Tellurite And Fluorotellurite Glasses For Active And Passive

2. Literature review; MDO 55
2.5.2. Fluorotellurite glasses
With conventional melting procedures, oxide-tellurite glasses cannot be prepared OH
(hyroxyl) free without an elaborate set-up (e.g. bubbling drying chemicals such as carbon
tetrachloride through the molten glass with platinum tubing [53]). This is problematic as
OH exhibits two fundamental absorption bands in the mid-IR. A simpler way of
removing the OH from the glass during melting is to add a metal fluoride to the batch and
the fluoride will liberate hydrogen species from the melt (‘drying’ the glass) [56, 57]. For
example, in the ternary glass system TeO2-ZnO-Na2O, which is know to be a good glass-
former [36], ZnF2 can be added partially or completely replacing ZnO, and will react with
accessible OH according to equation (2.16):
2[≡Te-OH] + ZnF2 ≡Te-O-Te≡ + ZnO + 2HF(g)↑ (2.16)
In addition, adventitious water in the glass melting atmosphere can be removed (equation
(2.17)) to prevent hydrolysis of the melt (equation (2.18)):
ZnF2 + H2O ZnO + 2HF(g)↑ (2.17)
≡Te-O-Te≡ + H2O 2[≡Te-OH] (2.18)
Fluorotellurite compositions and properties reported in the literature are summarised in
the next section (2.5.2.1).