Original Article Two-different ways of synthesis for EG: Study of mechanical, thermal, and electrical properties of epoxy composite for TIMs Sagar Kumar Nayak , Arjyama Mishra, Subhransu S Pradhan and Jyoti Agarwal Abstract The current study reports the synthesis of expanded graphite (EG) in two different ways and its fabrication with epoxy matrix to form composite at various filler fractions (5, 10, 12.5). One type EG (EG-C) is prepared by the electrochemical process using natural graphite flake (NGF), concentrated sulfuric acid, and ammonium persulfate, while the other (EG-P) is just mixing and heating of NGF with zinc nitrate hexahydrate. The functional groups of synthesized EG were confirmed by Fourier transform infrared spectroscopy. The surface morphology and microstructure of synthesized filler (EG-C, EG-P) were studied using X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. An optimum through-plane thermal conductivity (TC) of 2.04 and 2.22 W/mK was observed in the case of the composites containing 12.5 wt% of EG-C and EG-P, respectively. The obtained experimental TC was compared with three numerical thermal models, that is, inverse rule of mixture, Maxwell–Eucken model, and Agari model. Furthermore, the thermal stability of both composites was compared by using a thermogravimetric analyzer. The electrical resistivity of EG-P/epoxy composite at different formulations was higher than the EG-C-filled epoxy composites. Keywords Expanded graphite, epoxy composite, thermal conductivity, electrical conductivity, TIMs Introduction Carbon nanotubes (CNTs) and other carbon-based nanoma- terials such as graphene, graphene oxide (GO), reduced GO (rGO), and graphene nanosheets (GNSs) have a break- through in today’s era of multifunctional material research. 1–4 As reported by Ajayan et al., 5 the utilization of CNTs as thermal fillers for the fabrication of polymer nanocomposites has gained immense attention from various researchers. These CNT, GO, rGO, and GNSs have become suitable thermal fillers to fabricate high thermal conductive polymer composites in the field of electronics packaging with their utilization in cell phones, laptops, and many more miniaturized high efficient electronics devices. 6–8 Thermal interface materials (TIMs) are used as the thermal couple in the electronic devices that act as heat transfer medium from heat-generating source to the sink by creating an efficient path of phonon transfer (heat conduction). TIM includes thermal fluid, thermal pastes, phase change material, and resilient thermal conductors. 9–11 To avoid undesirable weight in devices, the fabricated thermal conducting poly- mer composite must be lightweight. The metallic thermal fillers, such as aluminum (Al), silver, copper (Cu), nickel, and so on, and ceramic thermal fillers, such as alumina (Al 2 O 3 ), silica (SiO 2 ), boron nitride (BN), silicon carbide, aluminum nitride (AlN), and so on, are utilized to improve the thermal conductivity (TC) of polymer composite. 12–15 However, to achieve high thermal conductive polymer com- posite, the high amount of filler (>70%) has been incorpo- rated due to their high specific weight. 7,16 At higher filler fraction, the composite became heavy and reduces its mechanical properties leading to unsuitability toward end application. It has been proved that carbon filler and poly- mer matrix are equally important in terms of lightweight, SARP-Laboratory for Advanced Research in Polymeric Materials, Central Institute of Plastics Engineering and Technology Bhubaneswar, Bhubaneswar, Odisha, India Corresponding author: Sagar Kumar Nayak, SARP-Laboratory for Advanced Research in Polymeric Materials, Central Institute of Plastics Engineering and Technology Bhubaneswar, Bhubaneswar, Odisha, 751024, India. Email: nayak.sagarkumar@yahoo.in High Performance Polymers 1–19 ª The Author(s) 2020 Article reuse guidelines: sagepub.com/journals-permissions DOI: 10.1177/0954008320945383 journals.sagepub.com/home/hip