (ADM) on 60 μm stainless steel substrates are fabricated and tested. The device is design to have natural fre...

Key features in the development of unimorph stainless steel cantilever with screen‐printed PZT dedicated to energy harvesting applications

“…Although high normalized power densities are obtained, their brittleness makes them unfeasible to exploit ubiquitous ambient vibrations. Thus, metal‐based substrates have received significant interest, especially stainless steel substrates over silicon ones, due to better mechanical strength, a lower Young's Modulus and the possibility to be prestressed in order to avoid tensile cracks of ceramic piezoelectric materials 3,14–21 …”

“…However, energy harvesting performance depends on a large extent on the microstructure, the properties of the active piezoelectric material and the quality of the interfaces, 16,18,24,25 parameters inherently dependent on the fabrication process of the device. In particular, correct densification step of the PZT layer is required to limit the detrimental effect of porosity (leakage current) and to ensure high piezoelectric properties.…”

One step densification of printed multilayers by SPS: Towards new piezoelectric energy harvester MEMS

“…Here, it is worth noting the comparison is complex because of many parameters influencing the total harvester powers. The normalized power in W.m -3 .g-2(Tang et al 2018) is often proposed for a fair comparison of the intrinsic properties of the EH.…”