Photochemical Sciences Ph.D. Dissertations

Title

Defect Studies in Metals, Alloys, and Oxides by Positron Annihilation Spectroscopy and Related Techniques

Date of Award

2021

Document Type

Dissertation

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Photochemical Sciences

First Advisor

Farida Selim (Advisor)

Second Advisor

Abby Braden (Other)

Third Advisor

Marco Nardone (Committee Member)

Fourth Advisor

Alexander Tarnovsky (Committee Member)

Abstract

Positrons administer a unique non-destructive approach to probe materials with atomic-scale sensitivity and provide reliable information about the nature and size of defects. This study reflects on the powerful capabilities of positron annihilation spectroscopy (PAS) to characterize point defects at atomic-scale, which can be crucial in the development of relevant material for a wide range of applications like nuclear reactors, medical sciences, optoelectronic devices, nanotechnology, polymers etc. The work presented in this dissertation aims to gain a fundamental understanding of the defect structures in three unique material systems: Fe metal, Fe-Cr alloy, and Ce:YAG oxide by applying the methodology and concepts of PAS. A wide range of other complementary characterization techniques has also been employed to enhance the understanding.

The depth-resolved PAS was used to identify vacancy clusters in ion irradiated Fe and measure their density as a function of depth. PAS measurements uncovered the structure of vacancy clusters and the change in their size and density with irradiation dose. Combining with TEM measurements led to discovering a novel mechanism for the interaction of cascade damage with voids in ion-irradiated materials.

The effect of Cr alloying on the formation and evolution of atomic size clusters induced by ion irradiation in Fe-Cr alloys was also investigated using depth-resolved PAS measurements. Combining with atomic probe tomography (APT), a possible explanation for the long-standing question about the well-known resistance to radiation in Fe-Cr alloys was addressed. It was attributed to the stabilization of vacancy clusters around Cr atoms that act as indirect sinks for radiation-induced defects.

The final part of this work focuses on studying the role of defects on the luminescence properties of an important photonic material, Ce:YAG. The work reports an interesting mechanism that modifies and completely reverses the photoluminescence (PL) temperature-dependent kinetics. Further, it is shown that PL temperature-dependent kinetics can be controlled by modifying microstructure and engineering defects.

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