Silicides reactivity

Clearly the best protection from oxidation by a silicide as a coating on a reactive substrate would be the disilicide, which has the highest silicon content, and could be expected to provide a relatively protective silica coating. 

Silicides of groups I and 2 are generally much more reactive than those of the transition elements (cf. borides and carbides). Hydrogen and/or silanes are typical products e.g. ... 

Synthesis in liquidAl Al as a reactive solvent Several intermetallic alu-minides have been prepared from liquid aluminium very often the separation of the compounds may be achieved through the dissolution of Al which dissolves readily in several non-oxidizing acids (for instance HC1). For a review on the reactions carried out in liquid aluminium and on several compounds prepared, see Kanatzidis et al. (2005) binary compounds are listed (Re-Al, Co-Al, Ir-Al) as well as ternary phases (lanthanide and actinide-transition metal aluminides). Examples of quaternary compounds (alumino-silicides, alumino-germanides of lanthanides and transition metals) have also been described. As an example, a few preparative details of specific compounds are reported in the following. 

All rare earth metals can be characterised as being electropositive with respect to most other metals this fact, coupled with their large atomic radius and high reactivity towards non-metals, points the way to their widespread use as alloying constituents. However, in the manufacture of ductile iron and in steelmaking it is practice to use mischmetal or mixed rare earth silicides as the rare earth additive and from purely cost considerations this situation is unlikely to change significantly. 

Iron(III) salts, and Grignard reagent reactivity, 9, 49 Iron seleno-terephthalates, for fungus and molds, 12, 458 Iron silicides, formation, 6, 19... 

Silicon is a very hard but brittle element having a melting temperature of 1422°C and a density of 2.40. This element is fairly reactive toward the halogens and solutions of strong bases such as potassium hydroxide. Silicon reacts less readily with oxygen to form silicon dioxide and with other elements similarly to form a class of binary compounds known as silicides. 

If SiH4 is added to the reactive gas mixture, then tungsten silicide can be deposited.18 In this case. He was used as a diluent, along with the WF6 and SiH4, and deposition was carried out in a parallel-plate, cold-wall plasma... 

A comparative study of microsilicas from 18 sources showed considerable variation in composition and properties, one of those examined containing as little as 23% of SiOj and having a specific surface area of only 7.5 m g (A21). The same study showed that in most of the samples the diffuse XRD peak from the glass accounted for 98-99.5% of the total diffracted intensity and that it peaked at the value of 0.405 nm characteristic of vitreous silica. The commonest crystalline impurities detected were KCl, quartz, metallic iron and iron silicide, and pozzolanic reactivity was found to depend more on the chemical composition and nature of impurities than on the fineness or SiOj content. A surface layer of carbon, if present, greatly decreased reactivity. 

Lithium silicide, LigSi2.—By heating excess of lithium with silicon, and expelling the uncombined metal at 400° to 500° C., the silicide is obtained as a very hygroscopic, dark-violet, crystalline substance 8 of density 1 12. It is a very reactive product and a powerful reducer. With concentrated hydrochloric acid it yields spontaneously inflammable silicoethane, Si2H3, of which it may be considered a derivative. 

It was shown that chromium disilicide (CrSi2, Eg= 0.35 eV) nanociystallites are embedded in monocrystalline lattice by reactive deposition epitaxy (RDE of Cr) and Si molecular beam epitaxy (MBE) [1]. Redistribution of CrSi2 NCs has been observed in silicon-silicide-silicon heterostructures with one embedded layer by HR XTEM data [2]. 

Summary Both in the Rochow synthesis of methylchlorosilanes and in the reaction of transition metal silicides with HCl, catalytic reactions of silicon, bound as metal silicide, with gaseous reactants are involved. With both reactions, the kinetic parameters ko and Ea exhibit consequent compensation effects, with the isokinetic temperature positioned within the range of reaction temperatures investigated. In this paper, we ply the model of selective energy transfer fiorn the catalyst to adsorbed species to the kinetic data. With Rochow synthesis Si-CHs rocking frequencies, and with hydrochlorination of silicides Si—H vibration frequencies could correspond to the isokinetic temperatures observed. An interpretation in terms of accessibility of the reactive silicon atom to reactant molecules is given. 

The basis for a common interpretation of the two compensation effects should be the control of the reactivity of silicon atoms by the nature of neighboring metal atoms as catalysts or promoters as well as by structural or morphological properties of the silicide phases involved. The reactivity of silicon atoms can vary in dependence on such influences however, the essential step of the reactions is independent of them. The variation of the reactivity of silicon atoms with their environment always results in compensation behavior. 

As can be concluded from Fig. 4, the initial reactivity of the surface of Sited, is very high The relevant copper species present, possibly copper chlorides, which can easily form copper-silicidic phases by reaction with silicon, can easily attack the whole of the surface and react with surface silicon, resulting in Cu-Si species and finally metallic copper, e g, [26]. Due to this very fast formation of catalytically active Cu-Si species and of precursors thereof, the reaction becomes very fast already after a short time. But, on the other hand, this overall attack on the silicon surface gives copper species the possibility to be deposited practically over the whole of the silicon grain This means in terms of our model of catalytically active Cu-Si surface species, explained in the first section of this paper, that there is a lack of still free silicon surface area, which is needed in order to form the active "two-dimensional" Cu-Si species. The surface is simply blocked by thick copper-containing layers. As consequence, the reaction goes down after a short time and the contact mass reaches only low stationary activity. 

Pure iron is a white, lustrous metal, m.p. 1528°. It is not particularly hard, and it is quite reactive. In moist air it is rather rapidly oxidized to give a hydrous oxide which affords no protection since it flakes off, exposing fresh metal surfaces. In a very finely divided state, metallic iron is pyrophoric. It combines vigorously with chlorine on mild heating and also with a variety of other non-metals including the other halogens, sulfur, phosphorus, boron, carbon and silicon. The carbide and silicide phases play a major role in the technical metallurgy of iron. 

Mo forms a silicide and a carbide. Geib et al [96] reported Auger sputter-profiling results for Mo sputter-deposited on chemically-cleaned P-SiC. Thermodynamic considerations indicate that reaction of Mo with SiC should not be favourable. Other than a thin reacted layer at the immediate interface, there is no indication of reactive intermixing even after an 800 °C anneal. Hara et al [97] reported AES, Rutherford backscattering (RBS), XRD and transmission electron microscopy (TEM) measurements on Mo layers formed either by plasma deposition or by evaporation on chemically-cleaned a- and P-SiC. Prolonged (1 hr) anneals at 1200°C give an outer layer of Mo carbide on top of a layer of Mo silicide. 

Ag does not form a silicide or a carbide. Niles et al [105] studied Ag on ion-bombarded P-SiC using core-level and valence-band photoemission. Three-dimensional island growth is observed during deposition near 25 °C. The interface is non-reactive and exhibits ohmic behaviour. Annealing in the 400 - 600 °C range leads to coalescence of the Ag, with bare SiC between clusters, and higher temperatures lead to evaporation of essentially all the metal. 

W forms a silicide and a carbide. Geib et al [96] reported Auger sputter-profiling results for W sputter-deposited on chemically-cleaned P-SiC. Thermodynamic considerations indicate that reaction of W with SiC should not be favourable. Other than a thin reacted layer at the immediate interface, there is no indication of reactive intermixing even after an 800 °C anneal. 

There seems to be a general consensus [282—285] that this is a non-reactive interface, i.e. while Ag might be chemically bonded to Si, the interface is essentially planar with no dissociation of the semiconductor and no anomalous diffusion. However, no definitive results have been reported to support this belief and there is evidence which suggests the formation of a silicide phase at room temperature [283]. 

The corresponding explanation for silicon, advanced by Ottaviani et al. [325], is that the specific reactive interface (i.e. silicide—silicon) determines the barrier height, but again it is the result of a metal—semiconductor interaction. 

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