Determination of the complex index of refractory metals at high temperatures: Application to the determination of thermo-optical properties

This paper outlines a measurement method of the complex index nr − jχ in the wavelengths field from ultraviolet (0.3 μm) to infrared (15 μm). The two components of the complex index are determined with error values 8% for nr and 5% for χ, both errors being linked. Spectra of nr and χ are presented for tantalum and molybdenum at various temperatures. These spectra show that the X point for refractory metals could be uniquely due to the variations of nr. We could then derive directly the two optical factors that are the emissivity ε and the reflectivity ρ. Using the spectra for indexes nr(λ) and χ(λ), and applying the theory of DRUDE, we obtained the plasma frequency ωp, the relaxation frequency ωτ as a function of temperature and wavelength and the effective optical mass. These three fundamental parameters enable us not only to model the radiation of a metal in the infrared but also to determine properties such as electrical conductivity γ and thermal conductivity κ. The temperature of the sample was measured using an original method developed in the laboratory: the ultraviolet photons counting pyrometry. Thanks to this method, we can minimize the error due to the poor knowledge of the emissivity value. The precision is in the order of 10 K around 2000 K.