High Thermal Conductivity Mesophase Pitch-Derived Graphitic Foams

1. Introduction In recent years there has been an increasing number of applications requiring more efficient and lightweight thermal management such as high-density electronics, hybrid diesel-electric vehicles, communication satellites, and advanced aircraft. The primary concerns in these thermal management applications are high thermal conductivity, low weight, low coefficient of thermal expansion, high specific strength and low cost (1). Such applications have focused on sandwich structures (a high thermal conductivity material encapsulating a structural core material) to provide the required mechanical properties (1). However, since structural cores (e.g. honeycombs) are typically low-density materials, the thermal conductivity of the overall composite through the thickness is relatively low (~3-10 W/m·K for aluminum honeycomb) (2, 3). One potential core material being explored is metallic foam: however, the thermal conductivities are still low, 5-50 W/m·K (3) and are not significantly greater than the out-of-plane thermal conductivities of typical carbon-carbon composites (see Table 1). Existing carbon foams are typically reticulated glassy carbon foams with a pentagonal dodecahedron structure (7-9), illustrated in Figure 1, and typically exhibit thermal conductivities less than 1 W/m·K (3, 10-12). Other pitch-derived carbon foams have been reported and explored. Unfortunately, these are also thermally insulating and are designed for structural reinforcement rather than thermal management. The pitch-derived graphitic foams reported here 2 exhibit a spherical morphology, and present a unique solution to this problem by offering high thermal conductivity with a low weight. In order to produce high stiffness and high thermal conductivity graphitic foams, a mesophase pitch invariably must be used as the precursor, thus assuring a graphitic-like (or turbostratic) structure in the ligaments (13-15). Typical foam forming processes utilize a blowing technique, or pressure release, to produce foam of the pitch precursor (14, 16-19). As the bubbles in the foam grow, bi-axial extension orients the mesophase domains parallel to the cell walls, similar to the uni-axial extension (shear) during melt extrusion (or spinning) of mesophase pitch-based carbon fibers. As with carbon fiber production, the pitch foam is then stabilized by heating in air or oxygen for many hours to cross-link the structure, and " set " the pitch, so it does not melt during further heat treatment (16, 21). Stabilization can be a very time consuming and expensive process depending on the part size. The " stabilized " pitch foam is carbonized in an inert atmosphere to temperatures as high as 1100°C, producing a structural material suitable for composite reinforcement …