Carbon from anisotropic mesophase pitch

Carbon fibers from anisotropic mesophase pitch High molecular weight aromatic pitches that are mainly anisotropic in nature are referred to as mesophase pitches. 

Fibers produced from pitch precursors can be manufactured by heat treating isotropic pitch at 400 to 450°C in an inert environment to transform it into a hquid crystalline state. The pitch is then spun into fibers and allowed to thermoset at 300°C for short periods of time. The fibers are subsequendy carbonized and graphitized at temperatures similar to those used in the manufacture of PAN-based fibers. The isotropic pitch precursor has not proved attractive to industry. However, a process based on anisotropic mesophase pitch (30), in which commercial pitch is spun and polymerized to form the mesophase, which is then melt spun, stabilized in air at about 300°C, carbonized at 1300°C, and graphitized at 3000°C, produces ultrahigh modulus (UHM) carbon fibers. In this process tension is not requited in the stabilization and graphitization stages. 

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. ... 

Mesopitch based carbon fibers exhibit a variety of microtextures depending on the structure of the specific precursor fiber and the spinning conditions. Mesopitch based fibers prepared from 100% mesophase pitch have a more homogeneous microtexture than those prepared from mixtures of anisotropic and isotropic pitches. Further, disturbing the flow pattern of the... 

The formation of melt-blown pitch webs is followed by stabilization in air and carbonization in nitrogen. Processes have been developed with isotropic pitches and with anisotropic mesophase pitches. The mesophase pitch-based and melt-blown discontinuous carbon fibers have a structure comprised of a large number of small domains, each domain having an average equivalent diameter from 0.03 to 1 mm, and a nearly unidirectional orientation of folded carbon layers assembled to form a mosaic structure on the cross section of the carbon fibers. The folded carbon layers of each domain are oriented at an angle to the direction of the folded carbon layers of the neighboring domains on the boundary. 

When the elementary spherules of mesophase coalesce to form anisotropic domains, the microtexture of the carbonaceous mesophase becomes more complex. Disdinations (rotational defects) in the arrangement of the discotic molecules are often present. Disdinations in 2-D media play a role similar to that of dislocations in crystals and the evolution of gas bubbles results from condensation reactions in a medium which is still fluid. Disdinations, which occur during the mesophase formation, remain after carbonization and are key to understanding the relationships between the microtexture and the properties of carbon fibers formed from mesophase pitches. 

When pitch binder is pyrolyzed during the carbon bake operation, it is converted from an isotropic liquid, with no structural order, to a liquid-crystal (called mesophase) having a layered structure which is finally converted to layers of carbon atoms in a hexagonal lattice of graphite crystallites. These crystallites of binder coke become more disordered and crosslinked into a more-isotropic coke as the pitch QI content increases. Such moderately-isotropic coke, in contrast to highly-anisotropic microstructure (10), is preferred binder coke because it forms both physical and chemical bonds between filler coke particles which are stronger and more oxidation-resistant (8,9). 

In liquid-phase carbonizations, the mechanisms are completely different from those in the solid phase. It is via liquid-phase carbonizations (but not all liquid phases) that graphitizable forms of carbon result. How does this come about The explanation takes us to a quite different subject area, that of anisotropic aromatic, discotic, nematic liquid crystals (called mesophase) formed as a result of growth and self-assembly of the constituent polycyclic aromatic molecules of the parent material. These usually are the highly aromatic coal-tar pitches, a liquid product from the making of metallurgical coke, from aromatic pitches synthesized by the petroleum industry as well as polycyclic aromatic model compounds. 

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