Fabrication Of Highly Thermally Conductive Composites Via Hierarchical Assembly Of Micro-/Nano- Building Blocks

The miniaturization and higher integration of modern electronics endow the devices with multi-function and improved effiency,which also significantly increase the power density dissipated by electronics.The dissipative heat generated in electronics would directly cause rising temperature and thermal stress,undoubtly deterioating the performance,stability and lifetime of electronics.It is commonly agreed that the limiting factor for the further advance of these devices is not the hardware itself,but rather the development of effective thermal-management materials.The current materials for dissipating heat in electronics are mainly polymer-based composites,which typically have a low thermal conductivity,hardly satisfying the growing demand for high-performance materials that are highly thermally conductive,electrically insulating,mechanically strong,and lightweight.Therefore,there is an urgent need to develop a new strategy to prepare desired materials with high thermal conductivity,which has become the key technology in the research field.The present dissertation will focus on the fabrication of high-performance thermally conductive composites via hierarchical assembly of micro-/nano-building blocks,based on the analysis of present research progress and the trend of development for the thermal management materials.The thermal conductivity,mechanical strength and electrically insulating properties of the obtained thermally conductive composites were investgated based on the relationship between microstructure and interfacial thermal conducting.The main results can be summarized as follows:1.In view of the balancing challenge between thermal and mechanical properties under high filler content,we demonstrate a novel-structured thermally conductive paper via bioinspired assembly of BNNS and GO.The strong lattice vibrational couplings and interactions between BNNS and GO as well as the highly aligned layered structure enable the bioinspired paper to achieve high thermal conductivity and excellent mechanical flexibility.As the weight fraction of BNNS reaches 95%,the as-obtained bioinspired paper exhibits a maximum in-plane thermal conductivity of 29.8 Wm^-1K^-1,superior to other BN-related composite films/papers.We attribute the high thermal conductivity to the following three aspects:1)the perfect orientation in BNNS-GO paper eliminates the interference between the through-plane phonon modes and in-plane modes primarily responsible for thermal energy transfer in strictly defined 2D channels;2)the phonon spectral match between BNNS and GO leads to a reduced interfacial thermal resistance;3)the strong interaction between BNNS and GO could further weaken phonon scattering.In addition,the BNNS-GO paper is an electrical insulator with a volume resistance far beyond 10¹³Ω·cm,and thus has potential applications in thermal management in modern electronic packaging.2.Conventional polymer composites that have been used widely as thermal-management materials suffer from undesired thermal conductivity lower than10 W m^-1K^-1.We report a novel thermally conductive composite paper based on the thought of bioinspired engineering.The composite paper was fabricated by a facile paper-making process based on two-dimensional（2D）BNNSs decorated with silver nanoparticles（AgNPs）,one-dimensional（1D）silicon carbide nanowires（SiCNWs）decorated with AgNPs,and PVA.Specifically,2D BNNSs,1D SiCNWs,AgNP,and PVA are mimicking the 2D aragonite plates,1D nanofibrillar chitin,the mineral bridges between microplatelets from nanometer scale,and protein in ternary natural nacre,respectively.The advantage of the bioinspired papers over conventional composites lies in that they possess a very high in-plane thermal conductivity up to21.7 W m^-1K^-1 along with good mechanical properties and high electrical insulation.We attribute the high thermal conductivity to the improved interfacial interaction between assembled components through the introduction of silver nanoparticles,and the oriented structure based on boron nitride nanosheets and silicon carbide nanowires.The composite paper exhibits a tensile strength of 39.80 MPa,Young’s modulus of11.50 GPa,and tensile toughness of 0.21 MJ m^-3.After the incorporation of AgNPs and SiCNWs,the volume resistance of the composite paper reaches 8.29×10¹³Ω·cm,which is still far beyond the critical resistance for electrical insulation（10⁹Ω·cm）.3.Polymer composites with high thermal conductivity have attracted much attention,along with the rapid development of electronic devices toward higher speed and better performance.However,high interfacial thermal resistance between fillers and matrix or between fillers and fillers has been one of the primary bottlenecks for the effective thermal conduction in polymer composites.We report on engineering interfacial structure of SiCNW/cellulose microcrystal（CMC）paper by generating AgNP.We show that AgNP-deposited SiCNWs as fillers can effectively enhance the thermal conductivity of the matrix.The in-plane thermal conductivity of the resultant composite paper reaches as high as 34.0 Wm^-1K^-1,which is one order magnitude higher than that of conventional polymer composites.Fitting the measured thermal conductivity with theoretical models qualitatively demonstrates the addition of AgNP reduce the thermal resistance at SiCNW/CMC interface from（7-8）×10^-5 m² KW^-11 to（5-6）×10^-5 m² KW^-1 ane the thermal resistance at SiCNW/SiCNW interface from2.9×10^-8 m² KW^-11 to 1.6×10^-8 m² KW^-1.This interfacial engineering approach provides a powerful tool for sophisticated fabrication of high-performance thermal-management materials.4.The conventional method to improve through-plane thermal conductivity of polymer composites usually yields an undesired value(below 3.0 Wm^-1K^-1).The lack of through-plane assembly technology has led to the low effciency of through-plane thermal conductivity enhancement in the polymer composites.Construction of a 3D phonon skeleton is reported composed of stacked boron nitride（BN）platelets reinforced with reduced graphene oxide（rGO）for epoxy composites by the combination of ice-templated and infltrating methods.At a low filler loading of 13.16vol%,the resulting 3D BN-rGO/epoxy composites exhibit an ultrahigh through-plane thermal conductivity of 5.05 Wm^-1K^-1 as the best thermal-conduction performance reported so far for BN sheet-based composites.Theoretical models qualitatively demonstrate that this enhancement results from the formation of phonon-matching 3D BN-rGO networks,leading to high rates of phonon transport.The simulation results show that the intrinstic thermal conductivity of BN-rGO network reaches 105¹15Wm^-1K^-1,the value for BN-CMC network is only 75⁸0 Wm^-1K^-11 for comparison.The incorporation of 3D BN-rGO skeleton into the epoxy resin also produces an increased storage modulus,e.g.,from 2230 MPa for the neat epoxy to 4690 MPa at13.16 vol%BN-rGO.The 3D BN-rGO/epoxy composite exhibits the minimum volume resistivity(2.9×10¹²Ω?cm)which is far beyond the critical resistivity for electrical insulation（10⁹Ω?cm）.5.Previously reported polymer composites exhibit limited enhancement of thermal conductivity,even when highly loaded with thermally conductive fillers,because of the lack of efficient heat transfer pathways.We report vertically aligned and interconnected SiCNW networks as efficient fillers for polymer composites,achieving significantly enhanced thermal conductivity.The SiCNW networks are produced by freeze-casting nanowire aqueous suspensions followed by thermal sintering to consolidate the nanowire junctions,exhibiting a hierarchical architecture in which honeycomb-like SiCNW layers are aligned.The composite obtained by infiltrating SiCNW networks with epoxy resin,at a relatively low SiCNW loading of2.17 vol%,represents a high through-plane thermalconductivity(1.67 W m^-1K^-1)compared to the pure matrix,which is equivalent to a significant enhancement of406.6%per 1 vol%loading.The orderly SiCNW network which can act as a macroscopic expressway for phonon transport is believed to be the main contributor for the excellent thermal performance.This strategy provides insights for the design of high-performance composites with potential to be used in advanced thermal management materials.6.In nanotechnology,the creation of new nanomaterials consistently feeds back into efforts to design and fabricate new macroscopic materials with specific properties.Despite recent progress in preparing numerous types of nanomaterials,it remains a difficult challenge to assemble the tiny building blocks into functional macroscale architectures suitable for practical applications.We report a facile strategy suitable for zero-,one-,and two-dimensional nanomaterials,in which ice-templated assembly and liquid nitrogen-assisted rotation are coupled,for fast and large batch of processing of macroscopic spongy spheres with tunable microstructures.The obtained spongy spheres possess a hierarchical structure with an interpenetrating,open-porous,urchin-like network and abundant external voids.The geometrical shapes of the frames,combined with the ice-assisted growing to all directions,facilitate the well-defined connections between materials in accordance with the innermost kernel.By employing this unique three-dimensional network,we demonstrate the great potential of spongy spheres in thermal management.Take SiCNW sphere as an example,the SiCNW sphere/epoxy composite exbihit a high thermal conductivity of0.91 Wm^-1K^-1 under a filler content of 4.6 vol%,larger than the composites incorporated with commericial thermal fillers.Automatic propulsion devices with syringes can be used for scalable production of spongy spheres.