Tellurite And Fluorotellurite Glasses For Active And Passive

3. Glass batching and melting; MDO 79
were added to 15 g of fluoride, which were mixed in a platinum crucible. This lidded
crucible was then placed in the melting furnace, inside a silica liner, as described in
section (3.3) and ramped from room temperature to 70°C, and held there for 17.5
hours. This stage was followed by a ramp to 400°C for 1 hour then cooled to room
temperature (a heating / cooling rate of 300°C.hr. -1 was used for all stages) [8]. The
same heating procedure was also used for fluorination in the glovebox, however, a
nitrogen rich atmosphere was used.
In the work on the fluorination of ZrF4.H2O described by Sanghera et al. [3, 4] the
mixture of ‘wet’ fluoride and ammonium hydrogen difluoride was held at around
70°C for around 18 hours to allow the (NH4)HF2 sufficient time to react with the
fluoride. An intermediate fluoride compound (identified by TGA and XRD) was
formed at this temperature, ammonium heptafluorozirconate ((NH4)3ZrF7), while
drying the fluoride. As the temperature was increased to around 400°C this compound
decomposed to the pure fluoride compound ZrF4. According to Sanghera et al. [3, 4],
in the first stage of fluorination (70°C in this study), if the temperature was too low
(e.g. 30°C), the reaction of (NH4)HF2 with the ‘wet’ fluoride was too slow, and if the
temperature was too high (e.g. 90°C) decomposition of the (NH4)HF2 occurred too
fast to enable the formation of the intermediate metal ammonium fluoride. In the
fluorination of ‘wet’ ZnF2, the intermediate compound formed is (NH4)2ZnF4 [7].
Equations (3.7) to (3.10) illustrate the proposed analogous decomposition reactions.
2(NH4)HF2 + Zn(OH)F (NH4)2ZnF4 + H2O↑ + HF↑ 70°C (3.7)
2(NH4)HF2 + Zn(OH)2 (NH4)2ZnF4 + 2H2O↑ 70°C (3.8)