Structural and mechanical characterization of graphite foam/phase change material composites

Abstract

We consider graphite/phase change material (PCM) composites for energy storage by latent heat in the context of power generation by solar concentration technologies. Based on X-ray computerized tomography 3D data, we calculate the cell-size distribution of polycrystalline highly ordered mesophase pitch-based graphitic foams (KFoam®) and semi-crystalline coal-based graphite foams (CFoam®). Compressive experiments show that the elastic modulus and strength are higher in the z-direction (foaming direction) than in the xy-plane. The elastic modulus Ez is found linearly related to the mean cell size d, while no simple relation is found between Exy and d. This supports that mechanical properties of graphite foams do not relate to the same geometrical characteristics depending whether the z-direction or the xy-plane direction is considered. After infiltrating the KFoam® and CFoam® host porous media with a molten PCM (sodium nitrate or a binary mixture of hydroxides), the response of the composite materials to compression tests is shown to be dependent upon a complex network of cracks all through the PCM, the occurrence of which is explained by the process whereby the PCM crystallizes within the host graphite foam and by the contrast between the thermo-mechanical properties of the PCM and those of the graphite.