A glass of the composition 2SrO?TiO 2 ?2.75SiO 2 was melted. Cooled samples were polished and thermally annealed at 970 uC for 10 min to 20 h to achieve surface crystallisation of Sr 2 TiSi 2 O 8 fresnoite. At the surface, the crystals were immediately oriented with their crystallographic c-axes perpendicular to the surface. Crystal growth occurred in the form of similarly oriented structures. Very homogeneous structures were also observed but they fray into areas of higher orientation diversity at some distance from the surface. The crystallised areas were highly permeated by nanoscale inclusions of residual glass. Kinetic selection occurs and leads to a preferred orientation with the crystallographic c-axis tilted by about 43 ¡ 5u from the surface normal after about 300 mm. Crystal growth is not governed by the fastest growing crystallographic direction or the formation of a diffusion barrier, but rather by the ability to circumvent the residual glass.
During the quenching of a melt with the composition 2SrO·TiO2·2.75SiO2, cubic SrTiO3- and tetragonal Sr2TiSi2O8-crystals are formed at the surface. Subsequent crystal growth leads to dendritic fresnoite structures which become increasingly finer until the mechanism changes to viscous fingering during further cooling. In the final stages of this initial growth step, the crystal orientations of these dendrites systematically change. Due to a complete absence of bulk nucleation in this system, crystal growth is resumed upon reheating to 970°C and fractal growth with the c-axis tilted by about 45° from the main growth direction is observed. The results are interpreted to confirm the link between viscous fingering and dendritic growth in the case of a true crystallization process.
Two glasses of the compositions 2 BaO - TiO2 - 2.75 GeO2 and 2 BaO – TiO2 –3.67 GeO2 (also known as BTG55) are annealed at temperatures from 680 to 970 °C to induce surface crystallization. The resulting samples are analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM) including electron backscatter diffraction (EBSD). Ge-Fresnoite (Ba2TiGe2O8, BTG) is observed at the immediate surface of all samples and oriented nucleation is proven in both compositions. After a very fast kinetic selection, the crystal growth of BTG into the bulk occurs via highly oriented dendrites where the c-axes are oriented perpendicular to the surface. The growth of this oriented layer is finally blocked by dendritc BTG originating from bulk nucleation. The secondary phases BaTiGe3O9 (benitoite) and BaGe4O9 are also identified near the surface by XRD and localized by EBSD which additionally indicates orientation preferences for these phases. This behaviour is in contrast with previous reports from the Ba2TiSi2O8 as well as the Sr2TiSi2O8 systems.
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