A glass with the composition 2BaO·TiO2·2.75SiO2 was annealed at 810 °C for 20 h. This led to surface crystallization. Immediately at the surface of the sample, a layer of Ba2TiSi2O8-type fresnoite crystals (layer I), with a thickness of approximately 7 μm, oriented with the crystallographic [101]-direction perpendicular to the surface, was formed. The pole of the (001)-plane rotates randomly around the [101]-direction. It is assumed that nucleation kinetics is decisive to the direction of growth. In the next layer (layer II), the crystals are oriented with the crystallographic [001]-direction (c-axis) perpendicular to the surface of the sample. This layer occurs at a distance of 7−60 μm from the surface. Here, crystals that are not oriented in that way hinder each other during crystal growth. At a distance > 60 μm, the orientation of the fresnoite crystals is random and is the result of volume crystallization. The main characterization method is electron backscatter diffraction/orientation imaging microscopy.
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.
A glass with the composition Ba 2 TiSi 2.75 O 9.5 was crystallized by electrochemically induced nucleation. A dc-voltage (4.8 V) was attached between a platinum crucible containing the melt and a platinum wire inserted into the melt. The platinum wire was the cathode. After 2 min, crystals were formed at the cathode, which grew toward the crucible. These crystals consisted of fresnoite Ba 2 TiSi 2 O 8 , were highly oriented, and showed dendritic growth. These structures were characterized in detail by using electron backscatter diffraction/orientation imaging microscopy. Only a few crystals deviate from the main orientation of the respective dendrite. They were located and quantified. Intermediate areas between dendrites were analyzed, and the true boundaries of areas with homogeneous orientation are presented. The orientation is not as pronounced in the first 600 μm next to the platinum wire.
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