Carbon crystallite

The other process is the transformation of an organic precursor into a continuous thin ceramic fiber. In the spinning process, polycarbosilane, a high molecular weight polymer containing Si and C, is obtained by thermal decomposition and polymerization of polydimethylsilane. The fiber thus produced consists of a mixture of P-SiC, carbon crystallite and SiO. The presence of carbon crystallite suppresses the growth of SiC crystals. Yajima and coworkers (Yajima et al., 1976, 1978, 1979) were the first to produce fine (10-30 pm in diameter), continuous and flexible fibers, which are commercialized with the trade name of Nicalon (Nippon Carbon Co.). 

Figure 6. Relation between interlayer spacing (c /2) and size of carbon crystallites in c-axis direction (Lc)> in Angstrom units. Black squares represent positions of minor peaks within the principal anthraxolite (002) band regiony which correspond to me values of Lc indicated. Curve after Blayden, Riley y and Taylor, (4)...

The similarity between x-ray diffractograms of Pennsylvania anthracite and the anthraxolite suggests a common metamorphic history rather than a common origin. The dimensions of the carbon crystallites in the anthraxolite have been approximated on the basis of published relationships between crystallite dimensions, carbon contents, and interlayer spacing. These data indicate... 

An ambitious review of carbon applications in chemical power sources (see also Section 5.3.5) was offered by Fialkov [94], It is disconcerting, however, that the author discusses the influence exerted by... surface properties without citing even one of the well-known—or well-cited or more recent—studies on carbon surface chemistry. And yet, in conjunction with the use of carbon in air (oxygen) electrodes, he speculates that the oxygen electroreduction kinetics depend on... the degree to which side faces of carbon crystallites are developed because base groups are formed there and presumably interact (e.g., with HjOj) in the following manner ... 

The Brunaurer Emmett Teller specific surface area (BET SSA) of carbon black is influenced by the size, the porosity, and the surface microstructure of the primary particles. Surface heterogeneities given by graphitic planes at the surface, amorphous carbon, crystallite edges, and slit-shaped cavities representing adsorption sites of different energies describe the surface microstructure. 

Fig. 14 Schematic view of the association between morphological arrangements of carbon crystallites and energetic characteristics of carbon black surfaces. Four different types of adsorption sites are distinguished that refer to the de-convolution shown in Fig. 13...

Heckman and Harling57 examined the gas-phase oxidation of carbon black micro-structures and showed that oxidative attack of carbon crystallites was concentrated on the small crystallites, at the edges of layer planes and at lattice defects. Partial graphitization of a carbon black, so that only the outermost surface layers are well-ordered, causes oxidative corrosion within the core of the carbon particle, leaving an outer shell . Consequently, similar behavior can be expected for ungraphitized and partially graphitized carbons in electrochemical environments. 

The foregoing discussion serves to show that disordered carbon structures are oxidized more readily than well-ordered graphite planes and that dislocations and active sites provide nucleation points for attack of the carbon crystallite. Another factor that must be considered is that dispersed electrocatalysts, such as platinum, on the carbon surface are not benign. The electrocatalysts interact with the carbon causing local oxidation or corrosion, i.e., the platinum catalyzes the corrosion of the carbon itself. In the presence of oxygen, which is the condition under which the electrocatalyst will operate, reduction intermediates from the oxygen (e.g., HOj) can have an accelerated corrosion effect. 

FIG. 1 Schematic repre.sentation of the constitution of a carbon crystallite where L and Li are the sizes and thickness of a crystallite and y is the angle that characterizes the intrinsic defects of graphite planes, in compliance with the data of Ref. 60. 

Mineralized tissues of skeletons and shells of molluscs, annelids, barnacles, and echinoderms may be affected by an enzymic dissolution (digestion) of the organic matrix which contains calcium carbonate crystallites (Carriker and Smith, 1969 Carriker et al., 1969). An ability to digest organic matter from within animal skeletons has been demonstrated for bacteria (DiSalvo,... 

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