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

The first part of this work is dedicated to an experimental investigation of the C-Hf-O system and in particular to the stability domain of the HfC x O y oxycarbide phase formed through the carbothermal reduction in HfO 2 by carbon black at 1650 and 1750°C. This study which has been carried out by coupling chemical elemental analysis of powder with a structural approach by X-ray powder diffraction and Transmission Electron Microscopy (TEM) shows that the solid solution of the oxygen ranges from HfC 0.98AE0.01 O 0.02AE0.03 to HfC 0.90AE0.01 O 0.10AE0.03 at 1650°C and from HfC 0.97AE0.01 O 0.03AE0.03 to HfC 0.89AE0.01 O 0.11AE0.03 at 1750°C. It was then concluded that contrary with earlier previous results from the literature, the stability field of the solid solution is very limited and not very sensitive to temperature. This behavior is also in contrast with results obtained on the ZrC x O y oxycarbide which shows an isostructural form of HfC x O y . The second part of the paper reports on the first modeling of the ternary thermodynamic diagram of the C-Hf-O system by the CALPHAD method. The accurate data experimentally obtained on the solid solution were integrated as diagrammatic data for the calculation. The modeling was based on from already published binary assessments after a selection of compatible binary models and the description of the HfC x O y solid solution was then added and optimized using the data of this work. As a result, a set of coherent parameters has been obtained, allowing to calculate isothermal sections of this C-Hf-O ternary system. K E Y W O R D Soxycarbides, solid solutions, thermodynamics, transmission electron micros, X-ray methods

Fully dense boron carbide monoliths exhibiting fine microstructure (i.e., submicrometric grain size) are sintered by Spark Plasma Sintering. Two different commercial powder batches, exhibiting different stoichiometries (i.e., B/C ratio and oxygen content) and various amounts of secondary phases (i.e., boric acid and free carbon), are used. Their chemical composition is well‐defined by coupling different methods (Transmission Electron Microscopy associated with XRD analyses, and Instrumental Gas Analysis), and are correlated with their mechanical properties, characterized from meso‐ to macro‐scopic scales by nano‐indentation and ultrasonic pulse echography. The presence of secondary phases (graphite and boric acid) is evidenced in various proportions in each powder batch. If the boric acid disappears during sintering, the graphite remains. However, for the considered amounts of graphite (lower than 1 wt%), the low variations in graphite content have no significant effect on hardness and elasticity values. At the opposite, the presence of oxygen in boron carbide lattice, leading to a boron oxycarbide phase, induces a decrease in both hardness and elasticity properties.

Pure powders of TiCxO(1-x) solid solution were synthesized through the carbothermal route. The chemical analysis of the light elements in the as-obtained TiCxO(1-x) oxycarbides powders were performed by Instrumented Gas Analysis (IGA). The cell parameters of the samples were determined with an accuracy of about 2% by means of X-ray powder diffraction and the internal standard method. As a result, a model correlating the cell parameters to the chemical composition was established. These reference TiCxO(1-x) oxycarbides powders were then sintered in order to obtain pellets of dense ceramics. After having determined that the sintering process does not change the chemical composition of the starting powder, chemical analysis of the different samples of the solid solution were successfully undertaken by Ion Beam Analysis techniques (IBA). The Nuclear Reaction Analysis (NRA) method -that was used to analyse light elements with very high sensitivity -was coupled with Rutherford Back Scattering (RBS) analysis in order to accurately determine the metallic over light elements ratio and to determine the stoichiometry of the phase on massive samples. Exhaustive simulations of the NRA spectra were performed and demonstrated that discrete compositions of the TiCxO(1-x) can be efficiently measured locally for bulk samples. Compared to IGA results, the relative amounts of carbon and oxygen of bulk materials were determined with a bias lower than 5%. This protocol being implemented for the TiCxO(1-x) system was then tested on HfCxO(1-x)with the same success.3

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The aim of this work is to provide some experimental information within the M-C-O ternary systems (where M is a transition metal belonging to the IVB group). More particular, this study is devoted to the determination of the reaction and specific heats of the ZrC x O y , TiC x O y , HfC x O y oxicarbides and of their stability domain in their respective ternary phase diagram, these data being useful to assess more accurately the corresponding ternary systems. These compounds which exhibit a FCC solid solution over a large range of temperature [1] are of interest in nuclear and high temperature applications.So, bulk carbides and oxides were synthetized by Spark Plasma Sintering, allowing us to obtain very dense materials (close to the theoretical density). These pellets were used as carbide / oxide couples and heated to high temperatures. Chemical compositions at the equilibrium of the oxicarbide phase were then analysed in order to determine the composition and the structural evolutions of these solid solutions (the lattice parameter being directly related to the C/O ratio).Another way to obtain oxicarbides with specific compositions is based on the carboreduction of transition metal oxides [2,3]. Thermal analysis (DTA and DSC) measurements were carried out during the carboreduction process from carbides and oxides powders mixtures to determine the heat of formation of the oxicarbide phase as a function of its C/M or O/M ratio. These results will be used for the optimization process through the PARROT module of the Thermo-Calc software.