SiAlONs structure

The stability of a -SiAlON at elevated temperatures (1100-1500°C) depends on the nature of the cations R entering the ot -SiAlON structure. For the trivalent rare-earth cations (i.e. La-Yb Y), the stability increases with decreasing atomic radius [14,81] i.e. the temperature below which a phase transformation a — P can thermodynamically take place decreases. The stability also increases with increasing substitution level. Thermodynamic stabilisation of the a -SiAlON with respect to the p -SiAlON can be achieved by the use of mixed cations [14]. 

In p-sialon, there are continuous channels that are parallel to the c direction as seen in Fig. 16.29. It is accepted that the several types of modifying cations can be accommodated in the ot-sialon structure for achieving charge neutralization, whereas no cations, except Al , can enter into the P-sialon lattice because the charge balance is not required. However, Hirosaki et al. have reported very intense luminescence of Eu in p-sialon, which obviously indicates that Eu " is indeed dissolved in the lattice of P-sialon [105, 107]. 

The covalently-bonded silicon carbide, silicon nitride, and sialons (alloys of Si3N4 and AI2O3) seem to be the best bet for high-temperature structural use. Their creep resistance... 

Five aspects of the preparation of solids can be distinguished (i) preparation of a series of compounds in order to investigate a specific property, as exemplified by a series of perovskite oxides to examine their electrical properties or by a series of spinel ferrites to screen their magnetic properties (ii) preparation of unknown members of a structurally related class of solids to extend (or extrapolate) structure-property relations, as exemplified by the synthesis of layered chalcogenides and their intercalates or derivatives of TTF-TCNQ to study their superconductivity (iii) synthesis of a new class of compounds (e.g. sialons, (Si, Al)3(0, N)4, or doped polyacetylenes), with novel structural properties (iv) preparation of known solids of prescribed specifications (crystallinity, shape, purity, etc.) as in the case of crystals of Si, III-V compounds and... 

Thompson DP, Korgul P, Hendry A (1983) The structural characterisation of SiAlON Polytypoids. In Riley FL (ed) Progress in Nitrogen Ceramics. NATO ASI Ser E 65. Kluwer Acad Publ, Dordrecht, p 61... 

O-SiAlON is another crystalline phase of interest. There is a limited solubility of alumina in silicon oxynitride structure to give O-SiAlONs, represented by the formula Si2 A-Aly01+A.N2 where ovaries from zero to 0.2. Formation of O-SiAION occurs in the same mechanism as P-SiAlON i.e. Si + N is replaced by Al + O. The lattice parameters of O-SiAION, Si2 AI/)i+aN2 a, increase in a very typical way with the x value.37... 

Since the macroscopic properties of a material are strongly dependent on its structure, studies of structure/property relationships are among the most important issues in material science. The properties of SiAlON ceramics are strongly influenced by their microstructure and chemical composition. 

Although the functionally graded materials concept has been known since 1972, the production of functionally graded SiAlON ceramics is still a subject to explore. Especially, the formation mechanisms of the functional gradient structure have not yet been fully understood. If diffusion mechanisms are explored, compositional design would be easy and the thickness of the graded... 

As resulting from XRD measurements, the lines of a-sialon (PDF-JCPDS 42-0251), together with a-Si3N4 (PDF-JCPDS 41-0360), mellilite (Y2Si3N403, PDF-JCPDS 30-1460) and small additions of P-Si3N4 (PDF-JCPDS 33-1160), can be observed (sample PLS-1). After 1% carbon addition (Fig. 19.6, samples PLS-3 and PLS-7) the same main structural lines could be observed as in the reference sample (PLS-1). After we increased the carbon to 10% (PLS-5 and PLS-9) the a-sialon and melilite lines disappeared. In this case the main constituents of the microstructures are a-Si3N4, P-Si3N4 and SiC (PDF-... 

Figure 14 Projection of the idealized structures of (a) the 15R and (b) the 12H phases in the sialon system, projected onto the (110) planes (Reproduced by permission from J. Mater. Sci., 1976, 11, 1135)...

Table 2 Compositions of the known sialon inter growth structures...

Silicon nitride, oxynitride and silicon aluminium oxynitride (SiAlON) compounds are of considerable technical interest as advanced ceramics because of their stability and chemical inertness at elevated temperatures, and their excellent mechanical properties. Si MAS NMR has proved to be useful in studying various aspects of these compounds, including their formation and structure, the processes by which they are... 

The sialons fall into several structural types ... 

Figure 4.37. Effect of 1 wt% of various metal oxide additives on the silicothermal formation of X-sialon at 1500°C monitored by Si NMR. As the reaction proceeds, the shape of the spectra change according to changes in the various regions corresponding to particular structural units as marked = silicon oxynitrides, O = Si3N4, = Si02, = mullite. Note that not all oxide additives produce the characteristic X-sialon spectrum at 1500°C. Adapted from Sheppard and...

The structure of these sialons is based on that of a-SisN4, but requires the presence of other metal ions (M) for stabilisation. The M ions are typically yttrium and rare-earths, but can also be Mg or Ca. The values of m and n in the formula depend on the metal ion present. The a-sialons, which have extremely good mechanical properties, also form technically useful solid solutions with P-sialons. The Si chemical shift of a-sialon (about - 48 ppm) is similar to that of Si3N4 and appears to be insensitive both to the sialon composition and the nature of the M ion (Leach et al. 1990). 

---