Carbonaceous mesophase

Complex aromatic raw materials such as petroleum resids, decant oils, coal, and coal tars have been employed for many years by the earbon industry and eontinue to be used extensively in the fabneation of eoke, earbon, and artifieial graphite [1], These same feedstoeks also have the potential for use in produeing "advaneed" earbon produets such as carbonaceous mesophase, fibers, and beads 12-4],... 

High porosity carbons ranging from typically microporous solids of narrow pore size distribution to materials with over 30% of mesopore contribution were produced by the treatment of various polymeric-type (coal) and carbonaceous (mesophase, semi-cokes, commercial active carbon) precursors with an excess of KOH. The effects related to parent material nature, KOH/precursor ratio and reaction temperature and time on the porosity characteristics and surface chemistry is described. The results are discussed in terms of suitability of produced carbons as an electrode material in electric double-layer capacitors. 

Figures 1-3 demonstrate the effect of KOH/precursor ratio, reaction temperature and reaction time, respectively, on porous structure parameters of carbon produced by KOH activation. While the presented relationships concern mostly carbonaceous mesophase, basically they are typical of all coal and pitch-derived materials of the study.

Chemical and plastic behavior of the carbonaceous mesophase, which is the liquid crystalline phase where the microstructure of coke and many other carbon products is established... 

The origins and development of the concept of carbonaceous mesophase, as derived from discotic aromatic nematic liquid crystals, enable applications to be made to industrial processes. The world availability of pitch materials is such that there is an abundance of pitch which produces cokes of little commercial value. A major incentive for research into the Chemistry of mesophase formation is the commercial up-grading of such pitches and the development of specialized pitches. Structure in cokes is described in terms of optical texture. 

The carbonaceous mesophase is the intermediate material formed during carbonization of parent pitch and leading to the resultant coke. The discovery and development of the concept of mesophase over the last twenty years must represent one of the most significant advances in carbon science, Mesophase is a term borrowed from the science of conventional liquid crystals and means "intermediate phase". The term "carbonaceous mesophase" is distinguished from the term 1nematic... 

The optical anisotropy has been shown by Brooks and Taylor ( 5) to be built in through the carbonaceous mesophase, the liquid crystalline state formed during the liquid-phase carbonization of those organic materials that can be pyrolyzed and heat-treated to the graphitic state. The mesophase is the critical intermediate state in which the quality of carbon products is determined. The chemistry of its characterization, preparation, and control is most relevant to modern carbonization technology. 

Figure 8 A schematic phase diagram for the carbonaceous mesophase ...

In the previous symposium, we reviewed mesophase mechanisms involved in the formation of petroleum coke ( 2 ). Since 1975, two significant developments have been the use of hot-stage microscopy to observe the dynamic behavior of the carbonaceous mesophase in its fluid state (3-6), and the emergence of carbon fibers spun from mesophase pitch (7-9) as effective competitors in applications in which high elastic modulus or good graphiticity is important. This paper focuses on mesophase carbon fibers as an example of how the plastic mesophase can be manipulated to produce fibers with intense preferred orientations and elastic moduli that approach the theoretical limit for the graphite crystal in the a-direction. 

This paper commences with evidence for lamelliform morphologies in mesophase carbon fiber, summarizes relevant information on disclination structures in the carbonaceous mesophase, and then reviews what we learn of disclination behavior from hot-stage observations and from deformation and carbonization experiments. The results indicate that disclination interactions that occur before the mesophase is fully hardened play an important role in determining the microstructures of mesophase carbon fibers, as well as those of cokes and graphites that form through the carbonaceous mesophase. 

The model of carbonaceous mesophase sketched in Figure 7 is based on the molecular architecture proposed by Mochida et al. 

Figure 7 Schematic models for the carbonaceous mesophase [after Mochida et al. (16)] and its curvature strains of bend, splay, and twi.st. The molecular and strain models differ in scale by 1000x.

Figure 9. Schematic models for the wedge and twist disclinations of the carbonaceous mesophase.

Generally the sweep width of broad-line NMR is about 10 to 10 Hz, while that of high-resolution NMR is about 10 Hz at 36.4 MHz for a proton. But the H NMR spectra of the mesophase for Kureha pitch (Figure 8) and ethylene tar pitch (Figure 9) were swept over 2.5 x 10" Hz. Thus the sweep width of the H NMR spectrum for the carbonaceous mesophase is intermediate between those of broad-line H NMR... 

Two categories of pitch-based fiber exist isotropic carbon fiber produced from an isotropic pitch precursor, and an oriented, anisotropic fiber produced from a mesophase pitch precursor. Isotropic fibers were developed from low melting point isotropic pitches The precursor was melt-spun into fibers, which were oxidized to render them infusible, and then carbonized. Their low strengths and moduli make these fibers unsuitable for use in advanced composites. Orientation was accomplished by a hot-stretching process (>2200°C), but it is accompanied by the same processing difficulties encountered in the rayon precursor process. A different approach was suggested by the discovery of carbonaceous mesophase. ... 

---