Improved heat-spreading properties of fluorinated graphite/epoxy film

Copyright © Korean Carbon Society http://carbonlett.org The use of electronic devices has rapidly increased, and such devices are required to be smaller, thinner, lighter, and with higher energy densities than ever before. These requirements can result in the generation of additional heat during device operation, and this heat has detrimental effects on performance, stability, running rate, and lifespan. For instance, if the temperature of an electronic device increases by 2oC when the operating temperature is higher than the stable operating temperature, the stability of the device will be reduced by 10% [1-3]. Thus, the additional heat that such devices generate must be spread to the surrounding environment via heat sinks, coatings, or films with high thermal conductivity. The use of coatings and films is an appropriate approach for improving the heat spread in electronic devices, particularly as these devices become lighter, smaller, and thinner. Such coatings and films are typically based on polymers such as epoxy, acryl, and polyurethane. Due to the low thermal conductivity of these polymers, it is difficult to use this approach to achieve effective heat spreading. To solve this problem, a great deal of research has addressed the addition of fillers with high thermal conductivity, such as aluminum nitride [4,5], silica (SiO2), alumina (Al2O3), boron nitride [1,6,7], graphite [8], expanded graphite (EG) [9-11], carbon nanotubes (CNTs) [11-13], graphene [9,14,15], and graphene oxide (GO) to polymer-based coatings. Recently, carbon-based materials such as graphite, EG, CNTs, graphene, and GO have been widely investigated because of their low density and high thermal conductivity. However, CNTs and graphene are not appropriate for large-scale applications due to their high production cost and complex processing. In addition, fillers with poor dispersive stability aggregate in coatings form voids, resulting in the degradation of thermal conductivity [16,17]. In this study, anionic fluorine functional groups are introduced into a graphite surface via direct fluorination to increase dispersive stability by generating electrostatic repulsion. We expect the enhanced dispersive stability of the modified graphite to lead to the formation of fewer voids, resulting in improved thermal conductivity of the prepared heat-spreading film. Diglycidyl ether of bisphenol A (DGEBA; YD-128, Kukdo Chemical Co., Korea) with a viscosity of 11,500 to 13,500 cP was used as the epoxy monomer, and polyamide resin (G-640, Kukdo Chemical Co.) with a viscosity of 8,000 to 12,000 cP was used as the curing agent for the epoxy resin. Acetone was used as a diluent for the epoxy resin. Fluorine gas (99.8%, Messer Griesheim GmbH, Germany) was used to introduce anionic fluorine functional groups into the graphite surface. The following fluorination method was used for this introduction. Graphite was loaded into a nickel boat and placed into a batch reactor. Degassing was performed for 1 h at room temperature using a vacuum pump. Subsequently, a mixture of fluorine gas and nitrogen gas (at a partial pressure ratio of F2:N2=1:9) at a total pressure of 1 bar was placed into the reactor, and fluorination was conducted for 10 min at room temperature. Pristine graphite and fluorinated graphite were labeled PG and FG, respectively. Filler-free coatings were prepared by mixing the epoxy resin, polyamide resin, and acetone (2:1:2 by wt%) using a planetary mixer at 2000 rpm for 1 min. Filler (PG or FG) was added (at a wt% of 10%) to the prepared coatings by continuous mixing at 2000 rpm for 5 min, and the coatings were sonicated in an ultrasonic bath for 10 min to achieve uniform dispersion. A copper plate was coated with the prepared coatings with a wet thickness of 250 μm, and then cured in an oven at 100oC for 30 min. To investigate the functional groups introduced into the surface of the PG and FG, X-ray photoelectron spectroscopy (XPS; MulDOI: http://dx.doi.org/ DOI:10.5714/CL.2018.28.096