C12A7 Electride Research Articles

Reproducibility of operating potentials for a C12A7 electride plasma-based cathode with constant keeper current control

Hollow Cathodes are fundamental components for electric propulsion systems. In recent publications, the performance of a planar C12A7 electride cathode has been presented with promising results. This includes the endurance operation of the cathode, the heaterless ignition cycling, and the general performance mapping over a wide range of parameters. The present publication will add to these publications, presenting results of only one defined set of discharge parameters, allowing a statistical evaluation of repeated discharge operations. Overall, exceptable repeatability of the performance could be evaluated, confirming the overall performance trends observed in previous campaigns. The cathode was operated in self-heating mode using krypton as propellant in a current range of 100 mA to 600 mA. A discharge potential of 30 V with a slight increase for lower discharge currents is reported. Furthermore, means to reduce the variation in the test results by increasing the keeper current have been identified.

Low-energy atomic and molecular hydrogen ion interaction with low-work function electride 12CaO · 7Al2O3

To efficiently generate H− ions from positive atomic or molecular hydrogen beams injected onto a solid surface, it has been suggested to use a material with a low-work function as the target material. However, it is not clear under what conditions the most efficient H− production is realized for incident beam parameters or reflection angles. Therefore, we studied the interaction between low-energy atomic and molecular hydrogen beams (less than 1 keV/nucleon) with a low-work function electride 12CaO⋅7Al2O3 (C12A7). The production ratio of H− to H+ ions from the C12A7 electride was much higher than Mo targets with higher work functions, especially at smaller incident and smaller reflection angles. The H− to H+ production ratio slightly increased as the incident energies were decreased, but there was no significant difference between the electride and Mo targets. These results indicate that smaller incident angles and lower beam energies of the incident hydrogen beam are favorable for the enhancement of the production ratio of H− to H+ ions in C12A7. The higher production ratio appeared at the vertical beam energies less than on the order of 100 eV, where quantum mechanical processes may become important.

Improvement of the thermionic emission properties of C12A7 electride

Calcium aluminate electride (named as C12A7) has become an active topic as a material for electron emission in the research field of electron sources, especially for electric propulsion, in recent years. Whereas the initially predicted, extremely low work function of 0.6 eV could not be confirmed experimentally at technically relevant surface current densities, the material remains an attractive emitter. Besides C12A7 emitting electrons at lower temperatures compared to LaB6, it appears to be resistant to poisoning and even iodine compatible as only known low work function material. On the other hand, the ceramic electride nature of the emitter introduces challenges related to the thermal, mechanical and electrical properties. Therefore, C12A7 has been investigated experimentally as a candidate material for thermionic emitters in space applications. The aim of this study was to enhance the thermionic emission by material modifications such as surface treatments, electrical contacting methods and doping of the C12A7. The performance improvement was quantified in a thermionic diode configuration, in which the emission current was determined as a function of temperature over multiple heating cycles to observe time dependent effects. The surface treatment such as grinding or polishing affects the emission properties of the C12A7 negatively. In contrary, the doping of the C12A7 lattice as well as the contacting of the back side (heater side) significantly improved the emission as well as the measured current density.

Concerning the ignition of a C12A7 electride plasma-based cathode

Hollow cathodes are crucial components for many electric propulsion systems used for space applications. However, they also represent a challenging and sometimes even limiting element for the propulsion systems. Particularly in recent times when alternative propellants are under evaluation for the thruster, and the design and the limits of the cathodes are being pushed. Consequently, significant improvement in the technology is desired. This publication tests a heaterless cathode using the emitter material C12A7 electride, particularly reviewing its ignition behavior and performance during an ignition cycling campaign. Stable heaterless ignition has been reliantly achieved in only a few milliseconds at an ignition potential of less than 400 V and 20 sccm Kr flow rate. Furthermore, two ignition cycling campaigns at different flow rates are presented, one reaching 3300 ignitions, the other igniting at only 6 sccm Kr flow rate. The degradation of the emitter materials and the influence on the ignition performance is discussed.

Characterization of a C12A7 electride plasma-based cathode using different keeper orifice sizes

For the operation of electric propulsion systems in space, efficient electron sources are crucial components. Hollow cathodes have been established for many applications since they allow sufficient current ranges at reasonable power requirements and have been proven to operate for several thousands of hours. New approaches are being evaluated to improve these cathodes’ general performance. This publication presents an extended characteristic of a heaterless plasma-based cathode using the emitter material C12A7 electride. The focus is on the relationship between the discharge potential and total discharge power over the discharge current. Furthermore, a characteristic of the discharge performance at lower mass flow rates is presented and discussed. The discharge potential is generally quite constant for a wide range of discharge currents, typical in the range of 30 V and only increases steeply for low discharge current ranges. Successful heaterless ignition and stable operation have been achieved down to 2 sccm krypton flow rate.

Endurance operation of a heaterless C12A7 electride plasma cathode and post operation material analysis

A heaterless plasma cathode using disc shaped C12A7 electride as emitter material is being developed for electric propulsion systems in space. C12A7 electride is a calcium-aluminate ceramic (12CaO · 7Al2O3) with a low work function of about 2.4eV. The material has been optimized for improved thermionic emission and discharge stability. Due to the low work function of the emitter material, the cathode can be easily ignited from room temperature, eliminating the need of a complex and delicate heater. The cathode has been operated continuously for more than 950 h with the results being presented. After this endurance operation, a detailed material analysis (phase composition and microstructure) has been conducted and is presented.

Characterization of Novel ALD Process for the Synthesis of CaO for 12CaO·7Al2O3 electride Applications

In this work we report on a novel CaO ALD process development with a little used solid ALD precursor for Calcium. This forms part of a larger effort to synthesize 12CaO·7Al2O3 [C12A7], which is a new electronic oxide compound with very low work function and applications potentials in displays and Field-Emission (FE) Cathodes. Currently there are no existing thin film technologies for the conformal deposition of C12A7 electride films on complex device surfaces with nanoscale precision. Use of ALD to obtain precise nanoscale interleaving of CaO and Al2O3 layers over Micro-Hollow Cathode Discharge devices MHCD or FE devices allows conversion to correct alloying of C12A7 in the desired nano-layers. Success in reproducibly obtaining these layers will significantly benefit applications using cathodes, such as light sources, nano-sat thrusters, vacuum electronics and small-format microwave amplifiers. For the final C12A7 film, the challenge is to develop two ALD processes for two binary films in this case CaO and Al2O3 which must be synthesized as alternating nanolaminates in the correct stoichiometric ratio and are subsequently reacted under high temperature annealing. Since ALD of Al2O3 is already a well-known established ALD process, we had to focus on developing an ALD process for CaO. For this study we decided to utilize the relatively novel solid ALD precursor Bis(N,N-di-iso-propylformamidinato)calcium(II) dimer [Ca(famd)2], which is a low vapor pressure solid Ca amidinate precursor and which has to be reacted with ozone or DI H2O vapors. The chemical structure of the Calcium precursor is shown schematically in Fig. 1. The Al2O3 binary compound will be synthesized with trimethylaluminum (TMA) and DI water vapor. The low vapor pressure of Ca amidinate precursor Ca(famd)2 in solid form constitutes a major technical challenge and cannot be reacted by ALD in vapor draw mode but necessitated as new hardware a 150mL booster bubbler with dip tube for bubbling the molten precursor and the installation of a Low Vapor Pressure Delivery (LVPD) kit with bypass. For this presentation we will report the physical characterization of the ALD CaO film with FE-SEM surface morphology studies, SEM EDS elemental analysis of the final composition, X-ray diffraction XRD analysis and atomic force microscopy AFM. Fig. 1: Schematic of chemical structure of the novel solid ALD Ca amidinate precursor Bis(N,N-di-iso-propylformamidinato)calcium(II) dimer [Ca(famd)2] Figure 1

Experimental investigation of electric propulsion systems using C12A7 electride hollow cathodes

The development and experimental investigation of two low-power electric propulsion concepts using compact heaterless C12A7 electride (C12A7:e-) hollow cathodes is presented. The first concept represents an electrothermal thruster, in which a cathode discharge is used to heat a gas that is subsequently accelerated in a nozzle-shaped anode. The second propulsion system is an attempt to develop a sub-500 W magnetoplasmadynamic thruster (MPDT) that uses a rectangular discharge channel that allows to increase the applied magnetic field and thus lower the necessary discharge current. Extensive parameter studies with both concepts were conducted, and the thrust and discharge properties of different geometric and operational configurations were determined. This work is a follow-up publication of a previous paper (Gondol and Tajmar in CEAS Space J 14:65–77, 2021).

Probe measurement of an ECR hydrogen plasma facing the C12A7 electride surface

Plasma parameters of an ECR discharge excited in a compact ion source with a replaceable plasma electrode (PE) were measured with a Langmuir probe. The PE was installed to investigate how an electride PE changes the plasma parameters from those with a Mo PE. The effect upon the plasma parameters in front of the PE due to the material will be elucidated by changing the probe position axially along the beam extraction direction. Analyses of the measured probe characteristics revealed that electride PE has a higher electron density (ne) and lower electron temperature (Te) than the Mo PE. When the PE was biased negatively, the ratio of the positive saturation current (Ii ) to the negative saturation current (Ie +Ih- ) for the electride PE was about 1.6 times of the ratio for the Mo PE.

Comment on Weber et al. Mayenite-Based Electride C12A7e−: A Reactivity and Stability Study. Catalysts 2021, 11, 334

In 2012, we reported that C12A7 electride (C12A7: e−) significantly promotes the catalytic activity of Ru nanoparticles for ammonia synthesis through the electron donation from the C12A7: e− with a low work function (2.4 eV) to Ru [...]

Ship-in-a-Bottle Synthesis of High Concentration of N2 Molecules in a Cage-Structured Electride.

We report the formation of neutral nitrogen molecules in the cages of [Ca12Al14O32]2+ (C12A7) framework compensated by extra-framework anions. NH3 treatment of C12A7 electride (C12A7:e-) at 800 °C leads to the formation of N2 and NH2- species in the C12A7 cages. N2 and NHx species in the cages are identified using the Raman spectroscopy of 14NH3 and 15NH3-treated C12A7:e-. The concentration of H and N in the C12A7 cages after NH3 treatment is ∼1021 cm-3. We propose a two-step mechanism, supported by density functional theory (DFT) modeling, of N2 incorporation into the C12A7 cages, i.e., incorporation of NH2- formed from decomposition of NH3 at C12A7:e- surface followed by the NH2- species reacting to form N2 molecules. Encapsulation of neutral molecules, as opposed to negatively charged species reported in C12A7 previously, offers new opportunities for trapping and storing gaseous substances in nanoporous materials.

Work function performance of a C12A7 electride surface exposed to low pressure low temperature hydrogen plasmas

The calcium aluminate electride [Ca24Al28O64]4+(e−)4 (C12A7:e−) is chemically and thermally stable in ambient atmosphere, offers strong electron donating capabilities and exhibits an intrinsic work function of 2.4 eV. Thus, it is an attractive material for electron emitter and low work function applications. For an implementation in negative hydrogen ion sources, where the electride could serve as a converter surface for the H− production, compatibility with moderate vacuum conditions and exposure to low pressure low temperature hydrogen plasmas must be ensured. Hence, the stability and work function performance of a polycrystalline C12A7:e− sample are investigated under ion source relevant vacuum (∼10−6 mbar) and plasma conditions (electron temperature ∼2 eV, electron density ∼1016m−3). The work function is measured via the photoelectric effect, showing that long-term hydrogen plasma exposure results in a steady-state surface work function of 2.75±0.10 eV. Since the electride is very sensitive to surface contaminations, an increase of the surface work function occurs as soon as the plasma is switched off. Vacuum heat treatment of the degraded C12A7:e− surface after plasma activation leads to a work function minimum of 2.9 eV for temperatures in the range of 670–770 K. Exceeding a temperature of ∼770 K leads to an increase of the C12A7:e− surface work function. The sample is stable during long-term hydrogen plasma exposure (up to 10 h total plasma-on time tested) and no plasma-induced erosion is observed.

Mayenite Electrides and Their Doped Forms for Oxygen Reduction Reaction in Solid Oxide Fuel Cells

The oxygen reduction reaction is an important reaction at the cathode in solid oxide fuel cells. Materials that exhibit high chemical and mechanical stability, high ionic and electronic conductivity, and are non-toxic are of great interest as cathodes for the reduction of oxygen. Here, we use density functional theory simulations to examine the efficacy of 12CaO·7Al2O3 and 12SrO·7Al2O3 electrides and their doped forms for the conversion of O2 gas to form O2− in their nanocages via encapsulation. Calculations show that encapsulation is exoergic in the un-doped electrides, and the formation of O2− is confirmed by the charge analysis. A stronger encapsulation is noted for C12A7 electride than the S12A7 electride. The C12A7 electride doped with B or Ga also exhibits exoergic encapsulation, but its encapsulation energy is slightly lower than that calculated for the un-doped C12A7 electride. There is an enhancement in the encapsulation for the S12A7 electride doped with B compared to its un-doped form. Doping of Ga in S12A7 electride exhibits only a very small change in the encapsulation with respect to its un-doped form. The present results can be of interest in the design of cathode material for solid oxide fuel cells.

Nature of Reactive Hydrogen for Ammonia Synthesis over a Ru/C12A7 Electride Catalyst

Recently, there have been renewed interests in exploring new catalysts for ammonia synthesis under mild conditions. Electride-based catalysts are among the emerging ones. Ruthenium particles supported on an electride composed of a mixture of calcium and aluminum oxides (C12A7) have attracted great attention for ammonia synthesis due to their facile ability in activating N2 under ambient pressure. However, the exact nature of the reactive hydrogen species and the role of electride support still remain elusive for this catalytic system. In this work, we report for the first time that the surface-adsorbed hydrogen, rather than the hydride encaged in the C12A7 electride, plays a major role in ammonia synthesis over the Ru/C12A7 electride catalyst with the aid of in situ neutron scattering techniques. Combining in situ neutron diffraction, inelastic neutron spectroscopy, density functional theory (DFT) calculation, and temperature-programmed reactions, the results provide direct evidence for not only the presence of encaged hydrides during ammonia synthesis but also the strong thermal and chemical stability of the hydride species in the Ru/C12A7 electride. Steady state isotopic transient kinetic analysis (SSITKA) of ammonia synthesis showed that the coverage of reactive intermediates increased significantly when the Ru particles were promoted by the electride form (coverage up to 84%) of the C12A7 support rather than the oxide form (coverage up to 15%). Such a drastic change in the intermediate coverage on the Ru surface is attributed to the positive role of electride support where the H2 poisoning effect is absent during ammonia synthesis over Ru. The finding of this work has significant implications for understanding catalysis by electride-based materials for ammonia synthesis and hydrogenation reactions in general.

A compact electron cyclotron resonance negative hydrogen ion source for evaluation of plasma electrode materials.

A compact ion source that produces hydrogen plasma with an electron cyclotron resonance (ECR) configuration combined with a 2-stage extraction system with a single aperture of 6 mm diameter has been designed and built to study the performance of different materials as the plasma electrode (PE) of a negative hydrogen ion source. The source has the capability to electrically bias the PE with respect to the ECR plasma. The first experiment with low ECR power input (less than 40 W) was carried out. The PE of the C12A7 electride showed the largest H- current among aluminum, molybdenum, and the C12A7 electride.

Negative Muonium Ion Production With a C12A7 Electride Film

Negative muonium atom (μ+e–e–, Mu–) has unique features stimulating potential interesting for several scientific fields. Since its discovery in late 1980’s in vacuum, it has been discussed that the production efficiency would be improved using a low-work function material. C12A7 was a well-known insulator as a constituent of alumina cement, but was recently confirmed to exhibit electric conductivity by electron doping. The C12A7 electride has relatively low-work function (2.9 eV). In this paper, the negative muonium production measurement with several materials including a C12A7 electride film will be presented.

Investigation on electrical conductivity and surface distribution of work function of C12A7 electride

In this work, a polycrystalline C12A7 electride target was prepared by one-step sintering method using a graphite crucible. X-rays diffraction confirmed the presence of the C12A7 phase. In the meanwhile, a carbon phase was founded in the target and this situation was never reported before. It can be considered that there is a little carbon left in the C12A7 crystal. The electrical and emission properties of carbon doped C12A7 electride were explored. In terms of the electrical characteristics, the electrical resistance measured by a four-pin resistance meter at 300 K is relatively large, which is similar to semiconductors, indicating a low-level electron concentration of about 1.0 × 1018 cm−3. For the emission performance, a Scanning Kelvin Probe was used to measure the surface distribution of work function. The result shows that the surface work function is relatively stable with a floating about ± 0.03 eV of the average value 3.7 eV. The work function is also smaller, although it is higher than other pure C12A7 electrides, making it has a great prospect in optoelectronics devices.

Structure and Electronic Properties of [Ca24Al28O64]4+·4e– Surfaces: Opportunities for Termination-Controlled Electron Transfer

We investigate the structure, thermodynamic stability, and electronic properties of (001), (110), and (111) surfaces of mayenite (12CaO·7Al2O3 or C12A7) in its oxygen-deficient phase known as mayenite-derived electride Ca12Al14O32 using simulations based on the density functional theory. Seven types of surface terminations were identified, depending on the orientation of the lattice nanosize cages with respect to the surface plane. We find that the surface termination has a strong effect on the distribution of electron anions in the near-surface region, as demonstrated by the range of the ionic charges and binding energies of adsorbed Au atoms and dimers. Notably, in all cases, Au species acquire a negative charge and preferentially adsorb as isolated ions rather than aggregate into clusters. These results show that C12A7 electride can provide a simple, yet versatile substrate to achieve high adsorption capacity and steric selectivity for single atom catalysts.

Rapid synthesis of inorganic [Ca24Al28O64]4+(e−)4 electride and its performance as an electron thermal emitter

We propose an effective and rapid route for synthesizing polycrystalline mayenite electride (C12A7:e−) in dense bulk. C12A7:e− bulks with high-density electron doping were obtained via a comprehensive method, which combined Ti powder treatment with spark plasma sintering (SPS) process under a 10−2Pa vacuum. The resulting electride, which exhibits an electron concentration of ∼2.3 × 1021cm−3, an electrical conductivity of ∼1.38 × 103 S/cm at 300 K, and an absorption peak at 2.5eV, was obtained via SPS process at 1150 °C for 20 min. Furthermore, thermionic emission from a flat surface of C12A7 electride was examined at temperatures of 973–1373 K and acceleration electrical field of 3000–35000 V/cm in a 10−5 vacuum. A large current density of 1.68 A/cm2 was obtained in the thermionic emission with an work function (φWF) of 2.13eV for an applied electric field of 35000 V/cm. Moreover, the emission with a current density of 1.68 A/cm2 was stably sustained for 30 h. These results not only suggest an efficient method for fabricating high-quality mayenite electrides but also pave a way for various applications including thermionic emission.

Negative-hydrogen-ion production from a nanoporous 12CaO ∙ 7Al2O3 electride surface

A high production rate of negative hydrogen ions (H−) was observed from a nanoporous 12CaO ∙ 7Al2O3 (C12A7) electride surface immersed in hydrogen/deuterium low-pressure plasmas. The target was negatively biased at 20–130 V, and the target surface was bombarded by H3+ ions from the plasma. The production rate was compared with that from a clean molybdenum surface. Using the pseudo-exponential work-function dependence of the H− production rate, the total H− yield from the C12A7 electride surface bombarded at 80 V was evaluated to be 25% of that from a cesiated molybdenum surface with the lowest work-function. The measured H− energy spectrum indicates that the major production mechanism is desorption by sputtering. This material has potential to be used as a production surface of cesium-free negative ion sources for accelerators, heating beams in nuclear fusion, and surface modification for industrial applications.