ABSTRACT
Carbon materials described in this chapter consist of carbon atoms bound by sp2 hybrid orbital, which result in the formation of hexagonal layers of carbon atoms, known as graphite. Even though graphite structure which is fundamentally composed of hexagonal carbon layers stacked in parallel with a regularity of so-called ABABAB... is a thermodynamically stable phase, parallel stacking of these layers without any regularity (turbostratic structure) (Warren, 1941) is possible, particularly in the products just after carbonization at low temperatures of 1000-1500°C. These parallel-stacked carbon layers construct a structural unit in carbonized materials, and govern the further development in structure and also in properties, therefore, being called basic structural unit, BSU (Oberlin, 1989). By heat treatment to temperatures above carbonization temperature, these BSUs grow to form so-called crystallites in the field of crystallography. Further heat treatment up to 3200°C makes these crystallites grow in size, and also the stacking of carbon layers in each crystallite is improved to be in three-dimensional regularity, i.e. ABABAB... stacking, and this structural change is in a strict sense called graphitization. However, this structural development is known to depend strongly on the starting organic materials and their carbonization conditions, in other words, the size and local orientation of BSUs in carbonized materials, and also on the graphitization condition at high temperatures. The carbonaceous precursors have been classified schematically into two groups, graphitizing
and non-graphitizing carbons, or soft and hard carbons, but it has to be pointed out that in many cases such rough classification is not appropriate, many carbon precursors showing an intermediate behavior between the two groups. Therefore, we have a wide range of structure of carbon materials, from completely turbostratic stacking to the three-dimensional graphite organization by selecting the precursor and the heat treatment conditions. Here, carbon materials of which structural change is on the way to reach perfect graphite are called pregraphitic carbons, especially those with a turbostractic structure.
