Structure Design And Performance Regulation Of Cellulose-based Thermally Conductive Composites

With the high integration and miniaturization of electronic devices,efficient thermal management system has become the technical guarantee for the stable operation and excellent performance of electronic devices.Among them,polymer-based thermally conductive composites（PTCs）have become an important part of the thermal management system due to their advantages of lightweight,electrical insulation,corrosion resistance,and easy processing.However,the matrices of most PTCs are derived from non-renewable petrochemical resources.Besides,with the renewal speed of electronic products continuously accelerating,the rapid growth of electronic waste has posed a severe challenge to the environment.Currently,under the dual threats of global energy shortage and environmental pollution,it has become crucially significant to develop sustainable thermally conductive composites by using the biomass resources.Cellulose is the most abundant polymer in nature.It has excellent degradability,renewability,low thermal expansion coefficient,and excellent mechanical strength.Therefore,researchers have combined cellulose and thermally conductive fillers to design various green and environmentally-friendly composites with high thermal conductivity.At present,despite a series of advances,the cellulose-based thermally conductive composites still exist several problems such as low thermal conductivity,strong hydrophilic and hygroscopicity,flammability,and deteriorated mechanical properties.And it is still facing many key scientific and technical problems to construct multifunctional cellulose-based thermally conductive composites with high thermal conductivity,flame retardancy,mechanical strength or hydrophobic properties through the interfacial design and modification of cellulose/fillers composites.In order to promote the practical application of cellulose-based thermally conductive composites,this paper focuses on the existing bottlenecks,and systematically investigated the construction of new nanohybrid fillers,interfacial regulation of composites,functionalized modification,and the relationship between structure and performances.The main research contents and conclusion are listed as following:（1）Water-dispersible hydroxylated boron nitride nanosheets（BNNS）were successfully prepared by alkali treatment of commercially available h-BN and subsequent sonication-assisted exfoliation.And inspired by the orderly hierarchical architecture from natural nacre and marvellous microscaled papillae structure from lotus leaves,a highly thermally conductive and superhydrophobic cellulose-based composite was successfully prepared via a vacuum-assisted assembly method and subsequent surface modification.The results showed that the as-obtained CNFs-based composite with 50 wt%OH-BNNS exhibit a high in-plane thermal conductivity(15.13 W·m^-1·K^-1)owing to the orderly hierarchical architecture and strong hydrogen bonding interaction.Moreover,the modified CNFs/OH-BNNS composites also achieve excellent hydrophobic properties（contact angle over 155^o）and a simultaneous water resistance.The research realized the effective integration of high thermal conductivity and superhydrophobicity in cellulose-based composites through dual bioinspired design,laying a methodological foundation for the bioinspired design of other multifunctional nanocomposites.（2）A new type of nanohybrid（BNNS@ND）was constructed through a simple electrostatic self-assembly of nanodiamond（ND）and BNNS.Furthermore,the obtained BNNS@ND was applied into the CNFs matrix,and explored the effect of BNNS@ND on the structure and performances of CNFs-based composites.The results showed that the ND with a particle size of 4-6 nm can be stably adsorbed on the surface of BNNS through electrostatic interaction.The prepared BNNS@ND can be closely stacked with CNFs and form a highly oriented"brick-mortar"structure along the plane direction,which provides a good thermal conduction pathway for the phonons between adjacent boron nitride nanosheets.After incorporating the same amount of thermally conductive fillers,the in-plane and through-plane thermal conductivity of CNFs/BNNS@ND-3 composite reached 18.54 W·m^-1·K^-1 and 0.77W·m^-1·K^-1,respectively,which were significantly higher than that of CNFs/BNNS composite.（3）A modified alumina（m-Al₂O₃）with positive Zeta potential was prepared by the chemical grafting of nano-alumina usingγ-aminopropyltriethoxysilane,and then the BNNS@m-Al₂O₃ nanohybrid was constructed by electrostatic interaction with BNNS.The effect of BNNS@m-Al₂O₃ nanohybrid on the structure and properties of cellulose-based composites were systematically studied.The results showed that,unlike the direct physical blending of BNNS and Al₂O₃,the m-Al₂O₃ can be uniformly and stably attached to the surface of BNNS without obvious agglomeration.After compounding the BNNS@m-Al₂O₃with CNFs,the in-plane thermal conductivity of CNFs/BNNS@m-Al₂O₃-3 composites is as high as 12.65 W·m^-1·K^-1,which is about 7.2 times that of pure CNFs film.Besides,the through-plane thermal conductivity of CNFs/BNNS@m-Al₂O₃-5 composite can reach up to0.58 W·m^-1·K^-1,which is significantly higher than CNFs/BNNS film(0.35 W·m^-1·K^-1)even containing the same amount of filler.The design of low-cost BNNS@m-Al₂O₃ further expands the scope of the“point-plane”structure nanohybrids,which provides an opportunity for the large-scale applications of nanohybrids in thermally conductive composites.（4）Carboxylated carbon nanotubes（c-CNTs）were prepared by acidizing carbon nanotubes,and they were compounded with BNNS and CNFs to construct CNFs/BNNS/c-CNTs thermally conductive composites.Furthermore,the CNFs-based composites were modified with calcium ions,and systematically analysed the cross-sectional morphologies,thermal conductivity,interfacial bonding,mechanical properties and thermal stability of the Ca²⁺-CNFs/BNNS/c-CNTs composites.The results showed that,benefiting from the double interfacial interaction of hydrogen bonds and coordination bonds in the Ca²⁺-CNFs/BNNS/c-CNTs composite and highly oriented structure,the prepared composite exhibits a high in-plane thermal conductivity of 13.46 W·m^-1·K^-1 even filling with only 50 phr（33.33 wt%）of thermally conductive fillers.Moreover,the strong interfacial interaction also endows the composites with excellent mechanical properties.The tensile strength and strain of the Ca²⁺-CNFs/BNNS/c-CNTs composite reached 134 MPa and 2.5%,respectively,which are significantly superior than CNFs/BNNS composite.Additionally,the thermal stability of composites was also effectively enhanced.This study may provide a new idea for the structural design of high-performance thermally conductive composites.（5）In view of the fire risk of cellulose-based thermally conductive composites,the ammonium polyphosphate functionalized BNNS（BNNS-p-APP）was compounded with CNFs to prepare a cellulose-based composite with a nacre-like layered oriented structure.The effect of BNNS-p-APP on thermal conductivity,flame retardancy,thermal stability and mechanical properties of CNFs-based composites were systematically explored.The results showed that the CNFs/BNNS-p-APP composite exhibited a high in-plane thermal conductivity of 9.1 W·m^-1·K^-1.The immobilization of ammonium polyphosphate and the unique“brick-mortar”layered architecture effectively suppresses the release of combustible volatiles and restrains the exterior heat and oxygen permeation into the inner matrix,which can combine with the catalyzed charring effect of flame-retardant molecules to significantly enhance the flame resistance of CNFs composites.The CNFs/BNNS-p-APP composite not only realized the synergistic integration of high thermal conductivity and flame retardancy,but also showed remarkable flexibility,folding endurance,and mechanical robustness,which promise huge potential in the thermal management applications of advanced electronics.