Experimental Characterization of Thermal Wadis in Support of Lunar Exploration

Abstract

This paper addresses the problem of science and exploration systems to survive the lunar night. We have proposed that a thermal mass can be manufactured from lunar regolith by electrical resistance heating. The thermal mass can then store heat during the lunar day and release it slowly throughout the night, providing protection to science and exploration systems from the detrimental effects of cold. When engineering a thermal mass, accurate characterization of how the mass conducts and emits heat is vital. Two regolith stimulants, JSC-1AF and NU-LHT-2M, and Columbia River Basalt BCR-2 were used to study the effect of experimental conditions (T, t, and atmosphere) on sintering and densification and to identify the optimal conditions for manufacturing the thermal mass material. The sintering and densification of small samples was performed in the electrical furnaces at different final temperatures, different cooling rates, under air or vacuum, or in an argon atmosphere. To determine the impact of sample microstructure (size, shape, and concentration of open and closed pores) and crystallinity on the range of thermal diffusivities for sintered/densified materials and to obtain emissivities of densified materials as a function of temperature, the manufactured samples were analyzed with high magnification, X-ray diffraction (XRD), a laser-flash thermal diffusivity (LFTD) system, and the millimeter-wave (MMW) system. The sintered/densified samples were analyzed for thermal diffusivity by the laser flash method in air and under vacuum in the temperature range 27 to 390°C. Thermal diffusivities of densified low-porosity JSC-1AF and BCR-2 samples were about the same in air or under vacuum and ranged from 0.46 to 0.76 mm 2 /s at 27°C. On contrary, thermal diffusivities of high open-porosity and fully crystalline NU-LHT-2M samples were approximately 2 to 3 times smaller under vacuum because of the longer conductive path through the pores boundary layer. The thermal diffusivities of tested samples decreased with temperature and were ≈ 20 to 25% lower at 390°C than at 27°C. The effect of temperature on radiative emissivity was monitored with a millimeter wave radiometer/interferometer during the temperature rise from 25 to 325°C. The emissivities for all the tested materials increased with temperature in the same way and ranged from 0.67 at 25°C to 0.96 at 325°C.