O-Sialons

A new compound formed during a novel silicothermal synthesis of O-sialon from clay, silica and elemental Si was found to have similar Al and Si NMR spectra to O-sialon. This NMR observation led to its subsequent identification as a low-temperature form which was previously unknown because other synthesis methods require higher temperatures, at which the low-temperature form is unstable, and conversion to the more usual high-temperature form has occurred (Barris et al. 1997). 

Because of their high Si02 content, O-sialons show superior resistance to oxidation. The oxidation of O-sialon powder has been studied by Al and Si NMR, which indicates that the octahedral Al thought to be present as an impurity phase in the unoxidised sialon is retained at all oxidation temperatures (MacKenzie et al. 1998). Some of this octahedral Al is present as mullite which forms at 1200-15(X)°C, but at higher temperatures, the oxidation is progressively hindered by the formation of a protective fused layer which is found by Al NMR to contain an unexpectedly high proportion of octahedral Al in the amorphous silica-rich material formed at 1600°C (MacKenzie etal. 1998). 

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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... 

O-sialon is technically important because of its superior resistance to attack by molten metals and to oxidation at higher temperatures. Its oxidation reactions have been studied by Si and Al MAS NMR which indicates that it is increasingly protected from oxidation at higher temperatures by the formation of a protective layer of Si02 containing some octahedral Al (MacKenzie et al. 1998). 

Figure 4.35. Comparison of the spectrum of a previously unknown sialon phase with that of O-sialon. The diagnostic presence of oxynitride units in the new phase (- 61 ppm) has led to its identification as a low-temperature form of O-sialon. From Barris et al. (1997), by permission of...

Figure 5.35. Typical Al MAS NMR spectra of sialons. A. 21R polytypoid sialon. B. p-sialon, z = 2.7, C. O-sialon, x = 0.24, D. X-sialon. Asterisks indicate spinning side bands. Spectra A-C from Sjoberg et al. (1992), by permission of the Royal Society of Chemistry. Spectrum D...

Figure 10. Si-N-Al-O (Sialon) Reference 7, Vol. X (1994), Figure 8759. Reprinted with permission.

AIN polytypes. = 3-sialon (Si Al O Ng O = sialon of Si20N2 type X = S1A102N ( nitrogen mullite ). ALON ss = aluminum oxynitride ss extending from approximately Al O N to Al O N. 

The potential appHcations of Si2N20 include roles as a high-temperature electric insulator [119,120], as anudear-reactor moderator or reflector [121], and as materials for dectrolytes [122]. O -SiAlON has the potential for similar engineering appUcations. 

A third family of sialons is based on the Si2N20 structure, and these are known as the o -sialons. 

SiALON. Sialon is synthesized by reacting together sihcon nitride, silica, alumina, and alnminnm nitride. Sialon exists in three forms j8 based on j8 silicon nitride a based on a sihcon nitride, and O sialon, which has the crystal strncture of silicon oxynitrides and obeys the formula SijAl2 j,03 Nj, with z taking values in the range of 1.3 to 2. Materials based on ji sialon or a mixture of a and sialons are well suited for high-temperature and high-stress applications. 

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