Influence of iso-structural substitutions on properties of germanium(arsenic, antimony)(sulfur, selenium) glasses

Chalcogenide glasses possess exceptional infrared transparency, large non-linear refractive indexes and low phonon energies, making them good candidates for infrared optical applications. Previous studies in the Ge-Sb-S glass system have shown an increase in the nonlinear refractive index with an iso-structural substitution of two-coordinated selenium (Se) for sulphur (S) and variation of other physical properties with substitution of other iso-structural species such as three-coordinated arsenic (As) for antimony (Sb). The role of such iso-structural exchange on properties important to the thermo-, visco- and mechanical attributes of glasses in the Ge-As/Sb-S/Se system has not been thoroughly evaluated and form the basis of this thesis.;This study reports results of a systematic study of the relationship of glass structure on the thermal, mechanical and optical properties of glasses in the system Ge28Sb12S(60-x)Se(x) and Ge28As12S(60-x)Se(x) with x = 0, 15, 30, 45 and 60. The aim has been to elucidate the effect of the substitution of S for Se and Sb for As on physical properties of importance for Precision Glass Molding (PGM) where such multicomponent glasses possess important refractive index and dispersion properties. Using a range of experimental characterization techniques to assess the glass' composition, thermal stability, thermal expansion, viscosity behavior, optical transparency and Vickers hardness, these property changes are interpreted based on the impact of the structural role of the group V (As, Sb) and VI (S, Se) constituents.;It has been found that most properties show linear variation where properties are dominated by a majority bond type. The glass' transition temperature, microhardness and the viscosity decrease with sulphur substitution by Se due to the variation of strength of the bonds. Mixed bonding occurs through a transition region of x ∼30-45 mol% Se and is characterized by a plateau in some properties, especially the glass transition temperature. This result is the signature of a switch of the dominant bonds in glasses. Furthermore, it is found that all glasses have superior crystallization stability, based on measured Tx-Tg stability data.;Extreme duration heat treatment studies of the glasses carried out to initiate crystallization (87 hours as compared to the ∼20-30 mins of a typical molding cycle) at the molding temperature (eta∼ 7-11 Pa s) show the onset of both bulk and surface crystallization. XRD indicates the formation of crystals within the bulk (GeSe), consistent with previous findings of other authors, and at the glasses surface (GeSe3) in the Ge28Sb 12Se60 glass composition. Sulphide glasses and mixed chalcogen glasses (Ge28Sb12S60 and Ge28Sb 12S30Se30, respectively) exhibit less devitrification when heat treated for similar duration.;Thus, within the context of the finite composition space examined in this study, the selenide glass in the Sb-based system is concluded to be the best candidate for precision glass molding due to its low hardness/more ductile behavior (required for conventional preform fabrication), its lower infrared absorption (due to intrinsic impurities in elemental starting materials), its long viscosity curve (offering a broader working range for pressing) and a large DeltaT (for resistance to crystallization upon reheat). Small sulphur additions further reduce the possibility of minor surface crystallization for materials undergoing very long PGM cycle times.