Ultralow Emittance Thermal Radiation Barrier Achieved by a High-Contrast Grating Coating

Thermal radiative emission in vacuum is minimized using metal-backed flexible "space blankets" that have a theoretical minimum infrared emittance of 0.03. However, their presence under oxygenated and degradation-prone environments rapidly increases emittance due to metal oxidation, surface pitting, and implantation of contaminants. A monolithic dielectric coating composed of microscale periodic metasurface gratings on multilayers and metal thin film can achieve sub-1% total emittance. The minimum emittance can be tailored to any temperature-function blackbody emission, so long as the selected dielectric coating materials have near-zero absorption. Using computational optimization and theoretical understanding of high-contrast grating phase-shift mode conditions, we identified characteristic at-wavelength germanium gratings and a near-quarter-wave layer above a low-refractive-index infrared-transparent Fabry-Perot multilayer interference cavity. This dual mechanism can achieve a room-temperature total emittance of 0.0085, paving a new theoretical minimum multilayer insulation effective conductance. As multilayer insulation, this coating offers total effective emittance of 0.0032 per pair of optimally mismatched grating surfaces. This ultrahigh reflection coating design can also be relevant in thermal management of refrigeration and electronic components.