The effects of carbon coating onto graphite filler on the structure and properties of carbon foams

Abstract

Carbon foams (CFms) are of crucial importance in electrical and mechanical devices due to their high thermal conductivity used for eliminating high heat flow and maintaining low temperatures [1]. The most common CFms are types of aluminum alloys, copper and diamond. However, the mechanical properties of metal-based heat sink materials are difficult to control at temperatures over 550°C [2,3]. Thus, CFms are actively investigated as heat sink materials because of their good properties, such as high thermal stability, high thermal/electrical conductivity, low density, high porosity and high specific surface area [4,5]. CFms are suitable materials for applications in phase-change materials, such as in latent heat thermal energy storage and lithium-ion batteries [6][7][8][9].CFms are prepared from thermoplastic graphitizable precursors, such as petroleum-derived, coal-derived and mesophase pitch (MP). In particular, MP has been widely used as an appropriate precursor for high-performance carbon materials due to its attractive properties, i.e., high coke yield, low softening point, and high fluidity [10]. Previous processes used a blowing technique, or pressure release, to produce foam from MP. These manufacturing techniques have many disadvantages: the volume fraction of generated cells cannot be precisely controlled, and controlling the shape and distribution of cells is difficult, if not impossible [11]. To better control and tailor the structure of such foams, polymer template methods are actively being researched. Hydrogel template methods are suitable for preparing high-strength CFms due to their large quantities of carbon precursors and the pressure generated through hydrogel shrinkage via heat treatment [10,12]. However, their mechanical strengths are low because of their high porosities.One method for improving the mechanical strength of CFms is adding fillers with a high mechanical strength. Several additives have been used as fillers, including carbon nanofibers, short carbon fibers, graphite and graphene nanosheets [13][14][15][16][17][18][19][20][21][22]. In particular, graphite has beneficial properties that include high lubricating ability, heat resistance, corrosion resistance and thermal/electrical conductivity. It has been widely applied in various fields as a highly functional component with efficient properties. Additionally, the theoretical thermal conductivity of graphite is high, with values ranging from 3000 to 5000 W/mK, and its mechanical strength is 1100 GPa [23,24]. The thermal/mechanical properties of CFms could be improved by the addition of carbon nanofillers with good thermal/mechanical properties through the formation of network structures comprising CFms and carbon nanofillers. Furthermore, carbon nanofillers can be carbon-coated (C-coated) to improve the interfacial bonding with metal. For example, Katzman [25] showed that carbon fibers could be C-coated using amorphous pitch to improve the interfacial bonding with metal oxide and prepare a carbon-fiber-reinforced metal-matrix compo...