Characterization of the anisotropy of pyrolytic carbon by Raman spectroscopy

The anisotropy of pyrolytic carbon is one of the most important properties for the development of TRISO (tristructural isotropic) coated fuel particles. Polarized Raman spectroscopy has been used to measure the anisotropy of pyrolytic carbon coatings ranging from high (highly oriented pyrolytic graphite) to low texture samples. Values obtained were correlated to the texture measured from the orientation angle (OA) of selected area electron diffraction patterns. The nodal behavior of the Raman intensity signals in graphite has been used to observe changes in the first order bands when the laser and scattered signal were polarized parallel or perpendicular to the graphene layers. Texture observed by Raman spectroscopy was largely affected by the orientation of the graphene layers inside growth features. Discrepancies in the values of texture measured between Raman spectroscopy and the OA was due to the difference of spatial resolution of each technique, 0.2 μm for OA and 2 μm for Raman spectroscopy. Both polarized Raman spectroscopy and nano-indentation were used to characterize the inner pyrolytic carbon in coated fuel particles before and after SiC deposition at 1500 °C.