La 0.8 Sr 0.2 )MnO 3 -Yttria Stabilized Zirconia (LSM-YSZ) cathodes fired at various temperatures were studied using Focused Ion Beam -Scanning Electron Microscopy (FIB-SEM) three-dimensional (3D) tomography and Electrochemical Impedance Spectroscopy (EIS). The total cathode polarization resistance, measured at 800 • C in air, showed a minimum versus firing temperature, T f , at 1175 • C. The EIS showed two dominant responses that were fit well using a two (R-CPE) element equivalent circuit. The higher frequency (10 4 -10 5 Hz) response, attributed to YSZ grain boundary resistance within the LSM-YSZ composite, decreased with increasing T f and was explained by grain size increases estimated from the 3D structural data. The main EIS response, attributed to the oxygen reduction process, decreased in characteristic frequency from 500 to 1 Hz as T f increased, while its magnitude was minimized at 1175 • C. An electrochemical model quantitatively predicted the resistance minimum based primarily on a maximum in the density of electrochemically-active three-phase boundaries (TPBs), measured using 3D tomography. The active TPB density maximum resulted from two factors: substantial particle coarsening and densification at high T f that yielded a low TPB density, and low LSM-particle percolation at low T f that yielded a low fraction of active TPBs.

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Colloidal aperiodic phases (i.e., entropy stabilized degenerate crystals, DCs) are realized via self-assembly of hollow fluorescent silica dimers under wedge-cell confinement. The dimer building blocks approximate two tangent spheres and their arrangements are studied via laser scanning confocal microscopy. In the DCs, the individual lobes tile a lattice and five distinct DC arrangements with square, triangular or rectangular layer symmetry are determined as a function of confinement height. Moreover, Monte Carlo simulations are used to construct the phase diagram for DCs up to two layer confinements and to analyze structural order in detail. Just as for spheres, the DC structural transitions under confinement are attributed to the ability or frustration to accommodate an integral number of particle layers between hard walls. Unlike spheres, dimers can also experience transitions involving changes in orientation. DCs are among the unconventional structures (e.g., semi-regular tilings, quasicrystals, plastic crystals) expected to enhance the properties of photonic solids.

(La0.8Sr0.2)MnO3-Y-stabilized Zirconia (LSM-YSZ) cathodes fired at various temperatures were studied using 3D Focused Ion Beam - Scanning Electron Microscopy (FIB-SEM) tomography and Impedance Spectroscopy (IS). The lowest resistance cathode, fired at 1175°C, had nearly the highest active three-phase boundary density. This optimum was where the firing temperature was sufficient to yield good LSM-particle percolation but not so high as to cause substantial particle coarsening and densification. Increasing the firing temperature above 1175°C caused a shift in the main impedance response from ~100-1000 Hz to 1-10 Hz; this low-frequency response was primarily an electrochemical process, with gas diffusion becoming significant only for low oxygen pressure. Focusing on the low frequency peak for cathodes fired at 1325°C, A-site-deficient LSM yielded lower resistance than stoichiometric LSM. Finally, cathodes were annealed at 1000-1100°C in order to accelerate structural evolution; changes in the impedance response were similar to those observed with increasing firing temperature.

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