Date of Award

12-2018

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

School of Materials Science and Engineering

Committee Member

Kyle Brinkman, Committee Chair

Committee Member

Ranjendra Bordia

Committee Member

Fei Peng

Committee Member

Lindsay Shuller-Nickles

Abstract

The ceramic phase hollandite is a component material for some multi-phase ceramic waste forms and has been a prominent ceramic waste form for the immobilization of cesium and strontium radionuclides. The immobilization of cesium is difficult due to its large size and water solubility. This dissertation is focused on understanding the fundamental structure of hollandite and the effect cesium doping has on the properties of hollandite in order to develop a more effective and efficient waste form for cesium immobilization. The topics cover the structure and stability, thermodynamic properties and phase formation, irradiation resistance, and leaching resistance of hollandite. A brief history, the current status, and advantages and shortcomings of hollandite as a waste form are given in the introduction and background chapter. In chapter 2, the effect of elemental doping on the hollandite structure, symmetry, and stability are described, and methods to improve these properties are presented. Different B-site dopants were tested to probe the monoclinic/tetragonal symmetry boundary of hollandite. Additionally, cesium doping into barium-zinc-titanium hollandite was performed to develop a better understanding of how divalent cations affect the stability of hollandite and how cesium doping and occupancy can be controlled to increase the stability of hollandite. In chapter 3, the thermal properties were studied and calorimetry was performed. The melting, thermal stability, and cesium loss behaviors were measured and found to be dependent on the cesium content. Calorimetry data was collected for a series of cesium doped hollandite. The drop solution enthalpy was measured and the formation enthalpy was calculated. Zinc hollandite formation was more favorable and energetic stability increased as the cesium content increased. In chapter 4, the radiation stability of hollandite was measured over a full range of cesium doping. The onset of amorphization did not vary with cesium content; however, the dose required for full amorphization doubled with full cesium substitution. Amorphization was also measured at an elevated temperature, and the critical amorphization temperature of hollandite was measured between 200 °C and 300 °C. A defect mechanism was proposed, and the amorphization model for hollandite was determined. In chapter 5, elemental leaching dependence on cesium content was studied. The amorphization mechanism and effect of irradiation on leaching rate were discussed. The leaching rate decreased with increasing cesium content. Leaching experiments were also performed on irradiation samples. Cesium leaching was found to increase after irradiation with the high cesium content intermediate compositions exhibited the lowest cesium losses. In summary, the stability of zinc hollandite was measured and found to be primarily dependent on cesium content with some occupancy effect. Cesium doping increased the stability of the tetragonal symmetry, reduced cesium loss during processing and leaching, stabilized hollandite formation, increased radiation tolerance, and reduced elemental leaching. Thus, high cesium content hollandite has improved the waste form properties.

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