Preparation of multiple-cation ?-SiAlON ceramics containing lanthanum

Abstract

Until recently, it was accepted that Ce 3+ cations, with an ionic radius (r) of 1.03 Å, were too large to form an ?-SiAlON structure. However, more-recent studies have shown that cerium cations can be incorporated into ?-SiAlON via quenching at a rate of 600°C/min, after sintering at 1800°C. Thus far, no ?-SiAlON formation has been observed for La 3+ cations with r = 1.06 Å. In the present work, the possibility of having the La 3+ species as a dopant cation in ?-SiAlON has been investigated by using La 2O 3 alone or in equimolar mixtures with CaO or Yb 2O 3. The resulting materials have been heat-treated at a temperature of 1450°C for up to 720 h to devitrify the grain-boundary glass into crystalline phases and also to observe the ? › ß SiAlON transformation. X-ray diffractometry on samples that were densified with single cations revealed that the La 3+ cation alone does not form an ?-SiAlON; rather, it forms the N-phase (La 3Si 8O 4N 11) with a ß-SiAlON phase. In the case of multiple cations, ?-SiAlON was observed only as a matrix phase. Energy-dispersive X-ray measurements have proven that La 3+ cations can be accommodated into the ?-SiAlON structure and this structure also does not transform to ß-SiAlON at lower temperatures.