Preparation Of Flexible Graphite Film And Study On Its Thermal Conductivity Property

Along with the rapid development of electronic technology,and the trend towards miniaturization,densification and high-power of electronic products and devices,an increasing amount of heat has been generated in a finite volume.Therefore,the efficiency of heat conduction and dissipation is critical and considered as an important factor which affects the stability of these products.High thermal conductivity flexible graphite film plays an important role in maintaining system stability,reducing energy consumption,as well as conducting and dissipating heat.In this paper,natural flake graphite was used as main raw material to prepare expandable graphite using HClO₄/KMnO₄/H₃PO₄ system,and then expanded graphite was obtained after high-temperature expansion.After that,high thermal-conducting flexible graphite film was prepared using compression molding method.The thermal conductivity of graphite film,calculated by measuring the room temperature resistivity,was then applied to investigate the effect of raw material properties and pressing process on thermal conductivity of graphite film.Moreover,XRD,XPS,SEM and Raman spectroscopy were applied to investigate the forming process and thermal conduction mechanisms of graphite film.Preparation of expanded graphite showed that the intercalation reaction was very fast when HClO₄/KMnO₄/H₃PO₄ system was applied,and temperature had a significant effect on the oxidation capacity in the solution system.Excessive HClO₄and KMnO₄ enhanced the oxidizing ability in the solution,leading to the decrease of expansion volume of expanded graphite.When the ratio of graphite?g?,HClO₄?mL?,KMnO₄?g?and H₃PO₄?mL?was 3:9:0.3:2,the reaction temperature was 50°C,and the reaction time was 30 min,a high expansion volume of 350 mL/g was obtained.Study on the preparation of flexible graphite film showed that the purity of raw material exhibited little effect on the expansion volume of expanded graphite,while a higher grade significantly increased the thermal conductivity of graphite film.Larger particle size of raw material resulted in the increase of both the expansion volume of expanded graphite and the thermal conductivity of graphite film.As the temperature increased,the expansion volume of expanded graphite increased,while the thermal conductivity of graphite film presented a downward trend.When the fixed carbon content was 99%,the size fraction was-0.500+0.300 mm and the expansion temperature was 600?,the expansion volume of expanded graphite was 300 mL/g and the thermal conductivity of graphite film was 303.15 W/m·K.The thickness,density,and surface flatness of flexible graphite film all affected its thermal property:the thicker the film,the lower the thermal conductivity.As the molding pressure increased,the thermal conductivity of graphite film improved with the increase of graphite film density.By multiple compression and changing the strength of each part,the thickness of graphite film was more uniform,and the surface was more smooth and flat,contributing to the enhancement of thermal conductivity.When fourth compression technique and the compression pressure of 26MPa was applied,and the thickness and density of graphite film were controlled at approximately 0.16 mm and 1.46 g/cm³,the thermal conductivity of graphite film was523.57 W/m·K.The forming process of flexible graphite film:the natural flake graphite was of high purity,crystallinity and of a rough surface with many defects such as micro holes and pits,facilitating the reaction of potassium permanganate with its surface or edge areas,and to some extent,undermining its lamellar structure.Perchloric acid and phosphate was intercalated into graphite layers easily,forming the graphite intercalation compounds,which was then expanded to bulky,porous and worm-like expanded graphite under high temperature.After compression molding,the expanded graphite layers were rotated and slipped,and then interlaced and tightly overlapped to form the flexible graphite film.The thermal-conducting mechanism of flexible graphite film:XRD and Raman spectroscopy demonstrated that the graphite film had a similar high degree of crystallinity and carbon atomic order as flake graphite.SEM analysis revealed that the microcrystal layer in the graphite film was preferentially arranged along the perpendicular direction to the molding pressure,and the layers were tightly laminated,which were all responsible for the excellent thermal conductivity of graphite film.