Phase relationship between β′- and O′-sialon at 1623 K

Abstract

β′-sialon whiskers and co-products of synthesis, such as β′-sialon powders and O′-sialon powders, were annealed at 1623 K for 8 h in a closed graphite reaction tube under 1 atm nitrogen. Phase stabilities, Si/Al ratios, and crystallographic features were investigated. The O′-sialon phase, which formed in the early stage of synthesis when oxygen partial pressure was relatively high, became less stable in the present annealing condition and decomposed. The majority of released aluminium and possibly oxygen from the decomposed O′-powder was incorporated into β′-sialon whiskers with little change in its lattice parameters, when the β′-sialon whiskers were included in annealing. The aluminium contents were always lower in the β′-whiskers than in the powders even after increasing its aluminium content during 8 h annealing. The lattice parameters of both β′-whiskers and powders increased with increasing aluminium content and became closer after annealing. The lattice parameters of β′-whiskers remained the same before and after annealing despite the increased aluminium content, while the lattice parameters of β′-powders decreased despite its aluminium content remaining unchanged. The lattice parameters of O′-sialon increased with increasing aluminium content, and the increase in the a direction is the largest when compared with other parameters.