Flexible graphite modified by carbon black paste for use as a thermal interface material
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Thermal interface materials for improving thermal contacts are investigated. Flexible graphite, which is made by compressing exfoliated graphite, is an attractive thermal interface material due to its resilience and thermal conductivity. Polyol-ester-based carbon black (Cabot/Tokai) pastes are used to coat or penetrate flexible graphite. The thermal contact conductance of flexible graphite between copper surfaces at a contact pressure of 0.46 MPa is increased by the paste introduction, as measured by the guarded hot plate method. Paste penetration by up to an effective paste thickness (which describes the amount of penetrated paste, as calculated by dividing the volume of the penetrated paste by the geometric area of the flexible graphite) of 5 μm increases the conductance by up to 350%, 98% and 36% for thicknesses of 50, 130 and 300 μm respectively. Paste coating up to 10 μm increases the conductance by up to 200%, 120% and 65% for thicknesses of 50, 130 and 300 μm respectively. The paste penetration is more effective than the paste coating in enhancing the conductance, when the thickness is 130 μm or below. At the thickness of 130 μm or above, paste penetration and paste coating give similar increases of conductance. These results stem from the relatively low interfacial thermal resistivity provided by paste penetration and the relatively high though-thickness thermal conductivity provided by paste coating. Paste penetration decreases the thermal conductivity of flexible graphite, but paste coating does not affect the conductivity. Both penetration and coating decrease the interfacial resistivity. The highest thermal contact conductance attained is 1.4×10 5 W/m 2 ·K, as provided by a 26-μm thick paste-penetrated flexible graphite, which is 100% higher than the reported highest value before this work. Due to its high viscosity, the 15 vol.% Tokai paste gives less paste penetration but larger paste coating thickness than the 2.4 vol.% Cabot paste. However, the introduction of these pastes in flexible graphite gives similar values of the thermal conductivity, interfacial resistivity and thermal contact conductance. The carbon black modification (penetration) of flexible graphite increases the thermal contact conductance, thereby enabling the modified flexible graphite to be comparable to or better than any of the previously reported thermal interface materials, which include thermal pastes, paste-coated metal foils, silicone-based thermal pads, carbon nanotube arrays and silver nanofiber arrays. Applications include microelectronics, LED lighting and solid-state lasers.