| Popis: |
It is definite that stabilization conditions such as stabilization temperature, time, and heating rate, as well as partial pressure of oxygen, are very influential in the formation of a skin-core structure in carbonized fibers about 10 μm in diameter, defining the thickness of the skin as well as the orientation degree of the carbon sheets in the core. The higher stabilization temperature and faster heating to stabilization were responsible for the formation of a distinct skin-core structure and wider sheets in the core. A lower partial pressure of oxygen in the stabilization appears to be a key for the introduction of a skin-core structure at conventional stabilization temperatures (230–270°C). Such results suggest that the formation of a skin-core structure originates from the oxygen gradient formed in the stabilized fibers by the competition between the oxidation progress and restricted supply of the oxidant along the radius of the pitch fiber. Such stabilized fibers were carbonized, using a heating rates from 1 to 30°C/ min, with or without strain. The growth of mesophase domains in the core was enhanced by increasing the heating rates to carbonization. Layers of unfolded or onionskin sheets were introduced in the core of carbonized fibers by carbonization under strain. |
