Investigation of Structural relaxation in IV - V -VI Chalcogenide glasses for Infrared applications

Abstract

Chalcogenide glasses are well known for their application in infrared transmission and sensing. These glasses are usually prepared by quenching the melt. The arrangement of atoms in the liquid state gets arrested into the solid state possessing high enthalpy in the structure and reaching a metastable state. Upon heating, these glasses undergo an endothermic reaction and exhibit a glass transition temperature (Tg) which is about 2/3 of the melting temperature. The viscosity of the glass decreases at Tg and the glass softens. This property is being exploited by glass technologists to mold the glass into a required shape according to the need of the application. Since glasses are kinetically arrested systems, they tend to relax with time to their most stable state in a thermodynamically non equilibrium state. Structural relaxation depends on the viscosity variation with temperature. Angell classified the glass-forming liquids into strong and fragile based on the viscosity variation with temperature. If the variation of viscosity with temperature is Arrhenius then the melt is strong and if it is non-Arrhenius it is a fragile melt. For device applications, the glass needs to be stable. To mold into the required shape, the temperature of the glass needs to be taken a little above Tg and pressures of the order of 30MPa applied. So, the knowledge of viscosity variation with temperature and structural relaxation is very much essential in designing the parameters for molding the glass. The other interesting aspect is that, unlike their crystalline counterparts, glasses can be prepared over a wide composition range offering flexibility in fine-tuning the properties according to the requirement of the desired application. Thus understanding structural relaxation and the structure property relations and their composition dependence forms an important aspect of glass science. Current work is focused on preparing chalcogenide glasses containing Te as one of the major components to extend the IR transmission range. Compared to S and Se, Te is known to increase the glass forming difficulty. Compositions of Te containing IV(Ge)-V(As)-VI(Se/Te) glasses are carefully designed to have a high glasse forming ability and a wide IR transmission range. Melt quenched GexTeySe(100-x-y) (10 ≤ x ≤ 40, 20 ≤ y ≤ 45) glasses covering the average coordination number (Zav) between 2.20 and 2.80 are found to have high thermal stability, high glaas forming ability and high activation energy for Tg. The fragility index varies between 20 and 35 indicating strong nature of the melts. The viscosity as a function of reduced temperature follows almost an Arrhenius behavior confirming the melts are strong in nature. The non-reversing heat flow (ΔHnr) calculated fron MDSC measurements shows that these glasses are non-ageing. Glasses in the pseudo-binary joint GeTe4-As2Se3. and GeSe4-As2Se3 (0 ≤ x ≤ 100) having a fixed connectivity of 2.40 were also prepared to study the composition effects keeping the network connectivity constant. In covalent network glasses, the properties are mainly determined by the network connectivity and Tg is expected to undergo minimal changes. On the other hand, a large variation in Tg has been observed indicating the effect of compositions in modulating the properties. The change in bond energy and the structural motifs are responsible for the observed variations in thermal, optical and structla properties. The fragility index varies between 15 and 32 indicating that the melts of these glasses are super strong in nature. The strong nature of the melts of these glasses indicates that the structural relaxation is minimal and are non-ageing. All the three systems studied in this work were found to transmit IR light up to 18 microns. There were absorption peaks due to oxide impurities. A distillation process using Al as an oxygen getter has been developed to purify these glasses. The glasses subjected to distillation are found to be free from absorption due to impurities. The strong nature of the melts and the non-ageing exhibited by these glasses indicate that these glasses are suitable for device applications.