Physical Science and Engineering (PSE) Division

Ammonia (NH3) is a promising carbon-free fuel and hydrogen energy carrier for decarbonizing power generation because it can be produced from renewable energy, has relatively high energy density, and is safer to store, especially when compared to hydrogen. Nonetheless, there are limitations to using NH3 as a fuel, such as limited reactivity, low flame speed, restricted flammability limits, and a tendency to produce significant levels of nitrogen oxides (NOx). This body of work examines the combustion of NH3-based fuel mixes to power a micro gas turbine and evaluates exhaust emissions and flame stability results. First, the stability and exhaust emissions of a commercial micro gas turbine fueled with NH3-based fuel blends were investigated. By substituting methane (CH4) with NH3 up to 63% in volume, stable functioning was obtained. This limit was increased to 75% replacement by adding hydrogen (H2) to a fuel combination of 90% NH3 - 10% H2. However, hardware modifications will be required to meet current NOx limits and ensure suitably low N2O emissions. The excessively high nitrous oxide (N2O) content in the exhaust gases was caused by the far lean equivalence ratios near the lean blowout. Next, an optically-accessible reduced-scale burner inspired by the micro gas turbine combustor was employed as an experimental platform. Experiments at the relevant elevated pressure demonstrated that stable far lean NH3-CH4-air combustion was attainable due to the improved flame stability afforded by the pilot flame. Although far lean NH3-based flames exhibit nitric oxide (NO) emissions that are much reduced compared to that found for lean equivalence ratios typically associated with lean premixed combustors, the NO concentration is still too high to satisfy current regulations. In contrast, N2O emissions are negligible for lean equivalence ratios, except for far lean equivalence ratios where N2O reaches unacceptably high values, as observed during the experiments in the micro gas turbine. The impact of the pilot flame characteristics on exhaust emissions and flame morphology was also investigated. It was found that the pilot power and pilot fuel composition must be tuned for different NH3-based fuel blends to ensure satisfactory flame stability and low emissions. For example, the fuel composition of the pilot flame has a complicated effect on exhaust emissions. This is because it changes the flame shape and the OH concentration in the inner recirculation zone, which has a positive correlation with NO concentration. Finally, a strategy to reduce NOx emissions, called two-stage rich-lean combustion, was investigated using the reduced-scale burner as an experimental platform. This strategy satisfactorily reduces the NO emissions by an order of magnitude. Since the reduced-scale burner is a smaller-scale replica of the actual AE-T100 mGT burner, further improvement might be expected at the elevated pressure and temperature conditions while running the mGT. On the other hand, high secondary airflow rates induce flame instabilities at low frequencies, resulting in flame blowout. This phenomenon is reduced by increasing the number of holes. However, it still occurs at high enough secondary airflow rates, implying that not only the high velocity of the jets in the secondary combustion zone causes the onset of unstable combustion but also cooling/heat losses.

Crystalline semiconductors with intrinsically low lattice thermal conductivity and high power factor are crucial for high-performance thermoelectrics. While it is common that materials with complex crystal structures, large unit cells, and heavy atoms exhibit low lattice thermal conductivity, materials with relatively light atoms can also possess low lattice thermal conductivity due to low group velocity, large scattering phase space, and high lattice anharmonicity. Investigation of the microscopic mechanisms behind this abnormal behavior is not only of fundamental interest, but it also helps to unravel the complex interplay between crystal structure, chemical bonding, and lattice dynamics. The results of such investigations provide new criteria to discover hitherto unknown materials with low lattice thermal conductivity and pave the way to engineering the heat transport in thermoelectric materials. The goal of this thesis is to understand the mechanisms behind the abnormal behavior of the lattice thermal conductivity in computationally predicted chalcogenides using density functional theory and semi-classical Boltzmann transport theory. We show that the weak chemical bonding in Ba4Sb2Se generates numerous low-frequency optical phonons with low group velocity. Consequently, the phonon scattering rate is increased by increasing both the number and the strength of the three-phonon scattering processes. As a result, the lattice thermal conductivity is increased by 24% from Ba4Sb2Se to Ba4Sb2Te despite the increase of the atomic mass from Se to Te. Incorporation of the four-phonon scattering processes decreases the lattice thermal conductivity of Ba4Sb2Se, for example, by 31%. Comparing two members of the AMM′Q3 (A/M/M′ = alkali metal, alkaline earth metal, post-transition metal, and lanthanide; Q = chalcogen) family of materials, KLiZrSe3 and KLiHfSe3, we demonstrate an increase of the lattice thermal conductivity by 14% from KLiZrSe3 to KLiHfSe3 in the a-direction, despite the fact that Zr is lighter than Hf. The abnormal behavior is attributed to weak chemical bonding in KLiZrSe3 which causes high lattice anharmonicity and strong phonon scattering. The figure of merit of KLiHfSe3 is found to be higher than that of KLiZrSe3 due to a higher power factor that overcompensates the higher lattice thermal conductivity. We show that a delicate interplay between structural, bonding, and band structure features determines the overall thermoelectric efficiency.

One of the challenges in NH3 synthesis catalysis research is the nature of the support effect on Ru nanoparticles and the reaction mechanism. In this work, we chose isostructural apatite-type Sr5(BO3)3X (X=F−, H−, OH−, mixed H−/OH−) as catalyst supports to study their activity in NH3 synthesis catalysis. Kinetic analysis of activation energy and reaction orders indicates a strong dependence of the activity in catalysis on the anionic composition. DFT calculations supported hypotheses regarding hydrogen spill-over from Ru nanoparticles to the supports and electron donation of the support. Partial hydroxides Sr5 (BO3 )3 (OH)x H1-x with a higher content of OH− or a higher content of H− were compared for their activity in catalysis. The x-value in the composition was estimated by Rietveld refinement of powder X-ray diffraction data and 1H solid-state NMR. This composition is interesting because the coex- istence of proton and hydride is rare and was reported only once in BaTiO3-xHx by Masuda et al. [1]. The apatite structure type family is large, and to the best of our knowledge, this work is the first to test these materials as supports for NH3 synthesis cata- lysts. We believe that this work will provide ideas on the design of apatites for NH3 synthesis catalysis for other researchers.

A brief account of photonics research activities in the selected countries in the Middle East and Africa is presented in this article. Though not comprehensive, we hope to provide a glimpse of the research landscape in the region, and the collaboration and connection with each other and the international partners.

With the increasing demand for more efficient and reliable electronics, a precise and damage-free etching technique is ever more crucial. Thermal atomic layer etching (ALE) might be the technique that pave the way for future advancements in magnetic device fabrications. The motivation behind this work is to avoid the complexities associated with using fluorinated compounds such as hexafluoroacetylacetone (hfacH). Instead, the focus is to utilize the more cost-effective acetylacetone (acacH). However, implementing acacH in thermal ALE results in higher degradation rates and lower efficiency compared to hfacH. Therefore, this study investigates the degradation disparity through first principles calculations on NiO during thermal ALE of Ni. The findings contribute to the advancement of thermal ALE as a promising atomic-scale device fabrication method. The research reveals that water interaction has minimal impact, while the breaking of C-C bonds is the primary degradation pathway. A unique peak in the IR spectrum could be used to confirm the C-C bond breaking mechanism. The disparity between acacH and hfacH degradation is attributed to energy barrier differences.