Microstructure And Properties Of Thermally Conductive Cellulose Nanocrystal Composites

With the increased integration degree and miniaturization of electronic devices,the thermal management has become the key to guarantee the performance,life-span and operating stability.It is significantly important to design high-performance thermal interface materials(TIMs)to ensure the effective heat dissipation of electronic device.However,due to the poor interfacial interaction and aggregation of nanofillers,the TIMs suffer from limited thermal conductivity and could not meet the strict requirements of modern electronic equipment for heat dissipation.Cellulose nanocrystal(CNC),a naturally abundant nanomaterial with excellent dispersibility,mechanical properties and low coefficient of thermal expansion,has attracted intensive attentions.In this dissertation,CNC was used to improve the dispersibility of reduced graphene oxides(RGO),boron nitride nanosheet(BNNS),and silver nanowires(AgNW)in composites,and enhance the interfacial interaction between fillers and matrices.Moreover,CNC could promote the orientation of nanofillers in composites,which is beneficial for constructing continuous thermal conductive pathways.This dissertation focuses on the effects of interfacial interaction and ordered structure on the thermal conductivity of composite.The main works are as follows:(1)A facile hybridization approach was implemented to prepare uniform CNC-RGO hybrids using CNC as the dispersant.Then the CNC-RGO hybrids were compounded with poly(ethylene oxide)(PEO)to fabricate PEO/CNC-RGO composites.The effect of the mass ratio of CNC to RGO on the dispersion of RGO was investigated.The relationship between the dispersion of CNC-RGO hybrids and the thermal stability,mechanical properties,and thermal conductivity of nanocomposites were studied.The results showed that the interfacial interaction between fillers and matrix was enhanced by the non-covalent functionalization of RGO with CNC,leading to improved mechanical properties and thermal conductivity of composites.When 15 wt% of CNC-RGO hybrids were loaded,the tensile strength,Young's modulus and thermal conductivity(TC)of the PEO/CNC-RGO composites rose by 90.1%,168.4% and 152.4%,respectively,compared with pure PEO.(2)By utilizing carboxylated CNC as the dispersant and liquid-crystal template,CNC/AA/RGOEISA composites with ordered structure were prepared from CNC matrix and RGO filler via the facile solvent evaporation induced self-assembly(EISA)approach.The epoxy based adhesive agent(AA)was utilized to enhance the interfacial interaction between the fillers and the matrix.The effects of interfacial interaction and alignment of the fillers on the mechanical properties and thermal conductivity were investigated.The results showed that not only the dispersibility of RGO was improved,but also orientation structure was constructed during the solvent evaporation process with the aid of CNC.Thermally conductive pathways were formed in the composite,resulted in a greatly improved thermal conductivity.The mechanical properties were also enhanced owing to the robust crosslinked structure induced by AA.When the RGO filling content is 7.3 wt%,the tensile strength and in-plane TC of the composite were 116.5 MPa and 20.2 W/m K,respectively.(3)The amino-functionalized BNNS was prepared through the urea-assisted ball milling method,then compounded with carboxylated CNC to form well-dispersed CNCBNNS hybrids.Using CNC as the template,CNC/AA/BNNS composites with ordered structure were fabricated through EISA method.The results showed that CNC could promote the uniform dispersion of BNNS in the water and induce the alignment of BNNS in the composite.The mechanical and thermal conductive properties were enhanced owing to the continuous pathways formed by aligned BNNS and reinforced the interfacial interaction by AA.The tensile strength of the CNC/AA/BNNS composites reach up to 197.3MPa,which is 118.7 % higher than of CNC/AA,when 20.0 wt% of BNNS was loaded.Meanwhile,the in-plane TC of CNC/AA/BNNS composites reach up to 10.9 W/m K.(4)The CNC/AA/AgNW composites with ordered structure were fabricated via EISA method using carboxylated CNC as both the dispersant and liquid crystal template.The effects of dispersibility and alignment of AgNW on the mechanical and thermal conductive properties were studied.It was found that CNC improved the dispersibility of the AgNW by the hydrogen bonding interaction.Meanwhile,thermal conductive pathways were constructed owing to the alignment of AgNW induced by CNC.When the loading of AgNW is 40 wt%,the tensile strength CNC/AA/AgNW composites are 47.3 MPa,which is 126.3 %higher than CNC.At the same loading,the in-plane TC of the CNC/AA/AgNW composite increase to 11.3 W/m K.