Thermodynamics and Reaction Kinetics During Mechanochemical Synthesis and Environmental Testing of Lanthanide and Actinide Refractory Materials

High energy ball milling is a broad family of well-known techniques for materials processing that includes mechanochemical synthesis, in which the milled materials react during milling. The latter may involve components reacting in the solid, liquid, or gas phases, and the operative kinetics have hardly been studied. Several manufacturers of high energy ball mills have recently made available instrumented milling vessel lids. These lids are able to monitor the temperature of the lid and the gas pressure within the vessel in situ during milling experiments. This ability is a significant improvement over the ad hoc instrumentation setups sometimes discussed in literature, and enables a level of precision in investigating milling processes not available previously. This dissertation describes work done investigating the mechanochemical synthesis and compound formation in the Ce-Si, Ce-S, U-Si, and Dy-N systems, including the thermodynamic and kinetics processes that govern mechanochemical synthesis of these materials. Particular attention is devoted to two distinct mechanochemical phenomena. The first process is a fast reaction that occurs after a characteristic milling time, termed mechanically-induced self-propagating reaction (MSR). This behavior is sometimes observed during milling of the elemental constituents of compounds with a high enthalpy of formation. The second process is compound formation during milling of elemental metals in reactive gasses such as oxygen, nitrogen, or hydrogen. Mechanochemical synthesis experiments have been conducted while monitoring the temperature and pressure of the milling vessel in situ to gain insight into the reaction kinetics. A new approach to analyzing this data in conjunction with simple physical models of the milling mechanics has been developed. The effects of compound formation on the system geometry and energy transfer were assessed by experiment. The kinetics model explicitly considers milling energy and energy transfer to form chemically active surface site generation, and should be applicable to many systems involving gas reactions with ductile solids. Furthermore, the model allows for the incorporation of thermally activated kinetics if appropriate experimental data is available. The results discussed in this dissertation are drawn from four research papers submitted or accepted by peer-reviewed journals. Several unique phenomena are observed due to a recently developed capability for in situ monitoring of temperature and pressure during milling. As such, this dissertation represents several significant steps forward in the understanding of the processes occurring during mechanochemical synthesis