ALBERT

Notes: An important aspect of previous sialon research in NZ has been the development of newsynthesis methods, including refinements in carbothermal reduction and nitridation (CRN) methodsand the use of mechanochemical activation of sialon precursors (either Al and Si nitrides and oxidesor CRN mixtures). Mechanochemical activation of CRN mixtures of clay and carbon heated in N2formed β-sialon (z = 2) at 1300oC (100oC lower than in unground mixtures) but 21R polytypoid andcorundum were also formed. More recently, our attention has focussed on the technique ofsilicothermal reduction and nitridation (SRN) to synthesise other sialons, including the AlNpolytypoids and Na and Li α-sialons. The interest in the polytypoids springs from their expectedphysical properties (thermal conductivity and good electrical insulation similar to AlN), theircovalent bonding and relatively light weight arising from their high Al and N contents and theirelongated crystal morphology which may improve the crack resistance of polytypoid compositeswith α-sialon. This paper describes the development of SRN single-step synthesis of high-puritydense 15R sialon from clay, Si and AlN, and the effect of additives on the synthesis and sintering ofthe product. A method is also described for SRN synthesis of Na and Li α-sialons from clay, Si andAlN using fluoride additives. Fluorides have the advantage of small size, high electronegativity,leading to their known facilitation of AlN synthesis. Furthermore, they do not readily enter thesialon structure but may toughen it by formation of glassy phases. Fluorides allow use of clay inthis SRN synthesis by introducing M+ without additional oxygen, but have the disadvantage ofgenerating SiF4 as a byproduct. The reaction using LiF proceeds readily at the very low temperatureof 1200oC via an O-sialon intermediate by a mechanism which probably involves Si migrationassisted by the formation of SiF4.The effect of mechanochemical activation (high energy grinding)on the SRN formation and sintering of Na and Li α-sialons, O and β-sialon has also beenstudied.Grinding the SRN O-sialon precursor promotes O-sialon formation in powders but not inpellets due to pre-reaction sintering, which is facilitated by the smaller particle size. Grinding Naand Li α-sialon SRN precursors forms a mixture of sialons rather than the target monophaseproduct, while sintering of all the sialons is assisted by grinding their SRN precursors