Silicides intermetallics

Two different types of reactors are used depending on the product synthesized. The first type can maintain pressures up to 150 atm, and is widely used for production of powders in gasless and gas-solid systems. Carbides, borides, silicides, intermetallics, chalcogenides, phosphides, and nitrides are usually produced in this type of reactor. The second type, a high-pressure reactor (up to 2000 atm), is used for the production of nitride-based articles and materials, since higher initial sample densities require elevated reactant gas pressures for full conversion. For example, well-sintered pure BN ceramic with a porosity of about 20-35% was synthesized at 100 to 5000-atm nitrogen pressure (Merzhanov, 1992). Additional examples are discussed in Section III. 

Suppose we are attempting to nucleate a solid-phase reaction between a metal and a silicon substrate to produce a silicide intermetallic compound and cany the reaction to completion. Further suppose that we have a dopant in the solid which we do not wish to have diffusing as the silicide reaction takes place. Suppose the two processes both have rates defined by Equation 4.26. Given the kinetic parameters for the reactions as follows ... 

When the coating metal halide is formed in situ, the overall reaction represents the transfer of coating metal from a source where it is at high activity (e.g. the pure metal powder, = 1) to the surface of the substrate where is kept less than 1 by diffusion. The formation of carbides or intermetallic compounds such as aluminides or silicides as part of the coating reaction may provide an additional driving force for the process. 

It is known about the existence of lithium silicide, Li6Si2, which is close to intermetallic compounds, and also that silicon is capable to form with lithium different alloys. We have calculated the theoretical specific capacity of such possible compounds, as well as pure silicon (Table 2). It is possible to explain from the Table 2 the noticeable increase of capacity for graphite electrodes (11%) even at the small content of Si (3wt%). We can suppose that some of such compounds (LixSiy) with high capacity may form... 

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. 

The oxides, hydrides, halides, sulphides, sulphate , carbonates, nitrates, and phosphates are considered with the basic elements the other compounds are taken in connection with the aoidio element. The double or complex salts in connection with a given element include those associated with elements previously discussed. The carbides, silicides, titanides, phosphides, arsenides, etc., are considered in connection with carbon, silicon, titanium, etc. The intermetallic compounds of a given element include those associated with elements previously considered. 

Several fiber types have been mentioned so far, and several other types have been neglected that have been worked on over the past few years. Some of those not discussed may become important fibers for reinforcement in the years ahead. To date though, they have not been available in sufficient quantity for thorough evaluation in composite specimens. Included in this group are boron carbide, spinel, polycrystalline alumina and silica, titanium diboride, and miscellaneous silicides and intermetallics. Ten years from now as we look back on the 70s we no doubt will have an entirely different view of some of these materials. 

Note that the silicide layer may grow not only between silicon and a transition metal, but also between a silicon-containing phase and a transition metal or an intermetallic compound. Such layers are known to occur in the process of brazing the transition metals by their own melts with Si3N4-base ceramics239 and also during the interaction of transition metals with silicon carbide.238 240 245... 

If the initial solid substance is a chemical compound (an intermetallic, a silicide, etc), then its oxidation can proceed via two different mechanisms, depending on the experimental conditions. Two oxides are formed in the severe oxidation (combustion) usually resulting in the disintegration of the compact solid phase. In the partial (soft) oxidation the chemical compound undergoes a partial decomposition giving another chemical compound of the same class and an oxide. 

Comparison of the consequences following from the physicochemical and purely diffusional approaches is given to show that the latter is one of the limiting cases of the former. Theoretical conclusions are illustrated by the available experimental data on the formation of intermetallics, silicides, oxides, salts and other chemical compounds. 

The improvement of existing materials as well as the development of new materials is often based on the use of a chemical reaction in which a solid reacts with another solid, a liquid or a gas to form a solid product (an intermetallic, a silicide, an oxide, a salt, etc) at the interface between initial substances. Therefore, kinetics of solid-state formation of chemical compound layers are of interest not only to chemists (researchers and technologists) but also to metal and solid-state physicists, materials scientists, metallurgists, specialists in the field of corrosion, protective coating, welding, soldering and microelectronics. 

The silicides occupy a hybrid position, intermediate between interstitial and intermetallic compounds. They usually behave like intermetallic compounds but often form genuine interstitial compounds with the larger transition metals of groups IV and V. 

The synthesis of several hundred materials, by both the SHS and VCS modes, has been reported in the literature. The types of compounds produced include carbides, borides, intermetallics, silicides, aluminides, composites, nitrides, hydrides, and oxides. The purpose of this section is to provide a description of the synthesized compounds, as well as the materials (i.e., powders, poreless materials, and functionally graded materials) and articles produced. The practical applications of experimental and technological methods are also described. 

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