Structural, mechanical and optical investigations in the TeO2-rich part of the TeO2–GeO2–ZnO ternary glass system
Résumé
Stable glasses are successfully synthesized in the TeO2-GeO2-ZnO system at 850 C by the meltquenching
method and the glass forming domain is determined in the TeO2-rich part of the diagram.
The thermal study, carried out using differential scanning calorimetry, reveals that the glass transition
temperature, as well as the thermal stability, increases with the addition of ZnO or GeO2. Bulk glass
samples are elaborated within two series of compositions, corresponding to fixed concentrations in GeO2
(respectively 5 or 10 mol. %), and to various contents in ZnO. Structural changes caused by the ZnO
addition are discussed based on Raman spectroscopy data. A progressive but very moderate network
depolymerization is shown with increasing amount of ZnO. However, two different regimes can be
identified, depending on the ZnO content. It is believed that ZnO acts as a network modifier for compositions
below 20 mol. %, and starts to participate as a glass network former over such concentration. It
is well evidenced that GeO2 contributes to the increase in Young's modulus E, evaluated from ultrasonic
echography measurements. In addition, this oxide favors the network reticulation detected by the
decrease of the Poisson ratio and the increase of the fractal bond connectivity. However, the role of ZnO is
more complicated and will be extensively discussed. The decrease in the atomic packing density Cg
probably explains the global evolution of E as a function of ZnO content. The refractive indices and optical
band gap energies are extracted from UV-Visible-NIR optical transmission data. For the studied glasses, it
is found that the transmission threshold decreases with larger ZnO contents, reflecting the increase in
the optical band gap value. Refractive index is finally seen to decrease as a function of both ZnO and GeO2
contents. Such variation is explained by the decrease of the molar electronic polarizability, and by the
lower optical basicity values known for TeO3 entities in comparison to TeO4 units.