Mesophase spheres

Fig. 12. A, Schematic representation of parallel arrays of polynuclear aromatic hydrocarbon molecules in a mesophase sphere. B, a) isolated mesophasc spheres in an isotropic fluid pitch matrix b) coalescence of mesophase c) structure of semi-coke after phase inversion and solidification.

In the course of liquid-phase carbonization of pitches, optically anisotropic spheres are formed first, which are called mesophase spheres. By further heating, the spheres grow and coalesce with... 

FIGURE 2.23 (a) Formation, (b) growth and (c) coalescence of mesophase spheres in a pitch. 

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

The spherical MCMB is formed in the mesophase of coal tar during carbonization. The heat treatment is stopped at temperatures at which the mesophase spheres formed in the coal tar pitch... 

The molecular structure and constitution of the anisotropic mesophase sphere have been analyzed (49,50) NMR, UV, IR, and... 

Figure 7. Models for the constituent molecules of mesophase spheres Reproduced with permission from reference 20,...

Coalescence of mesophase is often said to be determined by the mesophase viscosity. This aspect requires much further investigation. However, it is clear that, amongst other factors, the rheological behaviour (including viscoelastic effects) of each phase is important in mesophase growth and coalescence. Diffusion of molecular species through the isotropic pitch to the mesophase spheres is likely to be related to the viscosity of the isotropic med i urn. 

The Pittsburgh No. 8 semicoke that was cooled from 400 C contained remnant mesophase spheres, and these can be seen in Figure 4. This photomicrograph was taken in the secondary electron mode in the scanning transmission electron microscope. 

In this imaging mode, only surface features can be seen by topographic contrast. In the transmitted electron mode, most of the sample was too thick to pass electrons, and spheres were difficult to distinguish. In the Pittsburgh No. 8 sample, 400 C is the onset of fluidity, and mesophase spheres were sufficiently abundant to be observed in the STEM (Figure 4). 

Petroleum pitches and coal tar pitches are becoming increasingly important as raw materials for the production of low cost, high performance carbon fibers. This development has been primarily due to the ability of the pitch to form an oriented liquid crystalline mesophase. Since the 1st observation of mesophase spheres in pitch material by Brooks Taylor ( ), a number of studies on the kinetics of mesophase formation have been made by several different groups. 

Figure 4. (left) Mesophase sphere with section including polar diameter (right) cross... 

Precursor MCMB generally was separated from the heated pitch containing mesophase spheres before graphitization. There are different kinds of textures in MCMB such as Brooks-Taylor type, Honda type, Kovac-Lewis type, and Huttinger type. In Japan, there are two main companies, Osaka and Kawasaki... 

D. Auguie, M. Oberlin, A. Oberlin and P. Hyvemat, Microtexture of mesophase spheres as studied by high resolution conventional transmission electron microscopy (CTEM), Carbon, 18,337-346 (1980). 

Figure 4.23 Mesophase spheres in a partially converted chemical pitch (polarized light x400). Source.

The effect of mesophase pitches on the tensile modulus of the resultant pitch based carbon fiber has been studied [245] and carbon fiber prepared from an isotropic pitch containing mesophase spheres has also been studied [246],... 

Korai Y, Ishida S, Watanabe F, Yoon SH, Wang YG, Mochida I, Kato I, Nakamura T, Sakai Y, Komatsu M, Preparation of carbon fiber from isotropic pitch containing mesophase spheres. 

BULK MESOPHASE is a continuous anisotropic phase formed by the coalescence of mesophase spheres. BULK MESOPHASE retains fluidity and is deformable in the temperature range up to about 770K, and transforms into GREEN COKE by further loss of hydrogen or other low molecular weight species. 

Moriyama, R., Kumagai, H., Hayashi, J.L, Yamaguchi, C., Mondori, J., Matsui, H., and Chiba, T. Formation of mesophase spheres from a coal tar pitch upon heating and subsequent cooling observed by in situ H-NMR. Carbon 2000 38 749-758. 

The activated carbons prepared by chemical activation of olive stones and mesophase spheres could not be istinguished from each other by HRTEM. Nor could they be distinguished from olive stones activated thermally. 

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