Graphitization diamond corrosion

Diamond corrosion is usually a problem of oxidation and graphitization. Graphi-tization in inert atmospheres was observed at about 1500°C and becomes very fast at 2100°, where sizable diamonds are converted to graphite within minutes [44], An extrapolation of the data gives a F 1450°C for = 0. It has been reported that graphitization is prevented in pure H2 up to 2200°C [45]. 

Figure 1. P T diagram of the low-pressure, low-temperature labile equilibriums of carbon solution 1 = graphite-diamond equilibrium line, 2 = glassy carbon-diamond transition line, 3 = range of pneumatolytic hydrothermal processes, 4=oxidative corrosion of diamond, 5 = anticipated area of diamond hydrosynthesis, 6 and 7 = diamond synthesis from glassy carbon precursors, 8 = low-pressure, low-temperature hydrothermal homoepitaxy of diamond. Reproduced from [15] with permission from A. Szymanski.

Graphite paint n. A painting compound consisting of powdered graphite and oil used to coat metallic structures to inhibit corrosion. Pierson HO (1994) Handbook of carbon, graphite, diamond and fullerenes. Noyes Data Corporation/Noyes Corporation, New York. 

Carbon electrodes — Carbon is selected for many electrochemical applications because of its good electrical and thermal conductivity, low density, adequate corrosion resistance, low thermal expansion, low elasticity, and high purity In addition, carbon materials can be produced in a variety of structures, such as powders, fibers, large blocks, thin solid and porous sheets, nanotubes, fullerenes, graphite, and diamond. Furthermore, carbon materials are readily available and are generally low-cost materials. 

PTFE is a crystalline polymer consisting of twisted zigzag spirals with at least 13 repeating units per turn. This nonpolar polymer has a solubility parameter of 6.2 H, a high (327 C), and a heat deflection temperature of 121 C PTFE is a tough, flexible polymer which retains its ductility at extremely low temperatures (-269 0. The coefficient of friction of ptfe is the lowest of any known solid material (see Table 13.4). Films of ptfe can be bonded by adhesives to other surfaces if the polymer surface is treated with sodium. It also bonds to diamonds and graphite whose surfaces have been fluorinated. Liquid sodium removes fluoride ions from the surface and leaves free radicals on the polymer surface, ptfe is resistant to almost all corrosives and solvents, but it can be dissolved in hot perfluorinated kerosene, ptfe is difficult to mold or extrude. 

The oxidation of diamond is clearly an active corrosion process. At least up to 700°C diamond has a fast reacting lll -plane, an intermediate 110 and a slow 100 -plane, which indicates reaction control. At higher temperatures and/or lower oxygen pressures gas diffusion becomes rate determining in analogy with graphite [46], and this is indicated by a more even attack [47]. Hence corrosion rates are faster or start at lower temperatures for fine powders compared to films and the corrosion in air is faster than in low-oxygen environments [48]. 

However, there exists much information on corrosion of hexagonal graphite-like BN. Similar to the graphite and diamond phases of carbon, the reaction products are the same for all modifications of BN. However, hard modifications of BN typically show a higher corrosion resistance. Thus the available information on graphitic BN can be used for evaluation of the lowest corrosion resistance limit of hard BN. 

The support for the metal nano-particles turns out to be as important as the nano-particles for providing their dispersion and stability. Studies on electron transfer are particularly important for carbon-based materials because those materials, such as glassy carbon, graphite, fullerene, and diamond with different electronic and structural properties, have been proved to possess distinctly different electrochemical properties from each other (Ramesh and Sampath, 2003). Carbon supports provide high electronic conductivity, uniform catalyst dispersion, corrosion resistance, and sufficient access of gas reactants to the catalyst (Ismagilov et al, 2005). In addition to electrical conductivity and surface area, hydrophobicity, morphology, porosity, and... 

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