鉄および代表的な遷移金属の放射率の温度特性

抄録

To estimate the accuracy of the optical thermometry, data for emissivities are indispensable. Recently the reliability of the optical thermometry instruments has increased, as seen in the steel making processes. In these processes the near-infrared wavelength is commonly used for detection of the temperature of a radiating surface. But there are little studies about the emissivity of Fe in this wavelength region. In this paper the temperature dependence of spectral emissivity of Fe and related transition metals are studied experimentally and theoretically.
The analysis of temperature dependence may clarify the fundamental properties of the emissivity of metals. For example it is observed that the emissivity of Au increases with temperature both in the visible and infrared regions. In the visible region the increase in emissivity can be explained by a shift of the optical absorption edge of a bound electron and in the infrared region the increase is explained by a change in electrical conductivity.
The emissivity of transition metals such as Fe in the near-infrared region is affected doubly by the dispersion of bound electrons and by that of free electrons. To analyse this property, the author introduced a classical Drude-Lorentz dispersion model, based on the following study.
By the analysis of total emissivity, it is proved that the ratio λ_τ/σ in the Drude model is independent of temperature difference, where λ_τ is the characteristic wavelength and σ is the static optical conductivity.
By the measurement in the visible region, it is found that emissivities are proportional to λ^-2, Where λ is the wavelength. This property is similar to the reflectivity spectrum obtained by the Lorentz dispersion model. This may justify the adoption of Lorentz model to calculate the emissivity of metals.
The most interesting phenomenon of the emissivity in the near-infrared region is the presence of so called “X point”, or zero temperature coefficient, which is reported by many researchers. This phenomenon is recognized as the essential property of the emissivity of metals. By using Drude-Lorentz model, the X point can be interpreted as follows:
Under the constraint of the dispersion parameters (K_i>10, λ_τ/σ=constant), Lorentz dispersion acts on the emissivity in the visible region and Drude dispersion acts on it in the infrared region separately. The boundary of these two dispersion models appears in the near infrared region, and in this region, the emissivity is little affected by changing the dispersion parameters, or the minimum temperature variation occurs.
Thus it is concluded that the essential characteristics of emissivity for transition metals is not the presence of the X point, but the week-interaction between the Drude dispersion term and Lorentz dispersion term. It is clarified that the temperature variations for the observed emissivities of transition metals could be closely agreed from the visible to infrared wavelength regions with the Drude-Lorentz model.