Raman microspectroscopy and hyperspectral Raman line imaging of hydrothermal aluminosilicate glasses.

Abstract

Depth profiling Raman microspectroscopy and hyperspectral Raman line imaging were used to examine the structural effect of exposure to pressurized steam (autoclaving) and refluxing dilute acetic acid on a series of aluminosilicate glasses and a commercial dental restorative glass (Duceram LFC). These protocols are being investigated as a means of strengthening glasses, especially for dental restorations, while simultaneously reducing their hardness. The thousands of spectra collected in the low frequency (200-2000 cm$\sp{-1})$ and high frequency (1800-2000 cm$\sp{-1})$ spectral regions were analyzed using factor analysis. Significant factors represented a network of fully polymerized silicate tetrahedra, tetrahedra with one nonbridging oxygen (NBO), and residual bicarbonate starting material. Autoclaved samples also had a factor representing a partially depolymerized silicate network with 1-4 NBO per tetrahedron. Its score was significant only near the surface, but was observed increasingly deeper with longer autoclaving times. Autoclaving was found to add water to the glass from the surface inwards, confirmed by D$\sb2$O experiments. Water exists in the glasses in every form, including isolated SiOH groups, weakly and very strongly hydrogen bonded water molecules, and bulk, liquid water. Initially, the spectral response from the hydroxyl groups in the glass is broad and ill-defined. SiOH, molecular water, and a relatively sharp unassigned band at 3570 cm$\sp{-1}$ appear with longer autoclaving times. The scores for the water and depolymerized silicate network factors are coincident. The appearance of these factors corresponds to decreases in the bicarbonate and 1 NBO factors, implying that the water reacts with bicarbonate and with tetrahedra with 1 NBO preferentially. The high alkali content of the commercial dental restorative glass produces a predominately 1 NBO silicate network. A factor representing a more severed network (1-4 NBO) occurs in the samples exposed to refluxing acetic acid. Molecular water is observable in the samples with the lengthiest exposure. Different, nearly crystalline borate species, including metaborate, pyroborate, and boroxol groups were identified in different layers near the surface and may contribute to unusual mechanical properties. Hyperspectral Raman line imaging provided a necessary additional data dimension and permitted better interpretation of the relationships between species.