Study On The Preparation And Dielectric Properties Of MWCNT/PVDF Composite Films

The integration and miniaturization of electrical components requires dielectric materials with high dielectric constant,low dielectric loss,high breakdown strength and good mechanical properties.The multi-wall carbon nanotubes(MWCNTs)filled polyvinylidene fluoride(PVDF)composites become one of the most promising dielectric materials due to their high dielectric constant and good mechanical properties,but its high dielectric loss greatly reduces the energy storage efficiency.In order to reduce the dielectric loss of MWCNT/PVDF composite films and improve the energy storage density,MWCNTs were coated with polydopamine(PDA)for the first time in this thesis,and then added into PVDF matrix to form the composite films.By changing the thickness of PDA coating layers,the dielectric constant,dielectric loss and breakdown field strength of MWCNT@PDA/PVDF films could be controled.Moreover,the relationship between the dielectric properties and the percolation behavior of MWCNT/PVDF composite film was also studied,which provided the basis for the design of MWCNTs filling ratio in composite films.The effects of MWCNTs acidification process and film-forming process on the structure and dielectric properties of MWCNT/PVDF composite films were studied.The results show that the acidification process of MWCNTs improves its dispersibility in PVDF matrix and its compatibility with PVDF.The surface of acid-treated MWCNT/PVDF film was smooth and without microcrack,dielectric properties uniform and improved.When the film-casting condition was 50? and 6h,the content of ? phase and dielectric constant of MWCNT/PVDF composite films were the highest.Based on the optimized preparation process above,the effect of MWCNTs volume fraction on the structure and properties of MWCNT/PVDF composite films was investigated.The results show that the addition of MWCNTs improved the crystallinity,? phase content,electrical conductivity,dielectric constant and dielectric loss,reduced the breakdown field strength.When the MWCNTs volume fraction reached 8%,the dielectric constant was very high(1010),but the dielectric loss also reached 103,and the breakdown field strength decreased.The study also demonstrated the conductive percolation in the MWCNT/PVDF composite membrane.By fiting,The percolation threshold was 4.7vol%.When the volume fraction of carbon nanotubes reached the upper percolation limit,the dielectric properties of the composite film began to decrease.Therefore,for the design of PVDF composite film with high composite dielectric properties,the volume fraction of MWCNTs should be controlled below the upper percolation limit.For MWCNT/PVDF composite film,reducing the conductance loss is the key to improve its comprehensive dielectric properties.Coating the MWCNTs with PDA,the effect of PDA coating thickness on the structure and properties of MWCNT@PDA/PVDF composite films were investigated.The result shows that the PDA coating layer reduced the conductance loss,increased the breakdown field strength and energy storage density.When the coating thickness was 5.2nm,the dielectric loss was only 1% of that without coating.When the coating thickness was 12.5nm,the breakdown field strength was 20% higher than that without coating.When the coating thickness of 1.3nm,the storage density was 2.18 times higher than of pure PVDF film.In order to further reduce the conduction loss and improve the energy storage density of MWCNT@PDA/PVDF composite film,the effect of MWCNTs volume fraction and annealing process on the structure and dielectric properties of composite film was also discussed.The results show that the dielectric constant,dielectric loss and energy storage density increased with the increase of MWCNTs volume fraction.When the volume fraction of MWCNTs reached 12%,the dielectric constant was the highest at 691,and the energy density was the highest at 5.54 J?cm-3,which was 4.66 times the pure PVDF film,and the dielectric loss was maintained below 1.14.By annealing,the dielectric loss of the composite film was reduced,and the dielectric loss was the lowest at 0.044 after annealing 4h at 160?,only 1/4 that of the PVDF matrix.