Ternary boron nitrides

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

Ternary boron nitrides (i.e. compounds of type M B N ) are a relatively new addition to boron-nitrogen chemistry. The high-temperature reactions of hexagonal BN with Li3N or Mg3N2 lead to Li3BN2 and Mg3BN3 respectively. Reaction... 

In agreement with experimental information at a pressure of 1 bar the tieline Si3N4-BN exists up to the Si3N4 decomposition temperature of 2114 K. The Scheil reaction scheme as derived from the calculation is given in Fig. 16. Six invariant ternary reactions are calculated. All of them are of a degenerated type because boron nitride, formed at 2767 K, does not participate in the ternary reactions. At room temperature six three-phase equilibria are detected. 

The ternary system was calculated by extrapolation from the binary subsystems (Kasper, 1996) [33]. The calculations cover phase equilibria at one bar and do not assume any solubilities as no experimental evidence for stable sohd solutions between B4+5C and BN or a-BN and graphite exist. The section between graphite and boron nitride including the invariant reactions Uj, Dj and U2 (Fig. 23) is shown in Fig. 22. A calculated potential phase diagram (logpN2-T) can be found in [244], The complete Scheil reaction scheme (P = 1 bar) is shown in Fig. 23. 

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]. 

In addition to binary compounds made with elements from the 2d period, ternary compounds may also exhibit higher thermal stability that diamond [218]. Such compounds can be considered as solid solutions of carbon in boron nitride. Efforts to synthesize such compounds have been made both at low and at high pressures. 

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)... 

Besides diamond and cBN, the well known boron carbide B4C is among the hardest materials and has been comprehensively reviewed by F. Thevenot [103], In the present chapter, the latest developments concerning the binary and ternary systems B-N, boron carbide nitrides (B-C-N), and boron suboxides are discussed. Other hard materials based on boron are described by R. Telle et al. in Part III. 

---