Michel Billy

Oxidation mechanism of titanium nitride in oxygen

Despite the apparent complexity of the linear oxidation of titanium or titanium nitride resulting from the formation of a layered structure of the rutile scale formed, the uniqueness of the mechanism is proven. The limiting step appeared to be the predominant short-circuit diffusion of oxygen (E∼ 44 kcal · mole−1) through a rutile lamella of variable thickness growing at the nitride-oxide boundary and fracturing periodically to form a detached porous layer through which molecular oxygen can penetrate. The pressure dependence of the diffusion process in the case of the nitride was associated with the outward migration of nitrogen, while the undulations of the kinetics under certain conditions were caused by the growth of a sintered, recrystallized outer zone of oxide. The periodic exfoliation of the oxide was related to its poor adherence to the substrate, certainly due to the presence at the nitride-oxide interface of a thin gray film probably composed of intermediate phases between TiNx (or Ti) and TiO2.

Modifications structurales du nitrure de silicium en fonction de la temperature

Abstract The structural evolution of α and β - Si 3 N 4 has been studied up to 900°C by means of the time of flight neutron diffraction method. It has been shown that the relative evolution of lattice parameters is the same for both phases; the modifications versus temperature are isotropic along the cristallographic axas a and c. However, the atomic positions, bond angles and bond lengths at different temperatures, as well as the volume evolution of SiN4 tetrahedra compared to that of the unit cell volume, suggest a less stability of α Si 3 N 4 with regard to the β phase which explains the α → β transformation at higher temperatures.

Comportement du nitrure de niobium cubique δ en atmosphere d'oxygene

Abstract The oxidation of cubic δ NbN .87 plates and NbN .90 powder in oxygen at pressures of 25–730 Torr in the temperature range 425–600°C obeys pressure independent sigmoidal kinetics. There is formation of a low temperature form of Nb 2 O 5 . The reaction is governed by an inner interface process, the shape of the curves being associated to a chain fragmentation of the nitride substrate.

L'oxynitrure de silicium: Si2N2O: II. Evolution structurale et endommagement par irradiation aux neutrons rapides

Abstract Infrared spectroscopy and time of flight neutron diffraction methods have been used in order to study and to explain the structural modifications and damage effects in fast neutron irradiated silicon oxinitride. Atomic positions, bond lengths and bond angles are given for different irradiation doses, up to 3 10 20 n.cm −2 . An anisotropic behavior of the lattice parameters and a decrease of the volume of the unit cell have been set forth. This can be explained by a greater rigidity of the Si-1bN bonds compared to the Si-1bO bonds, and by the flexibility of the Si-1bO-1bSi bridge. During irradiation SiN 3 O tetrahedra undergo a rotation around the oxygen atom in association with a conter rotation of the opposite tetrahedron linked to the same oxygen. The destruction of the structure is a gradual process. The mechanism of amorphisation results from a more rapid breaking, in some cells, of the Si-1bO-1bSi bridge, in comparison with Si-1bN-1bSi, while the contiguous cells remain nearly undamaged.

The direct bonding between AlN ceramics and CaO-doped tungsten

Abstract The bonding of AlN ceramic substrates with tungsten can only be achieved by incorporating CaO into the metallic powder. This oxide improves the sintering process of W and leads to calcium aluminates at the AlN/W interface, whose enrichment in CaO through the metal gives compositions CA, C 12 A 7 and C 3 A with decreasing eutectic temperatures and viscosity. This allows a liquid phase to diffuse at firing temperatures. A good adhesion needs: (i) a CaO content threshold to form substantial amounts of liquid phase, which depends on the amount of alumina available from the nitride surface; and (ii) temperatures above 1490°C, a eutectic temperature in the system CaO/WO 3 , because the solid Ca tungstate film simultaneously formed near the W/AlN interface may prevent Ca aluminate from diffusing when not displaced towards the metal core by means of a dissolution-precipitation mechanism.

Modifications structurales de l'oxynitrure de silicium sous contraintes thermiques

Structural modifications of silicon oxynitride have been studied at high temperature up to 1200°C by time of flight neutron diffraction method. The thermal expansion along the three cristallographic axes are 1.4 ± 0.7 10−6 deg−1 along a, 4.4 ± 0.3 10−6 deg−1 along b, and 4.1 ± 0.4 10−6 deg−1 along c. The variation of the atomic positions are given, and also the evolution of bond angles and bond lengths between 20 and 1200°C. It is shown that the main contribution to the deformation is the evolution of the SiOSi angle which permits the rearrangement of SiN3O tetrahedra by rotation around oxygen atoms.

Suivi cinétique de la relaxation des contraintes de croissance au cours de l'oxydation d'un alliage FeCrAl☆

The oxidation behaviour of FeCrAl alloy in O2 was studied in the temperature range 800–1200°C for two types of sample: platelets and wires. In both cases the oxidation rate is controlled by a diffusion process. However, the induced growth stresses and their relaxation are very sensitive to the geometry of the oxidized sample. They have been evaluated as a function of time and temperature by following the resulting differences observed in the rate constants.

Resistance du nitrure de silicium a l'irradiation par neutrons rapides

Abstract Structural modifications of α and β silicon nitride have been studied under irradiation by fast neutron, by time of flight neutron diffraction method. The structure shows a great stability and after a dose of 3.10 20 n/cm 2 the relatives variations of the a and c parameters are 0,08 % and 0,16 % for the α phase and 0,15 % and −0,04 % for the β phase. The variation of the atomic position are given and also the evolution of bond angles and bond lengths up to 3.10 20 n/cm 2 .

Oxidation behaviour of vanadium nitride in carbon dioxide-carbon monoxide mixtures

Abstract The oxidation of vanadium nitride by CO 2 -CO mixtures, at a total pressure of 1 atm, for ratios 0.54 ⩽ P co 2 ,/ P co ⩽ 4 leads to the formation of a porous V 2 O 3 scale between 1050 and 1200°C. The kinetics are continuously decelerated. Their shape, intermediate between linear and parabolic, is well represented by a model based on the hypothesis of two parallel processes: a reaction regime slowed by the progressive closure of the open porosity of the scale and a diffusion process of the reactants through lattice defects or, more probably, low-resistance paths in V 2 O 3 . The effect of the nature of the substrate and atmosphere is discussed.

Equipment for studying hot corrosion processes in dynamic gas mixtures containing water vapour

Abstract A Setaram unit including a symmetric oven and a MTB 10 8 microbalance was chosen as a base equipment in order to reduce the impulse variations frequently occurring in dynamic regimes. The thermobalance and its surroundings were thoroughly thermostated owing to water vapour. To pull up and let down the sample quickly without damage for the torsion strip, a telehandling airtight apparatus with visual control has been worked out, allowing to make optional the connection between the suspension chain and the microbalance beam. The equipment here described has been conceived for hot corrosion kinetic studies (round 1000°C) concerning low weight samples (10–100mg) in such gaseous flows as X/H2O containing air, oxygen and hydrogen in the presence of water vapour (1–75 %).