Carbon diffusivity

Hall et a/. pointed out that carburisation is controlled by three independent processes, i.e. carbon deposition, carbon ingress (through the protective scale) and carbon diffusion through the matrix. Carbon deposition usually occurs by decomposition of CH4 adsorbed on the surface or the catalytic decomposition of CO (Boudouard reaction). Hydrogen... 

There are a number of differences between interstitial and substitutional solid solutions, one of the most important of which is the mechanism by which diffusion occurs. In substitutional solid solutions diffusion occurs by the vacancy mechanism already discussed. Since the vacancy concentration and the frequency of vacancy jumps are very low at ambient temperatures, diffusion in substitutional solid solutions is usually negligible at room temperature and only becomes appreciable at temperatures above about 0.5T where is the melting point of the solvent metal (K). In interstitial solid solutions, however, diffusion of the solute atoms occurs by jumps between adjacent interstitial positions. This is a much lower energy process which does not involve vacancies and it therefore occurs at much lower temperatures. Thus hydrogen is mobile in steel at room temperature, while carbon diffuses quite rapidly in steel at temperatures above about 370 K. 

Ni3C decomposition is included in this class on the basis of Doremieux s conclusion [669] that the slow step is the combination of carbon atoms on reactant surfaces. The reaction (543—613 K) obeyed first-order [eqn. (15)] kinetics. The rate was not significantly different in nitrogen and, unlike the hydrides and nitrides, the mobile lattice constituent was not volatilized but deposited as amorphous carbon. The mechanism suggested is that carbon diffuses from within the structure to a surface where combination occurs. When carbon concentration within the crystal has been decreased sufficiently, nuclei of nickel metal are formed and thereafter reaction proceeds through boundary displacement. 

Crystallization of CaCO with the Anionic PAMAM Dendrimers by Carbonate Diffusion Method... 

Crystallization of CaCOj is highly dependent on nucleation condition. The precipitation of CaCOj in the absence or the presence of the G4.5 PAMAM dendrimer was carried out by a carbonate diffusion method similar to the method described by Addadi et al. [35]. A solution of the dendrimer with calcium chloride in 200 ml of distilled water was adjusted to pH 8.5 with aqueous NHj, and then placed in a closed desiccator containing crushed ammonium carbonate (Fig. 5). Carbon dioxide was introduced to the solution via vapor diffusion. The critical point of the appearance in the turbidity of the solution was observed at around 5 min. These solutions were kept at 30 °C under N2 for one day. The crys-... 

Fig. 5. The experimental set-up of a carbonate diffusion method for the precipitation of calcium carbonate...

Spherical vaterite crystals were obtained with 4-mercaptobenzoic acid protected gold nanoparticles as the nucleation template by the carbonate diffusion method [51]. The crystallization of calcium carbonate in the absence of the 4-MBA capped gold nanoparticles resulted in calcite crystals. This indicates that the polymorphs of CaCOj were controlled by the acid-terminated gold nanoparticles. This result indicates that the rigid carboxylic acid structures can play a role in initiating the nucleation of vaterite as in the case of the G4.5 PAMAM dendrimer described above. 

Partly disoriented layers of graphite were formed when each of these faces was dosed with sufficient benzene and annealed at temperatures between 375° and 425°C. At higher temperatures, the graphitic and other structures broke down and carbon diffused into the bulk of the metal crystal. 

CHX and hydrocarbon wax are, respectively, the active intermediates formed by the hydrogenation of surface carbide and products of FTS formed by chain growth and hydrogenation of CHX intermediates. The hydrocarbon wax can contain molecules with the number of carbon atoms in excess of 100. Bulk carbide refers to a crystalline CoxC structure formed by the diffusion of carbon into bulk metal. Subsurface carbon may be a precursor to these bulk species and is formed when surface carbon diffuses into an octahedral position under the first surface layer of cobalt atoms. 

Colbow, Zhang, and Wilkinson [128] showed that the performance of liquid feed fuel cells could be increased by oxidizing the carbon diffusion layer. The DL was electrochemically oxidized in acidic aqueous solution (impregnated in some cases with proton-conducting ionomer) prior to application of the electrocatalyst. 

Figure 4.1 Microstructures of cross sections of group V transition metal - carbon diffusion couples. From top to bottom V-C, Nb-C and Ta-C. Left column C, phase band between the a and the 6 phase right column Absence of the Q phase above the decomposition temperature (compare Table 4.1). Polarized light.

Figure 4.1 shows these diffusion couples below (left) and above (right) the decomposition temperature of the , phases.10 It is obvious that the phase bands of the , phases are present between the fi and 6 phases the left column of the microstructures while it is absent in the right column showing the presence of an invariant phase reaction. The accordingly invariant temperatures are given in Table 4.1. For the Nb-C system the formation of a phase band was most difficult to observe because the peritectoid temperature is rather low and the carbon diffusivity is slow. 

FIG. 14-95 Comparison of bubbles from a porous septum and from a perforated-pipe sparger. Air in water at 70°F. (a) Grade 25 porous-carbon diffuser operating under a pressure differential of 13.7 in of water, (b) Karbate pipe perforated with 1/16-in holes on 1-in centers. To convert inches to centimeters, multiply by 2.54 °C = SA (°F - 32). (National Carbon Co.)... 

FIG. 14-96 Pressure drop across porous-carbon diffusers submerged in water at 70°F. To convert feet per minute to meters per second, multiply by 0.0051 to convert inches to millimeters, multiply by 25.4 °C = s/9 (°F-32). (National Carbon Co.)... 

Cementation process. This process, now little used, consists of heating wrought iron or low-carbon steel in powdered charcoal or leather dust for 6 to 11 days in a closed boxlike furnace at 650 to 700°C. At these temperatures, carbon diffuses slowly into the surface of the steel, thus producing a thin coat of high-carbon steel over a core of low-carbon steel. This is essentially a case-hardening procedure, and steel produced in this manner is used largely in the manufacture of tools. 

The production of high-pressure steam by cooling syngas creates the potential for metal dusting. The term metal dusting refers to the situation in which a metal surface in contact with carbon monoxide turns to powder. This takes place under certain process conditions in which carbon diffuses into a metal matrix and forms carbides. These carbide formations result in a buildup of stress that causes metal dusting177. 

The carbon diffuses into the molten silicon droplet and reacts to produces SiC ... 

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