A Global Glassy Layer on BaAl₂B₂O₇ Crystals Formed during Surface Crystallization of BaO·Al₂O₃·B₂O₃ Glass

Abstract

A glass with the composition BaO•Al 2 O 3 •B 2 O 3 was thermally annealed at temperatures from 700 to 780 °C for 1−8 h. This resulted in the crystallization of [0001]-oriented rhombohedral BaAl 2 B 2 O 7 at the surface of the glass. The crystals grow fastest perpendicular to their c-axes (parallel to the surface) and cannot circumvent barriers in the surface. The surface of samples encapsulated in alumina ceramics during annealing was covered by a thin layer of glass, indicating nucleation occurred below the surface. If the samples were not encapsulated in alumina, around 85% of the surface was glass covered. This was the result of surface contaminations such as cubic BaSO 4 detected on these samples. Crystal growth occurs toward the surface until the enrichment of impurities in the glassy layer is large enough to inhibit further crystal growth. The coinciding enrichment of impurities on the bulk side of the oriented surface crystals probably initiates the secondary nucleation, leading to the second layer of oriented crystals previously described.

Microstructure of Transparent Strontium Fresnoite Glass-Ceramics

“…In summary, the oriented nucleation already described for STS in Glass B22, BTS26, diopside29 and BaAl 2 B 2 O 7 crystals in glass ceramics30 was also detected in glass A after annealing at temperatures of 840, 940 and 970°C. Hence the detected 001 preference occurs over a wide range of temperatures similar to the 101 texture observed in BTS where additional textures were observed as the annealing temperatures were increased27.…”

Oriented growth of a β-quartz solid solution from a MgO–Al2O3–SiO2glass coated by a sol–gel ZrO2layer

“…[26][27][28][29] Whether the heterogeneous nucleation observed in the system presented here shows a preferred orientation with respect to the surface cannot be said because a direct measurement of the surface using EBSD was not possible. 7,10 On the other hand, oriented nucleation has also been observed in surface crystallizing systems.…”

Analysis of the cordierite X-phase and phase transformation by electron backscatter diffraction (EBSD)

“…The numbered crystals highlight "regular hexagons" (1, 2) and "elongated hexagons" (3)(4)(5). The orientations of the numbered crystals are visualized by the corresponding unit cell wire frames and the Euler Angles given in Table 1.…”

“…Fig. 5 also presents EBSD-patterns from the X-phase crystals (1-4) as well as from the matrix (5,6). While the matrix patterns may be indexed as μ-cordierite, the patterns of the X-phase could not be indexed at all.…”

“…with piezoelectric properties in the case of fresnoite [1,2]. The main mystery of this process is the surface nucleation itself, which has recently been realized to be more complicated than previously thought as oriented nucleation has been shown to occur in a number of surface crystallizing glass systems [1][2][3][4][5][6].…”