The effect of thermal etching on the emissivity of tungsten

Measurements have been made of differences of up to 2?5% in emissivity between grains found on a polycrystalline tungsten strip resulting from both grooving at the grain boundaries and roughening of the grain surfaces. These variations have been related to the magnitude of the surface roughening and to the depth of the grain-boundary grooves. It is shown that, because of the higher emissivity of a grain-boundary groove, emissivity measurements made on a surface of fine grain size can lead to a significantly higher result than those made on a surface of larger grain size owing to the greater number of grooves per unit area in the fine grain surface. Some of the differences between the published values of the emissivity of tungsten due to de Vos and Larrabee can be accounted for on this basis. No evidence has been found for real differences in emissivity between surfaces of different crystal orientation.