The physical properties of graphite are considered a limit of the crystallization degree of pyrolytic carbon deposits. For instance, the spacing of basal planes of graphite is 0.335 nm while for pyrolytic carbon deposits it varies from 0.344 to 0.335 nm as annealing temperature increases 1. The ratio of pi- and sigma-bonding electronic states in graphite is 1/3, and the deviation from this value can be related to different texture degrees of pyrolytic carbon matrices of infiltrated carbon felts 2. On the contrary to other optical investigations 3 this correlation between graphite and pyrolytic carbon is not valid for polarized light microscopy, which is based on the optical anisotropy of pyrolytic carbon deposits and has been a benchmark for the rapid characterization of the texture degree of such materials for a long time. The measured extinction angles A_e for deposits usually exhibit values up to A_e approx=24 deg 4, which are significantly higher than the value of 16.3 deg reported for graphite 5 (Fig. 1). As graphite is considered the limit of the texture degree of pyrolytic carbon, the extinction angle of pyrolytic carbon is supposed to approach the value for graphite which is seemingly not the case.
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