Performance Optimization And Applications Of Flexible Composite Piezoelectric Nanogenerators

With the rapid development of electronic technology,a variety of portable,wearable intelligent electronic devices have emerged to enhance people’s living standards and economic and social development.Meanwhile,to meet the special demand of these devices in terms of energy harvesting and utilization,it is of great significance to develop renewable,portable and low-cost energy that can provide sustainable power supply for electronic devices.Piezoelectric nanogenerators,which are based on piezoelectric effect to convert mechanical energy into electric energy,demonstrate great potential in harvesting ubiquitously mechanical energy in environment and continuously powering electronic devices.It is needed to design flexible nanogenerators which is compatible and stable for complex mechanical energy and collect these energy efficiently.Incorporating piezoelectric materials into flexible polymer matrix is a simple way to achieve flexible piezoelectric nanogenerators,which is simple,low-cost and suitable for large-scale production.In this thesis,we studied the optimization theory,design strategy and current output characteristics of piezoelectric nanogenerators.The influence of the mechanical and electrical parameters of the materials aswell as the composite structure on the performance of piezoelectric nanogenerators was investigated.Three kinds of flexible piezoelectric nanogenerators based on vanadium-doped zinc oxide（V-ZnO）/bacterial cellulose composite,barium titanate（BaTiO₃）nanoparticles/polydimethylsiloxane（PDMS）composite and BaTiO₃ nanoparticles/bacterial cellulose composite were designed and developed.The maximum peak current of piezoelectric nanogenerators is measured,which realized accurate characterization of the output current for piezoelectric nanogenerators.The influence of the factors such as the capacitance of the piezoelectric nanogenerators and the applied force on the maximum peak current was revealed.The performance of piezoelectric nanogenerators was simulated by finite element multiphysics simulation software COMSOL.It was found that increasing the Young’s modulus or Poisson’s ratio of the matrix can significantly increases the force transmitted on the piezoelectric phase and thus enhance the performance of piezoelectric nanogenerators.The performance can also be significantly improved by increasing the piezoelectric coefficient of piezoelectric phase or decreasing the permittivity of the piezoelectric phase.When the permittivity of the matrix is relatively low（εr<30）,it has enhancing effect on the performance of piezoelectric nanogenerators by increasing the permittivity of the matrix.While if the permittivity of the matrix is in high range（εr>30）,increasing permittivity of the matrix will have negative effect on the performance of piezoelectric nanogenerators.Augmented performance can be obtained through increasing the distribution density of the piezoelectric phase in the transverse and vertical directions of the matrix.Moreover,the performance of piezoelectric nanogenerators can be enhanced by integrating multiple units in series with distribution density of piezoelectric phase in each unit unchanged.Flexible V-ZnO/bacterial cellulose composite piezoelectric nanogenerators were developed.V-ZnO microballs featured with uniform size and distribution were in-situ synthesized in bacterial cellulose,which possess three-dimensional network structure and excellent hydrophilicity.Ferroelectric V-ZnO can be polarized with the external electric filed.There is no need to control the growth direction of V-ZnO crystal.Due to the elevated piezoelectric coefficient of V-ZnO,the output voltage,current density and power density of the flexible V-ZnO/bacterial cellulose composite nanogenerators under cyclic bending condition can reach 1.5 V,80 nA/cm2 and 60 nW/cm2,respectively,which is much higher than that of pure-ZnO based devices.The as-fabricated nanogenerators can be utilized as a self-powered sensor,which is capable of detecting page-turning motion.The response time of the sensor is 0.1 s and the generated signal can drive an LCD screen directly.The rectified output voltage and current can reach 0.9 V and 40 nA,which is capable of charging a 2.2 μF capacitor to 0.1 V in 350 s.Flexible BaTiO₃ nanoparticles/PDMS composite piezoelectric nanogenerators were developed.Devices with interdigital electrodes was designed.The output voltage and current density reached 1.5 V and 80 nA/cm2,which is 10 times of that obtained by a sandwich-structured device.Due the structure of the interdigital electrodes,a higher BaTiO₃ nanoparticle distribution density was obtained;meanwhile the piezoelectric coefficient d33 was used is significantly higher compared with a sandwich-structured device based on piezoelectric coefficient d31.Flexible BaTiO₃ nanoparticles/bacterial cellulose composite piezoelectric nanogenerators were developed.The device achieved output voltage and current density of 14 V and 190 nA/cm2 under vertical impact state and voltage of 1.5 V under cyclic bending state.A high BaTiO₃ nanoparticle distribution density can be rendered by the three-dimensional network structure and hydrophilic property of bacterial cellulose.In addition,the force exerted on the BaTiO₃ nanoparticles can be enhanced due to the high Young’s modulus of bacterial cellulose.All of above two factors plays a key role in improving performance of the devices.The maximum peak current of piezoelectric nanogenerators is successfully measured through a rational designed measurement procedure.The measurement is implemented by controlling the on/off state of the measurement circuit to separate the process of strain exertion and charge transfer,which can completely exclude the effect of strain rate on the measurement result.The expression describing the maximum peak current is established through theoretical derivation and experiment corrections,which revealed that the inner resistance of the device and the parasitic capacitance of the test circuit can significantly affect the maximum peak current.The charge-discharge time constant of capacitors can be used to quantitatively describe the time-dependent variation of the output current,which should be an important metric to assess the output current of nanogenerators.The influence of the capacitance of the device and the applied force on the maximum peak current and time constant are systematically studied.It is found that the increasing inner capacitance will reduce the maximum peak current and output power,prolong the time constant and reduce the inner resistance of nanogenerators.Increasing the magnitude of the applied force can give rise to the maximum peak current and output power,while has no influence on the time constant and inner resistance of nanogenerators.