Study On The Structure And Thermal Conductivity Of AlN/PVDF Dielectric Composite Materials

Polyvinylidene fluoride(PVDF)is an important dielectric polymer for building thin film capacitors.To solve the heat dissipation problem of thin film capacitor,it is necessary to add thermal conductive fillers into PVDF matrix.However,how to efficiently use fillers to improve thermal and dielectric properties of PVDF remains a big challenge.This article first uses aluminum nitride(AlN)as a filler.In order to solve the aggregation problem of the filler in PVDF,the surface of AlN was chemically modified using silane coupling agent(KH550)and dopamine(DA),and AlN/PVDF composite materials were prepared by solution pouring hot pressing method.In order to better construct thermal conductivity pathways,carbon fiber(CF)was introduced to prepare AlN/CF/PVDF composite materials.By exploring the effects of AlN and CF on the crystal form and crystallinity of PVDF,as well as the thermal,dielectric,and mechanical properties of composite materials,the influence of filler content and surface modification on the various properties of composite materials was clarified.The results are as follows:(1)In PVDF/AlN composites,although AlN particles can not promote PVDF to produce its polar phase crystals,it acts as the nucleating agent for a-spherulites,thereby increasing their nucleation densities and crystallinities and accelerating crystallization rate.Most importantly,AlN has a positive effect on the thermal conductivity of the composite.The more AlN content in the system,the greater the thermal conductivity of the composite.As the AlN mass fraction increases from 0 wt%to 50 wt%,the thermal conductivity increases from 0.190 W/mK to 0.438 W/mK.It is worth mentioning that the change trend of dielectric properties of PVDF is also positively correlated with AlN content.In the applied electric field at 103 Hz,the dielectric constant of pure PVDF is 10,while that of the PVDF/AlN composite containing 50 wt%AlN increases to 13.93.Moreover,the dielectric loss always remains in a relatively low range of 0～0.25,which is crucial to improve electricity conversion efficiency.(2)In order to improve the dispersion of AlN particles,the surface of AlN was chemically modified with KH550,DA,and both KH550 and DA,respectively.Then,PVDF composites containing three different modified AlN particles were prepared.The results show that the percentages of deposition weight of KH550,DA and DA-KH550 on AlN surface are 0.64%,1.57%and 6.19%respectively.When the load of modified AlN is 50 wt%,the optimum thermal conductivity of KH550-AlN/PVDF composite is 0.472 W/mK and that of DA-AlN/PVDF composite is 0.505 W/mK.However,it is worth noting that the optimum thermal conductivity of DA-KH550-AlN/PVDF composite reaches 0.659 W/mK,which is about 2.5 times higher than that of PVDF.In contrast,the synergistically modified AlN with DA and KH550 has the most significant improvement in the thermal conductivity of PVDF.Additionally,the co-modified AlN with DA and KH550 can also raise greatly the dielectric properties of PVDF.For the composite with 50 wt%modified AlN,the dielectric constant of the composite reaches 18.36,while the dielectric loss decreases to 0.135.(3)In order to utilize low load fillers in PVDF matrix to better construct thermal conductivity pathways,the carbon fibers with good thermal conductivity and AlN were simultaneously introduced into the PVDF matrix.The results show that the introduction of both CF and AlN results in double percolation structure,which significantly improves the thermal and dielectric properties of PVDF.When the contents of AlN and CF are 20 and 8 wt%,respectively,the thermal conductivity of PVDF/AlN/CF composites rises to 0.582 W/mK,which not only increases by about 3 times compared to that of neat PVDF,but also exceeds that of 50 wt%AlN/PVDF composites.This clearly indicates that the formation of a double percolation structure helps to significantly reduce the amount of fillers.Furthermore,the results also show that the dielectric constant of AlN/CF/PVDF composites can reach a maximum of 19 at the frequency of 103 Hz.As the CF content increases from 0 wt%to 6 wt%,the breaking strength of the ternary composite increases from 33.27 MPa to 48.61 MPa.In addition,by adjusting the lamination method during sample preparation,the multi-layer structure within the composites can be well controlled,thereby regulating the thermal conductivity of the composites.The thermal conductivity of the composite can reach a maximum value of 0.444 W/mK when the AlN/PVDF film and CF/PVDF film are overlapped 8 times.