Metal-substrate alloy formation

Traditional alloy design emphasizes surface and structural stability, but not the electrical conductivity of the scale formed during oxidation. In SOFC interconnect applications, the oxidation scale is part of the electrical circuit, so its conductivity is important. Thus, alloying practices used in the past may not be fully compatible with high-scale electrical conductivity. For example, Si, often a residual element in alloy substrates, leads to formation of a silica sublayer between scale and metal substrate. Immiscible with chromia and electrically insulating [112], the silica sublayer would increase electrical resistance, in particular if the subscale is continuous. 

Bismuth-film electrodes (BiFEs), consisting of a thin bismuth-film deposited on a suitable substrate, have been shown to offer comparable performance to MFEs in ASY heavy metals determination [17]. The remarkable stripping performance of BiFE can be due to the binary and multi-component fusing alloys formation of bismuth with metals like lead and cadmium [18]. Besides the attractive characteristics of BiFE, the low toxicity of bismuth makes it an alternative material to mercury in terms of trace-metal determination. Various substrates for bismuth-film formation are reported. Bismuth film was prepared by electrodeposition onto the micro disc by applying an in situ electroplating procedure [19]. Bismuth deposition onto gold [20], carbon paste [21], or glassy carbon [22-24] electrodes have been reported to display an... 

When surfaces of tribological systems are involved in the mechanical activity of rubbing, direct reactions of surface adsorbed films with solid surfaces take place. The mechanically activated clean surface (nascent surface) of the metals and alloys is extremely reactive. Tribofilm formation is caused by the interaction between the metal (M, substrate) nascent surface under high energy and chemisorbed molecules of additive (adsorbate) (Buckley, 1981). 

With metallic adsorbates very close-packed overlayers can be formed, because metal adsorbate atoms attract each other relatively strongly and coalesce with covalent interatomic distances. When the atomic sizes of the overlayer and substrate metals are nearly the same, one observes one-monolayer (lxl) structures, where adsorbate atoms occupy every unit cell of the substrate. With less equal atomic radii, other structures are formed, dominated by the covalent closest packing distance of the adsorbate. Beyond one close-packed overlayer, metal adsorbates frequently form multilayers or also three-dimensional crystallites. Alloy formation by interdiffusion is also observed in a number of cases, even in the submonolayer regime. 

Reid and Schey studied the role of substrate composition and other factors in the formation and performance of films on various metal substrates, including copper, aluminium, titanium and mild steel, tested against themselves and against an alloy steel. They used a twist-compression test to assess performance, and concluded that substrate hardness and composition had the greatest influence on film formation and life. They believed that film formation and especially durability are improved by chemical reaction if a substrate, such as copper or iron, has a strong tendency to react to form a sulphide, provided that the reaction kinetics are favourable. However, they found no direct evidence of reaction or of sulphide formation. Their conclusions were based on the fact that the durability of the films was found to be in the sequence aluminium, titanium, iron, copper, which is the same as the order of the free energies of formation of their sulphides. 

The initial step in the preparation of the mixed-metal films consisted of vapor deposition, one metal at a time, onto a refractory substrate sequential deposition was necessary in order to track the doser-calibration conditions. Alloy formation was then carried out by a high-temperature treatment. Figme 5 shows LEISS spectra of a Mo(llO) substrate on which ten monolayers of a Pt-Co mixtrrre, deposited in a 1 4 monolayer ratio, was heated to selected temperatures. In Fig. 5a, 2 ML of Pt were deposited first, followed by 8 ML of Co in Fig. 5b, the reverse was done in which 8 ML of Co were generated first. 

A great variety of structures are formed after deposition of one (or several) metals on the surface of another [1]. The deposited metals may form alloys with each other or they may form islands with some microstructure [7,8] with the substrate in the first or deeper layers [1-6]. Alloy formation at the surface may be observed even in those cases where there is phase separation in the bulk [9-11]. If the size mismatch between the deposited and substrate atoms is large, misfit dislocation structures may be formed [12-14]. 

Transition metal adsorption results in one of two types of behavior alloy formation and the initial growth of epitaxial structures. Mn on Pd(100), Au and Pd on Cu(100) and Sn on Pt(lll) can result in substitutional structures in which the adatom replaces an host atom in the top layer of the substrate. This results in the formation of an ordered alloy which is confined to the first atomic layer of the surface. Because of the size difference between the adatom and host atom, this substitution can result in a buckling of the alloy monolayer (see table 11). 

In this context, rare earths on transition metal substrates attracted considerable research attention from two directions i) to understand the overlayer growth mechanisms involved [3] and ii) to prepare oxide-supported metal catalysts from bimetallic alloy precursor compounds grown in situ on the surface of a specific substrate [4,5]. The later studies are especially significant in terms of understanding the chemistry and catalytic properties of rare earth systems which are increasingly used in methanol synthesis, ammonia synthesis etc. In this paper, we shall examine the mechanism of Sm overlayer and alloy formation with Ru and their chemisorption properties using CO as a probe molecule. 

The subtractive methods practically can only be used to produce single films with lower refractive indices than those of the multicomponent glass substrates. Consequently in optics their use is limited to reflection-reducing films. This limitation does not exist with the additive film formation methods. With additive methods, it is possible to deposit in any sequence low- or non-absorbing compound films with various refractive indices as well as absorbing an highly reflecting metal or alloy films and mixtures of both types of films. It is obvious therefore that the subtractive methods are almost completely superseded by additive ones. The additive film formation methods can be classified into chemical and physical methods and further into wet or dry processes the latter may run at atmosphere or under vacuum. 

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