Emissivity of Boron Nitride and Metals for the Solar Probe Plus Mission

Abstract

For application to the Solar Probe Plus mission (NASA), the behavior and the thermo-optical performance at very high temperatures (range 1100–2200 K) of candidate passive thermal control materials was assessed. On one hand, a pyrolytic boron nitride coating ( thick) was proved to be stable at high temperatures up to 2200 K in vacuum, as well as proved, via total and spectral emissivity measurements at high temperatures, to be able to effectively turn an initially selective solar absorber substrate (carbon/carbon composite) into a solar reflector. On the other hand, chemical vapor deposition coatings made of refractory metals with highly textured surfaces were proved to be able to significantly reduce the temperature of a metallic instrument facing the sun thanks to their high total hemispherical emissivity (up to 0.8 in the range of 1300–1700 K), although it was highlighted that their use showed limitation in term of stability at high temperatures as well as outgassing.