α′-Sialon ceramics [7]

OUR previous report¹ on Si-Al-O-N ceramics stated that expanded α-silicon 'nitride' structures had been obtained by reaction of lithium-silicon nitride, LiSi₂N₃, with alumina. The unit-cell dimensions of one example (a, 7.822; c, 5.677Å) gave a cell volume about 3% greater than that of α-silicon nitride. Subsequent work² showed a variation in dimensions for the α′-lithium sialon when lithium aluminate, LiAlO₂, was reacted in different proportions with Si₃N₄. However, other phases, such as β′-sialon and nitrogen-eucryptite, were always present and the product never contained more than ∼30% of the α′ material. αν-Sialons were also found in the Mg-Si-Al-O-N system³ but again never pure, and were observed by Masaki et al.⁴ during the nitriding of silicon with A1N and A1₂O₃ additions. A recent claim by Mitomo⁵ of α′ solid solutions of Si ₃N₄-Al₂O₃ and/or Si ₃N₄-Y₂O₃ occurring during the sintering of Si₃N₄ at 1,700-1,800 °C with Al ₂O₃-Y₂O₃ mixtures prompted this report of the preparation and characterisation of pure α′-phases in M-Si-Al-O-N systems where M is Li, Ca or Y. Such phases also occur, but have not so far been prepared in a pure form, in magnesium and other sialon systems.