International audienceThe ZrCxOy oxycarbides are well-known relevant ceramic materials for ultra-high temperature applications. The intrinsic macroscopic properties of ZrCxOy being closely related to the C/O ratio, a detailed analysis of the C–O–Zr system has been undertaken experimentally in order to accurately determine the extent of the solid solution of oxygen within the oxycarbide phase at different synthesis temperatures. The obtained results were then used as diagrammatic data to extrapolate the ternary C–O–Zr phase equilibria diagram by the CALPHAD method, providing a predictive tool for the oxycarbide synthesis. The model proposed in the temperature range 1650–2000 °C is in fair agreement with results obtained in the literature. The chemical determination of the relative ratio between light elements (oxygen (O) and carbon (C)) being a difficult issue for most of the general applications, an accurate determination of the cell parameters of the different oxycarbide compositions has been performed to propose an abacus reporting the evolution of the cell parameter against the C/O amount. The chemical composition of the oxycarbide is shown to be determined with an accuracy better that a few percent. It is also shown that the evolution of the cell parameter is not linear, indicative of a possible change of the ionocovalent character of the chemical bonds with the composition of ZrCxOy

International audienceThe synthesis of HfCxOy oxycarbides through the carbothermal reaction of hafnia with carbon black was undertaken. The obtained powders at different rates of advancement were studied by TEM and XRD in order to investigate the reaction mechanisms involved during such a transformation. The contact between the two starting reactants is shown to be non-reactive, attesting to the transformation operating through solid–gas reactions. The hafnia phase is destabilized by the CO(g) rich atmosphere and is consumed by the migration of ledges at the surface of the crystals acting as a zipper mechanism that liberates HfO(g) and CO2(g) species. The carbon dioxide thus released is used in return to oxidize the carbon black forming carbon monoxide through the Boudouard equilibrium. The liberated HfO(g) then reacts with the ambient CO(g) to form the oxycarbide phase which is shown to nucleate in the carbon black areas. The oxycarbide nuclei display a core–shell microstructure which is formed by a single crystal core embedded in an oxygen rich amorphous phase. During the final stage of the reaction, the atmosphere, which, saturated in CO(g), progressively reduces the oxygen rich gangue until it finally disappears. The accurate determination of the cell parameter of the oxycarbide phase during the reaction indicates that the first formed compound is nearly saturated in carbon, comparable to the metallic carbide. The small change in the lattice parameter indicates that the chemical composition is very restricted, so the solid solution of oxygen within the hafnium oxycarbide seems to be very limited