| In recent years,in order to satisfy the development trend of lightweight,portable and intelligent electronic devices,research on polymer-based energy storage composite materials has attracted much attention.The ceramic nanofibers with high aspect ratio can effectively increase the dielectric constant and breakdown strength of the material at a lower content,and obtain high energy storage density.In this paper,a variety of polyvinylidene fluoride(PVDF)-based composite films were prepared by designing core-shell fibers and preparing the order of filler distribution in the polymer matrix.The structure and morphology of fillers and composite films were characterized,and the effects of core-shell structure and filler orientation distribution on the dielectric,breakdown strength,energy storage,heat properties and mechanical properties of composite films were compared.Barium titanate(BT)fiber with a diameter from 180 nm to 230 nm was successfully prepared by electrospinning method,and a core-shell structure fiber(BT@Ti O2)coated with BT with titanium dioxide was obtained by sol-gel method.The thickness of the Ti O2 layer was about 60 nm.BT nanofibers and BT@Ti O2 core-shell nanofibers were used as fillers,blended with PVDF solution and cast into films to prepare BT/PVDF and BT@Ti O2/PVDF composite films.Both nanofiber fillers have better dispersibility in the polymer matrix.Compared with pure PVDF and BT/PVDF composite films,the dielectric,breakdown and mechanical properties of BT@Ti O2/PVDF composite films have been improved.At a frequency of 1k Hz,when the filler content is 4vol%,the dielectric constant of BT@Ti O2/PVDF is up to 24,which is 26%higher than the BT/PVDF composite film with the same filler content,and the dielectric loss is reduced by 28%.When the filler content is 2 vol%,the breakdown field strength and tensile strength of the BT@Ti O2/PVDF composite film are the highest,reaching 321 k V/mm and 39.24 MPa,respectively,which are 18%and19.6%higher than pure PVDF,and 122%and 102%of BT/PVDF.Although the thermal degradation temperature of BT@Ti O2/PVDF decreases,the residual weight rate increases with the increase of filler content,and is higher than that of pure PVDF and BT/PVDF composite films.By optimizing the electrospinning process,three composite fibers of oriented BT particles/PVDF,BT/PVDF and BT@Ti O2/PVDF were successfully prepared under the conditions of spinning voltage of 7.5 k V,advancing speed of 0.8 m L/h and drum speed of 2800 rpm.After hot pressing,three oriented composite films are obtained,and the parallel distribution of ceramic nanofibers in the polymer matrix can be clearly observed.Orientation is beneficial to improve the crystallinity,melting point of BT@Ti O2/PVDF composite films and compatibility of nanofibers with PVDF.Compared with other oriented composite films,the tensile strength,dielectric constant and breakdown strength of oriented BT@Ti O2/PVDF composite films are significantly improved,while the dielectric loss is the lowest.When the filler content is 2 vol%,the dielectric constant,dielectric loss and conductivity of the oriented BT@Ti O2/PVDF composite film at 1k Hz are 14.9 and 2.9×10-11 S/m,respectively.The breakdown strength and energy storage density are the highest among all composite films,achieve362 k V/mm and 5.4J/cm~3. |
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