Vibration Energy Harvesting Based On Piezoelectric Materials At Macro And Micro Scales

As the economy and society develop,the demand of energy is increasing.Thus,exploring renewable,sustainable and green energy resources becomes a hot topic.Normally electrical devices are powered by batteries.Traditional batteries have lots of defects,like limited service life,requirement of frequent replacement,and large size,which restrict the development of wireless and microelectronics techniques.Energy harvesting is an alternative to batteries.It can convert environmental sources into electrical energy.Energy harvesters have multiple structures.In this work,we focus on piezoelectric vibrational energy harvesters,which are based on piezoelectric effect to harvest environmental energy.Vibrational energy harvesters are widely used.Here we choose nonlinear piezoelectric cantilever energy harvesters and ZnO nanowire as our study topics,from macro and micro scales respectively.Theoretical analysis,numerical simulation,experimental verification and other methods are conducted to study the output performance.These researches are of great importance to structure optimization and efficiency improvement.As for piezoelectric cantilever energy harvesters at macro scale,we study the dynamic characteristics of nonlinear bi-stable generators at first.Comparison results show that although bi-stable generators is able to overcome the limitations of linear harvesters,they still have some defects.When the intensity of excitation is insufficient,the oscillator may be stuck in either potential well instead of transiting between two wells,which will lead to a low output voltage.In order to enhance the performance of bi-stable generators,we study a tri-stable piezoelectric energy harvesting system.Since its potential function is tri-stable,the potential barriers become lower and the distance between two wells become larger,which make it easier to achieve higher output voltage even when the external excitation is weak.Also,the corresponding optimum magnet intervals of tri-stable generators under low-or high-intensity excitation are nearly unchanged,which means there is no need of structure regulation to meet different environmental conditions.All the nonlinear systems in this work are based on nonlinear magnetic force.Hence,it is important to obtain an accurate calculation method of magnetic force firstly.It relates to not only the system’s potential,but also the optimization of system parameters,such as the size of magnets,the magnetization,and the magnet interval.A precise model for calculating magnetic force is built using equivalent magnetizing current theory.Experimental verifications indicate that the proposed method shows a higher degree of accuracy than the usual magnetic dipoles method,especially when the magnet interval is small.Therefore,the proposed magnetizing current method in this paper can be chosen as a priority to analyze the magnetic force of nonlinear piezoelectric energy harvesting systems with external magnetic field.To study piezoelectric energy harvesting systems at micro scales,we focus on ZnO nanowires.In the existing studies,many researchers were interested in experimental measurement,while some researchers used continuum equation to calculate the output voltage.However,the details of piezoelectric properties of the ZnO nanowire are still not clear.We choose molecular dynamics as our main method to study the piezoelectricity and the dynamic properties of ZnO nanogenerators.Some issues are mainly discussed,like electron transport characteristics,the mechanical stability of the metal–semiconductor contact junctions,and the piezoelectric potential distribution.To calculate the piezoelectric potential distribution of a ZnO nanowire,a traditional method is dividing the three-dimensional space uniformly.This kind of grid cannot describe the location of our research object precisely.Thus,we propose a new grid definition method.The geometric center of point charges in each unit cell is considered as a grid point,which is better than the traditional three-dimensional uniform spatial grid,due to automatically mapping related locations during deformation.This method makes it easier to describe the piezoelectric potential distribution exactly.Two kinds of piezoelectric energy harvesting systems at micro scales are studied in this paper.One is an axial compressed ZnO nanowire,and the other one is a bent ZnO nanowire slid by a Pt metal tip.Their piezoelectric potential and variations of contact forces are obtained through molecular dynamics simulation.We analyze the piezoelectricity and the dynamic properties of ZnO nanowire in different conditions.Molecular dynamics simulation builds a connection between laboratory experiment and theory.It can provide the technical guidance of design optimization for piezoelectric energy harvesting systems at nanometer scales.In conclusion,we study some representative piezoelectric vibrational energy harvesters from both macro and micro scales.This research provides new ideas for theoretical analysis of energy harvesting.The results also establish bases for practical application of energy harvesters.