Low-emittance copper-coating system using atomic-layer-deposited aluminum oxide

ABSTRACT Copper, due to its unique properties, has a huge technological importance to our society. However, the oxidation of copper remains an issue in numerous application areas. This is especially the case in visible and IR-band optics, where even minuscule oxide layers degrade the thermo-optical properties of copper surfaces. A solution possibly resides in the application of protective coatings, which can simultaneously impair the low thermal emittance of bare copper surfaces. The present paper examines the use of thin Al2O3 layers as a protective coating for copper. Al2O3 layers with thickness of 4.5, 9.1, 18.5 or 28.3 nm were deposited on polished copper discs using atomic layer deposition (ALD). The total hemispherical emissivity and absorptivity of these coated copper discs were measured from 20 K up to room temperature. The emissivity and absorptivity of the copper with ALD-deposited Al2O3 layers increased with rising temperature and layer thickness. Nonetheless, the observed values stayed below 1.8%, allowing the use of the coated copper in systems where low emission or absorption of thermal radiation is needed. Alongside the experiments, we present a computer-based analysis and interpretation, which may be generally applied for prediction of temperature-dependent emittance of metallic surfaces coated with a thin polar dielectric layer.