Flash Sintering for In-Situ Resource Utilisation of Lunar Regolith
Through the Artemis program, a multinational collaborative project, development has been proceeding on the Lunar Gateway space station that promises to function as a hub for communications, habitation, and research in orbit around the Moon. The Gateway will be an opportunity to run more frequent missions to the lunar surface, which leads naturally to questions of habitation of the Moon itself.
Requirements for this include oxygen, water, and food, as well as infrastructure. To maintain a stable and flexible supply of building materials, you can’t just load up a rocket with a few houses’ worth of bricks; the space and weight costs would be prohibitive and reliant on regular resupplies from Earth. The ability to use local resources and materials from the lunar surface is therefore a major priority, often referred to as in-situ resource utilisation (ISRU).
The lunar surface is composed of regolith produced by meteoroid impacts. For ISRU applications, regolith has been explored as both a construction material and for oxygen generation. For construction applications, regolith can be used as raw material to produce structurally sound bricks or pellets. The usual processing route for this is through pressing and sintering, but the huge energy requirements of conventional sintering methods are a glaring flaw that prevents them from being considered viable.
Any alternative processing or sintering method that could be introduced to process lunar regolith into viable construction material would make the goal of sustainable ISRU processes significantly more achievable. To this end, Lucideon has been developing its Flash Sintering process through a European Space Agency (ESA) funded programme.
What is Flash Sintering?
Flash Sintering, also known as Field Enhanced Sintering, is an advanced sintering technique that applies an electric field to a ceramic body. The customised electrodes are in direct contact with the green body, or unsintered ceramic, dissipating heat directly into it during the sintering process. It’s an attractive sintering method for its reduced furnace temperatures and processing time, and its ability to maintain microstructural control. Flash Sintering is also compatible with pressureless sintering, unlike methods such as hot-pressing or spark plasma sintering.
Figure 1 displays a YSZ (yttria stabilised zirconia) microstructural comparison of Flash Sintering to conventional sintering. In under 90 minutes and at a furnace temperature of 1050 C, Flash Sintering has been shown to potentially achieve the same microstructure and density as conventional sintering can achieve in 5 hours at 1500 C.
Figure 1 – Comparison of conventional sintering to Flash Sintering of a YSZ pellet
Application of Flash Sintering relies not just on the apparatus; thermal control and scalability are essential elements of program development and are key aspects of Lucideon’s work in commercialising this technology. Advancements have been made in electrode interfaces and real-time control software, resulting in over 20 different field variables that can be adjusted, ensuring that ceramics are produced repeatably and homogenously for a wide range of material types and geometries.
Figure 2: The Flash Sintering scale-up furnace
Regolith Simulant for ISRU
Given Lucideon’s capabilities, ESA funded a programme of work to demonstrate the feasibility of Flash Sintering for applications with regolith simulant, as well as full comparisons with conventionally sintered samples.
The project used the recently developed simulant EAC-1A. Given the newness of the material, existing sintering and processing information was limited. When Lucideon started producing pellets from the literature parameters of approximately 30 MPa, the pellets did not give satisfactory green state handling properties. Increasing the pressure to 275 MPa gave a compact pellet, 20 mm diameter x 5 mm height, shown in Figure 3.
Figure 3 – 20 mm diameter x 5 mm height EAC-1A pellet
Figure 4 – Stitched optical cross section of green body
Figure 4 demonstrates the variability in its microstructure. The stipulations imposed by the final application require a process that is simple, can be performed in vacuum, needs no terrestrial material additions, and minimises energy usage. Therefore, no binder or solvent additions were used, and milling was not completed.
To establish its electrical properties, impedance spectroscopy was completed across a temperature range, providing parameters for initial flash sintering trials. These initial trials showed that the material was prone to electrical flashover and/or localisation of current. A variety of different methods of applying the electric field were tried as well as electrode improvements, and the optimised field parameters led to the “pulse train” solution.
Eliminating localisation, where a majority of power is dissipated in a small area, is achievable by introducing a successive pulsed flash approach that incorporates an on and off time for successive pulses to allow thermal homogenisation of the dissipated power.
Figure 4, Figure 5, and Figure 6 compare the initial field parameters, with the optimised pulse train parameters, and the conventionally sintered part. Pulsed flash sintering for 5 mins at a furnace temperature of 850°C has reached comparable densification to conventionally sintered samples at 1080°C for 1 hr. From calculation, it is predicted that there is up to 50% energy saving and 35-40% speed reduction in comparison to conventional sintering.
Therefore, Flash Sintering demonstrated its feasibility for sintering lunar regolith for ISRU structural materials.
Next Steps
As a feasibility trial, the tests were completed in air. One of the key next steps is to demonstrate this in a similar atmosphere to the lunar environment. Since the current in air Flash Sintering system is limited to 500 V due to dielectric breakdown. A vacuum environment would allow a considerably larger voltage, allowing for larger samples, a quicker process, and further reduced temperatures.
Furthermore, increasing sample scale and geometry will allow mechanical tests to be performed, and refining powder processing methods and material homogeneity will likely improve the sintering outcomes of these samples.
The ESA project energy calculations showed that even with conservative estimates, flash sintering can lead to a significant decrease in energy consumption compared to conventional sintering, combined with reduced processing time and furnace temperatures. This reduced energy requirement may lead to not only efficient processes, but also a reduction in launch payload.
The project identified 3 promising avenues for future research: more thorough examination of the powder processing and homogenisation, scaling up of the flash sintered sample dimensions, and moving to a non-oxidising atmosphere for both conventional and flash sintering.
