Silicides refractory metal compounds

Silicides are useful compounds characterized by their refractoriness and high electrical conductivity. There are many silicides, since silicon reacts with most metals and often more than one silicide is formed. For instance, there are five known tantalum silicides. 

The silicides of major industrial importance are the disilicides of the refractory metals molybdenum, tantalum, titanium, tungsten, vanadium, and zirconium.pl] These compounds are of great interest par-... 

Line compounds. These are phases where sublattice occupation is restricted by particular combinations of atomic size, electronegativity, etc., and there is a well-defined stoichiometry with respect to the components. Many examples occur in transition metal borides and silicides, III-V compounds and a number of carbides. Although such phases are considered to be stoichiometric in the relevant binary systems, they can have partial or complete solubility of other components with preferential substitution for one of the binary elements. This can be demonstrated for the case of a compound such as the orthorhombic Cr2B-type boride which exists in a number or refractory metal-boride phase diagrams. Mixing then occurs by substitution on the metal sublattice. 

Although the results obtained can be considered as approximate rather than accurate, they give some information about the relative strength of oxocompounds of the Group VIb elements. Nevertheless, these results were used as initial data for the development of the processes of reduction of these oxocompounds in chloride melts [150-152]. Theoretical bases and principles for monitoring the electrochemical processes of deposition of the free refractory metals and their compounds with some non-metals (carbides, borides, silicides, etc.) from molten ionic media [153] were developed. 

Although carbides and nitrides as a group form the most refractory compounds, they are certainly not the only ones. Several borides, oxides, phosphides, silicides, and metals meet the refractory requirements mentioned above. To some degree, these materials complement the refractory carbides and nitrides and may be considered as competitors. 

Finally, three elements of the periodic system occupy an intermediate position with regard to the ability to form refractory metal-like and non-metallic compounds. These elements, beryllium, magnesium, and aluminum, are capable of forming fairly refractory semiconductor compounds with nonmetals (beryllium, magnesium, aluminum borides, aluminum nitride, magnesium silicides), and they may also enter into the composition of intermetallic compounds of the beryllide, aluminide, etc., type. 

In general (Table 14.1), silicides reacting at low temperatures (quasi-noble metals) and forming as first reacted phase metal-rich compounds (MejSi) exhibit a behavior characterized by a time dependence of the growth [26]. In turn, the thickness of refractory metals silicides, which require higher temperatures to react. 

Low-temperature solvents are not readily available for many refractory compounds and semiconductors of interest. Molten salt electrolysis is utilized in many instances, as for the synthesis and deposition of elemental materials such as Al, Si, and also a wide variety of binary and ternary compounds such as borides, carbides, silicides, phosphides, arsenides, and sulfides, and the semiconductors SiC, GaAs, and GaP and InP [16], A few available reports regarding the metal chalcogenides examined in this chapter will be addressed in the respective sections. Let us note here that halide fluxes provide a good reaction medium for the crystal growth of refractory compounds. A wide spectrum of alkali and alkaline earth halides provides... 

In the case of glass, however, no great variations in behaviour between different types are expected because of their very similar structure and surface composition. Chemical vapour deposition reactions had already been tried by the last century, for instance in the refinement and deposition of silicon by reduction of SiF4 and SiCU with alkali metals [71] and in the refining of Ni using Ni-carbonyl in the Mond process [72,73]. The major impact of chemical vapour deposition on thin-film technology took place, starting some 60 years ago, when refractory compounds such as metal carbides, nitrides, silicides, borides and oxides as well as mixed phases of... 

Aro] Aronsson, B., Lundstroem, T., Engstroem, I., Some Aspects of the Crystal Chemistry of Borides, Boro-Carbides and Silicides of the Transition Metals , Anisotropy in Single-Crystal Refractory Compounds, Vahldiek, F.W, Mersol, S.A., (Eds.), Proc. Int. Symp., Dayton, Ohio, June 13-15, 1967, Plenum Press, New York, 1, 3-22 (1968) (Crys. Structure,... 

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