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Interfacial Phenomena in CNT-MMCs
Published in Andy Nieto, Arvind Agarwal, Debrupa Lahiri, Ankita Bisht, Srinivasa Rao Bakshi, Carbon Nanotubes, 2021
Andy Nieto, Arvind Agarwal, Debrupa Lahiri, Ankita Bisht, Srinivasa Rao Bakshi
Not only is thermodynamics important, but also kinetics. Figure 7.10 shows the tCrit values for carbide formation as a function of alloy composition at 1700 K [17]. It also supports the experimentally observed results. The mechanism of growth of interfacial SiC layer has also been studied [19], which is shown by the schematic in Figure 7.11. The growth of the silicon carbide will occur in two directions, that is (1) lateral growth on the CNT surface and (2) growth perpendicular to the carbide layer. The lateral growth will be caused by the reaction at the triple point, whereas the perpendicular growth is caused by diffusion of atoms through the SiC layer to the CNT interface. The diffusion of larger Si atoms in β-SiC crystal occurs by vacancy migration in through regular Si sites rather than through the tetrahedral and octahedral sites. However, smaller carbon atoms (0.77 Å) can easily diffuse through the interstitial sites in the β-SiC crystal. The activation energy required for the diffusion of carbon atoms is lower. The diffusivity of carbon atoms (~12.36 × 10–12 cm2 s–1 at 2000 °C) is approximately two orders of magnitude higher than that of Si atoms (~8.66 × 10–14 cm2 s–1 at 2000 °C) through β-SiC layer in the temperature range of 2283–2547 K [35]. Thus, the perpendicular growth of the SiC layer occurs by diffusion of carbon atoms from the CNT surface to the interface of the SiC layer and the Al-Si matrix. However, the growth of SiC would be restricted by the rapid solidification involved in plasma spraying and the available carbon atoms from the very stable CNT structure. Growth of aluminum carbide is also governed by its crystal structure, which is rhombohedral (space group R3m) as shown in Figure 7.12 [17]. It is made up of alternating layers of Al2C and Al2C2 with Al atoms having tetrahedral C coordination. C atoms have octahedral (C1 in Figure 7.12) and trigonal bipyramidal (C2 in Figure 7.12) coordination with Al atoms [36]. The Al2C layer is close packed with C in octahedral voids formed by close packed aluminum atoms. Therefore, it is expected that the lateral diffusion of carbon atoms by an interstitial mechanism would be favored through the Al2C2 layer. As seen from Figure 7.12, (0003) plane of Al4C3 has a hexagonal arrangement of carbon atoms similar to that in graphite. However, it is to be remembered that the C-C distance in graphite is 1.42 Å while it is 3.33 Å in Al4C3. Thus, Al4C3/CNT interface is expected to be strained without any orientation relationship.
Novel multi-stage aluminium production: part 1 – thermodynamic assessment of carbosulphidation of Al2O3/bauxite using H2S and sodiothermic reduction of Al2S3
Published in Mineral Processing and Extractive Metallurgy, 2018
M. A. Rhamdhani, N. Huda, A. Khaliq, G. A. Brooks, B. J. Monaghan, D. A. Sheppard, L. Prentice
The most important alternatives to the regular Hall–Héroult process include a modified Hall–Héroult process utilising inert anodes (Kvande 1999), the direct carbothermic reduction of alumina (Motzfeld et al. 1989), and the multi-stage indirect Al production (including the use of AlCl3 as an intermediate step) (Russell et al. 1979). In the case of direct carbothermal reduction of alumina/aluminous ores, a mixture of aluminium carbide and metallic Al is formed when alumina/aluminous ores are reduced by solid carbon at higher temperatures. The main problems with this route were high operating temperature and Al vapour back reactions with carbon monoxide (Stroup 1964) to form aluminium oxides and carbides that reduced the Al yield.