Study On Multifunctional Conductive Flexible Composite Films For Electromagnetic Shielding Applications

With the development of modern electronics and the arrival of 5G communication era,the pollution problem of electromagnetic wave is becoming increasingly serious.Long-term exposure to electromagnetic wave environment will cause damage to the normal operation of precision equipment and human health.In order to prevent electronic instruments from being affected by electromagnetic interference radiation and freezing in cold weather,as well as the multiple needs of human body shielding electromagnetic radiation and personal thermal management,multifunctional thin film has become a new development direction in the field of electronic devices because of its characteristics of thinness.However,it is still a great challenge to prepare high-performance multifunctional films with good flexibility,mechanical strength and durability.Based on the bacterial cellulose（BC）and aramid nanofibers（ANF）for flexible substrate,intrinsic conducting polymers polypyrrole（PPy）,graphene nanosheets（GNs）,and nickel nanoparticles（Ni NPs）as conductive filler,the electromagnetic shielding composite films with excellent mechanical properties were prepared by vacuum assisted filtration and electron beam evaporation deposition,The application of Joule heating and photothermal conversion performances were explored,and the application field of composite films was broadened.The main research contents are as follows:（1）BC was used as flexible substrate material,PPy was in-situ polymerized on the surface of BC by solution in situ polymerization,and BC@PPy flexible multifunctional composite film was prepared by vacuum assisted filtration technology.The morphologies and structure of BC@PPy composite films were characterized by SEM and TEM,and it was found that PPy could be uniformly deposited on the surface of BC.BC@PPy composite film has good tensile strength（50 MPa）and elongation at break（3%）.Then,the influence of different polymerization time on the conductivity of BC@PPy composite film was analyzed.It was found that the electrical conductivity of BC@PPy composite film increased firstly and then decreased with the extension of polymerization time.When the polymerization time was 6 h,the conductivity of BC@PPy-6 h composite film reached the maximum 134 S m^-1.In addition,the EMI shielding and Joule heating performances of BC@PPy composite film show the same trend as the electrical conductivity.The best EMI shielding effectiveness of BC@PPy-6 h in X-band is 32 d B,and the saturation temperature is 95.4°C at 5 V input voltage.At the same time,BC@PPy composite film has a saturation temperature of 52.4°C under a solar intensity(100 m W cm^-2),showing good photothermal conversion performance.（2）Due to the low intrinsic electrical conductivity of PPy,its application prospect in the field of EMI shielding and low voltage driven Joule heating is seriously restricted.For this purpose,GNs were selected as conductive filler.The GNs/BC composite films with Janus structure were prepared by two-step vacuum assisted filtration method.The XRD and SEM results showed that the GNs/BC composite films with Janus structure were successfully prepared.When the GNs mass loading was 11 mg,the GNs/BC-1 composite film had the maximum tensile strength（40 MPa）and elongation at break（3.4%）.GNs/BC composite film also has adjustable air permeability and good hydrophobicity（CA=100°）.The electrical conductivity of GNs/BC composite film was adjusted by the GNs mass loading.When the GNs mass loading was 44 mg,the electrical conductivity of GNs/BC-4composite film could reach 1261 S cm^-1.In addition,the GNs/BC composite film also demonstrated EMI shielding effectiveness of up to 70 d B in the X-band and excellent Joule heating performance of low voltage driven（2.5 V,111°C）.（3）In order to further improve the mechanical properties and heat resistance of composite films,ANF was introduced into BC to prepare ANF/BC composite films.The experimental results show that when the mass ratio of ANF and BC is 7:3（AFN/BC-3）,the composite films exhibit the maximum tensile strength（95 MPa）,elongation at break（7.5%）and heat resistance as high as 219.1°C,indicating the synergistic enhancement effect of ANF and BC on mechanical properties and heat resistance.On this basis,the ANF/BC-3/GNs composite membrane was prepared by vacuum assisted filtration with the introduction of GNs with high electrical conductivity.With the increase of GNs mass loading,the EMI shielding and Joule heating performances of the composite film also improved synchronously.When the GNs mass loading was 55 mg,The ANF/BC-3/GNs-55composite film demonstrates 38 d B EMI shielding performance,high saturation temperature（145°C）at low input voltage（5 V）,fast temperature response and recovery time（<10 s）as well as good durability and long time operation stability.In addition,the ANF/BC-3/GNs-55 composite film also has good photothermal conversion performance at saturation temperature of 45°C under 100 m W cm^-2 light intensity.（4）The conductive magnetic Ni NPs was deposited on one side surface of ANF/BC-3/GNs-55 composite film by electron beam evaporation deposition technique.The experimental results show that:when the Ni NPs layer thickness is 300 nm,the electrical conductivity of ANF/BC-3/GNs-55/Ni-300 composite film can reach 328 S cm^-1.The increased electrical conductivity and magnetic loss further improve the EMI shielding performance.It shows an EMI shielding effectiveness of more than 50 d B in the X-band,.In addition,the ANF/BC-3/GNs-55/Ni composite film also maintained good Joule heating（5 V,135°C）and photothermal performances(100 m W cm^-2,45°C)of ANF/BC-3/GNs-55composite film.