Surface dynamics of high performance scandate cathodes
Thermionic cathodes are key components in a wide range of vacuum electron devices (VEDs), including traveling wave tubes, microwave devices, thermionic energy converters, and more. A high performance subset of these, scandate cathodes, are fabricated by impregnating porous Sc2O3-doped W matrices with xBaO-yCaO-zAl2O3. Experimental results show that W grains in low work function scandate cathodes are terminated with (001), (110), (112) facets. Previous work has calculated surface energies and effective work functions of a large number of configurations of adsorbed Ba, Sc, and O on W (001), (110), and (112) facets. Currently, I am using ab initio molecular dynamics to investigate conditions that instigate barium desorption from the aforementioned surface configurations.
Mechanical Behavior of Random Structures
Nanoporous (NP) materials are emerging as promising catalysts, photocatalysts and light-weight structural materials, chiefly due to their high surface-to-volume ratios and low reduced densities. A robust and predictive understanding of these material’s mechanical properties is needed before they can be widely adopted in technological devices and systems. The intrinsic complexity and randomness of NP structures has long represented a stumbling block to accurate and comprehensive modeling and simulation of NP materials behavior. Here, we have created a stochastic modeling approach for NP materials that allows for systematic study of mechanical properties as a function of quantifiable structural and geometric factors, including network coordination state, connecting ligament aspect ratios, and overall material reduced density. The modeling approach includes both a computational toolkit for building representative unit volumes of random NP structures, and a methodology for computing distributions of mechanical properties from sets of unit volumes. We are even exploring uses for this model beyond NP materials like fiber nanocomposites and membranes.
LSCF Microstructural Evolution
Lanthanum-strontium-cobalt-ferrite (LSCF) is a widely studied cathode material of solid oxide fuel cell (SOFC) for its high ionic and electronic conductivity. The main advantage of LSCF over alternative cathode materials, such as lanthanum-strontium-manganese oxide (LSM), is its superior oxygen ion conductivity at intermediate temperatures (about 800°C), which results in higher electrochemical performance. Despite its inherent advantage, LSCF also suffers from performance degradation during its operation. Two key mechanisms of degradation are the formation of strontium oxides on the surface of the cathode, which impedes oxygen reduction, and coarsening of the microstructure that reduces total chemically active surface area. Currently, I am focusing on the latter: microstructural changes of the LSCF cathode during coarsening.
While the importance of microstructural changes during coarsening on the performance of LSCF cathodes is well documented, our understanding of the evolution of the LSCF microstructure is still limited. In general, microstructures with intricate geometries often undergo noticeable changes in the length scale, morphology and topology during coarsening. To better understand the microstructural evolution of LSCF cathode at extended operation times. Here, we simulate coarsening of experimentally obtained LSCF microstructure using the phase-field method. To our best knowledge, this is the first coarsening simulation performed on the experimentally obtained LSFC microstructure.