Tellurite And Fluorotellurite Glasses For Active And Passive

2. Literature review; MDO 50
It can be seen the curves are shifted to much lower stress intensity factors for tellurite
glasses compared to silicates (silicates have KIC values around 0.8 MPa.m 1/2 ). This
indicates tellurite glass is far more brittle than silicate glass. The substitution of ZnO by
Na2O shifts the curve to lower stress intensity due to the decrease in bond strength
because of the introduction of weak non-bridging Na-O bonds.
The slopes of tellurite glass KI-v curves are larger as there is a relatively small
contribution of stress corrosion reaction (compared to silicates [50]) to subcritical crack
growth, the stress corrosion mechanism being based upon a stress enhanced chemical
reaction between water and the chemical bonds in the glass. This is possibly due to the
lack of ring structure in tellurite glasses which is seen in silicates. A chain like structure
is seen in zinc tellurite glasses which may not permit the strained bonds [49].
Watanabe et al. [51] studied the Vickers’ hardness (Hv) of a glass of composition
(mol. %) 70TeO2-15ZnO-15Na2O (Hv = 2.6 GPa at room temperature) around its glass
transition region (Tg = 264°C). There was a sharp decrease in hardness of this glass at
around T/Tg = 0.9 to 1.0. This can be attributed to the ‘fragile’ [46, 47] nature of TeO2-
based glasses, so the breaking of atomic bonds and atomic rearrangement occur easily
around Tg.
Aida et al. [52] studied the thermal stability of the compositions (mol. %) 80TeO2-
10ZnO-10Na2O and 70TeO2-15ZnO-15Na2O as well as many other ternaries. Fig. (2.11)
shows DTA traces of some alkali-containing zinc tellurite glasses.