Mechanistic Studies of Tellurite-Based Glasses for Enhancing Visible Transparency and Ionic Conductivity

Abstract

TeO2-based glasses have attracted growing attention due to their unique combination of properties (e.g., high refractive index, visible-to-infrared transparency window, high rare-earth solubility), which are promising for a wide range of applications. This thesis presents a mechanistic study on the relationship between glass-structure and properties of TeO2-based glasses with the aim of (i) enhancing visible transparency of TeO2-ZnO-Na2O (TZN) glass film and (ii) understanding ionic conductivity of TeO2-Na2O-NaX (TNN) glass system with X=Cl, Br, I. The first focus of this thesis is driven by the notorious issue that metallic-Te formed in TZN solgel films deteriorates the transparency. Previous research has revealed that metallic-Te is formed due to the chemical reduction of Te4+ to Te0 during the synthesis process. Applying O2 rich atmosphere during the thermal treatment (for converting organic sol to inorganic TZN glass) can suppress the formation of metallic-Te. However, the complete prevention of metallic-Te formation has not been achieved to date. In this thesis, the research on non-hydrolytic sol-gel (NHSG) process of making TZN transparent glass film elucidates the mechanism of metallic-Te formation, which reveals the use of O2 combined with a longer sol aging time (30~60 days) and an optimized sol heating temperature (150 oC) can completely prevent the formation of metallic-Te. The TZN glass film prepared via this optimized protocol has a transmission close to the theoretical limit. The achievement of high-transparent TeO2-based glass film paves the way to develop hybrid optoelectronic films by incorporating diverse nanoparticles with unique functionalities. The second part of this thesis focuses on ionic conductive glasses that rely on the migration of mobile alkali (sodium or lithium) ions. The conductivity of such glasses is determined by the concentration and mobility of mobile alkali ions. The research on traditional oxide glasses (SiO2- , B2O3, P2O5-based) has revealed that the incorporation of halides enhances the conductivity. This thesis investigates the impact of halides on the conductivity of the less-explored glass system, which is TNN glasses with varied (type and concentration) NaX. The conductivity depends on the mobility and concentration of the mobile ions. This research reveals the increase of free volume with increasing NaX concentration, which suggests an increase of space for ion migration thus mobile ion mobility. The Raman spectra confirm the expansion of glass network caused by the introduced NaX. The conductivities of NaCl and NaBr containing glasses remain constant while the mobile ion mobility increases with increasing NaCl or NaBr concentration, which infers a decrease of mobile Na+ ion concentration. The conductivity of NaI containing glasses significantly increases with increasing NaI concentration, which suggests an increase of mobile Na+ ion concentration in addition to the increase of the mobility. This research achieved the enhancement of conductivity while maintaining a good transparency of TeO2-based glasses. In addition, the mechanistic understanding of the relationship of halides, glass-structure and conductivity is a valuable guidance to the future research on ionic conductive glasses.