wileyonlinelibrary.comdensity and reduced resonance frequency have been explored, including advanced mechanical design, [ 5 ] maximizing intrinsic piezoelectric materials properties, [ 6,7 ] and using a fl exible passive layer. [ 8,9 ] In the last decade, microelectromechanical (MEMS) energy harvesters based on piezoelectric materials such as AlN and Pb(Zr,Ti)O 3 (PZT) have been studied for use in harvesting energy from small amplitude, mechanical vibration. [10][11][12] Improving the piezoelectric material for a piezoelectric energy harvester (PEH) can be pursued via an energy harvesting fi gure of merit (FoM). The maximum extractable electric power generated at the resonance frequency of a e 31,f mode cantilever structure is [ 13,14 ] where m is mass, ω is angular frequency, Y is Young's modulus, ε 0 is vacuum permittivity, and a is the acceleration. The in-plane piezoelectric coeffi cient ( e 31,f ) and the relative permittivity ( ε r ) control the energy harvesting FoM = ( e 31,f ) 2 / ε r when the mechanical energy is stored in the passive layer. The orientation of the PZT grains strongly infl uences the fi gure of merit. [ 7,15 ] In particular, strongly (001) oriented PZT fi lms, which have a signifi cant fraction of the ferroelectric polarization directed out-of-plane, have large FoM. [ 16 ] From the standpoint of improving the toughness and reliability of the piezoelectric energy harvester, a metal foil is preferable compared to Si, the typical substrate for MEMS. [ 17 ] Moreover, thin metal foils enable low resonance frequencies without etching, and are easily machined. Morimoto et al. [ 8 ] reported a cantilever piezoelectric energy harvester using epitaxial PZT thin fi lms transferred onto stainless steel that shows output power of 5.3 µW at 5.0 m s −2 and 126 Hz without a proof mass. In much the same way, (Na 0.5 K 0.5 )NbO 3 (NKN) piezoelectric thin fi lms coated on Ni-based metal foils were investigated for vibration energy harvesters. [ 18 ] Another benefi t of using metal substrates for PEH is that copper and nickel have larger thermal expansion coeffi cients than many perovskite fi lms. This produces compressive stresses in the tetragonal PZT fi lm on cooling, increasing the volume fraction of the out-of-plane polarization direction. [ 19 ] Thus, high compressive stresses in {001} PZT fi lms give rise to an improved FoM for energy harvesting. [ 7,[19][20][21] Typical PEH designs use a fi xed-free cantilever beam confi guration with or without a proof mass. While fi xed-free Extracting energy from low vibration frequencies (<10 Hz) using piezoelectric energy harvester promises continuous self-powering for sensors and wearables. The piezoelectric compliant mechanism (PCM) design provides a signifi cantly higher effi ciency by fostering a uniform strain for its 1st mode shape, and so is interesting for this application. In this paper, a PCM energy harvester with bimorph Pb(Zr,Ti)O 3 (PZT) fi lms on Ni foil deposited by rf magnetron sputtering is shown to have high effi ciency and large power for low...
Flexible metal foil substrates are useful in some microelectromechanical systems applications including wearable piezoelectric sensors or energy harvesters based on Pb(Zr,Ti)O3 (PZT) thin films. Full utilization of the potential of piezoelectrics on metal foils requires control of the film crystallographic texture. In this study, {001} oriented PZT thin films were grown by chemical solution deposition (CSD) on Ni foil and Si substrates. Ni foils were passivated using HfO2 grown by atomic layer deposition in order to suppress substrate oxidation during subsequent thermal treatment. To obtain the desired orientation of PZT film, strongly (100) oriented LaNiO3 films were integrated by CSD on the HfO2 coated substrates. A high level of {001} LaNiO3 and PZT film orientation were confirmed by X-ray diffraction patterns. Before poling, the low field dielectric permittivity and loss tangents of (001) oriented PZT films on Ni are near 780 and 0.04 at 1 kHz; the permittivity drops significantly on poling due to in-plane to out-of-plane domain switching. (001) oriented PZT film on Ni displayed a well-saturated hysteresis loop with a large remanent polarization ∼36 μC/cm2, while (100) oriented PZT on Si showed slanted P-E hysteresis loops with much lower remanent polarizations. The |e31,f| piezoelectric coefficient was around 10.6 C/m2 for hot-poled (001) oriented PZT film on Ni.
For piezoelectric energy harvesters, a large volume of piezoelectric material with a high figure of merit is essential to obtain a higher power density. The work describes the growth of highly (001) oriented sputtered lead zirconate titanate (PZT) films (f ≈ 0.99) exceeding 4 µm in thickness on both sides of an Ni foil to produce a bimorph structure. These films are incorporated in novel wrist-worn energy harvesters (<16 cm 2 ) in which piezoelectric beams are plucked magnetically using an eccentric rotor with embedded magnets to implement frequency up-conversion. 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). Measured at resonance, six beams producing an output of 1.2 mW is achieved at 0.15 G acceleration. For magnetic plucking of a wrist-worn nonresonant device, 40-50 µW is produced during mild activity.
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