TEM study of the reaction mechanisms involved in the carbothermal reduction of hafnia

Abstract

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

Novel insight into the chemical analysis of light elements in oxycarbides

“…It has been previously shown that slight substitution of carbon atoms by oxygen atoms leads to a significant decrease in the cell parameter, the covalent radius of oxygen (rO = 0.64 Å) being lower than that of carbon (rC = 0.74 Å) [2][3][4]23,24]. A small change of the cell parameter induces in turn peak shifts in the XRD patterns, the peaks typically shifting towards high 2 angles as the oxygen content increases within the oxycarbide.…”

“…Indeed, as the most used route to elaborate carbides, the carbothermal reduction reaction which consists in the progressive reduction of dioxides with carbon generally leads to an oxycarbide final product [2][3][4] whose composition is largely dependent on the synthesis conditions. Besides, these ultrarefractory carbides are ideally suited for ultra-high temperature applications [5], thanks to their favorable properties: high hardness, good wear resistance and high decomposition temperature [6].…”

Enhancement of the Carbothermal Reduction of Hafnium Oxide by Silicon

“…[17] For the same effect to occur in the HfC system, silicon substitution into the HfC lattice would have to occur. [16,30] Due to the unlikely formation of SiO x species discussed above and thermodynamic considerations which conclude that Si cannot reduce group IV metal oxides, [31] we assume that no increased formation of CO (g) takes place. As an alteration of the lattice parameter was not observed, we therefore exclude a change in diffusion behavior as the reason for the increased carbide formation.…”

“…The calculation of free energy of intermediate reactions taking place during the carbothermal reduction of hafnia, assuming gaseous HfO (g) as a reacting species, as proposed by R ejasse et al, [16] can be found in Figure 6. The calculation of free energy of intermediate reactions taking place during the carbothermal reduction of hafnia, assuming gaseous HfO (g) as a reacting species, as proposed by R ejasse et al, [16] can be found in Figure 6.…”

“…[14] Due to problems faced in producing pure HfC 1.0 it is, at least for carbothermal processes, more correct to refer to the produced carbide as an oxycarbide HfC x O y . Recently, R ejasse et al [16] demonstrated that the main reacting species in the carbothermal reduction of HfC is not, as so far believed, HfO 2 , but gaseous HfO (g) , this is analogous to the formation of SiC in which SiO (g) drives the reaction. Recently, R ejasse et al [16] demonstrated that the main reacting species in the carbothermal reduction of HfC is not, as so far believed, HfO 2 , but gaseous HfO (g) , this is analogous to the formation of SiC in which SiO (g) drives the reaction.…”