Surface Modification Of CNT And Research On The Preparation And Performance Of PVDF Composites

In recent years,composite dielectric materials have become a hot spot for research in cutting-edge fields such as electronics and energy storage because of the light weight,good processing properties and excellent voltage resistance ability.Among them,the conductive filler/polyvinylidene fluoride（PVDF）composites can improve the polarization strength of the composites significantly with a very small amount of filler addition,but a high dielectric loss would be introduced meanwhile.In order to improve the shortcomings above,the carbon nanotubes（CNT）was chosen as conductive fillers,and the weak conductor Dopamine（DA）and the insulator silicon dioxide（SiO₂）were selected to coat CNT as shells to achieve the purpose of improving the dielectric constant of the CNT core-shell particles/PVDF composite material and keeping the dielectric loss at a low level.The main research progress is listed as follows:The CNT@PDA（CNT@polydopamine）core-shell particles were firstly prepared and the optimal synthesize process was determined by analyzing the morphology of the core-shell particles under different react conditions,which showed the optimal react time was 6 h and the optimal temperature was 50℃.At this time,the thickness of the PDA shell layer was about 5 nm.The CNT-OH@SiO₂ core-shell particles were prepared with cetyltrimethylammonium bromide（CTAB）and ethyl orthosilicate（TEOS）.The results showed that the ratio of CNT-OH/TEOS=1 g/6 ml was the best,The shell thickness was 4.3 nm.Finally,core double-shell nanoparticles were prepared by CNT-OH@SiO₂ core-shell particles and PDA outer shell.A series composite films of CNT@PDA/PVDF,CNT-OH@SiO₂/PVDF and CNT-OH@SiO₂@PDA/PVDF with different filler additions by solution casting method were prepared.The effects of CNT surface modification methods and filler addition on the microstructure,dielectric properties and mechanical properties of CNT core-shell particles/PVDF composites were discussed.The experimental results showed that the addition of three kinds of nano-core-shell fillers promoted the transformation of the crystalline phase of PVDF fromαphase andγphase toβphase.When the test frequency was 100 Hz and CNT@PDA,CNT-OH@SiO₂and CNT-OH@SiO₂@PDA core-shell particles addition was 2 wt.%,the dielectric constant of the composite film of CNT@PDA/PVDF,CNT-OH@SiO₂/PVDF and CNT-OH@SiO₂@PDA/PVDF were 10.5,11.447,and 12.001,and the dielectric loss were 0.02696,0.02347,and 0.02341.The breakdown field strength of the CNT core-shell particles/PVDF composite film decreased with the increase of CNT core-shell particles.The addition of three kinds of CNT core-shell particles made a large number of nano-capacitor structures formed in the CNT core-shell particle/PVDF composite film.A large amount of space charge accumulates,which leading to the electric field distortion and reduce of the ability of the composite film to resist electrical breakdown.The breakdown field strength of 0.5 wt.%CNT-OH@SiO₂@PDA/PVDF composite film was 172.304 k V/mm.At this time,the energy storage density of the composite film reached 2.17 J/cm~3,which was 1.13 times that of pure PVDF film.The DMA test results of the CNT core-shell particle/PVDF composite film showed that the addition of fillers improved the storage modulus of the PVDF matrix.The storage modulus of the CNT-OH@SiO₂@PDA/PVDF composite film was 878.68 MPa,which was 2.22 times than that of pure PVDF.