The Optimization Of PVDF-TrFE Films On Piezoelectric Properties In Ultrasonic Transduction And Energy Harvesting

With the progress of science and technology,smart wearable devices are gradually developing towards miniaturization,lightweight,multi-function,flexibility and portability.For example,wearable medicine devices and implantable pressure sensors have higher requirements for the flexibility and handiness of piezoelectric materials.Different from traditional piezoelectric ceramics which are too brittle and toxic to the environment,poly(vinylidenedifluoride-trifluoroethylene)(PVDF-TrFE),as the most representative piezoelectric polymer material,has high piezoelectric coefficient and natural flexibility which attracts many attention of the researchers.They have kept an eye on improving the piezoelectric property of PVDF-TrFE.So in this paper,we focused on the preparation,process optimization and doping modification of PVDF-TrFE piezoelectric materials.And the performance of piezoelectric micro-devices in ultrasonic response and energy collection were also investigated.In terms of process optimization,this paper carried out relevant optimization verification for drying,annealing and polarization conditions.And as for doping modification,it also explored doped reduced graphene oxide(rGO).Due to the low ambient temperature and small airflow fluctuation in the vacuum drying process,PVDF-TrFE films dried by vacuum had fewer surface defects and more uniform thickness than the hot-baked samples.It was found that the crystallinity and Curie temperature Tc of the films would be affected by the annealing temperature and annealing time.After annealing at 140?for 1h,PVDF-TrFE could obtain a high purity of?-phase.The piezoelectric coefficient of PVDF-TrFE thin film could reach to24.7pC/N after the poling under 7.5kV of self-designed in-situ polarization equipment.As for PVDF-TrFE doped with rGO,the crystallization performance was significantly improved due to the interaction between rGO and PVDF-TrFE.When the doping ratio of rGO was 0.1wt%,the piezoelectric coefficient of rGO/PVDF-TrFE got the largest number of 34pC/N.However,as the doping concentration increasing,there would form an agglomeration which wouldl lead to circuit conduction and polarization failure ultimately.As for the application of energy collection and ultrasonic response,this paper had explored the relationship between stress and the response strength,ambient temperature and piezoelectric properties.The open circuit voltage and short circuit current were increased as stress boosting.The rGO/PVDF-TrFE films which had 0.1wt%rGO obtained a maximum open circuit voltage of 8.32V and short circuit current of 0.59?A under the stress of 5N.And the corresponding maximum charge density reached to28.7w/m~3.At the resonant frequency,the response voltage generated by the ultrasonic wave of the piezoelectric device had 0.017mV.However,as the environment temperature increasing,the piezoelectric effect of PVDF-TrFE would descend.Ultrasonic property of PVDF-TrFE attenuated by about 50%under the temperature of50?.The piezoelectric effect still existed when the temperature was restored to room temperature after heating to 90?.But as the temperature of environment close to Curie transition temperature,the piezoelectric properties of materials would undergo irreversible attenuation.