Kinetyka redukcji siarczku niklu NiS

Reduction mechanism and kinetics of NiS nickel sulphide obtained during the process of nickel sulphidation, have been studied as a function of temperature (723-873 K). It has been found that the reduction process follows linear kinetics with activation energy of 103 kJ/mol. It is important to note that during nickel sulphidation and after the reduction of nickel sulphide, the product sample shows highly developed surface, creating thus the potential possibilities to be applied in heterogeneous catalysis.Przeprowadzono badania mechanizmu oraz kinetyki redukcji siarczku niklu, NiS, w funkcji temperatury (723-873K). Siarczek ten został uzyskany w procesie siarkowania czystego metalu. Stwierdzono, że proces redukcji zachodzi zgodnie z prawem liniowym, a jego energia aktywacji wynosi 103 kJ/mol. Należy podkreślić, że w wyniku siarkowania niklu oraz redukcji siarczku, otrzymany został materiał charakteryzujący się bardzo rozwiniętą powierzchnią właściwą, co stwarza potencjalne możliwości jego wykorzystania do celów katalitycznych

On the early stages of scale development on Ni - 22Al - 30Pt with and without Hf additions at 1150°C

Platinum modified Ni - Al-based alloys play an important role in the aeronautical industry as materials applied as Bond Coats (BCs) in thermal barrier coating systems (TBC), which provide protection against high temperature oxidation. Therefore, it is crucial for the assessment of the performance of the TBC to understand the oxidation behaviour of the Bond Coat material and the properties of the thermally grown oxide developing on it. This paper reports on the scale growth mechanism of Pt-modified γ′-Ni 3 Al-based alloys with and without Hf additions at the early stages of oxidation. Samples were oxidized at 1150°C for up to 50 min. The consecutive stages of the scale evolution were followed by a systematic approach using a two-stage oxidation treatment in atmospheres containing different amounts of 18O 2 oxygen isotope. The scale growth mechanism was followed with the aid of the in-depth analysis of elemental distribution, in particular of the oxygen isotopes, across the scale using secondary ion mass spectrometry (SIMS). The surface morphology of the scales was observed using scanning electron microscopy, while its local phase composition was determined using photoluminescence spectroscopy. In addition, the imaging SIMS was used to generate distribution maps of the oxygen isotopes. Because of the limited lateral resolution of the SIMS technique (down to 0.5 mm) they were conclusive only for the most prolonged exposures (50 min). Typical stages of scale evolution were observed for the growing scales: initially flat layers were formed, followed by more or less developed blade-like surface grains, phase-transformation-related cracks and round patches and, finally, ridges. The latter were only found on the Hf-free material. The scale growth mechanism evolved from an initially predominant outward growth mechanism towards an increasing contribution of the inward mechanism. The relative extents of the outward and inward growth mechanisms depended strongly on the surface fraction of patches and through-scale cracks. The phase composition analyses showed that cracks and patches are regions where the transient aluminas were preferentially transformed into the α-Al 2O 3 phase which is the required protective scale. The ridges formed in cracks essentially consisted of α-Al 2O 3. For the most of studied exposure periods both types of phases: transient aluminas and α-Al 2O 3 co-existed in the scales. The results obtained indicate that: (i) phase transformations occur locally and not simultaneously in the entire scale; (ii) Hf additions retard the phase transformation; (iii) the phase transformation is completed only on Hf-free material and not after oxidation for 50 min; (iv) the two-stage oxidation approach should be carefully applied to study the growth mechanism of evolving scales in which regions of different phase compositions develop

Oxide phases and residual stresses in scales formed at early stages of oxidation of Β-NiAl at 1473 K and the effect of implanted yttrium

Oxide phases formed at early oxidation stages on unmodified and yttrium-implanted β-NiAl intermetallic compound were studied with the aid of photoluminescence spectroscopy in scales developed at 1473 K for oxidation periods of up to 4 h. Two-stage-oxidation exposures with18O2-tracer were applied. The PLS method was also used to assess the residual stresses in α-Al2O3scale at consecutive stages of its development. It was found that scale formation occurs in a very local way, related to inhomogeneous distribution of Cr in substrate: sometimes distances as short as 10–15 μm were sufficient to observe entirely different phases in the scale. In general, implanted yttrium was found to retard the formation of a stable and protective layer of α-Al2O3. Oxidation for 4 h was necessary for no transient aluminum oxides (mostly θ and/or δ) to be detected, while on non-implanted material they were not found after only 26 min exposure. Only compressive stresses were found in α-alumina scales. They were rather high, reaching even 5 GPa. However, their dependence on the regions in the scale and the scale evolution stage was observed. The results indicate that patches frequently observed in scales on β-NiAl are regions where the phase transformation of transient aluminas to the α phase occurs preferentially. Ridges, which fill the transformation-related cracks, consisted only of α-Al2O3. Combination of these results with those obtained using SIMS distribution analyses of oxygen isotopes as well as with SEM/EDX, enabled very precise conclusions to be inferred concerning the scale growth mechanism. In particular, the local nature of the scale development can be described in more detail. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei

The early Stages of the Scale Growth on FeCrAl(+RE)-Type Alumina Formers

International audienc

The early Stages of the Scale Growth on FeCrAl(+RE)-Type Alumina Formers

International audienc