Investigation On The Construction Of Trhree Dimensional Anisotropic Thermal Conductivity Networks For The Performances Of Epoxy Resin-Based Composites

With the development and increasing maturity of 5G technology,electronic components continue to develop towards high power consumption,miniaturization and integration,and the problem of heat accumulation in equipment has become increasingly serious.Ineffective thermal management will lead to electronic equipment jams,circuit damage,and prone to safety accidents.The design and preparation of high-performance thermal interface materials（TIM）has become the main trend to ensure the reliable operation of electronic components.Epoxy resin（EP）is favored in TIM field for its excellent mechanical properties,electrical insulation,and favorable heat resistance,but the inherently low thermal conductivity limits its development.Although the construction of three-dimensional（3D）thermally conductive networks within EP matrix can improve the thermal conductivity of composites to a certain extent,the construction of efficient thermally conductive networks remains a great challenge owing to the limitations of the preparation process and the nature of the filler itself.Herein,the graphene nanosheets（GNS）with ultra-high thermal conductivity is employed in this investigation as the main filler,combining chemical exfoliation,gel compression,hightemperature carbonization and ice-template method to construct a high-quality dense thermal conductive network to enhance the thermal conductivity of EP composites.The main contents of the investigation are as follows:（1）Multi-walled carbon nanotubes（MWCNTs）were radially oxidatively exfoliated and rechemically reduced to obtain graphene nanoribbon-carbon nanotube（GNR-CNT）hybrid materials by potassium permanganate（KMnO₄）under acidic conditions.Using carboxymethyl materials by potassium permanganate（KMnO₄）under acidic conditions.Using carboxymethyl chitosan（CMCS）as the gelation agent,3D GNR-CNT/CMCS-derived carbon（GNR-CNT/C）foams were prepared by mechanical compression combining high-temperature carbonization.The bridging effect of two-dimensional（2D）GNR enabled GNR-CNT to form a multi-branched neuron-like interconnected network structure,which promoted the rapid transmission of phonons.Mechanical compression and high-temperature carbonization not only led to an orderly internal orientation of the GNR-CNT/C foam,but also improved the foam denseness and connectivity,increasing the number of thermally conductive pathways to reduce the interfacial thermal resistance（ITR）.With the 2.43 vol%GNR-CNT/C network content,the out-plane thermal conductivity of obtained GNR-CNT/C/EP composites is 6.79 W·m^-1K^-1 and the thermal conductivity growth efficiency per unit volume（TCE）is as high as 1356%.In addition,GNR-CNT/C foam with high electrical conductivity made the volume resistivity of GNR-CNT/C/EP composite decreased by 1.04×103 Ω·cm,making the composite electrically conductive.（2）Although GNR-CNT/C/EP composites possess excellent thermal conductivity,the gel compression process limits the density of the thermally conductive network and hinders the further enhancement of the thermal conductivity of composites.Thus,the high-density thermal conductivity network was constructed by regulating the composition,structure and content of the thermally conductive fillers using the synergistic effect between different fillers and the ice-template method.The cation-modified GNS/polydopamine-modified MWCNT hybrid foam（GPMF）was prepared by electrostatic self-assembly between the cation-modified GNS（CTAB-GNS）and the polydopamine-modified MWCNT（PDA-MWCNT）in combination with the ice template method,which was incorporated into EP resin to prepare GPMF/EP composites.CTAB-GNS and PDA-MWCNT were tightly stacked under electrostatic interaction and formed a vertically oriented thermal conductive network induced by ice crystals,which provided a high-speed pathway for phonon transport and reduced phonon scattering.When the CTAB-GNS/PDA-MWCNT mass ratio is 9:1 and the hybrid foam content is 9 vol%,the through-plane thermal conductivity of the GPMF/EP composite is up to 30.09 W·m^-1K^-1 and the TCE reaches 1661%.In addition,the insulating cladding of PDA impeded the electron transport in the hybrid foam,causing the composites to exhibit semi-insulating properties.（3）Although GNR-CNT/C/EP composites and GPMF/EP composites have excellent thermal conductivities,the high electrical conductivities of these composites limit their application as TIMs in the field of electronic packaging.In order to make composites simultaneously possess excellent thermal conductivity and insulating properties,GNS with high thermal conductivity were combined with ceramic materials with low thermal conductivity and ternary hybrid filler of C/GNS/MgO with flower cluster structure was prepared by thermal reduction by the aid of the hydration reaction of magnesium oxide（MgO）with EHL as the interfacial layer.The vertically oriented C/GNS/MgO hybrid network was constructed by the ice template method using polyvinyl alcohol（PVA）as the cross-linking agent,which subsequently compounded with EP resin.The introduction of the C/GNS/MgO hybrid network provided an efficient dense thermal conductivity network for the thermal transport of the composites and reduced the ITR.The through-plane thermal conductivity of the C/GNS/MgO/EP composite reaches 4.87 W·m^-1K^-1 with 180%of TCE when the filler content was 12.96 vol%.The MgO encapsulation and the cross-linking of PVA impeded the electron transport in the C/GNS/MgO hybrid network,resulting in an electrically insulating composite with a surface resistivity（ps）and volume resistivity（pv）are 1.32×10¹¹ Ω and 8.13×10¹¹Ω·cm,respectively.（4）Although the combination of MgO coating and GNS successfully prepared EP-based composites with both insulation and high thermal conductivity,the phonon mismatch between the hybrid fillers increased phonon scattering,which seriously affected the quality of the thermally conductive network,resulting in limited improvement of the thermal conductivity of composites.Thus,singular filler GNS was used to construct an insulating thermal conductivity network to reduce phonon scattering and improve the quality of the thermally conductive network.High-quality,low-defect GNS was produced by electrochemical exfoliation,and vertically oriented 3D interconnected PDA-GNS foams（PGF）were successfully constructed by combining surface modification with the Fe²⁺ assisted ice-template method.The complexation between Fe²⁺and PD A-GNS can properly cross-link the PD A-GNS lamellae,reduce the interaction between them and ice crystals,promoting the orientation arrangement of PDA-GNS lamellae in the directional freezing process provides a convenient pathway for phonon transport and effectively reduces the ITR.After compounding with EP resin,the through-plane thermal conductivity of the PGF/EP composites reached 3.48 W·m^-1K^-1 with the TCE of up to 543%when the PDA-GNS content was 3.02 vol%.At the same time,the formation of PDA coating and the complexation of Fe²⁺ ensure that the composites also have good mechanical and electrical insulation properties with a compressive strength of 83 MPa,ρ_s and ρ_v of 2.7×10¹¹ Ω and 1.7×10¹¹ Ω·cm,respectively.This work effectively balances the thermal conductivity,insulation and mechanical properties of the EP composites,and provides a good opportunity for the "5G era" high performance TIM.