Recent Developments in SiAlON Research in New Zealand

Abstract

An important aspect of previous sialon research in NZ has been the development of new synthesis methods, including refinements in carbothermal reduction and nitridation (CRN) methods and the use of mechanochemical activation of sialon precursors (either Al and Si nitrides and oxides or CRN mixtures). Mechanochemical activation of CRN mixtures of clay and carbon heated in N2 formed β-sialon (z = 2) at 1300oC (100oC lower than in unground mixtures) but 21R polytypoid and corundum were also formed. More recently, our attention has focussed on the technique of silicothermal reduction and nitridation (SRN) to synthesise other sialons, including the AlN polytypoids and Na and Li α-sialons. The interest in the polytypoids springs from their expected physical properties (thermal conductivity and good electrical insulation similar to AlN), their covalent bonding and relatively light weight arising from their high Al and N contents and their elongated crystal morphology which may improve the crack resistance of polytypoid composites with α-sialon. This paper describes the development of SRN single-step synthesis of high-purity dense 15R sialon from clay, Si and AlN, and the effect of additives on the synthesis and sintering of the product. A method is also described for SRN synthesis of Na and Li α-sialons from clay, Si and AlN 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 the sialon structure but may toughen it by formation of glassy phases. Fluorides allow use of clay in this SRN synthesis by introducing M+ without additional oxygen, but have the disadvantage of generating SiF4 as a byproduct. The reaction using LiF proceeds readily at the very low temperature of 1200oC via an O-sialon intermediate by a mechanism which probably involves Si migration assisted 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 been studied.Grinding the SRN O-sialon precursor promotes O-sialon formation in powders but not in pellets due to pre-reaction sintering, which is facilitated by the smaller particle size. Grinding Na and Li α-sialon SRN precursors forms a mixture of sialons rather than the target monophase product, while sintering of all the sialons is assisted by grinding their SRN precursors.