Publication Date

5-2015

Date of Final Oral Examination (Defense)

2-24-2015

Type of Culminating Activity

Dissertation

Degree Title

Doctor of Philosophy in Materials Science and Engineering

Department

Materials Science and Engineering

Major Advisor

Darryl Butt, Ph.D.

Advisor

Amy J. Moll, Ph.D.

Advisor

Dmitri Tenne, Ph.D.

Advisor

Kenneth McClellan, Ph.D.

Abstract

This dissertation presents the effects of atmosphere, time, and energy input (thermal and mechanical) on the kinetics of gas-solid and solid-solid reactions in systems of dysprosium, uranium, oxygen, and nitrogen. Accordingly, Chapter Two presents a novel synthesis technique used to form DyN via a gas-solid reaction between dysprosium and nitrogen. DyN was rapidly formed through a mechanochemical process in a high energy ball mill at ambient temperatures. The progress of the reaction was quantified using in situ temperature and pressure measurements coupled with microscopy and x-ray diffraction. It was found that the rate of the nitridation reaction is controlled by the creation of fresh dysprosium surfaces, which is a function of milling intensity and the number of milling media.

Chapter Three describes how UN was synthesized prior to mixing with UO2 in the fabrication of accident tolerant nitride fuels for nuclear reactor applications. High purity and carbon free UN was synthesized using a hydride-dehydride-nitride thermal synthesis route. In order to fabricate an accident tolerant nitride fuel, it is postulated that the addition of small amounts of UO2 (up to 10 w%) to UN will significantly increase its corrosion resistance. UN-UO2 composite pellets were sintered in Ar-(0-1 at%) N2 to study the effects of nitrogen concentration on the evolved phases and microstructures. Further studies were conducted where UN and UN-UO2 composites were sintered in Ar-100 ppm N2 for five hours at various temperatures (1700-2000 °C) and the final grain morphologies and phase concentrations were compared. Electron micrographs showed that the oxide inclusions remained homogenously dispersed throughout the microstructure and were located on UN grain boundaries. Increasing UO2 additions resulted in microstructural coarsening, as did the sintering temperature. Higher sintering temperatures also resulted in lower sintered densities, presumably due to a preferential formation of oxygen-stabilized nitrides or uranium oxynitrides.

The high temperature oxidation kinetics of dysprosium is presented in Chapter Four. Dysprosium particles were isothermally oxidized from 500 - 1000 °C in N2 - (2, 20, and 50%) O2 and Ar - 20% O2 using simultaneous thermal analysis techniques. Two distinct oxidation regions were identified at each isothermal temperature in each oxidizing atmosphere. Initially, the oxidation kinetics are very fast until the reaction enters a slower, intermediate region of oxidation. The two regions are defined and the kinetics of each are assessed to show an apparent activation energy of 8 - 25 kJ/mol in the initial region and 80 - 95 kJ/mol in the intermediate oxidation reaction region. The effects of varying the oxygen partial pressure on the reaction rate constant is used to show that dysprosium oxide (Dy2O3) generally acts as a p-type semiconductor in both regions of oxidation (with an exception above 750 °C in the intermediate region).

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