Why does cross-extinction property of pyrolytic carbon of C/C composites manufactured by CVI never disappear

Pyrolytic carbon of C/C composites (carbon/carbon composites) manufactured by CVI (chemical vapor infiltration) is an opaque mineral. Like other opaque minerals its reflection property is good. Unlike other opaque minerals, which do not exhibit cross-extinction property, its cross-extinction property is conspicuously good. In fact, its cross-extinction property never disappears. Why? We aim to explain this very interesting phenomenon, which has significant engineering importance. Extinction is an important characteristic for pyrolytic carbon in C/C composites when the polarization microscope is right-angled. When sample is revolved, its cross-extinction property never disappears. We examined and analyzed it on the basis of crystallography and its reflection. Pyrolytic carbon consists principally of very small graphite crystallines. Because its atomic arrangement is similar to that of ideal graphite, their optical characteristics shown under polarization microscope should also be similar. Like the concentration of small graphite crystalline in pyrolytic carbon, the elliptical slices of indicatrix of optical crystal are also concentrated around the carbon fibers in C/C composites. Whenever sample was revolved, we observed that there are always some elliptical slices of parts of small graphite crystallines of pyrolytic carbon parallel to the two cross directions of polarized light of polarizer and they induce extinction there under the right-angled polarzation microscope and form a cross that never disappears. We should like to emphasize that this is what we think is the answer to the question asked in the title of this paper. The extinction explanation of pyrolytic carbon may be beneficial for properties and characterization of pyrolytic carbon in C/C composites manufactured by CVI.