Liquid/glass immiscibility in yttria doped mullite ceramics

Abstract

5wt.% yttria doped mullite ceramics were sintered using nanocomposite powders which consisted of a Y2O3–SiO2 core coated with pseudoboehmite. Firstly, the powders were heated up to 1300°C; mullite began to form at 1050°C which is ∼150°C lower than in previous diphasic precursors. Then, the powders were sintered at 1550°C for 0.5, 5 and 48h, and characterized using XRD, SEM and TEM with EDS.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.