Liquid/glass immiscibility in yttria doped mullite ceramics

The phases present were mullite and two intergranular glasses comprising silica-rich droplets embedded in a Y-Si-Al-O glassy matrix, providing direct evidence of liquid-liquid phase separation at the grain boundaries during sintering. This liquid-liquid separation is facilitated by the prior existence of yttrium disilicate which moves the overall sintering liquid compositions toward the two immiscible liquids region of the Y2O3-Al2O3-SiO2 phase diagram. The presence of the silica-rich phase in the grain boundaries will effectively increase their viscosity and thus the resistance of the Y2O3 densified mullite to creep.