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Novel Electride Material for Enhanced electrical propulSIon Solutions

Periodic Reporting for period 2 - NEMESIS (Novel Electride Material for Enhanced electrical propulSIon Solutions)

Reporting period: 2020-10-01 to 2023-03-31

Electric propulsion (EP) is an increasingly adopted technology for spacecraft propulsion in station keeping, orbit raising, or primary propulsion applications. A common need for most of the EP thruster technologies is to operate a device for electron emission either for plasma generation or for charge neutralization of impellent positive ions. Permanent research on new materials and architectures is mandatory to optimize efficiency of such devices in the extreme operation conditions and limited resources in space.

Improving the performance of on board EP devices from NEMESIS project will improve competitiveness and strength of the European space industry, with associated employment and scientific level improvements. On the other side, it will accelerate the availability of simpler and cheaper platforms for small satellites, and even to pave the way for enabling brand new exploration, scientific and commercial missions not yet available due to actual constraints of traditional thermionic emitters technology.

Other potential impacts for society will be driven by the faster and wider deployment of satellites and constellations, that will bring many different applications like environmental surveillance, or safety and security applications based on Earth Observation. Tele-medicine and similar tele-assistance applications for isolated population will also be enabled by large deployment of telecom satellites.

The overall objective of the NEMESIS project is to demonstrate performance improvements of EP devices based on a novel thermionic emitter material, namely C12A7:e- electride, vs present state of the art with traditional emitters such as LaB6 or BaO. For this purpose, NEMESIS will be developing electride-based cathode technology, which is compatible with all kinds of electric propulsion (EP) systems requiring neutralization or electron emission.

Chemical inertness, low work function, much lower operational temperatures, and lifetime/duration are some of the characteristics of the C12A7:e- electride material that will drive the performance improvements. During project execution, these anticipated advantages of the new thermionic electride material have been verified, and a set of cathode devices with different architectures using this new material as electron emitter have been developped and tested, reaching very good performance.

Summarizing, a new multi-propellant (Ar, Kr, Xe, I, NH3) cathode technology based on C12A7:e- material has been matured up to TRL 4 for different architectures and designs.
Continuous improvements on processes and techniques for C12A7 ceramic synthesis and its transformation into C12A7:e- electride have been investigated during this first period of NEMESIS project. A synthesis and transformation method leading to high electron concentration electride has been achieved, with resulting sample resistances in the low milli ohm range.

Many findings of C12A7:e- behaviour and interaction have been shared amongst the Consortium members for consideration to the design of C12A7:e- devices and test set-up equipment. Three technical notes on different topics have been produced, periodically updated, and shared with the team members. Issues have also been detected in extreme conditions of use, and actions have been anticipated to guarantee the durability and reliability of C12A7:e--based systems.

The C12A7:e- compatibility study with alternative propellants is completed. Initial tests show none or only little degradation in presence of alternative propellant candidates.

A hollow cathode has been designed at JLU for LaB6 and C12A7:e- operation with alternative propellants. C12A7:e- characterization tests for 5A and 1A class cathode tests have been completed, and studies of C12A7:e- and LaB6 comparison regarding work function, emission, and 5 A-class cathode performance were also completed.

The iPott test-chamber adaption to the requirements of the HET was made according to the provided HET characteristics. Subsequently, the HET thruster was tested and characterized with iodine in the adapted test chamber.

IP rights protected for 7 key inventions, grouped into 2 patents for operative convenience, and a wide know how generated through project teams members and dissemination actions:
24 presentations at 9 international Conferences / Workshops
5 peer reviewed publications (see and 3 more under preparation:
C12A7:e- workshop has been organized by JLU Giessen (04-05.10.2022) with the participation and presentations from academic and industrial entities

Three follow on projects awarded and 2 industrial collaboration agreements signed, so that R&D will continue on new configurations of C12A7:e- based cathodes beyond the NEMESIS project
C12A7:e- properties are superior to those of conventional ceramics currently employed in EP neutralizer technology. Apart from less required thermal energy, C12A7:e- lower work function enables devices operating at working temperatures half of the typical operating temperatures of BaO and LaB6 based neutralizers. Additionally, lower heating power is essential to decrease the system power-to-thrust ratio.

Within the NEMESIS project, LaB6 and C12A7:e- comparison has shown an order of magnitude higher emission current density at 900 ºC for a 8 mm diameter disc sample: 0,01 mA vs 0,2 mA.

Thermionic emission from novel electride material C12A7:e- deposited thin films has been documented by the very first time worldwide. C12A7:e- deposited thin film (250 nm) on graphite has shown thermionic emission, thus opening the door to many possible additional applications.

Material synthesis processes have been finetuned, enabling production in Europe of high quality electride from locally abundant unexpensive precursors. Unlike other termionic materials like LaB6 or BaO, both the C12A7:e- electride and its precursors are non toxic. It has also been demonstrated that the material has a long term stability, remaining fully operational after more than two years with just a simple plastic bag storage.

One of the most complex issues to solve was the emission barrier effect and the charge accumulation caused by the thin dielectric layer present at material surface, which is randomly released as sparks, causing instabilities and material damage. Charge coupling technique using a pulsed operation mode was verified as the solution to such sparks and instabilities, which additionally provides twice as much anode current, and a slightly better anode to cathode current ratio. Patent for pulsed operation mode for C12A7:e- based cathodes has been granted to ATD (ES-2897523), and its worldwide extension is in progress

Several engineering model prototypes developed, successfully tested with Xe, Ar, Kr, I, and NH3, coupled with HET thrusters, and reaching high performance figures of merit, reaching performance ratios of up to 10 mA/W, and losses at keeper lower than 5% (Ianode/Icathode > 95%). Some of them operated with powers below 1 W in heater less configurations reaching operating temperatures < 200 ºC and providing anode extracted currents of some tenths of mA.

In only 3 years and with less than 1 M€ budget, a new multi-propellant cathode technology has been matured up to TRL 4 for different designs and propellants in the NEMESIS project and is now ready to take off to higher TRLs to replace of old LaB6 technology and serve the European EP space industry preserving its non-dependance.
ATD test setup
FOTEC test setup
ATD test setup