Tellurite And Fluorotellurite Glasses For Active And Passive

8. Fibre drawing; MDO 365
Fig. (8.20): Crystal grown at 435°C in TeO2-LiCl glass [28].
These crystals in fig. (8.20) were an order of magnitude larger than those seen in the
fluorotellurite fibre (40 µm compared to 2 µm), and occurred at the surface of the glass (4
to 9 µm deep). The crystal shown in fig. (8.20) was identified by EDX analysis and XRD
to be tetragonal α-TeO2 (paratellurite) [28].
Fig. (8.9) shows an electron micrograph of crystals near the surface (10 to 20 µm) of
the glass fibre of composition MOF005ii (70TeO2-10Na2O-20ZnF2 mol. %), mounted
length-ways in epoxy resin, and cross-sectioned. It can be seen that most of the crystals
were found around 10 µm from the fibre surface. The fibre was around 200 µm in
diameter, pulled from a preform 10 mm in diameter. Therefore, this region corresponds to
the outer 0.5 mm of the preform (due to a 50 times reduction in diameter). This surface
crystallisation phenomenon could be due to changes in surface chemistry mentioned
earlier in this section. It could also be due to excess heating at the preform / fibre surface.
The surface of the glass will inevitably be hotter than the interior, which may be enough
to nucleate and grow crystals in this region. This local heating could also lead to
degradation of the fibre, also destabilising the glass. These crystals seem to be primarily