Oxidation Behavior of RE-α-Sialon Ceramics (RE=Nd, Sm, Gd, Dy, Y, Ho, Er, Yb)

Eight kinds of RE-α-sialon (RE=Nd, Sm, Gd, Dy, Y, Ho, Er, Yb) were prepared by the sintering Si3N4, AlN and RE2O3 mixtures in a composition of RE0.5(Si, Al)12(O, N)16 in nitrogen, and characterized by X-ray diffraction (XRD) analysis. The lattice constants, a and c, of Nd- to Dy-α-sialons were almost constant. When ionic radius was smaller than that of Dy3+, lattice constants increased with decreasing ionic radius of RE3+. The oxidation behavior of powders prepared by pulverizing these sialons was also examined in an air stream by differential thermal analysis (DTA) and XRD techniques. The DTA curves of these powders exhibited one or two exothermic peaks associated with the oxidation reaction in the range of 1200 and 1400°C. These peaks of Gd-, Dy-, Y-, Ho-, Er- and Yb-α-sialons shifted upwards with decreasing ionic radius of RE3+. The oxidation products formed by heating each sialon powder up to 1400°C were identified as α-cristobalite, RE2Si2O7 and Al2SiO5. Among these products, the amount of RE2Si2O7 in Er- and Yb-α-sialons was larger than that in other sialons. The amount of α-cristobalite was the smallest in the Yb-α-sialon.