Preparation And Properties Of Hydroxyl Boron Nitride Based Microwave Absorbing Materials With Core-Shell Structure

With the advancement of electronic information technology and the rise of the 5th electronic communication technology,various electronic devices are developing rapidly in the direction of integration and miniaturization.While they bring convenience to people’s lives,they also make the problem of electromagnetic pollution increasingly prominent.Absorbing materials are an effective way to solve electromagnetic pollution because they can be dissipated by converting electromagnetic wave energy into heat energy.Traditional microwave absorbing materials cannot meet the new requirements of current electronic equipment due to their high density,narrow effective absorbing width,and poor absorbing strength.It is realistic and urgent to develop lightweight absorbing materials with solid performance and wide effective absorbing frequency.The practice has shown that the heat dissipation capacity cannot be sacrificed for high-power electronic equipment when solving the problem of electromagnetic pollution.Otherwise,the high thermal load will reduce the operating reliability of the electronic equipment and reduce its service life.It is necessary to design and obtain a lightweight material with good thermal conductivity and microwave wave absorbing performance.This thesis considers the primary research object of light and high thermal conductivity hydroxylated boron nitride（HO-BNNS）.The thesis considers the primary research object of light and high thermal conductivity hydroxylated boron nitride（HO-BNNS）.HO-BNNS is combined with one or more magnetic materials,conductive polymer materials,and graphene oxide materials through synthetic methods.Obtain a composite absorbing material with high impedance matching and strong electromagnetic loss ability with high thermal conductivity and low density.The composite material’s structure,composition ratio,and morphology are discussed,and the effect on performance is concerned.The main research contents are as follows:1.Commercial BN was successfully exfoliated into an atomic layer thin BNNS（thickness about 0.7 nm）by steam-assisted melting hydroxide,and BNNS was functionalized with concentrated alkali to prepare HO-BNNS.Because the stripping and function enhance the polarization ability of HO-BNNS,the microwave absorption ability of HO-BNNS is more potent than that of BN.However,the optimal reflection loss has not reached-10 d B due to the single absorption mechanism.In addition,the thermal conductivity enhancement ability of BN,BNNS,and HO-BNNS as thermal conductivity additives was evaluated.When the addition amount of thermal conductivity additives was 0.2 wt%,the thermal conductivity enhancement effect on the matrix material was33.94%,64.77%,and 143.27%in the order of BN,BNNS,and HO-BNNS.2.With dopamine as the morphology control agent,Fe₃O₄ was synthesized in situ on the surface of HO-BNNS by ion co-precipitation method and the HO-BNNS@Fe₃O₄composite material with core-shell structure was prepared.Due to the introduction of the magnetic loss mechanism and the enhancement of dielectric loss performance,the absorbing performance of composite material HO-BNNS@Fe₃O₄ has been dramatically improved compared with HO-BNNS.When the content of HO-BNNS is 20 wt%,the RLmin of the prepared HO-BNNS@Fe₃O₄ composite material is-45.31 d B,the matching frequency is 8.64 GHz,and the thickness of the absorber is 2 mm.In addition,when the mass fraction of HO-BNNS is 20%,the composite HO-BNNS@Fe₃O₄ shows a wide effective absorption bandwidth of 2.5 GHz（7.6-10.1 GHz）,and the thermal conductivity is 1.5 W/（m?K）,which is nearly 5 times higher than that of pure Fe₃O₄.3.HO-BNNS@PANI nanocomposites with different doping degrees were prepared by in-situ polymerization.The effects of varying doping degrees and different addition amounts of HO-BNNS on the electrical conductivity,thermal conductivity,and microwave absorption capabilities of the composite were studied.Compared with HO-BNNS,the dielectric properties of the HO-BNNS@PANI composite have been greatly improved,showing excellent microwave absorption performance.When the HO-BNNS content in the composite material is 15 wt%,and the hydrogen ion doping concentration is 10^-2 mol/L,the HO-BNNS@PANI composite material exhibits good microwave absorption performance,and the thickness of the absorber is 3.5 mm When the RLmin value of HO-BNNS@PANI is-38.60 d B,the matching frequency is 7.28 GHz,and the corresponding effective absorption width is 2.4 GHz（6.24-8.64 GHz）.Probably because HO-BNNS@PANI has proper conductivity at this time,it exhibits a good surface reflection coefficient and attenuation constant.When electromagnetic waves enter the HO-BNNS@PANI absorber,they can be sufficiently dissipated.In addition,the thermal conductivity of HO-BNNS@PANI composite material is 0.96 W/（m?K）,which is317%of pure PANI.4.The HO-BNNS@Fe₃O₄@PANI nanocomposite with a core-shell structure was designed and synthesized.The thermal performance and microwave absorption performance test results show that the absorber of HO-BNNS@Fe₃O₄@PANI is 3 mm thick,matching When the frequency is 11.36 GHz,the best RLmin value is-49.85 d B,and the effective absorption width is up to 8 GHz.The sea anemone-like structure design plays a particular auxiliary role in microwave absorption.Electromagnetic waves can be reflected multiple times through the HO-BNNS path,and then the magnetic loss,dielectric loss,and polarization processes are converted into heat.What is more interesting is that when the combined content of HO-BNNS is only 13 wt%,the thermal conductivity of HO-BNNS@Fe₃O₄@PANI composite material reaches 0.98 W/（m?K）.5.Two-dimensional HO-BNNS/r-GO heterogeneous materials modified with r-GO were prepared,and then HO-BNNS/r-GO@Fe₃O₄@PANI quaternary absorbing materials were designed and synthesized.Through the coordination between Fe₃O₄,PANI,and r-GO,the impedance matching of the material is enhanced,and the loss mechanism is increased.The test results of composite materials with different PANI content show that when the PANI content is 40 wt%,the composite material HO-BNNS/r-GO@Fe₃O₄@PANI leads the best impedance matching.When the matching frequency and thickness are 7.2 GHz and 3.5 mm,respectively,the RL_minof the absorber is-54.72 d B,and the effective absorber width is 3.12 GHz.At the same time,due to the high thermal conductivity of the HO-BNNS/r-GO heterostructure,the thermal conductivity of the HO-BNNS/r-GO@Fe₃O₄@PANI composite material reaches 1.69W/（m?K）.