Mesoscopic Structure and Properties of Liquid Crystalline Mesophase Pitch and Its Transformation into Carbon Fiber

Abstract

The history and present state of the art in the chemistry of mesophase pitch, which is an important precursor for carbon fiber and other high-performance industrial carbons, are reviewed relative to their structural properties. The structural concepts in both microscopic and macroscopic views are summarized in terms of the sp(2) carbon hexagonal plane as a basic unit common to graphitic materials, its planar stacking in clusters, and cluster assembly into microdomains and domains, the latter of which reflect the isochromatic unit of optical anisotropy. Such a series of structural units is described in a semiquantitative manner corresponding to the same units of graphitic materials, although the size and stacking height of the hexagonal planes (graphitic sheets) are very different. Mesophase pitch is a liquid crystal material whose basic structural concepts are maintained in the temperature range of 250 to 350 degrees C. The melt flow and thermal properties are related to its micro- and mesoscopic structure. The structure of mesophase-pitch-based carbon fiber of high tensile strength, modulus, and thermal conductivity has been formed through spinning, and has inherited the same structural concepts of mesophase pitch. Stabilization settles the structure in successive heat treatments up to 3000 degrees C. Carbonization and graphitization enable growth of the hexagonal planes and their stacking into units of graphite. Such growth is governed and controlled by the alignment of micro- and mesoscopic structures in the mesophase pitch, which define the derived carbon materials as nanostructural materials. Their properties are controlled by the nanoscopic units that are expected to behave as nanomaterials when appropriately isolated or handled.

Preparation of Nitrogen‐Rich Nanolayered, Nanoclustered, and Nanodendritic Carbon Nitrides

“…Unlike many other organic compounds, such as mesocarbon microbeads, [23,24] mesophase pitch-based carbon fibers, [25][26][27] non-azido-substituted triazines [4,13,14] and nonazido-substituted heptazines, [3,[15][16][17] TAAT [18] and other polyazido compounds containing only C and N atoms [28,29] are ideal precursors for nitrogen-rich carbon nitrides because of their clean and thermodynamically favorable decomposition, which presumably extrudes nitrogen gas as the only byproduct [Eqs. (1) and (2)].…”

Lignin-based Carbon Fibers

“…Dave et al 86 emphasized that in order to produce CFs with good mechanical properties, the precursor should have the ability to form a fluid, yet organized, liquid-crystalline state when spun into fibers in order to generate a highly oriented graphitic structure upon thermal treatment. This type of behavior is characteristic of mesophase pitch 72 and interestingly, was demonstrated for some lignin model compounds, 86 although the temperature range of the anisotropic phase was extremely low, and processing prohibitive. Impurities such as inorganic salts and carbohydrates are also an important issue for lignin-based CF production.…”

Preparation, Characterization, and Applications of Carbonaceous Mesophase: A Review

“…have been widely used to synthesize spinnable mesophase pitch by catalyzing with superacid, HF-BF 3 . The obtained naphthalene-derived mesophase pitch possesses characteristics of high purity, controllable molecular structure, and ideal physical property [3,[9][10][11]. However, the severe corrosion problem and potential operating risk of using HF-BF 3 as a catalyst will unfortunately limit its widespread use (and such a mesophase pitch product named "AR" as shown in Figure 2(a) is now no longer available from, e.g., Mitsubishi Gas Chemical Company).…”

“…The carbonaceous mesophase pitch prepared by AlCl 3 catalytic thermal polymerization of naphthalene has a relatively high aromaticity (the aromatic index is about 0.70) and a regular planar molecular structure constructed by a number of naphthenic structure, as well as a relatively large molecular weight of ~2600 g/mol, consisting of mesogen units (a ladder-shaped molecular structure) formed by ~20 naphthalene molecules through thermally induced aromatic growth [3,10,11] according to the analyses of Figures 6-8. The suitable softening point (260-280°C) and appropriate H/C mol ratio (0.52-0.60), as well as high liquid-crystalline mesophase content (100 vol.%) and ideal fine flow texture as displayed in Figures 9 and 11, are the significant characteristics of such carbonaceous mesophase.…”