Tellurite And Fluorotellurite Glasses For Active And Passive

8. Fibre drawing; MDO 366
cross-shaped, indicating that the fibre was not held for sufficient time or temperature to
promote the growth of secondary dendrites described above.
Fig. (8.10) and (8.11) show electron micrographs of crystals near the surface of the
glass fibre of composition MOF005ii (70TeO2-10Na2O-20ZnF2 mol. %) as drawn.
Crystals can be clearly seen protruding from the fibre surface. The glass appears to have
flowed around the crystals during fibre drawing. The nodular crystals have darker
regions. These ‘holes’ are filled by glass or voids. This is likely to be due to preferential
growth described earlier in this section. The rounded edges are possibly the result of re-
dissolution of any sharply defined edges by the flowing glass during the fibre drawing
process. Therefore, it is reasonable to propose that the crystals grew on the neckdown
region of the preform, and the reflow occurred at the tip of the preform as the fibre drew.
Fig. (8.12) depicts an electron micrograph of a snowflake shaped crystal in the glass
fibre of composition MOF005ii (70TeO2-10Na2O-20ZnF2 mol. %). The fibre was
mounted end-on in epoxy resin, and polished-back to reveal the transverse plane to the
fibre axis, i.e. cross-section. Again, this type of crystal showed the primary and secondary
dendrite structure described above. Fig. (8.13) displays the EDX analysis from points Q
and R in fig. (8.12). The crystal contained significantly more Na and F than the bulk glass
and less O. This indicates once more that it is the orthorhombic NaZnF3 phase.
Fig. (8.14) and (8.15) present electron micrographs of a crystals 10-20 µm from the
surface in the glass fibre of composition MOF005ii (70TeO2-10Na2O-20ZnF2 mol. %).
The fibre was mounted end on in epoxy resin, and polished-back. A number of different
crystal shapes were observed. This is likely to be due to orientation from polishing and
different stages in the growth of the crystals.