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Real-time measurement of dielectric properties of lunar regolith simulants

This study utilizes a dual-mode measurement technique to characterize the temperature-dependent complex permittivity of lunar regolith simulants (JSC-1A and NU-LHT-4M) up to their melting points, providing critical data to optimize microwave heating processes for future lunar construction and in-situ resource utilization.

Original authors: C. O’Connell, B. Garcia-Baños, J. M. Catalá-Civera, J. D. Gutiérrez-Cano, A. Lucca Fabris, S. Lim

Published 2026-10-26

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CC BY 4.0

Original authors: C. O’Connell, B. Garcia-Baños, J. M. Catalá-Civera, J. D. Gutiérrez-Cano, A. Lucca Fabris, S. Lim

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Building a permanent home on the Moon requires more than just landing a spacecraft; it demands the construction of durable infrastructure to shield astronauts and equipment from the harsh lunar environment. The surface is covered in a fine, abrasive dust called regolith, which poses a constant threat to machinery and human health, while the lack of atmosphere leaves everything exposed to radiation and extreme temperature swings. To solve this, scientists are looking to use the lunar soil itself as a building material. By heating the regolith until it melts or fuses together, engineers can create solid landing pads, roadways, and protective shelters without needing to transport heavy construction supplies from Earth. The most promising method for this involves using microwaves, the same type of energy used in kitchen appliances, to heat the material directly from the inside out. This approach is far more efficient than traditional ovens, which struggle to heat the poor-conducting lunar dust evenly. However, to design the machines that will perform this task, engineers need to know exactly how the soil reacts to microwaves as it gets hotter, a piece of information that has been difficult to pin down until now.

In a recent study, researchers set out to measure these changing properties in real time. They focused on two types of lunar soil simulants, which are Earth-made materials designed to closely mimic the chemical and mineral makeup of actual moon dust. One simulant represents the dark, volcanic plains known as maria, while the other represents the lighter, ancient highlands. Instead of heating these samples in a conventional furnace and then moving them to a measuring device—a process that introduces errors because the sample cools down during the transfer—the team built a special system that heats the material with microwaves while measuring its properties simultaneously. This setup allowed them to watch how the soil's ability to absorb and store microwave energy changed continuously as it warmed from room temperature all the way to the point where it began to melt.

The results revealed that the lunar soil behaves very differently depending on how hot it gets. At lower temperatures, the material absorbs microwave energy quite weakly. However, as the temperature rises, the soil becomes increasingly efficient at capturing that energy. For the highland simulant, this change happened in a dramatic, sudden burst. At a specific critical temperature, the material's ability to absorb microwaves jumped sharply, causing the heating rate to accelerate rapidly on its own. This suggests that once the process gets going, the material essentially helps heat itself, a phenomenon that could be both a powerful tool for construction and a challenge to control. The volcanic simulant showed a more gradual but still significant increase in absorption, tripling in its ability to store energy and increasing its heat-generating loss by over a hundred times as it approached melting.

The researchers also discovered that the history of the material matters. When they let the melted samples cool back down, the solidified blocks retained a higher ability to absorb microwaves than the original loose powder. This indicates that the act of melting and fusing the soil permanently alters its structure, making the resulting building blocks even more responsive to microwave energy. Furthermore, the two types of simulants behaved differently from one another, likely due to their distinct mineral compositions. The volcanic simulant contained more iron and titanium, which contributed to its higher density and stronger interaction with the microwaves compared to the highland material.

These findings provide a crucial foundation for designing the future of lunar construction. Because the soil's ability to absorb energy changes so drastically with temperature, engineers cannot rely on simple, static models to design their heating systems. Instead, they will need to build machines that can adapt in real time, adjusting their power and frequency to match the changing behavior of the soil as it transforms from dust to solid rock. By understanding these dynamic shifts, scientists can optimize the energy efficiency of these processes, ensuring that the first permanent structures on the Moon are built safely and effectively using the resources already available on the surface.

Technical Summary: Real-time Measurement of Dielectric Properties of Lunar Regolith Simulants

Problem Statement
Establishing a permanent lunar settlement requires infrastructure capable of withstanding the harsh lunar environment. Lunar regolith, a dielectric material, offers a promising feedstock for In Situ Resource Utilisation (ISRU) construction via microwave heating, which provides volumetric heating and energy efficiency compared to conventional furnace methods. However, accurate numerical modelling of microwave heating processes is hindered by a lack of reliable, temperature-dependent dielectric property data. Previous studies often assumed constant permittivity or relied on measurements taken after transferring samples from conventional furnaces to microwave cavities. These indirect methods introduce uncertainties due to sample cooling during transfer and fail to capture the specific dielectric interactions occurring under direct microwave heating, where electromagnetic field-material interactions and heating rates differ significantly from conventional thermal processing.

Methodology
The study employed a dual-mode microwave cavity system to enable the simultaneous heating of lunar regolith simulants and the in-situ measurement of their complex permittivity.

Simulants: Two high-fidelity simulants were tested: JSC-1A (low-Ti mare simulant) and NU-LHT-4M (highland simulant).
Experimental Setup: A single-mode microwave system operating at approximately 2.45 GHz heated the samples, while a separate measurement source at ~2.3 GHz characterized the dielectric properties. A cross-coupling filter isolated the two systems.
Procedure: Approximately 2.2 g of powdered samples were loaded into quartz tubes and heated under a nitrogen atmosphere from room temperature to melting, followed by cooling. Temperature was monitored via an external pyrometer, with bulk temperature correlations validated using Differential Thermal Analysis (DTA) and Raman spectroscopy.
Measurement Technique: The Cavity Perturbation Method (CPM) was used to derive complex permittivity ( $\varepsilon = \varepsilon' - j\varepsilon''$ ) from shifts in resonant frequency and quality factor. The system maintained a heating rate of approximately 0.25 °C s⁻¹, with automatic adjustments to frequency and power to ensure stable coupling.

Key Contributions
This work presents the first real-time measurements of the real ( $\varepsilon'$ ) and imaginary ( $\varepsilon''$ ) components of complex permittivity for lunar regolith simulants during direct microwave heating from ambient conditions to melting. Unlike previous studies that utilized indirect heating methods, this approach eliminates uncertainties associated with sample transfer and cooling, providing a continuous dataset that captures the dynamic evolution of dielectric properties as thermal and phase transformations occur.

Results

Temperature Dependence: For both simulants, $\varepsilon'$ and $\varepsilon''$ increased significantly with temperature. The imaginary part ( $\varepsilon''$ ), representing dielectric loss, increased by more than 100-fold for JSC-1A and approximately 300-fold for NU-LHT-4M.
Material Differences: JSC-1A exhibited higher permittivity values than NU-LHT-4M across the temperature range, attributed to its higher bulk density and greater abundance of Fe- and Ti-bearing phases (e.g., ilmenite).
Heating Dynamics:
- JSC-1A: Showed a gradual increase in permittivity up to ~470 °C, followed by an accelerated rise. A sharp increase in $\varepsilon''$ indicated the onset of melting. During the microwave heating process, $\varepsilon'$ more than tripled, while $\varepsilon''$ increased by more than 100-fold.
- NU-LHT-4M: Displayed a distinct "critical temperature" behavior between 750 °C and 800 °C. At this threshold, $\varepsilon''$ increased by an order of magnitude, accompanied by a rapid temperature rise (from ~0.25 °C s⁻¹ to >5 °C s⁻¹) and a temporary decrease in absorbed power before a second rise. This suggests a change in dielectric loss mechanisms, potentially linked to glass phase transitions or volatile release. Overall, $\varepsilon'$ increased by a factor of 3, while $\varepsilon''$ increased by a factor of 300.
Cooling Behavior: Upon cooling, the solidified samples (sintered shells with central voids) exhibited higher permittivity values than the initial powders. Samples that reached higher permittivity during heating generally retained higher values upon cooling, suggesting irreversible microstructural changes such as densification and phase modification.
Consistency: Measurements were highly consistent at lower temperatures but showed increased variability above the glass transition temperature ( $T_g$ ), likely due to local compositional variations affecting electric field concentration.

Significance and Claims
The authors state that these findings provide essential input data for improving the accuracy of coupled numerical models of microwave heating in lunar regolith. The study demonstrates that dielectric properties are not only temperature-dependent but also influenced by the heating process itself (microwave vs. furnace) and the resulting thermal/microstructural state of the material.

Specifically, the paper claims:

Model Accuracy: Models based solely on room-temperature properties or constant permittivity assumptions will fail to accurately predict energy deposition, temperature evolution, and the onset of thermal runaway in lunar processing.
Process Optimization: The observed increase in microwave absorption at high temperatures suggests that once processing conditions are established, the material becomes a more efficient absorber, potentially aiding in maintaining elevated temperatures for sintering and melting.
Control Strategies: The dynamic feedback between temperature, permittivity, and energy absorption necessitates control strategies such as dynamic frequency tuning, impedance matching, or power modulation to maintain stability, particularly as materials approach sintering and melting points.
ISRU Implications: Quantifying the differences between mare and highland simulants is critical for optimizing processing conditions for different lunar feedstocks. The data supports the development of more efficient microwave heating devices for lunar ISRU, enabling better energy efficiency and process control for future construction activities.

Technical Summary: Real-time Measurement of Dielectric Properties of Lunar Regolith Simulants

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