Search

Oxidation of uranium carbide (UC) small fragments from sintered pellets was experimentally tested to better understand UC safe-handling procedures given the renewed interest of non-oxide fuels for high temperature gas or liquid metal cooled reactors. Transformation from UC to U 3 O 8 via a self-ignition reaction was observed at partial pressure of oxygen as low as 10 Pa. The heat output from UC self-ignition in fragments (not-free from UO 2 contamination) previously stored in either air atmosphere or inert-atmosphere during a three months period was monitored at 973 K and 1073 K in air atmosphere in a TGA/DTA and no difference could be observed. Residual carbon content, measured as amorphous carbon, carbide or CO/CO 2 , decreased with exposition temperature in U 3 O 8 resulting oxide, this is in contrast with zirconium carbide resulting oxide, ZrO 2. Cracking and stresses accumulated in the oxide were highest for UC to U 3 O 8 compared to UC to UO 2 reactions and ZrC to ZrO 2 reactions. [ABSTRACT FROM AUTHOR]

The oxidation behaviour of four model alloys with the composition Ni-25Cr-xMn (with x = 0, 0.5, 1 and 1.5 wt%) was investigated at 1050 °C in air by thermogravimetry and by in situ observations in an environmental scanning electron microscope (ESEM). The addition of manganese modifies the oxidation rate of Ni-25Cr alloys by (1) increasing the parabolic constant kp compared to that for Ni-25Cr and (2) lowering the short-time oxidation rate. Regardless of the Mn concentration, in situ ESEM observations indicated the formation of spinel crystallites from the very beginning of the oxidation process. The size of the spinel crystallites was directly linked to the initial Mn concentration. The obtained results suggested that the formation of the spinel layer at the top of the chromia oxide scale rather than at the metal-oxide interface as thermodynamically expected must be attributed to the higher diffusion rate of Mn than Cr both in the lattice and at the grain boundary. [ABSTRACT FROM AUTHOR]

The transformation of Al microparticles into hollow and broken Al 2 O 3 microspheres was investigated by in situ environmental scanning electron microscopy (ESEM) up to 1150 °C under different heating rates and 120 Pa of gas atmospheres. Slow heating rates (2 °C min − 1 ) resulted in a better coverage of the particles shell than with fast heating rates (20 °C min − 1 ) that delayed the threshold temperature at which the particles opened. Above this threshold, the amount of opened spheres increased with heating rate, with the coarser particles opening more than the small ones. It appeared that inert atmospheres (He–4%H 2 ) increased the temperature at which the particles transformed, while air and pure oxygen tended to lower it. In contrast, the temperature interval was larger and was size-dependent when using water vapour. Irrespective of the gas atmosphere, opening of the spheres allowed molten Al to flow out from the core and aluminise the substrate while leaving behind a top coat of hollow alumina spheres. [ABSTRACT FROM AUTHOR]

Abstract: The total chromium oxide (Cr2O3) solubility is measured in the liquids belonging to the Na2O–SiO2 system using a thermochemical reactor which allows the independent control of temperature (1125°C

Abstract: Nb3Co3CrSi6 (isostructural to Nb3Fe3CrSi6) and Nb4Co4Si7 (isostructural to Zr4Co4Ge7) intermetallic compounds have been studied from the oxidation point of view. Isothermal oxidation results at 1300°C are discussed. The volatilisation of the external chromia layer formed by oxidation on Nb3Co3CrSi6 was quantified by analysing the thermogravimetric curves on the basis of the differential equation that describes the oxidation kinetics. Nb4Co4Si7 is a typical SiO2-forming compound with a parabolic oxidation constant k p′=6×10−13 g2/cm4 s. Nb3Co3CrSi6 oxidises by formation of a duplex SiO2–Cr2O3 layer but is mainly a Cr2O3-forming compound with k p′=1.8×10−11 g2/cm4 s and the linear volatilisation constant k l′=1.85×10−9 g/cm2 s. Transformation of Nb3Co3CrSi6 into Nb4Co4Si7 slows down the diffusion of Cr towards the surface leading to an overall decreasing of the Cr2O3 formation rate. [Copyright &y& Elsevier]

Microstructure phenomena resulting from high temperature oxidation of a simple nickel alloy and two cobalt-base superalloys were studied by microstructure examinations and thermodynamic calculations. The latter allowed to better quantify what happened to carbon during oxidation, which was not possible with microstructure examination and microanalysis alone, and to explain the microstructure changes due to oxidation. At 1000 ∘C, due to the Cr oxidation, the grain boundary carbides dissolve and the carbon diffuses inwards, where new carbides form. However, at 1100 and 1200 ∘C, the carbon diffuses outwards to the surface. Thermodynamic calculations were used to explain the seemingly different mechanisms of carbon diffusion. [Copyright &y& Elsevier]