Exfoliation Of Mica And Its Utility For Enhancing The Dielectric Properties Of Polymer-based Composites

Dielectric energy storage materials have important application value in the fields of electrical and electronic,such as embedded capacitors,electrical stress control and high energy storage devices.The study on the composite materials with high dielectric energy storage density will promote the development of national economy and improve the national security and defense capabilities.Polymer-based composite dielectric energy storage materials have attracted much attention because of their ease of processing and better mechanical properties compared with other dielectric energy storage materials.Mica is a kind of layered silicate with high breakdown strength,low dielectric loss,and large volume resistivity.It is possible to prepare high breakdown strength and high dielectric constant composite materials by composition of mica and polymer,which is of great significance in developing composite materials with high dielectric energy density.In this thesis,mica was exfoliated via ultrasonic stirring to obtain nanoscale mica.Polypropylene(PP)/mica nanocomposites were prepared by melt blending and polyvinylidene fluoride(PVDF)/mica nanocomposites were prepared by solution blending.The structures of the nanocomposites were characterized through scanning electron microscopy(SEM),infrared spectroscopy(FTIR)and X-ray diffraction(XRD).In this thesis,the structure of PP/mica nanocomposites and PVDF/mica nanocomposites were studied,meanwhile the mechanical properties,dielectric properties,breakdown and energy storage properties of PP/mica nanocomposites and PVDF/mica nanocomposites were also discussed.The results were as follows.(1)The mica was exfoliated by liquid-phase ultrasonic stirring in N-N dimethylformamide(DMF).Transmission electron microscopy(TEM)and atomic force microscopy(AFM)analysis of the exfoliated mica sheets shows that the mica sheets with a thickness of 9 nm could be prepared after ultrasonic stirring at the speed of 800 r/min for 12 h using DMF as the dispersing agent.(2)The micas were added into PP by melt blending method.The results of XRD and TG analysis shows that after compositing,the?crystal form of polypropylene remained unchanged and there was little change in decomposition temperature.With the increase in mica content,the dielectric constant of the composites increased.When the volume fraction of mica reached 20%,the dielectric constant was 3.5.When the amount of nano-mica sheet was 5%,the tensile strength of the composites was improved by 7%and the energy storage density was improved by 52%,compared with those of PP.(3)The PVDF/mica composite was prepared by adding the mica into the polyvinylidene fluoride matrix by solution blending method.SEM results show that the nano-mica sheets were well dispersed in PVDF.XRD analyses show that the crystal form of PVDF changed from?crystal to?crystal after the addition of mica.With the increase in mica content,the tensile strength and breakdown strength of the composites increase.When the amount of nano-mica was 6%,the tensile strength of the composites increased by 41%compared with the neat PVDF,while the dielectric constant increased by 87.5%.The relevant breakdown strength reached 352 MV/m and the energy density increased from 1.29 J/cm~3to 7.12 J/cm~3.