Tellurite And Fluorotellurite Glasses For Active And Passive

7. Surface properties; MDO 314
cleaved surface, during sample synthesis and preparation. As expected, the oxygen levels
were higher on this polished sample compared to the cleaved glass (71.5 and 50.4 at. %
respectively), most likely due to hydrolysis, with OH groups replacing fluorine, which
was lower at the surface than the cleave (5.9 and 10.1 at. % respectively). Sodium was
also lower (6.3 to 2.9 at. %), as this is easily leached from the glass after contact with
atmospheric moisture. Fig. (7.22) shows these trends graphically.
7.3.2. Chemical and environmental durability
7.3.2.1. Durability of oxide tellurite glasses
Fig. (7.23) shows the surface quality of glass MOF009 (77TeO2-12ZnO-10Na2O-1Yb2O3
mol. %) before, and after 60, 120, and 180 sec. immersion in a 1M solution of NaOH at
21°C. An uneven surface layer formed, indicating preferential attack of components in
the glass. Initially all component of the glass will be attacked by the alkali and taken into
solution, such as ZnO (2NaOH + ZnO Zn(OH)2 + Na2O) and TeO2 (2NaOH + TeO2
Na2TeO3 + H2O) [22]. However, it is attack of the glass network by OH which will
strongly degrade mechanical and optical properties, as illustrated by equation (7.9).
NaOH
≡Te-O-Te≡ + H + OH - 2(Te-OH)
(7.9)
After 180 seconds, the surface has begun to break up, exposing fresh glass below. This
indicates the hyrdolysed layer was not mechanically rigid, or had been attacked by the