The formation of nanocrystalline ZrO2 nuclei in a Li2O-Al2O3-SiO2 glass – a combined XANES and TEM study

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.

Inversion of quartz solid solutions at cryogenic temperatures

“…Concerning the data points pertaining to glass‐ceramics containing crystallization additives (Figure 3), a higher scattering of the values is clearly observable. The nucleation of TiO 2 ‐ and ZrO 2 ‐bearing seeds has been previously shown to induce a heterogeneous distribution of other chemical species in the glass matrix at the nanoscale, 35‐39 possibly affecting the composition of the subsequently forming Qss crystals. Moreover, an overall reduction in the lattice parameters of Qss was observed in situ during isothermal ceramization of LAS glass‐ceramics and connected to a gradual Si‐enrichment of the crystals 40 .…”

X-ray Absorption Fine Structure (XAFS) Studies of Oxide Glasses—A 45-Year Overview

“…Very recently, Kleebusch et al presented a detailed study about the crystallization behavior of Zr as a nucleating agent in Li 2 O-Al 2 O 3 -SiO 2 (LAS) using a combination of different techniques such as X-ray absorption fine structure spectroscopy (XAFS), X-ray diffraction (XRD) and Scanning Transmission Electron microscopy (STEM) [ 104 ]. These authors used the fact that the intensity ratios of the doublet peaks of both the Zr L 2 -edge and Zr L 3 -edge are dependent on the coordination number to determine the local coordination around Zr atoms [ 105 ].…”

“…These authors used the fact that the intensity ratios of the doublet peaks of both the Zr L 2 -edge and Zr L 3 -edge are dependent on the coordination number to determine the local coordination around Zr atoms [ 105 ]. Figure 15 shows a comparison of Zr L-edges XANES spectra of LAS glass containing zirconia with the XANES spectra of the sample at the final stage of heat-treatment, as well as the XANES spectra of crystalline reference compounds where Zr atoms are sixfold [6] Zr 4+ in SrZrO 3 ) and eightfold coordinated [8] Zr 4+ in t-ZrO 2 [ 104 ]. From this figure, it is clear that in the glassy sample, Zr ions are mainly six-fold coordinated by oxygen atoms whereas, at the final stage of heat-treatment, the XANES spectra of the glass-ceramic sample are similar to the XANES spectra of the t-ZrO 2 where Zr ions are eightfold coordinated by oxygen atoms.…”

“… ( a ) Zr L 2 -edge XANES spectra and ( b ) Zr-L 3 edge XANES spectra of LAS glass-ceramic sample compared to the spectra of the glassy sample and [6] Zr 2+ and [8] Zr 2+ crystalline reference compounds [ 104 ]. “Reproduced with permission from published © Nature Publishing Group.” (2017).…”

Structural evolution at short and medium range distances during crystallization of a P2O5‐Li2O‐Al2O3‐SiO2 glass

“…From heterogeneities or amorphous phase separation preexisting in the as-cast glasses, [2][3][4] the agents precipitate heterogeneously upon heat treatments, leading to structural and compositional modifications of the glass that allow nucleation of a major phase. [4][5][6][7][8] Though TiO 2 and ZrO 2 are the most commonly used nucleating agents, P 2 O 5 has been well recognized to aid nucleation in various silicate glasses [9][10][11][12][13] and is particularly effective when introduced to LiO 2 -SiO 2 glasses. [5][6][7][8][9][10][11][12][13] Deep insights on the role of P 2 O 5 have been mostly obtained using NMR techniques, showing that P 2 O 5 can precipitate as a phosphate phase, usually Li 3 PO 4 , but this phase is not considered as a precursor for the nucleation of Li 2 Si 2 O 5 .…”