| Carbon foam has excellent potential for aerospace applications where thermal management is critical. The open-cell architecture and thermally conductive cell walls are the key characteristics for heat management. Aerospace structures also demand enhanced mechanical properties. Carbon foams have modest crush strength. The objective of this dissertation was to improve both thermal conductivity and crush strength of carbon foams by introducing carbon nanofibers into the carbon foam. This dissertation reports on the effects of low volume fraction (up to 0.04) carbon nanofibers on foam structure, thermal conductivity and crush strength. The bulk density of the carbon foam increased linearly with the fiber volume fraction, reflecting the morphological changes in the cell structure. Thermal conductivity increased at low fiber volume fractions, but dropped significantly at higher fiber volume fractions. Crush strength increased linearly with fiber fraction for 10 mum length fibers, but decreased for the 100 mum length fibers. Analyses employing scanning electron microscopy (SEM), petrography, and x-ray diffraction (XRD) illustrated the complex effects of the carbon nanofibers on the foam characteristics. Available models for thermal conductivity and crush strength have been extended to account for these effects incorporating cell structure and morphology (macro effect), presence of fibers (micro effect), and graphite crystal d spacing (nano effect). This research has shown that the nano-fibers have a complex role in the composite foam properties by affecting the structure at the macro, micro, and the nano levels. |
