Silicides thermodynamics

If the rules for volatiles and thermodynamics of the haUdes are followed, the reaction can be used for aluminizing, silicidizing, chromizing, and similar processing. 

The expected oxidation mechanisms of carbides and silicides can be analysed from a thermodynamic viewpoint by a comparison of the relative stabilities... 

I. Tomaszkiewicz, G. A. Hope, C. M. Beck II, P. A. G. O Hare. Thermodynamic Properties of Silicides. VI. Pentamolybdenum Trisilicide (Mo Sf). Fluorine Combustion Calorimetric Determination of the Standard Molar Enthalpy of Formation at the Temperature of 298.15 K. J. Chem. Thermodynamics 1997, 29, 87-98. 

Miodownik et al. 1979, Watkin 1979). Irradiation can cause void-swelling, suppression of a formation in stainless steels and non-equilibrium precipitation of silicides. These phenomena are complex and occur by a combination of thermodynamic and kinetic effects. However, it was shown by Miodownik et al. (1979) that a thermodynamic analysis could be used to good effect to rationalise the effect of radiation on silicide formation. Although the work was done for a simple alloy system, it demonstrates how thermodynamics can be used in unusual cirounstances. 

Consider the thermodynamic data on the formation of a few transition metal silicides presented in Table 3.1. The values of AfG° were taken from the work by V.N. Yeremenko et al. When comparing the free energy changes calculated per mole of the compound, the silicides of the Me5Si3 type should be regarded as most stable. However, in none of the systems under consideration the compound of such a composition was found to form first. 

It should be noted that the thermodynamic data for the same binary system often reveal a very considerable scatter. This can readily be seen from the results compiled by J. Gulpen260 for the nickel silicides. At 850°C,... 

V.N. Yeremenko, G.M. Lukashenko, V.R. Sidorko. Thermodynamic properties of vanadium, chromium and manganese silicides at elevated temperatures // Rev.Intern. Haut.Temp.Refract.- 1975.- V.12, No.3.- P.237-240. 

The generation of all silicide phases in dependence on temperature can be thermodynamically explained by means of equilibrium calculations. Taking into accoimt our experimental results for the H2/SiCl4/Ni system, a picture of the mechanism of hydrodehalogenation is developed which combines thermodynamic and kinetic results. 

In accordance with our thermodynamic calculations we have observed the generation of the NiSi phase at temperatures above 700 °C. The incorporated silicon atoms originate from the gas phase. After the generation period the trichlorosilane conversion increases to the level determined by the equilibrium calculations. Taking into account the proposed mechanism of hydrodehalogenation, this leads to the same consequences as in the case of Ni2Si silicide catalyst, but the selectivity is less. Traces of H2SiCl2 are found. These are attributed to noncatalyzed subreactions in addition to HSiClj (Eq. 10, 11). 

The generation of the silicide phases is reversible. According to thermodynamic calculations, the NiSi phase was changed to the NizSi phase below 700 °C in a SiC /Hz gas phase. 

The silicides of the actinides have not been widely characterized. A uranium sihcide of the form U3Si2 has received the maximum amount of study owing to its nse as a dispersion fuel in research reactors. Another common phase observed for actinide silicides is AnSi2. One of the common synthetic routes to the silicides is the reaction of actinide fluorides, for example, Anp4, with elemental Si at high temperatures. One of the thermodynamic driving forces for this reaction is the release of Sip4. 

Ni, Pd and Pt do not react with SiOj, which is in accordance with thermodynamic calculations showing that metals with electronegativities <1.5 react with the SiOj to form a silicide. The silicide phase is also more metal rich than the one found during reaction with a Si substrate. 

It is a well known phenomenon that metals can significantly influence the crystallization of amorphous Si. The temperature required to crystallize bare a-Si is about 600°C. The contact of a-Si with metals usually leads to a strong reduction of this crystallization temperature (metal-induced crystallization). Metal/Si systems can be divided into (1) compound forming systems, which form stable metal silicide phases in thermodynamic equilibrium (e.g., Ni/Si, Pd/Si, Pt/Si) and (2) simple eutectic systems, which do not form stable... 

In the case where a silicide is the substrate there is not much known about the initial step a reaction of WF6 with the silicide under formation of W or an adsorption of hydrogen to form activated atomic hydrogen. It can be shown [HSrsta et al.78] that thermodynamically the following reactions can proceed ... 

In table 3.1 the data of the X-ray spectra is gathered and compared with solid state phases as predicted by thermodynamic calculations. There appears to be a reasonable agreement between the thermodynamic predictions and the experimental (X-ray) data. Of course we should keep in mind that because of the low deposition temperature, the silicide phases stay micro-crystalline. Therefore, we see no silicide reflections. In disagreement with the prediction is the formation of the meta-stable/3-W phase at the 1.5-1.6 flow ratio. However, after annealing the /3-W sample at 800°C in vacuum, a-W reflections plus those of W5Si3 appear. Thus the reaction proceeds far from equilibrium and the beta-W phase is only kinetically stable. The formation of /3-W and its conversion into a-W is described extensively in the literature [Morcom et al.116, Tang et al.117, Paine et al.118, Davazoglou et al.119]. 

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. 

Ta forms a silicide and a carbide. Geib et al [96] reported Auger sputter-profiling results for Ta sputter-deposited on chemically-cleaned P-SiC. As expected from thermodynamic considerations, Ta reacts during annealing, forming a macroscopically rough layer of intermixed Ta carbide and Ta silicide. 

Bolgar AS, Blinder AV, Muratov VB, Makarenko GN (1990) Thermodynamic properties of boron silicides. In Silicides and their application in technology Collection of scientific papers (in Russian). Inst Probl Materialoved, Akad Nauk UkrSSR, Kiev, p 78... 

Carbides, Silicides, and Germanides. The standard heat of formation and the dissociation energy of TiCjCg) have been determined as — 730 9 and 160 + 8 kJ mol , respectively, using the mass spectrometric Knudsen effusion technique. The standard heat and entropy of formation of TijSijis) have been reported as — 580 kJ moP and 272 e.u., respectively. A thermodynamic analysis of the co-reduction of TiOj with Si02 by carbon at elevated temperatures has shown that the formation of TiSi is more probable than TiSij. Phase equilibria in the Ti-Nb-Ge ternary system have been investigated. ... 

The thermodynamic characteristics of the chromium silicides Cr3Si, Cr5Si3, CrSi, and CrSi2 have been determined electrochemically. ... 

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