Oxidation and Equilibrium Morphology of Zirconium Carbide Low Index Surfaces Using DFT and Atomistic Thermodynamic Modeling
2016
Osei-Agyemang, Eric | Paul, Jean-François | Lucas, Romain | Foucaud, Sylvie | Cristol, Sylvain
ZrC is a nonoxide high-temperature ceramic used for applications in harsh environments with corrosive and oxidizing conditions like nuclear reactor and nozzle flaps of jet propulsion engines. Moreover, easy oxidation of the crystal surfaces affects the excellent mechanical and physical properties. It is however imperative to study the oxidation processes on the different low index surfaces. DFT studies in conjunction with atomistic thermodynamic modeling have been used for this purpose. The (100) surface completely dissociates molecular oxygen into atomic species, and the surface is fully covered with oxygen atoms at a wide range of temperatures and pressures, recovering the bare surface at temperatures in excess of 1500 K. The (111) surface interacts strongly with O₂ by completely dissociating it into atomic species which adsorb at 3-fold fcc hollow sites. This surface is extremely difficult to clean of oxygen by manipulation of temperature and pressure alone. The bare surface cannot be recovered even at temperatures above 1800 K. The (110) surface on the other hand has a complex oxidation process, accumulating eight O atoms leading to the formation of surface ZrO₂ units with the release of CO₂. This ZrO₂-covered surface is highly stable and cannot be removed at temperatures below 2000 K by manipulating temperature and pressure alone. Wulff construction of the equilibrium morphology of the nanocrystallites upon oxidation reveals only the ZrO₂-covered (110) surface.
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