Wolfgang Wisniewski

The degradation of EBSD-patterns as a tool to investigate surface crystallized glasses and to identify glassy surface layers.

Surface crystallized samples of glass-ceramics containing cordierite, rhombohedral BaAl₂B₂O₇ and fresnoite were analyzed using electron backscatter diffraction (EBSD). The first two materials were chosen because surface crystallized samples of these materials have previously been shown to contain crystals covered by a very thin layer of glass. In all materials, EBSD pattern degradation occurs if the step size of a scan is chosen to be small. It is shown that the minimum step size enabling an evaluable EBSD-scan increases notably, if the crystals are covered by a thin layer of glass. It is also shown that pattern degradation may be utilized to prove the existence of such a thin glass or otherwise thermally sensitive layer. This provides significant information concerning the nucleation process of glasses also with respect to nucleation theory of glass-ceramics. It is also possible to describe the quantity of crystalline surface covered by the thermally sensitive layer.

Dendritic growth of yttrium aluminum garnet from an oxide melt in the system SiO2/Al2O3/Y2O3/CaO

Glasses with the compositions x CaO·1CeF3 (11−0.2x) Y2O3·(49.2−0.8x) Al2O3·28.8 SiO2 (with x = 10, 20, 30 and 40) were thermally annealed at 1200 °C for 6 h. This led to the crystallization of yttrium aluminium garnet (YAG) as the only occurring phase. The crystals formed dendrites instead of monolithic bodies with large quantities of glassy phase between the dendrite arms. Large, cube shaped crystals occurred in the samples with the highest and the lowest calcium concentration (10 and 40 mol%). Interpenetrating dendrites were detected in samples with calcium concentrations of 20 and 30 mol% using electron backscatter diffraction (EBSD). The glass-ceramics show intense fluorescence due to Ce3+ and might be suitable for lighting technology to convert the blue light of light emitting diodes to white light.

Electron backscatter diffraction of BaAl2B2O7 crystals grown from the surface of a BaO·Al2O3·B2O3 glass

A glass with the composition BaO·Al2O2·B2O3 was annealed at 780 °C for 8 h. The samples were investigated by electron backscatter diffraction (EBSD). At the surface of the samples a first layer composed of BaAl2B2O7 crystals oriented with their crystallographic c-axes perpendicular to the surface was formed. This layer was highly oriented and up to 20 µm thick. In a subsequent layer, the crystals were oriented with their crystallographic c-axes parallel to the surface. While the first layer was caused by oriented nucleation at the surface, the orientation of the secondary layer was caused by kinetic selection of spherulitically grown crystals.

Sr-fresnoite surface crystallisation in a 2SrO·TiO2·2.75 SiO2 glass studied by EBSD

A glass of the composition 2SrO·TiO2·2.75SiO2 was melted. Cooled samples were polished and thermally annealed at 970 °C for 10 min to 20 h to achieve surface crystallisation of Sr2TiSi2O8 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 ± 5° from the surface normal after about 300 μm. 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.

Viscous Fingering and Dendritic Growth of Surface Crystallized Sr2TiSi2O8 Fresnoite

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.

EBSD and EDX analyses of a multiphase glass-ceramic obtained by crystallizing an yttrium aluminosilicate glass.

A glass with the mol % composition 23.82 Y2O3·39.82 Al2O3·28.50 SiO2·2.91AlF3·4.95 ZrO2 doped with 2 mol % CeF3 is crystallized at 1250 °C for 20 h. The crystallized samples are studied using X-ray diffraction (XRD), the SEM-based methods EBSD and EDX, as well as fluorescence microscopy. Six crystalline phases are detected in the residual glass including alumina, YAG, Y-stabilized zirconia (YSZ), and three different yttrium silicates of the composition Y2Si2O7. Chemistry-assisted indexing (ChI) is successfully applied to separate YAG and YSZ in EBSD-scans. YAG displays polygon as well as dendritic growth. Some crystals show both mechanisms at opposite ends, indicating that the growth mechanism is influenced by the chemistry of the surrounding glass matrix.

Surface crystallisation of yttrium aluminum garnet from a silicate glass

Glasses with the mol% composition 17Y2O3·33Al2O3·40SiO2·2AlF3·3Na2O·2CeF3·3B2O3 were thermally annealed at 1000 °C for different periods of time. This resulted in the surface nucleation and subsequent growth of Ce3+-doped yttrium aluminium garnet (YAG) which shows an intense broadband fluorescence. In the bulk, the nucleation and subsequent growth of an X-ray amorphous star-shaped and alumina enriched phase leads to a depletion of alumina from the glass. The nucleation of monoclinic and orthorhombic Y2Si2O7 adjacent to the YAG layer is observed in addition to pore formation. These pores serve as centres for enhanced nucleation near the surface. The nucleation and growth of the orthorhombic phase and a different monoclinic Y2Si2O7 are additionally observed in the bulk. The phase identification and localization was performed by X-ray diffraction and electron backscatter diffraction (EBSD).

Microstructure of Transparent Strontium Fresnoite Glass-Ceramics

Glass-ceramics grown from a glass of the composition Sr2TiSi2.45O8.9 (STS 45) are analyzed by scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). Oriented nucleation with the c-axes preferably perpendicular to the surface is detected. A very strong 001-texture is observed after only 10 μm of growth into the bulk, making this the first system in which an orientation preferred during nucleation prevails during growth into the bulk in glass-ceramics. Piezoelectric measurements are performed and d33-values presented and discussed. The obtained results are critically viewed with respect to the two growth models describing Sr2TiSi2O8 growth in glasses.

Piezoelectric glass-ceramics produced via oriented growth of Sr2TiSi2O8 fresnoite: thermal annealing of surface modified quenched glasses

Glass samples with the composition 2SrO·TiO2·2.75 SiO2 were polished, subsequently covered with graphite or alumina powder and finally annealed at 970 °C for 20 h in order to induce surface crystallisation. The effect of the surface treatment on the preferred crystal orientations during growth into the bulk is analyzed using X-ray diffraction (XRD) and electron backscatter diffraction (EBSD). Furthermore, the effect of the surface treatment on the piezoelectric d33-value is described and referenced to polished samples annealed without previous surface treatment.

Oriented crystallization of a β-Quartz Solid Solution from a MgO/Al2O3/SiO2 glass in contact with tetragonal ZrO2 ceramics

Model experiments concerning the nucleation of MgO/Al2O3/SiO2 glass are reported. A glass with the composition 22.5MgO·22.5Al2O3·55SiO2 (in mol%) is brought into contact with ceramic samples of tetragonal ZrO2 at a temperature of 1500 °C. This leads to a heavy corrosion of the ceramics and the diffusion of zirconia into the glass. Subsequent thermal treatments at 820/850 °C and 1050 °C provoke the formation of different phases at the glass/ceramic interface: monoclinic and tetragonal ZrO2, β-quartz solid solution (β-QSS), and spinel. At some distance from the ceramics, the only detected crystalline phase is the highly oriented β-QSS. Only mullite is observed at the air/glass interface where it also grows into the bulk. A sample directly crystallized at 1050 °C shows a very different behavior and only mullite is formed at both the air/glass as well as the glass/ceramic interface. The thermal treatment at the nucleation temperatures of 820/850 °C is thus essential for the precipitation of zirconia and β-QSS in the glass. X-ray diffraction, high resolution (scanning) transmission electron microscopy, and scanning electron microscopy including electron backscatter diffraction are performed to elucidate the underlying mechanisms.