Influence of process parameters on the phase composition of SIAION ceramics from polish fly ash

The fly ash with high calcia content (Lagisza power station) has been chosen for the present study. Possible nitrogen for oxygen substitution was estimated on the base of the Si:Al ratio or Ca:Si:Al ratio in fly ash and necessary carbon amount was calculated from the stoichiometric reaction. Fly ash with addition of clay and carbon was reacted at 1520°C. After sintering at 1350-1450°C XRD, EDS and SEM examinations were applied to the specimens sintered with yttria addition. It has been found that phase composition of the carbothermal reduction product depended on the amount of carbon and resulted in β- or α/β-sialons besides 15R and 12H phases. Subsequent sintering involved changes of the phase composition, dissolution of Ca-α-sialon and crystallization of YAG or secondary β-sialon, depending on temperature of sintering.