Piezoelectric Cantilever for Mechanical Energy Harvesting

Abstract

Renewable energy based power sources are of great interest in mobile and wireless devices due to limitations of capacity and disposal in chemical reaction based batteries. Piezoelectric energy harvester (PEH) is one of the most appropriate devices which can replace or back up the battery in such applications. The PEH converts vibration energy, typical form of mechanical energy, to electrical energy. However, harvested power density of the PEH is relatively low compared to other renewable energy based devices, which may be insufficient to operate current mobile and wireless devices. In addition, the operational frequency range of a linear PEH is too narrow for practical applications. Therefore, the operational frequency range needs to be expanded to harvest a wider range of environmental vibration. In this dissertation, the improvement of the output power of PEH was investigated as functions of temperature and piezoelectric modes. Hybridization of piezoelectric material with magnetic material was also studied to expand the frequency range of harvested vibration. The temperature effect on PEHs resulted in the degradation of the output power. Soft PZT based PEH degrades faster than the hard PZT based device up to about 100 ˚C. Above 100 ˚C with approaching Tc, the hard PZT based device appears to show faster degradation. In general, MEMS PEH degrades slower than bulk scale PEH. The most influential material parameter responsible for such degradation was the dielectric constant. Therefore, the reduction of the power density of the PEH at higher operating temperatures can be minimized by controlling the dielectric constant of a piezoelectric material MEMS scale PZT cantilevers based on d33 and d31 mode were designed, fabricated and evaluated to compare the output power density of the devices depending on the piezoelectric mode; d33 and d31. Since output power estimation model for d33 mode PEH has been rarely reported, a theoretical model was derived by combining Roundy’s analytical model and conformal mapping method. The derived mathematical model was also verified by the experiment results. The d31 mode device produced higher output power density than the d33 mode device in the fabricated devices. However, theoretical modeling suggested that higher output power can be generated from d33 mode device when finger electrodes become narrower than those of the fabricated devices. Under optimal electrode design, the d33 mode device can offer higher efficiency output power conversion due to higher voltage than d31 mode device. The d33 mode device can exhibit higher output power with the narrow finger electrode design, so that the effective spacing/actual spacing of piezoelectric layer is less than one. Piezoelectric ZnO cantilevers grown on magnetic substrates were fabricated to demonstrate harvesting vibration energy in a wide frequency range. Cu was doped to enhance piezoelectricity of ZnO thin films by lowering leakage current. The ZnO cantilever/magnetic substrate was vibrated driven by external magnetic force and showed energy harvesting from 20 Hz to 180 Hz vibration. Such a range covers almost the entire frequency range that is available in our environment.