Graphitic matrix, crystallite

TEG macrostructure differs from that of natural graphite it possesses abnormally high porosity and highly developed active surface (40-50 m2/g) (Figure 1). The performed thermochemical treatment leads to an essential exfoliation of graphite matrix with a formation of cellular structure. The thickness of cell s walls is equal to 20-25 nm. The surface of cell s walls contains a lot of macrocracks, outcrops of crystallites, etc. The thermochemical re-treatment was applied to enhance TEG dispersivity. 

X-Ray Diffraction Data. The QI components found in the products of hydrocarbon pyrolysis are generally brittle, infusible solids. Because of this appearance the QI are usually classified under the general heading of coke. However, to more clearly define the nature of the QI produced in this work, x-ray diffraction patterns were obtained. The crystallite parameters for a graphitic matrix, as defined in Table II, were calculated for the QI and are compared in Table II with the same parameters determined for a sample of /3-resins and for a sample of coke obtained from a test carbon anode. The x-ray diffraction pattern for the sample of /3-resins was not well defined, so the value of Lc could not be determined reliably. The values of the other /3-resin parameters are... 

Crystallite parameters La, crystallite diameter in the plane of the a axis Lc, crystallite thickness in the C axis a, graphite unit cell dimension in the plane of the layers C, twice the interlayer spacing in the graphite matrix. 

Fig. 8.4 Phase-contrast TEM image of SiC whisker in alumina matrix that has undergone oxidation. The silicate glass (G) has begun to react with alumina to form a mullite nucleus (M), and graphitic carbon (C) surrounds the crystallite.14...

It is worth to mention that under the same conditions (1540 °C, N2) leading to surface crystallization via gas phase processes, Kleebe et al. were able to show by TEM and XRD that, within an amorphous SiCN matrix, globular inclusions also exist where crystallization has taken place and where crystallites of graphite, silicon carbide, as well as silicon nitride were found side by side. However, the thermodynamic stability of this system vanishes at higher temperatures [162]. At T = 1600 °C silicon nitride has disappeared and, e.g., the Si NMR spectrum reveals only [SiC4] (and no longer [SiN4] which had been detectable at T < 1500 °C.)... 

Attempts were made to discover the correlation between the crystalline structure of carbonaceous materials and their capability to reversibly intercalate lithium. This correlation has not been definitely established, but still, one can assume as a certain general principle that the optimum materials would contain an amorphous matrix with inclusions of a mesophase nuclei of graphite crystallites. Such materials are various cokes, pyrographite, and products of pyrolysis (carbonization) of various polymers. For practical purposes, the industry mastered some special materials providing high characteristics of negative electrodes in lithium ion batteries. The most popular material is manufactured by the Japanese company of Osaka Gas Co. under the name of mesocarbon microbeads, MCMB it represents the carbonization product of pitchy resins under a certain temperature regime. 

---