Thermal Management Material: Graphite

Abstract

Graphite has pronounced anisotropy in its properties. Thermal conductivity of graphite crystal is extremely high along its layer plane, as high as 2000 Wm À1 K À1 , much higher than metals, and very low across its layer plane, as low as 7 Wm À1 K À1 . This anisotropy gives high possibility for graphite as thermal management materials. Here, graphite membranes with high thermal conductivity along the surface for heat dissipation and with low conductivity perpendicular to the surface for heat insulation, in addition to graphite foams as container of phase change materials for heat energy storage, are reviewed, particularly referring to the preparations of various graphite materials. Thermal conductivity of diamond crystal is also high, 2500 Wm À1 K À1 , but it is very sensitive for structural defects, but that of graphitic materials is less sensitive for structural defects, suggesting that the latter is more practical for thermal management than the former.(along the layer plane) as %2000 Wm À1 K À1 , [1] much higher than metals, such as Cu and Ag, but very small k RT along the c-axis (perpendicular to the layer plane) as around 6-9 Wm À1 K À1[2] : graphite crystal can be highly conductive along the layer and be highly insulating perpendicular to the layer. This pronounced anisotropy is due to graphite structure, graphite layer consisting of strong s bonding, and parallel stacking of these layers by van der Waals bonding due to the interaction between p-electron clouds on neighboring layers. Diamond crystal, which consists of carbon atoms using sp 3 CÀ ÀC bonding, is isotropic and has the highest value of k RT in solids as %2200 Wm À1 K À1 . [3] Highly graphitized carbon materials exhibit very high thermal conductivities. A highly-oriented pyrolytic graphite (HOPG) is reported to have k RT of 1950 Wm À1 K À1 . [1,4] Recently, graphite films with high thermal conductivity have become commercially available for dissipating heat from electronic devices containing integrated circuits, such as personal computers and mobile phones. [5] The k RT of the commercially available graphite films is reported to be more than 1600 Wm À1 K À1 , more than four times higher than the values of Ag, which is known to have the highest k RT among metals. Carbon/carbon composites prepared from natural graphite flakes by using mesophase pitch as binder, followed by heat treatment at high temperatures, gives k RT of about 650 Wm À1 K À1 , higher than Ag. In order to develop practical materials having k high enough, the preparation of highly oriented and highly crystallized graphite is essential. Flexible graphite sheets prepared from natural graphite via [*] Prof. M. InagakiProfessor Emeritus

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