Abstract
Glass ceramics based on Li2O/Al2O3/SiO2 are of high economic importance, as they often show very low coefficients of thermal expansion. This enables a number of challenging applications, such as cooktop panels, furnace windows or telescope mirror blanks. Usually, the crystallization of the desired LAS crystal phases within the glasses must be tailored by a careful choice of crystallization schedule and type of nucleation agents to be used. The present work describes the formation of nanocrystalline TiO2 within an LAS base composition that contains solely TiO2 as nucleating agent. Using a combination of scanning transmission electron microscopy as well as X-ray absorption spectroscopy, it is found that a mixture of four- and six-fold coordinated Ti4+ ions exists already within the glass. Heating of the glass to 740 °C immediately changes this ratio towards a high content of six-fold coordinated Ti, which accumulates in liquid-liquid phase-separation droplets. During the course of thermal treatment, these droplets eventually evolve into nanocrystalline TiO2 precipitations, in which Ti4+ is six-fold coordinated. Thus, it is shown that the nucleation of nanocrystalline TiO2 is initiated by a gradual re-arrangement of the Ti ions in the amorphous, glassy matrix, from a four-fold towards a six-fold coordination.
Highlights
Crystal phases within the glasses must be tailored by a careful choice of crystallization schedule and type of nucleation agents to be used
In previous studies we investigate different LAS glass compositions to verify the mechanism of nucleation especially in the early stages
The chemical composition was controlled by energy-dispersive X-Ray spectroscopy (EDXS) spectroscopy and agreed well with that calculated from the batch composition
Summary
The casted glass has a brown coloration frequently denoted as ilmenite coloration which is due to an Fe3+-O-Ti4+ charge transfer formed by trace impurities of iron. The XRD results as presented above allow general assumptions concerning the course of crystallization upon thermal treatment of the LAS glass with solely TiO2 as nucleation agent: starting from an amorphous glass sample, at first, TiO2 is precipitated, followed by a crystallization of the LAS phase later on. The TiO2 nanocrystals start to crystallize from these phase separation droplets
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