Tellurite And Fluorotellurite Glasses For Active And Passive

7. Surface properties; MDO 319
lower rate by the HCl. Fig. (7.31a) shows the wt. % loss versus (t) 1/2 for the glasses
shown in fig. (7.31a), with linear fitting, again indicating diffusion controlled attack.
Fig. (7.32) shows micrographs of glass MOF001 (65TeO2-10Na2O-25ZnF2 mol. %)
after etching with 40% HF at 21°C. It can be clearly seen, that although this etchant
produced some pitting, (preferential attack), the acid solution did not appear to deposit
any kind of surface layer, leaving fluorotellurite glass at the surface even after 30 min.
This would seem to indicate hydrolysis was avoided, compared to other etchants studied.
Fig. (7.33) shows the linear etch rate for 4% HF at 21°C on glass MOF005 (70TeO2-
10Na2O-20ZnF2 mol. %) on this glass (MOF001), which also left a clean surface (see fig.
(7.34)). The linear etch rate was around 0.5 wt. % per min. This rate is reasonable to
perform a controlled etch on the glass. Fig. (7.34) shows the effect of this treatment on
the infrared spectra. It can be seen, that the OH bands increased significantly with etching
time. The strongly H-bonded OH band can also be seen, which was not apparent in the
bulk polished ZnF2 containing samples (see chapter 6), indicating these latter glasses
were very dry. Therefore, any wet etching techniques may not be suitable for
fluorotellurite preform preparation, as this OH content, presumable concentrated near the
glass surface, may introduce stresses, and crystal nucleation sites in the preform, as well
as exhibiting dissimilar rheological behaviour to the ‘dry’ glass.
Fig. (7.35) shows a micrographs of the effect of various etchants on a similar glass to
MOF001, of composition MOF005 (70TeO2-10Na2O-20ZnF2 mol. %). Hydrofluoric,
acetic and phosphoric acids produced good surface finishes and the latter two acids
should be investigated further, as weight loss was not measured, and would need to be