Tellurite And Fluorotellurite Glasses For Active And Passive

2. Literature review; MDO 22
the Zachariasen and Dietzel) highly thermally stable glasses can result from tellurite
systems. Stanworth [13] also predicted some tellurite glass properties from knowledge of
the chemical bonding in TeO2. Equation (2.4) can be used to roughly calculate the
contribution of oxide MmOn to the refractive index (n) of the glass:
∑
( n −1) V0
= aM
N M
(2.4)
where aM is the partial refractivity of component MmOn , Vo the volume of the glass
containing 1 g.atom of oxygen and NM the number of atoms of M in the glass per atom of
oxygen. Using this equation, Stanworth predicted the contribution of tellurium dioxide to
the refractive index of the glass is, aM ≈ 1.93 (wavelength not stated) [13]. Stanworth also
predicted that tellurite glasses would have softening points well below silicates due to
their much lower melting temperatures and the lower Te-O bond strength compared to Si-
O. Softening points were in the range of 250 to 400°C [13].
Furthermore, Stanworth melted the first fluorotellurite glasses, based on the ternary
system TeO2-PbO-ZnF2 [14]. Glass formation was shown from 8.2 to 33.2 mol. % ZnF2,
for glasses melted in a zirconia crucible. These compositions were shown to have
improved infrared transmission (extinction coefficient < 0.5 from 1.5 to 5 µm), compared
to the oxide compositions studied (TeO2 with PbO, or BaO, and one of the following:
SO3, Li2O, Na2O, B2O3, Nb2O5, P2O5, MoO3, WO3, V2O5, MgO, CdO, TiO2, GeO2,
ThO2, Ta2O5, La2O3). Absorption bands at around 3.2 and 4.5 µm due to OH [16, 17]
were significantly reduced with ZnF2 addition [14], due to self drying during glass
melting.