Hindering the Kinetic Selection of Dendritic Ba-Fresnoite by Phase Separation in a Glass of the Near-Eutectic Composition Ba₂TiSi₂O₈–2.625SiO₂

Abstract

A glass with the near-eutectic molar composition of 2BaO·TiO2·4.625SiO2 (BT2.625S) in the Ba-fresnoite–SiO2 system is produced and crystallized at 850, 880, 910, or 1100 °C. The resulting surface crystallized layer is analyzed using scanning electron microscopy (SEM) including electron backscatter diffraction (EBSD) and X-ray diffraction (XRD). In agreement with related glass compositions, this glass shows oriented nucleation at the surface. However, a phase previously not observed in the Ba-fresnoite–SiO2 system is detected and indicated to be brookite (TiO2) by XRD, as well as EBSD results. Crystal growth into the bulk occurs via the dendritic mechanism, but when crystallizing at 1100 °C, the dendrites are severely disturbed by droplets resulting from phase separation, which also hinders kinetic selection.

How Can Surface-Crystallized Glass-Ceramics Be Piezoelectric?

“…In both cases, the degree of orientation increases with an increasing distance from the surface, and the growth selection can occur relatively fast, i.e., within the first 15 μm of growth, or relatively slow, e.g., over 300 μm of growth . Growth selection may also be hindered, e.g., by barriers resulting from phase separation or other types of crystals that nucleate in the bulk.…”

“…Unless the growth mechanism changes, e.g., induced by nonisothermal conditions during growth , or varying chemical compositions of the matrix, the fastest-growing crystal direction usually becomes oriented perpendicular to the surface via kinetic selection, also known as “survival of the fastest”. This is, e.g., usually observed after oriented surface nucleation in glasses and has been intensively studied for (polar) fresnoite glass-ceramics. ,,− The growth mechanisms leading to the polar fresnoite layers are often dendritic for glass-ceramics containing Ba-fresnoite − or Ge-fresnoite, , while the oriented layers of Sr-fresnoite can result from what appears to be polygon growth , or what has been proposed to be viscous fingering. ,,,, …”