Preparation SiAlON

Carbothermal synthesis and its variant, silicothermal synthesis, have proved attractive routes for preparing sialons from readily available clay mineral raw materials. The clay is mixed with fine carbon and/or silicon powder and reacted in a stream of purified nitrogen at > 1400°C. The 3-sialon product carbothermally synthesised from kaolinite has the composition Si3Al303N5 (z = 3), controlled by the Si02 Al203 ratio of the clay... 

Silicon-containing ceramics include the oxide materials, silica and the silicates the binary compounds of silicon with non-metals, principally silicon carbide and silicon nitride silicon oxynitride and the sialons main group and transition metal silicides, and, finally, elemental silicon itself. There is a vigorous research activity throughout the world on the preparation of all of these classes of solid silicon compounds by the newer preparative techniques. In this report, we will focus on silicon carbide and silicon nitride. 

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... 

For the powder bed method, a composition rich in P-SiAlON (named B2) was selected as a compact composition to observe compact-powder bed interaction. Two different powder bed compositions, a-SiA10N (100 wt%) and A1N BN mixture (50 50 wt%), were prepared. P-SiAlON-rich pellets were embedded into the powder bed compositions, as schematically shown in Fig. 6.21. Both green and sintered pellets were embedded into the same powder bed composition in order to compare the effect of presintering on the interaction. Sintering of the pellets was carried out under 22 bar nitrogen gas pressure at 1800°C for 1 hour. To understand the effect of pressure on the interaction zone, pressureless sintering was also carried out for comparison. 

Reprinted from J. Eur. Ceram. Soc., 21(5), Demir A and Thompson D P, High-performance SiC-fibre reinforced fi-sialon CMCs prepared from heat-treated Nicalon fibres, 639-647 (2001). Copyright 2001, with permission of Elsevier. 

Ekstrom, T., (1992a), Preparation and properties of alpha-sialon ceramics , J. Hard. Mater., 3, 109-118. 

Yu, Z.B. and Thompson, D.P., (1998), Preparation of carbon fibre reinforced Li-alpha-sialon composites , in Gibson, G., Consolidating New Applications, Seventh International Conference on Fibre Reinforced Composite, Cambridge, UK, Woodhead, 264-270. 

Yu, Z.B., Thompson, D.P. and Bhatti, A.R., (1998a), Preparation and homogeneity region of Li-alpha-sialon ceramics , Brit. Ceram. Trans., 97(2), 41 -6. 

Diaz, A., Hampshire, S. (2002), Crystallisation of M-SiAlON glasses to Iw-phase glass-ceramics preparation and characterization , J. Mater. Sci., 37, 723-30. 

Lemercier, H., Ramesh, R., Besson, J.-L., Liddell, K., Thompson, D.P., Hampshire, S. (1997), Preparation of pure B-phase glass-ceramic in the yttrium-sialon system , Key Eng. Mater., 132-136, 814-7. 

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... 

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). 

Carbothermal synthesis has been used to prepare yttrium a-sialon from mixtures of halloysite clay and Y2O3 with either Si02 or elemental Si. The reaction mechanism, particularly the role played by intermediate SiC phases, was monitored by Si NMR (Ekstrom et al. 1998). The preparation of fine-grained Ca a-sialon has also been studied by 2 Si NMR (Hewitt et al. 1994). 

A kind of translucent but tough Dy-SiAlON was successfully prepared by HP-sintered at 1600-1650 °C for 60-90 min using LiF as a co-additive to reduce sintering temperature. The resultant samples possessed a higher transmittance of 65% and also kept a higher fracture toughness of 5.21 MPa m at the same time. 

ROD-LIKE P-SIALON POWDER PREPARED BY A NEW N2-ASSISTED CARBOTHERMAL... 

Rod-Like p-Sialon Powder Prepared by a New Nj-Assisted Carbothermal Reduction... 

In conclusion, a kind of rod-Iike P-Sialon (P-SijAl303N5) powder was prepared through a carbothermal reduction-nitridation method using mesoporous silca as the starting material, combined with a carbon and aluminum nanocasting procedure. The resultant P-Sialon powder can be expected to find its use in preparing high performance Sialon ceramics. 

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