Anisotropic domains

Polarized light photomicrographs were taken of the green and calcined cokes, as well as their corresponding test graphites. The untreated extract cokes are characterized by very small anisotropic domains on the order of 3 microns or less. This type of optical structure is believed to be highly desirable for nuclear graphite applications. 

Needle coke Ribbonlike parallel-ordered anisotropic domains that can also occur as folded structures... 

Lenticular/granular Lenticular anisotropic domains of various sizes that are not aligned parallel to the particle surface... 

Mixed layer Ribbon and lenticular anisotropic domains of various sizes in curved and irregular layered arrangements... 

Shot Ribbon and lenticular anisotropic domains arranged in concentric patterns to form shotlike coke... 

A severely weathered bituminous coal from eastern Canada was treated by thermal hydrogenation under various reactor conditions. The coking properties of this coal were found to be restored under appropriate hydrogenation conditions. The semi-coke of the hydrogenated coal exhibited an anisotropic coke structure. The size of the anisotropic domains in the semi-coke was found to depend on reactor temperature and hydrogen pressure during hydrogenation. 

M.-Th. Mackowsky. Have you recognized a relation between rank and the size of the anisotropic domains We believe that we have found an increase in the size of the anisotropic domains with rank and the degree of plasticity. In cokes from anisotropic coals we have two types of anisotropy, one related to the coal and the other formed in the plastic zone. 

Ribbonlike parallel-ordered anisotropic domains that can also occur as folded structures Lenticular anisotropic domains of various sizes that are not aligned parallel to the particle surface Ribbon and lenticular anisotropic domains of various sizes in curved and irregular layered arrangements Porous microstructure with walls that are generally anisotropic but with pores and walls that vary in size Ribbon and lenticular anisotropic domains arranged in concentric patterns to form shothke coke Isotropic carbon form closely associated with parent hquor. Higher in volatile matter than incipient mesophase... 

Phase diagram studies by NMR are generally based on the fact that the rapid molecular dynamics causes an elimination of any spin interactions to an extent that is directly related to the degree of anisotropy of the structure. The most important use of NMR is to distinguish between phases that give residual dipolar or quadrupolar interactions and phases that do not in general, this also marks the difference between optically anisotropic and optically isotropic phases. Whether residual couplings are seen depends on the extension of the anisotropic domains (microcrystallite sizes) in combination with the rate of molecular diffusion. 

A second stage, which reveals strongly time dependent annealing, relaxation and phase separation (large scale demixing of isotropic and anisotropic domains) phenomena, as illustrated in subsequent sections. 

The X-ray scattering is only sensitive to density flnctnations whereas seatler-ing of polarized light (so-called // -component) reacts to anisotropy fluctuations as well. It has been observed that the // -component does not show aity angular dependence for amorphons polymers in eqitihbrirrm. It demonstrates that there are no anisotropic domains resulting from alignment or order of ehains. 

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. 

As-spun mesophase pitch fibers consist of large and elongated anisotropic domains aligned almost parallel to the fiber axis [15] [25]. Since a high degree of orientation is achieved during melt extrusion, they do not need to be further stretched. Moreover, the transverse microtexture of these fibers can be tailored by controlling the flow of the discotic molecules in... 

Fiber Spinning and Mechanical Properties Aside from providing the critical concentrations at which liquid-crystalline behavior appears. Table m also lists the processing concentrations of the candidate block copolymers. Since rigid molecules would aggregate together to form anisotropic domains as their concentration exceeds Cer the processing of molecular composites must be controlled within this limit. 

MC moves that can be made in these types of systems inclnde rigid body rotations and translations besides the local displacements of atoms, and these larger moves greatly facilitate the exploration of the anisotropic domains in phase space. Moves of this type cannot be nsed for the amorphous phase, as rigid body rotations, for example, will most likely precipitate very energetic overlaps with neighboring molecules as has been discussed above. 

---