Glass surface layer density by neutron depth profiling

Several lithium-disilicate ( Li₂O{bullet operator}2SiO₂ ) glass samples were synthesized, polished and subjected to static leach tests for varying periods of time in an HCl solution (pH 4) at 90°C. The presence of an alteration layer on the leached specimens was independently confirmed using infrared spectroscopy. These samples were then analyzed with neutron depth profiling (NDP), and for all exposure times between 1 and 20 h, a leached layer was detected by NDP. The residual concentration of ⁶Li in the alteration layer was observed to be approximately 10% of the bulk glass value. The 7.5 h and 17.5 h samples were cross-sectioned using focused ion beam milling, and the thickness of the alteration layer on the samples was measured directly using scanning electron microscopy to be 9.8 and 22.9 μm, respectively. The areal density of the gel layers - determined by deconvolution of the NDP spectra - was coupled with the SEM measurement to give densities of 1.46 ± 0.08 g/cm³ for the 7.5 h specimen and 1.46 ± 0.05 g/cm³ for the 17.5 h specimen. This preliminary work shows that NDP can be used to determine the average density of gel layers formed owing to leaching of alkali-silicate glass.