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Heat Treatment Temperature (°C)

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Heat Treatment Temperature (°C)

Fig. 13. Increase in stack width parameter, L„ with heat treatment temperature, HTT, for some graphitising cokes, [Adapted from 112],

6.4. Electron microscopical studies of engineering carbons

The microstructural model for disordered carbons has been greatly elaborated following the application of high resolution transmission electron microscopy. The early work by Ban [114] and Jenkins et al [115] lead to the development of the ribbon model for glassy carbon, Fig. 14, which envisages the non-graphitic structure as a network of twisted and folded carbon layer planes. Interestingly, this microstructural model for carbons was perhaps the first to depart from the flat graphite layer model and introduce concepts of curvature that can now be rationalised using microstructural elements borrowed from Fullerenes and nanotubes. However, the Jenkins model is essentially intuitive and later workers [116] have cautioned against the use of such simplistic readings of electron microscopical images.

Perhaps the most elaborate and extensive electron microscopical studies of carbonaceous materials were carried out by Agnes Oberlin and her group [116] who showed that a great deal of microstructural information on carbons can be obtained using a combination of selected area diffraction and dark field and light field imaging. For all carbons, Oberlin defines a basic structural unit, BSU, as a parallel stack of two to four layer planes each containing less than 10-20 aromatic rings. A related concept is local molecular ordering, LMO, which consists of an array of BSU with a near-common orientation, Fig. 15. In non-

graphitising carbons there is a high degree of misorientation of BSU so that LMO is small or non-existent, whereas in graphitising carbons the misorientation between adjacent BSU is small and consequently there is extensive LMO extending to the order of microns.

Fig. 14. The ribbon model for the microstructure of a glassy carbon [115].
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