Ternary Silicon Nitrides

Only a few ternary silicon nitrides have been prepared in a pure form and characterized. The isotypic compounds MSiN2 (M = Be, Mg, Mn, Ca, Zn), with the same valence electron concentration of 4, can be considered as ternary substitution variants of AlN [247]. These compounds contain three-dimensional (3-D) infinite network structures, with SiN4 tetrahedra linked through all four vertices by comer-sharing, which forms condensed [SieNe] twelve-membered rings in MgSiN2 [248]. 

Silicon-based ternary nitrides such as MgSiN2 and LaSiaNs have been intensively studied as an alternative material for the substrates of integrated circuits [249-253], as host lattices for light-emitting phosphors [254—260], and as semiconductors. 

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Rogi, P. and J.C. Schuster Phase Diagrams of Ternary Boron Nitride and Silicon Nitride Systems, ASM International, Materials Park, OH. 1992. 

There are many other binary and ternary silicon compounds of commercial importance. For example, silicon nitride (Si3N4) occurs in two hexagonal forms the a-form and a denser 3-form. The crystal stmctures are very complex, but may be thought of as close-packed nitrogen atoms, with three-eighths of the tetrahedral vacancies occupied by silicon atoms. In this respect it is like Si02, being a three-dimensional network of... 

The thermodynamics of the above-elucidated SiC/C and SijN Si composites are determined by the decomposition of silicon carbide and silicon nitride, respectively, into their elements. The chemistry of ternary Si-C-N composites is more complex. If producing Si-C-N ceramics for applications at elevated temperature, reactions between carbon and silicon nitride have to be considered. Figure 18.2, which exhibits a ternary phase diagram valid up to 1484°C (1 bar N2) displays the situation. The only stable crystalline phases under these conditions are silicon carbide and silicon nitride. Ceramics with compositions in the three-phase field SiC/Si3N4/N are unknown (this is a consequence of the thermal instability of C-N bonds). Although composites within the three-phase field SiC/Si3N4/Si are thermodynamically stable even above 1500°C, such materials are rare. The reasons are difficulties in the synthesis of the required precursors and silicon melting above 1414°C. The latter aspect is of relevance, since liquid silicon dramatically worsens the mechanical properties of the derived ceramics. 

However, a systematic investigation of the chemistry of ternary and multinary silicon nitrides in analogy to the phosphoms nitrides has been prevented by the extraordinary chemical stability of Si3N4. For this reason, SisN4 has only been used in the past by way of exception as an educt in chemical reactions. 

In principle, the simplest way to produce preceramic polymers for ternary silicon boron nitrides is to coammonolize mixtures of silicon and boron chloride. Dietz has applied for a patent for such a process, with Si/B ratios ranging from 9 1 to 1 9. There are two major disadvantages of this approach (1) the polymer is only accessible as a mixture with the by-product, ammonium chloride, and (2) the ceramics obtained are composites constituted of the binaries BN and Si3N4 [49]. 

During their genesis all precursor derived ceramics pass through an amorphous state which in certain cases is stable up to the respective decomposition temperature. The propensity to crystaUize is strongest for the binary silicon and boron nitrides and carbides, while in particular quaternary materials out of the Si/B/N/C system show the strongest resistance towards crystallization. It is interesting to note that in any case investigated, so far, the amorphous multinary ceramics decompose into the binary border phases upon crystallization - crystalline ternary silicon boron nitrides or carbides are not known to date cf. [9]. 

Historically, the development of nitrides proceeded slowly until the emergence of silicon nitride during the late 1950s. From that time onwards, the intricacies of the relationship between the two forms of silicon nitride, combined with the observation that, when hot-pressed or sintered into dense products, silicon nitride displayed excellent mechanical properties at temperatures in excess of 1000 °C, propelled silicon nitride into a subject area of intense interest. As a result, it now resides in a well-defined niche as a stmctural material for wear parts and related applications. At the same time, this provided a catalyst for the development of other nitrides, both of a binary and ternary character. Increasing complexity was provided by the observation that, just as with mineral silicates, aluminum could replace silicon in... 

All the ternary lanthanide silicon nitrides exhibit an excellent thermal and chemical stability which predestines them for use as high-temperature materials. 

Rog] Rogl, R, The System B-N-Fe m Phase Diagrams of Ternary Boron Nitride and Silicon Nitride Systems, , Rogl, P. Schuster, J.C., (Eds.), ASM, Materials Park, OH 1992, 33-36 (Crys. Structure, Thermodyn., Phase Diagram, Phase Relations, Experimental, Review, 9)... 

In the past, attempts to prepare such ternary nitrides by reaction of the respective binary nitrides always have failed because the binary nitrides do not melt congruently and also because of the low self-diffusion coefficients of these materials. However, for the synthesis of SiPNj a molecular precursor Cl3SiNPCl3 has been proven to be specifically useful [5]. In this compound the required structural element of two vertex sharing tetrahedra centered by phosphorus and silicon and connected via a common nitrogen atom is pre-organized on a molecular level. The precursor compound is obtained (Scheme 3) in a three-step synthesis starting from ((CH3)3Si)2NH which is commercially available. 

The mechanisms of whisker growth vary depending on the substrate used for the reaction. For example, the Si3N4 whiskers grown on mullite show axial defects, characteristic of crystallization by an axial-screw-dislo-cation mechanism [29], In the presence of a catalyst, whisker growth occurs by means of a VLS mechanism, involving crystallization of the nitride from drops of binary or ternary alloys of silicon with iron and aluminum. The iron and aluminum act as solvents for silicon. 

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