Surface crystallization of low thermal expansion Ba_0.5Sr_0.5Zn₂Si₂O₇ from an 8 BaO·8 SrO·34 ZnO·50 SiO₂ glass

Abstract

Thermal treatment of a glass with the composition 8 BaO$8 SrO$34 ZnO$50 SiO 2 has led to the crystallization of a Ba 1Àx Sr x Zn 2 Si 2 O 7 solid solution. This solid solution has a very low or even negative thermal expansion. The glass system possesses a strong tendency towards surface crystallization, while bulk nucleation is negligible. The surface crystallization behavior was characterized using differential scanning calorimetry, X-ray diffraction, optical microscopy, and scanning electron microscopy including electron backscatter diffraction. The observed morphology strongly depends on the type of surface pretreatment. Two different surface qualities were compared: a polished surface and a surface obtained by cutting. The as-cut surface shows a significantly enhanced nucleation rate, which leads to smaller crystals caused by a growth selection near to the surface. The crystal orientation related to the inward growth starting from the surface was successfully investigated by EBSD, for the first time, using a recently reported crystal structure. The crystals show a preferred orientation of the c-axis perpendicular to the surface; this has a significant effect on the crack formation.

Role of Tin Oxide as a Nucleating Agent with Low Solubility in BaO–SrO–ZnO–SiO2 Glasses Studied by Electron and X-ray Microscopy

“…This can be seen in Figure (lower four tomograms), where also both morphologies can be reached solely by changing the viewing direction. This behavior was already suspected after an EBSD analysis of a surface-crystallized sample in which a dependence of the crystal appearance of their orientation was found . It should be mentioned that stresses occurring during sample preparation (e.g., SEM analyses), i.e., cutting and polishing, may provoke cracks at the surface due to stress relaxation.…”

“…This behavior was already suspected after an EBSD analysis of a surface-crystallized sample in which a dependence of the crystal appearance of their orientation was found. 31 It should be mentioned that stresses occurring during sample preparation (e.g., SEM analyses), i.e., cutting and polishing, may provoke cracks at the surface due to stress relaxation. If these stresses are too high, cracks are formed which will propagate into the volume.…”

“…However, the composition of the base system BaO−SrO−ZnO−SiO 2 has to be modified, because in this quasiquaternary system, solely surface crystallization takes place. 31 The resulting microstructure of surface crystallized samples is full of cracks, and the mechanical properties are hence fairly poor. In order to achieve volume crystallization with homogeneously dispersed crystals in the sub-micrometer range, nucleation agents have to be evaluated.…”

“…Therefore, at first a glass has to be synthesized, and afterward, this glass can be transformed into a glass ceramic by applying appropriate heat treatments. However, the composition of the base system BaO–SrO–ZnO–SiO 2 has to be modified, because in this quasiquaternary system, solely surface crystallization takes place . The resulting microstructure of surface crystallized samples is full of cracks, and the mechanical properties are hence fairly poor.…”

Solid solutions based on BaZn2Si2O7 with thermal expansions from negative to highly positive – a review

“…In the BaO/SrO/ZnO/SiO 2 system, nucleation at the surface already leads to an orientation of the a-or b-axis parallel to the surface. 50 The highest growth velocity, however, is along the crystallographic c-axis (see.…”