Date of Award
Doctor of Philosophy
Materials Science and Engineering
Claudia J. Rawn
Brett G. Compton, David J. Keffer, David G. Mandrus, Kurt E. Sickafus
Ca12Al14O33, also known as mayenite or C12A7, is a functional material with many applications; most uniquely is the ability to form a conductive oxide. The performance of the material is altered with the addition of dopants, both cationic and anionic, as well as with the control of sample microstructure. Obtaining further performance increases requires a robust, rapid, and low temperature synthesis method for doped C12A7, an understanding of the kinetics of formation to control microstructure, and development of direct electronic material synthesis method.Conventional synthesis of ceramic materials follows the solid-state synthesis method which homogenizes a heterogenous mixture of reactants with high temperatures and long process times. Sol-gel synthesis methods utilizing atomically homogenous reactants lead to a decrease in process time and temperature yielding a controllable microstructure. A polymer assisted sol-gel method is developed which overcomes chemical constraints of the amorphous citrate sol-gel method and allows for robust, rapid, and low-temperature synthesis for large cationic systems.Elucidation of the kinetics behind12A7 synthesis and formation traditionally adopts a thermodynamic approach that characterizes the end-point and provides little understanding of the journey to that thermodynamic equilibrium. In-situ X-ray diffraction is employed to study the kinetics of formation as a function of reactant homogeneity and atmosphere to elucidate the kinetic pathways in real time. The polymer-assisted reactants lead to a 30% decrease in formation temperature, order of magnitude reduction in formation time, and two orders of magnitude reduction in grain size.The current process to form electrically conducting C12A7 requires the synthesis, consolidation, and then reduction of oxy-C12A7. This consists of multiple long duration high temperature processing steps prohibiting microstructural control. Herein a direct electride synthesis technique is developed and a theory behind the kinetics of formation is presented. Consolidated semi-conducting C12A7 is directly synthesized in under 12 h with room for process refinement to further reduce this time.The refinement of a robust homogenous reactant synthesis method, elucidation of the kinetic pathways of formation, and the development of a direct electride synthesis method allows for the formation of doped C12A7 electronic materials with a controlled microstructure leading to an increase in performance of a functional material with nearly endless applications.
Salasin, John Robert, "Formation Studies of Ca12Al14O33. " PhD diss., University of Tennessee, 2018.