Graphitizability of carbonaceous materials as studied by TEM and X-ray diffraction

SUMMARY Progressive graphitizationis measured from X-ray patterns by the probability P for finding a carbon layer pair reaching the graphiteorder in the solid. TEM studies show that all carbons (whether graphitizingor not) contain small basic structural unitsless than 10 A both in thickness and in diameter which acquire, locally, a preferred parallel orientation during the carbonization process, i.e. at the end of tar release. However, tilt and twist boundaries remain between adjacent units which create wrinkled layers. All the materials are porous; from microporous if the orientation is less than 50–100 A (non- graphitizingcarbons) to macroporous if the preferred orientation is due to coalesced mesophase spheres ( graphitizingcarbons). For graphitizationto occur it is necessary to have small basic structural unitsdistributed in large areas of preferred orientation. Graphitizationoccurs only after a sudden elimination of the defects situated at the boundaries between two adjacent structural units, i.e. after development of perfect two dimensional order by dewrinkling and stiffening of the carbon layers (turbostratic structure made of large layers). If both Lamax and Lcmax are too small (micropores) graphitizationis impossible. There is a continuous range of materials extending between those with the smallest regions possessing the same molecular orientations (non- graphitizingcarbons) and the largest ( graphitizingcarbons). A nearly continuous increase of P (from P = 0 to P = 0.8) was found by studying a great variety of kerogens, coals and asphalts. For a given sample, the maximum possible value of P depends only on the size of the oriented areas which itself depends only on the elemental composition at the very moment of its production. There is an opposite influence of the hydrogenated functional groups and the cross-linking functional groups such as oxygen, sulphur and nitrogen.