Study On The Dielectric Properties Of PVDF-based Composites

High dielectric materials have received more and more attention due to their important applications in the electrical and electronic industries.Although inorganic dielectric materials have high dielectric constants,they are limited due to their low breakdown strength,high dielectric loss.,high brittleness,inconvenient processing,poor life and stability,etc.It is widely used.Although high-molecular polymer materials have high toughness and extremely high breakdown resistance,their relatively low dielectric constant also limits the use of polymer materials.Therefore,preparing inorganic/organic polymer composite materials to have the advantages of both inorganic dielectric materials and organic dielectric materials is an important direction of current dielectric materials research.However,due to.the poor interface compatibility of the composite materials prepared by the traditional blending method,problems such as agglomeration and uneven dispersion of the inorganic dielectric materials in the organic polymer matrix will be caused,which further affect the dielectric properties of the composite materials.In this paper,BT/PVDF-based composites were prepared by changing the barium titanate(BT)particle size,addition amount,surface modification,cross-linking and polyvinylidene fluoride(PVDF)structure adjustment methods,and studied the above factors on the composite material.The impact of electrical properties.(1)BT/PVDF composites with different addition amounts and different particle sizes were prepared,composite films were prepared by solution casting and hot pressing methods,and the effects of addition amount and particle size on the dielectric properties of the composites were studied.The grain size is greatly affected by the amount of BT added.For BT of the same particle size,the grains first increase and then decrease with the increase of BT content,and reach the maximum at 2wt%addition.When the same amount is added,the 30nm BT composite has the largest particle size.With the increase of BT filling amount.The crystallinity of PVDF gradually decreases,the dielectric constant and conductivity of the composite film continue to increase,and the dielectric loss increases slightly at low frequencies.The dielectric properties of the composites prepared with 100nm BT are the best.Under 40wt%addition,the dielectric constant is increased from 7.89 to 20.3,and the dielectric loss is increased from 0.058 to 0.098;the breakdown performance of the composites prepared with 30nm BT Optimally,at 40wt%addition,30nm BT prepared composite material,the breakdown strength can reach 208 MV/m.(2)The MD-PVDF polymer film with different HF removal time was prepared by solution casting method and hot pressing method.AIBN was used to cross-link MD-PVDF polymer films with different HF removal times to study the influence of the introduction of double bonds and moderate cross-linking on the dielectric properties of the polymer.With the progress of HF removal,a double bond structure is formed in PVDF,which can promote the transformation of PVDF from a crystal form to β crystal form,and the crystallinity ranges from 44.6%-22.7%.As the degree of cross-linking increases,the polymer’s β-crystalline form changes to α-crystalline form,and the crystallinity will increase,from 34.1%-41.3%.With the increase of HF removal time,the dielectric constant,dielectric loss and conductivity of the polymer continue to increase.At 100Hz,the dielectric constant increases from 7.89 to 14.9,the dielectric loss increases from 0.058 to 0.085,and the conductivity from 2.17*10-9(S/m)increased to 6.16*10-9(S/m).With the increase of AIBN,the dielectric constant,dielectric loss and conductivity of composite materials have a certain decrease.After PVDF is removed from HF for 6h,0.24g of AIBN is added for cross-linking to obtain a moderately cross-linked polyvinylidene fluoride polymer.The energy storage density can reach 9.33 J/cm3 under the electric field strength of 350MV/m,which is higher than that of PVDF.By about 42%.(3)VTS@BT was prepared by modifying the surface of BT ceramic particles with hydrogen peroxide and vinyl dimethoxy silane,and then adding 0.24 g of AIBN initiator and reacting it with modified polyvinylidene fluoride(MDPVDF)to prepare Moderately cross-linked XL-VTS@BT/PVDF composite material.VTS@BT has a certain effect on the crystallization of PVDF.In the composite material,the α crystal form and β crystal form of PVDF coexist.Cross-linking will promote the transformation of β crystal form to a crystal form.As the content of VTS@BT increases,the crystallinity of PVDF gradually decreases,and the crystallization rate decreases slightly.With the increase of the amount of VTS@BT,the dielectric constant of the composite film continues to increase and the dielectric loss increases slightly at low frequencies.With the addition of 40 wt%,the dielectric constant of the composite material increased from 7.89 to 19.1.The dielectric loss only increased from 0.058 to 0.075,and the breakdown strength decreased from 390MV/m to 167MV/m.Appropriate cross-linking greatly increases the energy storage density of the composite material.With the addition of 10wt%,the energy storage density of the composite material can reach 10.36 J/cm3 under an electric field strength of 340MV/m,which is nearly 57%higher than the energy storage density of PVDF.