The crystallization of MgO–Al₂O₃–SiO₂–ZrO₂glass-ceramics with and without the addition of Y₂O₃– a combined STEM/XANES study

Abstract

Glasses with the mol% composition of 51.9 SiO 2 / 21.2 Al 2 O 3 / 21.2 MgO/ 5.7 ZrO 2 without and with the addition of 2.5 mol% Y 2 O 3 were melted and subsequently transformed into glass-ceramics via annealing. Both glass-ceramics show strong differences in the microstructure and in the phase composition after crystallization at 950 °C for 5 h and subsequently at 1060 °C for different annealing times. In the glass without Y 2 O 3 , the main crystal phase is a quartz solid solution accompanied by the precipitation of ZrO 2 and spinel. By contrast, glass-ceramics without the presence of a quartz solid solution were observed after the crystallization of the Y 2 O 3 -containing glass, using the same heat treatment. This is confirmed by analytical scanning transmission electron microscopy analysis and Xray absorption near-edge structure spectroscopy data gathered at the Zr L 2 -, Y L 2,3 -, Si K-and Al Ledges. Furthermore, using X-ray absorption spectroscopy the coordination of the respective elements is analysed, and changes of the coordination during crystallization are monitored.

Characterizing the residual glass in a MgO/Al2O3/SiO2/ZrO2/Y2O3 glass-ceramic

“…The crystallization of ZrO 2 in glass A is thus accompanied by a change in the coordination number: while Zr 4+ is incorporated into the glass network in a 6-fold coordination, it is 8-fold coordinated in the crystallized, tetragonal (or cubic) ZrO 2 33. However, the transition from the 6- to the 8-fold coordination is not completed in the glass-ceramic A2, because a certain amount of Zr remains in the residual glass after crystallization623. In previous investigations, it was assumed that the Zr in the residual glass still exhibits the 6-fold coordination23.…”

“…This was especially shown in a glass with the mol% composition 50.6 SiO 2  · 20.7 MgO · 20.7 Al 2 O 3  · 5.6 ZrO 2 2.4 Y 2 O 3 6723. A considerable quantity of residual glass was detected after crystallizing at 950 °C for 5 h, cooling to room temperature (RT) and a second crystallization step at 1060 °C for 1 h623. However, the produced glass-ceramics, containing tetragonal (or cubic) ZrO 2 (CTE = 10.5 · 10 −6  K −1  24) and spinel (MgAl 2 O 4 , CTE (20–800 °C)  = 8 · 10 −6  K −1  22), show excellent mechanical properties despite the absence of any QSS7.…”

“…This was especially shown in a glass with the mol% composition 50.6 SiO 2  · 20.7 MgO · 20.7 Al 2 O 3  · 5.6 ZrO 2 2.4 Y 2 O 3 6723. A considerable quantity of residual glass was detected after crystallizing at 950 °C for 5 h, cooling to room temperature (RT) and a second crystallization step at 1060 °C for 1 h623.…”

“…In another study, these glass-ceramics have been intensively investigated using analytical scanning transmission electron microscopy (STEM) and X-ray absorption at near edge spectroscopy (XANES) at the Zr L 2 -, Y L 2,3 -, Si K - and Al L - edges23. The composition of the residual glassy phase was determined using STEM in combination with energy-dispersive X-Ray spectroscopy (EDXS), and it was found that a considerable amount of Zr is still present in the residual glass, showing that not all Zr contributed to the crystallization of ZrO 2 in these glass-ceramics.…”

“…Additionally, the coordination of the respective elements in the samples was analysed, and the change of the Zr coordination from 6-fold (in the glass) to 8-fold (after the crystallization of ZrO 2 ) was reported. However, a direct measurement of the coordination of the remaining Zr in the residual glassy phase was not possible due to the too small areas of residual glass with respect to the lateral resolution of the XANES analyses23.…”