Carbonaceous phases

A final example of induced mesogenicity in a multicomponent system is the well studied, but less well understood, carbonaceous mesophases which are comprised of a myriad of unidentified molecules which are created in situ as petroleum pitches are heated to temperatures where chemical transformations occur [163]. The processes leading to a mesophase involve decreases in both the elemental weight fraction of hydrogen and the group fraction of aliphatic carbon atoms [164]. Model studies have demonstrated that the component molecules of these phases are fused, polycyclic aromatic molecules with disk-like shapes the exact structures of the components depend upon the natures of the precursor molecules which are heated [164-167]. All of the carbonaceous mesophases somewhat resemble discotic nematic phases [168]. At least some of them probably represent another example of liquid crystallinity induced by mixing molecular components which, when separated, are not mesogenic. 

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Although the FTS is considered a carbon in-sensitive reaction,30 deactivation of the cobalt active phase by carbon deposition during FTS has been widely postulated.31-38 This mechanism, however, is hard to prove during realistic synthesis conditions due to the presence of heavy hydrocarbon wax product and the potential spillover and buildup of inert carbon on the catalyst support. Also, studies on supported cobalt catalysts have been conducted that suggest deactivation by pore plugging of narrow catalyst pores by the heavy (> 40) wax product.39,40 Very often, regeneration treatments that remove these carbonaceous phases from the catalyst result in reactivation of the catalyst.32 Many of the companies with experience in cobalt-based FTS research report that these catalysts are negatively influenced by carbon (Table 4.1). 

Peptides interact with the carbonaceous phase of RP-HPLC supports and are eluted by increasing the strength (organic phase, most often acetonitrile) of the buffer. 

Carbon in the carbonaceous chondrites does not exist as polymer or organic molecules alone. Carbonates are also present in relatively small amounts 20,23) and the same is true for elemental carbon. Elemental carbon seems to exist as carbynes (triple-bonded allotropes of carbon). At least three types of carbynes have been described in Murchison 341 but these results were questioned in 1982 by Smith and Buseck63). According to these authors, sheet silicates mixed with elemental carbon could be misidentified as carbynes in X-ray diffraction patterns. These particular carbonaceous phases (carbynes or otherwise) and other carbonaceous phases (polymer and amorphous carbon phases called C-oe and C- 3) are carriers of noble gases trapped in the chondritic material. Some of these carbynes seem to be condensates from the protosolar nebula while others are probably of presolar origin34 >. 

We now have a fairly adequate understanding of the different properties, including the particle diameter i/p, the pore size, the degree of permeability, and the chemical composition of the surface of the support matrix, to know which type of stationary phase can be successfully used with a particular class of peptides. Most of the HPLC packing materials now in use for peptide separations are based on the wide pore microparticulate silica gels with polar or nonpolar carbonaceous phases chemically bonded to the surface of the matrix. Methods for the preparation of these chemically bonded stationary phases, their available sources of supply. 

Active carbons, cokes and other substances exhibiting in their structure the presence of pores with wide range of diameters, can be the components of carbonaceous phase [1,2]. The following pores exist in the structure ultramicropores with diameters below 0.4 nm, micropores - accessible for benzene particles - with diameters within 0.4 - 2 nm, transport mesopores (2 - 50 nm) and macropores (above 50 nm). 

The theoretical model ing of the kinetic aspects of diamond nucleation processes is indeed scarce in published literature. Although attempts have been made to model the nucleation kinetics,as reviewed above, the approaches require an accurate estimation of the kinetic rate constants, necessitating fitting the kinetic model to experimental data, thereby making the model system- (or experiment-) dependent. In addition, the kinetics of surface diffusion of adatoms and the formation of intermediate carbonaceous phases were not considered in these studies. As indicated in Ref. 217, a kinetic model is expected to contribute to a better understanding of the role of SiC formation in the nucleation of diamond on a Si substrate. However, the kinetic scheme employed in these studies was, in fact, unable to distinguish between a Si and a SiC surface. To capture the possibil ity that an intermediate carbonaceous phase (such as DLC, carbide or graphite) may form prior to diamond nucleation, the kinetic model should be modified to includea time dependence of the density of nucleation sites determined by the kinetics of the formation of the intermediate phase. Further studies are therefore needed to construct a clear picture of the kinetics of diamond nucleation processes in CVD. 

Fig. 9 shows the XRD patterns of LSCF powder both as-prepared and calcined at 900 °C for 4 h. Prior to calcination and after only 1 minute of microwave irradiation, the powder already exhibited mainly the perovskite-type structure. In addition, some peaks of deleterious phases (LaCo04 and SrCOs) were detected before calcination. The formation of SrCOs can be attributed to the reaction of Sr(NQ3)2 and CO2. Compared with the as-paepared pwwder, the LSCF powder after calcination displayed higher crystallization. Since carbonaceous phases... 

Fig. 10.29 TEM images of (a) the ZnO-coated LiNi Miij jO particle in [110] zone, (b) uncoated LiNijjMnjjO particle after cycling at 55°C arrows indicate the carbonaceous phases), (c) uncoated LiNi jMnj O particle after cychng at 55°C at a higher magnification (002 graphitic and 111 spinel spacings are shown for comparison), and (d) ZnO-coated LiNi Mn, 0 particle after cycling showing no carbonaceous phases on the particle surface...

A semiquantitative analysis carried out by integrating the silicon-related peak intensities revealed that at the beginning of the first lithiation Li preferentially saturated the graphite phase (intercalation), and later, silicon became involved in the reaction (alloying). However, due to the milder alloying conditions, in the presence of the carbonaceous phase, the formation of lithium-silicon alloys with high lithium content on the surface of the alloying silicon particles has been suppressed. Therefore, the carbonaceous media led to a more uniform distribution of Li within the bulk of the active silicon particles. 

The carbon structures formed under ion implantation reinforce the polymer surface and result in a drastic increase in its hardness (Fig. 4). These carbon-rich structures, with sizes ranging up to 40 nm in the case of energetic ion implantation [34], were found to be the aggregates of much smaller carbon clusters several nanometers in size [34,54]. The hardness of the carbonaceous phase produced by ion implantation does not depend directly on the concentration of sp sites (the... 

Optical spectroscopy thus enables us to trace the major stages of carbonaceous phase formation in the course of the ion bombardment of a polymer (1) formation of the initial carbon clusters and conjugated double bonds in the polymer host at doses below 4>f, (2) growth of ir-bonded clusters at higher doses, and (3) completion of cluster growth at high fluences. At the last stage most of the clusters reach... 

Slow degradation of the conducting carbonaceous phase, resulting in electrode deactivation, also occurs when oxidation of the redox species proceeds at the surface of ion-implanted polymers. 

Although conductivities up to 10 S cm can be achieved by this method [238] the high-energy ions used in this process lead to severe chemical degradation of the polymeric material with the formation of carbonaceous phases, responsible for the conductivity increase, as was shown for polyimides [238-243]. 

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