Degree

Doctor of Philosophy (PhD)

Department

The Gordon A. and Mary Cain Department of Chemical Engineering

Document Type

Dissertation

Abstract

Rare earth (RE) based phosphor materials are widely used as luminescent probes in high-temperature optical thermometry where standard methods are unsuitable. The current generation of luminescent thermometers lacks stability and energy transport within the operating temperature ranges. The challenge is developing an oxide-based material with high luminescence intensity and thermal sensitivity/stability, suitable for an optical temperature sensor/thermal barrier coating. This work aims to develop a fundamental understanding of the local environment surrounding RE or actinide dopants in metal oxide hosts to engineer their luminescence property. To maximize the luminescence, the local symmetry, doping concentration, and radiative/non-radiative relaxation pathways must be optimized simultaneously.

A two-step synthesis method of co-precipitation and molten salt was employed to prepare complex RE-doped metal oxides. First, Tb doped Y2Zr2O7 (YZO) was applied to study the Ce sensitizing effect on the local structure and luminescence intensity. After co-doping with Ce ions as the sensitizer, the Tb3+ emission is quenched instead of the expected sensitization and was attributed to Ce-driven oxidization to a non-luminescent Tb4+ state. This was then extended to La2Hf2O7 (LHO) and Gd2Hf2O7 (GHO) host crystals to study the effect of structure on the dopant state. A U-driven phase transfer in LHO and GHO was observed to stabilize the local coordination environment surrounding the dopants, resulting in two distinct U clusters based on the available lattice site size. This response was also used to enhance the Er upconversion luminescence by co-doping the host with smaller cations (Sc3+). PL results and Judd-Ofelt calculation showed that YScO:Er with Sc concentration less than 50% of (Y+Sc) have improved efficiency and intensity and decreased site symmetry level, indicating that incorporating a small amount of Sc drives Er ions to occupy lower symmetry sites (C2) and reduces the overall site symmetry. Finally, these spatial and structural controls of RE dopants were applied in designing a qualifying material for the high-temperature application, such as optical thermometers. This work offers an idea about the relation between dopants and the structure-property change of different host materials, which can be applied to designing materials for various applications such as phosphors, solid-state lasers, and temperature sensors.

Date

6-24-2022

Committee Chair

Dorman, James A.

Available for download on Monday, June 23, 2025

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