SPECTRALLY SELECTIVE THERMAL COATINGS FOR IMPROVED COOLER PERFORMANCE

摘要

The GOES Imager and Sounder Radiant Coolers are controlled to run at temperatures around 100 K. Future instruments may have added detectors and additional detector heat which will cause the radiant cooler temperatures to rise if design changes are not implemented. Thermal analyses show that lowering the radiant energy from the cooler sun shield (temperatures range between 170 K and 250 K) and/or the Solar Sail Astromast (temperatures range between 270 K and 310 K) adsorbed by the 100 K cooler emitter can significantly lower cooler temperatures if the emitter hemispherical emittance is not lowered substantially. The existing cooler emitter is an open honeycomb with black paint (Z-307) and had an extremely high emittance even at 100 K. The proposed approach is to replace the open honeycomb with a coating that is spectrally selective with low absorptance out to 10 micrometers and high absorptance beyond 20 micrometers. Several coating formulations were developed and parametric thermal analyses were conducted to select the coating formulation for final coating verification. The coating formulation selected was Ag/Al2O3 (14,000 A)/TiO2 (6,000 A)/Al2O3 (14,000 A) vacuum deposited to a highly specular substrate. The thermal radiative properties were: solar absorptance, 0.09, hemispherical emittance at 100 K, 0.80, IR absorptance (200 K blackbody), 0.78, and IR absorptance (300 K BB), 0.65. To take advantage of the low solar absorptance of this cooler emitter coating, a change in the Astromast coating was proposed that would keep its solar absorptance/emittance ratio the same (approximately 1.0), but significantly lower the emittance and thereby lower the IR irradiance on the emitter. The net results reduce the emitter temperature by approximately 9 K. The paper will also contain descriptions of the environmental tests and measurements conducted on the coatings and the results of the thermal parametric studies on the cooler emitter.