Investigating Micropatterned Thermochromic Coatings for Space Vehicle Thermal Management

Thermochromic variable emittance coatings (VECs) facilitate passive, dynamic thermal management of radiators for space vehicles due to their temperature-dependent optical properties. Ideally, a thermochromic material for a space vehicle’s radiator should exhibit low mid-IR emissivity for lower temperatures and switch to high mid-IR emissivity for higher temperatures; one promising material that demonstrates these characteristics is Lanthanum Strontium Manganite (LSM). The transition temperatures of LSM are dependent on the Lanthanum-to-Strontium ratio <i>x</i>, La_1-<i>x</i>Sr<i>_x</i>MnO₃. However, the dielectric function for LSM is not widely available. Here we measure the dielectric function of two common LSM compositions, as well as, their temperature dependence using ellipsometry. We report the dielectric function of La_0.9Sr_0.1MnO₃and La_0.8Sr_0.2MnO₃ from 263K to 363K. Additionally, LSM has high solar absorptance which has a negative impact on space vehicle radiators. To combat this issue our developed VEC will be micropatterned on top of a highly solar reflective layer of Barium Sulfate (BaSO₄). The objective of the micropatterns is to create a mid-IR thermal switch by engineering the feature sizes to create an effective scattering area larger than the LSM pattern which exposes a portion of the BaSO₄ layer to reflect the solar irradiation spectrum. Consequently, this work will elucidate the tradeoffs between solar absorptance and variable emittance for this technology since the micropattern adversely couples the solar absorptance to the mid-IR emittance. Thermochromic VECs will ultimately lead to simplifications of active cooling systems which synergistically correlates to mass reduction and less energy consumption for space vehicles.