Si-Al-O-N Compounds

The sialons are a group of ceramic compounds of increasing technological importance for their thermal, chemical and mechanical properties. These are similar to Si3N4 but can be more readily tailored to specific requirements because of the wide range of possible sialon compositions. 

The sialons fall into several structural types  

Of the various methods which can been used to prepare (3-sialon, several have been investigated by Si NMR. Thus, Yue et al. (1996a) reported that the products of 

However, the synthesis process most extensively studied by solid-state NMR is that of carbothermal reduction of aluminosilicate minerals such as kaolinite, which are mixed with finely divided carbon and heated in nitrogen at 1400°C (Neal et al. 1994, MacKenzie et al. 1994a). Under carbothermal conditions the clay decomposes to a mixture of mullite and amorphous silica (MacKenzie et al. 1996b), the latter forming SiC which reacts with the mullite to form P-sialon, in some cases via other sialon phases such as X-sialon (see below). The precise reaction sequence and the nature of the intermediates has been shown by the NMR studies to depend on various factors including the nature of the aluminosilicate starting mineral (MacKenzie er a/. 1994a). 

In a variation on this process which has been used to prepare P-sialons with low z-values the additional Si can be supplied by the addition of either SiOa or elemental Si. This reaction, which is assisted by the addition of a small amount of Y2O3, has been studied by NMR, which shows that the sequence is unaffected by the Y2O3, which facilitates the formation of both mullite and P-sialon and promotes the formation of Al-N units (MacKenzie etal. 1997). 

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