Tellurite And Fluorotellurite Glasses For Active And Passive

9. Conclusions; MDO 389
• These models were used to predict the fibre drawing temperature (Tη ≈ 330°C
corresponding to a viscosity of 10 4.5 Pa.s) of the fluorotellurite core / clad pair,
occurring at least 60°C < Tx [7].
Crystals in fibre
• Dendritic snowflake shaped crystals were seen in optical fibres of composition
70TeO2-10Na2O-20ZnF2 mol. % pulled at around 300°C under a ‘wet’
atmosphere.
• These Na, and F rich, and Te and O (most likely NaZnF3) deficient crystals were
seen near the surface of the fibre indicting that degradation due to moisture at
elevated temperatures has promoted their nucleation and growth. Fibre drawn
under dryer conditions did not devitrify.
• A proposed growth mechanism for these crystals is initial primary cross shaped
dendrites (in the 2-dimentional plane), followed by secondary dendrites growing
along a different crystallographic plane from these primary dendrites.
Optical loss of fluorotellurite fibre
• The following melting, casting and fibre drawing parameters resulted in the best
quality final fibre (strength, appearance, and optical loss) for fluorotellurite
glasses: ≥ 10 hr. melting time, a dry oxidising melting atmosphere, fluorinated
ZnF2, gold crucible, brass mould, and dry air / N2 fibre drawing atmosphere.