Vibrational piezoelectric energy harvesters based on thinned bulk PZT sheets fabricated at the wafer level

We present a complete wafer level microfabrication process for the production of unimorph MEMS energy harvesters based on thinned bulk piezoelectric ceramic, lead zirconate titanate (PZT), sheets. This process eliminates the need for individual bonding of PZT pieces and proof masses at the chip level while still benefitting from the excellent properties of bulk PZT. With the process presented in this paper, 20 piezoelectric energy harvesters have been fabricated in parallel at the wafer level by bonding a single bulk (PZT) sheet onto a silicon-on-insulator (SOI) wafer using a low-temperature process and structuring the bonded stack with standard microfabrication techniques including thinning of the bulk PZT sheet using mechanical grinding as well as electrodeposition to deposit a thick nickel proof mass on the tip of each cantilever. A single fabricated harvester with an effective volume of 47.82 mm(3) is capable of generating a normalized power density of 3346 mu W cm(-3) g(-2) with an average power of 1.6 mu W under an excitation of 0.1 g (1 g = 9.81 m s(-2)) at a resonant frequency of 100 Hz through an optimal resistive load of 11.8 k Omega Thinned bulk PZT exhibits high power and a useable voltage while maintaining a low optimal resistive load, demonstrating the potential of high performance piezoelectric MEMS energy harvesters using bulk PZT sheets fabricated at the wafer level. (C) 2014 Elsevier B.V. All rights reserved

On the experimental determination of the efficiency of piezoelectric impact-type energy harvesters using a rotational flywheel

This paper demonstrates a novel methodology using a rotational flywheel to determine the energy conversion efficiency of the impact based piezoelectric energy harvesters. The influence of the impact speed and additional proof mass on the efficiency is presented here. In order to convert low frequency mechanical oscillations into usable electrical energy, a piezoelectric harvester is coupled to a rotating gear wheel driven by flywheel. The efficiency is determined from the ratio of the electrical energy generated by the harvester to the mechanical energy dissipated by the flywheel. The experimental results reveal that free vibrations of the harvester after plucking contribute significantly to the efficiency. The efficiency and output energy can be greatly improved by adding a proof mass to the harvester. Under certain conditions, the piezoelectric harvesters have an impact energy conversion efficiency of 1.2%

An automatic test bench for complete characterization of vibration-energy harvesters

This paper presents an automated test bench, based on a rigorous measurement procedure, used to fully characterize vibration energy harvesters including determination of the resonance frequency, impedance, optimal load, and output power as a function of both frequency and acceleration. The potential of this method and the performance of the automated test bench allows systematic data acquisition which is essential for a good comparison of all harvesters. A dedicated automation circuit was designed and fabricated. It uses stepper motors to mechanically control trimmers to vary the resistive load and reed relays to switch between the measurement sequences. With this, the setup is able to determine the optimal load of the device-under-test at its resonant frequency for a given acceleration. The test bench was used to fully characterize several types of vibration harvesters fabricated on both silicon and polymeric substrates. A comparison of the characterized devices is discussed using a figure of merit proposed here. A survey and compilation of current practices used to benchmark vibration harvesters is also reported

Flexible and Robust Multilayer Micro-Vibrational Harvesters for High Acceleration Environments

This paper presents the fabrication and characterization of multilayer PVDF resonant micro-vibrational energy harvesters designed to withstand environments in which high levels of acceleration are present. The multilayer cantilevers are fabricated by combining two folded PVDF stacks into a multilayered, bimorph structure. This acts to increase the overall capacitance of the harvester, a problem that plaques PVDF cantilevers as a result of its low dielectric constant. Moderate powers (7 mu W) are produced from the cantilevers even at high acceleration levels (20 g) due to the limited piezoelectric coefficient of PVDF; however, as a result of the high tensile strength and low elastic modulus of PVDF, the cantilevers are able to survive extremely high accelerations (> 4000 g) without breakage - a critical problem for harvesters based on brittle piezoelectric materials and substrates

Vibration energy harvesters on plastic foil by lamination of PZT thick sheets

This paper presents a low-complexity and low temperature (85 degrees C) fabrication process for vibration energy harvesters. The process employs lamination steps to transfer thinned PZT thick sheets onto flexible polymeric substrates, using dry film photoresist. The influence of geometrical parameters on the device performance were assessed by FEM simulations (using COMSOL) and supported by experiments. Optimization of the output power was performed by modifying the neutral plane within the device and by using a localized seismic mass at the tip, which has resulted in an output power of 30 mu W at 52 Hz and an acceleration of 1g. Finally, a low-complexity and fully polymeric package is proposed, which together with the harvester process are compatible with large area fabrication methods

Design optimization of vibration energy harvesters fabricated by lamination of thinned bulk-PZT on polymeric substrates

The design optimization through modeling of a thinned bulk-PZT-based vibration energy harvester on a flexible polymeric substrate is presented. We also propose a simple foil-level fabrication process for their realization, by thinning the PZT down to 50 mu m and laminating it via dry film photoresist onto a PET substrate at low temperature (<85 degrees C). Two models, based on analytical and finite element modeling (FEM) methods, were developed and experimentally validated. The first, referred to as the hybrid model, is based mainly on analytical equations with the introduction of a correction factor derived from FEM simulations. The second, referred to as the numerical model, is fully based on COMSOL simulations. Both models have exhibited a very good agreement with the measured output power and resonance frequency. After their validation, a geometrical optimization through a parametric study was performed for the length, width, and thicknesses of the different layers comprising the device. As a result, an output power of 6.7 mu W at 49.8 Hz and 0.1 g, a normalized power density (NPD) of 11 683 mu W g(-2) cm(-3), and a figure of merit (FOM) of 227 mu W g(-2) cm(-3) were obtained for the optimized harvester

Electrical fatigue behavior of Ba0.85Ca0.15Zr0.1Ti0.9O3 ceramics under different oxygen concentrations

Fatigue degradation is a significant problem in piezo/ferroelectric materials and their commercial applications. The major causes of electrical fatigue degradation are a domain pinning effect and physical damage such as microcracking. This work reports the fatigue behavior of barium calcium zirconate titanate (Ba0.85Ca0.15Zr0.1Ti0.9O3) under regular and low oxygen concentration silicone oil. Impedence analyzer and LCR meter are employed to analyzer the dielectric properties and it also revealed the relationship between activation energy and oxygen vacancy. X-ray diffraction, synchrotron X-ray absorption spectroscopy, and scanning electron microscope techniques were employed to study the local structural changes, defect development, physical damage and microcracking in the ceramics. FEFF-8.4 simulations were used to determine the oxygen vacancy creation. The study reveals the relationship of oxygen vacancy creation, domain wall pinning, microcracking and the fatigue behavior of the ferroelectric ceramic. The work investigated the dielectric and ferroelectric properties of BCZT ceramics intermes of applied electric field

Flexible and robust multilayer micro-vibrational harvesters for high acceleration environments

This paper presents the fabrication and characterization of multilayer PVDF resonant micro-vibrational energy harvesters designed to withstand environments in which high levels of acceleration are present. The multilayer cantilevers are fabricated by combining two folded PVDF stacks into a multilayered, bimorph structure. This acts to increase the overall capacitance of the harvester, a problem that plaques PVDF cantilevers as a result of its low dielectric constant. Moderate powers (7 mu W) are produced from the cantilevers even at high acceleration levels (20 g) due to the limited piezoelectric coefficient of PVDF; however, as a result of the high tensile strength and low elastic modulus of PVDF, the cantilevers are able to survive extremely high accelerations (> 4000 g) without breakage - a critical problem for harvesters based on brittle piezoelectric materials and substrates