Cokes optical properties

Although Taylor (5) discussed the development of optical properties of anisotropic material formed during carbonization in 1961, and together with Brooks (6) reviewed the concept of liquid crystals as an intermediate to coke formation in 1968, it was not until well into the 1970 s that the potential of this knowledge was realised. 

Optical microscopy, using polarized reflected light, has been recognized as a very useful tool and has played a major role in studies of carbonization mechanisms and coke quality properties. Examination of a coke polished surface under a microscope reveals the presence of certain parts without optical activity (isotropic coke) and others with optical activity (anisotropic coke). The anisotropic carbon exhibits yellow, blue, and purple areas of various sizes, <1-200 )tm, and shapes (optical texture) with an interchange of color on rotation of the specimen. 

Regular coke is a petroleum coke with good graphitizability and is characterized by a combination of properties which differ significantly from those of metallurgical coke but do not reach the quality level of premium coke These properties are optical anisotropy, medium reversible thermal expansion, and low ash content See metallurgical coke, petroleum coke, premium coke... 

It is my contention that the optical and physical properties and the optical structure produced during the destructive distillation or thermal decomposition of vitrinite is closely related to mode of carbonization and, in the case of pitch, is intimately related to the method of pitch preparation. For instance, a pitch may be produced from a high or low temperature tar, from a primary cooler tar, or from a flushing liquor tar. In addition, it may be air blown, thermally or chemically treated, straight distilled, or cut back, just to mention a few. Under similar carbonization conditions almost any one of these pitches will produce a coke which has certain characteristics that are related to the parent pitch. Even pitches similarly processed from the tar can differ in the content of quinoline- and benzene-insoluble material and P-resin, and can contain more than one distinct liquid phase. None of these points of difference has been discussed by Dr. Taylor or even recognized in the preparation. To interpret the structure of pitch coke divorced from a knowledge of the pitch source and/or carbonization conditions can lead to erroneous conclusions. These are pertinent data omitted by the authors. 

Bacha et al. (62) used cyclohexane, acetone, toluene and tetrahydrofuran to fractionate petroleum pitches and related the properties of the fractions to optical texture of resultant cokes. For Ag hland A240 pitch the toluene extracted fraction showed a higher temperature of mesophase formation than the tetrahydrofuran fraction. The carboniza-... 

Mochida et al. (76) report that in carbonizations of several heterocyclic compounds containing nitrogen, sulphur and oxygen, the presence of the heteroatoms markedly affects optical texture of resultant cokes. When co-carbonized with aluminium chloride the heteroatoms exert a less marked effect where heteroatom evolution becomes important. Again, Mochida et al. (76) emphasise that the rate of the carbonization process, by influencing viscosity, is dominant in determining optical textures of cokes and related physical properties, i.e. CTE. 

This influences the structural features of the mesophase which remains more disordered, a point made by Cranmer et al. (43). Stadelhofer (107) found that the presence of QI did not change rates of formation of mesophase. Romovacek et al. (108) consider that pyrolytic particles in pitch (primary QI) retard the development of mesophase and suppress coalescence. Decrease in size of optical texture, as brought about by mechanical modification as distinct from chemical modification of pitch properties can increase both the strength and reactivity to oxidising gases of the resultant coke, as recently put forward by Markovic et al. (109). ... 

The optical anisotropy observed in most carbon materials reflects the ordered stacking of graphite-like microcrystalline units that has been recognized to be essential in determining their properties. Pitch-based carbon fiber, electrode and metallurgical cokes, and carbons for nuclear reactors are characterized by their anisotropic texture since this structural factor is fundamentally related to their mechanical, thermal, electronic, and chemical properties (1-5) ... 

The optical texture of mesophase and resultant carbons is observed readily by means of a reflected polarized light microscope and may be classified according to the shape and size of the isochromatic units. Such a classification is useful to evaluate the properties of mesophase and carbons such as needle cokes. The mesophase has been defined as the intermediate state which shows optical anisotropy and is quinoline-insoluble at room temperature (5,51) (liquid crystal glass), although it is a viscous liquid crystal during the carbonization process (6). ... 

Aromatic compounds have not only been of academic interest ever since organic chemistry became a scientific discipline in the first half of the nineteenth century but they are also important products in numerous hydrocarbon technologies, e.g. the catalytic hydrocracking of petroleum to produce gasoline, pyrolytic processes used in the formation of lower olefins and soot or the carbonization of coal in coke production [1]. The structures of benzene and polycyclic aromatic hydrocarbons (PAHs) can be found in many industrial products such as polymers [2], specialized dyes and luminescence materials [3], liquid crystals and other mesogenic materials [4]. Furthermore, the intrinsic (electronic) properties of aromatic compounds promoted their use in the design of organic conductors [5], solar cells [6],photo- and electroluminescent devices [3,7], optically active polymers [8], non-linear optical (NLO) materials [9], and in many other fields of research. 

Significance of Optical Texture In metallurgical and foundry cokes, subject to stringent specifications of strength, size and reactivity, it is necessary to know whether the development of anisotropy during the mesophase is incidental or essential to coke properties (14, 15, 16). 

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