Structure Design And Properties Of Cellulose Nanofibers Based Conductive Composite Papers

Cellulose nanofibers,derived from natural plant fibers,are a novel type of onedimensional(1D)nanomaterials with excellent features of extensive sources,green and renewable,and excellent physical and chemical properties.In particular,composite papers with CNFs as the skeleton have gained tremendous attention because of great flexibility,lightweight,excellent mechanical properties,and easy processing.However,limited by insulation and single function,CNFs-based composite papers are difficult to meet the requirements of practical application.Silver nanowires(Ag NWs)and transition metal carbide and/or nitride(MXene)are the typical 1D and 2D conductive nanomaterials featuring excellent electrical conductivity,thermal conductivity,and easy processability in aqueous solution,respectively,are widely applied in the fields of flexible electronics,electromagnetic interference(EMI)shielding,thermal management,and energy storage.Introducing CNFs in cooperation with Ag NWs and MXene to construct the complete phonon/electron transmission networks is one of the effective strategies to enhance the mechanical properties and electrical and thermal conductivity of the composite papers.Nevertheless,the conductivity of the composite papers is heavily weakened due to the inherent insulation of CNFs,consequently compromising the conduction associative functions.This dissertation starts with the basic structure of CNFs,and aims at exploring the construction approach of CNFs-based composites applied in high-tech fields and addressing the issues in balancing high electrical and mechanical properties of CNFs-based composite papers.The work constructs a series of multifunctional CNFs-based conductive composite papers based on the idea of “materials selection-structure design-performance optimization”,and illuminates the mechanism of the composite papers with synchronously improved electrical and mechanical properties.Potential applications involving EMI shielding and thermal management for the composite papers are also discussed.It can provide theoretical basis for high value and functionalization utilizations and deep development of plant nanocellulose.The paper mainly includes the following aspects:(1)To address the problem of low conductivity of CNFs-based composite papers,a successive vacuum filtration-assisted self-assembly strategy is employed to assemble Ag NWs onto CNFs substrate,successfully preparing highly conductive CNFs-Ag NWs composite papers with Janus structure.Thermal welding is further performed to effectively eliminate the interfacial resistance caused by the cylindrical structure and poly-(vinylpyrrolidone)(PVP)layer on the surfaces of Ag NWs,enhancing the junction connection and constructing the complete 3D-interconnected conductive networks.The obtained CNFs-Ag NWs composite papers present excellent mechanical strength(176.75 MPa),low sheet resistance(0.60 Ω/sq),and superb electrical stabilities.The composite papers can be applied in electrical heaters and possess fast electro-thermal response(< 10 s),high heating temperature(～ 220 °C),and longterm stability(> 186 h)and repeatability(> 20000 cycles)with low Ag NWs contents and driving voltages(0.5-5 V).Moreover,large-scale production feasibilities of the composite papers are demonstrated and assembled into multifunctional applications,including personal thermal management,healthcare thermotherapy,multifunctional cups,indoor heating and so on.(2)This work aims at addressing the problems of the inherent structure defect of 1D Ag NWs and poor interfacial interactions between Ag NWs and CNFs.A two-step vacuum filtration method is used to assemble conductive fillers constructed by the complementary integration of 1D Ag NWs and 2D MXene onto CNFs substrate,fabricating Janus structural CNFs-MXene/Ag NWs composite papers with highly efficient 3D conductive networks.MXene is hybridized with Ag NWs to improve the integrity of the conductive functional layer and its adhesion with CNFs substrate.Benefitting from the synergistic effect of Janus structure design,3D MXene/Ag NWs conductive networks,and high-performance CNFs substrate,the resultant papers possess excellent mechanical strength of 224.0 MPa,superb Young’s modulus of 10.0 GPa,and high electrical conductivity of 2069.3 S/cm.In views of high conductivity and layered conductive network,the composite papers present excellent EMI shielding effectiveness(EMI SE)of 54.1 d B.Compared with the composite papers prepared by a onestep vacuum filtration method,the composite papers with Janus structure possess greater advantages in electrical,mechanical,and EMI shielding performances.Besides,the composite papers also possess desirable Joule heating performance,including low input voltage,short response time(< 10 s),and superb heating stability and reliability,which makes them suitable for high-performance EMI shielding and thermal management applications.(3)This work aims at addressing the problems of limited mechanical performance improvement of the composite papers and the inflammability of CNFs.Inspired by natural nacre,natural montmorillonite(MMT)nanosheets are introduced into CNFs to construct the CNFs/MMT substrate with “brick-mortar” layered structure by a vacuum filtration method,which greatly enhances the mechanical properties of CNFs substrate via covalent bonding(Al-O-C)and hydrogen bonding interactions and endows it with outstanding fire resistance.2D MXene flakes are further deposited on the CNFs/MMT substrate using a vacuum filtrationassisted self-assembly method,manufacturing Janus structural CNFs/MMT-MXene conductive composite papers with great fire safety.Wherein,the CNFs/MMT layer ensures the excellent mechanical properties of the composite papers while the MXene layer endow them great conductivity.The designed papers possess excellent mechanical properties(tensile strength and Young’s modulus are 272.7 MPa and 10.8 GPa,respectively),high conductivity(2222.5 S/cm),and impressive EMI shielding performance(56.1 d B).In addition,the composite papers also present impressive fire resistance and electro-/photothermal heating features,demonstrating their reliability,safety,and versatility.(4)This work aims at addressing the defects of weak interlayer interaction and easy oxidation of 2D MXene.A strategy of mussel-inspired biomimetic chemistry is proposed to construct dopamine(DA)crosslinked high strength and toughness MXene films and successfully address the above problems by improving interflake interaction and arrangement of MXene.DA undergoes in situ polymerization and binding at MXene flakes surfaces by spontaneous interfacial charge transfer,forming an ultrathin polydopamine(PDA)adhesive layer.Ambient stability of MXene and its assembled films is greatly improved by efficient screening of oxygen and moisture.In particular,PDA decoration further promotes the innate high electrical conductivity of MXene films.Then,PDA-decorated MXene(PDDM)is deposited on the CNFs/MMT substrate using a two-step vacuum filtration-assisted selfassembly strategy,constructing Janus structural CNFs/MMT-PDDM conductive composite papers with great ambient stability and resulting in significantly improved mechanical properties with tensile strength of 325.5 MPa and Young’s modulus of 15.4 GPa while maintaining excellent conductivity(3307.7 S/cm).Moreover,Janus structure design enhances the EMI shielding and Joule heating performances of the composite papers,while PDA decoration ensures their reliability of long-term applications.This work provides a new idea for the construction of the composite paper materials with high conductivity,high strength and ambient stability.