Structural Construction And Performance Research Of Cellulose Nanofiber-Based Electromagnetic Interference Shielding Composite Films

With the flourishing development of information technology,especially the explosive advance of miniaturized,highly integrated and high-power electronic devices in the fifthgeneration（5G）communication era,which brings convenience to human life while causing severe electromagnetic radiation.The undesired electromagnetic radiation not only deteriorates the accuracy and dependability of electronic equipment during operation,but also poses a serious threat to human health.Therefore,it is urgently required to exploit high-performance electromagnetic interference（EMI）shielding materials.Polymer-based EMI shielding materials have aroused considerable attention in the field of EMI shielding owing to their advantages of lightweight,good processability,corrosion resistance and tunable electromagnetic parameters.Nevertheless,traditional polymer-based EMI shielding materials mainly rely on surface severe electromagnetic waves（EMWs）reflection to realize high-efficiency EMI shielding performance,which irreversibly causes secondary EMWs pollution.Such EMI shielding materials cannot fundamentally tackle the harm generated by electromagnetic radiation.It has become an effective strategy to reduce EMWs reflection by using interface polarization,dielectric loss and magnetic loss to dissipate and attenuate EMWs.In addition,the increasingly prominent energy dilemma and environmental pollution make it imperative that petroleum-based polymers are gradually replaced by renewable biomass materials with green,environmentally friendly and biodegradable feature.Furthermore,the rapid growth of portable electronic devices and wearable electronics as well as the complexity of application scenarios of EMI shielding materials have put forward higher requirements for the flexibility and versatility.Herein,this research employs cellulose nanofiber（CNF）as matrix,CoFe₂O₄@MXene and silver nanowires（AgNWs）as functional fillers to fabricate highly efficient EMI shielding composite films with low EMWs reflection characteristics through structural regulation.Meanwhile,the application of composite films in the electro-thermal conversion field is explored.The main contents and conclusions are summarized as follows:（1）To solve the problem of unsatisfactory EMI shielding performance of conventional EMI shielding materials,fillers with different dimensional advantages are used to synergistically construct high-efficiency conductive/magnetic network,and thus improving EMI shielding effectiveness（SE）.CoFe₂O₄@MXene,AgNWs and CNF were first uniformly blended.Then,ultrathin and flexible CoFe₂O₄@MXene/AgNWs/CNF composite films with homogeneous structure were prepared by a facile vacuum-assisted filtration strategy.Scanning electron microscopy analysis reveals that CoFe₂O₄@MXene and AgNWs are evenly distributed in the CNF matrix,and the one-dimensional AgNWs acts as conductive "bridges" to connect CoFe₂O₄@MXene to form perfectly conductive pathways.As the mass ratio of AgNWs to CNF is 5:10,the homogeneous composite film with a thickness of only 0.1 mm exhibits excellent electrical conductivity（29.3 S/cm）and EMI SE（36.2 dB）,but its reflection coefficient is as high as 0.92.Moreover,outstanding electrical conductivity endows the prepared composite films with brilliant electro-thermal response ability.The prepared homogeneous structured CoFe₂O₄@MXene/AgNWs/CNF composite films with flexible and ultrathin characteristics present broad application prospects in the fields of wearable electronics and thermal management.（2）The reflection coefficient of the homogeneous CoFe₂O₄@MXene/AgNWs/CNF composite films prepared based on the impedance mismatching strategy is still high,which results in seriously secondary EMWs pollution.To address the issues of irreconcilable impedance matching and severe surface EMWs reflection of homogeneous structured composite films,the design strategy of constructing asymmetric layered architecture in composite films is proposed.CoFe₂O₄@MXene-AgNWs/CNF composite films with asymmetric layered architecture consisting of a low-conductive CoFe₂O₄@MXene/CNF layer（impedance matching layer）and a highly conductive AgNWs/CNF layer（shielding layer）were prepared via a sequential vacuumassisted filtration approach.Profiting from the rational arrangement of the impedance matching layer and shielding layer,as well as the coupling effect of electric and magnetic losses,the resultant asymmetric layered composite film achieves an ultrahigh EMI SE of 73.3 dB at only 0.1 mm thickness,which is 102.5%higher than that of homogeneous structured composite film（36.2 dB）at the same filler loading.The reflection coefficient of asymmetric layered composite film is 0.68,which is 26.1%lower than that of homogeneous structured composite film（0.92）.At the same time,CoFe₂O₄@MXene-AgNWs/CNF composite films with asymmetric layered architecture reveals excellent EMI shielding reliability.The EMI SE retention of composite films exceeds 95%even suffering from continuously physical deformations and long-term chemical attacks.Additionally,the prepared composite films present extraordinary mechanical properties and thermal management capability.This work provides a feasible strategy for exploiting highperformance EMI shielding composite films with attractive thermal management capacity.The prepared composite films present extensive application potential in aerospace,artificial intelligence,stealth technology and the national defense industry.（3）Although the construction of asymmetric layered structure could greatly enhance EMI shielding performance and alleviate the surface EMWs reflection,the specific relationship between the number of layers and EMI shielding performance of multilayered composites remains unclear.Herein,alternating multilayered CoFe₂O₄@MXene-AgNWs/CNF composite films were fabricated through a controlled alternating vacuum-assisted filtration process by employing CNF as matrix,CoFe₂O₄@MXene and AgNWs as functional fillers,which are used to clarify the specific effect of the number of layers on EMI shielding performance.CoFe₂O₄@MXene/CNF layer and AgNWs/CNF layer are arranged alternately in the alternating multilayered CoFe₂O₄@MXene-AgNWs/CNF composite films.The electrical conductivity and EMI shielding performance of the composite films enhance with the increasing internal interfaces（i.e.,number of layers）.Based on the layered design of asymmetric EMI shielding network,the electrical conductivity and EMI shielding performance of the alternating six-layered composite films are 793.4 S/cm and 87.8 dB,respectively,which are 96.9%and 19.8%higher than those of asymmetric layered composite films.However,the reflection coefficient of alternating multilayered CoFe₂O₄@MXene-AgNWs/CNF composite film rises to 0.72,which is 5.9%higher than that of the asymmetric layered composite films（0.68）.This is probably ascribed to the reduction of the thickness of CoFe₂O₄@MXene/CNF impedance matching layer.Moreover,the alternating multilayered composite films present controlled Joule heating performance,which offers insights for the multifunctional design of EMI shielding materials.（4）In order to further reduce the reflection coefficient of the composite films and realize the absorption-dominated EMI shielding mechanism,the strategy of constructing a double-gradient structured EMI shielding network is proposed.CoFe₂O₄@MXene with different CoFe₂O₄ contents were synthesized by solvothermal method.A double-gradient EMI shielding network with positively conductive gradient and negatively magnetic gradient along the incident direction of EMWs is constructed in the composite films by using CNF as matrix,CoFe₂O₄@MXene with various CoFe₂O₄ contents and AgNWs as functional fillers.Benefiting from the design of dualgradient EMI shielding network and the "absorption-reflection-reabsorption" EMWs attenuation mechanism,the CoFe₂O₄@MXene（90-50-10）-50AgNWs/CNF composite film with doublegradient EMI shielding network obtains a high-efficiency EMI SE（82.9 dB）and a low reflection coefficient of 0.42.The prepared composite films simultaneously realize highly efficient EMI shielding performance and low EMWs reflection.Meanwhile,the increase of CoFe₂O₄@MXene/CNF absorption layer（impedance matching layer）thickness could remarkably decrease reflection coefficient.The increasing AgNWs/CNF reflection layer（shielding layer）thickness could significantly improve EMI SE.In addition,outstanding electrical conductivity enables the prepared composite films to achieve excellent and low-voltage driven Joule heating performance.The surface temperature of the composite film reaches 108℃in 10 s under an applied voltage of 3.0 V.This study not only provides a novel idea for the design and preparation of flexible EMI shielding composite films with high-efficiency EMI SE entailing low EMWs reflection and satisfactory Joule heating performance,but also broadens the applicability of multifunctional composite films in next-generation wearable electronic devices and personal thermal management systems.