Structure and properties of thermally poled lithium alumosilicate glasses and glass-ceramics

• Struktur und Eigenschaften von polarisierten Lithiumalumosilikatgläsern und Glaskeramiken

Sander, Malte; Roos, Christian Hans-Georg (Thesis advisor); Deubener, Joachim (Thesis advisor)

Aachen : RWTH Aachen University (2023)
Dissertation / PhD Thesis

Dissertation, Rheinisch-Westfälische Technische Hochschule Aachen, 2023

Abstract

A two-step process consisting of a thermal poling and a crystallization treatment was developed to modify the surface chemistry, structure, and properties of glass-ceramics. A lithium alumosilicate glass was used to study fundamental processes inside the glass throughout the treatments. During the thermal poling treatment, a voltage (50 to 1000 V) was applied to the glass at an elevated temperature (200 to 500 °C), resulting in cation migration toward the cathode. Underneath the anode-faced surface, the depletion of lithium cations and an arising strong internal electric field enabled electrons to discharge into the anode. As a result, neutral non-bridging oxygens reacted with each other, forming bridging oxygen and thus polymerizing the network. ToF-SIMS element mappings and SNMS depth profiles revealed an entire lithium depletion up to a depth of 18.7 μm. Furthermore, Raman spectra of this region pointed toward an increase in the extent of the mid-range order and polymerization of the network. As a second charge compensation mechanism, the formation of oxygen tri-clusters was discussed based on atomic pair distribution functions. The thermally poled glasses were subsequently crystallized. During the treatment, thermally activated relaxation processes enabled lithium cations to re-enter the depletion layer. The relaxation processes were investigated by measuring thermally stimulated depolarization currents (TSDC). The TSDC measurements, in combination with thermal analysis, revealed that the relaxation process was interrupted by the high-quartz s.s. phase formation. During the relaxation, only 25 % of the initial lithium cations could re-enter the surface because sodium and potassium occupied the interstitial charge-compensation sites in the vicinity of non-bridging oxygen and charge deficit tetrahedra. The lower lithium concentration underneath the anode-faced surface limited the high-quartz s.s. phase formation and caused the incorporation of a higher silica content into the solid solution. Furthermore, grazing incidence XRD revealed a characteristic bi-layered structure underneath the anode-faced surface. Here, on top of the surface, a low-crystalline layer was found that is followed by an amorphous intermediate layer. SEM images and Raman investigations showed that structural modifications underneath the anode-faced surface of the glass-ceramics extend deeper into the surface than the depletion layer in the uncrystallized as-poled glasses. In layers beneath the former depletion layer, high compressive stresses with values up to −600 MPa were measured inside the crystalline phase via RS-XRD. The stresses arise due to the layered surface structure as well as different chemistry and thermal expansion coefficients. Finally, a parameter study showed that the depletion layer thickness is a state function of the poling voltage as well as temperature and that its chemical composition is identical in all poled samples. Furthermore, it could be shown that the crystalline phase content and silica content in the high-quartz s.s. phase is only a function of the depletion layer thickness in the as-poled glass.

Institutions

• Division of Materials Science and Engineering [520000]
• Chair of Glass and Glass-ceramic [524210]