Faculté des Sciences

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Browsing Faculté des Sciences by Author "AIDROUS, Hakima"

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    Oxydation du nickel fritté dopé a l'aluminium au calcuim au cérium
    (Universite Mouloud MAMMERI Tizi-Ouzou, 2020-10-10) AIDROUS, Hakima
    The present study is concerned with the influence of sputter-coatings CaO on to the oxidation behavior of Ni polycrystals. The experiments were performed in air, in the temperature range 800°-1200°C. Below 1200°C, CaO coatings reduce the oxidation rate, while this beneficial effect disappears at 1200 °C. The oxidized specimens were examined by X-Ray diffraction, but also by EPMA depth profiling to evaluate the scale composition. Furthermore, electrical conductivity measurements and kinetic demixing studies were carried out on Ca-doped NiO single crystals, to get a better insight regarding the transport processes involved during oxidation. These last results show that the main characteristics that control the oxidation mechanism and contribute to the beneficial effect of CaO coatings are: - the lower driving force of diffusion (F) due to the shift of the Ni/NiO phase boundary to higher PO2, which increases with the calcium amount, - the blocking effects due to CaO precipitates near the position of the original metal surface, all the more important that the temperature is low and the oxidation time short. Furthermore, in agreement with previous experimental findings, the decrease of the vacancy fluxes which arrive at the metal/oxide interface allows to explain the improvement of the scale adhesion to the substrate observed experimentally.