Oriented nucleation and crystal growth in SrO–Al2O3–SiO2 tectosilicate glasses

It has been shown that transparent SrAl2O4–SrAl2Si2O8 ceramics can be synthesized from glasses by complete crystallization. This article features three tectosilicate (SrO/Al2O3 = 1) glasses in the system Sr1−x/2Al2−xSixO4 with decreasing amounts of SiO2 (x = 0.9, 0.7 and 0.5), i.e. with an increasing amount of SrO. The glasses are doped with Eu and produced via aerodynamic levitation coupled to laser-heating. Further application of the appropriate single-step heat treatment leads to the complete and congruent crystallization of these glasses via surface nucleation and subsequent crystal growth into the bulk. Bulk nucleation does not occur. Analysing the resulting transparent ceramics by scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and X-ray diffraction (XRD) shows that these visually very similar materials exhibit very different microstructures. All three materials show oriented nucleation but the orientations preferred during nucleation never prevail during growth into the bulk where these materials show strong crystallographic textures, i.e. they are highly oriented. A previously unknown monoclinic phase with the composition Sr0.55Al1.1Si0.9O4 is detected at the immediate surface of the corresponding glass after crystallization.