Factors influencing thermal radiative properties of metals

TL;DR

This paper reviews how surface conditions, material purity, and coatings affect the thermal radiative properties of metals, emphasizing measurement challenges and variability in data used for cryogenic heat flow calculations.

Contribution

It provides an analysis of factors influencing metal emissivity and absorptivity, based on 20 years of experimental experience, highlighting the importance of surface and material conditions.

Findings

Surface treatment significantly affects emissivity and absorptivity.

Measurement conditions at low temperatures contribute to data dispersion.

Material purity and surface layers influence thermal radiative properties.

Abstract

Data on thermal absorptivity and emissivity are important for calculations of heat flows in cryogenic devices. Although thermal radiative properties of materials used in cryogenics are measured, published and used for more than sixty years we have noticed that sometimes users of that data do not well understand the significance of individual factors influencing thermal radiative properties. The sensitivity of the thermal radiative properties of metals to the condition of the surface and the difficulty of measurement, especially at low temperatures is probably the reason of dispersion of the published values. The effect of the material and its purity, finishing (machining, mechanical or chemical polishing) and of the protective coatings or accidental layers (water, oxidation) on the thermal radiative properties is not always obvious. Influence of the material, surface treatment, roughness and layers on the surface is discussed on the basis of our 20 year experience with measurement of the emissivity and absorptivity in the range from 20 K to 300 K.