Reactive-element segregation

However in the present study the first quantitative results of reactive element segregation have been obtained. A comparison with the results of the SIMS investigations [7] indicates that the segregated reactive elements (Zr, Y) reduce the cation diffusion along the a-Al203 grain boundaries, as has been proposed by Przybylski et al. [14]. 

After oxidation at 1200 C a-A1203 was the only A1203 phase present in the oxide scale. Analytical studies revealed the segregation of reactive elements (Zr, Y) at the oxide grain boundaries as well as at the metal/oxide interface. Analytical studies on NiAl-0.1 Y further more indicate that Y can getter the S in the alloy. 

Exposure of freshly deposited permalloy films to the atmosphere or oxidation in pure O2 up to 250°C results in surface segregation and preferential oxidation of Fe to Fe203 [144-146], However, anodically formed films are enriched in Ni and consist of an inner layer of NiO and an outer layer of mixed nickel and iron hydroxides [147,148]. The more reactive element Fe oxidizes first and is enriched at the surface during atmospheric oxidation but dissolves into solution during anodic oxidation. 

B.A. Pint, Experimental observations in support of the dynamic segregation theory to explain the reactive element effect. Oxidation of Metals, 45, 1/2, 1-31 (1996). 

One important point is the combined action and effect of interface structure and interface reactions. The development of this kind of model could lead to a new approach to questions linked to the effect of impurities and their segregation, to the effect of reactive elements or to the coupling between mechanical and oxidation behaviour, which could be of major importance in the determination of service lifetime for high-temperature components. 

Once more, the presence of reactive elements allows protective oxide scales to form, even in the presence of sulphur. It is generally proposed that the reactive elements react with S and form sulphides, then stopping their detrimental effect on the oxide scale adherence [208,209]. However, very few studies were able to identify such compounds [210,211], even if S was detected [212], The role played by the RE then is preventing sulphur segregation at the oxide scale-metal interface [213]. 

Pint B A (1996), Experimental Observations in Support of the Dynamic Segregation Theory to Explain the Reactive Element Effect, Oxid Met, 45, 1-37. 

Alloy nanoparticles have attracted increasing attention because they display structures and properties that are distinct from those of the pure elemental clusters. A recent review by Eerrando et highlights that their chemical and physical properties may be tuned by varying the composition, atomic ordering, and size of the clusters. Surface structures, composition, and segregation properties of nanoalloys are important in determining chemical reactivity and especially catalytic activity. Synergism is sometimes observed in catalysis by bimetallic nanoalloys. 

This study deals with the preparation, properties and reactivity of high-surface-area Ni/Mg/Al mixed oxides featuring different Ni g ratios obtained from HT anionic clays. In HT precipitates all cations are present inside the brucite- pe layers, therefore the specific properties of each element may be evidenced without any interfraence due to phase segregation and/or physical dishomogeneity. 

As mentioned in Section 5.3.2.3, depth resolution may also be affected by primary ion beam-induced modification of the substrate chemistry. This is typically noted for reactive primary ion beams. As an example, 02 primary ion beams can oxidize the outer surface of certain materials. This can then facilitate the movement of the elements of interest within the solid, i.e. induce segregation to/from the surface. This effect is referred to as oxidation-induced segregation. Radiation-induced segregation discussed in Section 3.2.3.1 may also occur. Concomitant secondary ion yield variations resulting from these surface chemical modifications will further complicate matters. 

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