Flexible Physical Sensor Transducers For Human-Activity Monitoring By Vinylidene Fluoride-based Polymers

Vinylidene fluoride-based(PVDF-based)polymers are representative examples of electroactive polymers.The superior performance of PVDF-based polymers associates with changes of the electrical dipoles inside them.To enhance the response of the dipoles can improve their performance in multifunctional applications.Similar to other ferroelectric materials,PVDF-based polymers are applicable in various areas due to their multi-field coupling ability.How to manipulate the structure or design the materials,and thus obtaining optimized performance is one of the concerns in the research.In this thesis,we aim to optimize the electromechanical conversion properties of the PVDF-based polymers.Afterwards,the applications of PVDF-based ferroelectric materials and their composites are studied.The main results are summarized as follows.First,we use the finite element analysis method for determining the distribution of stress and strain in both truncated pyramidal and pyramidal structure.When the structure was subjected to an external stress stimuli,the internal stress in truncated pyramid was more concentrated,because the diffusion in an appropriate space of internal stress was limited by the structure of edges and surfaces,compared to the pyramidal structure.The strain transfered further in the structure and enhanced the intensity,which would increase the strain gradient.Based on the results of the analysis,we fabricated the inverted pyramidal and inverted truncated pyramidal silicon templates by wet etching.We also fabricated an inverted pyramidal soft template through transfer method.Finally,we fabricated pyramid or truncated pyramid arrays of ferroelectric polymer poly(vinylidene difluoride-trifluoroethylene),P(VDF-TrFE),with ordered and smooth surface through the simple yet effective ways of nano-imprinting and solution-cast method.Then,to facilitate the application of the ferroelectric polymer P(VDF-TrFE)in monitoring human activities and physiological signals in view of its excellent electromechanical conversion,we developed self-powered ferroelectric transducers with pyramidal or truncated pyramidal structured P(VDF-TrFE)films.They would enhance the electromechanical conversion performance based on flexoelectric effect and stress concentration effect.To recognize and distinguish various activities caused by different parts of the human body,such as different amplitude and frequency of motions,we apply the devices on the hands,elbows,neck,face and so on.In addition,we achieved the distinction and recognition for physical behavior of different people doing the same action.In the electromechanical conversion process,the ferroelectric polymer transducer can reach a maximum output voltage of 4 V,which can be used for driving portable electronic devices.Finally,to enhance the transducer performance and realize multi-functionality,we fabricate the ferroelectric polymer composite materials with two-dimensional materials,such as graphene and molybdenum disulfide.Wherein,the P(VDF-TrFE)and graphene composite are capable of increasing its response to 1-2 times of the original ones with a simple truncated pyramids structure.The ferroelectric polymer composite with molybdenum disulfide is applied in multi-functional integration of piezoelectricity and luminescence.