Preparation And Properties Of Low Dielectric Aramid Film

Compared with traditional inorganic low dielectric materials such as silica,polymer-based dielectric materials have been widely used in power devices,flexible electronics and energy storage due to their advantages of light weight,easy processing and designability of molecular chain structure.However,the intrinsic dielectric constant of most polymers is not up to the standard of low dielectric materials,which fails to meet the requirements of dielectric material in the age of 5G communication(ε≤2.5).Since the dielectric constant of air is as low as 1,introducing porous structure into the polymer matrix is an effective strategy to prepare low dielectric materials.However,when a large number of pores are introduced into the polymer matrix,the mechanical properties of materials tend to decrease significantly,which severely limits the practical application of dielectric materials in the field of microelectronics.Morever,the processing temperature of metal wire interconnection processing on the interlayer dielectric is as high as 400 ℃,while the conventional polymer dielectric materials with flexible chain segments have low glass transition temperature and poor thermal stability,which is difficult to meet the processing requirements.In addition,the rapid development of electronic technology makes the integration of electronic equipment is very high,and the large amount of heat generated in the operation process is an important reason for the failure of electronic products.Good heat conduction is an effective means to extend the life of devices.Unfortunately,most dielectric polymers have low intrinsic thermal conductivity(0.1-0.3 W/(m·K))and poor heat dissipation capacity,making them difficult to use as cooling devices.In view of the above situation,this paper firstly selected aramid nanofiber(ANF)with high mechanical strength and excellent thermal stability as the polymer matrix,and prepared ANF aerogel film with hierarchical layered porous structure from the perspective of structural design.Due to the unique polymer structure skeleton,ANF aerogel films have ultra-low dielectric constant while maintaining good mechanical properties.Furthermore,using high performance ANF as polymer matrix and introducing thermal conductivity filler boron nitride nanosheet(BNNS)and interface reinforcing agent polyvinyl alcohol(PVA)to construct efficient thermal conductivity and insulation network,the ANF/PVA/BNNS composite film with high thermal conductivity(λ)and low dielectric constant is prepared.The main research contents of this paper are as follows:(1)ANF aerogel films with hierarchical layered porous structure were prepared by vacuumassisted filtration and ambient drying.In the whole process of ambient drying,the surface tension of solvent,the affinity between solvent and polymer skeleton and the solvent drying rate were used to regulate the porosity of the aerogel film.And the influence of porosity on the mechanical and dielectric properties of ANF aerogel films was explored.It was found that solvents with lower surface tension and polymer affinity could significantly improve the porosity of ANF aerogel films.Under natural drying conditions,n-hexane with the lowest surface tension and the worst affinity contributed the most to the porosity of ANF aerogel films,making the porosity of ANF aerogel films reach 68%.Secondly,with the increase of drying rate,the porosity of ANF aerogel films increases first and then decreases.Under the drying rate of 1.0 g/(g·min)with n-hexane as solvent exchange,the porosity of ANF aerogel films is as high as 79%.ANF aerogel films have both ultra-low dielectric constant and excellent mechanical properties.The dielectric constant and dielectric loss of ANF aerogel films with 79% porosity are as low as 1.56 and 0.0055 respectively at 1 MHz,and the mechanical strength and modulus are 22 MPa and 444 MPa,respectively.These excellent properties are attributed to the successful construction of hierarchical layered porous structure.(2)ANF/PVA/BNNS nanocomposite films with low dielectric constant and high thermal conductivity were prepared baseing on hot-pressing induction orientation technology,using ANF as polymer matrix,choosing boron nitride nanosheet(BNNS)as thermal conductivity filler,and by introducing PVA to strengthen the interface binding force between BNNS and polymer matrix.The influence of pressure during hot-pressing on BNNS orientation,the influence of BNNS content onλ and dielectric properties,and the influence of interfacial interaction on λ was investigated successively.The results show that the optimal BNNS were arranged along the plane height under the pressure of 15 MPa.Secondly,the addition of BNNS make the thermal conductivity of ANF/PVA/BNNS composite film increase first and then decrease after hot-pressing.The thermal conductivity of ANF/PVA/BNNS composite films with BNNS content of 20 vol% was the best.The in-plane and through-plane thermal conductivity were 16.5 W/(m·K)and 1.94 W/(m·K),respectively.Since the dielectric constant of BNNS was higher than that of PVA,the introduction of BNNS increased the dielectric constant of the material.With the increase of BNNS content from6 vol% to 36 vol%,the dielectric constant of ANF/PVA/BNNS composite film at 1 MHz increased from 2.05 to 2.61.At the same time,due to the strong interfacial interaction between BNNS and ANF/PVA,the interface polarization was inhibited,and the dielectric loss of the composite film was kept at low level(0.013-0.017).