Thermal Properties of Lunar Regolith Simulant Melting Specimen

THERMAL PROPERTIES OF LUNAR REGOLITH SIMULANT MELTING SPECIMEN. P.-M. Kost1, S. Linke2, B. Gundlach1, A. Lethuillier3, J. Baasch2, E. Stoll2, J. Blum3, 1Institute for Planetology, Universität Münster, Wilhelm-Klemm-Str. 10, D-48149 Münster, Germany, 2Chair of Space Technology, TU Berlin, Marchstr. 12-14, 10587 Berlin, Germany, 3Institute for Geophysics and Extraterrestrial Physics, TU Braunschweig, Mendelssohnstraße 3, 38106, Braunschweig, Germany. philipp-marius.ko[email protected] Introduction: The Moon provides the possibility to train and practice technologies like In-Situ Resource Utilization (ISRU) directly in our front yard. ISRU requires the use of local resources, which can be provided by the widely available and easy to access lunar regolith and applications can be extensively tested using regolith simulants. This research focuses on the thermal properties of two types of lunar regolith simulants, called TUBS-M and TUBS-T, originally developed by the Institute of Space Systems (IRAS) at the TU Braunschweig [1]. Experimental investigations were carried out to determine the specific heat capacity and thermal conductivity of these materials in their sintered state. Several specimen of TUBS-M and TUBS-T were sintered at different temperatures with the same holding time to investigate the influence of the sintering temperature on the thermal properties. Experimental Procedure: To determine the specific heat capacity, the sample was heated in an oven to a temperature of 80 ℃. Afterwards, it was placed in a beaker filled with water at room temperature, resulting in heat transfer from the sample to the water. During the whole process, the temperature of the water was measured and recorded. Using Richmann’s law, the specific heat capacity of TUBS-M and TUBS-T was calculated. Eight measurements were carried out for each sample. To obtain the thermal conductivity, the thermal diffusivity was determined first. To do this the cooling behavior of the samples was measured. Therefore, the samples were placed in a vacuum chamber and a laser placed outside the chamber was used to heat the top surface of the sample from a distance. Fig. 1 shows the experimental setup. The temperature profile of the sample surface was continuously recorded by an infrared camera system. The chamber pressure was reduced to 10−6 mbar and the sample was heated by the laser for 5 s. The temperature profile of the sample surface was recorded every 0.16 s until the sample returned to room temperature after the laser was switched off. This procedure was carried out ten times for each sample. As the temperature distribution around the center of the heating point was found to be nearly radially symmetric, only one radial temperature profile was determined by averaging eight equiangular temperature profiles of the samples top surface to compensate for the difference from a radially symmetric distribution. Fig 1: Experimental setup. This one-dimensional temperature distribution was used for the numerical model. The recorded cooling behavior of the sample was simulated by solving the heat equation for a given three-dimensional sample geometry using MATLAB. The emissivity was set to 𝜖 = 0.95 for all measurements and all samples. While the emissivity is temperature and composition dependent [2], we assumed identical values for all samples and materials. This assumption was justified by performing several analyses with emissivity as a free parameter, showing only little impact on the derived thermal conductivity due to the short heating and cooling phases. Density measurements on TUBS-M and TUBS-T were performed in previous work [3] for samples sintered at the investigated temperatures. The simulation was run with a preset range of values for the thermal diffusivity. By comparing the measured temperatures of the sample surface with those of the simulation using a 𝜒 2-test, the thermal diffusivity was determined as the value used to achieve the greatest match between the measured and simulated temperature curves. Results: The specific heat capacity of both sintered simulants showed an almost constant trend independent of the sintering temperature, as shown in Fig. 2 and Fig. 3. Values of 555 ± 65 J/(kg K) for TUBS-M and 540 ± 80 J/(kg K) for TUBS-T were determined. Thermal conductivity results in Fig. 4 and Fig. 5 showed an increase with increasing sintering temperature for both simulants. After reaching a maximum at high sintering temperatures, the thermal conductivity started to decrease. For TUBS-M, thermal conductivity starts at 0.3 ± 0.025 W/(m K) for a sintering temperature of 1100 ℃ and reaches a global maximum at 1350 ℃ with 1.1 ± 0.25 W/(m K). Values for TUBS-T reach from 0.1 ± 0.025 W/(m K) at 1300 ℃ up to a maximum of 0.65 ± 0.3 W/(m K) at 1500 ℃. Fig 2: Specific heat capacity of sintered TUBS-M as a function of sintering temperature. Fig 3: Specific heat capacity of sintered TUBS-T as a function of sintering temperature. Fig 4: Thermal conductivity and thermal inertia of sintered TUBS-M as a function of sintering temperature. Fig 5: Thermal conductivity and thermal inertia of sintered TUBS-T as a function of sintering temperature. Acknowledgement: Most of the work was part of the project X-Science (MOONRISE extended Science FK 50WM1951), which is funded by German Aerospace Center (DLR) and Federal Ministry for Economic Affairs and Energy, Germany. This work was also carried out in the framework of the CoPhyLab project funded by the D-A-CH program, Germany (GU 1620/3-1 and BL 298/26-1/SNF 200021E 177964/FWF I 3730-N36). References: [1] S. Linke et al., Planet. Space Sci. 180 (2020) 104747. [2] P.J. Timans, J. Appl. Phys. 74 (10) (1993) 6353–6364. [3] J. Baasch, Bachelorthesis, TU Braunschweig, Braunschweig, 2019.