Spherules, mesophase

Figure 1. Three steps in the development of optical anisotropy via the mesophase. (a) Nucleation of mesophase spherules. (b) Growth and coalescence of mesophase. (c) Flow anisotropy.

Figure 13 depicts a coalescence event that took place in 7 sec. Like other coalescence events in which new disclinations are produced, a 2tt disclination appeared at the position of the former boundary between the mesophase spherules. Frenkel (27) has pointed out that the time constant t for such coalescence phenomena is given approximately by... 

Figure 13. Coalescence of mesophase spherules to produce a new 2tt disclination. Hot-stage microscopy, crossed polarizers.

Figure . The Brooks and Taylor Model of the structure of a mesophase spherule [16] reproduced with permission.

FIGU RE 2.37 Models of nanotexture for small-sized carbon black, mesophase sphere, and carbon spherule. 

During the course of the pyrolysis of a petroleum-derived residue (pyrolysis tar, cat-cracker residue) or a filtered coal tar pitch it is possible to observe, under the polarisation microscope and at a certain temperature, the formation of anisotropic spherules, which grow as the reaction time lengthens and the temperature increases, coalesce and, at around 500 to 600 °C, are transformed into a semi-coke phase with marked anisotropy. Figure 13.2 shows photomicrographs of a filtered coal tar pitch pyrolyzed at 400 °C with the formation of spherulitic mesophases after reaction times of 2, 6,10 and 16 hours. 

When the elementary spherules of mesophase coalesce to form anisotropic domains, the microtexture of the carbonaceous mesophase becomes more complex. Disdinations (rotational defects) in the arrangement of the discotic molecules are often present. Disdinations in 2-D media play a role similar to that of dislocations in crystals and the evolution of gas bubbles results from condensation reactions in a medium which is still fluid. Disdinations, which occur during the mesophase formation, remain after carbonization and are key to understanding the relationships between the microtexture and the properties of carbon fibers formed from mesophase pitches. 

The term Brooks and Taylor structure is recommended to describe the particular lamellar morphology of the spherules most commonly precipitated from pyrolyzed PITCH. The term honors the workers who first recognized the significance of caxbonaceous mesophase to carbon science and technology and who first defined this spherical morphology. The term Brooks and Taylor structure does not cover all structures found in the spherical mesophase, because other lamellar arrangements have been observed. 

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