The crystallisation of Er-Si-Al-O-N B-phase glass-ceramics

The development of microstructure during crystallisation of a glass with composition (e/o) 35Er:45Si:20Al:83O:17N has been studied by analytical transmission electron microscopy, including electron spectroscopic imaging. The crystals take up a wide range of composition after crystallisation heat treatment at 1050 and 1150°C; the erbium cation percentage varies around that of the expected B-phase composition (Er₂SiAlO₅N), but the aluminium content is slightly lower and the silicon content higher than that. In addition, the erbium content is strongly anti-correlated with the silicon content. A comparison of B-phase composition after crystallisation of equivalent glasses formed with either yttrium, erbium or ytterbium showed that the B-phase solid solution range depends on the particular cation radius. As a consequence of this, the degree of crystallisation and the composition of the residual glass will, for equivalent glasses, also depend on the cation radius. The crystal shape is lenticular after growth at 1050°C, while heat treatment at 1150°C results in crystals with an irregular shape. In addition to crystal growth, the crystallisation heat treatment results in a phase separation of the residual glass whereby smaller silicon and nitrogen rich amorphous features form. These features are effectively pinning boundaries of growing crystals.