Alexandre Maitre

Solid state reaction of zirconia with carbon

Abstract The solid–solid reaction between zirconia and carbon under flowing argon produces zirconium carbide via the intermediate formation of an oxycarbide ZrC0.84O0.06. In the temperature range 1623–1823 K, its final transformation into carbide is slow. The reaction producing the oxycarbide obeys the kinetic law between the degree of conversion α and time: F(α)=1−(1−α) 1/3 =K e −E/RI T , associated with an activation energy of 208±15 kJ·mol−1. This reaction occurs via two solid–gas reactions: ZrO 2 +0.84 CO=ZrC 0.84 O 0.06 +1.39 O 2 and 2.78 C+1.39 O 2 =2.78 CO . The oxycarbide appears on the surface of the oxide whose grain shape is not modified.

Spark plasma sintering of zirconium carbide and oxycarbide: Finite element modeling of current density, temperature, and stress distributions

A combined experimental/numerical approach was developed to determine the distribution of current density, temperature, and stress arising within the sample during spark plasma sintering (SPS) treatment of zirconium carbide (ZrCx) or oxycarbide (ZrCxOy). Stress distribution was calculated by using a numerical thermomechanical model, assuming that a slip without mechanical friction exists at the interfaces between the sample and the graphite elements. Heating up to 1950 � C at 100 � Cm in � 1 and a constant applied pressure of 100 MPa were retained as process conditions. Simulated temperature distributions were found to be in excellent agreement with those measured experimentally. The numerical model confirms that, during the zirconium oxycarbide sintering, the temperature measured by the pyrometer on the die surface largely underestimates the actual temperature of the sample. This real temperature is in fact near the optimized sintering temperature for hotpressed zirconium oxycarbide specimens. It is also shown that high stress gradients existing within the sample are much higher than the thermal ones. The amplitude of the stress gradients was found to be correlated with those of temperature even if they are also influenced by the macroscopic sample properties (coefficient of thermal expansion and elastic modulus). At high temperature, the radial and angular stresses, which are much higher than the vertical applied stress, provide the more significant contribution to the stress-related driving force for densification during the SPS treatment. The heat lost by radiation toward the wall chambers controlled both the thermal and stress gradients.

Influence of the grain size on the reactivity of TiO2/C mixtures

Abstract Mixtures of TiO 2 and C of different grain sizes do not show the same reactivity during the synthesis of titanium carbide TiC. The finest powders (∅≈0.2 μm) lead to the serial reaction TiO 2 →Ti 3 O 5 →Ti 2 OC→TiC, while the coarsest powder leads to the intermediate formation of Ti 2 O 3 . For temperatures between 1503 and 1873 K, the diffusional oxygen loss leading to the suboxides is very rapid. It is followed by a complex carburization forming Ti 2 OC via a rate limiting process governed by the consumption of carbon via C+CO 2 →2CO. CO 2 comes from an equilibrium associated with the suboxide grains, such as 2Ti 3 O 5 +13CO=3Ti 2 OC+10CO 2 , this explains the coupling observed between temperature and partial pressure of carbon monoxide. The last stage of the reaction, associated with the diffusion of carbon inside the grains of the oxycarbide, is slowed down probably because of the inhomogeneity of the mixtures at the end of the reaction.

Aging and overaging processes in Pb-0.08%Ca-x%Sn alloys using transmission electron microscopy and differential scanning calorimetry

Abstract The precipitation hardening and overaging mechanisms in three Pb-0.08%Ca- x %Sn alloys were investigated by means of hardness and calorimetric measurements (DSC) and transmission electron microscopy (TEM) observations. It is found that the highest tin content, close to 2 wt.%, allows delaying the overaging phenomenon in ternary Pb–Ca–Sn alloys. DSC studies show that tin has a favourable effect on the aging by providing further nucleation sites. Furthermore, the precipitate coarsening due to overaging shifts to higher temperatures upon increasing the Sn content. Using TEM to observe the microstructural transformations shows the presence of an epitaxial relationship between precipitates and the matrix during both aging and overaging. In the course of overaging, coarse precipitates adopt a lamellar morphology and their growth occurs preferentially along the [111] compact atomic plane direction. The tin content of coarse precipitates of (Pb 1− x Sn x ) 3 Ca increases with rising overaging time and follows the tin content of the initial alloy.

Carbon oxidation at high temperature during carbothermal reduction of titanium dioxide

Carbothermal reduction of titanium dioxide is a very complex reaction. It comprises several successive or simultaneous reactions, interest being focused on the sequence: which is itself composed of two reactions: the first being a quasi-equilibrium which fixes the partial pressure of carbon dioxide for the second reaction, slow and kinetically limiting. The overall reaction rate is identical to that of carbon oxidation. Hence, this method allows a study of the kinetic behaviour of carbon at high temperatures (here, 1523 K) under low partial pressures of carbon dioxide (10-2–103 Pa) which are unusual conditions. The degree of conversion of carbon α vs. time t follows the kinetic law: where the constant K determines the specific rate v of the reaction. Changes of the partial pressure of carbon dioxide, to which carbon is exposed, are obtained by imposing variable partial pressures of carbon monoxide (0–105 Pa). The resulting relationship v=f(P): shows that the oxidation of carbon is governed by the desorption of the carbon dioxide. This work proves the feasibility of a study of the carbon oxidation at low partial pressures of CO2 and for high temperatures and casts new light on the kinetics of the carbothermal reduction of titanium dioxide.

Gain structuration in dual-wavelength Nd:YSAG ceramic lasers

We demonstrate a dual-wavelength Nd:YSAG ceramic laser in which the gain volume is structurated into two different regions providing gain at the wavelength of 1061 nm and 1064 nm respectively. We discuss the role of the nonuniform distribution of the temperature in structurating the gain region via the Boltzmann effect. We show that the two laser wavelengths can be switched by adjusting the size of the pump beam or by slightly modifying the geometrical parameters of the laser cavity, either the length of the cavity or the orientation of a mirror. Additionally, we demonstrate that the transverse modes at the two wavelengths are shaped according to the effect of gain filtering caused by the structuration of the gain region.

Plasma-jet coating of preoxidized XC38 steel: influence of the nature of the oxide layer

Preoxidation of alloy substrates before spraying an oxide powder improves the adhesion of the coating. In the case of a low carbon steel (XC38) coated by plasma sprayed alumina, three preoxidation treatments in a classical furnace have been undertaken, in order to form respectively FeO, Fe3O4 and Fe2O3 layers. The best adhesion of alumina coating is obtained with FeO interlayers, and the worst with Fe2O3. The preliminary thermal treatment giving FeO implies a controlled atmosphere of carbon dioxide, but it points to some interesting prospects for coating complex parts. The origin of the improvement of adhesion may be a diffusion bonding at the FeO/Al2O3 interface or more simply it might be due to the increase of the substrate roughness with the time of oxidation. Besides, the epitaxial growth of FeO on steel probably explains the good mechanical resistance of the steel/FeO interface .

Experimental investigation and thermodynamic evaluation of the C–O–Zr ternary system

The 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.

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

The 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.

Synthesis of zirconium oxycarbide powders using metal–organic framework (MOF) compounds as precursors

ZrCxOy oxycarbides were synthesized for the first time by using metal–organic framework (MOF) compounds as precursors. These MOFs, based on zirconium carboxylates, are derived from the UiO-66 type structure. Three different Zr-MOF compounds were synthesized, differing by their C/Zr ratio, due to the use of terephthalic acid (C/Zr = 8, or UiO-66), naphthalene-2,6-dicarboxylic acid (C/Zr = 12) and biphenyl-4,4′-dicarboxylic acid (C/Zr = 14 or UiO-67). The oxycarbide crystallites obtained through appropriate thermal treatments (under Ar atmosphere) of the Zr-MOF precursors show an average size of a few hundred microns. They are surrounded by an outer rim of turbostratic carbon, whose thickness is directly relevant to the C/Zr ratio coming from the nature of the organic linker. The composition of the oxycarbides obtained was estimated on the basis of the cell parameters refined from the powder XRD patterns. The oxycarbides synthesized from naphtalene-2,6-dicarboxylic acid and biphenyl-4,4′-dicarboxylic acid are close to ZrC0.925O0.075, while that of the oxycarbide obtained from the terephthalic acid precursor is ZrC0.944O0.056. It results that the carbon richer oxycarbide product is observed for the UiO-66 precursor synthesized with terephthalic acid, and exhibiting the lower C/Zr ratio of the series. The composition of the oxycarbide powders is shown to be directly relevant to the mechanisms of decomposition of the Zr-MOF compound involved during heating.