Theoretical Study And Sensitive Features Of Piezoelectret Nanogenerators

The‘Internet of Things'?IoT?,known as the third wave of global information industry after computer and the Internet,will redefine people's lifestyle in the next decade.In the human-centered intelligent world,wearable sensing network is a branch of IoT closest to human body.Wearable sensors bridge the communication between human and the intelligent world.Through wearable sensors,we can collect information from human body,deliver instructions to IoT,etc.Pressure is the most common explicit signal of human body,and mechanical energy is the most common energy form in the surroundings.Therefore,it is of great significance to study flexible&wearable electromechanical transducers and collect-analyze-utilize the mechanical signals/energy.In this thesis,piezoelectret materials and the fundamental physics were lucubrated first,followed by the theoretical study and simulation of piezoelectret nanogenerators?PENGs?.We theorized PENGs and guided the subsequent device designs.Moreover,we proposed two wearable sensors based on piezoelectret and expanded the applications in wearable electronics.The main results are listed below:1.Piezoelectret is a class of porous electret material that possesses piezoelectricity.Piezoelectret,like traditional piezoelectric crystals,exhibits both piezoelectric effect and inverse piezoelectric effect,but the fundamental physics follows electrostatic effect of the stored charges in electret.Charges,as the core of piezoelectret,are usually generated by the disruptive discharge of gas in the high-voltage polarization process.The charges with opposite polarities are captured by electret on the upper and lower pore walls,respectively,forming electric dipoles which lead to piezoelectricity.The piezoelectricity of piezoelectret is directly related to its compressibility.In this chapter,the relationship between the elastic modulus and the structure is analyzed,providing an idea for future structural designs.The expressions of piezoelectric coefficient d₃₃,open-circuit piezoelectric coefficient g₃₃ and inverse piezoelectric coefficient K are derived,revealing the influences of charge density,Young's modulus,etc.on the piezoelectric properties of piezoelectret.2.Theoretically,the basic physical models of PENG are established.The charge density,electric field and potential inside PENG are calculated,and the output formulas are derived.Through simulating the uniform and cosine compression modes,the influences of various parameters?structure,material,load,external stimulation,etc.?on device performance are studied.Moreover,finite element simulation is employed to further study the distributions of internal stress,strain,electric field and potential under an external force,revealing the nature of physical changes inside piezoelectret in the working process.3.A wearable active voiceprint sensor is fabricated based on cellular polypropylene piezoelectret prepared by the expansion-polarization method.The polarization in the voids is adjusted by the dipole orientation control method to enhance the output current and sensitivity of the sensor.In the single-frequency measurement,the acoustic frequency response and sound pressure level response are studied,and the bending-state property,directivity and stability of the device are characterized.In the complex-frequency measurement,the device exhibits high signal-to-noise ratio?up to 20?and high energy ratio?about 10?,revealing the great potential in acoustic sensing and voiceprint recognition.As an instantiation,a voiceprint recognition system is constructed based on the voiceprint sensor.The dual recognition of voiceprint features and speech content is realized,demonstrating the broad prospect in non-contact biorecognition and security management.4.A THV/COC piezoelectret pressure sensor with standard structure is fabricated by COC?THV?electret with excellent positive?negative?charge storage ability,combined with PDMS elastomer as the supporting layer.The piezoelectret with imbalanced charge distribution is theoretically analyzed,revealing that open-circuit voltage is directly related to the initial electric field inside the device.Therefore,a high charge density can effectively enhance the electric field and the output voltage.According to the charge storage mechanism,a quenching method is employed to improve the negative charge storage ability of THV.According to the theory of piezoelectret,the structure of PDMS supporting array is adjusted to reduce the Young's modulus of the device and improve the output voltage.The final pressure sensor exhibits a high sensitivity of 30 mV/kPa and a wide linear pressure range up to 150 kPa.The THV/COC piezoelectret pressure sensor is applied in wearable wide-range pressure sensing,which broadens the application of flexible high-pressure sensors and promotes the development of wearable sensing network.