Phase Dependence (Liquid/Solid) of Normal Spectral Emissivities of Noble Metals at Melting Points

Normal spectral emissivities of liquid and solid Cu, Ag, and Au have been determined at their melting (freezing) points in the visible region using a cold crucible as the heating method. The use of the cold crucible enables the solidification front to be moved on the molten metal surface slowly enough to measure the emissivities of liquid and solid phases separately at the freezing point. Combined standard uncertainties of the spectral emissivities and wavelengths have been estimated. In silver, the spectral emissivity obtained for the liquid is systematically larger than that for the solid over the visible region, which is consistent with the prediction from a classical free-electron model. In copper and gold, the spectral emissivities at wavelengths around their absorption edges do not change for the solid-to-liquid transition. The wavelength range where the emissivity of copper is independent of the phase is unexpectedly broad (the width is greater than 40 nm), which differs significantly from classical experimental studies on the so-called X-point in the emissivity of copper. A qualitative explanation is provided for the difference in the phase dependence (liquid/solid) of the emissivity between copper and gold.