Design And Modulation Of Three-Dimensional Graphene Architecture For Using As Thermal Interface Materials

Along with the reduction in the feature size of IC chips and the increase in the packing density of electronic components,the power density per unit volume inside electronic devices has also increased dramatically.If the heat generated during the operation of devices cannot be extracted in time,the accumulation of heat will cause a sharp rise of the internal temperature for electronic components,lead to the serious deterioration of its working efficiency and service life.Highly efficient thermal design is the key to address this issue,and the central part is the selection of thermal interface materials（TIMs）,which are applied to bridge the heat generating components（heater）and the heat sink/spreader to enhance the interfacial thermal conductance.In order to maximally fill the microgaps and to replace the near-adiabatic air(0.026 W m^-1 K^-1)between the mating surfaces of heater and heat sink,TIMs need to have both high through-plane thermal conductivity and good compressibility to achieve a good interfacial heat transfer efficiency.The most commonly used TIMs in the industry are made of polymer-based composites filled with thermally conductive materials,such as BN,Al N,Al₂O₃ etc,to achieve thermal conductivity values of 1-5 W m^-1 K^-1（50-70 wt%filler content is required）.However,with the rapid progress of the semiconductor industry,the accompanying demand for highly efficient heat dissipation is beyond the ability of such conventional TIMs to deal with.Therefore,there is an urgent need to develop new TIMs to meet the ever-increasing thermal management requirement.Graphene is a monolayer of covalently sp²-hybridized carbon atoms with a honeycomb lattice,exhibiting ultra-high intrinsic thermal conductivity of 3,500–5,300 W m^-1 K^-1,by which a large amount of academic interest has been focused on the development of high thermally conductive graphene-based TIMs.Nevertheless,due to the two-dimensional（2D）structure based on weak van der Waals interactions between graphene layers,the thermal conductivity of graphene is anisotropic,that is,graphene has a great capacity to transfer heat along the basal plane（in-plane）direction but a much poorer capacity along the out-of-plane direction(≈2 W m^-1 K^-1).On the contrary,the application of TIMs are putting more emphasis on the heat conduction toward the out-of-plane direction;so the contradiction between this point and the anisotropic thermal conductivity of graphene forms the key scientific and technical issue for the preparation of graphene-based TIMs.Given that,this thesis is aimed at enhancing the thermal conductance capacity of graphene-based materials along the out-of-plane direction for meeting the cross-plane heat transfer from the TIM application.The main research direction of this thesis is focused on the design and modulation of three-dimensional graphene architecture to develop graphene-based TIMs with ultra-high through-plane thermal conductivity.Simultaneously,this thesis studies relationship of the microscopic arrangement of graphene and the component of the interface versus the intrinsic thermal conductivity and interfacial heat conduction property of graphene-based TIMs.The specific research contents mainly includes two chapters,and the results were summarized as follows:Chapter 1:Hierarchically structural graphene/Si C nanowires based TIMIn view of the low through-plane thermal conductivity of conventional graphene paper and the heat dissipation requirements of TIM application along the out-of-plane direction,in this chapter,we proposed a kind of hierarchically structural graphene paper based on graphene-silicon carbide nanowire hybrid structure.The graphene hybrid paper（GHP）was developed by the interaction of Si O₂ nanoparticles（Si O₂NPs）between graphene layers,based on a facile and easy to scale-up filtration method,followed by rapid heat treatment in air.As a result,The Si C nanowires could be in-situ grown and covalently bonded on the surface of graphene sheets based on the carbothermal reduction reaction.The crystal structure of as-grown Si C nanowires is cubic 3C-Si C with a preferred growth orientation along the[111]direction,and the arrangement of Si C nanowires tend to be roughly perpendicular to the graphene surface,contributing to the architecture of GHP composed of loosely stacked graphene sheets bonded with Si C nanowires interlayer.Based on this characteristic structure,and the in-situ growth of Si C nanowires on graphene surface leading to a high interfacial thermal conductance at the graphene/Si C interface,the through-plane thermal diffusivity of GHP(18.4 mm² s^-1)can be improved by≈1.2 times compared to conventional graphene paper(8.2 mm² s^-1).In addition,conventional graphene-based papers lose their through-plane thermal conductivity as a compressive force is vertically applied to them,which is not capable of developing practical TIM applications.In contrast,due to the formation of relatively strong covalent C–Si bonding,the through-plane thermal conductivity of compressed GHP can be even higher(up to 17.6 W m^-1 K^-1)with a compression force of 75 psi.To the best of our knowledge,the GHP possesses the highest through-plane thermal conductivity compared to the other graphene-based papers in the previous literature,and also much better than that of commercial TIMs,such as thermal gels,pads and greases.In TIM performance test we demonstrated that the heater temperature of compressed GHP by a decrease of 18.3°C in TIM performance test（heating power:30W）is superior than 9.8°C of commercial thermal pads(κ:5 W m^-1K^-1)with the cooling efficiency improving by 37.7%.This result is also verified under actual operating conditions of CPU cooling,indicating the outstanding heat transfer properties of GHP for TIM application.Chapter 2:Vertically aligned graphene TIMAiming at the requirements of interfacial heat conductance including high through-plane conductivity and good compressibility for the applied TIM,in this chapter,we developed a soft graphene TIM（named as HLGP）having the compressive modulus of 0.87 MPa comparable to that of silicones,and the through-plane thermal conductivity achieved is a thousand times as high as the silicones,up to 143 W m^-1K^-1,which is also over order-of-magnitude greater than that of most conventional TIMs,and exceeds that of many metals and ceramics.To the best of our knowledge,the proposed HLGP has the highest through-plane thermal conductivity with a similar level of compressive modulus,as compared to various materials,showing great potential for TIMs application.Our samples were made by a feasible machining-based process based on a self-developed automation equipment,which manipulated the stacked architecture of graphene paper from horizontal to vertical orientation.The obtained HLGP show a hierarchical microstructure composed of mainly vertically aligned graphene in the middle and a thin cap of horizontal graphene layers at both top and bottom sides.We demonstrated that the former with the volume fraction over90%endows the HLGP with a high through-plane thermal conductivity up to metal-level.And,the later（cap layer）contributes to a low contact thermal resistance comparable to metallic solders of(5.8 K mm² W^-1),based on an in-depth experimental investigation,followed by a non-equilibrium molecular dynamics（NEMD）simulation analysis.As a result of TIM performance measurement,the system cooling efficiency using HLGP is≈3 times as high as that of state-of-the-art thermal pads(≈17 W m^-1K^-1),deriving from the decrease of the heater temperature by 65°C（our samples）versus 38°C（counterparts）.This result qualifys HLGP a promising candidate for next generation TIM applications.Moreover,due to all-inorganic composition of graphene,the HLGP has excellent thermal cycling stability（<0.5%decay after 2,500 cycles）and wide operating temperature range in air（-196–500°C）,and both are far beyond those of silicone-based thermal pads（-50–200°C）,suggesting potential heat transport applications in extreme environments.Our finding not only provides an efficient and scalable method for the development of graphene sheets having superlative thermal transfer properties,but also insight to the construction of others two-dimensional materials,such BN nanosheets and MXenes,into vertically aligned architectures,to expand their real-world diverse applications.