Project title: Materials and Devices for Piezoelectric Energy Harvesters

Transcription

1 Project title: Materials and Devices for Piezoelectric Energy Harvesters Team Members: Project Leader: Susan Trolier-McKinstry Graduate Students: Charles Yeager and Hong Goo Yeo Undergraduate Student: Nico Mesyngier Collaborators: T. N. Jackson, J.Israel Ramirez, Chris Rahn, Xiaokun Ma, Tiancheng Xue and Shad Roundy Statement of Project Goals: The goal of this program is to fabricate and test a piezoelectric energy harvesting system for scavenging power from motion of the human body. 1,2 A piezoelectric energy harvester (PEH) utilizes mechanical motion as a source of energy. Increase in the harvested power level allows more sophisticated sensor systems, and/or enables the duty cycle for data transmission to be increased. The approach being taken is to combine optimized mechanical design to couple energy from the body into the harvesters structure with an efficient piezoelectric to convert from mechanical to electrical energy, and an efficient energy extraction system. Project s Role in Support of the Strategic Plan: Self-powered systems require some means of harvesting energy. ASSIST platforms will utilize a combination of solar (from commercially available sources), thermoelectric, and piezoelectric harvesters. This effort (coupled with those of the rest of the mechanical energy harvesting team) is intended to significantly increase the amount of energy that can be scavenged mechanically in order to power the ASSIST platforms. Discussion of Fundamental Research, Educational, or Technology Advancement Barriers and the Methodologies Used to Address Them: There are a number of key challenges associated with harvesting energy from the human body. The frequencies of most human motion are low frequency (<10 Hz) and small in amplitude (<1 g): Work this year has concentrated on fabrication of non-resonant harvesters that do not require resonant excitation. The structures are being fabricated using high efficiency piezoelectrics with superior energy harvesting figures of merit. While human motion is not strongly tonal, reducing the efficacy of resonant-based harvesting systems, future ASSIST platforms, as well as many associated technologies would benefit from more efficient low-frequency resonant harvesters. The approach being adopted here is to use a mechanical design in which the piezoelectric is strained uniformly, coupled with mechanical nonlinearities to increase the bandwidth of the mechanical system. Many mechanical energy harvesters utilize fragile thin films: This was addressed by switching from brittle passive elastic materials to robust metal foil substrates. This necessitated development of processing methods that enable high figure of merit strongly oriented piezoelectrics on substrates where epitaxy is not possible. The limiting figures of merit for piezoelectric energy harvesting were not known in the key PbZr1-xTixO3 system. This was approached by growing nano-domain state controlled films on a variety of substrates for several x values. 31

2 Foreign Collaborations: Graduate student Charles Yeager spent summer 2013 working at the Tokyo Institute of Technology in the group of Prof. Hiroshi Funakubo. ASSIST team members demonstrated that engineering of the nano-domain state of PbZrxTi1-xO3 (PZT) films enables a significant increase in the energy harvesting figure of merit, FoM = e31,f 2 /εr, where e31,f is the relevant piezoelectric coefficient for a piezoelectric film with top and bottom electrodes which is bent to produce a transverse strain, and εr is the relative permittivity. 3 The true composition dependence of this figure of merit was unknown for two primary reasons. First, previous data had unreported variations in the level of c- domain texture for the films grown. Secondly, calculations of the composition dependence from phenomenological values is complicated by the unknown elastic Figure 1: Energy harvesting figure of merit, FoM, of PZT films as a function of the volume fraction of c-domain orientation, f001. stiffness values for oriented films. To address this epitaxial {001} textured PZT thin films were grown by chemical vapor deposition (CVD) at the Tokyo Institute of Technology, Japan with systematic changes in [Zr] and thermal stress. Substrates with systematically varied thermal expansion coefficient, α, were used to control the domain texture via the thermal stress between the growth temperature and the Curie temperature. These films were then characterized using x-ray diffraction to determine the volume fraction of c-domain orientation, f001. Extensive electromechanical and dielectric characterization was also conducted. Figure 2: Process flow for PZT films on Ni foils 32

3 Figure 1 demonstrates that the FoM for piezoelectric energy harvesting is a much stronger function of the degree of c-domain orientation than the composition itself, 4 which is an unexpected finding based on the existing phenomenological calculations. 3 It is also noted that when c-domain texture was very high (>85%), e31,f > -10 C/m 2 were observed for films with [Zr] as low as This work shows that the optimal PZT composition for MEMS energy harvesting can be pushed toward more tetragonal composition as the c-domain texture increases, and suggests that new data need to be collected for the electrostrictive and elastic constants of oriented PZT films and ceramics. Furthermore, Fig. 1 definitively answers the question on the limiting values for the FoM of PZT films. Control of the domain state enables a factor of improvement relative to AlN. Figure 3: Microstructures of dense and nanoporous PZT films for energy harvesters Extensive work was conducted in year 3 on fabrication of mechanical energy harvesters using these optimized films using MgO and polymer passive elastic layers. Processing procedures enabling the integration were developed, but the resulting low frequency harvesters (~5 Hz) were found to be mechanically brittle. Consequently, for integration into ASSIST non-resonant harvesters, PZT films on Ni metal foils were developed. Fig. 2 shows the optimized process flow for preparation of oriented PZT films with high FoM. 5 Fig. 3 shows a comparison of PZT films with dense microstructures compared to another with engineered nano-scale porosity intended to explore whether the figure of merit could be increased further via porosity-induced reductions in the permittivity. 6 It was found that the dense films provided superior properties as a result of a combination of improved orientation control, reduced motion of ferroelastic domain walls, and higher elastic stiffness. 33

4 Reference Table 1: Benchmark of Mechanical Energy Harvesters Device Power FoM = Active Material, Acceleration Frequency Powerrms area Density ( Power mode [cm 2 [G] [Hz] [μw] ] [μw/cm 2 *g 2 ] density/fr Aktakka Bulk-PZT5A d Andosca AlN film d Morimoto (001)PZT on steel, d31 Hayakawa Watch Dynamo n/a n/a Ringgard (Meggitt) PZT thick film, d Kamel AlN film, d Funakubo KNN on Ni, d Defay 5 µm AlN, d Durou Bulk PZT-5H, d PSU cant. PZT on Ni d PSU (compliant) 1.5 µm PZT bimorph on Ni d Figure 4: (Left) Nonresonant energy harvester schematic (right) PZT bimorph on shaped Ni foil with flexible connect cables for ASSIST energy harvester Bimorph samples were grown with PZT on both sides of the Ni foil. To benchmark this work, the bimorphs were incorporated into resonant energy harvesters using the ASSIST-developed piezoelectric compliant mechanism (Rahn group). Table 1 demonstrates that ASSIST harvesters have the highest efficiency when normalized to area, acceleration level, and resonant frequency. It was found that there was a significant enhancement in response associated with device nonlinearity, leading to unusually power large power levels at very low accelerations. We anticipate being able to further increase by another factor of two for thicker piezoelectric layers. This approach was then used to prepare prototype devices for nonresonant energy harvesting. Close collaboration between the groups of Roundy, Jackson, Rahn, and Trolier- McKinstry produced the device described in the report of Roundy. A process for laser cutting the 34

5 nickel foil was developed to enable the shaped piezoelectric section. Figure 4 shows the piezoelectric component of that device with a flexible connector developed by the Jackson group. Work is ongoing now to complete assembly of the magnets head measure the response of the nonresident system. This is expected to be a key component of the self-powered ASSIST testbed. Summary of Other Relevant Work Being Conducted Within and Outside of the ERC and How This Project is Different: All of the ASSIST piezoelectric harvesting efforts in Thrust 1 are very closely connected. In addition, we are now pursuing a small demonstrator that couples the mechanical harvester to an ASSIST supercapacitor to demonstrate storage. Work has also been conducted with Thrust 2 for mechanical harvesting for the non-volatile processor, Thrust 5 for the energy harvesting subsystem, and the testbeds. Table 1 gives a comparison of this work to previous reports of piezo-mems (as well as some macroscale piezoelectric) energy harvesters. The use of high FoM merit materials with a good mechanical design yields the best-reported performance to date. In terms of other work on mechanical energy harvesting we have initiated a collaboration with the group of Dr. Shashank Priya and the NSF I/UCRC Center for Energy Harvesting Materials and Systems, at Virginia Tech. We plan a proposal for joint I/UCRC ERC funding. Plans for the Next Year: There are several key targets for this next year. 1) We will measure the generated power from the non-resonant harvester. 2) We will redesign to optimize the mechanics, based on the initial data, and fabricate additional harvesters. 3) We will collect data on energy harvesting of body-worn non-resonant harvesters for different activities, 4) We plan to examine use of piezoelectric energy harvesting for shoe or joint mounted devices, as well as in strain based harvesters such as piezoelectric compression shirts. Simple calculations suggest that it should be possible to harvest 1 mw from breathing. Expected milestones and deliverables for the project: Subthrust Yrs. 1-2 Gen 1 Yrs. 3-5 Gen 2 Yrs. 6-7 Gen 3 Yrs Gen 4 Piezoelectric 2 µw/cm 2 g original target; 30 µw/cm 2 g achieved in ~50 Hz resonant device 10 µw/cm 3 normal walking (nonresonant) Initiation of strain-based devices 40 µw/cm 3 normal walking (non-resonant) Lead Free Piezoelectric >100 µw on joint based devices 25 µw/cm 3 average power output during daytime activities breath harvesting 35

6 Member company benefits: Several member companies have expressed interest in the technology, for either on-body or offthe-body applications. Nike has committed money for a separate program in shoe-based mechanical energy harvesting based at Penn State (with the groups of Jackson and Trolier- McKinstry). We anticipate a start date during spring There have also been communicating with Tyco on mechanical energy harvesting for sensing in ductwork. If relevant, commercialization impacts or course implementation information: N/A References: 1. Anton, S.and H. A. Sodano A review of power harvesting using piezoelectric materials, Smart Materials and Structures, 16, R1. 2. Cook-Chennault, K. A., N. Thambi, and A. M. Sastry Power MEMS portable devices a review of non-regenerative and regenerative power supply systems with special emphasis on piezoelectric energy harvesting systems, Smart Material Structures, 17, Yeager C. B. and S. Trolier-McKinstry Epitaxial Pb(Zrx,Ti1-x)O3 (0.30 < x < 0.63) Films on (100) MgO Substrates for Energy Harvesting Applications, J. Appl. Phys. 112, Yeager, C. B., Y. Ehara, N. Oshima, H. Funakubo and S. Trolier-McKinstry Dependence of e31,f on Polar Axis Texture for Tetragonal Pb(Zrx,Ti1-x)O3 Thin Films, J. Appl. Phys (2014). 5. Yeo, H. G. and S. Trolier-McKinstry {001} Oriented Piezoelectric Films Prepared by Chemical Solution Deposition on Ni Foils, J. Appl. Phys.116 (1) R.L. Johnson-Wilke, R.H.T. Wilke, M. Wallace, A. Rajashekhar, G. Esteves, Z. Merritt, J. L. Jones, and S. Trolier-McKinstry Ferroelectric/Ferroelastic Domain Wall Motion in Dense and Porous Tetragonal Lead Zirconate Titanate Films, IEEE Trans. Ultrason. Ferroelec. Freq. Control 62 (1)