Effect of roughness on the emissivity of the precious metals silver, gold, palladium, platinum, rhodium, and iridium
Wladimir Sabuga, Reinhard Todtenhaupt
The effect of roughness on the directional spectral emissivity as well as the directional and hemispherical total emissivities was systematically studied for the precious metals silver, gold, palladium, platinum, rhodium, and iridium in order to avoid or minimise the effect of oxidation and other chemical reactions. The microtopography of these surfaces, which were mechanically treated by rolling, turning, grinding, lapping and polishing and subjected to an appropriate thermal treatment to remove crystal structure defects caused by roughening, was characterised by roughness measurements yielding roughness profiles and their statistical characteristics such as roughness height, roughness transfer length and average slope of surface elements. The emissivities were measured by radiometric techniques in the wavelength range 2.5 μm to 51 μm and in the angular range 0° to 75° at a temperature of 150°C. The results of the emissivity and roughness measurements are discussed within the scope of the existing theories for the emissivity of rough surfaces. It was found that roughness with heights smaller than a fraction 0.01 of the wavelength can still affect the radiative properties of the surface. However, the emissivity does not monotonically increase with the roughness height but rather correlates with the slope of parts of the surface profile. For the same metal with different surface finish, the directional total emissivity was observed to vary by at most a factor of 20. These changes are much larger than those caused by structural defects due to roughening and should be attributed to the surface topography.