Order carbon vacancies

As above-mentioned, the oxidation of ZrC is diffusion controlled. The oxygen diffusion should be prompted in presence of carbon vacancies, especially ordered carbon vacancies. It has been reported that the starting temperature of oxidation should be at 300 °C for stoichiometric or nearly stoichiometric ZrC, independent of oxygen pressures (Shimada Ishii, 1990). At the initial stage of oxidation, a thin layer of oxycarbide can be observed to form. At an elevated temperature, the oxycarbide layer can be transformed to an amorphous one, from which the cubic/tetragonal zirconia nanocrystals can be developed. To date this process is still not clear. Though the formation of the oxycarbide layer can be proposed by the composition analysis, the structure is not completely identified. Many authors proposed that the structure of the formed oxycarbide should be similar to rocksalt structured ZrC with a small... 

To date extensive investigations have been carried ont on the syntheses and applications of the transition metal carbides. Abont the non-stoichiometric carbides with ordered carbon vacancies, however, the stndies are still insufficient, especially on their synthesizing route and potential applications. As to the ordered Zr(Ti)C, the following issues need further study in detail ... 

In summary, by spark plasma sintering of the mechano-chemically synthesized ZrC nano-powders with certain carbon content, the ordered carbon vacancies in sintered ZrC(, can be achieved simultaneously during the sintering process, forming a cubic... 

Based on those observations, we can conclude that the oxidation temperature and process of nonstoichiometric ZrC, especially that with ordered carbon vacancies, are strongly dependent on the concentration and state of carbon vacancies. The ordered vacancies provide the additional fast tracks for the oxygen diffusion, inducing the fast oxidation of non-stoichiometric ZrC. The ordered carbon vacancies are not conducive to the application of carbides at high temperature and under aerobic conditions. However, the fast oxidation of ordered ZrC provides a new route to fabricate the metastable tetragonal zirconia nano-powders. 

Xiang, J. Y, Hu, W. T., Liu, S. C., Chen, C. K., Zhang, Y, Wang, P. et al. (2011). Spark plasma sintering of the non-stoichiometric ultrafine-grained titanium carbides with nano superstructural domains of the ordered carbon vacancies. Materials Chemistry and Physics, 130, 352-360. doi 10.1016/j.matchemphys.2011.06.048. 

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