Oriented Nucleation of both Ge-Fresnoite and Benitoite/BaGe4O9 during the Surface Crystallisation of Glass Studied by Electron Backscatter Diffraction

Abstract

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.

Oriented Nucleation and Crystal Growth of Ba-Fresnoite (Ba2TiSi2O8) in 2 BaO·TiO2·2 SiO2 Glasses with Additional SiO2

“…20°. Considering this tilt, it can be concluded that the microstructure in Figure features Ba-fresnoite dendrites growing perpendicular to the surface, similar to the observations made for BT0.75G in a related glass-ceramic . Bulk nucleation was not detected in this BT3S glass-ceramics after the applied heat treatment.…”

“…Figure presents an SEM micrograph obtained from a cross section of the BT0.75S glass-ceramic showing an area where the growth fronts from the top and right side of the sample collided. In analogy to Ge-fresnoite, Ba-fresnoite shows no growth front interaction. Hence, the dendritic crystals growing in the BT0.75S-glass at 810 °C simply collide and prevent each other from further growth, similar to the observations made for Ge-fresnoite …”

“…While the growth front interactions of the related systems STS and BTG have been analyzed by SEM and EBSD to see whether the local growth velocity is increased, reduced, or unaffected, , this has not been done for Ba-fresnoite growing in glasses. Figure presents an SEM micrograph obtained from a cross section of the BT0.75S glass-ceramic showing an area where the growth fronts from the top and right side of the sample collided.…”

“…13 This observation is in contrast to the basic assumption of randomly oriented nuclei in the classical nucleation theory for glasses. Oriented nucleation has been described to occur in glass-ceramics containing Ba-fresnoite, 11,13 Sr-fresnoite, 14,15 or Ge-fresnoite. 16 It has also been detected in other systems during the surface crystallization of BaAl 2 B 2 O 7 , 17,18 diopside, 19 Sr 1−x/2 Al 2−x Si x O 4 (x = 0.9, 0.7, and 0.5), 20 walstromite, 21 probably ε-Y 2 Si 2 O 7 , as well as yttrium stabilized ZrO 2 22 and after the crystallization of mullite at the surface of a glass melt during cooling.…”

Surface Crystallization of a MgO/Y2O3/SiO2/Al2O3/ZrO2 Glass: Growth of an Oriented β-Y2Si2O7 Layer and Epitaxial ZrO2

“…4 indicate an oriented nucleation of ε-YS with the c-axis preferably parallel to the surface and of Y-ZrO 2 with a {111}-plane parallel to the surface. Oriented nucleation has currently been detected in more than five glass systems, most recently in Ge-fresnoite 43 where multiple phases showed up to three orientation preferences at the immediate surface after crystallization 43 . As the ε-YS dendrites in the EBSD-scan of Fig.…”