Gilles Trolliard

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.

TEM investigation of forsterite dendrites

Abstract Transmission electron microscopy (TEM) has been used to investigate dendritic forsterite systematically, in order to understand its morphology in three dimensions. TEM images of olivine dendrites are reported for the first time. Crystals have been obtained by dynamic crystallization in the CaO-MgO-Al2O3-SiO2 (CMAS) system in a one-atmosphere vertical furnace. A fast cooling rate (1890°C/h) has been used with degrees of undercooling varying from 156 to 356 ∞C. Different microstructures were observed depending on their location in the dendritic crystal. The external part of the crystal reveals true olivine dendrite propagation, as classically observed in other materials (succinonitrile, alloys). In the inner part of the crystal, this microstructure changes to the formation of single crystalline units with well-defined crystalline faces. The shapes observed are very reproducible. These units are limited by the usual forms of olivine [(010), {021}, {110}, {120}, {101}] and also by less common forms [(001), {130}, {140}] as additional faceting. All these elementary units present the same morphology, the hopper shape that corresponds to the skeletal form of olivine. These units are linked in a peculiar spatial organization giving rise to dendrite branches. It is shown that the [101] preferential direction of growth of dendrites is a mean direction and corresponds to the juxtaposition of these elementary units. In the innermost part of dendritic crystals, a dissolution-recrystallization process of elementary units occurs and a typical textural ripening microstructure is observed. This textural ripening starts early and is coincident with the real dendritic growth (external part of the crystal). Thus, the final appearance of dendritic forsterite crystals mainly results from textural ripening. This study emphasizes that dendritic solidification is a complex phenomenon, and the various microstructures suggest that multiple mechanisms are involved during the formation of so-called dendritic crystals

Nano-petrographic investigation of a mafic xenolith (maar de Beaunit, Massif Central, France)

The thermal history of a mafic xenolith from the Beaunit maar (Massif Central, France) is reconstructedatthe basisofa transmissionelectronmicroscopystudy. The protolith isa meta-microgabbro(opx- 1,cpx-1, pl-1)sampledinthelowercontinentalcrust(T=870-970°C,P 0.7-0.8 GPa).The incorporation inthe basalticmagmaproducedfivereactionsaround orthopyroxene: opx-1 ficpx-2fi cpx-3(augite+highpigeonite) fi liq fi cpx-4. The final reaction is the transformation of residual cpx-3 (augite + high pigeonite) into cpx-5 (augite+lowpigeonite).The calculationofthetimerequiredforeachtransformationyieldsaminimumresidence time of the enclave in the host magma of 16 hours and a magma ascent velocity of 1.8 km.h -1 . Exsolutions are produced by pressure decreaseas the xenolith is brought up to the surfacein the host basalt. Fracturesobserved in primary minerals are interpreted as a consequence of xenolith shocks against the wall of the magma conduit.

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.