| With the rapid development of modern social science and technology,the advanced electronic equipment reduces the size of the circuit a lot for small size.However,this design makes the equipment highly integrate,which leads to a large amount of heat accumulation in the equipment and results in relatively short life of the equipment.Polymer packaging materials have low thermal conductivity and reduce heat dissipation efficiency.Therefore polymer composites with high thermal conductivity are very important and have good prospects for application.Boron nitride have attracted much attention of domestic and foreign researchers cause of its physical properties including excellent oxidation stability,outstanding thermal conductivity,superior thermal stability,good corrosion resistance,chemical stability,electrical property and good lubricity.As one of the inorganic thermal conductive fillers,boron nitride is easy to reunite in the polymer matrix,which affects the performance of thermal conductivity.So,improving the compatibility and dispersion of boron nitride and polymer matrix is very important.Boron nitride is widely used in the oxidation resistance field because of its excellent antioxidant properties.It can be used as antioxidant coating on the surface of carbon fiber.However,there are twodefects in the traditional solution impregnation method.One is that the boron nitride precursor can't be evenly distributed on the fiber surface because the precipitation of precursor is random.The other is gravity and capillary force greatly weakens the covering ability of liquid products in the intermediate process of heat treatment.Therefore,it is of great significance to solve the two defects exiting in nitride boron coating on fiber surface to improve the antioxidant property.Hexagonal boron nitride was non-covalently functionalized with tannic acid The effect of tannic acid content and modification time in the modified system was studied.It was found that tannic acid content(0.6g,1.2g,1.6g)and the modification time(3h,6h,12h)cannot affect hexagonal boron nitride,and there was nearly same tannic acid content on the surface of boron nitride.The effect of tannic to hexagonal boron nitride morphology was investigated and found tannic acid was randomly attached to the surface.The effect of tannic acid to the hexagonal boron nitride structure was studied and found that tannic acid had no effect on the crystal structure of hexagonal boron nitride.The thermal stability analysis of hexagonal boron nitride before and after modification was studied and found boron nitride after modification exhibited good stability when the temperature was less than 250?.The dispersivity of modified hexagonal boron nitride in the solvents including water,ethanol,acetone,tetrahydrofuran and dimethylsulfoxide was investigated.The dispersivity of the modified hexagonal boron nitride in these solvents was better,which could help expand the application area of hexagonal boron nitride.Polyvinyl alcohol/hexagonal boron nitride composites were prepared.The effect of tannic acid dispersibility of hexagonal boron nitride in polyvinyl alcohol matrix was studied.The influence of thermal conductive filler content and filler before and after modification for the composite thermal conductivity was investigated,and the properties of the composites were characterized.The thermal conductive model were analyzed systematically.The results showed that after modification,the dispersibility of hexagonal boron nitride microplatelets in polyvinyl alcohol matrix was prominently improved,and modified filler improved the thermal conductivity of composite materials effectively.This is beneficial to reduce the amount of fillers for fabricating light weight and high thermal conductive composites.The modification and fabrication methods are facile handling,ecofriendly,and suitable for large scale production.The preparation process of boron nitride coating on carbon fiber was systematically studied in this paper.The influence of the monomer concentration,impregnation time and the number of impregnation-drying cycles on the surface morphology of carbon fiber was investigated.The preparation process of boron nitride coating was determined by the research of boron nitride precursor gel.Morphologies about the traditional solution impregnation method and the improved were also compared.The results showed that boric acid and urea precipitated little on the surface of carbon fibers through less impregnation and drying cycles by the traditional solution impregnation.Even if the drying cycles were increased,the precipitation of boron nitride precursor on the fiber surface is still not enough.After many experiments,when the monomer concentration of boron nitride impregnated sol was 0.3 76mol/L,the sol can form a membranous substance on the surface of the fiber.The urea would be dispersed uniformly in the network gel structure of polyacrylamide during the gelation process.When the concentration of the monomer was too low,the sol did not have ability to form the film.The high concentration of the monomer could make the film lose ability of forming membranous substance and cause adhesion between the fibers.After studying the impregnation time and the cycles of impregnation,it was found that the immersion time was 3 min,the monomer concentration was 0.376moI/L,and the cycles was twice could be used as fiber impregnation process.According to the gel research of boron nitride precursor,it was found that when the temperature was 300?.the extending of holding time was beneficial to improve the crystallinity of boron nitride.However,too long time had a negative effect on the product.The extending of the heat preservation time in nitriding stage and the increase of the nitriding temperature were beneficial to the crystallinity of boron nitride.Above all,the boron nitride coating preparation process was selected at 300? for 90min and 850? for 5h.The advantage of this preparation process was the introduction of acrylamide monomer.A gel film formed and evenly distributed the precursors of boron nitride on the surface of carbon fibers in the dipping process and retained the evenness in the heat treatment process.The results showed that the boron nitride coatings,prepared by two cycles of impregnation in the modified dip-coating process,covered the carbon fibers well. |
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