Heavy petroleum products: Microtexture and ability to graphitize

Abstract

Various asphalts extracted from crude oils by light n-alcanes were studied by elemental analysis, conventional transmission microscopy and X-ray diffraction. Like most carbonaceous materials, asphalts contain planar aromatic structures (4–12 rings in dia.) single or piled up by twos or threes (basic structural units). These units are distributed at random in the bulk and linked by non-aromatic groups. Heat-treatment was carried out under inert gas flow from room temperature to 3000°C. In the early stages of carbonization, the structural units acquire a local parallel orientation (just before the end of the tar release). The extent of the molecular orientation depends only on the opposing influence of the hydrogens and of heteroatoms such as oxygen, sulfur and nitrogen. The extent of orientation increases drastically from less than 50 Å to a few μm as the percentage of cross-linking agents (oxygen and sulfur) and of nitrogen decreases. At the same time the ability to graphitize increases progressively. All kinds of intermediates between non-graphitizing and graphitizing carbons are found after heat-treatment.