Tellurite And Fluorotellurite Glasses For Active And Passive

7. Surface properties; MDO 318
Table (7.8): Proposed regimes of water attack on fluorotellurite glasses.
Temperature regime Main reactions Comments
High T
(≈ 60°C)
Low T (≈ 21°C) -Zn + F - + H + OH - -Zn-OH + H + F - F - / OH - exchange
≡Te-O-Te≡ + H + OH - 2(≡Te-OH) Bridging bond severance
≡Te-O-Na + + H + OH - ≡Te-OH + Na + OH - Cationic leaching
The leaching of sodium ions and severance of Te-O-Te bonds could take place at around
a 1:1 ratio at lower temperatures and the weight loss and gain would approximately
cancel each other out (RAMNa = 23.0 g.mol -1 , and RMMH2O = 18.0 g.mol -1 ), with little
effect seen on the durability curve, fig. (7.29a). If this was the case, higher temperature
leaching also occurs with sodium ions, as well as heavier cations, i.e. tellurium and zinc.
The durability curves levelled out eventually, perhaps as an impermeable hydroxide layer
will form, protecting the material away from the surface hydrolysing further. Fig. (7.29b)
shows the weight loss from (fig. 7.29a) plotted against (t) 1/2 . For both temperatures, 21
and 60°C, the plot was highly linear, suggesting diffusion controlled attack of the
fluorotellurite glass by the water.
Fig. (7.30) shows the surface quality of glass MOF001 (65TeO2-10Na2O-25ZnF2 mol.
%) after immersion in 3M HCl, with agitation at 21°C. These glasses exhibited different
surface morphology after etching to the oxide glasses reported in the previous section.
Initially (30 sec.) the spherical pitting was relatively smooth, but after 1 min. the surface
quality became very poor. Fig. (7.31a) shows the exponential etch rate of this solution,
and the linear etch rate (≈ 3 wt. % per min.) of the 1M solution on glass MOF005
(70TeO2-10Na2O-20ZnF2 mol. %). This pronounced pitting may be due to the
preferential attack of TeO2 / Na2O / ZnO, leaving more fluoride material, etched at a