Tellurite And Fluorotellurite Glasses For Active And Passive

7. Surface properties; MDO 315
solution which would have decreased in alkalinity with increasing time. The linear etch
rate of this solution (fig. (7.24)) was around 0.012 wt. % glass per second. It would
appear NaOH is an unsuitable etchant for cleaning the surface (e.g. preform) of oxide
tellurite glass.
Fig. (7.25) shows optical micrographs of glass MOD012 (75TeO2-10ZnO-10Na2O-
5GeO2 mol. %) after immersion in 3M H2SO4 at 15°C for 60, 120, 180, and 240 seconds.
Qualitatively, the surface was not attacked as severely as by the NaOH at 21°C, however
this 3M H2SO4 etch was performed at a lower temperature. TeO2 has also been shown to
be completely insoluble in concentrated (100%) H2SO4 [25]. The Na2O in the glass
would be more readily attacked in this glass when in contact with an acidic solution, and
Zn +2 also leached into solution (ZnO + H2SO4 ZnSO4_+ H2O). Again this solution will
hydrolyse the glass network as shown by equation (7.10).
H2SO4
≡Te-O - Na + + H + OH - ≡Te-OH + Na + OH - (7.10)
If the sodium was leached from the glass, and the solution donated a proton (H + ) to the
oxygen ion attached to Te, this would have formed OH at the glass surface. This reaction
would have proceeded, increasing the pH of the solution. Hydrolysis of bridging oxygens,
shown by equation (7.9), probably also occurred. The etchant was again poor, due to
severe surface hydrolysis forming an opaque surface layer.
Fig. (7.26) shows the surface of the glass from fig. (7.25) immersed in 1M HCl at
15°C for 60, 120, 180 and 240 sec. Surface quality was again poor. Sodium ions will