| Thermally conductive polymers are widely used in microelectronics,mobile devices,aerospaces,electrical power,energy and heat exchange fields due to their good processing property and low cost.However,the thermal conductivity of most polymers is too poor to meet practical applications,and it is necessary to add fillers with high thermal conductivity to improve them.In this paper,epoxy resin(EP)and hexagonal boron nitride(h-BN)was used as matrix and the main thermal conductive filler,respectively.The thermal conductivity of composites was systematically studied from the following aspects:non-covalent and covalent modification of h-BN,influence of the addition of polyethylene glycol(PEG)on the properties of EP,and the construction of a three-dimensional thermally conductive network with cotton and sepiolite fibers as skeletons.The main contents are as follows:(1)Polyvinyl alcohol and benzaldehyde were reacted in an aqueous solution to synthesize polyvinyl benzaldehyde(PVB)to non-covalently coat modified h-BN.The thermal diffusivity of epoxy composites prepared by modified particles with different PVB contents indicated that the non-covalent coating was easily too thick to cause interfacial thermal resistance and agglomeration.In order to solve this problem,a new"solvent-assisted dissolution-diffusion" method was proposed,which can make h-BN secondary disperse,reduce the thermal resistance caused by the excessively thick coating layer,and at the same time promote the filler to form a thermally conductive network.When the content of h-BN@PVB particles was 40 wt%,the thermal conductivity of composite reach 0.89 W/m·K,which was 4 times higher than that of raw EP.(2)We found that thionyl chloride can etch h-BN at room temperature and introduce a large number of hydroxyl and amino groups on its surface,and the content of amino groups was greater than that of hydroxyl groups.We then proposed the etching mechanism,proving that the formation of boric acid makes the content of amino groups higher than that of hydroxyl groups,and the H+and OH-could affect the defect structure of etched boron nitride by affecting the generation of boric acid.The thermal conductivity of the composite material showed that the interface between the etched TBN and EP resin was improved due to the rough surface and the existence of a large number of functional groups,which compensated the decrease of its own performance caused by the defect,so the thermal conductivity of composite material was increased accordingly.(3)Based on the previous studies,we summarized and predicted that some reagents with X=O(X=C,S,N,P,Cr)structure could be used to etch h-BN,and confirmed that phosphorus oxychloride and sulfonyl chloride have this capability,and the etching mechanism was the same as thionyl chloride.Then,the S-BN obtained by etching with sulfonyl chloride was reacted with furoyl chloride to introduce furan groups,and PEG was covalently introduced on the surface of S-BN by Diels-Alder reaction.When the covalently modified filler content was 50 wt%,the thermal conductivity of composite reach 1.14 W/m·K,which was about 5 times higher than that of raw EP.(4)EP was modified by long chain molecules such as PEG,polypropylene glycol(PPG),etc.with different molecular weights.The results showed that with the increase of PEG,PPG content,the cured product changed from hard to soft state,and with the lower Tg and worse mechanical properties,and the thermal conductivity decreased accordingly.In order to obtain a composite material with high thermal conductivity,the surface of the absorbent cotton fibers was dissolved by urea-sodium hydroxide solution and h-BN was adhered to it.A foam material was obtained after rapid drying,and then impregnated into the modified resin to prepare composite material.When the content of h-BN was 20 wt%,a large amount of h-BN was coated on the surface of cotton fibers and overlapped with each other to form a thermal conduction path,which made the thermal conductivity of the composite material reach 0.58 W/m·K.(5)After the sepiolite fiber,h-BN and metal tin(Sn)powder were mixed uniformly,a filter cake was obtained by suction filtration,and then the low melting point Sn was heated and melted to penetrate into the surrounding voids.The Sn particles formed a layered structure under the action of sepiolite,and after the filter cake was heat treated,the h-BN and sepiolite around Sn would be embedded into its surface to improve the interface between the fillers.Therefore,when the Sn content was about 47.2 wt%,the thermal conductivity could reach 3.37 W/m·K.The h-BN added to the system could prevent the molten Sn from contacting each other and conduct electricity,but did not hinder the heat conduction,so the volume resistivity of the sample was still greater than 10~9Ω·m. |
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