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Abstract.Segregation of impurities to grain boundaries plays an important role in both the stability and macroscopic behavior of polycrystalline materials. The research objective in this work is to better characterize the energetics and length scales involved with the process of solute and impurity segregation to grain boundaries. Molecular statics simulations are used to calculate the segregation energies for carbon within multiple substitutional and interstitial grain boundary sites over a database of 125 symmetric tilt grain boundaries in Fe. The simulation results show that there are two energetically favorable grain boundary segregation processes: (1) an octahedral C atom in the lattice segregating to an interstitial grain boundary site and (2) an octahedral C atom and a vacancy in the lattice segregating to a grain boundary substitutional site. In both cases, lower segregation energies than appear in the bulk lattice were calculated. Moreover, based on segregation energies approaching bulk values, the length scale of interaction is larger for interstitial C than for substitutional C in the grain boundary (≈ 5Å compared to ≈ 3Å from center of the grain boundary). A subsequent data reduction and statistical representation of this dataset provides critical information such as about the mean segregation energy and the associated energy distributions for carbon atoms as a function of distance from the grain boundary, which quantitatively informs higher scale models with energetics and length scales necessary for capturing the segregation behavior of alloying elements and impurities in Fe. The significance of this research is the development of a methodology capable of ascertaining segregation energies over a wide range of grain boundary character (typical of that observed in polycrystalline materials), which herein has been applied to carbon segregation to substitutional and interstitial sites in a specific class of grain boundaries in α-Fe.

Solar thermochemical energy storage has enormous potential for enabling cost-effective concentrated solar power (CSP). A thermochemical storage system based on a SrO/SrCO3 carbonation cycle offers the ability to store and release high temperature (≈1200 °C) heat. The energy density of SrCO3/SrO systems supported by zirconia-based sintering inhibitors was investigated for 15 cycles of exothermic carbonation at 1150 °C followed by decomposition at 1235 °C. A sample with 40 wt % of SrO supported by yttria-stabilized zirconia (YSZ) shows good energy storage stability at 1450 MJ m(-3) over fifteen cycles at the same cycling temperatures. After further testing over 45 cycles, a decrease in energy storage capacity to 1260 MJ m(-3) is observed during the final cycle. The decrease is due to slowing carbonation kinetics, and the original value of energy density may be obtained by lengthening the carbonation steps.

The use of an intermediate reactive material composed of cerium (IV) oxide (ceria) is explored for solar fuel production through a CO 2-splitting thermochemical redox cycle. To this end, powder and porous ceria samples are tested with thermogravimetric analysis (TGA) to ascertain their maximum fuel production potential from the 22 CeO CeO    cycle. A maximum value of the non-stoichiometric reduction factor δ of ceria powder was 0.0383 at 1450°C. The reactive stability of a synthesized porous ceria sample is then observed with carbon dioxide splitting at 1100°C and thermal reduction at 1450°C. Approximately 86.4% of initial fuel production is retained after 2000 cycles, and the mean value of δ is found to be 0.0197. Scanning electron microscopy (SEM) imaging suggests that the porous ceria structure is retained over 2000 cycles despite apparent loss of some surface area. Energy dispersive x-ray spectroscopy (EDS) line scans show that oxidation of porous ceria becomes increasingly homogenous throughout the bulk material over an increasing number of cycles. Significant retention of reactivity and porous structure demonstrates the potential of porous ceria for use in a commercial thermochemical reactor.