Date of Award
Doctor of Philosophy (PhD)
School of Materials Science and Engineering
Hollandite is one of the promising crystalline ceramic materials to immobilize the radioactive cesium (Cs) elements in nuclear waste stream. Hollandite waste forms have superior chemical durability, energetic stability, and Cs loading comparing to the most widely used borosilicate glasses. In this dissertation, various properties and performance of different hollandites are investigated including structure, energetic stability, leaching-resistance, and electrical properties. More importantly, some underlying correlations among these scientific fundamentals are also discussed. As comprehensive understanding of this unique tunnel-structured material class is developed, general principles might be summarized and be further used to more effectively direct materials design for a broader range of applications (e.g., fast-ionic conductors and electrode materials in battery systems) instead of the sole nuclear waste disposal. The structure of this dissertation is developed as follows.In chapter 1, history and applications of hollandite-type materials, background of important methodologies and approaches, and scientific objectives are given. In chapter 2, general experimental procedures are described from sample synthesis, characterization, and analytical techniques. In chapter 3, the effect of Cs substitution on hollandite structure is studied and the results indicate that a monoclinic-to-tetragonal phase transformation could be induced by increasing Cs content in the tunnels. In chapter 4, the energetic stability of hollandite waste forms is evaluated by using high-temperature oxide melt solution calorimetry. In general, higher Cs-containing compositions exhibit higher stability which might be attributed to tetragonal symmetry, smaller RB/RA, larger tolerance factor and optical basicity. In chapter 5, it is found that Cs release in an aqueous environment is also significantly suppressed by increased Cs content, which is supported by elution studies of radioactive 137Cs. In chapter 6, the mechanisms of radiation damage and thermal recovery are investigated. Distinct radiation stability observed in different hollandites are resulted from the constituent binary oxide form of corresponding B-site dopants (e.g., Ga2O3). Specifically, the Ga-doped hollandite exhibited superior radiation-resistance comparing to Fe-and Zn-doped analogues. The mechanism of thermal recovery is complex and decoupled at different length-scales based on the combined thermal and structural analysis. In chapter 7, effects of crystal structure, A-site cations, A-site occupancy, and B-site cations on electrical properties of Na, K, and Cs hollandite systems are investigated. Generally, tetragonal symmetry, smaller tunnel cations, and intermediate A-site occupancy (e.g., ~75%) would enhance ionic conductivity. In contrast, severe electronic conductivity might be introduced if transition metals or multivalent elements are doped on B-sites. In chapter 8, the correlations among different properties and performance of hollandite-type materials are discussed and developed. In chapter 9, the most important highlights of numerous establishments achieved in this dissertation are summarized.
Zhao, Mingyang, "An Investigation of Structure, Thermochemistry, Electrochemistry, and Stability of Tunnel Structured Hollandite Materials" (2021). All Dissertations. 2805.