Micro Piezoelectric Power Generator Driven By Ambient Vibration

In recent years, with the development of the integrated circuit and micro/nano electric-mechanical technology, the demand for wireless sensors nodes and portable electronics has been spreading speedily. The wireless devices are usually designed to run on batteries; however as the networks increase in number and the micro systems decrease in size, it is inconvenient to replace the exhausted batteries periodically. So the longtime micro power sources have been a pivotal issue in their diversified applications. To prioritize micro power options a comparison has been performed about the energy conversion theories of micro batteries. The micropower generator based on ambient vibration energy sources will be the most promising micro power sources and there are three conversion methods to achieve this type of energy, which are piezoelectric, electromagnetic and electrostatic conversion. As a micro power source the piezoelectric power generator is capable of the higher power output densities, reliable and adapts more fields. The specific scope of this thesis includes theory model and finite element analysis for micro piezoelectric power and developing a novel prototype fabrication process based on silicon films for integration. The experiment is carried out to show the batteries characters.The piezoelectric cantilever theory model is well known as a sensors and actuators, but which was used in a new scope of batteries to harvest ambient vibration has not been yet fully developed. In this thesis the theory analysis models were considered to calculate the output power and voltage from the micro cantilever piezoelectric batteries. And the calculation results showed that the output values appeared maximal values when the ambient vibration frequency approaches the natural frequency of micro power structures. As the forcing frequency deviated from the natural frequency the output power will decrease rapidly. The theory models also reflect the output values changing with dimensions of micro piezoelectric power.The relation of output voltage to micro piezoelectric cantilever based on silicon films is simulated by finite element method (FEM). The analysis models were done using ANASYS because of its capability to work with coupled-field elements. The simulation results indicated that the voltages would increase as the piezoelectric cantilever length and proof mass at the free end increasing, and the thickness decreasing. The open circuit voltage is about 0.02V with dimension about 2mm×0.4mm×10μm. The simulation results can be useful for micro power generator design.The micro fabrication processes are investigated for the piezoelectric cantilever power generator and this work includes the preparation of the ferroelectric lead zirconate tianate (PZT) thin films, pattern the electrode and PZT films and the free end release of microcantilever. Firstly to deposit uniform PZT thin films on Pt/Ti/SiO₂/Si substrates by Sol-Gel method the precursor sol solution is pivotal and allows to gel at low temperature. A novel stepped annealing method is presented to control the crystalline state forming. The low electric plats pattern by wet etching with aqua regia. The top electrics do by lift-off technology and which are covered with amorphous PZT film to isolate from air. The wet and dry bulk micromachining techniques are adopted to release the micro silicon cantilever. These micro micromachining processes realize the compatibility and integration of micro piezoelectric batteries.The output power and voltage characters of micro prototype generators are surveyed. The test results indicate that the micro piezoelectric cantilever power generators based on metals can resonate with a frequency about 100Hz at the ambient vibration and produce approximately voltages ranging from 10V to 20V. The output power is about 100μW induced on the pure resistance. The output voltage is about 15mV from the micro piezoelectric cantilevers generator based on silicon film. The test results are consistent with the theory analysis. The research results indicate that a piezoelectric generator for ambient vibration energy harvesting is a promising long lifetime electric power source to meet the needs of wireless sensors nodes with micropower consumption.