Abstract
The high economic importance of glass ceramics based on Li2O/Al2O3/SiO2 (LAS) is mainly due to their low coefficients of thermal expansion (CTE), which make these materials suitable candidates for a number of applications. The exact mechanism of the crystallization processes in LAS glasses is still not fully understood. The present work focuses on the formation and development of nanocrystalline ZrO2 within an LAS base composition which contains only ZrO2 as nucleating agent. Using a combination of transmission electron microscopy and X-ray absorption spectroscopy techniques, the temporal evolution of the ZrO2 nanocrystal formation is described. It is found that the formation of ZrO2 is initiated by liquid-liquid phase separation droplets with high Zr content, which eventually evolve into the nanocrystalline ZrO2 precipitations. This process is accompanied by a gradual change of the coordination of the tetravalent Zr ions from sixfold in the glass to eightfold in the crystals. The diameters of the ZrO2 crystals stay well below 4 nm, even at late stages. The degree of crystallization at each step of the crystallization process is deduced, and from that, the Avrami coefficient n is determined to be n ≈ 1, which describes a barrier-limited crystal growth process.
Highlights
Nowadays, materials with low, i.e. near to zero thermal expansion find many applications, such as high temperature windows, cook top panels, telescope mirrors, as well as numerous devices in optics and photonics[1]
We have shown that it is possible to determine and describe the crystallization mechanism of the nucleation agent ZrTiO4 in LAS glass ceramics, with a combination of X-ray absorption near edge structure spectroscopy (XANES) and analytical transmission electron microscopy (TEM)[26]
As the focus of this study is on the temporal evolution of the nucleation agent ZrO2 within the LAS glasses, rather than on the subsequent LAS phase formation itself, several samples of the LAS glass with the composition as stated in Table 1 were thermally treated at 725 °C for several periods of time between t = 15 min and t = 24 h
Summary
Enrico Kleebusch[1], Christian Patzig[2], Michael Krause[2], Yongfeng Hu3, Thomas Höche2 & Christian Rüssel[1]. We have shown that it is possible to determine and describe the crystallization mechanism of the nucleation agent ZrTiO4 in LAS glass ceramics, with a combination of X-ray absorption near edge structure spectroscopy (XANES) and analytical transmission electron microscopy (TEM)[26]. We reported on the LAS phase formation and microstructure evolution as a function of the temperature and time of thermal treatment in glass ceramics that are solely nucleated with the nucleating agent ZrO227. By using a combination of X-ray Absorption Near Edge Structure spectroscopy (XANES), X-ray Diffraction (XRD), and (Scanning) Transmission Electron Microscopy (S)TEM including Energy-Dispersive X-Ray Spectroscopy (EDXS), the evolution of nanoscaled, Zr-rich liquid-liquid phase separation droplets into nanoscaled ZrO2 crystals is followed. The samples were selectively carbon coated using a specific coating mask (CoatMaster, 3D- Micromac AG)[34]
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