Biomimetic Preparation Of Boron Nitride And Graphene-based Nanocomposites For Thermal Management And Electromagnetic Shielding

With the advent of the 5G era,modern electronic products are rapidly developing towards functionalization and miniaturization.In a limited area,the high-speed operation of electronic devices will inevitably generate a large amount of heat,which will not only affect the safe and stable operation of electronic devices,but also reduce the life of electronic devices if they cannot dissipate heat effectively in a timely manner.In addition,the electromagnetic waves emitted by electronic devices interfere with neighboring electronic components,affecting the reliable operation of electronic devices.Polymer materials are widely used in daily life because of their light weight,flexibility,ease of processing and excellent corrosion resistance.However,most polymeric materials exhibit extremely low thermal and electrical conductivity,which greatly limits their application in electronic devices.Therefore,the development of the preparation of multifunctional polymers with high thermal and electrical conductivity,endowed with excellent thermal management and electromagnetic shielding properties,will greatly promote their application in electronics,which has far-reaching academic significance and extensive practical value.Inspired by the interface of natural shell structure,this thesis constructs the shell brick mud structure and intra-jurisdictional framework structure by adding boron nitride and graphene fillers to endow the polymer material with excellent thermal conductivity,electromagnetic shielding properties and high mechanical properties.The specific study is divided into three elements as follows:(1)Boron nitride nanosheets(BNNS)without planar defects and with fewer layers were prepared by exfoliation in N-methyl-2-pyrrolidone(NMP)solvent using bulk boron nitride as raw material and a mixed ball milling technique with small and large spheres.The exfoliated BNNS dispersion was concentrated by centrifugation and homogeneously mixed with polyvinylidene fluoride(PVDF)to prepare large-area,layered BNNS/PVDF nanocomposite films by a one-step blade coating technique.The high quality boron nitride nanosheets are arranged in the nanocomposite in a directionally ordered layered structure,which gives the composite films a high in-plane thermal conductivity and a low through-plane thermal conductivity.When 60 wt%of boron nitride nanosheets were doped,an in-plane thermal conductivity of 11.88 W m^-1K^-1was achieved for the composite films,which was two magnitudes superior to that of pure polyvinylidene fluoride films.Its through-plane thermal conductivity is only 0.41 W m^-1K^-1,which is comparable to that of pure polyvinylidene fluoride film.In addition,the BNNS/PVDF composite film is highly flexible and can be folded without breaking,making it extremely promising for thermal management in electronic devices.(2)The BNNS/ANF nanocomposite films with layered structure and nanofiber framework structure were successfully constructed by using aramid nanofibers(ANF)and boron nitride nanosheets(BNNS)as raw materials and preparing large-area sol-gel films by blade-coating technique with sol-gel transformation and drying process.When the mass fraction of BNNS was 20 wt%,the films exhibited a high thermal conductivity(14.03 W m^-1K^-1),which was 103 times higher than that of pure ANF films,attributed to the formation of a large number of continuous thermal conductivity pathways between BNNS nanosheets,which facilitated the fast phonon transport.The films have excellent mechanical properties(stress 97.14±5.17 MPa,strain 19.36±0.35%),high decomposition temperature(?542°C),medium dielectric constant(?6.9 at 10~4Hz)and low dielectric loss(?0.026 at 10~4Hz).At the same time,the films have a high breakdown voltage(310 m V m^-1)and volume resistivity(10¹³?cm^-1).Remarkably,these dielectric properties remain essentially constant over a wide temperature range(25-200°C).The preparation of high-performance biomimetic boron nitride based nanocomposites provides the material basis for their applications in flexible devices,fold-resistant electronic devices,etc.(3)High-strength,flexible and superhydrophobic graphene nanosheet/aramid nanofiber(GNS/ANF)nanocomposite films with hierarchical structure were successfully prepared by blade-coating technique using aramid nanofiber(ANF)and graphene nanopaste as raw materials,and after the sol-gel process and spraying process.The films exhibited excellent mechanical properties with tensile strength(131.17±2.77 MPa),strain at break(9.58±0.58%)and good toughness(8.84±0.74 MJ m^-3),which are 26.2,7.5 and 221.0 times higher than those of pure graphite films,respectively.These results are attributed to the synergistic effect between the tensile elongation of the 3D nanofiber network and the extensive sliding of the GNS,producing an effective energy dissipation.When the GNS content was 70 wt%,the EMI shielding effectiveness of the film was 31.3d B,and the reflection coefficient R was greater than 0.89,exhibiting a reflection-based shielding mechanism.The film also has superhydrophobic properties(158.7°±1.1°)and flame retardant properties.The multilayer nanocomposite films(thickness:39-57?m)possesse many amazing properties such as low density,good flexibility,and good corrosion resistance,which provide a material preparation pathway for nanocomposite materials in the field of EMI shielding applications.