The Investigation Of Energy Harvestering Performance Of Flexible Composites Regulated By Topological Filler

In the field of emerging science and technology,flexible energy harvester,which is applied to wearable devices for power supply,has put forward higher requirements on the electrical performance and processability of energy harvesting materials.Based on cooperative principle,we can combine ferroelectric ceramics and polymer because of the excellent electrical properties of the ceramic material and excellent mechanical properties of the flexible polymer materials with proper methods.A new type of organic and inorganic composite material can be built with the excellent energy harvesting performance and good mechanical properties and processability,which has wide application prospect in the field of flexible energy harvester.What needs to be emphasized here is that ferroelectric ceramic powder plays a key role in the composite material as the filler,and its morphology and orientation will affect the electrical and mechanical properties of the composite material,which related with the energy harvesting performance.This work mainly takes ferroelectric materials such as BaTiO₃ as the target system,which can be synthesized and shape controlled by molten salt method.The correlation between energy harvesting performance and the morphology,distribution of filler have been studied.The energy harvesting performance has been improved through design and process control of the composites based on the piezoelectric effect and triboelectric effects.BaTiO₃ is a widely studied ferroelectric material as filler of composite.Compared with spherical shape,the synthesis of ferroelectric filler with low dimensions(one-dimensional,two-dimensional,etc.)is very difficult.For material with obvious anisotropic structure,it can grow spontaneously into low dimensional morphology in the molten salt environment.However,perovskite materials with highly symmetrical structure tend to grow spontaneously to globular or cubic block.Therefore,it is very important to find a simple method for synthesis the ferroelectric materials with low dimension morphology.In this work,based on the principle of local topochemical conversion,BaTiO₃ with various morphologies were prepared by molten salt method.Materials with similar structure and low-dimensional morphology as targeted BaTiO₃ are selected as the template(rod-shaped Ba Ti₂O₅,platelet-shapped Ba₆Ti₁₇O₄0),and the basic unit[TiO_n]polyhedron could locally reorganized,so that the target product can inherit the morphology of the template well.In addition to controlling the morphology of products,the composition of product elements can also be controlled by molten salt method.Ba Ti₂O₅ nanorod was used as template to adjust the proportion of A site elements and prepare rod-shaped(Ba_x,Sr_1-x)Ti O₃(x=0.5?0.9).Flexible composites can be constructed by using ferroelectric materials as fillers and polymer as matrix.First,based on piezoelectric effect,PVDF-based composites were prepared with BaTiO₃ of different morphology as filler.The morphology of BaTiO₃ filler will affect the poling efficiency of the composites in the process of high-voltage DC polarization.The Potential-across model shows that the fillers with a large aspect ratio is conducive to the stronger distribution of electric field intensity,so that it has higher polarization efficiency and is conducive to the improvement of piezoelectric properties.On the other hand,the fillers with a large aspect ratio can better respond to external forces,so that it has a higher electromechanical conversion efficiency.The experimental results show that the rod-shaped BaTiO₃/PVDF composite has the best piezoelectrical performance among the spherical,rod-shaped and platelet-shaped BaTiO₃ based composites.Furthermore,based on the above results and the requirements of energy harvesting performance,BaTi₂O₅ nanorod with large aspect ratio,good piezoelectric response and low dielectric permittivity was selected as the filler to construct a flexible piezoelectric composite material with improved performance.In cantilever beam mode,Ba Ti₂O₅/PVDF-5%has a power density of 27.4 W/cm³,and can work stably at 10 g acceleration,with good fatigue characteristics.Moreover,it can harvest the energy of wheel rotation during cycling,which is expected to provide energy for various on-board low-power electronic devices.In addition to the choice of filler,the material structure will also affect the energy harvesting performance of composite materials.Based on the required parameters of piezoelectric energy harvesting performance,the piezoelectric,dielectric and mechanical properties of the material can be adjusted simultaneously by introducing ferroelectric semiconductor FeTiNb O₆ and constructing the sandwich structure with FeTiNbO₆/PVDF as intermediate layer.By introducing rich interfacial polarization and the increased?content of PVDF,the piezoelectric constants of composites were effectively increased.Sandwich structure also effectively prohibited the increase of dielectric permittivity,according to the mixing laws.In addition,the rigid fillers can also increase the Young's modulus of composites in a certain extent,which is advantageous to the force transmission.So,the composite with 15% FeTi NbO₆/PVDF as the middle layer(P-FTN15%-P)has the optimal energy harvesting performance,power density reached 110?W/cm³,charge density 75?C/m².Excitingly,it can also effectively response to the weak force,such as the pulse of human body neck,which is expected to be used in medical equipment.New energy harvester for bicycles based on magnetic effects have improved durability.Flexible energy harvesting materials based on triboelectric effect have also received extensive attention.The dielectric permittivity and surface state of the composite material significantly affect the harvesting performance.Therefore,the effects of different morphology of BaTiO₃ on dielectric properties of composites were studied.It demonstrated that the dielectric properties are affected by three key factors with different shapes of fillers:1)spontaneous polarization of BaTiO₃,2)interface polarization,and 3)distribution of electric field intensity.The results show that one-dimensional rod-shaped BaTiO₃/PVDF has the maximum dielectric permittivity due to its maximum interface area and the maximum distribution intensity of electric field on the fillers.PDMS based composites were prepared with one-dimensional bar BaTiO₃ as filler,and the effects of filler orientation on dielectric properties and energy harvesting properties of composites were further studied.The results show that the one-dimensional rod-shaped BaTiO₃/PDMS composite processed by dielectrophoresis has excellent power generation performance.The open-circuit voltage can reach 200V,the power density is 46.95?W/cm²,and about 100 LED lights can be lit instantly.In this paper,the structural design of composite materials and the theoretical analysis of related flexible energy harvester have a good reference value for the development of new flexible energy harvesting materials,and the related material system is expected to be further optimized for the application of wearable devices.