Preparation And Interface Modification Of Flexible Pyroelectric Thin Films

Flexible electronic devices,such as wearable devices,flexible electronic screens and flexible multi-functional sensors,are emerging in recent years as a new type of electronic devices.These devices have unique properties such as flexibility,biocompatibility,and low material cost.They have broad application prospects in information,energy,medical,and national defense and other fields.Polyvinylidene fluoride(PVDF)material is one of the most widely used flexible electronic materials,but the traditional three-step preparation process has greatly limited the industrialization of the material.Therefore,this paper starts with the preparation of pyroelectric PVDF film to study the influence of film forming process on film properties.(1)Firstly,crystallinity,crystal phase and pyroelectric voltage response of PVDF films cast on different substrates are investigated.It is found that the PVDF films prepared on hydrophilic substrates has higher β phase content,the higher crystallinity and higher pyroelectric voltage response.For crystalline substrates with little difference in hydrophilicity,there is a lattice mismatch between the PVDF film and the substrate.The smaller mismatch between substrate and a certain crystalline phase of PVDF,the easier it is to produce this crystalline phase.The lattice misfit between copper plate and PVDF film is very low,less than 0.2%,so the PVDF film cast on Copper Plate has high phase content.(2)Secondly,this work improves the hydrophilicity of the substrate by modifying hydroxyl groups on the surface of amorphous glass.With the increasement of the number of modified hydroxyl groups,the crystallinity of PVDF film is higher,and the proportion ofβ phase increase is much higher than that of a phase.The residual polarization strength,relative dielectric constant and dielectric loss also gradually increase.More and more hydrogen bonds are formed between the films and the substrate,which led to the peeling resistance of the film from the substrate increasing too.More importantly,the hydroxyl-modified PVDF film produce spontaneous polarization,and the pyroelectric voltage response can be directly measured without thermal polarization,which indicates that the dipoles in this film have been arranged orderly.So,one-step method to prepare the spontaneously polarized PVDF pyroelectric film is realized.(3)Finally,the polarization characteristics of spontaneously polarized PVDF films are investigated from the aspects of polarization direction,polarization degree and mechanism.It is found that the polarization direction of hydroxyl-modified PVDF films is from the substrate to the films.Moreover,this paper proposes a "seed layer" model to explain the spontaneous polarization mechanism of hydroxyl-modified PVDF films:the hydrogen bonding between the film and the substrate induces alignment orderly of the PVDF molecular chains of the first layer.What’s more,the PVDF molecular chains of the first layer act as a"seed layer"so that the subsequent molecular chains are aligned orderly from bottom to top under the action of electrostatic force.The polarization degree of spontaneous polarization film is studied by thermal polarization treatment.The pyroelectric response of PVDF films modified by hydroxyl groups is equivalent to the thermal polarization response of conventional films under the condition of 23 MV m-1.However,the pyroelectric response of the films decreases continuously under reverse polarization,and the polarization direction of the films reverses when the field strength is greater than 27 MV m-1.