Carbonaceous mesophase: History and prospects
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The chemical and physical processes involved in formation of mesophase pitch are described. Carbonaceous mesophase exhibits properties of both nematic liquid crystals and glass-like pitches. Mesophase development requires polymerization of aromatic pitch components to a sufficient molecular size, and a proper concentration balance between molecules with different mesophase forming tendencies.
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The effect of ferrocene in pitch on nucleation and growth processes of mesophase was investigated from the behavior of iron. The quinoline soluble coal-tar pitch to which ferrocene was added, was heat-treated at 430°C for a certain period. Quinoline insoluble part in the heat-treated pitch was separated by solvent extraction with quinoline. Then, the quinoline insoluble part was treated with HCl. Iron contents in the heat-treated pitch, the quinoline insoluble part, and the quinoline insoluble part treated with HCl were examined. The obtained results are as follows.Ferrocene added in pitch was already decomposed at 430°C and changed into the fine crystals of α-Fe and FeS, which covered the surface of mesophase spherules formed in pitch. The mesophase spherules were found to have almost the same size. Amounts of mesophase formation in the ferrocene-added pitch were larger than those in the pitch having no ferrocene content. From these results, it is considered that the iron crystals controll the size of mesophase spherules and accelerate the mesophase formation. Furthermore, when the mesophase coalesces each other, the crystals not only covers the surface of the coalesced mesophase but also exists inside the mesophase.It is also made clear that the shape of mesophase spherules changes from lemon-like form to spherical form with growth of mesophase spherules.
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A new 13C-NMR measurement technique, named fused-state 13C-NMR, has been developed. Using this technique, many aspects of carbonaceous mesophase have been elucidated: Constituents of soft mesophase have methyl and/or naphthenic groups whereas those of hard mesophase have no aliphatic groups. The order parameter of carbonaceous mesophase in the nematic phase is about 0.7. Very small molecules such as hexamethylbenzene and pyrene orientate in a carbonaceous mesophase. In a liquid crystalline phase, all oriented molecules in a carbonaceous mesophase (including the small molecules trapped in the mesophase) fluctuate around the orientation axis in the same manner. That is, an oriented molecule behaves as if it were trapped between two walls the distance between which is about 1.0 nm. The analyzed results rule out the assertion that soft mesophase is a gel and also rule out the existence of basic structural units. A constructed model for molecular ordering in a mesophase has clarified some pending questions on carbonaceous mesophase.
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A method in determination of the mesophase contents of pitches are presented. The method is based on the reflectivity difference between a mesophase part and an isotropic part. In order to get clear contrast between a mesophase part and an isotropic part, image processing is used. The method achieved accurate determination of the mesophase content from 0% to 100% in several minuets.
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Carbonaceous mesophase with a nematic liquid crystal structure possesses an easily graphitizable characteristic and can be used as a promising raw material to prepare anisotropic carbon and graphite materials with high performance and multifunction. Therefore, the carbonaceous mesophase occupies a pivotal and irreplaceable position in many frontier and cutting-edge fields. The controllable preparation and characterization of carbonaceous mesophase derived from a model molecule (i.e., naphthalene) are presented, especially the formation, development, and transformation of anisotropic liquid crystalline mesophase in the synthetic naphthalene pitch during the process of liquid-phase carbonization (350–450°C). The increasing applications of naphthalene-based carbonaceous mesophase as an ideal precursor material for fabricating representative advanced carbon materials with high added value (e.g., mesophase pitch-derived coke, mesocarbon microbeads, mesophase pitch-based carbon foam, high-modulus mesophase pitch-based carbon fibers, and high-thermal-conductivity carbon-based composites, etc.) are reviewed in detail in this chapter.
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Mesophase pitch is the pioneer body of the new carbon materials.This paper reveals recent research aspects of mesophase pitch such as transform mechanism,the composition,properties,and the influencing factors of mesophase pitch and the latest application of mesophase pitch.
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