Tellurite And Fluorotellurite Glasses For Active And Passive

7. Surface properties; MDO 324
This provided a reasonable fit for the EDX data, with all points lying within the error bars
(+1 at. %).
Fig. (7.37) shows the diffusion profile of Ag + for glass ST08 (79TeO2-5ZnO-10Na2O-
5PbO-1Yb2O3 mol. %), for which one flat side was dipped into a molten salt solution
(2AgNO3-49NaNO3-49KNO3 mol. %) at 270°C for 5 hours. A much higher silver content
was incorporated into the glass, around 9.1 at. % at 0.25 µm, and 0.5 at. % 1.5 µm,
although the depth of diffusion was not greater than the heat treated sample. This high
level over shorter time of treatment is to be expected, as the molten salt solution will
result in a more vigorous exchange, and higher refractive index contrast in the exchanged
region compared to the bulk glass. The first data point at 0 µm was not included in the
exponential decay fit. The surface contained some pitting from the molten salt, therefore
the electron beam will sample some of these voids when performing EDX analysis,
lowering the overall silver content. However, if the sodium exchanged for the silver 1:1,
this would result in 6.8 at. % at the surface, similar to the value seen in fig. (7.37) at 0
µm. Around 7 at. % silver was also seen at 0.5 µm depth, possibly indicating the
measurement at 0.25 µm was incorrect, and the ion-exchange followed a more complex
profile than shown by equation (7.14).
For this glass N0 = 0.125 (N = 0.091 (9.1 at. %) at x = 0.25 µm was used as a
constraint, due to the spurious EDX value at x = 0 µm), t = 18000 seconds (i.e. 5 hours),
and D = 14.38×10 -6 m 2 .s -1 (using constraint: silver fraction = 4.636×10 -3 (0.46 at. %) at x
= 1.5 µm). Fig. (7.41) shows the EDX data and modelling for glass ST08 (79TeO2-5ZnO-
10Na2O-5PbO-1Yb2O3 mol. %), where one flat side was dipped into a molten salt
solution (2AgNO3-49NaNO3-49KNO3 mol. %) at 270°C for 5 hours.