Tellurite And Fluorotellurite Glasses For Active And Passive

7. Surface properties; MDO 317
exponentially for both temperatures. As expected, the 60°C glass degraded more rapidly,
as water more readily dissociates into H + and OH - at higher temperatures. This will result
in the severance of bridging bonds, and hydrolysis.
≡Te-O-Te≡ + H + OH - 2(≡Te-OH) (7.12)
OH will also replace fluorine in the glass due to their similar ionic radii and
electronegativities, and sodium ions probably leached into solution as described above.
The glass treated at room temperature, showed relatively low weight loss, 0.34 wt. %,
over the 21 day period. This may indicate two different temperature regimes of water
attack. At lower temperatures (21°C), very few cationic species (Te +4 , Na + and Zn +2 )
were leached into solution (based on oveall weight loss), and severance of bridging bonds
by hydrolysis, was also low. The replacement of fluorine with OH is common, resulting
in a smaller net weight loss over time; fluoride was taken into solution, with RAM = 19.9
g.mol -1 , and OH added, with RMM = 17.0 g.mol -1 , a 2.9 g.mol -1 difference. At higher
temperatures (60°C), cationic species were leached into solution more readily, bridging
bonds were severed by hydrolysis, along with the attack occurring at lower temperatures,
which will have resulted in a higher weight loss over time, as shown by fig. (7.29a). The
severance of bridging bonds will have resulted in a gain in weight (H2O RMM = 18.0
g.mol -1 ), and leaching of species, a loss, the weight depending on the cation leached
(RAMNa = 23.0 g.mol -1 , RAMZn = 65.4 g.mol -1 , and RAMTe = 127.6 g.mol -1 ). Table (7.8)
summarises these reactions.