X-Sialons

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). 

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...

NMR has shown that X-sialon powder is less stable to high-temperature oxidation than 0-sialon, beginning to lose nitrogen from the Si02N2 units below 940°C and forming mullite at 120(>-1300°C (MacKenzie et al. 1998). 

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 5.36. Modification of the silicothemial synthesis of X-sialon by metal oxide additives monitored by A. changes in the tetrahedral Al shift as a function of synthesis temperature, and B. changes in the tetrahedralroctahedral Al ratio as a function of synthesis temperature. The heavy line in both graphs refers to the additive-free control sample. From Sheppard...

Figure 8.14. MAS NMR spectra of nitride-based ceramic bodies sintered with 10 wt % MgO and related magnesium silicon nitride compounds. A. Si3N4, from MacKenzie and Meinhold (1994), B. (J-sialon, from MacKenzie and Meinhold (1996), C. X-sialon, from MacKenzie et al. (2000), D. Crystalline MgSiNi (upper) and MgAlSiNj (lower), from MacKenzie and Meinhold (1994). All spectra used by permission of the copyright owner.

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