MECHANICAL ENERGY HARVESTERS UTILIZING {001} TEXTURED PZT FILMS ON FLEXIBLE METAL FOILS

Open Access
Author:
Yeo, Hong Goo
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
May 03, 2017
Committee Members:
  • Susan Trolier-McKinstry, Dissertation Advisor
  • Susan Trolier-McKinstry, Committee Chair
  • Clive A Randall, Committee Member
  • Roman Engel-Herbert, Committee Member
  • Christopher Rahn, Outside Member
Keywords:
  • Piezoelectric material
  • PZT film
  • Piezoelectric energy harvesting
  • Flexible metal substrates
Abstract:
This thesis describes the optimization and utilization of piezoelectric Pb(ZrxTi1-x)O3 PZT thin films on flexible metal foils for various types of mechanical piezoelectric energy harvesters. Flexible metal foil substrates with high fracture strength are useful in some microelectromechanical systems (MEMS) applications, including wearable piezoelectric sensors or energy harvesters based on Pb(Zr,Ti)O3 (PZT) thin films. Full utilization of the potential of PZT film on metal foils requires control of the film crystallographic texture to achieve a high figure of merit (FoM) for energy harvesting application. A high level of {001} film orientation enables an increase in the energy harvesting FoM due to the coupling of strong piezoelectricity and low dielectric permittivity. In this study, {001} oriented PZT thin films were grown by chemical solution deposition (CSD) and rf-magnetron sputtering on Ni foil. To overcome the issue with thermodynamic incompatability between the metal substrate and the oxide film during subsequent thermal treatment, pretreated Ni foils were passivated using HfO2 grown by atomic layer deposition (ALD). Highly {001} oriented PZT films were successfully deposited on Ni foils with (100) oriented LaNiO3 seed layers on HfO2 buffer layers using either chemical solution deposition or rf sputtering. The (001) oriented PZT thin films achieve an e31,f piezoelectric coefficient of -10 ~ -12 C/m2 (which is comparable with (100) textured PZT films on silicon substrate) coupled with a low dielectric constant (330 ~ 530 at 1kHz) after hot poling. Piezoelectric energy harvesters with cantilever type structures consisting of PZT films on Ni foil revealed a strong correlation between the FoM and the harvester output. Both increasing the volume of the piezoelectric element (e.g. the film thickness in the 31 mode) as well as inducing a high FoM via hot poling improve the performance of PZT cantilevers. As a result, it is possible to achieve a power density of 1036 W/cm2G2 with 3 m thick sputtered PZT film on Ni below 100 Hz. Extracting energy from low vibration frequencies (<10Hz) is essential for wearable harvesters. Two main types of mechanical energy harvesting devices (e.g. resonant and non-resonant harvesters) were investigated. The Piezoelectric Compliant Mechanism (PCM) design proposed by Ma and Rahn was targeted for high efficiency operation at 5 Hz by fostering a uniform strain for its 1st mode shape. In particular, a PCM energy harvester utilizing {001} textured bimorph PZT films on Ni foil provided a large power level of 3.9 mW/cm2·G2 and 65% mode shape efficiencies at ~ 6 Hz. Frequency-up conversion for non-resonant piezoelectric energy harvesters is an excellent strategy to extract electrical energy from human motion for self-powering portable electronics. Three different designs related to magnetic plucking suggested by Xue and Roundy were explored using bimorph PZT films on flexible nickel foils for wrist-worn harvesters (for which the desired device size is less than 16 cm2). Thick PZT films on Ni foil deposited by high temperature sputtering enable the production of multiple piezoelectric beams with complex designs (e.g., star shaped cantilever beams). Based on analysis of the plucked beam dynamics (simulated by Xue, and Roundy), PZT beams and permanent magnet configuration were selected and demonstrated. The resulting devices successfully convert low frequency vibration sources (i.e. from walking. rotating the wrist, and jogging) to higher frequency vibrations of the PZT beams (100 ~ 200 Hz). Additionally, the devices are able to generate 40~50 W power during mild activities. In summary, strongly {001} oriented bimorph PZT films on flexible metal foils are promising for implementation of high efficiency harvesters with a variety of shapes and dimensions from mm2 to cm2.