Efficient electron sources are crucial for any space-based mission, especially when using electric thrusters. In many respects, hollow cathodes are a baseline technology due to their power-efficient electron emission in the desired current range and the potentially long lifetime of these emitters. However, the delicate design of the heater, with the associated constraints on its operation, and the high degradation of state-of-the-art materials to new propellant options under evaluation for electric space propulsion systems, are severe limitations of current systems. To address some of the most pressing challenges with cathodes, a heaterless plasma-based cathode using the emitter material C12A7 electride has been developed and is described in this thesis. The cathode has been developed with the requirements of an electrodynamic tether demonstration mission in mind. C12A7 electride is an electrically conductive ceramic that has recently attracted much attention as a potential electron emitter in hollow cathodes. However, there appear to be significant challenges with the material itself, requiring careful design evaluation and thorough testing to gain a sufficient understanding of the material's behavior. Most importantly, material degradation in the harsh environment of a plasma. Throughout the thesis, an optimized electride material was developed and tested, yielding a ceramic-metal composite with greatly improved plasma performance compared to pure C12A7 electride material. In addition, a special design of a plasma-based cathode was developed and described, which respects the unique properties of the material and allows convenient operation, and thus characterization and optimization of the cathode. Several milestones have been achieved, including endurance operation for nearly \num{1000} hours, successful operation with a Hall-effect thruster, characterization of the cathode in the discharge current range of \qtyrange{0.2}{2}{\A}, reduction of the flow rate required for ignition and operation down to \qty{2}{\sccm}, and heaterless ignition cycling for up to \num{3300} cycles with a single insert. The observed performance of the cathode was eventually compared with performance data reported in the literature using state-of-the-art materials and showed reasonable comparability. In particular, advantages over state-of-the-art cathodes were identified in terms of ignition behavior: Requiring only \qty{2}{\sccm} of krypton and a potential of less than \qty{400}{\V}, and reaching steady-state operation in less than a few tens of milliseconds, the performance was better than reported in the literature. Combined with the acceptable discharge performance, these results motivate the further development of such an electride cathode for space applications. Due to the simplicity of such a cathode, applications for a wide range of industrial processes may also be considered.:1 - Introduction 2 - Cathode Theory 3 - C12A7 Electride 4 - Scope of Development 5 - Design Development 6 - Thruster Operation 7 - Endurance Operation 8 - Electride Cathode for Low Current EDT Operation 9 - Additional Tests with the Electride Cathode 10 - Discussion of Results and Further Steps 11 - Conclusion Bibliography Appendix

The first-principles density functional theory (DFT) was implemented to investigate the structural, electronic and magnetic properties of vanadium (V) substituted and chalcogen (Se) vacancies in tungsten diselenide (WSe 2 ) monolayer, novel two dimensional (2D) monolayer (ML) structures in binary compounds ZnX (X= As, Sb, and Bi), and novel 2D electrides on transition metal-rich mono-oxide or chalcogenides, based on Perdew-Burke-Ernzerhof (PBE) exchange functional employed in Vienna Ab-Initio Simulation Packages (VASP). The inherent defect in 2D transition metal dichalcogenides (TMDCs) contains unavoidable substitutional defects and a certain amount of chalcogen vacancies. This type of defect affects the electronic and magnetic properties of 2D-TMDCs. To account for this fact, we demonstrated using DFT that the V-doped WSe 2 monolayer exhibits long-range ferromagnetic order. Further, the chalcogenide (Se) vacancies clustered around V-atom enhance the ferromagnetic properties of the system consistent with experimental findings. This dissertation explores the important role of Se-vacancies in the magnetic properties of the V-doped WSe 2 monolayer and proposes a method to enhance the magnetic properties of such 2D non-magnetic van der Waal (vdW) materials. In the second study, we have attempted theoretically to engineer the monolayer structure in II-V binary compounds ZnX with orthorhombic symmetry. We proved the dynamical stability of the bulk and ML structures manifested by the absence of imaginary frequencies in phonon dispersion curves. Our calculations on the density of states (DOS), and band structures using GGA indicate the increasing value of bandgap as well as the transition from indirect to direct bandgap while going from bulk to monolayer structure of ZnX. Our theoretical calculations will represent an archetype of novel 2D semiconductors on ZnX. Next, we have tailored using DFT, the structural and electronic properties of the 2D electrides that belong to transition metal-rich mono-oxide and chalcogenides with hexagonal (Hf 2 X; X = O, S, Se, Te), and orthorhombic (Ti2S and Zr2S) symmetry thereby introducing novel electrides to the electride family. The Bader charge analysis, electron localization function (ELF), projected DOS, and the calculated value of low work functions provides sufficient theoretical shreds of evidence to prove these materials as electrides.

Functional materials, particularly metal oxides, have been the focus of much attention in solid state chemistry for many years and impact every aspect of modern life. The approach adopted in this thesis to access desirable functionality for enhanced fundamental understanding is via modifying existing materials by deploying reducing synthetic procedures. This work spans several groups of inorganic crystalline materials, but is unified by the development of new properties within host compounds of particular relevance to solid oxide fuel cell technology, which allow interstitial oxide ion conduction at elevated temperatures. The Ca₁₂Al₁₄O₃₂e₂ electride was successfully synthesized by replacing the mobile extra-framework oxygen ions with electrons acting as anions. The high concentration of electrons in the C12A7 electride gives rise to an exceptionally high electronic conductivity of up to 245 S cm⁻¹ at room temperature. Making use of the high density of electrons in Ca₁₂Al₁₄O₃₂e₂ electride, the strong N-N bonds in N₂ was found to be broken when heating Ca₁₂Al₁₄O₃₂e₂ in a N₂ atmosphere. A reaction between silicate apatites and the titanium metal yielded another completely new electride material La₉.₀Sr₁.₀(SiO₄)₆O₂.₄e₀.₂ which was found to be a semiconductor. To fully understand the role of oxygen interstitials in silicate apatites, high-resolution transmission electron microscopy (HRTEM) was employed as the main technique in probing how the oxygen nonstoichiometry is accommodated at the atomic level. Atomic-resolution imaging of interstitial oxygen in La₉.₀Sr₁.₀(SiO₄)₆O₂.₅ proved to be a success in this thesis. Substitution of oxygen in 2a and interstitial sites with fluoride ions in La[subscript(8+y)]Sr[subscript(2- z)](SiO₄)₆O[subscript(2+(3y-2z)/2)] (0<y<2, 0<z<2) could be an approach to enriching the functionalities in the apatite structure. A wide range of fluoride substitution levels was tolerated in La[subscript(10-x)]Sr[subscript(x)](SiO₄)₆O[subscript(3-1.5x)]F[subscript(2x)] (x= 0.67, 1, 1.5, 2) and AC impedance measurements were found in support of a tentative conclusion that fluoride ions could be mobile in fluorinated apatites. The last part of this thesis was focused on a new class of fast oxide ion conductors based on Ge₅P₆O₂₅ whose performance was superior to both La₉.₀Sr₁.₀(SiO₄)₆O₂.₅ and Ca₁₂Al₁₄O₃₃ in the low temperature range.