C12A7 a is a ceramic insulating material (bandgap of approx. 7 eV) with a basic structure composed of 12 nano-cages where 2 out of them contain oxygen ions (O2-), These ions are somewhat loosely bound and fairly mobile. They can be extracted easily and substituted by other types of negative ions or even electrons.
If pairs of electrons are used for replacing the oxygen ions, then the material is transformed from a ceramic isolator into a conductive electride with semiconducting behavior, or even metallic conduction depending on the % of ions that are substituted by electrons, and its work function is reduced to 2.4 eV.
Figure 1. Basic crystal structure of C12A7 (left) and current density emitted by several traditional thermionic materials as a function of temperature.
This allows the usage of C12A7:e- electride as a good thermionic electron emitter, especially because it has demonstrated operating temperatures for emission much lower than present state of the art thermionic materials used in cathode devices for EP. It enables a reduction in the operational temperature from the 1200-1500 ºC in case of LaB6 to 800 ºC or even lower in case of C12A7:e–.
This reduction not only means a reduction in the heating power required to start electron emission, but also decreases significantly the thermal load and the thermal stress imposed on the rest of thruster subsystems and the rest of the spacecraft, leading to design simplification and thus cost reduction.
Successful development of C12A7:e– neutralizers is then a highly disruptive technology, both regarding the materials properties and the anticipated device characteristics. Its main advantages, compared to current technologies, are:
- Lower operational temperatures
- Lower power consumption
- Lower dissipation losses
- Higher reliability;
- Longer lifetime (to be confirmed);
- Compatibility with more propellants, including iodine (alternative propellants).
Synthesis of C12A7 ceramics
Figure 2. Synthesis diagram of C12A7 ceramic (12CaO 7Al2O3)
Methods and processes for the synthesis of the basic ceramic material are crucial, since the successful transformation into the electride will only work with a starting material of very high purity.
Due to the complex binary phase diagram of (CaO) and (Al2O3) shown in Figure 2., obtaining pure C12A7 material without undesired secondary phases (e.g., CA, C3A) is challenging. However, it is a necessity as otherwise the material will not transform into the electride C12A7:e– which is the actual emitter material. The secondary phases will cause a progressive deterioration of the emitter material during thermionic emission in vacuum.
Fortunately, the other undesirable phases, mainly CA and C3A, have higher densities. Therefore, it is comparatively easy to determine the degree of purity even without having to refer to Raman and X-ray structural analysis. This is advantageous because the structural methods only probe small volumes and not an entire sample batch. However, the structural methods available within the consortium are important as a backup, in particular, for detecting microscopic causes of materials modifications during processing or device degradation. Their application is inherent for obtaining a full microscopic understanding of the processes underlying the thermionic emission in C12A7:e– based neutralizer technology. ATD has matured the synthesis methods and fabrication processes over the last three years and are now able to provide bulk C12A7 materials with purity greater than 98% and mass density of 2.67 +/- 0.01 g/cm3.
Transformation of C12A7 ceramics into C12A7:e- electrides
Methods and processes for transformation of the ceramic into the electride and the control of the “electron doping” density are other material-specific technological challenges. “Electron doping” is achieved by extracting O2− ions from the C12A7 structure via thermal-chemical reduction. Titanium is a good option, but there are other candidates that can work also with a good control, e.g., the “getters” used traditionally in ancient electronic vacuum tubes. The control of the substitution is paramount in order to obtain the desired degree of “electron doping”. In the case of C12A7:e– for EP applications, the maximum possible doping is desirable.
In the literature, many authors are attempting to obtain C12A7 ceramic material and transform it into C12A7:e– electride in one step. They fail because they do not realize that processes required in temperature, atmospheres and processes time are totally incompatible. ATD applies a three-step process for obtaining > 98% of purity of the basic ceramic material, and then a two-step process for transforming the ceramic into the desired electride with the maximum possible “doping”.
Figure 3. Samples of C12A7:e- electride bulk samples and films deposited on sapphire by ATD